Materials design and processing concepts for efficient and stable organic, hybrid, perovskite and dye solar cells

Resume : Degradation in organic solar cells combines a plurality of various processes which become relevant under specific external stress conditions, like heat, light, humidity or current density. We have studied the degradation of a variety of organic photovoltaic (OPV) materials as function of the microstructure as well as of the interface materials. Using charge extraction and photocurrent spectroscopy on the one hand, and long time light soaking tests on the other hand allows us to distinguish between the principle mechanisms of the burn-in effect at short time scales (10 - 100 hours) versus oxygen and hydrogen induced interface and bulk degradation mechanisms which occur on the scale of 1000 – 10000 hrs, depending on the quality of the package. Various learnings are drawn from these studies (i) Crystalline materials less sensitive towards degradation induced energetic disorder and thus the burn-in losses in the open-circuit voltage are fairly small or even negligible. (ii) Fullerene dimerization, which is dominantly operating on the short circuit losses, has an expressed microstructure correlation but is also sensitive to the operation mode of the solar cell (iii) ZnO type interfaces frequently fail in long time light soaking tests. Detailed transport measurements reveal interface as well as bulk related transport limitations for various ZnO or Aluminium doped ZnO (AZO) layers. Smart interfaces like PEI or Ba(OH)2 on top of ZnO majorly reduce these degradation processes. Overall, by combining the right combination of bulk as well as interface materials we are able to demonstrate tandem solar cells with extrapolated operational lifetimes under 1 sun of over 25000 hrs.

Resume : Practical applications of organic solar cells (OSCs) demonstrating now efficiencies above 10% are restricted mainly by their insufficient stability. Many research groups investigated photooxidation of organic semiconductors drawing the conclusions on their relative stability. However, one can hardly expect that OSCs will sustain operation under ambient conditions for any reasonable period of time. It is likely that action of oxygen/moisture will be avoided by using appropriate encapsulation. However, intense light and elevated temperatures might also cause significant degradation of the materials limiting the OSC performance. Here we present results of our study of ?intrinsic stability? of a series of conjugated polymers under anaerobic conditions. We have demonstrated that ESR spectroscopy represents a highly sensitive and informative technique for monitoring the degradation of conjugated polymers [1-2]. Independent experiments were performed to differentiate effects of heat and light ablation. Relative stability of different materials was quantified by comparing the rates of the trap accumulation (dCR/dt) estimated from their ESR profiles. The revealed correlations between the intrinsic stability of conjugated polymers and their molecular structures might contribute to rational design of novel highly stable materials for OSCs. [1] D.Susarova, P.Troshin et. al. Chem. Comm. 2015, DOI:10.1039/C4CC06197G [2] L.Frolova, P.Troshin et. al. Chem. Comm. 2015, DOI:10.1039/C4CC08146C

Resume : Understanding the working mechanism of electrical doping in organic semiconductors is essential for optimizing organic semiconductor devices such as organic light emitting diodes, solar cells or transistors. In contrast to molecular p-doping of organic semiconductors, n-doping creates the additional problem of air instability. The successful transfer of an electron to the lowest unoccupied molecular orbital (LUMO) of typical matrix materials requires n-dopants exhibiting shallow highest molecular orbitals (HOMO), rendering them prone to reactions with e.g. oxygen. Hence, air stable precursor compounds, enclosing the actual n-dopant species, have been suggested to circumvent the challenging preparation and storage of the materials under inert atmosphere. In this study, the doping effects of three different types of n-dopants on the transparent electron transport material Bis-HFl-NTCDI are compared: air stable cationic DMBI dopants, halogen-free DMBI dimers, and the established but air sensitive Cr2(hpp)4. Fermi-level shift and conductivity of co-evaporated thin films at different doping concentrations as well as their stability during air exposure are investigated by ultraviolet photoelectron spectroscopy and electrical measurements. Contrary to expectations, all doped Bis-HF1-NTCDI layers exhibit a high air stability, even though the different pure dopants have different sensitivities to air.

Resume : Heliatek is the global technology leader in the field of organic photovoltaics (OPV) based on small molecules. Having set a new world record for OPV in the lab with a cell efficiency of 12% on >1cm², Heliatek has successfully transferred this complex vacuum process and produced tandem modules in R2R production line with 7% efficiency on active module area. Small molecules are particularly suited for multijunction solar cells as, both, stacking and thickness variation can be done easily and reproducible in vacuum evaporation. It will be discussed how different materials with non-overlapping absorption can be used in high efficient tandem and triple junctions to optimize open circuit voltage while at the same time decreasing the influence of the series resistance of the device on device performance and lifetime.

Resume : Recently, conversion efficiency of organic thin-film solar cell exceeded 10%. In 1991, I proposed pin junction incorporating co-deposited i-interlayer consisting of two kinds of organic semiconductors (so-called bulkheterojunction), which is an indispensable for present organic solar cells [1,2]. In this paper, bandgap science for organic thin-film solar cells [3], including (i) ‘seven-nines’ purification of organic semiconductors [3], (ii) pn-control of organic semiconductors by impurity doping [4-6], (iii) Doping mechanism investigated by Kelvin band-mapping [7], (vi) pn-control of the photovoltaic co-deposited films [8,9], (v) Ionization sensitization of doping [10], will be presented. 1. M. Hiramoto et al., Appl. Phys. Lett., 58, 1062 (1991). 2. M. Hiramoto et al., J. Appl. Phys., 72, 3781 (1992). 3. M. Hiramoto, M. Kubo, Y. Shinmura et al., Electronics, 3, 351 (2014). 4. M. Kubo et al., Appl. Phys. Lett., 98, 073311 (2011). 5. M. Kubo et al., AIP Advances, 1, 032177 (2011). 6. Y. Shinmura et al., AIP Advances, 2, 032145 (2012). 7. Y. Shinmura et al., Appl. Phys. Express, 7, 071601 (2014). 8. N. Ishiyama et al., Appl. Phys. Lett., 99, 133301 (2011). 9. N. Ishiyama et al., Org. Electron, 14, 1793 (2013). 10. Y. Shinmura et al., Appl. Phys. Lett., 105, 183306 (2014).

