Emerging photovoltaics: strategies for more stable devices

Resume : PbS Colloidal quantum dots offer a unique combination of low cost, extended spectral coverage and therefore the opportunity for synergism with other established photovoltaic technologies. We will present recent progress towards improving the efficiency of infrared harnessing solar cells based on PbS QDs and the opportunities that arise towards future tandem cells. The bandgap tunability of colloidal quantum dots offers the possibility to realize readily tandem solar cells. The challenge of developing the appropriate intermediate recombination layer is addressed by using an atomically thin layer of graphene leading to the demonstration of PbS QD tandem cells. The talk will also touch upon the very crucial matter of photostability of those materials that remains an open question. We will discuss the role of surface passivation of PbS QDs not only on the performance but also on the photostability of these materials as an initial point towards robust competitive solutions.

Resume : In this work we are presenting experimental results and analysis of luminescence spectra and decay curves of individual organometal trihalide perovskite nanocrystals. Such measurements on ultra-small particles with quantum confinement have been elusive so far due to poor photo stability of this important compound for emerging photovoltaics. The stability of perovskite quantum dots in a polymer matrix has been the most pressing issue from the viewpoint of fundamental and application studies. It was the main focus of this work, where we systematically varied different parameters, such as ambient oxygen, temperature, and halide atom composition, while monitoring emanating light properties in spectral and time domains from single nanocrystals [1]. As a result we found coexistence of two different photodegradation pathways. The first mechanism appears to be related to the oxygen-induced photochemical etching reaction. We verified this hypothesis by TEM measurements and also demonstrated its suppression by proper selection of the oxygen-scavenging polymer protection layer. The other process turns out to be of photophysical nature. It is related to charge trapping, similar to blinking effect known for other semiconductor quantum dots, which is also possible to mitigate by matrix and passivation selection [2]. [1] L. Liu, L. Deng, S. Huang, P. Zhang, J. Linnros, H. Zhong, I. Sychugov, Photodegradation of Organometal Hybrid Perovskite Nanocrystals: Clarifying the Role of Oxygen by Single-dot Photoluminescence, submitted (2019). [2] L. Liu, F. Pevere, J. Linnros, H. Zhong, I. Sychugov "Single methylammonium lead halide perovskite nanocrystals: temperature-dependent luminescent spectroscopy", submitted (2019).

Resume : Colloidal inorganic quantum dot (CQD) solar cells offer many advantages. They can be processed from solution at low temperatures and their optoelectronic properties can be easily tuned during synthesis. One of the most investigated system, lead sulfide (PbS) quantum dots has recently reached a power conversion efficiency of 11.6%, making it promising for application. However, the effects of environmental conditions on the performance of PbS devices remains unclear. For example, it became routine in literature to expose the devices to air prior to measurement, as that has been observed to improve their performance. In this work we investigate the effect of different environments (N2, humidity, oxygen and air) affects the performance of PbS solar cells. While exposure to N2 or humidity lead to a quick decline in performance, exposure to oxygen or air first result in a significant improvement followed by deterioration in PV performance. We show that the latter process is dominated by the oxidation of the PbS extraction layer, rather than the active layer of the device. Therefore the stability of a common PbS CQD cells is mainly limited by its extraction layer and can be significantly improved with the development of new ligands or extraction materials.

Resume : The stability of solution-processed semiconductors remains an important area for improvement on their path to wider deployment. Inorganic cesium lead halide perovskites have a bandgap well-suited to tandem solar cells; but suffer from an undesired phase transition in the vicinity of room temperature. Colloidal quantum dots (CQDs) are structurally robust materials prized for their size-tunable bandgap; yet they too require further advances in stability, for they are prone to aggregation and surface oxidization at high temperatures as a consequence of incomplete surface passivation. Here we report lattice-anchored materials that combine cesium lead halide perovskites with lead chalcogenide CQDs and achieve as a result stability exceeding that of the constituent materials. We find that CQDs keep the perovskite in its desired cubic phase and suppress the transition to the undesired, lattice-mismatched, phases. We achieve an order of magnitude enhancement in air stability for the perovskite, reporting greater than six months? stability in room ambient; and also document more than five hours at 200oC in air. The perovskite prevents oxidation of the CQD surfaces and reduces the nanoparticles? agglomeration under 100oC by a factor of five compared to CQD controls. The matrix-protected CQDs exhibit 30% photoluminescence quantum efficiency for a colloidal quantum dot solid emitting at infrared wavelengths. The lattice-anchored CQD:perovskite solid exhibits a doubling in charge carrier mobility as a result of a reduced energy barrier for carrier hopping compared to the pure CQD solid. These benefits indicate the potential of this new materials platform in solution-processed optoelectronic devices. M. Liu, et al. Lattice anchoring stabilizes solution-processed semiconductors. Nature, under revision.

Resume : The demand for low-cost and flexible photovoltaics is rapidly increasing, creating a need for a new generation of transparent electrodes (TE). Indium Tin Oxide has so far dominated the field of TE but Indium?s scarcity and brittleness have prompted a search into alternatives. Metallic nanowire networks appear to be one of the most promising emerging TE. Randomly deposited silver nanowire (AgNW) networks present sheet resistance values below 10 ?/sq, optical transparency of 90% and high mechanical stability under bending tests. Still, for efficient integration into emerging solar cells (OPV, QDs or perovskites), a large number of additional requirements have to be considered. In this work, we report the impact of the electrical homogeneity of AgNW networks intended for integration in solar cells, through experimental and modelling approaches. Electrode stability is another crucial issue and it involves electrical, thermal and mechanical aspects, ageing and chemical degradation. We investigate the origin of failure in AgNW networks by electrical and thermal stress measurements. Finally, we propose the encapsulation of AgNW networks by a very thin conformal zinc oxide layer deposited by Atmospheric Pressure Spatial Atomic Layer Deposition (AP-SALD). This nanocomposite shows much better thermal and electrical stability; moreover, it is fabricated by low-temperature and up scalable methods compatible with emerging photovoltaic technologies such as roll-to-roll.

Resume : The surface of hybrid perovskites plays a crucial role in the performance and stability of devices, as it strongly influences the recombination rate of excited charge carriers. Recently, it has been reported that molecular ligands such as benzylamine are capable of significantly reducing the surface trap state density in thin films. Here I will report on the mechanism that governs the surface passivation of hybrid perovskites by benzylamine. To this end, we developed a versatile approach to investigate the influence of benzylamine passivation on the well-defined crystal surface of freshly cleaved methylammonium lead tribromide single crystals. We show that benzylamine is capable of permanently passivating surface trap states in these single crystals, resulting in enhanced photoluminescence intensities and charge carrier lifetimes. Additionally, we show that exposure of the perovskite surface to benzylamine leads to replacement of the methylammonium cations by benzylammonium, thereby creating a thermodynamically more stable two-dimensional perovskite (BA)2PbBr4 on the surface of the 3D crystal. This conversion from a 3D to 2D perovskite drives an anisotropic etching of the crystal surface, with the {100} planes being most prone to etching.

Resume : Among the aspects of Halide Perovskites, HaPs, which make them such fascinating materials the different time-scales of the dynamics of interconnected processes stand out. Short-time behavior (< sec) is determined by electronic charge carrier dynamics, while longer time effects are typically due to atom/ion dynamics, characteristic of halide perovskites, such as ion / defect movement, self-healing and others. These slower processes are likely interdependent and hitherto not (well) elucidated, both as phenomena per se and in terms of effects on the performance of HaP-based devices. Our recent work provides clear evidence for self-healing, i.e., under certain conditions, damage in optical properties can be reversed and, in particular, the status quo ante can be re-established, as measured by 2-photon confocal microscopy. We demonstrated that the effect is an intrinsic property of the materials, as it was measured in the bulk of single crystals with typical times of minutes to hours. Here we report on the products of decomposition and possible chemical pathways that can lead to self-healing, paying particular attention to the kinetics of the phenomena. We include now quantitative results of our experiments following the degradation / healing process kinetics in situ and of the energy-dependent damage threshold. Furthermore, we measure, analyze and explain the crucial differences between the light-induced damaging and recovery mechanisms in the bulk and at the surface under different atmospheres. Finally, we measure and analyze the temperature dependence of the healing process, as it provides information about the energy of formation of the material from their binary halide constituents and, possibly, their entropic stabilization.

Resume : Perovskite-based photovoltaic (PV) cells have high power conversion efficiencies, but suffer from rapid degradation. While their excellent performance is often attributed to high defect tolerance, the presence of defects has been linked to their lack of stability. Here, we gradually tune the type and density of defect in MAPbI3 perovskites by fractionally changing their precursor stoichiometry. [1] The layers are exposed to various environments of N2, O2, H2O, and light, and the evolution of their PV performance is monitored. In parallel, we track changes in chemical composition via X-ray photoelectron spectroscopy, and changes in optoelectronic properties via photoluminescence and photothermal deflection spectroscopies, allowing us to correlate the evolution of the defects with the changes in performance. We find that storage stability is linked to the overall density of defects, with highly defective samples degrading fastest. On the other hand, degradation in a pure N2/O2 environment depends predominantly on the density of iodine vacancies, such that samples with a low density of iodine vacancies are most stable, despite the abundance of other defects. Our work elucidates how defects affect degradation and aims to develop guidelines and mitigation strategies for enhanced stability. [1] “Fractional Deviations in Precursor Stoichiometry Dictate the Properties, Performance and Stability of Perovskite Photovoltaic Devices" P. Fassl, et al, Energy Environ. Sci. 11, 3380 (2018).

Resume : Before realizing the large-scale deployment of perovskites solar cells, their instability under different conditions needs to be investigated, although perovskites solar cells have evolved unprecedentedly as a promising photovoltaic technology. In principle, the instability issues associated with the perovskite solar cells could arise from the structural and/or chemical degradation of the absorber layer, interfaces, and charge extraction layers. To realize promising stability, we have explored several approaches, including employing all-inorganic charge extraction layers [1,2], using foreign moieties to passivate the perovskite surface and encapsulate the perovskite grains; as the ionic nature of organic-inorganic perovskites renders them inherently susceptible towards reactive species [3], (c) identifying absorber materials which contains less electrophilic organic cations, and thermodynamically more stable inorganic framework [4], and (d) stabilization of thermodynamically unstable crystal structure [3]. I will discuss in-detail these promising strategies, which allowed the mitigation of stability issues associated with perovskites solar cells. References 1. N. Arora, M. I. Dar* et al., Science (2017), 358, 768-771. 2. M. I. Dar* et al., submitted. 3. T. Zhang, M. I. Dar et al., Science Advances (2017), 3, e1700841. 4. N. Arora, M. I. Dar* et al., Nano letters (2016), 14, 6991-6996.

Resume : The photovoltaic efficiencies of hybrid perovskite materials have reached as high as ~23%; however, the long-term stability of these perovskite materials is still a hurdle for its applications and commercialisation. The degradation of hybrid perovskite materials depends on several factors such as: humidity; heat; trapped charge and UV light. However, the local chemical composition is another key factor that influences the degradation mechanism. Our results reveal localised degradation in these hybrid perovskite materials which is highly dependent on the local composition of the individual grains. Using Secondary Electron Hyperspectral Imaging (SEHI), we followed MAPbI3 local degradation in response to different degradation triggers on the micron to nano-scale. We find individual grains of MAPbI3 that do not have the ideal ABX3 stoichiometry degrade faster than stoichiometric grains under the identical test conditions. We also show that this technique is also applicable to other mixed cation- (Cs and FAI) and halide- (I/Br) based organic-inorganic hybrid perovskites photovoltaic materials and can detect the presence of a non-uniform Br distribution with variation in concentration within the grain interiors and boundaries. These results, therefore, show that precise control of local stoichiometry is required to improve the stability of these hybrid perovskite materials and thus the overall lifetime of perovskite solar cell devices.

Resume : With an unprecedented rise in power conversion efficiency (PCE) to a record of 23.7%, perovskite solar cells (PSCs) are becoming star candidates for the next generation photovoltaics. In this work, we report a systematic study on the spontaneous improvements in photovoltaic performance of PCSs with a composition of Csx(FAyMA(1-y))(1-x)Pb(IzBr(1-z))3. The power output efficiency measured at constant voltage close to the maximum power point on the day of device preparation is initially very low (<13%), but increases gradually over a timescale of some days towards a reasonable stable PCE of ~19%. This improvement is spontaneous and occurs simply upon storage in the dark at room temperature (25 °C) in inert atmosphere (N2). Based on a systematic study on the role of different functional layers of the PSCs, we report that the observed spontaneous improvement in the performance of the PSCs is not a consequence of a perovskite composition, but in general, an intrinsic property of the studied perovskite active layer. Moreover, this improvement is independent from the electron transport material, the presence of the hole transport layer and electrode, or the specific characteristics of doped spiro-OMeTAD. A combination of transient and steady state photoluminescence spectroscopy, as well as ideality factor measurements suggests a reduction in the Shockley-Read-Hall recombination most likely due to a reduction of the charge carrier density within the perovskite layer as cause for the above phenomenon. This reduction, based on an X-ray diffraction study, seems to be related to a change in the crystallographic properties of the perovskite film.

Resume : Organic-inorganic hybrid perovskite solar cells are currently considered as one of the most promising solution-processed solar cell technologies. However, despite their high power conversion efficiency (> 23%) achieved, stability is still an important issue to be resolved preventing their large-scale applications. Toward more stable perovskite solar cells, so far various strategies have been proposed engineering either the composition or the morphology of the perovskite layer itself[1] or the electron/hole transport layers[2]. In particular, the additive strategies, i.e. by incorporating a small quantity of inorganic (K+ , Rb+ )[3] or organic compounds (Gu+ , SCN-)[4] into the perovskite layer, have been shown effective on solar cell stability by different research groups. In this study, via precisely controlling the device working atmosphere (inert gas, O2, and moisture levels), we compared the effects of different inorganic and/or organic additive strategies on the stability of MAPbI3 perovskite solar cells. Relevant materials, optical and device characterizations will be discussed to shed light on the mechanisms of these additive strategies. [1] Zhang, Y.; et al., Adv Mater 2018, 30 (22), e1707143. [2] Tavakoli, M. M.; et al., Journal of Materials Chemistry A 2019, 7 (2), 679-686. [3] Hu, Y.; et al., ACS Energy Letters 2017, 2 (10), 2212-2218. [4] Zhang, Z.; et al., Journal of Power Sources 2018, 377, 52-58.

Resume : The inherent instability of UV-induced degradation in TiO2-based perovskite solar cells was largely improved by replacing the anatase-phase compact TiO2 layer with an atomic sheet transport layer (ASTL) of two-dimensional (2D) Ti1-dO2. The vital role of microscopic carrier dynamics that govern the UV stability of perovskite solar cells was comprehensively examined in this work by performing time-resolved pump-probe spectroscopy. In conventional perovskite solar cells, the presence of a UV-active oxygen vacancy in compact TiO2 prohibits current generation by heavily trapping electrons after UV degradation. Conversely, the dominant vacancy type in 2D Ti1-dO2 ASTL is a titanium vacancy, which is a shallow acceptor and is not UV-sensitive. Therefore, it significantly suppresses carrier recombination and extends UV stability in perovskite solar cells with a 2D Ti1-dO2 ASTL. Other carrier dynamics, such as electron diffusion, electron injection, and hot hole transfer processes, were found to be less affected by UV irradiation. Quantitative pump–probe data clearly show a correlation between the carrier dynamics and UV aging of perovskite solar cells, thus providing profound insight into the factors driving UV-induced degradation in perovskite solar cells and the origin of its performance.

Resume : Catalytic mechanism of seeded PbS-nanocrystals on enhanced crystal growth, texture, and thermal stability of methylammonium lead trihalide perovskite MAPbI3-xClx are elucidated using time-resolved grazing incidence X-ray scattering, X-ray diffraction, and scanning electron microscopy. Analyses of the temperature-dependent and time-resolved crystal formation kinetics obtained from the X-ray results reveal dual roles of the PbS nanocrystals, including accelerating the nucleation of an intermediate phase of highly oriented 2D perovskite layer and subsequently catalyzing the intermediate phase into perovskite crystals. The catalytic role originates from a promoted anion exchange via the surface of MAI-capped PbS nanocrystals at high-temperature annealing, leading to oriented 2D-to-3D perovskite transformation. The hence formed cubic phase of perovskite crystals can be further stabilized by their crystallographic aligned interfaces with the PbS nanocrystal of a similar lattice. The results provide hints on tailoring halide-capped quantum dots for formation of specific perovskite crystalline films.

