Halide perovskites: low dimensions for devices

Resume : High compressive forces are typically applied to three-dimensional perovskite crystals to generate lattice distortions, and thus induce changes to their optical and transport properties. Low-dimensional perovskites can achieve similar effects with much lower pressures by exploiting structure anisotropy rather than not uniaxial cell distortions. Here, we introduce an architecture of layered perovskite microplatelets, referred to as flakes, and demonstrate this principle by studying in-situ their photoluminescence while mechanically loading and unloading in the range of tens of MPa. We find an drastic change in their photoluminescence, from near-white to an enhanced blue emission under pressure; a reversible process denoted by a hysteresis loop in the photoluminescence intensity of the flakes. Combining experimental analysis and computational modelling we conclude that such observations cannot be attributed to changes in the crystallographic structure of the flakes. Instead, the simulations suggest that, thanks to their structural anisotropy, flake alignment and reorientation are responsible for the observed photoluminescence modulation.1 These findings open an avenue for the exploitation of layered perovskite crystals in mechatronic systems, stress sensors, and actuators. In addition, further investigation of similar structures, where a preferential orientation is induced through mechanical exfoliation, leads to an enhanced blue emission, extending their use in devices where ultraflat deep blue emitters are needed. 1. A. Castelli. Adv. Mater. 2019, 31, 1805608.

Resume : The demand for large area high-energy radiation detection systems for security and medical imaging, has pushed the research to develop novel materials combining high sensitivity and low-cost fabrication. Hybrid organic-inorganic perovskites are excellent materials for X- and γ-photon direct detection, thanks to their high Z atoms, combined with superior semiconducting properties. Impressive detection performance has been recently reported employing thick wavers1 and single crystals2. However, these devices require high voltage operation and do not exhibit flexibility peculiar to thin films. Here, we report on highly sensitive direct X-ray detectors based on solution-processed Cs-containing mixed triple cation perovskite thin films. Despite being in a thin film form, the devices exhibit a remarkably high X-ray sensitivity of 82 µCmGy-1cm-3 under short circuit conditions. At a small bias of 0.4 V, the sensitivity further increases by orders of magnitude reaching a record value of 2190 µCmGy-1cm-3 which approaches state-of-art direct X-ray detectors (like CdTe). Based on detailed structural, electrical and spectroscopic investigation, we attribute the exceptional sensitivity of the triple cation Cs perovskite to its high ambipolar mobility-lifetime product as well as to the formation of a pure stable perovskite phase with a low degree of energetic disorder, due to an efficient solution-based alloying of individual n- and p-type perovskite semiconductors. [1] Y.C. Kim et al., Nature, 550, 87, 2017 – S. Shrestha et al., Nat Phot., 11,436, 2017 [2] W.Wei, Nat. Phot., 11, 315, 2017 – Y. He, Nat.Comm., 9, 1609, 2018

Resume : The natural, low-cost, self-assembled class of soft materials, named hybrid organic − inorganic semiconductors (HOIS), appears to be ideal for novel optoelectronic devices due to its inherent stable excitonic states, even at room temperature. We report here on some new HOIS concepts for single layer LEDs and possibly other devices. In some cases, defects appear to induce and/or enhance the functionality of the devices which will be primarily LED and chemical sensors. Some new ideas for compounds will be presented.

Resume : Following the introduction of perovskites for photovoltaic solar energy conversion, the use of these materials as a general purpose optoelectronic material for displays, lighting, and lasing has been explored. However, while reports on stimulated emission and lasing by perovskites show great promise, a comprehensive quantification of their optical gain characteristics is lacking. Here, we measure gain coefficients, clarify the gain mechanism, and explore the gain dynamics of colloidal CsPbBr3 nanocrystals by deploying a unique combination of broadband transient absorption and ultrafast fluorescence spectroscopy. Opposite from current literature, we show that optical gain in such nanocrystals is supported by stimulated emission from free carriers, and not from excitons or biexcitons. Importantly, we demonstrate that the concomitant gain coefficients and thresholds agree with literature results reported for perovksite thin films. Finally, we show that, even in the case of fully inorganic lead halide perovskites, a cooling bottleneck hampers the development of net stimulated emission at high excitation density. Based on these results, we propose that bulk-like colloidal nanocrystals in general offer a unique testbed to quantify optical gain of novel photonic materials and in particular for lead halide perovskites.