Resume : Interest in the hybrid perovskite solar cells lies in their high performances and also in the low temperature that can be used for the layer deposition. These devices are mainly prepared by solution-processing and, consequently, they open the gate to a mass production at a very low-cost. In this context, ZnO is a major candidate for the electron transport layer (ETL).[1] ZnO is a wide bandgap semiconductor which can be grown with a high structural quality with tailored nanostructures at low temperature by various techniques. We have explored the use of electrochemical techniques for the deposition of efficient ETL.[2,3] The advantages of the technique include the deposition at low temperature of high quality material, the precise control of the (nano)structure morphology and thickness, the control of the electrical properties and the excellent electrical contact between the deposited layers and the substrate. Various ZnO nanostructures have been prepared and tested to perovskite solar cells. The effects of the structure and growth electrochemical bath on the cell performance and functioning have been investigated by various techniques. They have also been compared to more classical TiO2 based PSC. [1] D. Liu, T. L. Kelly, Nature Photonics, 2014, 8, 133-138. [2] J. Zhang, P. Barboux, T. Pauporté, Adv. Energy Mater., 4, (18) (2014) 1400932. [3] J. Zhang, E. J. Juárez-Pérez, I. Mora-Seró, B. Viana, Th. Pauporté, J. Materials Chemistry A. submitted

Resume : Small-molecule donor/polymer acceptor bulk-heterojunction films with both compounds strongly absorbing have great potential for further enhancements of performance of organic solar cells. By employing a newly synthesized small molecule donor with a commercially available polymer acceptor in a solution-processed fullerene-free system, we report a high power conversion efficiency of close to 4%. Detailed investigation reveals that the record FF value of the small-molecule donor/polymer acceptor system of over 60% is related to the good charge carrier transport properties and low recombination losses; while efficient exciton dissociation/charge transfer comes along with a remarkably low energetic driving force of ~0.06 eV, which ensures the most optimized open-circuit voltage of all organic solar cells studied to date. Despite the more limited exciton dissociation compared to that in a small molecule/fullerene solar cell, the stronger absorption of the N2200 acceptor, allows for a rather comparable photocurrent to be extracted, and thus an overall superior photovoltaic performance.

Resume : In this work, we developed the bilayer solar cells with a new p-type amorphous material [poly(4-(5-(1,5-bis(octyl)naphthalen-2-yl)thiophen-2-yl)-7-(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PONTBT). We have demonstrated that the pre-located polymer layer morphology has significant effect on the charge dissociation and collection efficiency. Also, nanoscale morphology of bilayer solar cell can be systematically controlled by tuning the fraction of additive in the prepared polymer and PCBM solutions, respectively. Benefiting from architecture of bilayer solar cell, control the sub-layer nanoscale morphology separately giving a more straightforward concept of P-N junction solar cell and deeply insight in understanding the role of the donor and acceptor, respectively. It is proved that the performance of bilayer solar cell is comparable with that of the bulk heterojunction solar cell.

Resume : Small conjugated molecules are gaining momentum as an alternative to semiconducting polymers for the production of solution-processed bulk heterojunction (BHJ) solar cells. One of the main issues with SM-BHJs is the low carrier mobility caused by the lack of preferential percolative pathways for electrons and holes to the electrodes due to insufficient and uncontrolled phase segregation. In this talk, I will present a new paradigm for fine tuning the phase-segregation based on the post-deposition exploitation 
of latent hydrogen bonding in binary mixtures of PCBM with functionalized electron donors.[1] The strategy consist 
in the protection of the H-bond forming sites of the donors with a thermo-labile functionality whose controlled thermal/photo cleavage leads to the formation of stable, crystalline, phase-separated molecular aggregates. This approach allows the fine tuning of the nanoscale film connectivity and thereby to simultaneously optimize interfacial charge generation and extraction via ordered phase-separated domains. As a result, the PV efficiency undergoes an over 20-fold increase with respect to control devices. This strategy, can be applied to a wide variety of molecular blends for achieving highly efficient SM-BHJ solar cells. Furthermore, recent results demonstrate the possibility to combine this strategy to photo-lithographic technique to fabricate nano-patterned semiconducting features. [1]Bruni,F. et al. Adv. Funct. Mater. 24, 7410-7419

Resume : Solvent additives, used to either control the morphology of the active layer or to form an ultrathin buffer layer between the metal cathode and the active layer, can effectively enhance the performance of bulk heterojunction (BHJ) organic solar cells. In this work, we introduce a new solvent additive, chloropropyltrimethoxysilane (CP3MS), to improve the performance of solar cells based on the bend of 2,5-Di-(2-ethylhexyl)-3,6-bis-(5''-n -hexyl-[2,2',5',2''] terthiophen-5-yl)-pyrrolo[3,4-c ]pyrrole-1,4-dione (SMDPPEH) and Phenyl-C61-butyric acid methyl ester (PC61BM). The change of active layer morphology is observed while incorporating CP3MS as solvent additive. In addition, CP3MS migrates from bulk to interface between the active layer and Al cathode to form ultrathin buffer layer upon post annealing treatment. The morphology change and buffer layer formation lead to enhance the power conversion efficiency (PCE) of SMDPPEH/PC61BM BHJ solar cells from 2.75% to 4.55% upon addition of 0.1% of CP3MS combined with post-annealing. To demonstrate the general applicability of CP3MS, we have also incorporated CP3MS additive into other small molecule binary and ternary systems. The PCEs of a benzodithiophene-based small molecule (BDT6T)/PC61BM BHJ solar cells and SMDPPEH/PC61BM/BDT6T ternary BHJ solar cells can be improved from 3.05% to 3.8% and from 4.7% to 6.3% with incorporation of CP3MS, respectively.

Resume : In the last decades, many efforts were focused on the design of electron donor conjugated polymers for organic solar cells. Nevertheless, a number of the reported benzoxadiazole-containing polymers remains very limited in spite of their advanced optical and electrochemical properties. Recently, very promising copolymers (-X-TTBTBTT-)n (X – fluorene, carbazole; B -benzothiadiazole, T – thiophene) were designed in our group [1]. It was shown that they have lower LUMO energies and smaller band gaps in comparison with the conventional (-X-TBT-)n copolymers. In this work we report a family of similar (-X-TTATATT-)n copolymers, where A represents benzoxadiazole. According to the UV-Vis spectra and electrochemistry data, the synthesized polymers have narrow band gaps of~ 1.6 eV and deep-lying HOMO energy levels of ca. -5.5 eV. It was revealed that solubilizing alkyl side chains such as EtHex, BuOct, HexDec and OctDodec affect the morphology of the polymer composites with [70]PCBM and influence significantly on the solar cell performance. Power conversion efficiencies above 4.0% were achieved in the preliminary experiments using novel polymers as electron donor materials in organic solar cells. There is a potential for significant increase in the solar cells performance via further optimization of the materials (e.g. molecular weights) and devices (active layer morphology, buffer layers and etc.). [1] A. V. Akkuratov et al., J.Mater.Chem. C, 2015, 3, DOI: 10.1039/C4TC02432J