Resume : With a power conversion efficiency over 23%, perovskite solar cells (PSCs) are considered a rising star in the solar energy. Nowadays a lot of research is focused on the improvement of the device performance through the employment of new materials or architectures, but stability remains one of the biggest issues. Moreover, the importance of interfaces in such a system is well known and in particular a crucial role is played by the energy level alignment of the different layers [1]. A good match between the electronic bands is required in order to obtain high performance PSCs structures, to this purpose we functionalize the interface between the perovskite and the charge selective contacts within the device. We make use of specific molecule-to-substrate interactions to self-assembly perfluorinated small molecules on the perovskite surface. This not only leads to a shift in the perovskite work function and therefore to a reduction of the bands offset, but also to passivation and reduction of recombination [2]. Moreover, the formation of a nanometer thick perfluorinated layer results in hydrophobicity of the perovskite surface, which enhances stability by preventing the ingress of water from the atmosphere. Notably, such a functionalization can be done through scalable solution processing methods, which are compatible with fast output production including roll-to-roll and inject printing. We investigate the impact of the functionalization on material and device by characterizing the change in the energetics of the system and correlating them with the PSCs performance. Our results show that the interface functionalization with perfluorinated molecules is an effective new approach to improve energy levels alignment and enhance PSCs performance. Our aim is to implement this technique in order to create a universal tool to tune the work function and control its shift, which would lead to more flexibility in the choice of materials and structure. [1] P. Schulz, E. Edri, S. Kirmayer, G. Hodes, D. Cahen, A. Kahn, A. Energy Environ. Sci., 2014,7, 1377-1381. DOI: 10.1039/C4EE00168K [2] C.M. Wolff, L. Canil, N.L. Nguyen, P. Caprioglio, L. Fiedler, M. Stolterfoht, A. Abate & D. Neher, submitted

Resume : Organic-inorganic perovskite materials are widely used for solar cell applications due to their extraordinary optoelectronic properties and their panchromatic response as light absorbers [1]. Perovskite solar cells (PSCs) have rapidly evolved and reached power conversion efficiency (PCE) exceeding 23%. However, several issues including further efficiency increase and performance stabilization have not been effectively addressed yet. Thus, several strategies, such as perovskite interfaces engineering have been developed towards optimizing the overall performance of a PSC [2]. A very interesting approach recently introduced by our group includes the dye sensitization of the titania compact layer. Thus, the use of D35 organic dye led to more efficient and stable PSCs [3]. In this work we expanded our approach to metallated dyes. In fact, a Zn-porphyrin (ZnTPP-CC-BDP) [4] comprising a boron-dipyrromethene as the electron acceptor moiety, was spin-coated on the TiO2 electron transport layer of planar perovskite solar cells. The insertion of the porphyrin thin layer, between the compact layer and the perovskite absorber (CH3NH3PbI3) reduces the interfacial defects, increases the electron transfer towards the TiO2 and improves the crystal growth of the perovskite film. The photoelectrochemical characterization confirmed that the porphyrin-based devices present a PCE of 17.33%, outperforming that of the reference cell (16.62%). This significant efficiency increase is mainly attributed to short-circuit photocurrent density enhancement and is accompanied by greater stability, opening new perspectives in the field of PSCs. ACKNOWLEDGMENTS This work was supported by European Union’s Horizon 2020 Marie Curie Innovative Training Network 764787 “MAESTRO” project. ΙΚΥ Scholarship Programs, Strengthening Post-Doctoral Research Human Resources Development Program, Education and Lifelong Learning, co-financed by the European Social Fund – ESF and the Greek government is also acknowledged. References: [1] H. S. Jung, N.-G. Park, Small, 2015, 11, 10–25. [2] H. Zhou, Q. Chen, G. Li, S. Luo, T.-B. Song, H.-S. Duan, Z. Hong, J. You, Y. Liu, Y. Yang, Science 2014, 345, 542. [3] N. Balis, A.H. Zaky, D. Perganti, A. Kaltzoglou, L. Sygellou, F. Katsaros, T. Stergiopoulos, A.G. Kontos, and P. Falaras. ACS Appl. Energy Mater. 2018, 1, 6161–6171. [4] N. Balis, A. Verykios, A. Soultati, V. Constantoudis, M. Papadakis, F. Kournoutas, C. Drivas, M.-C. Skoulikidou, S. Gardelis, M. Fakis, S. Kennou, A. G. Kontos, A. G. Coutsolelos, P. Falaras, M. Vasilopoulou, ACS Appl. Energy Mater. 2018, 1, 3216-3229.

Resume : Solution-processed organic solar cells (OSCs) are promising owing to ease of processability, low cost, flexibility, light weight, and large-area fabrication. Particularly, small molecule (SM) active materials have been recently developed using straightforward synthesizing methods, exhibiting the least batch-to-batch variation in physical and optoelectronic properties and highly reproducible efficiency. In this study, we focused on a series of bis-benzodithiophene (BDT2)-based active materials which have different aromatic bridges between BDT2 as the electron donating core and the electron-withdrawing rhodanine unit and the effects of the aromatic bridges on the physical, optical, electrochemical, and photovoltaic properties. Along with changing the pi-conjugated bridge structure through rearrange bridge building block, the resulting compounds exhibited different photophysical properties, HOMO/LUMO energy levels, charge carrier mobilities and morphologies of blend films. Moreover, the effects of solvent vapor annealing on molecular structures exhibited a significant difference in photovoltaic properties. One of them shows an excellent power conversion efficiency of 9.16%. Detailed synthetic scheme, optical, electrochemical, and photovoltaic properties of the oligomers will be presented.

Resume : The effects of the molecular weight (MW) of an environment-friendly processable semiconducting polymer (asy-PBTBDT) on its photovoltaic results and thermal stability of the devices are firstly shown. The asy-PBTBDT with a high MW had the largest μh values because of increase in the π–π stacking along with MW relative to low-MW asy-PBTBDTs. The high-MW asy-PBTBDT with a large μh achieved the highest power conversion efficiencies of 18.2% and 20.0% for the non-doped and doped states in PerSCs, respectively, and 5.7% in PSCs in green processed devices. Moreover, the glass transition temperature increased with an increase in MW; this showed an effective decrease in heat-induced morphological degradation in the photovoltaic devices. As well, as the chain density with MW increases, the robustness against humidity and oxygen is improved.

Resume : Instruments for high-resolution continuum source absorption spectrometry (HR-CSAS) developed at ISAS extended the range for highly sensitive element analysis to numerous non-metal elements, which became possible by measuring absorption spectra of diatomic molecules. DMSO (dimethyl sulfoxide) is one of the common solvents for the deposition of hybrid perovskite films for solar cells. By applying HR-CSAS equipped with a graphite furnace, the amount of residual DMSO in CH3NH3PbI3 films was precisely determined. For this purpose, the DMSO molecules were separated from Pb and converted into CS at high temperature. The molecular absorption of the separate solutions of CS and Pb was measured at 258.056 and 261,418 nm, respectively. DMSO / Pb molar ratios were precisely determined. The mass limit of detection of S is 2.3 ng for the given method and can be reduced to less than 0.5 ng S by using mini graphite tubes.

Resume : The active layer of organic photovoltaics (OPV) devices, with a bulk heterojunction morphology, is a complex and highly disordered system, and thus raises issues on the reproducibility (difficult to control the domains between different devices), stability (dimensions of the domains change with time and also the oxygen quenching) and characterization (donor:acceptor properties are usually obtained from experiments in solution). In this communication we will present a new electron donor:acceptor (D:A) interface architecture, by tailoring the confinement in mesoporous silica nanoparticles (MSNs). This strategy relies on the structural versatility of MSNs, allowing the incorporation of the acceptors on the pore walls of the silica structure, while entrapping the donors within the pore volume. This way, we could control the loading and the location of the D:A, the pair mobility will be restricted due to the confinement, avoid the oxygen interference, and have an accurate characterization of the D:A properties in the same environment as they are used in the active layer. The versatility of our strategy is based on our recently discovered ability to accurately tune the diameter of the particle to a few tens of nanometers, as well as their pore width and geometry.[1] Additionally we have used MSNs as nanocontainers and carriers due to their high surface areas, large pore volumes, high loading capacity, and versatile surface functionalization. This synthetic versatility allow i) the incorporation of small molecules into the silica structure; ii) the modification of the external surface; and iii) the pore volume loading with molecules of interest.[2] We have successfully incorporated a perylenediimide derivative (acceptor) in the nanocontainer structure and a conjugated polymer (donor) inside the pore volume of MSN. The photophysics and photochemistry characterization shows large charge transfer lifetimes even in the presence of oxygen (shielding effect from the nanocontainer structure). References [1] S.V. Calderon, T. Ribeiro, J.P.S. Farinha, C. Baleizão, P.J. Ferreira, Small 2018, 14, 1802180 [2] T. Ribeiro, E. Coutinho, A.S. Rodrigues, C. Baleizão, J.P.S. Farinha, Nanoscale 2017, 36, 13485

Resume : Through optimization of fabrication procedures and material composition, remarkable progress has been achieved in perovskite solar cells research during the last years now reaching power conversion efficiencies above 23 %. Reproducing published fabrication procedures is, however, not trivial as process conditions may vary from lab to lab. Adapting methodologies developed for small-area devices based on spin-coating to larger area devices is challenging and requires re-engineering of the deposition method. Understanding of the film formation process during different stages of processing allows for a more rational approach to translate perovskite deposition strategies to scalable processing methods. We carried out in-situ spectroscopic measurements on the temporal evolution of cesium-containing triple cation perovskite (3CAT) and MAPbI3 on PTAA-covered during spin-coating using a fibre-optic based photoluminescence and reflection spectroscopy setup. Through varying the time of a crystallization-inducing anti-solvent drip, we identified the process window for growing MAPbI3 that would translate to high device performance to be narrower compared to 3CAT. Our results prove that the composition strongly influences the crystallization kinetics in perovskite processing. Systematic in-situ studies to rationalize film formation as a function of composition and processing conditions enable knowledge-based engineering of high quality perovskites compatible with larger area processing.

Resume : The discovery of hybrid organo-inorganic perovskite semiconductor materials paved a way to a new generation of solar cells delivering impressive power conversion efficiencies. However, the intrinsic instability of the active materials restricts further industrial application of this emerging PV technology. A promising solution to extend the lifetime of perovskite solar cells (PSCs) is based on tailoring the properties of adjacent charge transport layers. In this work, we systematically investigated the impact of the electron-transport layer (ETL)/perovskite interface on the stability and efficiency of PSCs in n-i-p solar cell configuration. Five families of materials including polymers (PS, PVA, PMMA), perylene (PTCDA, PDI) and fullerene derivatives (PCBM, PCBA), metal sulfides (ZnS, SnS2, CdS, In2S3) and thioacids were explored as thin (<20nm) passivation coatings for oxide ETLs: ZnO and SnO2. It was shown that the type of the coating affects significantly both thermal and photostability of PSCs. A combination of complementary techniques such as optical spectroscopy, XRD, XPS, AFM, etc. allowed us to reveal the main degradation pathways. Finally, some important correlations were found between the chemical composition and morphology of the passivation coatings and the PSCs operational lifetime. These findings should facilitate the development of highly efficient and stable perovskite solar cells suitable for commercial applications.

Resume : Perovskite solar cells represent one of the most promising photovoltaic technologies. Over the past few years, the performance of perovskite solar cells was gradually improved up to >23%, which is close to the characteristics of crystalline silicon photovoltaics. However, high toxicity and low stability of complex lead halides used as absorber materials hamper commercialization of this technology. Compositional engineering of lead perovskites was actively pursued in order to improve their stability and/or performance. In particular, a partial or full replacement of Pb2+ in MAPbI3 is highly desirable in terms of developing more environmentally friendly materials. Here we present a systematic study of lead substitution in MAPbI3 with >20 different cations introduced in atomic concentrations ranging from 10-5 to 20-30%. It was shown that replacing even minor fraction of lead could change significantly the perovskite film crystallinity and morphology. Importantly, the efficiency and stability of p-i-n and n-i-p perovskite solar cells were improved considerably by an appropriate modification of MAPbI3 films. Moreover, important relationships were established between the nature of the substituting ions (e.g. ionic radius or charge) and their effects on electronic properties and photovoltaic performance of the resulting hybrid perovskites. The obtained results should facilitate the rational design of more stable and less toxic absorber materials for advanced perovskite solar cells.

Resume : In this study, we investigated the characteristics of amorphous V2O5-InZnO (IZVO) combined electrodes to apply as an anode in perovskite solar cells (PSCs) with p-i-n structure. To modify the work function, IZVO combined electrodes as an anode were fabricated using a simultaneous sputtering process of V2O5 and IZO targets as an anode for PSCs. The electrical, optical, morphological, and structural properties of IZVO films were examined as a function of RF power on V2O¬5 target in co-sputtering system. Especially, the optimal IZVO film which has a high figure of merits shows a sheet resistance of 43.37 Ohm/sq, resistivity of 4.34×10-4 Ohm-cm, and average transmittance (wavelength 400-800 nm) of 89.25 %. Through SEM image, the morphologies of IZVO films indicate smooth and amorphous surfaces. In addition, we verified that the optimal IZVO electrode demonstrated amorphous structure through cross-sectional TEM image and XRD analysis. To compare the reference ITO and IZVO films, we investigated the flexibility using various bending tests such as critical radius, fatigue, rolling, and twisting tests. From these results, the amorphous IZVO films evaluated the various properties as a transparent conductive electrode, and expected to be applied to a flexible PSCs to enable a low cost and large area process.

Resume : Optically thin silver films are attracting growing attention as a viable alternative to conducting oxides as the transparent electrode in organic photovoltaics (OPVs), not least because they are compatible with flexible substrates. This talk will show how plasmonic excitations at the surface of evaporated optically thin silver films with very low sheet resistance can be harnessed to enhance light trapping in high performance OPVs. The talk will present device performance studies, in conjunction with modelling of the near-field optical intensity, detailed morphological characterization of the silver film electrodes and time resolved photoluminescence measurements to support the conclusions. An ultra-thin organic interface layer will also be presented that enables the significant potential of plasmon-active silver electrodes to be harnessed in OPVs with an inverted device architecture.

Resume : Nanostructuring modest materials without exceptional physical properties is a key step to compete with established materials in the photovoltaic field. In particular, sensitized solar cells (SSC) are based on materials with poorer electrical and optical properties than silicon and thin film based solar cells. SSC are typically characterized by mesoporous TiO2 layers, as electron transport material, coated by a light absorber and a hole transporter material. A promising type of SSC is the extremely thin absorber (ETA) solar cell. Sb2S3 has received increasing attention as ETA material due to its unique properties (α ≈ 1.8 × 105 cm–1; Eg = 1.7 eV). Typically, Sb2S3 is synthesized by chemical bath deposition (CBD) on mesoporous TiO2 electrodes. CBD brings formation of oxides and non-uniform coatings, and mesoporous TiO2 lack of control over geometrical parameters presenting non-defined pathways. We present a strategy to overcome these limitations by using parallel coaxial nanostructures whose geometrical parameters can be tuned. These nanostructures are functionalized as template for growing heterojunctions of thin films. TiO2 and Sb2S3 are deposited by atomic layer deposition allowing for uniform and conformal coatings all over the surface. This strategy allows for individualized study of the geometrical parameter effects in the optical and electrical properties of these materials, aiming for their optimization in solar cell devices.

Resume : The engineering of broadband absorbers to harvest white light in dye sensitized solar cells (DSSCs) is a major challenge in developing renewable materials for energy harvesting. In this study, gold (Au) rod-like sub-branches are successfully attached to the FTO substrate using an adhesive to produce the gold electrode. The proposed approach for fabricating photocathode is demonstrated to be facile and cost-effective, as opposed to existing techniques. Compared with electrodes prepared with sputtered Au, the Au rod-like sub-branches photocathode demonstrates significant increase of the power conversion efficiency (PCE) of a photoelectrochemical solar cell of 65% and higher catalytic activity with a cobalt-complex electrolyte. More importantly, the PCE of Au rod-like sub-branches photocathode improved by 16.6% compared to standard platinum (Pt) counter electrode. The increased efficiency is attributed to broadband absorption in the visible light region which due to incredible broadband plasmonic effect and presented lower lose in light-mater interaction of the Au. We have developed an electrochemical deposition process that enables scaled-up production of this nanostructures for large-scale energy-harvesting applications.

Resume : To establish a low-cost and easy-process type photovoltaics, our team has proposed carbon nanotube-silicon (CNT-Si) heterojunction solar cells. And to improve the performance of pristine CNT-Si solar cells, many methods have been explored. Although some progress was made, it is still demanding to develop a simple method with effective materials to achieve this goal.Many metal chlorides are soluble in common organic solvents which ensures good wetting to both CNTs and Si (which are usually in hydrophobic state), an important factor for promoting mutual interaction between these components and tailoring the solar cell behavior. Based on this, it is convenient to form a uniform metal chlorides layer onto CNT-Si surface by simple spin-coating technique. Here, we explored the effect of 17 kinds of metal chloride spin coated on CNT-Si solar cells. After correlation analysis we first time found metal chlorides’ effect on open-circuit voltage (Voc) has close relation with standard electrode potential of corresponding redox couple. By the combination use of FeCl3 and ZrCl4, whose function majorly in improving Voc and short-circuit current density, respectively, a typical solar cell with PCE of 15.6% and good stability (remained ~96% after 36 hours) was established. With general and specific study, we largely raise solar cell efficiency with good stability by a simple method. At the same time, present a guidance for choosing effective optimization materials in the future.

Resume : Hybrid lead halide perovskites are promising materials for future photovoltaics applications. Their spectral response can be readily tuned by controlling the halide composition, while their stability is strongly dependent on the film morphology and on the type of organic cation used. Mixed cation and mixed halide systems have led to the most stable perovskite solar cells reported, so far mostly prepared by solution-processing. This might be due to the technical difficulties associated with the vacuum deposition from multiple thermal sources, requiring a high level of control over the deposition rate of each precursor during the film formation. Here we present multi-component materials obtained by using multiple sources (3 and 4) thermal vacuum deposition. Different stoichiometry were studied, including A-site cations such as methylammonium, formamidinium and cesium, as well as mixed halide systems. In particular, mixed iodide-bromide perovskites with bandgap up to 1.8 eV were prepared, as they are ideal as front cell in perovskite-perovskite tandem devices. These thin-film absorbers are implemented into fully vacuum deposited solar cells using doped organic semiconductors, obtaining power conversion efficiency in excess of 16%. We highlight the importance of the control over the film morphology and composition, which differs substantially when these compounds are vacuum processed. Avenues to improve the stability and to maximize the open circuit voltage using additives and novel charge transport layers will be discussed.

Resume : Depth-dependent growth of perovskite crystals remains challenging for high-performance perovskite solar cells made by a two-step spin-coating method. Effective morphology engineering approaches that enable depth-independent perovskite crystals growth and facile characterization technique to monitor subtle yet influential changes accompanied are urgently required. Here, a porous and intercrossed PbI2-(CsI)0.15 nanorods scaffold is prepared by integrating CsI incorporation with toluene dripping in ambient air, and the mechanism underlying is uncovered. With this porous scaffold and moisture-assisted thermal annealing, depth-independent growth of FA0.85Cs0.15PbI3 is achieved, as evidenced in the photoluminescent (PL) spectra acquired by exciting the perovskite film from the top and bottom side, individually. It is of broad interest that PL spectroscopy is demonstrated as a sensitive technique to monitor the depth-dependent growth of perovskite. Moreover, the resulting inverted planar FA0.85Cs0.15PbI3 perovskite solar cells deliver an efficiency of 16.85%, along with superior thermal and photo stability. By incorporating 2% large-sized diammonium cation, propane-1,3-diammonium, the efficiency is further increased to 17.74%. Our work not only proposes a unique porous PbI2-(CsI)0.15 nanorods scaffold to achieve high-quality perovskite films in a two-step method, but also highlights the distinctive advantage of PL spectroscopy in monitoring the depth-dependent quality of perovskite films.