Resume : Halide perovskites solar cells have experienced an impressive enhancement of performance in the last decade. Despite the outstanding performance not just on solar but in other optoelectronics devices as LEDs, based on this family of materials, the working principles of these devices are not fully understood. Impedance spectroscopy is a non destructive characterization technique that allows the characterization of devices at the working condition at different applied bias and under illumination. Impedance spectroscopy is a characterization method in the frequency domain that permits decoupling physical processes occurring at different time scales. Despite there is not a complete model for the impedance spectroscopy pattern, some important trends can be determined by this technique as rembination resistance or low frequency capacitance. In this talk we analyze by impedance spectroscopy response of different kinds of solar cells, including hybrid and inorganic perovskite and perovskites with lower dimensionality.

Resume : The remarkable solar performance of lead halide perovskites can be attributed to their excellent physical properties that present many mysteries, challenges, as well as opportunities. Better control over crystal growth of these fascinating materials and better understanding of their complex solid state chemistry of various complex 3D and 2D perovskite phases would further enhance their applications. Here I will first briefly summarize our results on the solution and vapor-phase growth of single crystal nanowires and nanoplates of methylammonium (MA), formamidinium (FA), and all-inorganic cesium (Cs) lead halides perovskites (APbX3). High performance room temperature lasing with broad tunability of emission was demonstrated with these single-crystal perovskite nanowires. I will focus my discussion on the growth of multi-layered multi-colored vertical heterostructures of 2D Ruddlesden–Popper (RP) layered halide perovskites with defined n phases and atomically sharp interfaces, and use them to study the carrier transfer mechanisms between different RP phases. The excellent properties of these single-crystal perovskite nanostructures of diverse families of perovskite materials with different cations, anions, and dimensionality make them ideal for fundamental physical studies of carrier transport and decay mechanisms, and for enabling high performance lasers, LEDs, and other optoelectronic applications.

Resume : The contribution of synthetic solid state chemistry covers two fields. One part is the synthesis of new materials with specific and, hopefully, superior properties. A combination of different cations can enable a bandgap tuning. The introduction of protonated thiourea as a “new” organic cation leads to new perovskites without change of the underlying cubic closest packing of organic cations and halide anions. Within the series Tu/MA/Pb/I there is a continuous transition from 3D via 2D to 1D including several intermediates. This transition is accompanied by a shift of the bandgap. A similar bandgap tuning is possible based on a 3D structure when MA is substituted by the dication [H3N(CH2)2NH3]2+. These examples leave the impression that bandgap engineering by combination of specific pairs of organic cations A/A’ are predictable. Although the knowledge has increased, much more details on structure-property relations are still needed. A second aspect deals with the critical monitoring of experimental procedures. One example are non-innocent solvents that seem to be involved but have a fundamental influence. So we have shown that conc. aqueous hydroiodic acid, frequently simply denoted as HI, reacts with solid PbI2 via the gas phase to the oxonium compounds (H18O8)Pb3I8/ (H3O)0.5(H2O)1.5Pb0.75PbI2 which can be transferred by CH3NH2 to MAPbI3. The reaction of gaseous CH3NH2 with MAPbI3 (“molten salts approach”) leads to unexpected intermediate if traces of water are present. This intermediate contains the dimeric cation (CH3NH3•H2NCH3]+.

Resume : These last years, perovskite based solar cells (PSC) have shown an impressive potential, combining advantages of the thin film generation with high power conversion efficiency (PCE) up to 23.7%. However, stability of the perovskite material need to be improved in order to hope a future commercialization. Recently, our group have reported a novel hybrid perovskites family, possessing an intermediate structure between two- and three-dimensional. This new low-dimensional family arises from some substitutions of lead- and iodide-units by large organic cations inside the well-known methylammonium lead triiodide (MAPI) compound. This family has been named deficient hybrid perovskite (abbreviated d-HP) and been obtained as crystals, powders, and thin films. The incorporation inside the bulk of this large organic cations resulted in a improved air-stability compared to MAPI. Moreover, first deficient-MAPI (d-MAPI) solar cells showed an efficiency up to 6%, that proved the great potential of this d-HP family. More recently, in order to develop this d-HP family, our group obtained the deficient formamidinium lead triiodide perovskite (d-FAPI), which revealed a impressive improvement of the perovskite-phase stability under room conditions, one order of magnitude higher than FAPI compound. References: - National Renewable Energy Laboratory, N.R.E.L. - Leblanc, A.; Mercier, N. ; Allain, M. ; Dittmer, J. ; Fernandez, V. ; Pauporté, T. Angew. Chem. 2017, 129, 16283-16288. - French patent.