Resume : Conjugated polymers used as active materials in organic solar cells (OSC) undergo rather facile degradation under the device operation conditions. A considerable attention was paid to the photochemical degradation of conjugated polymers under ambient conditions. However, the conjugated polymers are known to be highly sensitive to the oxygen and moisture as well as many inorganic photoactive materials. Therefore, organic solar cells have to be protected from the environment by using appropriate barrier coatings. At the same time, we cannot protect the active layer of OSC from the action of light and elevated temperatures. We focused our research efforts on investigation of intrinsic photochemical and thermal stability of conjugated polymers under anaerobic conditions. Recently, we have shown that ESR spectroscopy represents a highly sensitive tool which is ideally suitable for studying photothermal degradation of conjugated polymers [L.A. Frolova, P.A. Troshin et.al. Chem. Comm., 2014, DOI: 10.1039/C4CC08146C]. Here we report investigation of a series of conjugated polymers comprising TTBTBTT unit (T - thiophene, B - benzothiadiazole) in combination with different building blocks such as carbazole, silafluorene, fluorene, indolocarbazole, benzodithiophene and cyclopentaditiophene. The correlation between the molecular structures of conjugated polymers and their intrinsic stability is presented.

Resume : Well-defined light-soaking effect is crucial for reliable measurements of efficiency and analysis of degradation dynamics. We investigate light-soaking effect of aged ITO-free inverted Organic Solar Cells (OCS) with Aluminum-doped Zinc Oxide (AZO) electrode. AZO layers on glass with various aluminum doping level (from 0% up to 5%) were grown by Atomic Layer Deposition and were used as a cathode in the inverted structure: glass/AZO/P3HT:PCBM/MoO3/Ag. The efficiency of analyzed solar cells is in the range of 0.5-1%. The AZO doping level had significant impact on shape of I-V curves. Differently pronounced S-shape of I-V curves for different Al doping level of AZO layers were observed. Samples were stored for 9 months in the dark and air. For the light-soaking experiments the devices were illuminated under sun simulator (AM1.5, 1000 W/m2) for a certain period of time and then the dark and light I-V curves were measured. This procedure was repeated up to 15 min of total light-soaking time. Light-soaking effect saturated after approximately 5-6 minutes of illumination and has a strong impact on reduction of S-shape. Due to light-soaking the relative changes of FF is increasing with increasing the dopant in AZO even up to double the absolute FF value. The efficiency of the representative sample (2.5% Al dopant) increased from 0.38% to 0.7% due to the light-soaking. This behavior is related to the traps states in AZO and miss-match energy levels of AZO and P3HT:PCBM.

Resume : Advances in the efficiency of OPVs have been enabled essentially by the discovery of novel organic materials: from polyphenylenevinylene polymers (e.g. MEH-PPV), to thiophene-based materials with greater air stability (e.g. P3HT), and currently, to low band-gap materials of donor-acceptor structure, such as benzothiadiazoles (e.g. PCPDTBT) and diketopyrrolopyrroles (e.g. DPP). In parallel, the development of better interface or buffer layers permitted the enhancement of device lifetime. Initially, low-work function metals or organic compounds with metallic properties were used, but more recently wide band gap semiconductors, including transition metal oxides, TMOs (e.g. ZnO, TiO2, V2O5, MoO3 and NiO), have shown to provide higher stability. TMOs can be obtained by low-cost solution processing techniques, and their payback time (PBT) is lower if compared to organic semiconductors, which make them promising barrier layer materials for OPVs. In this work, we present the synthesis of transition metal oxides, V2O5 and NiO, obtained by low-cost water-based solution processed methods at low-temperature. The oxides can be obtained by synthesis methods in water, and processed temperatures below 120 ºC, eliminating the use of toxic solvents and high processing temperatures. Indoor and outdoor stability analyses of the final OPV devices were carried out following the ISOS protocols, demonstrating above 3000 h of stability. Contrary to expected, the normal configuration OPV applying the V2O5 oxide demonstrated higher stability than the inverted configuration OPV. The latter has been related to the UV-filtering effect of the oxide layer. The OPVs applying NiO layer was shown to be highly stable with T80 maintained after more 1200 h of outdoor testing. The bur-in process observed for the solar cells, was related to interlayer mixing of the oxide layers and the active layer. The later is highly dependent on the UV light irradiation and was studied by depth-profile Tof-SIMS analyses. References: 1.- G. Teran Escobar, J. Pampel, J. Caicedo, M. Lira-Cantu. Energy Environ. Sci., 2013, 6, 3088–3098. 2.- Reese, M. O et al. Solar Energy Materials and Solar Cells 2011, 95, 1253-1267.

Resume : Water-based V2O5 hydrate has been applied as the hole transport layer (HTL) in organic solar cells (OSCs). V2O5 is obtained from a sodium metavanadate solution in water under ambient conditions, resulting in a final thin film of formula V2O5 0.5H2O. The HTL was used in OSCs in the normal and the inverted configurations, applying metallic Ag as the back-metal electrode in both cases. Fabrication of both OSC configurations completely by solution-processing printing methods in air is possible, since the Al electrode needed for the normalconfiguration OSC is not required. he work function (WF) and band gap energy (BG) of the V2O5 thin films were assessed by XPS, UPS and optical analyses. Different WF values were observed for V2O5 prepared from a fresh V2O5?isopropanol (IPA) solution (5.15 eV) and that prepared from a 24 h-old solution (5.5 eV). This difference is due to the gradual reduction of vanadium (from V5 to V4 ) in IPA. The OSCs made with the V2O5 thin film obtained from the 24 h-old V2O5?IPA solution required photoactivation, whereas those made with the freshly obtained V2O5 did not. Outdoor stability analyses of sealed OSCs containing a V2O5 HTL in either configuration revealed high stability for both devices: the photovoltaic response at T80 is still observed after more than 3200 h. The water-based V2O5 has been applied in ITO-free flexible Organic Solar Cells and Tandem OPVs. In this work we also present our recent results on the scale up of OPV from lab scale made by spin coating to the fabrication of ITO-free flexible devices made by roll to roll and Mini roll coater printing methods.

Resume : Star-shaped oligothiophenes (SSO) are promising materials for high-efficient and stable organic photovoltaics. The SSO-based solar cells have shown the efficiency higher than 5%; and its further enhancement needs optimization of the SSO molecular structure. In this work, we study new SSOs with increased solubility due to addition of methoxygroups to the triphenylamine core. The SSO molecular structure was tuned via variation of the number of thiophene rings and the length of alkyl end groups. The dependence of photovoltaic properties of new SSOs on their molecular structure was studied. The efficiencies of organic solar cells based on the SSOs with alkyl end groups blended with PC71BM were near 3%. It was found that efficiency increases with the number of thiophene rings from 2 to 3 in the star ray and with alkyl end group length from 1 to 6 carbon atoms mainly due to increase of the short-circuit current and fill factor. However, the dependence of the open-circuit voltage is reverse. For the SSO without alkyl groups the open-circuit voltage is maximal and the leakage currents are very low, but the efficiency and photocurrent are 1.5 – 2 times lower compared with other SSO. All studied SSOs showed high stability during storage in bookshelf conditions. The effect of the SSO molecular structure on the photovoltaic performance and further optimization pathways are discussed. The work was supported by Russian Science Foundation (grant 14-13-01380).