Resume : Semiconducting metal oxide films have been widely used as charge extraction layers in photovoltaic devices because of their superior electronic properties and stability compared to the organic counterparts. Among these, MoOx, WOx, GO and NiOx are well studied for hole extraction. These oxides suffer from high resistivity which limits their application to only very thin layers. However, their conductivity can be improved by creating metal vacancies and doping with some elements. Several chemical and physical methods were reported for the preparation of these oxides but to meet the requirements for large-scale, low cost and roll-to-roll production, the solution processable methods are more desirable. Therefore, we focus on the chemical synthesis of pre-formed nanocrystals to produce solution-processed films not requiring annealing treatment and thus compatible for both normal and inverted structures, and more specifically on doped NiOx using different dopants (Li, Cu, and Sn) in order to obtain thick films having acceptable conductivity by robust solution processing. The initial low work function of the materials was increased by doping the nanocrystal solutions with an organic acceptor, namely F4-TCNQ. We applied these hybrid materials to high efficiency polymer solar cells using both fullerene and non-fullerene acceptors. The impact of the doping on the performance, stability, air and thick layer processing will be discussed.

Resume : Perovskite solar cells (PSCs) have shown a rapid development in efficiencies from 6% to over the 22 % during the last decade. Their excellent photovoltaic performance is based on such physical material properties as a tunable band gap, good absorption properties and the formation of mainly energetically flat defect states in the band gap, which in turn lead to long diffusion lengths and a large charge carrier lifetime. However, aside from the Pb issue, the main challenge of perovskite photovoltaic devices is still their poor stability under working conditions, caused e.g. by moisture, high temperature and light. One of the degradation mechanisms is the decomposition of the perovskite layer itself, leading to reduced absorption properties and strong charge carrier recombination. Moreover, such decomposition compounds may react with charge carrier extraction layers or the contact metallization, resulting in unbeneficial high serial resistances and parasitic capacitances. Therefore, in this contribution, we have investigated the influence of different contact metallizations, such as Al, Ag, Au and Ni on the stability of inverted planar methylammonium lead iodide based perovskite solar cells without encapsulation. For this study, we have used IV and impedance measurements in combination with SEM and TOF-SIMs analysis to examine and correlate the electrical properties of PSCs with a perovskite / metal reaction, as well as the diffusion of metals into the extraction and perovskite layers. While we detected only a weak metal diffusion, a strong corrosion of the contacts and consequently a significant solar cell degradation was detected for devices with Al, Ag and Au contacts, implying the diffusion of perovskite decomposition products to the contacts and a metal dependent reaction. An exception from rule are Ni contacts, which demonstrate no metal corrosion or solar cell efficiency decrease after 1-month if storage and select AM1.5 testing in the ambient atmosphere, which is a positive step towards stable perovskite solar cell operation.

Resume : Advanced in materials development and device engineering have led to power conversion efficiencies (PCEs) of bulk-heterojunction (BHJ) organic photovoltaics (OPV)s. The surface modification of zinc oxide plays a significant and unique role in the enhanced performance of OPVs. Many surface modifiers have been tested successfully, including non-conjugated organic materials like as polyethylenimines (PEIs) and their ethoxylated derivatives (PEIEs), and organic and inorganic hybrid materials. However, applying biomaterials in optoelectronic devices have recently received great research interests since they not only possess economic benefits but also can facilitate the sustainable development of technology. We herein investigated the effectiveness of conjugated, amphiphilic and eco-friendly polymers as zinc oxide surface modifiers in inverted OPVs. Based on the manipulating the surface modification of ZnO, leading in the PCE enhancement of OPVs. In particular, the eco-friendly polymers, their abundant availability in the environment renders them to be easily accessible and more economical as compared to other commonly used polymeric interlayers. Our results reveal the eco-friendly polymer, methyl-cellulose, was demonstrated as the most efficient modifying interlayer for ZnO ETL, which enables 9.47% and 6.34% enhancement in PCE for the representative fullerene- and NFA-based BHJ systems (PTB7-Th:PC71BM and PBDB-T:ITIC), respectively, as compared to the control devices.

Resume : Time- related degradation effects have been rarely investigated to develop accurate current–voltage characteristics of photovoltaic devices. The development of new models including time- related degradation effects is crucial to predict the device behavior as function of stress time and provide the possibility to estimate the degradation behavior of the solar cell operating under stress conditions such as: prolonged radiations and long term reverse biasing. In this paper, we propose new electrical modeling approach including time-related degradation effects to predict the solar cell behavior working under stress conditions. The obtained results demonstrated that the accurate modeling of solar cells must be considered as a time-related degradation effect instead of static mechanism since the defect generation and annihilation is a dynamic process. Moreover, the results show the importance of the time-related degradation effect-based modeling in investigating the device physics of solar cells under prolonged radiations for spatial applications.

Resume : Organic Solar Cells (OSC) based on fullerene with diluted donor content has gained attention in recent years to be an efficient alternative to provide substantial generated photocurrent and increased open circuit voltage. The main challenge using such peculiar organic solar cell architecture is to realize hole extraction pathways to generate current as there are no such percolation pathways towards the contacts as compared to the bulk heterojunction donor: acceptor blends. The current generation mechanism explained by different researchers is not yet very clear and needs further development to provide exact insight into the physics of such architecture. In this study, we set up Kinetic Monte Carlo (kMC) model for fullerene based OSC with donor material (~1%) distributed in the form of polymer chains of different lengths. The generated photocurrent by hole escape mechanism is then investigated based on the highest occupied molecular orbitals difference between acceptor and donor [1]. Furthermore, the study is extended to the different configuration and distribution of the polymer chains. As the diluted configuration has no successive percolation pathways towards the electrodes, we get confirmation that the charge can still be transferred to the electrodes without the requirement of percolation pathways and substantial current is generated only at smaller highest occupied molecular orbitals difference. Ref: [1] T. Albes et al., J. Phys. Chem. C, 2018, 122 (27), pp 15140?15148

Resume : Processes taking place at contacts are of particular importance in organic and perovskite solar cells where selective contacts that are able to efficiently collect majority carriers, simultaneously blocking minority carriers are desired. However, a comprehensive understanding of the processes taking place at the contacts in organic thin-film semiconductor devices is still lacking. The surface recombination velocity S_R, describing the quality of the contact interface, is a key parameter in obtaining an increased understanding of the kinetics taking place at contacts in thin-film devices [1]. We have also extended the analytical framework of the charge extraction by linearly increasing voltage (CELIV) theory taking the effect of built-in voltage, diffusion and band-bending into account and show how we can experimentally quantify loss mechanisms in charge collection [2-3]. We have derived analytical expressions describing the effective reduction of the built-in voltage and the (effective) open-circuit voltage providing means to quantify and distinguish various (loss) mechanisms for contact related effects in thin film solar cells. We have used the technique to experimentally measure surface recombination velocities at collecting contacts in interface-limited organic semiconductor devices [4]. References [1] O.J. Sandberg, et al., Physical Review Applied 1, 024003 (2014) [2] O.J. Sandberg, et al., Organic Electronics 15, 3413-3420 (2015) [3] S. Dahlström, et al., Physical Review Applied 10, 0554019 (2018). [4] O.J. Sandberg, et. al, Physical Review Letters, 118, 076601 (2017).

Resume : Despite a significant progress in organic photovoltaics (OPV), the development of stable and efficient organic solar cells still represents a big challenge. Inducing optimal active layer morphology remains the most non-trivial technological step in production of OPV cells. Implementation of block-copolymers with donor and acceptor parts linked by covalent bond might facilitate controlling the size of domains and their arrangement in the films. Although this approach has been proposed long time ago, block-copolymers with optimal characteristics have not been developed yet. Here we report a series of six model block-copolymers designed and investigated with the aim to identify some correlations between the structure and properties of these materials and their performance in OPV cells. Donor and acceptor blocks were loaded with the solubilizing chains of different nature, e.g. alkyl vs. fluoroalkyl or oligoethylene glycol units, in order to facilitate their demixing and phase segregation in thin films. Probe microscopy revealed the nanoscale morphology of block copolymers in thin films and confirmed the formation of separate donor and acceptor domains. A clear relationship has been established between the structure and film morphology of the block-copolymers and their photovoltaic performance. These findings highlight a promising strategy for future design of new photoactive block copolymers with improved characteristics to enable efficient and stable organic solar cells.

Resume : For the past two decades, various conjugated polymers such as Poly (3, 4-ethylenedioxythiophene) (PEDOT) polyaniline (PANI), polypyrrole (PPY), have been tried to fabricate optoelectronic (OE) devices (organic photovoltaic (OPV), organic light emitting devices (OLED)) due to their good conductivity, environmental stability, flexibility and ease of synthesis [1]. However, commercialization of their practical applications is still under consideration due to some drawbacks such as poor processability, inhomogeneity, less lifetime etc. Presently poly-styrene sulfonate (PSS) doped PEDOT is widely using as hole transport layer (HTL) in OE devices to improve their performance by reducing free charge carrier recombination and surface defect induced loss. However, it has some serious disadvantages like, degradation effect on active layer, vulnerability to UV illumination and shorter lifetime. Therefore, several alternative materials like organic acid (CSA, DBSA, and PSS) doped PANI, PPY have been tried to replace PEDOT: PSS. Where the doping reagents have helped to improve their processability and conductivity [2]. Review reveals that there is a strong effect of the doping concentration of HTL material on the performance of OE device. Therefore, in this work we have prepared PSS doped PANI with various concentration of PSS. The transmittance of PANI: PSS films has been observed above 90%. The work function of the PANI: PSS films has been estimated within 4.9 eV-5.2 eV. Further, we have fabricated OPVs having poly (3-hexylthiophene) (P3HT) and [6, 6] - indene-C60 bisadduct (ICBA) as active layer and PANI: PSS as HTL to investigate the effect of doping concentration of HTL on the overall performance of the OPVs. References 1. Ziyang Hu, Jianjun Zhang, Zhihong Hao and Ying Zhao, Solar Energy Materials and Solar Cells 95, 2763-2767 (2011). 2. Omar Abdulrazzaq, Shawn E. Bourdo, Myungwu Woo, Viney Saini, Brian C. Berry, Anindya Ghosh, and Alexandru S. Biris*, Appl. Mater. Interfaces 7, 27667−27675 (2015).

Resume : Chalcopyrite-based chalcogenides have drawn great attention as a photovoltaic (PV) absorber for thin-film solar cells since the Cu(In,Ga)Se2 (CIGS) record PV device has exceeded more than 22% power conversion efficiency (PCE).[1] Despite such a high performance of the CIGS photovoltaic device, the market share of CIGS solar cell(<5%) is quite smaller than that of crystalline silicon solar cells (>90%). In order to compete with crystalline silicon solar cells, various strategies have been suggested such as the use of abundant materials (i.e. Cu2ZnSn(S,Se)4), further enhancement in device performance (i.e. alkali PDT), etc. However, none of them have been successfully demonstrated resulting in expansion of CIGS solar cell deployment. Therefore, the development of flexible CIGS solar cells used for building-integrated photovoltaics could be one the plausible ways to increase competitiveness in the PV market. Up to now, several flexible substrate materials have been utilized to realize flexible CIGS solar cells. For example, the CIGS solar cell on a flexible plastic substrate yielded 20.4% PCE.[2] However, given that high process temperature (>550C) for high-quality CIGS film deposition is required, polyimide with lower decomposition temperature (<500C) enables to limit process window to prepare CIGS film compared to metal foils. In this presentation, stainless steel (STS, SUS430 with thickness of 30 μm) as a flexible substrate was used. Particularly, the Cr or Ti barriers of impurities diffused from STS substrate were optimized by identifying trace of Fe concentration through SIMS analysis. For optimal bandgap grading in CIGS films, Ga elemental distribution in the CIGS films was adjusted through modification of well-known three-stage process. Na and K post-deposition treatments were also applied at the end of CIGS film deposition to improve device performance. As a result, we have achieved >18% PCE of flexible CIGS solar cell on STS substrate. Details on fabrication procedures and characterizations of CIGS PV devices are discussed.

Resume : Metal oxide perovskites (MOPs) are wide bandgap materials. Some MOPs are likely to have electron affinities (EAs) that make them suitable as Electron-selective Transport Layers (ETLs) in halide perovskite (HaP)-based solar cells. The rich substitution possibilities of oxide perovskites (regarding the A2+ and B4+ cations in the materials with ABO3 stoichiometry) allow tuning the EA to optimize it for use as ETL with HaPs. Sr- and Ba-based MOPs are particularly interesting as SrTiO3 has energy band positions similar to those of TiO2, the most commonly used ETL today. For BaTiO3 and BaSnO3, the EA is just 0.1 eV smaller and 0.3 eV larger than that of SrTiO3, respectively. We deposited MOPs solid solutions with a compositional spread by pulsed laser deposition (PLD) as ETLs and measured the resulting distribution of lattice parameters and energy band positions. Onto this library, a uniform layer of mixed halide perovskite (HaP) absorber is deposited, and the effect of the different ETL compositions on the interface with the HaP absorber and transport across it is studied. We aim to achieve control over the interfacial energy band alignments and to find the extent to which the electronic energy level and structural measurements of separate layers can be used to optimize electron transport processes across the interfaces in complete solar cells.

Resume : Two new A-D-A non-fullerene small molecule acceptors based on perylenediimide dimmer connected through different donor units (dipyridine –fluorene-di thiophene, PDIPFT and fluorene, PDIF) at bay positions were synthesized and investigated their optical and electrochemical properties. Both of them showed strong absorption in the shorter wavelength and suitable electrochemical energy levels and are complimentary to the most of the polymer donors. These two non-fullerene acceptors were employed as acceptor along with the polymer PTB7 (having optical band gap 1.58 eV) having complementary absorption spectra to both of the non-fullerene PDI based small molecules. Both of these two non-fullerene acceptors exhibit suitable frontier molecular energy levels for efficient exciton dissociation and charge transfer at donor –acceptor interfaces present in the bulk heterojunction active layer consists of a mixer of PDIPFT or PDIF and PTB7. After the optimization of active layer, the polymer solar cells based on PTB7:PDIF and PTB7:PDIPFT showed overall power conversion efficiency of about 6.17 % (Jsc =13.08 mA/cm2, Voc =0.83 V and FF = 0.57) and 8.66 % (Jsc =15.14 mA/cm2, Voc =0.88 V and FF= 0.65), respectively. The higher power conversion efficiency of the polymer solar cell based on PDIPFT may be related to the broader absorption spectra and high charge carrier mobilities and more balanced charge

Resume : In recent years, the particular attention in the problems of thermoelectricity has been given to the composite and nanostructured materials, demonstrating the sufficiently large value of Ioffe coefficient ZT. Sometimes these two aspects (composition and nanostructure) can be combined in one single material; the samples of this type of materials are the microcrystallites of granular semiconductor Si, separated by the SiOx oxide film. For such a material, where the values of the effective electrical conductivity,. of the Seebeck coefficient and of the thermal conductivity can be determined by use of the Landauer effective medium theory. Note that necessary positive significant changes and can be expected under modification of the SiOx layer in a certain way, so that they must contain the certain types of vacancy complexes in sufficient concentration, with special alloy impurities. Requirements for such vacancy-impurity complexes are the following: 1) complexes should make the local levels EN in the electronic spectrum, satisfying; 2) the ratio of the parameters of two-barrier potential relief of the electrons in the SiOx layer should be such that the interference at their electronic de Broglie waves create the resonance levels EN inside; 3) local properties including compressibility and density of these complexes in SiOx interlayers must differ from the maximum of crystalline Si ones. It is clear that in such a material at the same time the conditions for unique high probability of tunneling through the SiOx layer are realized, because it is not "Gamow” tunneling, but the "resonance" ones, for which the coefficient of transmittance through the layer can be very close to unity, that leads to the large increase. On the other hand, the largest possible cross-section of the scattering of long-wave phonons on the vacancy complexes with bonded impurities has been achieved that is the classical property of skutterudite materials. It is obvious that most low thermal conductivity takes place for the vacancies, because the phonon scattering cross section is here, where λ is the phonon wavelength, v is the volume of vacancy complex. Introducing the soft local vibrational mode by introduce of impurities inside the complex, can be slightly reduced, but the depth of EN level must be choosen using the necessary method. Concerning the Seebeck coefficient for the SiOx layer, it turns out that and the values of the numerator and of the denominator of the last fraction depend equally in resonant tunneling case on a small parameter for the two-barrier potential, so that αSiO(x) is a rather complex function of EN. Choosing the types of impurities in the cavities of SiOx, the concentration of cavities in the SiOx and crystallite size, it is possible to optimize the value of Ioffe coefficient ZT. Formation of such material requires, of course, the difficult combination of the conventional thermodynamic technology with the controlled radiation exposure, and the existing experimental results are very encouraging. In conclusion we note, that the discussed variant of the thermodynamic material combines the properties of scutterudites and properties of highly conducting electronic materials.

Resume : During the past two decades, the performance of organic solar cells has improved dramatically with the development of new active materials such as donors and acceptors. However, commercialization is at an early stage, and the reason is relatively low efficiency and low stability compared to inorganic solar cells. In this presentation, we propose a strategy for improving the efficiency and stability of organic solar cells, that is to say, the interface control technology between the cathode electrode and the photoactive layer. Typically, ZnO is widely used as a cathode buffer material in inverted organic solar cells, however, which showed limited device lifetime due to contact resistance and bad compatibility between the organic photoactive layer and ZnO layer. In order to solve this problem, we designed and synthesized several kinds of high dipole moment conjugated organic materials having binding functional group with oxide. As a result, we were able to obtain not only almost 20% improved power conversion efficiency but also increased lifetime of over 80% even after 1,000 hours under 1 sun light soaking and 85oC/85%RH damp heat condition in organic photovoltaic cells. In addition, our cathode buffer materials can be directly applied to hybrid photovoltaic cells such as quantum dot solar cells and Perovskite solar cells as well as organic photodiodes. Therefore, we are introducing new cathode buffer materials and discussing their applications in this presentation.

Resume : Perovskite solar cells (PSCs) have reached their highest efficiency with spiro-OMeTAD. However, this material can cause problems with respect to reproducibility and stability due to the use of dopants. To overcome these problems, we tried to introduce inorganic hole transport material (tungsten oxide, WO3) instead of spiro-OMeTAD. However, WO3 forms porous structure on the perovskite layer, and this property prevents the sole use of WO3 as an HTL. Accordingly, we report an inorganic-organic double layer based on WO3 and spiro-OMeTAD as a hole transport layer (HTL) in PSCs. The device equipped with a WO3/spiro-OMeTAD layer achieves the highest efficiency (21.44%) in the tin oxide planar structure. The WO3/spiro-OMeTAD layer exhibits better hole extraction ability and faster hole mobility. The WO3 layer particularly improves the open-circuit voltage (VOC) by lowering the quasi-Fermi energy level for holes and reducing charge recombination, resulting in high VOC (1.17 V in the champion cell). In especial, the WO3 layer as a scaffold layer promotes the formation of a uniform and pinhole-free spiro-OMeTAD overlayer in the WO3/spiro-OMeTAD layer. High stability under thermal and humid conditions stems from this property. Our study presents a facile approach for improving the efficiency and stability of PSCs by stacking an organic layer on an inorganic layer.