Resume : Recently, two-dimensional (2D) layered perovskites (An 1BnX3n 1, n = 1, 2, …) have caught the eyesight of researcher owing to their enhanced ambient stability compared with the conventional 3D counterparts. In addition to the A-site cation engineering, choosing an asymmetric pseudo-halide, SCN, anion in X-site anion is an alternative approach to comprise 2D perovskite. Among SCN-based perovskites, 2D (MA)2Pb(SCN)2I2 was the most widely researched one. Though privileged as a potential material because of its decent optoelectronic properties, (MA)2Pb(SCN)2I2 presented poor stability, which rouses worries from the latest researches, hindering its widespread applications. In this study, a systematical structure engineering of A2Pb(SCN)2X2 (A= FA , MA , Cs and X= Br-, I-) was conducted. Our results unveiled that the rod-like SCN- anion in structure may dictate crucial limitation on composing ions of its derivative 2D (PbX4(SCN)2) framework, which remain obscure for this sort of perovskites. Cs2Pb(SCN)2I2 was proved to be a favorable structure with the improved stability and photo-response compared with (MA)2Pb(SCN)2I2. Besides, albeit the akin crystal structures, the electronic band structures between Cs2Pb(SCN)2I2 and (MA)2Pb(SCN)2I2 might be different due to their discrepant photoluminescence (PL) spectra, for which the former exhibits a rather intense SE emission at ~594 nm overshadowing the emission at ~688 nm correlated with triplet or defective states at room temperature, as opposed to the latter.Furthermore, Finally, a series of mix-halide 2D Cs2Pb(SCN)2(I1-xBrx)2 (x = 0, 1/3, 1/2, 2/3, 1) with various colorful film was investigated by both simulation calculation and experimental works to verify the boosted stability as x value rises. Our results reveal that all inorganic 2D Cs2Pb(SCN)2X2 perovskite system is a promising class of material with nice stability and color-tunability, and can be considered as proper candidate for future applications.

Resume : The emergence of metal halide perovskite solar cell (PSC) has re-energized solution-processed photovoltaics research with material properties rivaling that of crystalline semiconductors and recently reported device efficiencies of 22.7% [1], comparable to that of multi-crystalline silicon. Methylammonium lead iodide perovskite (MAPbI3) is one of the most promising material for perovskite solar cells [2], however, it still doesn?t meet requirements of durability and stability in ambient conditions. Specifically, temperature and moisture induce breakdown of MAPbI3 perovskite crystal into its non-photoactive components (CH3NH3I and PbI2) deteriorating solar cell efficiency and long-term stability. Recently, the use of mixed dimensional perovskites with general formula (M)2(CH3NH3)n-1PbnX3n 1 (M=longer/bulky alkylammonium cation, n=integer number), have gained attention due to their stability against moisture [3]. In this work, we show that thermal stability of MAPbI3 is significantly improved by combining methylammonium (MA) and the bulky organic ligand naphthalene-methylammonium (NMA) to form (NMA)2(MA)n-1PbnI3n 1 mixed dimensional perovskite. Thanks to the effective passivation of traps and grain boundaries, we have achieved high PCE (~17%) solar cells with improved thermal stability, which maintain 70% of the initial efficiency when heating to 85°C for 360 hours, compared to 25% PCE retention of the pristine MAPbI3 perovskite. References [1] W. S. Yang, B.-W. Park, E. H. Jung, N. J. Jeon, Y. C. Kim, D. U. Lee, S. S. Shin, J. Seo, E. K. Kim, J. H. Noh, Science 2017, 356, 1376-1379. [2] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Journal of the American Chemical Society 2009, 131, 6050-6051. [3] T. M. Koh, K. Thirumal, H. S. Soo, N. Mathews, ChemSusChem 2016, 9, 2541-2558.

Resume : Low-dimensional perovskites offer more opportunities for compositional exploration as compared to the limited 3D counterparts. The reduced connectivity in low-dimensional perovskites, however, has negative impact on the charge transfer between the inorganic sheets. This reduced dimensionality also results in increased band gaps and reduced conductivity. Polyiodides is a class of compounds known for rich structural diversity and high conductivity. We have hence explored and synthesized new lead-based organic–inorganic hybrid perovskite compounds containing polyiodide species and subsequently characterized them. The crystal structures reveal the presence of polyiodide units acting as linkers between the inorganic anion slabs. One of the compounds contain triiodide anions, I3–, uniquely coordinated into the coordination sphere of two different lead atoms in two different sheets, thereby connecting the 2D inorganic slabs to form a 3D network. Optical spectroscopy indicates low band gaps and the optoelectronic properties were further examined with band structure calculations. Additional investigations of different systems involving ionic liquids have been made to explore structural possibilities and to find methods of making thin films of such compounds. Furthermore, robotized screening is applied in the quest for new photovoltaic materials including polyiodide hybrid perovskites.