Resume : We investigate performance limiting factors of high efficiency polymer:fullerene bulk heterojunction solar cells using intensity modulated photocurrent spectroscopy (IMPS) and impedance spectroscopy (IS). Inverted solar cells were prepared using blends of the low band gap polymer PTB7 and fullerene acceptor C70BM and novel transport layers between the active layer and contact materials, resulting in power conversion efficiencies of over 8 %. No loss in performance was observed in the solar cell parameters of devices stored under ambient conditions for several weeks. Frequency-resolved techniques such as IMPS and IS are very powerful for extracting highly detailed information related to variations in the electronic structure of materials and interfaces due to performance or environmentally induced ageing. We used IMPS to monitor changes in transport and recombination phenomena in the active layer, and found that environmental exposure did not influence carrier mobility or trapping processes. IS was applied to monitor the stability of the material interfaces [1,2], and we did not observe any decreases charge extraction efficiency due to ageing. These results demonstrate the potential for realizing high efficiency, stable polymer:fullerene solar cells. 1. B. Ecker, J. Posdorfer, E. von Hauff, Sol. Energ. Mat. Sol. Cells, 116 (2013) 176-181. 2. B. Ecker, J. Nolasco, J. Pallarés, L. Marsal, J. Posdorfer, J. Parisi, E. von Hauff, Adv. Funct. Mat. 21 (2011) 2705–2711.

Resume : Because of reproducible syntheses and high material purities, light harvesting molecules with low molecular weight may enable organic solar cells with improved long-term stability. In contrast to polymers, however, low molecular weight organic compounds often show poor film forming properties due to strong molecular aggregation. In this work, we present 5% organic solar cells and corresponding mini-modules, fabricated from solution, comprising light harvesting BODIPY molecules. The devices exhibit long-time stable bulk-heterojunction morphologies and hence enhanced device life-time. The morphology can be adjusted by applying proper processing temperatures or suitable additives. We investigated the bulk-heterojunction morphology and performance by means of AFM, KPFM and conductive AFM. Photophysical investigations revealed the charge carrier dynamics within the active layer.

Resume : Despite the essential role of fullerenes in achieving best-performance organic solar cells (OSCs), fullerene acceptors have several drawbacks including poor light absorption, high-cost production and purification. For this reason, small molecule acceptor (SMA)-based OSCs have attracted much attention due to the easy tunability of electronic and optical properties of SMA materials. In this study, polymers with temperature dependent aggregation behaviors are combined with various small molecule acceptor materials, which lead to impressive power conversion efficiencies of up to 7.3%. The morphological and aggregation properties of the polymer:small molecule blends are studied in details. It is found that the temperature-dependent aggregation behavior of polymers allows for the processing of the polymer solutions at moderately elevated temperature, and more importantly, controlled aggregation and strong crystallization of the polymer during the film cooling and drying process. This results in a well-controlled and near-ideal polymer:small molecule morphology that is controlled by polymer aggregation during warm casting and thus insensitive to the choice of small molecules. As a result, several cases of highly efficient (PCE between 6-7.3%) SMA OSCs are achieved. The second part of this presentation will describe the morphology of a new small molecule acceptor with a unique 3D structure. The relationship between molecular structure and morphology is revealed. Reference: 1. Liu, Y., Zhao, J., Li, Z., Mu, C., Ma, W., Hu, H., Jiang, K., Lin, H., Ade, H. and Yan, H., “Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells.” Nat. Commun., 2014, 5, 5293. 2. Liu, Y., Jiang, K., Zhao, J., Li, Y., Zhang, L., Mu, C., Li, Z., Lai, Joshua., Hu, R., Huang, X., Tang, B. and Yan, H., “A tetraphenylethylene core-based 3D-structure small molecule acceptor enabling efficient non-fullerene organic solar cells.” Adv. Mater., (2014), DOI: 10.1002/adma.201404152, in press. 3. Zhao, J., Li, Y., Lin, H., Liu, Y., Jiang, K., Mu, C., Ma, T., Lai, Joshua. and Yan, H., “High-Efficiency Non-Fullerene Organic Solar Cells Enabled by a Difluorobenzothiadizole-Based Donor Polymer Combined with a Properly Matched Small Molecule Acceptor.” Energy & Environmental Science, (2014), DOI:10.1039/C4EE02990A, in press. 4. Mu, C., Liu, P., Ma, W., Jiang, K., Zhao, J., Zhang, K., Chen, Z., Wei, Z., Yi, Y., Wang, J., Yang, S., Huang, F., Facchetti, A., Ade, H. and Yan, H., “High-Efficiency All-Polymer Solar Cells Based on a Pair of Crystalline Low-Bandgap Polymers.” Adv. Mater., (2014), DOI: 10.1002/adma.201402473. 5. Yan, H.; Chen, Z.; Zheng, Y.; Newman, C.; Quinn, J.; Dotz, F.; Kastler, M.; Facchetti, A. “A high-mobility electron-transporting polymer for printed transistors.” Nature，2009, 457, 679-686.

Resume : Flexible clear ultra-barriers still remain a challenge due to performance limitations and their cost for a wide range of 'plastic' electronic applications such as flexible thin-film photovoltaics, lighting and displays. The high sensitivity of the materials used in these types of devices to the environment, especially moisture requires barrier performances to 10-6 g/m2/day or better depending on application. The presentation will review the technological approaches to achieve this level of performance and the potential to apply them in batch and R2R platforms. Particular focus will be related to the Atomic Layer Deposition (ALD) of transparent thin barrier layers applied in batch systems and also the early stage development of the production of these barrier materials on a 0.5m wide pilot coater using spatial ALD deposition. Barrier performance will be discussed and issues related to the production of them.