Resume : Inkjet printing promises to be an invaluable technique for processing organic solar cells (OSCs) with key advantages such as low material consumption, freedom of design and compatibility with different types of flexible substrates making it suitable for large area production. However, one concern about inkjet printed OSCs is the common use of halogenated solvents during the ink formulation process. While halogenated solvents suit the inkjet printing process due to their high boiling points, suitable viscosity, and excellent solubility of organic donor and acceptor compounds, they also pose some risks for both human health and the environment, excluding them from being the ultimate choice for large area production. As an alternative, we demonstrated for the first time the engineering of an environmentally friendly ink using a blend of non-halogenated benzene derived solvents optimized to meet the viscosity and surface tension requirements for the inkjet printing process with the non-fullerene acceptor O-IDTBR (rhodanine-benzothiadiazole-coupled indacenodithiophene) combined with the commercially available P3HT (poly3- hexylthiophene) , yielding in a solar cell device efficiencies of up to 4.73% - the best efficiency achieved by the P3HT:O-IDTBR system processed with all non-halogenated solvents. This breakthrough in solvent engineering paves the way for environmental and health conscious mass production of organic photovoltaics through inkjet printing.

Resume : Organic photovoltaics have seen a tremendous increase of performance over the past years, now surpassing 14% power conversion efficiency. However, many of the novel high-efficiency materials are prone to severe degradation in ambient conditions, limiting their commercial applicability. Here, we present a comprehensive study of the environmental stability of the high-performance system PffBT4T-2OD:PC(71)BM. By applying fs-pump-probe techniques, ultra-sensitive photo-thermal deflection -and photo-electron spectroscopy in combination with organic field effect transistors, we conclude that oxygen-induced p-doping is responsible for the severe device degradation observed. We find that this process is accelerated by the presence of light, without inflicting major photo-oxidation of the materials.

Resume : To date, the performance of all emerging PV technologies has typically been determined using the test methods described in the IEC 60904 series. However, these technologies in particular present some additional measurement challenges that are not dealt with in that series. The nature of the challenge varies slightly among them, but in general the challenge is due to a relatively long transient response to applied illumination or applied voltage. This can lead to the cell output in laboratory testing being significantly different to the output that would be observed in a real application. In this work we will give a general and complete overview of the main aspects influencing the characterization of emerging PV devices and the best practices proposed by the experts of this community. We will then describe part of the work done by an international group of experts involved in the preparation of a new Technical Report in consultation with IEC Technical Committee 82, Solar Photovoltaic Energy Systems, Working Group 2. This document provides an overview of the challenges and current best practices for measuring the efficiency of PV devices with transient responses to light and/or applied voltage. The steps toward the achievement of steady state conditions and stabilization of the devices are also described. It seeks to highlight where the existing standards fail to accommodate the requirements of these technologies, to identify what additional measures may be needed for accurate determination of the device efficiency, and how these measures might be standardised in the future. This will remove ambiguity for the test laboratories in making measurements of record devices and will simplify exchanges between test laboratories, meaning stronger validation of devices and ultimately better measurement practices.

Resume : Perovskite solar cells (PSC), characterized by solution processed, facile and low-cost fabrication techniques, have achieved high power conversion efficiencies (PCEs) in only a few years of research.[1-3] However various drawbacks, such as the performance degradation under ambient conditions is one of the main reasons preventing them from full commercialization. In this context, stability against thermal stress is a very important issue to enable large scale applications, as the PV modules should be stable when operating at 80oC under 1 sun illumination (100 mW cm-2) for more than 1000h [4]. The dye-sensitization of the titania compact layer through the triphenylamine-based metal-free organic (E)-3-(5-(4-(bis(2',4'-dibutoxy-[1,1'-biphenyl]-4yl) amino) phenyl) thiophen-2-yl)-2-cyanoacrylic acid (D35) has been recently proposed. This sensitizer modifies the compact layer/perovskite interface, thus improving electron transfer to the anode and favoring the growth of the CH3NH3PbI3 perovskite crystal [5]. Apart from PCE enhancement, the presence of D35 also improves the thermal stability of the solar cells. The PSCs were evaluated after thermal exposure at 100oC for 1h, including devices with and without D35. The performed analysis indicated that the dye monolayer protects the perovskite film and contributes significantly to performance stabilization of the corresponding PSCs, which retained 80% of their initial efficiency, contrary to D35-free ones which preserve only 25% of their PCE. ACKNOWLEDGMENTS This work was supported by European Union’s Horizon 2020 Marie Curie Innovative Training Network 764787 “MAESTRO” project. ΙΚΥ Scholarship Programs, Strengthening Post-Doctoral Research Human Resources Development Program, Education and Lifelong Learning, co-financed by the European Social Fund – ESF and the Greek government is also acknowledged. References [1] M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Science, 2012, 338, 643. [2] H.-S. Kim, C.-R. Lee, J.-H. Im, K.-B. Lee, T. Moehl, A. Marchioro, S.-J. Moon, R. Humphry-Baker, J.-H. Yum, J.E. Moser, M. Grätzel, N.-G. Park, Sci. Rep., 2012, 2, 591. [3]National Renewable Energy Laboratory (NREL), http://www.nrel. gov/ncpv/images/efficiency_chart.jpg (as of January 2019). [4] T.A. Berhe, W.N. Su, C.H. Chen, C.J. Pan, J.H. Cheng, H.M. Chen, M.C. Tsai, L.Y. Chen, A.A. Dubale, B.J. Hwang, Energy Environ. Sci. 2016, 9, 323. [5] N. Balis, A.A. Zaky, D. Perganti, A. Kaltzoglou, L. Sygellou, F. Katsaros, T. Stergiopoulos, A.G. Kontos, P. Falaras, ACS Appl. Energy Mater., 2018, 1, 6161.

Resume : The packing of the donor:acceptor molecule highly influence the performance of an organic solar cell. In general, an additive is used to boost the efficiency of solar cells which improves the short circuit current and fill factor but, at the cost of open circuit voltage. We have studied the ordering of bulk and donor:acceptor interface, which affect the short circuit current and the open circuit voltage, respectively. We have modulated the morphology by varying the concentration of solvent additive in poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopental[2,1-b;3,4-b’]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) blended with [6,6]-Phenyl-C71-butyric acid methyl ester (PC71BM). The bulk and CT ordering is studied in terms of Urabch energy, electroluminescence and EL quantum efficiency. We found that, when the concentration of solvent additive changes from 0 vol% to 10 vol%, the Urbach energy varies from 80 meV to 39 meV for bulk, whereas it varies from 39 meV to 51 meV for D:A interface. Our studies clearly indicates that, in case of bulk heterojunction solar cell, there exists an inverse relation between bulk and D:A interface property. Furthermore, using the scanning photocurrent microscopy measurements, we found that the effective hole transport length is two times higher in case of 3 vol% additive as compared to that with 0 vol%. This increase in effective transport length is explained by increased delocalization of the electronic state HOMO of the donor polymer, which reflects in dark J-V knee voltage. Thus, we provide a functional relationship between the additive induced bulk heterojunction morphology and the optoelectronic properties of PCPDTBT-based solar cells.

Resume : In this work, We fabricated ternary blend polymer solar cells ( PSCs) with two donors, including, one low bandgap polymer Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b] thiophene-)-2-carboxylate-2-6-diyl)] (PBDTTT-EFT), one wide bandgap polymer poly[N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)](PCDTBT), and one acceptor [6,6]-phenyl C71 butyric acid methyl ester (PC71BM). The photoconversion efficiency (PCE) of 8.89 % was achieved with 20 % of PCDTBT incorporation into to the blends and a high Jsc of the ternary PSCs was observed than those of the binary PSCs with enhanced light absorption, efficient energy transfer and better blend morphology. The increased PCE attributed to not only non-radiative Forster resonance energy transfer (FRET) and enhancement of absorption between two donors (PBDTTT-EFT/ PCDTBT) but also the formation of a bicontinuous interpenetrating network in the ternary system. A detailed morphology of binary and ternary systems was studied by using atomic force microscopy (AFM), cryo-scanning electron microscopy (Cryo-SEM) and transmission electron microscopy (TEM). Furthermore, the electrochemical impedance spectroscopy (EIS) measurement was carried out for the binary as well as ternary PSCs. Our results clearly indicate that the incorporation of a wide band gap polymer as a third component is a simple, effective and promising way to achieve high performance PSCs.

Resume : Inkjet printing is a promising route towards an industrial production of perovskite solar cells. It offers a material- and cost-efficient deposition of the perovskite absorber on a wide range of substrates with the possibility for arbitrary printing patterns. However, a common problem for devices with inkjet-printed perovskite layers is the yet low power conversion efficiency, short stable power output efficiency and significant hysteresis effects. In this work, inkjet-printed triple-cation (Methylammonium, Formamidinium and Cesium) perovskite absorber layers with thickness larger than 1µm are demonstrated and employed in solar cells. High power conversion efficiencies as high as 18.5% with stable power output are demonstrated by optimization of the printing and annealing process. Record devices were produced on evaporated nickel oxide layers, showing almost no hysteresis in J-V-measurements. Characterization of the perovskite layers shows large columnar crystals without vertical grain-boundary for the printing and annealing process leading to the best-working devices. The presented results are the highest stable efficiencies for inkjet-printed perovskite solar cells and are considerably closer to state-of-the-art values achieved with other deposition methods than any preceding reports.

Resume : Organic solar cells have many advantages; they are lightweight and flexible and allow for scalable material production and cost effective roll-to-roll coating and printing techniques. Efficiency of >15% was disclosed in small area solar cells with various device architectures, which indicated the rapid progress over the last several years and the great potential of organic solar cells as an alternative source of energy. In order for organic solar cells to fully mature from research and development into cost effective products, solar cell technology at a small size needs to be realized at a large scale with high device stability. And for this, it is highly important to develop new conjugated polymers and understand the fundamental working mechanism of the materials in terms of device performance and stability. In the presentation, we developed new conjugated polymers for large-area solar cell technologies and studied the effects of conjugated polymers on photovoltaic performance and long-term stability in large area organic solar cells.

Resume : Low temperature processability of the photoactive layers of polymer solar cells (PSCs) along with their highly reproducible power conversion efficiencies and intrinsic thickness tolerance are extremely desirable for the large area roll-to-roll production of these PSCs. However, most of the highly efficient PSC materials, due to their strong aggregation and higher crystallinity, require elevated processing temperatures which are unfavorable for the industrial R2R manufacturing of PSCs. Recently many scientists are interested in the stability of the polymer solar cells, but most of the researches are hovering at thermal stability. But real challenges are in photo-stability and the burn-in loss during exposed on the sun light is one of the biggest challenges to go to the market. Lost more than 40% of their initial efficiency in just 24 hours after exposing to 1 sun condition is common for polymer solar cell devices. This burn-in loss does not come from the decomposition of the photoactive materials but rather caused by the electro- and physical-properties of the photoactive materials. To overcome the burn-in loss, we tried two approaches. First one is inserting an appropriate interlayer for inverted solar cells. Second approach is design new photoactive materials which reduce the burn-in loss. Here we present the improved photo-stability and reduced processing temperature results by applying elaborately designed photoactive materials.

Resume : In the rapidly developing field of organic-inorganic halide perovskite solar cells (SCs), CH3NH3PbBr3 (MAPbBr3) has recently emerged as an attractive material candidate due to its excellent photovoltaic properties and environmental stability. However, so far, there exists no systematic study on the surface properties of MAPbBr3 and its degradation mechanisms, which are vital information in achieving high-performance SCs. Herein, carrying out density functional theory calculations, we explore the developments of unstable freshly cleaved surfaces of cubic MAPbBr3 with various termination morphologies into their stable degraded counterparts in the controlled environmental conditions. We first find that MABr- and vacant PbBr2-terminated surfaces are the most stable surface structures of MAPbBr3. Moreover, we predict that a flat PbBr2-terminated surface can also be maintained by keeping a PbBr2-rich environment. We next present plausible degradation pathways from the several unstable surfaces to these stable surfaces under H2O, OH, O2, and CO-rich conditions, together with consideration of the bright and dark environment. Additionally, we investigated the defect engineering at different MAPbBr3 surface terminations and find MABr-terminated surface as a good host for deep level defects compared to PbBr2-flat surface that prefers to remain defect free due to high defect formation energies. Interestingly, the electronic structure of the stable defect-free surfaces in MAPbBr3 revealed the absence of midgap states similar to their bulk structural form, which will prevent the non-radiative recombination processes and improve the SC performance. Clarifying the atomistic details of the degradation of perovskite surfaces, our work will contribute to the design of advanced hybrid halide perovskite SCs.

Resume : It is reported the synthesis of two bis-benzodithiophene (BDT2)-based small molecules, SM1 and SM2, and the effects of solvent vapor annealing (SVA) on their molecular ordering, optical, and photovoltaic properties. Although SM1 and SM2 have similar backbones, they exhibit different photophysical and electrochemical properties, charge carrier mobilities, and morphologies in blend films. SVA profoundly affected the molecular structure and photovoltaic properties. Solvent vapor annealing remarkably enhanced the photovoltaic properties of SM2, increasing the power conversion efficiency (PCE) from 1.47% to 8.66% by inducing a favorable molecular orientation, a miscible morphology, and enhanced mobility. The relationship between the morphologies of the active layers comprising SM1 or SM2 with PC71BM submitted to SVA and the device performances were elucidated using 3D transmission electron microscope tomography.

Resume : Semiconductor organometallic halide perovskites (MHP) have emerged as low cost-effective materials with outstanding properties for optoelectronics. In addition to the high conversion efficiencies (> 23 %) in photovoltaic devices, other applications like lasers with thresholds smaller than 1 μJ/cm2 have been reported.[1] Recently, MHP materials have also demonstrated promising nonlinear properties, [2] which would pave the road on a broad range of applications, such as switching or data storage. The high excitation fluencies (~GW/cm2) required to observe the nonlinear optical process, however, can damage (or even destroy) the MHP material,[3] usually fabricated on a polycrystalline thin (<1 μm) films. Nevertheless, the solution process nature of MHP allows the incorporation of other organic/inorganic materials looking for improved performances. In particular, this work proposes Bi-doped CH3NH3PbI3 thin films as a MHP material with improved robustness, stability and novel properties. Bi3+ in introduced up to the limit for obtaining tetragonal I4cm space group. We analyze the impact of Bi-doping on the nonlinear absorption and refraction parameters, together with electrical transport in Al/MHP/Al lateral devices. The benefits of Bi on MHP films are demonstrated with an improvement of the damage threshold, without degradation of electrical properties. [1] Eur. Phys. J. Appl. Phys. 75, 30001 (2016). [2] J. Phys. Chem. Lett. 9, 5612−5623 (2018). [3] E. Climent-Pascual, et al. J. Mater. Chem. A 4 (2016) 18153.

Resume : Low operation stability of hybrid perovskite solar cells currently is the main obstacle for their practical implementation. Replacing fragile organic moieties with robust inorganic cations represents a promising approach to designing more stable absorber materials. Inorganic lead-halide based perovskites CsPbI3-xBrx (x=0-3) have demonstrated improved thermal and photochemical stability. Among them, CsPbI3 is considered as the most promising inorganic perovskite for photovoltaic cells due to its narrow band gap. Unfortunately, the cubic α-phase of CsPbI3 undergoes fast transition to a non-active yellow δ-phase under realistic solar cell operation conditions thus limiting practical application of this material. Efforts of many research groups working on improving CsPbI3 perovskite phase stability resulted in a certain progress in this field achieved very recently. Here we addressed the problem of poor α-phase stability of CsPbI3 films by using properly designed additives. A group of additives was identified that suppress significantly the phase transition effects in CsPbI3. The lifetime of the stabilized CsPbI3 perovskite exceeded 1000 h under 1 sun illumination at elevated temperatures. In addition, solar cells based on CsPbI3 films modified with optimal additives showed high power conversion efficiencies. These results pave a way to design of stable and efficient all-inorganic perovskite solar cells. This work was supported by Russian Science Foundation (project 18-13-00353).

Resume : Infrared colloidal quantum dots (IR CQDs) capable of absorbing sunlight beyond the band gap of crystalline silicon (cSi) are of interest in augmenting cSi photovoltaics. Today’s best IR CQD solar cells rely on the use of ligand passivation strategies based on iodide, which however fails to fully passviate the surface of the CQDs. Here we present three avenues to improve the passivation of IR CQDs and enable high performance IR-CQD solar cells. We first introduce a ligand exchange that avoids CQD agglomeration in the solution phase; long oleic acid ligands on the PbS surface were exchanged with medium chain ammonium anionic chloride ligands; we then re-shelled the surface using short halides and pseudohalides. Next, we simplified the procedure by introducing smaller halide (Cl, Br) ligands in an iodide-based exchange, which allows for full passivation of the IR-CQDs, as revealed by improved surface halide packing. Finally, we modified the previous ligand exchanges and fabricated solar cells with ultra-small band gap CQDs (Eg < 0.75 eV), enabling harvesting of most of the available solar spectrum. All three methods lead to improvements in IR-PCE, moving the field forward on the roadmap to cSi-CQD tandem devices.

Resume : The final properties of a composite hybrid organic/inorganic material strongly depend on all the characteristics of the materials that form the interface, especially in thin nanostructured porous configurations. As a matter of fact, we studied a porous silicon/eumelanin hybrid and we both used experimental and computational methods to investigate the role of the interface in the temporal stability of photocurrent production properties. From our research, we show that we could improve the temporal stability by increasing the pore diameter; putting together computational and chemical arguments, we could explain that behavior as attributed to a modified diffusion of the hydrogen peroxide, formed during the polymerization process, toward the silicon/eumelanin interface. In fact, the diffusion of the chemical species within the polymer molecule can be strongly modified by increasing the polymer density close to the hybrid interface, and this will affect the oxidation of the pores inner Si surface. In conclusion, we demonstrated that to fully understand the properties of hybrid nanoscale structures we should take into account the overall behavior of the two in-contact materials, not only the interface formation mechanisms.