Resume : In colloidal nanocrystal form, lead halide perovskites possess high photoluminescence quantum yields, while in bulk single-crystal form they exhibit long charge-carrier diffusion lengths. However, without proper strategies to diminish crystal surface defects and manage surface quality, the desired characteristics of perovskites cannot be effectively exploited for photovoltaic and optoelectronic devices. Here I discuss novel strategies to passivate the surface defects and improve the surface quality of perovskite nanocrystals and bulk single-crystals, enabling the fabrication of efficient devices. We demonstrate the passivation of CsPbX3-type nanocrystals with molecular ligands and metal dopants leading to stable near-unity quantum yield emitters, as well as efficient blue and red light-emitting diodes (LEDs). We also show the importance of designing crystal growth conditions, such as solvent, temperature, and substrate in order to grow bulk single-crystals with low-defect densities and good surface quality. Depending on the composition, MAPbX3-type single crystals grown (tens of microns thick) under optimal conditions were used to realize: a) very sensitive visible-blind UV-photodetectors with nanosecond response time; and b) single-crystal solar cells with ~21% power conversion efficiency. Unlike thin film polycrystalline solar cells, efficient cells with a grain-free single-crystal absorber are an ideal unobstructed system for investigating the device physics and chemistry of perovskites.

Resume : The past decade, metal halide perovskite nanocrystals have attracted great scientific and technological interest due to their promising application in diverse fields ranging from photovoltaics to lasing and LEDs. Despite the great success of organic metal halide perovskite material, several problems remain to be resolved such as their moisture, oxygen, light and heat intrinsic sensitivity. The replacement of the organic group with inorganic ions found that are more stable and some of these problems seems to be solved. In this work, we focus on the synthesis of new all-inorganic perovskite nanocrystals with various morphologies in solution form or directly grown on substrates. These synthesis methods are based on simple and low-cost room- temperature processes without using complex apparatus and inert gas flow. [1] Morphological and structural features, as well as their physicochemical properties of these materials will be presented. Furthermore, the potential utilization of these nanostructures in energy conversion and storage will be discussed. [2, 3] [1] A. Kostopoulou et al, Nanoscale 2017, 9, 18202-18207. [2] A. Kostopoulou et al, J. Mater. Chem. A 2018, 6, 9765-9798. [3] A. Kostopoulou et al, Nanoscale 2019, DOI: 10.1039/C8NR10009H. Acknowledgements. This research is co-financed by Greece and the European Union (European Social Fund- ESF) through the Operational Program «Human Resources Development, Education and Lifelong Learning 2014-2020» MIS 5004411

Resume : We demonstrate that single crystals of methylammonium lead bromide (MAPbBr3) could be grown directly on vertically aligned carbon nanotube (VACNT) forests. The fast-growing MAPbBr3 single crystals engulfed the protogenetic inclusions in the form of individual CNTs, thus resulting in a three-dimensionally enlarged photosensitive interface. The obtained devices worked as sensitive photodetectors being able to detect low light intensities (~20 nW) from 550 nm down to UV range. They served as operationally stable gamma-ray detectors, detecting irradiation sources in good agreement with commercially available and calibrated devices for gamma dose-rate measurements. Moreover, bright green electroluminescence of the MAPbBr3 single crystals, using symmetrical VACNT electrodes, was observed at room temperature for both polarities. The electroluminescence spectra and light intensity was recorded from room temperature to cryogenic temperatures (20 K). The underlying mechanism behind the light emission is the well documented ion migration. Charged ions or vacancies inside the perovskite, drift under an external electric field accumulating at the cathode and anode, forming a p-i-n heterojunction structure. The decreased interface energy barrier assisted by an enhanced electric field at the CNT tip allows an increased injection of charges. The effect of a 2D (ethylenediammonium lead bromide) layer coverage on the 3D single crystal was also assessed with respect to ion migration and light emission properties of halide perovskite light-emitting electrochemical cells with carbon nanotube forest contacts.