Resume : Polymer solar cell (PSC) technology has attracted much attention due to its promise as low-cost conversion of solar energy. Despite recent progress, several limitations are holding back PSC development. For instance, current high-efficiency (>9.0%) PSCs are restricted to materials combinations that are based on limited donor polymers and only one specific fullerene acceptor, PC71BM. Furthermore, best-efficiency PSCs are mostly based on relatively thin (100 nm) active layers. Thick-film PSCs generally exhibit lower fill factors and efficiencies compared to the best thin-film PSCs. Here we report multiple cases of high-performance thick-film (300 nm) PSCs (efficiencies up to 10.8%, fill factors up to 77%) via the formation of a near-optimal polymer:fullerene morphology that contains highly crystalline yet reasonably small polymer domains. This morphology is controlled by the temperature-dependent aggregation behavior of the donor polymers during casting and is insensitive to the choice of fullerenes. Our comparative study shows that the choice of alkyl chains is critically important in enabling optimal aggregation and morphology control. The uncovered aggregation and design rules yield three high-efficiency (>10%) donor polymers and will allow further synthetic advances, process optimizations, and matching of both the polymer and fullerene materials, potentially leading to significantly improved performance and increased design flexibility. The second part of this presentation will describe highly efficient non-fullerene PSCs with power conversion efficiencies up to 6.3%. There are several attractive features of our non-fullerene PSCs. 1) An exceptionally high Voc of nearly 1V can be achieved for a small molecule acceptor-based PSC with an optical bandgap of 1.6 eV. 2) A new small molecule acceptor with a unique 3D structure was developed that exhibits similar morphological and electronic properties to those of PCBMs. 3) Efficient all-polymer solar cells can be achieved using a pair of crystalline polymers that maintain their crystallinity in the polymer/polymer blend. Reference: 1. Liu, Y., Zhao, J., Li, Z., Mu, C., Ma, W., Hu, H., Jiang, K., Lin, H., Ade, H. and Yan, H., “Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells.” Nat. Commun., 5, 5293, (2014). 2. Liu, Y., Jiang, K., Zhao, J., Li, Y., Zhang, L., Mu, C., Li, Z., Lai, Joshua., Hu, R., Huang, X., Tang, B. and Yan, H., “A tetraphenylethylene core-based 3D-structure small molecule acceptor enabling efficient non-fullerene organic solar cells.” Adv. Mater., (2014), DOI: 10.1002/adma.201404152. 3. Zhao, J., Li, Y., Lin, H., Liu, Y., Jiang, K., Mu, C., Ma, T., Lai, Joshua. and Yan, H., “High-Efficiency Non-Fullerene Organic Solar Cells Enabled by a Difluorobenzothiadizole-Based Donor Polymer Combined with a Properly Matched Small Molecule Acceptor.” Energy & Environmental Science, (2014), DOI:10.1039/C4EE02990A. 4. Mu, C., Liu, P., Ma, W., Jiang, K., Zhao, J., Zhang, K., Chen, Z., Wei, Z., Yi, Y., Wang, J., Yang, S., Huang, F., Facchetti, A., Ade, H. and Yan, H., “High-Efficiency All-Polymer Solar Cells Based on a Pair of Crystalline Low-Bandgap Polymers.” Adv. Mater., (2014), DOI: 10.1002/adma.201402473. 5. Yan, H.; Chen, Z.; Zheng, Y.; Newman, C.; Quinn, J.; Dotz, F.; Kastler, M.; Facchetti, A. “A high-mobility electron-transporting polymer for printed transistors.” Nature，(2009), 457(7230), 679-686.

Resume : Since the seminal Nature paper by O?Regan and Gr?tzel in 1991 [1] the highest efficiencies of dye-sensitized solar cells (DSC) have been achieved using the iodide/tri-iodide redox system. A disadvantage of this mediator is the large internal losses caused by the fact that it is a two-electron redox couple. In 2010 we made a breakthrough by using one-electron transfer redox systems such as cobalt-complexes, in combination with a new generation of organic dyes, which efficiently prevents recombination losses. This discovery was quickly embraced by Gr?tzel and co-workers, and the new world record for DSC is at present 13.0% by using a Co-complex redox couple and a porphyrin dye. Besides liquid DSC we have recently developed solid-state DSC (ssDSC). Conducting polymers are used as hole conductors prepapred by in situ photopolymerization of the monomers in a photoelectrochemical cell. ssDSCs based on organic dyes gives together with PEDOT efficiencies up to 7%. A recent breakthrough has come from replacing the dye layer adsorbed on the mesoporous oxide surface with an ultrathin inorganic perovskite layer. In our laboratories we have obtained high efficiencies for perovskite solar cells on different nanostructured oxide substrates (TiO2, ZrO2 and Al2O3). We have developed new HTMs also for perovskite solar cells that give similar efficiencies as Sprio-MeOTAD. References [1] B. O?Regan, M. Gr?tzel, Nature, 353 (1991) 737.

Resume : A traditional counterelectrode in dye-sensitized solar cell (DSC) is platinized F-doped SnO2 (FTO). However, the cost of FTO glass is estimated to be about >20-60% of the cost of the DSC-module, which is a strong motivation for FTO replacement by cheaper materials. Recently, nanocarbon and graphene-based materials attracted considerable attention, particularly for Co-mediated DSCs. Another alternative, which also works well with the I3-/I- redox mediator, is the woven fabric consisting of transparent PEN fibers in warp and electrochemically platinized tungsten wires in weft. This electrode outperforms the thermally platinized FTO in serial ohmic resistance, Rs (1.5 vs. 8.2 Ωcm2), charge-transfer resistance for triiodide reduction (0.59 Ωcm2 vs. 0.76 Ωcm2) and offers comparable or better optical transparency in the visible and particularly in the near-IR spectral region (≈80%). The Pt-W/PEN cathode exhibits good stability during electrochemical load with the maximum (diffusion-limited) current both in cathodic and anodic directions, and during long term (≈month) storage at open circuit. The practical dye-sensitized solar cells with either Pt-W/PEN or Pt-FTO cathodes show similar performance, confirming that the former is a promising alternative for replacement of conductive glass in the DSC counterelectrodes. Acknowledgement: This research was supported by the Grant Agency of the Czech Republic (contract No. 13-07724S), by the Swiss Commission for Technology and Innovation (CTI) project No. 16452.2 PFNM-NM and by the European Research Council through the Advanced Research Grant no. 247404 ‘Mesolight’.