Resume : Lead halide perovskite solar cells have demonstrated impressive efficiencies exceeding 23% for the best laboratory samples, while their practical application is still hampered by poor stability. Besides the intensively investigated photochemical and thermal degradation effects, a particular attention should be paid to the electrochemical stability of absorber materials and completed photovoltaic devices. Here we report a systematic comparative study of the electrochemical stability of lead halide based perovskite solar cells assembled with different hole-transport materials such as PEDOT:PSS, NiOx, PTAA, etc. Devices were exposed to a stepwise potentiostatic polarization under anoxic conditions in dark. The evolution of the solar cell performance induced by different bias voltages was analyzed using ToF-SIMS profiling to unravel ion migration and parasitic redox processes occurring at the cathode and anode. The obtained results showed that the electrochemical stability of perovskite solar cells is largely affected by the used HTL materials. The revealed degradation pathways featured a crucial importance of controlling interfacial electrochemistry while designing highly efficient and stable perovskite solar cells. This work was supported by Russian Science Foundation (project 18-13-00353).

Resume : Perovskite solar cells (PSCs) have demonstrated impressive power conversion efficiencies going beyond 23% for the best laboratory samples, while their operation stability still requires substantial improvements before this technology can be successfully commercialized. Many publications provided evidences that stability of PSCs is strongly dependent on the interface chemistry between the absorber and charge transport materials. In particular, we have shown recently that perovskite solar cells incorporating fullerene-based electron transport layers (ETLs), e.g. based on [60]PCBM or [70]PCBM, undergo rapid degradation under illumination. The revealed mechanism is featuring the initial dissociation of MAPbI3 to PbI2 and MAI followed by accumulation of the latter component in the fullerene-based ETL (A. F. Akbulatov et al. Adv. Energ. Mater. 2017, 7, 170047). In the present work, we explored the impact of the hole-transport materials (HTLs) on the operation stability of p-i-n perovskite solar cells under continuous light soaking. In addition, we investigated the degradation effects induced at the interfaces formed by MAPbI3 with such HTLs as PEDOT:PSS, PTAA, NiOx, and NiOx/PTAA blend using UV–vis optical spectroscopy, X-ray diffraction, steady-state photoluminescence (PL) and TOF-SIMS analysis. The revealed degradation pathways open new opportunities for rational design of new HTL materials for efficient and stable perovskite solar cells.

Resume : The exceptionally rapid growth of power conversion efficiency of photovoltaic cells based on the family of organo-lead halide perovskites has recently reached 23 %. As a promising perspective for pushing the performance towards and beyond silicon (27%), tandem perovskite-perovskite solar cells have been proposed, with low band-gap (~1.2 eV) Lead-Tin (Pb:Sn) hybrid perovskite promising absorbers for the bottom cell [1]. However, the Pb:Sn perovskite compounds are still at a nascent stage of development, and fundamental properties critical for optimisation of these materials are not yet known. In this work, we present a comprehensive study of perovskite alloys with varying Lead-Tin composition in the MAPbxSn1-xI3 and (FA,Cs)PbxSn1-xI3 (x=0 -1) material families. We use optical spectroscopy measurements in high pulsed magnetic fields to show that the binding energy of the exciton is as small as ~15 meV, explaining the outstanding photovoltaic performance of these materials. We further observe that exciton reduced mass slowly follows the material band gap; a trend which can now be generalized to all halide perovskite absorbers [2,3]. Finally, both our optical and X-ray diffraction studies show that even with recent optimization of fabrication methods, a segregation of chemical phases still occurs; indicating that the full potential of this materials is yet to be reached. [1]G.E. Eperon et al. Science, 20, (2016) [2] K. Galkowski et al., Energy & Environmental Science 9, 962-970 (2016) [2] Z. Yang et al., ACS Energy Letters 7, 1621-1627 (2017)

Resume : Solar cells based on ultrathin Cu(In,Ga)Se2 (CIGS) absorbers are an effective way to reduce production costs while providing the potential to improve electrical performances, due to the application of a passivation layer with sub-micrometre line contacts at the rear electrode. An optical lithography process was developed using a Direct Laser Writing system to produce line contacts in a 25 nm SiO2 passivation layer. An optimization of the contact dimensions was done to obtain high passivation areas for enhanced solar cell performances. We show that if the distance between two consecutive line contacts is too long, the solar cells performance gets reduced mainly due to increased contact resistance leading to poor charge carrier extraction. Hence, it is important to maintain the distance of the contacts in the same order of magnitude as the minority carriers diffusion length to prevent hole accumulation. Atomic Force Microscopy was used to measure the width (0.7 ?m, 1.4 µm) and depth of the line contacts on SiO2 layer, while cross-section Scanning Electron Microscopy images were taken to analyse its influence on the morphology of the CIGS layer. J-V and EQE measurements were done to study solar cells performance and they revealed a power conversion efficiency improvement of nearly 1 % for rear passivated cells with the line contacts compared with unpassivated devices. A detailed comparison between solar cells produced by different line contacts is done and presented.

Resume : Despite the enormous progress that has been made improving the intrinsic stability of hybrid organic-inorganic lead halide perovskite films, future perovskite devices will require encapsulation. Indeed, the reactivity of perovskite films with water is one of their prominent degradation mechanisms (i). As such, it is necessary to test and develop encapsulation materials and procedures at a high rate. We present an experimental method to efficiently monitor the degradation of a perovskite film over a large area, using digital photography inspired by calcium testing procedures. The calcium test has been largely used to characterize encapsulation and sealing materials (ii) but its use is limited by strict sample fabrication conditions due to the extreme reactivity of calcium. We have developed an original testing procedure using perovskite layers, which may be fabricated in air, instead of calcium. We tested different sealing materials for use with glass/glass encapsulation and identified different gas permeation mechanisms, both along the interfaces as well as through the bulk of the adhesive layer. Key measurement parameters have been identified for a comprehensive understanding of the gas permeation pathways. This work was validated with damp heat aging (85°C/85%RH) of perovskite solar cells encapsulated with similar materials and designs then tested using the Perovskite test. (i) Taame Abraha Berhe& al. Energy Environ. Sci., 2016, 9, 323 (ii) J. J. Michels & al. J. Mater. Chem. C, 2014, 2, 5759–5768

Resume : Deposition of perovskite solar cell absorbers by co-evaporation offers a variety of advantages over solution based preparation such as homogeneous coating of large substrates and conformal coverage of textured substrates. Up to date the gas-like evaporation behavior of organic precursors such as methylammonium halides makes the rate measurement and consequently the process control difficult as volatile species might re-evaporate from chamber walls. To overcome these challenge we implement an evaporation setup with a thermal management system to directly co-evaporate volatile precursors for perovskites. Through a combination of actively cooled and heated surfaces, a direct and controlled evaporation process is achieved. This way, high quality Methylammonium Lead Iodide perovskite films are obtained as confirmed by XRD, PL and optical measurements. These perovskite films are implemented into p-i-n solar cells utilizing different hole transport materials (HTMs) such as PTAA, self-assembling monolayer (SAM) molecules and Spiro-TTB. Moreover, the relationship between HTM and solar cell stability is examined. Although the coarse grain morphology measured by SEM is very comparable, the solar cell performance is strongly affected by the used HTM and stabilized efficiency over 20% are realized with the SAMs only. The herein achieved efficiency is comparable to the highest reported value for evaporated perovskite solar cells.

Resume : Wide-bandgap perovskites are recently drawing tremendous attention in the community for high-efficiency all-perovskite tandem solar cells. However, both the formamidinium (FA+) and methylammonium (MA+) based wide-bandgap mixed halide perovskites are either suffered from high density of traps or pin-holes. Fundamental understanding on the crystallization and film formation processes are not yet clearly understood. In this study, an in-situ photoluminance technique was used to investigate the perovskite crystallization during the thermal annealing process. It is found that the crystallization of a mixed halide perovskite with bromide (Br-) and iodine (I-) ions following the Ostward ripening crystal growth. Interestingly, the initial nucleation reaction is quickly completed in the first few seconds, however, leaving the small crystals with inhomogeneous composition. The different aggregation affinities of such inhomogeneous small crystals provoke the formation of pin-holes during the thermal annealing process. By engineering the precursor solution to control the nucleation rate, the chemical composition of the small crystals has become homogenous. Uniform pin-hole free high Br- composited wide-bandgap MA0.9Cs0.1Pb(I0.6Br0.4)3 perovskite films with bandgap energy of 1.8 eV have been realized, which exhibits an encouraging device efficiency of 15.1% with superb photostability.

Resume : Lead-based perovskites, APbX3, achieved over 23 % of efficiency within only 7 years after first their usage in solar cells. Although these lead-based perovskites have rapidly achieved the high efficiency, they have two intrinsic problems to be addressed to be commercialised; toxicity and instability. To address the problems of lead at the same time, new lead-free and air-stable perovskites such as bismuth- or antimony-based perovskites are emerging. However, the efficiency of the solar cells employing these new materials is relatively low below 5% with intrinsic problems; large binding energy, high defect, and large band gap.1-4 Recently, it was reported that 10 times higher photo luminescence quantum yield (PLQY) was achieved by 7% of tin substitution on lead-based perovskite.5 The reason for this improvement is not clear yet but we expect the similar effect can be brought when bismuth and antimony are mixed in a perovskite structure because the relationship between lead and tin is like that between bismuth and antimony. In a report, the improvement in defect density and varied defect states with the ratio of bismuth in the antimony perovskite, leading to doubled solar cell performance.6 In this talk, we will present some of our results from the investigation on the bismuth and antimony mixed perovskites in (MA)3(BixSb1-x)2I9 structure. In particular, we will focus on the changes in structure and electronic and opto-electronic properties as changing the ratio of two atoms in the 0D perovskites by both experimental and theoretical methods. References 1. B.-W. Park, B. Philippe, X. Zhang, H. Rensmo, G. Boschloo and E. M. J. Johansson, Advanced Materials, 2015, 27, 6806. 2. M. Scholz, M. Morgenroth, K. Oum and T. Lenzer, Journal of Physical Chemistry C, 2018, 122, 5854. 3. S. M. Jain, D. Phuyal, M. L. Davies, M. Li, B. Philippe, C. De Castro, Z. Qiu, J. Kim, T. Watson, W. C. Tsoi, O. Karis, H. Rensmo, G. Boschloo, T. Edvinsson and J. R. Durrant, Nano Energy, 2018, 49, 614. 4. P. Karuppuswamy, K. M. Boopathi, A. Mohapatra, H. C. Chen, K. T. Wong, P. C. Wang and C. W. Chu, Nano Energy, 2018, 45, 330. 5. A. B. F. Vitoreti, S. Agouram, M. S. de la Fuente, V. Munoz-Sanjose, M. A. Schiavon and I. Mora-Sero, Journal of Physical Chemistry C, 2018, 122, 14222. 6. P. C. Harikesh, B. Wu, B. Ghosh, R. A. John, S. Lie, K. Thirumal, L. H. Wong, T. C. Sum, S. Mhaisalkar and N. Mathews, Advanced Materials, 2018, 30, 1802080.

Resume : Hybrid organic-inorganic perovskite research continues gaining efforts to achieve high overall photoconversion efficiencies due to their unique optoelectronic properties. [1] An intense activity is dedicated to overcome their high sensitivity to ambient atmosphere and to visible irradiation that strongly handicap their stability and performance. Obtaining more stable compounds as well as understanding the complex behavior of the emission evolution upon illumination and time are main concerns to be addressed. The photoluminescence behavior is strongly sensitive to different parameters, mainly to the presence of defects and traps whose evolution with time is related to ion migration and perovskite transformations.[2,3] Here, we demonstrate that incorporation of BiI3 in the MAPbI3 (MA=CH3-NH2) perovskite precursor solution induces totally novel effects. A doping limit is demonstrated by x-ray diffraction and their effect on phase purity, symmetry, lattice parameters, average crystalline domain and microstrain are carefully evaluated. A spurious MA3Bi2I9 phase is detected for nominal Bi > ca. 15% while the usual spurious PbI2 phase in undoped films is not observed for any Bi doping. Influence on optical properties are also studied. Bi3+ incorporation lead to a slight increment of the optical gap due to the reduction of lattice parameters. However, for the photoluminescence emission, the phenomenology for the Bi doped samples is drastically different at high power irradiation. An extraordinary photo-stability enhancement in ambient conditions, compared to that of undoped MAPbI3 thin films is observed. We propose a mechanism to explain the observed trends using a model based on the migration of Bi3 upon irradiation and its effect on the MAPbI3 cell volume and bandgap energy. These results provide a new path for obtaining highly stable materials which would allow an additional boost of hybrid perovskite based optoelectronics. 1 Wei Zhang, et al. Nature Energy, 1, 16048 (2016). 2 E. Climent-Pascual, et al. J. Mater. Chem. A 4 (2016) 18153. 3 D. W. de Quilettes, et al. Nature Commun. 7 (2016) 11683.

Resume : Solar cells based on solution processed molecular and hybrid organic - inorganic semiconducting materials are currently generating significant interest. With recent advances in the power conversion efficiency of such devices, attention is now shifting to other factors such as long-term operation stability. In particular, more recent studies of perovskite solar cells have shown that there is formation of reactive oxygen species (ROS) such as superoxide when the material is exposed to light and oxygen. This is in large part responsible for the degradation observed in perovskite materials and solar cells [1,2]. However, the generation and role of ROS in the potential degradation of organic materials such as semiconducting polymers has not been addressed. This study will focus on the ROS superoxide, and its generation from various materials used within organic solar cells. Moreover, we will explore the effect of energetics and film morphology on the yield of superoxide generation. The goal is to study the potential correlation between the structure of the polymer material, the superoxide yield and the polymer semiconductor film’s overall stability. 1.N. Aristidou, C. Eames, I. Sanchez-Molina, X. Bu, J. Kosco, M. Islam and S. Haque, Nature Communications, 2017, 8, 15218. 2.N. Aristidou, I. Sanchez-Molina, T. Chotchuangchutchaval, M. Brown, L. Martinez, T. Rath and S. Haque, Angewandte Chemie International Edition, 2015, 54, 8208-8212.

Resume : Metal halide perovskite materials have been attracting much attention due to their outstanding in optic and electronic properties. Among various applications of the perovskite materials, photovoltaic devices have been intensively studied in the past decade, and achieved a power conversion efficiency of 23.7%. However, the stability issue has hindered the commercialization of perovskite photovoltaics. To improve the stability, researchers have tried to use different compositions of the perovskite materials, and different contacts over the perovskite film. Here, we propose that the stability and performance of the perovskite photovoltaics can be improved at the same time by adopting a lateral structure. In this structure, the electrodes of the devices are arranged at one side of the perovskite layer. Therefore, the devices will benefit from less light loss due to the contacts and the transparent electrodes. We have confirmed using simulation methods that the proposed structure is 11% more efficient than the traditional sandwich structure.[1] Also, it is possible to coat an insulating and hydrophobic layer on the top of the perovskite layer to passivate surface defects and to prevent reaction of perovskite materials with the atmosphere for improved stability. In the presentation, we will share our preliminary results about the lateral photovoltaics. [1] T. Ma, Q. Song, D. Tadaki, M. Niwano, A. Hirano-Iwata, ACS Applied Energy Materials, 1, 970-975 (2018).

Resume : Complex metal halides are intensively explored primarily in the context of their potential applications as absorber materials for perovskite photovoltaics. Despite a significant progress achieved with perovskite solar cells based on complex lead halides, it is still unclear if they can be practically useful because of the unresolved so far severe operational stability issues. At the same time, the success and challenges faced with lead-based perovskites stimulate active screening of other complex metal halides in order to develop more stable and, probably, even more efficient absorber materials. In this work we explored for the first time a family of Te(IV) polyhalides A2TeX6I2, where A represents organic cation and X denotes Br or I. The crystal structures of these compounds feature [TeX6] octahedra interconnected by I2 bridge molecules. These compounds can be synthesized using a simple single-step procedure, which makes them available on a large scale. The tellurium complex bromides and, particularly, iodides exhibited interesting optical properties favoring their photovoltaic applications (e.g. band gaps below 1.5 eV). Thin films of the tellurium halides showed semiconductor behavior and revealed strong photoconductivity effects enabling their application as active materials in efficient photodetectors.

Resume : Organic-inorganic hybrid perovskite solar cells have attracted much attention due to their high power conversion efficiency (?23%) and low-cost fabrication. Directions to further improve these solar cells include strategies to enhance their stability and their efficiency by modifying either the perovskite absorber layer or the electron/hole transport layer. For example, the transparent electron transport layer (ETL) can be an important tuning knob influencing the charge extraction1, light harvesting2, and stability3 in these solar cells. In this work, a series of TiO2 nanocolumn photonic structures have been fabricated by glancing angle deposition with sputtering and subsequent thermal oxidation. We studied in detail the effects of these TiO2 nanocolumns on the performance and the stability of Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 perovskite solar cells. Compared to devices with planar TiO2 ETLs, we found that TiO2 nanocolumns can significantly enhance the power conversion efficiency of the Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 perovskite solar cells by 17 % and prolong their shelf life. By analyzing the optical properties, solar cells characteristics, as well as transport/recombination properties by impedance spectroscopy, we observed light-trapping and reduced carrier recombination in solar cells associated with the use of TiO2 nanocolumn arrays. The possible origins contributing to enhanced efficiency and stability of Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 perovskite solar cells by TiO2 nanocolumn arrays will be discussed. References: 1. Mali, S. S.; Shim, C. S.; Park, H. K.; Heo, J.; Patil, P. S.; Hong, C. K., Ultrathin Atomic Layer Deposited TiO2 for Surface Passivation of Hydrothermally Grown 1D TiO2 Nanorod Arrays for Efficient Solid-State Perovskite Solar Cells. Chemistry of Materials 2015, 27 (5), 1541-1551. 2. Liu, C.; Zhu, R.; Ng, A.; Ren, Z.; Cheung, S. H.; Du, L.; So, S. K.; Zapien, J. A.; Djuri?i?, A. B.; Lee Phillips, D.; Surya, C., Investigation of high performance TiO2 nanorod array perovskite solar cells. Journal of Materials Chemistry A 2017, 5 (30), 15970-15980. 3. Salado, M.; Oliva-Ramirez, M.; Kazim, S.; González-Elipe, A. R.; Ahmad, S., 1-dimensional TiO2 nano-forests as photoanodes for efficient and stable perovskite solar cells fabrication. Nano Energy 2017, 35, 215-222.