Resume : All-inorganic cesium lead halide perovskites without volatile organic cations has emerged as a new prototype semiconductor for high performance photovoltaic and optoelectronic devices, exhibiting significantly enhanced environmental stabilities. In particular, cubic a-CsPbI3 (black phase) has a direct bandgap of 1.73 eV, which is the suitable for practical applications among the all cesium-based lead halide perovskites. However, despite of its desirable inherent optoelectrical properties, CsPbI3 immediately converts into a non-photoactive orthorhombic phase δ-CsPbI3 (yellow phase) with a wide bandgap of 2.82 eV. To overcome this phase instability issue, we proposed a strain engineering induced by anodized aluminum oxide (AAO) templates as highly effective strategy for stabilizing the desirable black a-phase CsPbI3 at room temperature. The uniaxially aligned pores of the AAO templates acted as a spatial confinement, inducing anisotropic strains on the perovskite lattice as determined by the Williamson–Hall analysis. The density functional theory (DFT) calculation also clearly confirmed that the strain energy in the perovskite lattice enables the phase stabilization of the cubic a-CsPbI3 phase at room temperature. Upon exposure to the ambient air, the a-CsPbI3 confined within the AAO template with a pore size of 41 nm remained stable even after a three month-duration storage.

Resume : In this work, inverted cesium-containing perovskite light-emitting diodes (PeLEDs) based on zinc oxide nanocrystals (ZnO NCs) as the electron transport layer were demonstrated. The synthesized ZnO NCs showed uniformed size and excellent opto-electrical properties. Moreover, polyethyleneimine ethoxylated (PEIE) and an ionic polyfluorene (PF) derivative containing trimethylammonium hexafluorophosphate groups were introduced between ZnO NCs and CsPbBr3 film to enhance electron injection. We found that the introduction of the PEIE/ionic PF bilayer effectively improved CsPbBr3 coverage and morphology, thereby reducing current leakage in PeLEDs. Meanwhile, the improved CsPbBr3 film showed stronger photoluminescence, owing to anti-quenching capability of the PEIE/ionic PF and prolonged carrier lifetime. The PeLEDs with the configuration of ITO/ZnO NCs/PEIE/ionic PF/CsPbBr3/ TFB/Au were fabricated, employing TFB as the hole transport layer. The optimized PeLED based on the PEIE/ionic PF showed a low turn-on voltage of 2.8 V, a max luminance of 3,927 cd/m2 and max current efficiency of 0.2 cd/A, which was significantly higher than the one without PEIE/ionic PF bilayer. To conclude, ZnO NCs, PEIE and our synthesized fluorene-based polyelectrolyte was successfully combined as the ETL to enhanced device performance of inverted PeLEDs for the first time.

Resume : A promising class of lead-free perovskites for photovoltaic applications include the bismuth halides, such as MA3Bi2I9, BiOI, and Cs2AgBiBr6. Although these materials have been predicted to exhibit defect tolerance, as seen in lead-halide perovskites, and already display improved stabilities and long charge carrier lifetimes, the power conversion efficiencies of the corresponding devices have not reached the level of lead-based perovskites. Potential reasons for this are explored, for example, the disconnected nature of the bismuth halide octahedra in the crystal structure, which limits carrier mobility, and the lower levels of absorption due to indirect bandgaps. We probe the behaviour of excited states in many bismuth-halide compounds with various effective dimensionalities using ultrafast transient absorption, Raman, and teraherz spectroscopy. Overall, this work indicates that bismuth-based materials have the potential to be used in efficient optoelectronic devices, but there is a need to account for the effects of strong carrier-phonon coupling and localisation of electronic states on carrier scattering rates. We therefore present charge carrier-lattice interaction strength as an important design criterion for efficient next-generation solar cells.

Resume : Following an experimental breakthrough in 2016 [1], layered hybrid organic−inorganic perovskites (HOPs) have re-emerged as potential technological solutions for next-generation photovoltaics. Solar cell efficiencies are continuously improving and reaching currently over 15% [2]. In the meantime, new methodologies relying on first-principles calculations and composite approaches have been developed to scrutinize their optoelectronic properties [3]. In this contribution, we will present results of recent DFT calculations on new series of multilayered halide perovskites yielding high photovoltaic efficiencies. References 1. H. Tsai et al, Nature 2016, 536, 312. 2. H. Lai et al, J. Am. Chem. Soc. 2018, 140, 11639−1164. 3. B. Traoré, L. Pedesseau, L. Assam et al, ACS Nano, 2018, 12 (4), 3321-3332.