Resume : A novel polymeric ionic liquid, poly(oxyethylene)-imide-imidazolium selenocyanate (POEI-IS), was synthesized as a functional gel electrolyte for quasi-solid-state dye-sensitized solar cells (QSS-DSSCs). Herein, POEI-IS acts simultaneously as a redox mediator and a gelling agent in the electrolyte. In the structure of POEI-IS, the POE segment can chelate potassium cations within the electrolyte, and thereby improve the open-circuit voltage (VOC) of a QSS-DSSC. Also, the POE segment provides multiple lone-pair electrons, which enable the strong dipole-dipole interactions with selenocyanate; therefore, selenocyanate redox couples are able to provide good ionic conductivity and diffusivity within the gel electrolyte. Additionally, the selenocyanate anion possesses a large standard potential, which renders a high open-circuit voltage to its QSS-DSSC and benefits dye regeneration. By coupling with a high efficient tetrathienoacene-based organic dye, a QSS-DSSC with POEI-IS gel electrolyte reaches a high VOC of 0.8 V and a good power conversion efficiency of 7.5%. This properly designed polymeric ionic liquid paves a promising way for developing highly efficient QSS-DSSCs. The data of Fourier transform infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA), ionic conductivity, electrochemical impedance spectroscopy (EIS) and incident photon–to–current conversion efficiency (IPCE) are employed to substantiate the explanations.

Resume : Organic-inorganic perovskites are quickly overrunning research activities in new materials for cost-effective and high-efficiency photovoltaic technologies. Since the first demonstration from Kojima and co-workers in 2009, several perovskite-based solar cells have been reported and certified with rapidly improving power conversion efficiency. Recent reports demonstrate that perovskites can compete with the most efficient inorganic materials, while they still allow processing from solution as potential advantage to deliver a cost-effective solar technology. Compare to the impressive progress in power conversion efficiency, stability studies on perovskite solar cells are rather poor and often controversial. In the present talk we will focus on the stability of perovskite solar cells. We will discuss the impact of the device architecture, preparation procedure and materials composition on device ageing in working condition. We will present new procedures and materials which improve the device lifetime without giving up on high power conversion efficiency.

Resume : Recently Organic-inorganic Perovskite based solar cells have gained tremendous interest among the photovoltaic (PV) community due to their high power conversion efficiency and low cost fabrication process. However, laboratory tests show that such devices lose their performance under exposure to sun. The main objective of this study is to replace the expensive organic Spiro-OMETAD layer by an inorganic hole transport material (HTM) to enhance the durability of the device and lower its cost. We numerically analyze Perovskite device structures using two device simulators: wx Analysis of Microelectronic and Photonic Structures (wxAMPS). Five types of hole transport materials were considerd, namely Spiro-OMETAD, CuI, NiO, CuSCN and Cu2O. Cu2O shows the highest efficiency exceeding 25% as compared to other types of HTMs. Various Cu2O thin films were deposited by dc-reactive plasma sputtering using Argon with different oxygen contents. The temperature of the substrate was varied between 100 and 250oC. XRD, XPS, UV-Vis, and SEM investigations revealed the detailed structural, optical, and morphological properties, which are very promising for the design of Perovskite solar cells with Cu2O as HTM.

Resume : To minimize surface recombination in organic solar cells charge carrier selective layers are used between the photoactive layer and the electrode. These layers (e.g. PEDOT:PSS, TiOx, ZnO, WoOx, MoOx) often have a rather large band gap and thus can block the “wrong” type of charge carrier quite efficiently. However, surface states within the band gap can still act as recombination centers. Recently, there are numerous examples of polar (organic) molecules providing a high degree of charge carrier selectivity. We used cheap and easy-to-process organic molecules with dipole moments which alter the effective work function of the electrode and thus strongly increase (decrease) the electron (hole) concentration in the adjacent photoactive layer. This is the real reason for the enhanced selectivity leading to an increase of Voc for different active layers. We performed scanning Kelvin probe microscopy and found a corresponding shift of the surface potential. A theoretical model was set up and the results of the numerical simulations are in full accordance with the experimental data. Interestingly, DFT-calculations prove that the energy levels of the dipole molecules are not suited to conduct charge carriers from the photoactive layer to the electrode. Using a tunneling mechanism in our model, it is found that there is no necessity to assume preferential tunneling for electrons. Their accumulation due to the lowered work function is sufficient to explain the observed behavior.

Resume : Titanium dioxide (TiO2) and zinc oxide (ZnO) have been widely used as cathode interlayers in organic photovoltaics due to their high electron extraction rates. However, charge recombination occurring at trap states that are present on their surface is an important factor limiting device performance. Our group has recently suggested a strategy to passivate these trap states by using conformal coatings of alumina (Al2O3) or zirconia (ZrO2) films deposited with Angström-level precision by atomic layer deposition on top of TiO2 layers, resulting in a significant enhancement in power conversion efficiency of bulk heterojunction (BHJ) solar cells. Herein, we propose controlled hydrogen annealing as an alternative strategy to achieve highly conductive TiO2 and ZnO with defect-free surfaces. The choice of hydrogen is based on its ability to strongly affect the electronic and structural properties of many materials. It can be incorporated in three dimensional structures forming multicenter bonds and providing electrons that fill up the partially occupied mid gap states originating from oxygen vacancies and other impurities causing their passivation. Our results show that BHJ solar cells with an inverted structure based on hydrogen annealed TiO2 and ZnO films outperform the untreated control devices. The influence of hydrogen annealing of titanium and zinc oxides on solar cells performance is of great importance for obtaining more effective solar energy harvesting in the near future.

Resume : The purpose of this study was to investigate the structural,electronic and magnetic properties of the cubic praseodymium oxides perovskites PrMnO3. It includes our calculations based on the use of the density functional theory (DFT) with both generalized gradient approximation (GGA) and GGA+U approaches, The spin polarized electronic band structures and densities of states as well as the integer value of the magnetic moment of the unit cell (6 μB) illustrate that PrMnO3 is half-metallic ferromagnetic. The study prove that the compound is half-metallic ferromagnetic however the results obtained, make the cubic PrMnO3 a promising candidate for application in spintronics Keywords: Perovskite, DFT, Electronic properties, PrMnO3, Spintronics.

Resume : Organometal halide perovskites (CH3NH3PbI3) solar cells have triggered a rapid development of new photovoltaic devices with power conversion efficiencies raised from 3.8% in 2009 up to 19.3% in 2014, as they possess many excellent characters, such as appropriate direct bandgap, high absorption coefficient, excellent carrier transport, and so on. However, most of the devices are mainly based on hard substrate like glass coated ITO/FTO because they need the processing temperatures up to 500 ℃ to sinter the compact and mesoporous layers of metal-oxide support TiO2. In recent years, Perovskite solar cells with a flexible substrate have become hotspot research, which are useful for various wearable and portable electronic devices, and it is very important to investigate the preparation of high-performance perovskite films at low temperature below 150 ℃. In the present work, TiO2 compact layer was prepared on flexible substrate below 150 ℃ by spin coating method, and CH3NH3PbI3 light absorber film was deposited on the compact TiO2 film by a modified vapor-assisted solution process. At first PbI2 film was spin coated on the compact TiO2 film, then an in-situ reaction process with CH3NH3I vapor in N2 atmosphere was used to prepare the the CH3NH3PbI3 film, where the film thickness and grain size were tuned by the reaction time. The Ag/FTO/PEN film was employed as the counter electrode to fabricate the cells. Their optical absorption was studied by UV-Vis and photoluminescence spectra. The photovoltaic conversion efficiency of the solar cells was also investigated under AM 1.5 illumination. In conclusion, flexible perovskite thin films solar cells with full surface coverage and small surface roughness was sucessfully prepared by low-temperature vapor-assisted solution process, which was used to fabricate the flexible TiO2-perovskite solar cells with high efficiency.