Resume : ALD is particularly well-suited to fabricate encapsulation layers for sensitive devices. ALD allows control of thickness at the sub-nm level, high conformability on high aspect-ratio substrates and easy switch from one material to another to make multilayers. Furthermore, ALD can be readily integrated into industrial production chains of organic devices. It has also been seen as a method particularly well fitted for temperature sensitive devices: synthesis of oxides such as Al2O3 and TiO2 is possible at 200°C and below, temperatures much lower than other CVD or PVD methods. The synthesis can be done with water and inexpensive chemical precursors (trimethylaluminum for Al2O3 and titanium tetraisopropoxide for TiO2) for cost-effective processes. Al2O3 is a material of major interest in encapsulation technologies: as a monolayer, it is unequaled in term of barrier to water vapor and oxygen properties. However, the barrier performances of Al2O3 become poorer and poorer as the temperature of synthesis decreases towards the values required for organic devices on supple substrates (<100°C). Likewise, TiO2 made from low-temperature ALD has also poor barrier properties. The multilayers made from Al2O3 and TiO2 fail to provide the ultra-barrier level required for long-lasting organic devices. In this work, the reasons for the degradation of the barrier properties of Al2O3 and TiO2 grown with low-temperature ALD are investigated. Strategies to improve the oxide quality are also studied.

Resume : Low operation stability of hybrid perovskite solar cells currently is the main obstacle for their practical implementation. Replacing fragile organic moieties with robust inorganic cations represents a promising approach to designing more stable absorber materials. Inorganic lead-halide based perovskites CsPbI3-xBrx (x=0-3) have demonstrated improved thermal and photochemical stability. Among them, CsPbI3 is considered as the most promising inorganic perovskite for photovoltaic cells due to its narrow band gap. Unfortunately, the cubic α-phase of CsPbI3 undergoes fast transition to a non-active yellow δ-phase under realistic solar cell operation conditions thus limiting practical application of this material. Efforts of many research groups working on improving CsPbI3 perovskite phase stability resulted in a certain progress in this field achieved very recently. Here we addressed the problem of poor α-phase stability of CsPbI3 films by using properly designed additives. A group of additives was identified that suppress significantly the phase transition effects in CsPbI3. The lifetime of the stabilized CsPbI3 perovskite exceeded 1000 h under 1 sun illumination at elevated temperatures. In addition, solar cells based on CsPbI3 films modified with optimal additives showed high power conversion efficiencies. These results pave a way to design of stable and efficient all-inorganic perovskite solar cells. This work was supported by Russian Science Foundation (project 18-13-00353).

Resume : Fused‐ring electron acceptors (FREAs), as a family of non‐fullerene (NF) acceptors, have achieved tremendous success in pushing the power conversion efficiency of organic solar cells. Here, the detailed molecular packing motifs of two extensively studied FREAs—ITIC and ITIC‐Th are reported. It is revealed for the first time the long‐range structure ordering along the backbone direction originated from favored end group π–π stacking. The backbone ordering could be significantly enhanced in the ternary film by the mutual mixing of ITIC and ITIC‐Th, which gives rise to an improved in‐plane electron mobility and better ternary device performance. The backbone ordering might be a common morphological feature of FREAs, providing explanations to previously observed small open circuit voltage loss and superior performance of FREA‐based devices and guiding the future molecular design of high‐performance NF acceptors.

Resume : Advanced photovoltaic devices with a high performance/cost ratio is a major concern nowadays. In the present study, we investigate the energetic efficiency of a new concept based on an indirect (instead of direct) photovoltaic and thermoelectric coupling. Using state-of-the-art thermal transfer calculations, we have shown that such an indirect coupling is an interesting alternative to maximize solar energy exploitation. In our model, a concentrator is placed between photovoltaic and thermoelectric systems without any physical contact of the three components. Our major finding showed that the indirect coupling significantly improve the overall efficiency which is very promising for future photovoltaic developments.

Resume : As promising lead-free perovskite candidate, tin-based halide perovskites have attracted extensive attention. However, the low open circuit voltage and unsatisfactory long term device stability still remain unsettled issues. Here, inorganic hole transporting material (HTM) CuSCN was used as hole extraction layer for inverted FASnI3 solar cells to reduce the energy band mismatch between FASnI3 and hole transporting layer. Moreover, ammonium hypophosphite (AHP) was introduced into the perovskite precursor to suppress the oxidation of Sn2+ and retard the fast crystallization of FASnI3 film to get better film quality. With improved perovskite film quality and reduced defect density, high efficiency of 6.24% with open circuit voltage up to 0.57 V was achieved. The CuSCN-based FASnI3 solar cells exhibited enhanced long term stability, 70% of the initial efficiency was retained after storing in N2 for 1000 h. Our work suggests the combination of suitable charge extraction layer and effective reducing additive can help to achieve high performance Sn-based perovskite solar cells with enhanced long term device stability

Resume : Solution processed triple-cation perovskite solar cells (PSCs) typically involve the use of mesoporous titanium dioxide (meso-TiO2) electron transport layer (ETL) and toxic antisolvents such as chlorobenzene or toluene to achieve high performance. Herein, we employed planar tin oxide (SnO2) film as ETL and designed a green antisolvent composed of ethyl acetate (EA) solvent and hexane (Hex) additive to produce perovskite film with the finest quality. The use of EA-Hex mixed antisolvent has successfully regulated the crystal growth dynamic of perovskite, forming perovskite film with larger grain size and smoother surface without pinholes and cracks. The reduced grain boundaries and surface defects in perovskite film have resulted in lesser carrier recombination and more efficient carrier transport, yielding a champion cell with an outstanding power conversion efficiency (PCE) of ~20% with negligible hysteresis. The PSC produced with EA-Hex antisolvent herein holds an unprecedentedly high PCE among the state-of-the-art SnO2 based triple-cation PSC produced with green antisolvent. Furthermore, the PSC produced with EA-Hex antisolvent also demonstrated superior environmental stability after aging for more than 1000 hours (>42 days), retaining ~92% and 100% PCE in pristine and encapsulation state, respectively.

Resume : A lead-free OHP is critical for avoiding toxicity and environmental impacts from future technologies. One promising candidate in this direction is Sn (instead of Pb). However, many researchers have reported the instability of Sn-based hybrid perovskite due to the presence of the 2+ oxidation state of tin (Sn²⁺) in OHP. This implies that there is a possibility of Sn-basded OHP instability. To understand the instability of Sn-based perovskite (CH₃NH₃SnI₃), we performed the surface- and bulk-sensitive photoelectron spectroscopy (PES) experiment with the synchrotron radiation and theoretical calculations such as density functional theory and ab initio molecular dynamics. Findings from this experimental and theoretical study highlight the crucial changes of surface chemical states, the broken chemical bondings in Sn-I, and the depletion of a CH3-NH3⁺ cation on the surface region. We found and confirmed that the surface-chemical instability is closely associated with a critical reason for Sn-based OHP instability.

Resume : : The main problems preventing wide spreading of solar cells as alternative energy sources are their high cost and low efficiency. Efficiency of solar cells based on semiconductors materials is limited due to high electrical and optical losses and to recombination process. Therefore and in order to surmount these problems, we propose in this work to use ZTO nanoparticles deposited on porous silicon layer as antireflection coatings. We found that the formed layer decrease the optical losses which consequently contribute to the improvement in the conversion efficiency of solar cells.

Resume : The science and technology of organic electronics have made consistent progress. However, the long-term stability of organic devices is a critical issue that remains to be resolved. Encapsulation is a straightforward and practical means to protect organic materials from oxygen or moisture and thus improve air stability. Here, we report a high-performance flexible inorganic SiNx/SiOxNy/SiOx hybrid barrier film for application in both organic solar cells (OSCs) and quantum dots light emitting diodes (QD LEDs). This hybrid barrier film shows average transmittance of 85.5% and a water-vapor transmission rate of 7.1×10?5 g m?2 day?1. In OSCs comprising poly[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-bA]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl] thieno[3,4-b]-thiophenediyl] (PTB7) and [6,6]-phenyl-C70-butyric acid methyl ester (PC70BM), which were encapsulated by the SiNx/SiOxNy hybrid barrier film, the power-conversion efficiency remained above 86% of the initial value even after 2000 h of storage in air, which is comparable to that obtained for a device encapsulated by a glass lid. Moreover, two triad SiNx/SiOxNy/SiOx barrier films were also fabricated. They showed average transmittance (400 - 700 nm) of 84.3 % and ultra low water vapor transmission rate (WVTR) of 2 × 10?6 g m?2 day?1. To evaluate our barrier films, we fabricated QD_LEDs (Figure 5a) consisted of the patterned ITO, ZnO NPs/PEIE film as EIL/ETL, CdSe-ZnO QDs as the emission layer (EML), poly(N-vinylcarbazole) (PVK) blend layer as the hole transport layer (HTL), molybdenum trioxide (MoO3) as the hole injection layer (HIL), and Ag (anode). ZnO NPs were synthesized by solution precipitation method and other materials were commercially available. PEIE, ZnO NPs, CdSe-ZnS QDs, PVK layers were prepared by spin coating. QD-LEDs devices were encapsulated with glass lip and two triad barrier films. And the lifetime (T50), taken to reach half of the initial luminescence, of QD-LEDs encapsulation with the two triads SiNx/SiOxNy/SiOx barrier film is 354 h which is compatible with 396 h for QD-LEDs with glass lid.

Resume : Numerous new materials and device structures have been widely explored in order to cut the cost of photovoltaic (PV) manufacture. Organic-inorganic hybrid solar cells based on nanostructured semiconductor have built up in few years ago, which may promise the low cost and high performance. However, the device performances are relatively lower than its pristine all-inorganic PV devices, resulting from the numerous surface defect and improper organic-inorganic phase segregation. Here, we demonstrate that hybrid PVs based on organic conjugated molecular and nanostructured silicon nanowire arrays can achieve a high PCE by controlling the phase separation as well as surface passiviation. An advantage of hybrid devices presents the excellent light harvest capability of nanostructured as well as simple fabrication process. Especially, hybrid composites of conjugated organic materials and nanostructured inorganic materials are potential candidates for cost-effective and efficient solar-energy-harvesting devices1. This device can be worked by a light-modulated field effect solar cell, which can be potentially integrated with halide perovskite2. In addition, this type device can be easily integrated with other type energy device, for example, an energy harvesting structure that integrates an organic-Si heterojunction solar cell and a triboelectric nanogenerator (TENG) device is built to realize power generation from both sunlight and raindrop3. In addition, a self-charging power unit based on an organic-Si heterojunction solar cell and a polypyrrole supercapacitor, which simultaneously achieved both photoelectric conversion and energy storage4. Key Words: Silicon Solar Cell, Heterojunction, Flexibly, Integrating Device 1. Sun, B.; Shao, M.; Lee, S.-T. Advanced Materials 2016, 28, (47), 10539-10547. 2. Wang, Y.; Xia, Z.; Liu, L.; Xu, W.; Yuan, Z.; Zhang, Y.; Sirringhaus, H.; Lifshitz, Y.; Lee, S. T.; Bao, Q.; Sun, B. Advanced Materials 2017, 29, (18), 1606370. 3. Liu, Y.; Sun, N.; Liu, J.; Wen, Z.; Sun, X.; Lee, S.-T.; Sun, B. ACS Nano 2018 12 (3), 2893-2899. 4. Liu, R.; Wang, J.; Sun, T.; Wang, M.; Wu, C.; Zou, H.; Song, T.; Zhang, X.; Lee, S.-T.; Wang, Z. L.; Sun, B. Nano Letters 2017, 17, (7), 4240-4247.

Resume : Perovskite solar cells demonstrate an enormous potential for the next generation of photovoltaics because of their ease and low cost of fabrication in combination with their formidable light-harvesting properties. The devices usually consist of ca. 500 nm thick layer of an organometallic halide perovskite between two charge-transporting layers. While charge generation in these perovskites is fast and efficient, bulk recombination is slow, meaning that the device properties are largely limited by processes at the internal interfaces or within the charge-transporting layers. Here, we present the results of detailed studies on hybrid perovskite solar cells comprising all-organic charge transport layers. We show how the energetics of the organic semiconductor affects the device performance through the rate of interfacial charge recombination, and how these recombination processes are severely reduced through the proper choice of the organic material and introduce interlayers that mitigate this recombination [1, 2]. We investigate the impact of such interlayers on the stability of p-i-n type perovskite solar cells and achieve simultaneous efficiency and stability improvements by the introduction of specific perfluorinated self-assembled monolayers. Through the proper design of all layers and interfaces, perovskite solar cells with high fill factors (>80%) and high open circuit voltages (1.18V), exceeding a power conversion efficiency of 21% could be realized. Notably, the efficiency remained stable under MPP operation at 85°C for more than 250 h, while the device exhibited also enhanced humidity resilience maintaining ~95% device performance even if stored in humid air in the ambient over months [3]. [1] C.M. Wolff, F. Zu, A. Paulke, L.P. Toro, N. Koch, and D. Neher, Adv. Mater, 1700159 (2017) [2] M. Stolterfoht, C.M. Wolff, J.A. Márquez, S. Zhang, C.J. Hages, D. Rothhardt, S. Albrecht, P.L. Burn, P. Meredith, Th. Unold & D. Neher, Nat. Energy, 3, p. 847?854 (2018) [3] C.M. Wolff, L. Canil, N.L. Nguyen, P. Caprioglio, L. Fiedler, M. Stolterfoht, A. Abate & D. Neher, submitted

Resume : Most of the best-performing perovskite solar cells are used with the organic hole transporting materials (HTMs) in a doped state. Use of the dopants is often associated with the decreased stability of the devices. In particular, an interaction of the oxidized HTM with tert-butylpyridine can decrease a concentration of the cation-radicals, thus reducing transport ability of the layer. In our work we have synthesized new carbazole-based HTM, functionalized with anchoring phosphonic acid group (V1036). Based on this material, the deposition method for the formation of the hole-selective monolayer was developed. With this material, the highest efficiency of 17.8% was achieved [1]. Following the initial success, the concept was developed by means of molecular engineering. A new generation of the monolayer HTMs (V1193, V1194) ensured high open-circuit voltage values of up to 1.19 V, which in a combination with over 80% fill factors resulted in close to 21% efficiencies. Initial stability assessment showed good shelf life-time of the monolayer HTM-based devices, with the drop in performance of less than 3% after more than 4000 h. The ongoing research is focusing on the study of the impact of the monolayer HTM on the operational device stability. [1] A. Magomedov et al. Adv. Energy Mater. 2018, 8 (32), 1801892

Resume : In the pursuit for higher stability in perovskite solar cells not only the absorber needs to be considered but also the charge extraction layers. Up to now the highest efficiencies are achieved by employing organic compounds such as Spiro-OMeTAD, PTAA and PCBM. As these organic compounds are prone to degradation under extended irradiation, a pathway to improve stability is to replace them with oxides that are expected to show a higher durability. Nickel oxide is an p-type semiconductor, with reported valence band edge energy close to the one of MAPbI3. Also it was reported that the surface recombination velocity at the NiO/MAPbI3 interface is below a critical threshold above which device performance is drastically impaired. Additionally it was found that doping the NiO with Li and/or Mg improves the device performance. In this respect nickel oxide is a promising hole extraction layer. In this work, the interface between MAPbI3 and nickel magnesium oxide deposited by co-sputtering is investigated. Time-resolved photoluminescence and temperature-dependent J-V measurements are used to gain insight on the recombination processes and charge transport at this interface and to understand the effect of the doping with magnesium.

Resume : Two-dimensional halide perovskites (HaP) are solution-processed quantum wells with unique structure and photo-physical properties, which has led to several proof-of-concept high efficiency optoelectronic devices such as photovoltaics and light emitting devices with technologically relevant stability. Many approaches to control the phase purity, crystallinity and orientation have been used and have led to results that are often not easily reproducible or are often contradictory. In this talk, I will describe the design principles for tailoring achieving phase-purity, crystallinity and orientation in layered 2D perovskites and its implications on the specific optoelectronic application. Finally, I will demonstrate proof-of-concept optoelectronic devices that validate these findings and lay the path for tuning the structure and properties of 2D perovskites all the way from crystals to thin-film devices.

Resume : Semiconducting metal-halide perovskites present various types of chemical interactions which give them a characteristic fluctuating structure sensitive to the operating conditions of the device, to which they adjust. This makes the control of structure-properties relationship, especially at interfaces where the device realizes its function, the crucial step in order to control devices operation. In particular, given their simple processability at relatively low temperature, one can expect an intrinsic level of structural/chemical disorder of the semiconductor which results in the formation of defects. Here, first I will present our results on the role of structural and point defects in determining the nature and dynamic of photo-carriers in metal-halide perovskites. Then, I will discuss our understanding of key parameters which must be taken into consideration in order to evaluate the suscettibility of the perovkite crystals (2D and 3D) to the formation of defects, allowing one to proceed through a predictive synthetic procedure. Finally, I will show the correlation between the presence/formation of defects and the observed semiconductor instabilities/degradation.

Resume : Hybrid perovskite solar cells exhibit remarkable power conversion efficiencies, yet their limited stability and molecular-level engineering remain challenging.[1-5] In contrast to hybrid three-dimensional perovskites, their layered two-dimensional (2D) analogues have demonstrated promising stabilities, though at the expense of the corresponding efficiencies.[2,6] Our strategy provides stabilization without compromising the efficiency by employing judiciously designed multifunctional molecular modulators (MMMs) through fine-tuning noncovalent interactions complemented by structural adaptability.[4-6] These systems are devised to interact with the perovskite surface in a manner uniquely assessed by solid-state NMR spectroscopy.[4-5] As a result, we obtain perovskite solar cells with superior properties and efficiencies exceeding 20% for formamidinium cesium mixed lead iodide compositions, accompanied by enhanced stabilities.[5] Moreover, extending the MMM design into layered 2D architectures leads to further stability enhancement.[6] This approach has been investigated using a combination of techniques complemented by solid-state NMR to unravel the design principles and exemplify the role of molecular modulation in advancing perovskite solar cells. Reference: [1] Y. Rong, Y. Hu, A. Mei, et al. Science 2018, 361, eaat8235. [2] G. Grancini, M. K. Nazeeruddin, Nat. Rev. Mater. 2018. [3] J. V. Mili?, M. I. Dar, N. Arora, et al. Adv. Mater. Interfaces 2018, 1800416. [4] M. M. Tavakoli, W. Tress, J. V. Mili?, et al. Energy Environ. Sci. 2018, 11, 3310. [5] D. Bi, X. Li, J. V. Mili?*, et al. Nature Commun. 2018, 9, 4482. [6] Y. Li, J. V. Milic?*, A. Ummadisingu, et al. Nano Lett. 2018, doi:10.1021?acs.nanolett.8b03552.