Resume : Silicon nanopatterning technique has received significant attention due to its various applications in Si-based optoelectronics, chemical and biochemical sensors, and solar cells. Metal-assisted chemical etching (MacEtch) is very promising because of its low-cost and structure-controllable feature. In this research, we fabricated Si nanopatterns with high aspect ratios by imprinting & contact transfer methods combined with MacEtch. First, a PUA mold was casted using a commercial Si nano-stamp and the PUA mold was filled with PMMA by spin coating. The PMMA was then transferred onto the Si wafer by contact printing method and the transferred pattern was coated with a thin metal layer. After removing the PMMA by lift-off, MacEtch process was performed to produce a Si nanopattern on the wafer. This stamp was utilized to incorporate a diffraction grating into the dye-sensitized solar cell (DSSC). A refractive-index grating could be embedded in the bottom of TiO2 absorption layer of DSSC via imprinting combined with TiCl4 treatment. When the period of pattern was under 800 nm, total internal reflection occurred in the main absorption wavelength range of DSSC. This architecture highly increased light harvest by confining the incident light within the absorption layer.

Resume : Perovskite solar cells have generated much excitement recently due to rapid efficiency gains, ease of fabrication and material versatility. While efficiencies above 20% have been reported for small area cells (<0.2cm^2), many challenges remain to be solved before perovskite cells become a practical photovoltaic option. Two major challenges are film instability and degradation, and film non-uniformity (which limits cell areas). In this work we apply scanning microscopy tools to study the spatial variation of optoelectronic properties of CH3NH3PbI3 perovskite thin films and solar cells. We map the intensity and spectral variation of photoluminescence (PL) of different solution-processed perovskite films on various substrates, and in complete cells. We also perform photocurrent mapping of perovskite cells with micron-scale resolution to assess cell uniformity and identify local defects that reduce the cell performance. During these measurements, we also observed local changes (reversible and non-reversible) in the film properties induced by the focused laser light. A detailed discussion of these effects will be presented. In summary, we use high-resolution characterization tools to provide new insights into the opto-electronic and material properties of perovskite solar cells. Gaining a better understanding of the uniformity and stability of these materials on a micron scale will be essential for developing efficient and stable large-area perovskite solar cells.

Resume : Perovskite solar cells have gained great attention due to their high efficiency and low cost fabrication. There are two well known types of perovkskite solar cells structure based on the device geometry. The first one involves a mesoporous layer such as TiO2 or Al2O3 and perovskite layer forms in this structure [1-2]. The second one is planar geometry involving layer by layer deposition of semiconductor materials [3]. The formation of uniform perovskite layer is an important key parameter to obtain high performance solar cells during fabrication procces. Different techniques have been reported to prepare uniform perovskite layer by several authors. One of these technique is modification of TiO2 surface by some small molecules. In this work, we modified mesoporous TiO2 surface with Keggin type α‐(nBu4N)3[PW9O34(tBuSiOH)3](PW9) heteropolytungstates (POT)s. We aimed to facilite and lead the fomation of uniform perovksite layer. The surface morphology were investigated by AFM and SEM technique. The results were compared with non-modified mesoporous TiO2 surfaces. The results showed that modification of TiO2 surface lead to decrease in surface roughness. The results were optimized and presented. 1- Burschka, J. Et al. .Nature, 2013, 499, 316−319 2-Michael M. Lee, et al. Science, 2012. 338, 643. 3- Liu, M. Et al. Nature, 2013, 501, 395.

Resume : Dye Solar Cell technology is considered the best candidate for innovative application scenarios such as BIPV (Building Integrated PhotoVoltaics). However, industrialization and commercialization of Dye Solar Cells requires the stabilization of such technology. In this work, we will present the effort made by Dyepower researchers to finalize the production process of large area DSC modules and to stabilize their photovoltaic performance with respect to accelerated ageing stresses. A pilot-line has been designed and installed able to produce 20cm x 30cm modules and to automatically assemble the module in a panel. Advanced encapsulation technologies was developed to ensure, at the same time, an optimal sealing of the module and color uniformity over the entire device. An improved formulation of electrolyte was identified to improve thermal stability of the module without penalizing the photo-conversion efficiency. A cold lamination process was developed to assemble lightweight and transparent DSC panels for BIPV applications. We show that such optimizations lead to pilot-line produced modules able to pass UV preconditioning, Humidity-Freeze and Damp-Heat tests included in the IEC 61646 ?Thin-film terrestrial photovoltaic (PV) modules - Design qualification and type approval? standard. Reliability of DSC modules was achieved with several dyes and for both semitransparent (T>30%) and opaque modules.

Resume : We investigate the structural, electronic and magnetic properties of the orthorhombic Perovskite oxyde NdMnO3 through density-functional-theory (DFT) calculations using both generalized gradient approximation GGA+U approaches, where U is on-site Coulomb interaction correction. The electronic band structure, the partial and total density of states (DOS) and the magnetic moment are determined. The results show a giant magnetic moment and a half-metallic ferromagnetic ground state.

Resume : Perovskite based solar cells haveattracted a huge scientific interest due to their high power conversion efficiencies (PCE) expected to reach over 20%. However, for solution processed perovskite photovoltaics there are several issues like for i.e. ink concentration, deposition process, temperature, humidity of the environment, etc that have to be addressed in order to obtain devices with high efficiencies and reproducibility of the fabrication process. In addition, it seems that perovskite based photovoltaics will be a very challenging topic for the future and especially for large scale roll-to-roll (R2R) printed OPVs. In this work, a systematic investigation of the morphological and the structural features of perovskitebased thin filmshas been performed using different parameters during the deposition process. Initially, methylammonium iodide (MAI) has been synthesized in our lab and mixed with PbCl2in dimethylformamide (DMF) to form the mixed halide perovskite precursor ink. Perovskite thin films were fabricated by spin coating onto cleaned glass substrates, and different speeds(3000 and 5000 rpm) as well as humidity levels of the environment(inside the glovebox and in a room with ~40% RH)were used. X-Ray diffraction (XRD) and atomic force microscopy (AFM) have been found to give valuable structural and morphological information,and revealed thus the optimum experimental parameters that the perovskite ink has to be depositedfor high performance devices.