Resume : Organic−inorganic perovskites are receiving substantial attention during the recent years of their development due to their meteoric rise in efficiency. The family of mixed-halide perovskites exhibits a tunable bandgap from 1.6 -3.1 eV simply by adjusting the ratio of halides, makes it an excellent candidate for low-cost multijunction PV. In particular, wide bandgap perovskites (WBP) with a bandgap ranging between 1.7 - 1.8 eV are attractive top-cell materials to improve the efficiency of thin‐film solar cells in tandem structure. However, obtaining high open-circuit voltages (Voc) is still a key challenge for WBP solar cells to achieve sufficient power conversion efficiency. Despite the remarkable progress in PCE, the maximum Voc of all WBP solar cells in the optimal optical bandgap range for perovskite based tandem PV (1.7 - 1.8 eV) remains limited to 1.24 eV. In this work, a planar WBP (1.72 eV) solar cell with impressively improved Voc of 1.31 V along with a stable PCE of 19.4% is achieved. The Voc-to-bandgap ratio is the highest reported for WBP solar cell. This considerable progress was made by creating a hybrid 2D/3D perovskite heterostructure. By deposition of an ammonium derivative on top of the (Cs0.17FA0.83Pb(I0.6Br0.4)3) perovskite, a thin 2D Ruddlesden-Popper perovskite between the absorber layer and the hole transport layer (spiro-OMeTAD) is formed. The results obtained by time resolved photoluminescence showed a combination of excellent passivation and maintaining fast carrier extraction by this interlayer, leading to the significant enhancement in Voc and overall photovoltaic performance.

Resume : Perovskite solar cells (PSCs) provide an avenue for next-generation thin-film photovoltaics. I will show how ALD-grown tin-oxide (SnOx) as impermeable electron extraction layer for PSCs enables impressive stability of the cell against heat and moisture. Its outstanding barrier properties [1] protect the perovskite against ingress of moisture or migrating metal atoms, while simultaneously the metal electrode is protected against leaking halide compounds. PSCs with an efficiency of >20% and outstanding long-term stability can be achieved (> 4,500 h in air). [2,3] ALD grown SnOx is also excellently suited to sandwich and protect ultra-thin metal layers (Ag or Cu) as cost efficient Indium-free semitransparent electrodes (SnOx/metal/SnOx) in PSCs. Moreover, SnOx grown by using ozone affords hysteresis-free devices with a remarkably high open circuit voltage of up to 1.19 V.[4] We provide a study of the interface electronic structure of SnOx and the perovskite using photoelectron spectroscopy. Ultimately, SnOx grown by spatial ALD at atmospheric pressure is presented.[5,6] This work paves the way towards roll-to-roll fabrication of stable, indium free PSCs. [1] A. Behrendt et al., Adv. Mater. 2015, 27, 5961. [2] K. O. Brinkmann et al., Nat. Comms. 2017, 8, 13938. [3] J. Zhao et al., Adv. Energy Mater. 2017, 7, 1602599. [4] T. Hu et al., Adv. Mater. 2017, 29, 1606656. [5] L. Hoffmann et al., J. Vac. Sci. & Technol. A 2018, 36, 01A112. [6] L. Hoffmann et al., ACS Appl. Mater. & Interf. 2018, 10, 6006.

Resume : Metal hybrid perovskite solar cells (PSCs) have attract great attention owing to the excellent photovoltaic properties. However, instability issue is still one of the great challenges for the large-scale application. Here I would like to introduce our work on defect engineering to achieve highly stable and efficient PSCs. We proposed new strategies to improve the crystallization of perovskite films by controlling the molecular interactions within perovskite precursors. Large-area and pin-hole free perovskite films were obtained, which enabled stable PSCs. We also control the defect diffusion by developing nano-carbon electron transporting layer in PSCs. It successfully enabled better stability by hindering the reaction between the diffused iodide and the metal electrode. The device stability was enhanced with the efficiency improved by a better diffusion of electrons. Our findings indicate an effective way to obtain stable and efficient PSCs for future application. [1] E. B. Bi; H. Chen; F. X. Xie; Y. Z. Wu; W. Chen; Y. J. Su; A. Islam; M. Gratzel*; X. D. Yang*; L. Y. Han*. Nature Communications 2017, 8: 7. [2] H. Chen, F. Ye, W. Tang, J. He, M. Yin, Y. Wang, F. Xie, E. Bi, X. Yang, M. Gratzel, and L. Han, Nature, 2017, 550, 92. [3] Y. B. Wang; Y. F. Yue; X. D. Yang*; L. Y. Han, Advanced Energy Materials 2018, Doi.org/10.1002/aenm.201800249. .

Resume : We report a new method by which we ascertain the spatial distribution of photogenerated charge carrier collection, operando, in colloidal quantum dot (CQD) and perovskite solar cells. This non-destructive method allows for a visualization of the collection efficiency as a function of position in the active layer, and at bias conditions ranging from short-circuit to open-circuit and in between - especially at the maximum power point. To achieve this, we used a Gaussian-noise regularized least-squares numerical calculation, which provides a systematic approach for computing physically realistic solutions and does not involve hyperparameters. The method only requires as an input the refractive indices of each layer in the device and biased external quantum efficiency spectra. Using this newly developed tool on CQD solar cells, we find that they are not fully depleted at zero bias and that the hole-transport layer widely used across the CQD community provides a trap/barrier to hole collection. We also investigate the evolution of the collection efficiency in perovskite solar cells over time in different aging conditions to elucidate the origin and spatial distribution of degradation in the active layer.

Resume : In the past few years, hybrid perovskite solar cells have attracted a considerable amount of research and have undergone rapid development as next generation photovoltaics [1]. The power conversion efficiency has then been rapidly increasing and has recently exceeded 23.3% [2]. In order to enhance the device performance it is important to elaborate high-quality perovskite films by controlling their crystallinity, grain size, and morphology. The other layers such as the electron transporting layer (ETL), N-type Layer, the hole transporting layer (HTL), P-type Layer, and the interfaces also play a fundamental role on the initial performances and stability. Their chemical composition and structural defects are expected to affect critically the device electrical characteristics. In this work, the studied structure is the N-I-P type Perovskite solar cells based on CH3NH3PbI3 (MAPI) or double cations Perovskite active layer. The main aim of this study is to investigate the origin of devices instability when stored under illumination. In order to determine the key parameter responsible of this degradation, a differential aging of solar cells is performed. It consists in aging the different layers under illumination (1 sun / 35°C) before the deposition of top layers and/or evaporation of top electrode. The impact of the nature of 2 different oxide-based N-type layers (Al-doped ZnO and SnO2) on the formation of the Perovskite (grain size, morphology and crystallization) was first studied before aging. Differential aging results show that the stability of MAPI thin films is not the main parameter responsible of perovskite solar cells degradation. Moreover, structural and optical characterizations have shown that MAPI thin films are stable under illumination for more than 1000 hours. Therefore, the N layer/ Perovskite interface is the first leading reason causing degradation under illumination. To get further insight into the role of this interface on device behavior, XPS & TOF-SIMS have been combined to probe the chemical composition of the interface all along aging. These studies further confirmed the critical role of the outer surface of the metal oxide and especially of the carbon content of the outer surface. Improved system with stable SnO2 layer and double cations perovskite Cs0.05FA0.95Pb(I 0.83Br0.17)3 have also been studied and show better stability under illumination of complete cells. ( References: [1] T.A. Berhe et al, Energy Environ. Sci. 2016, 9, 323. [2] P. Meredith et al, Nature Communications, 2018, 9, 5261)

Resume : Recently, scientific works on perovskite solar cells (PSCs) have increased exponentially, reporting the synthesis of new materials while ensuring higher efficiencies and improved long-term stability. However, in most cases, manufacturing and measuring processes take place under specific strict environmental conditions (like glove boxes), in order to prevent the materials and devices from being degraded. It is obvious that the commercialization of the 3rd generation solar cells is going to be quite challenging, if these controlled conditions are considered as a prerequisite. Hence it would be desirable, more convenient and cost-effective to be able to fabricate PSCs through a solution-based deposition process under ambient air conditions of humidity, temperature and pressure. Due to the well-known issues of perovskite decomposition attributed mainly to humidity, we started a thorough study to provide a manufacturing protocol for the fabrication of efficient mesostructured PSCs in ambient air. For each layer, different parameters affecting its thickness and uniformity were examined (i.e. spin coating speed, deposited solution volume, loading time). Furthermore, various annealing conditions for the perovskite layer were tested (different drying time or gradual annealing before the final annealing) to control the crystallization, along with interface engineering with buffer layer.

Resume : A few years ago, issues around stability loomed as possible roadblocks to the successful implementation of high-efficiency perovskite solar cells. Subsequently, however, advances in engineering devices and understanding degradation mechanisms has made many stability problems seem solvable. As a result, perovskite solar cells of several types have now passed benchmark 1000-hour tests of stability. In this talk, I outline our studies of long-term stability of perovskite solar cells. I show how the use of a capping electrode of sputtered Indium Tin Oxide prevents egress of volatile organic cations and enables thermal stability while functioning as a conductive top electrode or as a recombination layer for tandem solar cells. In addition, the use of a metal electrode necessitates a good diffusion barrier to prevent reactions with the iodide in the perovskite, which we achieve by solution-processed interlayer before the ITO sputter. By adapting methods common in the industry to package perovskite solar cells, using a low-elastic modulus encapsulant and a butyl rubber edge seal, we pass a 1000-hour test of stability under damp heat and a test of stability under temperature cycling. While wide band gap lead halide perovskites have passed several tests of stability, low band gap devices based on tin-lead perovskites have until now been perceived as unstable. We study the mechanisms of oxidation of tin-lead perovskites and conclude that alloying tin with lead drastically stabilizes it toward oxidation. Using a perovskite with 40% tin, and by designing a device architecture that uses band bending at a heterojunction in place of the commonly used but unstable hole contact PEDOT:PSS, we successfully demonstrate 1000-hour thermal, atmospheric, and operational stability with a low band gap perovskite, thus removing the main perceived obstacle to implementation of highly efficient perovskite-perovskite tandem photovoltaics.

Resume : Solar cells based on lead halide perovskites have recently emerged showing a tremendous increase of power-conversion efficiency which exceeded 23%. In this contribution, the device physics of perovskite solar cells is addressed. The focus is on slow irreversible and reversible transient processes that reveal specific operation mechanisms and affect the power-conversion efficiency. The resulting consequences for operation under real weather conditions are discussed as well. A puzzling effect on the timescales of milliseconds is an extremely high apparent capacitance, which can show even negative values. This effect has been related to the mixed ionic-electronic conductivity of the perovskite material. Here, it is discussed in terms of charge carrier injection modulated by accumulated ionic charge. On the timescales of minutes to hours, reversible processes are observed such as a decrease of efficiency upon light-soaking. The open-circuit voltage is found to be mainly responsible for this effect which is investigated using transient electroluminescence measurements and analyses of the diode ideality factor. Temperature- and light-intensity dependent characterization indicates changes in surface recombination. In a degradation study spanning several weeks, real weather data for temperature and light intensity are used to investigate the effect of reversible and irreversible losses on the energy yield of perovskite solar cells. References 1. Tress, W. et al. Energy Environ. Sci. 11, 151–165 (2018). 2. Domanski, K. et al. Nat. Energy 3, 61–67 (2018).

Resume : In efficient perovskite solar cells, the perovskite thin-film absorber is sandwiched in between charge selective transport layers, which ensure rectification and avoid non-radiative charge recombination. Their conductivity is commonly enhanced by doping in order to form ohmic contacts to the external electrodes, enhancing the solar cell fill factor. In vacuum deposited solar cells, this is realized by simultaneous co-sublimation of an organic semiconductors and the corresponding molecular dopant. One drawback of the use of doped charge transport layers is their detrimental effect on the device lifetime, as most dopants are very sensitive to moisture and oxygen. In this work we discuss simple alternatives for doped organic contacts in highly efficient vacuum-deposited perovskite solar cells. We studied the performance of n-i-p solar cells based on methylammonium lead iodide using very thin-films (<10 nm) of ionic and neutral materials as interlayer between the front transparent conductor and the electron transport layer. We studied different interlayers which can be processed either by solution- or vacuum-deposition. Key to the efficient electron extraction is the reduction of the electrode work function, in order to decrease the energy barrier between the electrode and the electron transport layer. This can be achieved by using ionic materials, which introduce dipoles at the electrode interface, or by selection of molecular materials with small ionization potential. We demonstrate perovskite solar cells with 18% efficiency and without hysteresis, and present initial stability studies demonstrating the potential of our approach.

Resume : Organic-inorganic hybrid perovskite solar cells (PSCs) have attracted worldwide attention due to an unprecedented increase in power conversion efficiency (PCE) which reached 23.3% within less than ten years. Most efficient systems usually involve hole transport materials such as spiro-OMeTAD doped using complex mixture made of LiTFSI salt, TBP and/or Cobalt coordination compounds. However, doping process relies on controlled ambient conditions such as oxygen and light exposure which leads to reproducibility and stability issues. In this context, we herein report a metal-free formulation of spiro-OMeTAD based on an ionic liquid polymer efficient as hole transporter in n-i-p PSCs. Thus, the conductivity of spiro-OMeTAD was significantly improved by several orders of magnitude, up to 1.7 x 10-3 S.cm-1, using poly(N-vinyl-3-alkylimidazolium bis(trifluoromethane) sulfonamide (PVBI-TFSI) as dopant. In the FTO/c-TiO2/mp-TiO2/K+ doped MA0.15FA0.85PbI2.55Br0.45/HTM/Au PSC configuration, PVBI-TFSI-doped spiro-OMeTAD showed record power conversion efficiency as high as 20.4 %, versus 18.4 % for LiTFSI-doped spiro-OMeTAD, with considerably reduced hysteresis. Mechanistic investigations suggest that PVBI-TFSI acts as a source of protons promoting the spiro-OMeTAD oxidation and that the doping mechanism depends upon irradiance and tBP concentration. To the best of our knowledge, this is the best PSC performance reported so far with an alternative dopant in spiro-OMeTAD. Acknowledgments: This work was performed within the framework of LIA NextPV and was supported by IDEX Bordeaux, ELORPrintTec ANR-10-EQPX-28-01 and LCPO/Arkema/ANR INDUSTRIAL CHAIR “HOMERIC” ANR-13-CHIN-0002-01.

Resume : Perovskite solar cells (PSCs) have set themselves apart from their dye-sensitized (DSSC) and organic (OSC) predecessors, with impressive efficiencies at 24%. Despite the key-role that charge transport layers (CTLs) have played in this success, researchers are still debating the importance of parameters that govern charge transfer at the PSC interfaces, namely: (i) the interfacial energy offset ∆E, (ii) CTL structure and (iii) intrinsic perovskite properties. This talk will detail our progress in developing a complete picture of electron/hole transfer at PSC interfaces using a combination of time-resolved spectroscopy and device work.1,2 Specifically, we will outline our observation of highly efficient (>75%), nanosecond charge transfer to CTLs at remarkably low values of ∆E (<0.1eV). We will also show that careful selection of CTLs with electron-donating functional groups can slow interfacial recombination by passivating surface traps. The resulting perovskite/CTL interaction can result in a 3-fold improvement in charge transfer. These results have profound implications for the design of CTLs and explain the superiority of triarylamines such as PTAA over other polymeric HTLs of the same HOMO energy. Finally, we will show how such surface passivation can greatly improve the stability of PSCs.3 1. R. J. E. Westbrook et al, J. Phys. Chem. C 2018, 122, 1326−1332 2. R J. E. Westbrook, W. Xu et al, Manuscript in Preparation 3. Aristadou et al, Nat. Commun., 2017, 8, 15218

Resume : Tin oxide (SnO2) is a commonly used electron transport material in n-i-p (so called “regular”) perovskite solar cells (PSC) which enables high open circuit voltages (Voc) above 1.15V. In this study, compact layers of SnO2 processed from a SnCl2 precursor solution are investigated with regard to their performance in regular PSCs. We report the crucial importance of SnO2 surface treatments to obtain efficient and stable PSCs. In particular, we find that O2-plasmas instead of UV-O3 treatment strongly enhance performance, reproducibility and stability. The best efficiency from JV-scans is 18.1% with virtually no hysteresis and a stabilized power output of 17.9% for plasma-treated samples whereas UV-O3 treated samples yield 17.5% and 16.2%, respectively. X-ray-Photoelectron Spectroscopy (XPS) measurements show that plasma treatment reduces OH shoulders and the Cl-peak. If the exposure to plasma is too harsh, we observe a reduction in Voc up to 200mV. From Ultraviolet Photoelectron Spectroscopy (UPS) we find a plasma-induced shift in work function of up to 0.3eV which may cause an energetic misalignment and thus explain the observed Voc drop. Moreover, time-resolved photoluminescence (TRPL) measurements reveal increased carrier lifetimes and thus indicate less defects at the perovskite/SnO2 interface upon plasma-treatment. Our work demonstrates that detailed understanding of surface modifications is important to optimize device performance parameters and stability of PSCs.

Resume : The stability issue is a big challenge in the current halide-perovskite (HP) based optoelectronic techniques. Together with experimental studies, we explored a series of model systems using first-principles methods. To provide systematic experimental-theoretical guidelines for materials design towards high stability and proper perforrmance of optoelectronic devices, our study covers different approaches of HP stabilization. (i) Cesium-doping: the introduction of Cs enhances the thermodynamical stability of FASnI3 via unit-cell-volume reduction and mixing entropy [1]. (ii) Surface engineering: (a) surface passivation with the novel organic ligand 3-phenyl-allylamine (PAA) significantly improves the stability of MAPbI3 due to surface geometry, and assists the charge transfer from perovskite to the electron-transporting interlayer [2]; (b) we developed a high-throughput database-driven scheme to efficiently select proper inorganic semiconductors as protective coating materials [3]. (iii) 2D perovskites: We found that PAA is a proper ligand for 2D perovskites PAA2FA(n-1)Sn(n)I(3n+1) as the formation energy is significantly lower than using conventional ligands such as 1-butylamine. Devices with the stable PAA-mediated 2D perovskite can retain ~80% efficiency with self-healing in N2 after degradation in heat [4]. [1] Gao et al., J. Phys. Chem. Lett. 9 6999 (2018). [2] Dong et al., Adv. Funct. Mater. submitted. [3] Seidu et al., to be submitted. [4] Ran et al., to be submitted.