Resume : Inorganic–organic perovskite compounds (CH3NH3PbI3) have attracted attention as light-harvesting materials for solar cells. These compounds are advantageous for solar cells because of their wide light-absorption and high extinction coefficients. Moreover, solar cells based on these compounds could achieve high efficiency and low-cost. Therefore, the aim of this work is to focus on improving the characteristics of the solid state perovskite based solar cell in the route of high efficiency, by using a novel chemical approach. Indeed an original meso structuration will be used to improve and to control the porosity of the discontinuous TiO2 active layer. Increasing the porosity of the perovskite will help in increasing the contact area between the perovskite and TiO2 particles and consequently to enhance the charge extraction and then the quantum efficiency. This consists of using a self-assembling, at the anode surface, of polymer beads or microfibers spherical particles. We have therefore used sol-gel process to grow all the films of the perovskite based solar cell. It is interesting to note that the perovskite presents a cubic crystalline structure when grown on the porous TiO2 layer while an orthorhombic crystalline structure is obtained for the perovskite film grown directly on the glass substrate. In addition to the high absorption coefficient of the obtained perovskite, photoluminescence measurements show an intense line around 800 nm. In order to understand the effect of the meso structuration on the photovoltaic performance I-V characteristics on the complete devices will be presented and discussed.

Resume : Recently, perovskite based solar cells have revolutionized the emerging photovoltaic technologies with more than 19% power conversion efficiency. However, the toxicity of the lead perovskites is still a big challenge to implement it commercially. Here, we have replace lead with tin. Mixed halide tin perovskites (CH3NH3SnI3–xBrx, CH3NH3SnI3–xClx) have been prepared by a combined solid state/solution chemisty method by mixing stoichiometric amounts of CH3NH3I and SnX2 (X= Br, Cl) followed by proper heat treatment. Films were prepared by drop casting perovskite solution in DMF onto glass substrates. The perovskite films have been characterized by x-ray diffraction, scanning electron microscopy and absorption spectroscopy. The optical properties have been investigated by temperature dependent photoluminescence (PL) spectra. At room temperature, the perovskites exhibit a band gap emission close to 1.3 eV. However, at low temperature, we observe two bands below the band gap energy, which are associated with defect related transitions. Moreover, the emission behavior changes strongly with changing excitation intensity and becomes increasingly inhomogeneous when samples degrade over time.

Resume : A ruthenium polypyridyl dye containing a hexasulfanyl-styryl modified bipyridyl group as ancillary ligand, coded TG6, is investigated as a sensitizer for ZnO-based dye-sensitized solar cells (DSSCs).[1] The advantages of this dye are a broad wavelength absorption spectrum, a large loading in ZnO photoelectrodes, a significantly larger extinction coefficient compared to more classical Ru-polypyridyl dyes and the formation of less agglomerate in the pores of the ZnO layers. TG6 has been used to sensitize ZnO nanorod particle layers of high structural quality and ZnO layers made of sub-micrometer spheres composed of aggregated nanocrystallites and that develop an internal surface area.[2] The latter are highly light-scattering in the visible wavelength region but more difficult to sensitize correctly. The TG6 dye has been compared with the metal-free D149 dye [3] and has been shown more efficient photoconversion. The best performances have been obtained by combining TG6 with the nanorod layer, the optimal power conversion efficiency being measured at 5.30% in that case. The cells have been investigated by impedance spectroscopy over a large applied voltage range. We especially show that the sub-micrometer sphere layers exhibit a less conductivity and a less charge collection efficiency as compared to the nanorod particle ones. [1] M. Hosni, M.; Y. Kusumawati, Y.; Farhat, S.; Jouini, N.; A. L. Ivansyah, A.L.; M. A. Martoprawiro, A.M.; Th. Pauporté, Th. ACS Applied Mater. Interfaces 2015, DOI: 10.1021/am5068645. [2] Hosni, M.; Kusumawati, Y.; Farhat, S.; Jouini, N.; Pauporté, Th. J. Phys. Chem. C 2014, 118, 16791–16798. [3] Magne, C.; T. Moehl, T.; M. Urien, M.; M. Grätzel, M.; Th. Pauporté, Th., J. Mater. Chem. A 2013, 1, 2079–2088.

Resume : The efficiency and stability of bulk-heterojunction organic solar cells strongly depend on morphology of the active layer at the molecular scale. This morphology can be essentially modified by active layer processing and degradation. In this work, we present in-situ Raman spectroscopy technique to probe the molecular order in organic bulk heterojunctions. The technique was applied to monitor and optimize the thermal annealing process in bulk heterojunctions based of poly-3-hexylthiophene (P3HT) and various fullerene derivatives (methanofullerenes PCBMs, metal complexes of fullerenes and fluorinated fullerenes). The polymer Raman spectrum was recorded during thermal annealing and processed to calculate the degree of polymer order determined as the ratio between the semicrystalline and amorphous phases of the polymer. We have found that the dynamics of polymer ordering strongly depends on the type of fullerene and solvent used. This in-situ technique allowed us to determine the optimal annealing time and temperature for the blends studied. We show a clear correlation between the energy conversion efficiency and the degree of polymer order in polymer-fullerene devices processed according to different annealing protocols. We discuss the potential of the developed in-situ Raman technique for study of processing and degradation in organic solar cells.

Resume : One of the major remaining challenges for solution-processed bulk heterojunction (BHJ) organic devices is to increase their long-term operational stability. Indeed, donor-acceptor phase segregation?leading to diminished photovoltaic performance?can be persistent even at operating temperatures, especially for emerging small-molecule BHJ devices. Thus strategies to afford the long-term thermal stability of the ideal BHJ morphology are necessary. To this end, a series of novel compatibilizers and nucleation promoters we designed and prepared by linking high-performance small molecules (e.g. DPP-(TBFu)2, SMPV-1, and perylene diimide derivatives) together by a flexible alkyl bridge. Interestingly, while this flexible-linker strategy [1] does not significantly affect the optical properties of the materials in the solid-state, when used as an additive (at 1?5 wt%) it showed a clear improvement in thin-film formation through the inhibition of micrometer scale aggregation in as-cast films and after thermal annealing. These morphological effects were found to correlate directly to increased device thermal stability. Furthermore, by investigating the connection geometry of the aliphatic linker in a series of dimeric flexibly-linked small molecules we have gained significant new insight into the role of self-assembly on the charge transport and morphological stability in small-molecule BHJ devices. [1] A. Gasperini, S. Bivaud, K. Sivula, Chem. Sci. 2014, 5, 4922-4927.