Resume : Organic-inorganic hybrid perovskite has gotten lots of attention for fabricating low-cost and high-efficiency solar cells owing to the tremendous optoelectronic properties, including a broad and tunable light-harvesting range, low exciton binding energy, and decent ambipolar charge-transporting capability. With only a few years’ development, the record PCE of perovskite solar cells (PVSCs) has reached 23.2% last year. However, it suffers from a critical intrinsic disadvantage of low long-term stability, such as poor moisture and ultraviolet light resistance that dramatically lower the long-term stability of PVSC device. Accordingly, here we introduce the metal-organic frameworks (MOFs), which are three-dimensional (3D) porous crystalline materials consisting of multipodal organic linkers and secondary building units (SBUs) based on high-valent ions/clusters, and perform excellent moisture and chemical stabilities that have been widely employed for gas separation and catalysis. Meanwhile, MOFs are also introduced into photovoltaic field, including the application in dye-sensitized solar cells, or being served as the charge-transporting layers (CTLs) in PVSCs. In this study, We demonstrate the applications of using perovskite/Zr-MOF heterojunction in the stable inverted p-i-n PVSCs. We conduct two types of Zr-MOFs, UiO-66 and MOF-808, with different properties and the outstanding moisture and chemical stabilities. While serving as an interlayer in conjunction with the perovskite film, the MOFs reveal the abilities of UV-filtering and defect passivation by enhancing the crystallinity of perovskite. Wherefore, the UiO-66/MOF-808-modified PVSCs receive enhanced PCEs of 17.01% and 16.55%, comparing with the control device (15.79%). Moreover, While we further hybrid perovskite and Zr-MOF as the heterojunction to fabricate the devices, the hybrid MOFs are found to distribute over the perovskite grain boundary that providing the grain-locking effect, which simultaneously possess the ability of defect passivation and reinforcing the film’s robustness against the moisture invasion. As a result, the PCEs of the UiO-66/MOF-808-hybrid PVSCs are further enhanced to 18.01% and 17.81%, respectively. Besides, over 70% of initial PCE is retained after being stored in air (25 oC and relative humidity of 60 ± 5%) for over 2 weeks in contrast to the quick degradation observed for the control device. This study demonstrates the promising potential of using perovskite/MOF heterojunction to fabricate efficient and stable PVSCs.

Resume : Organic photovoltaic (OPV) devices now regularly reach 10% power conversion efficiency, especially now with the development of non-fullerene acceptors.[1] However, those performances are not always very stable and, in order to improve the lifetime of these devices, a deep understanding of the different degradation mechanisms is needed. Degradation pathways are complex and diverse: they can be induced by numerous intrinsic and extrinsic factors which can affect different regions of the device (active materials, interfaces, morphology, etc…). In the first part of this presentation, I will specifically focus on the degradation occurring at different interfaces in the multilayer which forms the OPV device. We identified several degradation mechanisms occurring at the bottom interface (ETL/Active layer)[2] as well as at the top interfaces (Active layer/HTL/Electrode).[3] In a second part, we studied the potentiality of graftable molecules such as self-assembled monolayers (SAM) to improve the stability of OPV devices. A solution to replace PEDOT:PSS in direct architecture was developed involving a thin layer of P3HT grafted on the ITO.[4] These different studies highlight the role of the architecture and, more specifically, of the ETL, HTL and electrode in the stability of OPV devices. They also showed that functionalization of the metal oxides can be a solution to improve the stability of the devices. [1] D. Baran et al., Nat. Commun. 9, 2059 (2018) [2] A. Tournebize et al., ACS Appl. Mater. Interfaces 9, 34131 (2017) [3] W. Greenbank et al., J. Mater. Chem. A 5, (2017) [4] H. Awada et al., Org. Electron. Physics, Mater. Appl. 57, (2018)

Resume : In the present study, protoporphyrin IX (PPIX) and squarine (SQ2) have been used in a co-sensitized dye-sensitized solar cell (DSSC) to apply their high absorption coefficients in the visible and NIR region of the solar spectrum and to probe the possibility of Förster resonance energy transfer (FRET) between the two dyes. FRET from the donor PPIX to acceptor SQ2 was observed from detailed investigation of the excited-state photophysics of the dye mixture, using time-resolved fluorescence decay measurements. The electron transfer time scales from the dyes to TiO2 have also been characterized for each dye. The current?voltage (I?V) characteristics and the wavelength-dependent photocurrent measurements of the co-sensitized DSSCs reveal that FRET between the two dyes increase the photocurrent as well as the efficiency of the device. From the absorption spectra of the co-sensitized photoanodes, PPIX was observed to be efficiently acting as a co-adsorbent and to reduce the dye aggregation problem of SQ2. It has further been proven by a comparison of the device performance with a chenodeoxycholic acid (CDCA) added to a SQ2-sensitized DSSC. Apart from increasing the absorption window, the FRET-induced enhanced photocurrent and the anti-aggregating behavior of PPIX towards SQ2 are crucial points that improve the performance of the co-sensitized DSSC.

Resume : As the efficiencies of organic photovoltaic devices steadily improve, understanding their degradation mechanisms becomes increasingly important. Commonly, degradation of device performance upon exposure to environmental factors (oxygen, water, light and heat) is attributed to degradation of contacts, interfaces and that of the active material, however little is known about the effects on the energetic level alignment in the device. Understanding how the energetic landscape evolves upon degradation is critically important as it affects fundamental processes such as charge generation, dissociation, transport and extraction. Herein, we demonstrate that combining ultra-violet photoemission spectroscopy with gas cluster ion beam sputtering allows for a damage-free depth profiling and hence, an accurate determination of the vertical energetic landscape of organic layers. With this we simultaneously obtain compositional information and energy alignment of contributing materials with a high vertical depth resolution, which is of particular interest for bulk heterojunction solar cells. Our technique allows for investigation of these properties both before and after degradation and thus to monitor the change of the energetic landscape upon degradation. We focus on the high performance system DRCN5T:PCBM70 and investigate the relationship between the evolution of the energetic landscape to the deterioration in the open circuit voltage of the device upon degradation.

Resume : Poly (3-hexylthiophene) was an early success in the development of donor polymers for organic photovoltaics. A simple and inexpensive synthesis suggests that it may be the most viable donor polymer to use in large-scale commercial organic solar cells. Replacing fullerenes with new electron acceptors has led to significant improvements in device performance and stability, with P3HT devices now able to exceed an efficiency of 7%. Previous studies have reported a dependence of device performance on polymer molecular weight in fullerene-based blends, however this has not been explored in nonfullerene acceptors. P3HT blends, with two nonfullerene acceptors (O- and EH-IDTBR), were probed in this work, using a number of P3HT batches with varying molecular weights. O-IDTBR was shown to exhibit a dependence on the polymer molecular weight, with optimal performance achieved with a 34 kDa polymer, whilst the less crystalline EH-IDTBR displayed an independence in performance with varying polymer molecular weight. Probing the thermal and morphological behavior of the P3HT:O-IDTBR blends suggest an optimal morphology, with pronounced donor and acceptor domains was only achieved in with the 34 kDa polymer batch; likely to be dictated by the competition between the crystallization of P3HT and O-IDTBR. The insensitivity of EH-IDTBR to polymer molecular weight can be attributed to its less crystalline nature, which does not impact the aggregation of the P3HT during the film forming process.

Resume : Bulk-hetrojunction (BHJ) organic solar cells (OSC) based on blends of conjugated polymers and non-fullerene acceptor (NFAs), have attracted attention due to their unique advantages of solution processability, flexibility, lightweight, large area, and low cost production. Recent breakthroughs in the field of (OSCs) based on NFAs have developed very fast, reaching power conversion efficiencies (PCEs) of 14%[1,2]. To meet the requirements of industrial commercialization, a proper balance among PCE, stability/life time and production cost has to be considered. Recent advancements in the PCEs and extremely low production cost of 0.05 euro/Wp exhibit great potential for commercialization compared with other PV technologies as well as other non-renewable energy technologies[3]. But, the stability of OSC has been still a big concern. In this work, we had fabricated highly efficient OSC based on PCE10, PCE12 as donor polymer and ITIC, ITIC-2F, ITIC-4Cl; IEICO, IEICO-4F, IEICO-4Cl as NFAs, with best working cell among the combination with 12% efficiency. It is found that subtle end-group modifications of NFAs significantly influence the photo-stability of OSCs. Breaking of conjugation of during photo-aging in an inert atmosphere leads to lesser molecular stability. Halogenation of the peripheral group stabilizes NFA’s against light soaking. In both NFA families, chlorinated NFA’s are more stable than their fluorinated versions, followed by their unmodified counterparts under continuous 1 sun illumination. A similar trend is observed in the respective OSC’s, and it’s independent of the donor polymer. The implications of substituents on the breaking of the conjugation of NFA will be discussed, providing guidelines for stable NFA’s and thereby OSC’s. References: (1) Zhang, S.; Qin, Y.; Zhu, J.; Hou, J. Over 14% Efficiency in Polymer Solar Cells Enabled by a Chlorinated Polymer Donor. Adv. Mater. 2018, 30 (20), 1800868. (2) Li, S.; Ye, L.; Zhao, W.; Yan, H.; Yang, B.; Liu, D.; Li, W.; Ade, H.; Hou, J. A Wide Band Gap Polymer with a Deep Highest Occupied Molecular Orbital Level Enables 14.2% Efficiency in Polymer Solar Cells. J. Am. Chem. Soc. 2018, 140 (23), 7159–7167. (3) Du, X.; Heumueller, T.; Gruber, W.; Classen, A.; Unruh, T.; Li, N.; Brabec, C. J. Efficient Polymer Solar Cells Based on Non-Fullerene Acceptors with Potential Device Lifetime Approaching 10 Years. Joule 2018, 1–12.

Resume : Ternary organic solar cells (TSCs) contain a single three-component photoactive layer with a wide absorption window, which is obtained without the need for multiple-stacks. Subsequently, TSCs have attracted great interest in the photovoltaics field. Through careful selection of the three (or more) active components that form the photoactive layer, all photovoltaic parameters can be simultaneously enhanced within a TSC — a strategy that has resulted in record single-junction efficiencies along with improved photo and thermal stability. In this talk, we outline key developments in TSCs, with a focus on the central role of the third component in achieving record efficiencies as well as stable bulk-heterojunction and reliable devices. We analyse the effects of the third component on the nanomorphology of the bulk heterojunction and the photovoltaic parameters of TSCs. We consider both polymer and small-molecule donors as well as fullerenes and recently developed non-fullerene acceptors. In addition, we summarize the recent success of TSCs in mitigating the stability issues of binary solar cells. Finally, we provide a perspective on the advantages of ternary blends and suggest design strategies for highly efficient and stable devices for commercial photovoltaics.

Resume : In the past, most organic solar cells (OSCs) employed fullerenes as electron acceptors. Recently, the synthesis of novel non-fullerene acceptors (NFA) resulted in higher power conversion efficiencies (PCE) of over 13 %. It has been shown, that in fullerene-based OSCs the formation of triplet excitons (TE) opens up an additional recombination pathway and may cause enhanced degradation. In this work, the formation of TEs in NFA-based OSCs is investigated. We studied optical, electrical and spin properties of the polymer donor PBDB-T and the NFA acceptor ITIC in pristine films, in blend films and OSC devices based on the blend. OSCs show a PCE of 9.8 % and external quantum yield of up to 73 %. Low-temperature spin-sensitive photoluminescence measurements reveal the formation of highly localized TEs in pristine films of donor (D) and acceptor (A), as well as in the blend films. This can occur either via intersystem crossing (ISC) or electron back transfer (EBT). ISC is likely for low mobility singlet excitons that cannot reach D-A interfaces and dissociate into charge transfer (CT) states. The EBT forms TE from the CT states. However, no TE signal was observed by electrically detected magnetic resonance measurements in OSCs under working conditions, thus no significant ISC or EBT occurs and the TE formation loss channel is not very efficient. The lack of TEs matches well with the high PCE of NFA-based OSCs and their stability. This work was supported by H2020-MSCA-ITN-2016 SEPOMO.

Resume : Owing to the rapid development over the last few years, the power conversion efficiencies (PCEs) of solution-processed organic solar cells (OSCs) are approaching the 15% efficiency milestone, exhibiting great potential for future energy supply.[1] The performance of OSCs is governed by the delicate, optimized bulk-heterojunction (BHJ) microstructure, where organic donor and acceptor are fine-mixed in the nm regime to facilitate exciton dissociation. For the same material composition, the quantum efficiencies may vary from close to 100 % to even less than 10 % as a function of processing. Therefore, the morphology evolution and the stability of BHJ microstructure have to be well investigated and analysed. In this contribution, we will examine the stability of BHJ microstructures for various model OSCs.[2-5] We recently found that selected high-performance OSCs gain their high excellent performance from a meta-stable microstructure, which quickly relaxed to the thermodynamic equilibrium state under external stress, such as light or thermal stress. The so-called burn-in degradation is identified as a spinodal de-mixing due to the low miscibility of donor and acceptor, which is turned out to be a major challenge for the development of stable OSCs.[2] To overcome this microstructural instability, we introduce a top-down approach to rationally screen molecular phase stabilizers from a database with more than 10000 small molecules.[3] According to the simple but elegant selection criteria, 21 small molecules are finally identified as promising candidates based on their interaction parameters with organic donor and acceptor. By adding a small amount of molecular stabilizer, the intrinsic instability of metastable OSCs can be successfully overcome without compromising device performance. [1] N. Li et al., Nature Energy 2017, 2, 772. [2] N. Li et al., Nature Communications 2017, 8, 14541. [3] C. Zhang et al., 2019, submitted. [4] X. Du et al., Joule 2019, doi:10.1016/j.joule.2018.09.001. [5] B. Fan et al., Nature Energy 2018, 3, 1051.

Resume : Recent significant achievements in materials development and device engineering have made remarkable breakthroughs in the field of nonfullerene accepter (NFA)-based organic solar cells (OSCs) with high power conversion efficiencies (PCEs) over 15%, certifying their potentials for commercialization. However, despite such promising advances, device instability of OSCs under operational conditions (e.g., light and heat) remains a prominent challenge which impedes more widespread applications. Recently, a ternary blend concept has been suggested to be an efficient way to simultaneously enhance the PCE and stability of OSCs. Here, by introducing small amount of fullerene additive into narrow bandgap NFA-based bulk heterojunction nanocomposite, we developed efficient ternary photoactive systems, leading to the enhanced PCE of 10.55% compared with reference device (PCE of 9.44%). More importantly, ternary blend devices exhibited prolonged lifetime that retained about 80% of their initial PCEs after 500 hrs operation under continuous illumination whereas binary counterparts underwent rapid degradation, losing most of their original efficiencies less than 50 hrs. It appears that fullerene additive provides excellent compatibility for efficient electron transfer and balanced charge transport with suppressed recombination.

Resume : This seminar presents recent work on unravelling the material-morphology-performance relationships in eco-friendly processed polymer solar cell photoactive layers with the use of advanced synchrotron X-ray characterization techniques, STXM and NEXAFS [1]. Organic photovoltaics (OPV) offer a competitive alternative to existing conventional solar cells and other emerging third generation solar cells due to their light weight, low cost, semi-transparency, mechanical flexibility, and ability to be printed with similar manufacturing techniques to newspapers and labels [2]. The primary architecture utilised as the photoactive layer of OPV, the bulk heterojunction (BHJ), is processed almost exclusively from toxic organic solvents, such as chloroform, chlorobenzene, and 1,2-dichlorobenzene [3]. The need to move away from processing with chlorinated solvents is pressing and hence the development of eco-friendly processing technologies for solar cells and other printed electronics is a recent target of the organic electronics research community [4-6]. Unfortunately, a reduction in solar cell performance often results when chlorinated solvents are eliminated, which is attributed to the non-optimal microstructure of the light absorbing layer. Here we explore the microstructure of green solvent-cast BHJ films and aqueous colloidal nanoparticle films for the poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1) : phenyl-C61 butyric acid methyl ester (PC61BM) and poly(3-hexylthiophene) (P3HT) : PC61BM donor-acceptor material systems. With additive engineering and customising nanoparticulate colloidal inks we are able to reduce the material domain size to approach the typical exciton diffusion length in these organic semiconductor systems, demonstrating that eco-friendly processing of printed electronics is a realistic goal for the near future. [1] Natalie P. Holmes, et al. Chem. Mater. 30, 6521−6531. (2018). [2] Frederik C. Krebs, et al. Adv. Mater. 26, 29–39. (2014) [3] Shaoqing Zhang, et al. Adv. Mater. 30, 1800868. (2018). [4] Natalie P. Holmes, et al. Nano Energy. 19, 495–510. (2016). [5] Chen Xie, et al. ACS Appl. Mater. Interfaces. 10, 23225–23234. (2018). [6] Chen Xie, et al. Nat. Commun. 9, 5335. (2018).

Resume : With the introduction of new copper complexes as promising redox mediators or HTMs in DSCs both criteria are satisfied to enhance power conversion efficiencies and stability.1 Due to the small reorganization energy between Cu(I) and Cu(II) species, this copper complexes can sufficiently regenerate the oxidized dye molecules with close to unity yield at driving force potentials as low as 0.1V. The high photovoltages of over 1.0 V were achieved by the series of copper complex based redox mediators without compromising photocurrent densities with efficiencies over 10 %. Our photosystem combines two judiciously designed sensitizers, coded D35 and XY1, with the copper complex Cu(II/I)(tmby) as a redox shuttle and hole transport material, and features a high open-circuit photovoltage of 1.1 V. The DSC achieves an external quantum efficiency for photocurrent generation that exceeds 90% across the whole visible domain. This translates into a PCE of 28.9%. Until recently, there have been no viable alternatives for Spiro-OMeTAD as a hole-transport material (HTM) for ssDSCs We show that copper complexes in the solid phase can act as efficient molecular hole conductors in DSCs. We report a record 11% stable solid-state molecular photovoltaic based on copper complex HTM under standard AM1.5G conditions. We demonstrate that rapid hole hopping and the amorphous state of copper complexes are pivotal for achieving such a high conversion efficiency in the solid-state molecular photovoltaics. (1) Freitag, M.; Teuscher, D. J.; Saygili, Y.; Zhang, D. X.; Giordano, D. F.; Liska, D. P.; Hua, P. J.; Zakeeruddin, S. M.; Moser, J.-E.; Grätzel, M.; Hagfeldt, A. Dye-Sensitized Solar Cell for Efficient Power Generation under Ambient Lighting. Nat. Photonics 2017.