Resume : The most popular holes transporting layer (HTL) in perovskite solar cells (PSC) is 2,2′,7,7′-tetrakis (N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD). However, despite the many advantages it has also some drawbacks where the main of them are high cost, complicated and long both synthesis and purification [1]. Due to this, the design and synthesis of new, efficient HTMs is still an important research topic [2]. In the literature, numerous examples of organic [3,4] and inorganic [5,6] compounds in the role of holes transporters were described. Among them, the low molecular weight compounds are dominated. Therefore, the aim of this study was testing of selected compounds consists of carbazole (1 and 2) or phenoxazine units (3), in perovskite solar cells. The investigated compounds are characterized by optimal value of HOMO levels 5.36, 5.35 and 5.1 eV for molecule 1, 2 and 3 respectively. What is similar to the most commonly used Spiro-OMeTAD. As simulations have shown, the tested molecules should lead to improvement of electrical parameters of PSC compared to a reference devices without HTM.

Resume : High temperature superconductors (HTSC's) are considered to be low carrier density materials. Therefore, light can penetrate the superconductor and can effectively excite the quasiparticles in it.The study of light detection by a "HTSC - semiconductor(SC)" hybrid contact structures(HCS's) is very perspective for fabrication of multifunctional photonic circuits. In the "HTSC - SC" system can be formed three principal types of HCS's: tunneling(1), proximity(2) and combined(3). The realization of "HTSC-SC" HCS's assumes the following conditions: -semiconducting layers should be p-type electroconductivity (only for the HCS's of type (1); -the thermal expansion and the lattice constants mish-match in the (001)base plane should not exceed the values of 15% and 2-5%, respectively; -the presence of oxygen ions in the anion sublattice of semiconductor is desirable; -the crystal structure of semiconductor should be very similar to the symmetry of space groups P/mmm and/or I4/mmm, which is usual for the YBa2Cu3O7-d and Bi(Tl)2Sr(Ba)2Can-1CunO2n+4 HTSCs. The unique temperature behavior of the resistance in heterojunctions consist in an N- shape anomaly around the superconducting transition temperature (interval 85- 105K for YBa2Cu3O7-d/BiOCl HCS; Tc=92K) It is interesting that these anomalies in the dependence R(T) appear at temperature T>Tc, and the appreciable deviation (decreasing) of the differential resistance appeared long before the approach to Tc (at T~85K).

Resume : Methylammonium (MA) lead halides form quasi-2D nanocrystals, referred to as nanoplatelets, under the right synthetic conditions. We investigated the excitonic properties of a dispersion of MAPbI3 nanoplatelets containing nanocrystals of thicknesses ranging from 4 to 1 PbI6 octahedra planes. We observed multiple absorption features. Those at energies around 1.6eV are attributed to bulk-like MAPbI3 nanoplatelets. We assigned the energy dips comprised between 1.9eV and 2.4eV to the excitonic absorption of nanoplatelets of varying thickness. The highest energy absorption at 2.5eV is assigned to an excited excitonic state. We performed systematic magnetotransmission measurements up to 70T, and we observed a quadratic magnetic field dependence for all the dips related to quasi-2D nanoplatelets, which confirms the excitonic nature of their absorption. Interestingly, we observed a well-defined trend of the diamagnetic coefficient, which decreased with decreasing nanoplatelet thickness. We estimated the exciton radius ⟨r⟩ via its relationship with the diamagnetic coefficient a=e2⟨r2⟩B2/8m, where m is the exciton reduced mass, B is the applied magnetic field and e is the electron charge. Our results demonstrate that the dip observed at 2.5eV is related to an excited excitonic state and can be used to estimate the exciton binding energy in these quasi two-dimensional systems.

Resume : Increases in the global energy demand necessitate the development of new approaches to energy conversion and storage. In particular, the utilisation of solar energy could provide a basis of evolving technologies capable of meeting modern demands. By combining the photovoltaic and newfound electrochemical properties of organo-metal hybrid perovskite materials in a single device, a novel photobattery technology is proposed. Utilising the photovoltaic performance of bulk 3D perovskite materials in combination with the intercalation and conversion mechanisms available to Lithium ion species of layered 2D perovskite materials, a device with the ability both to convert light to electrochemical energy and store it is discussed. The motivation for such a device will be demonstrated, with its inherent impact in areas such as off-grid energy solutions and the internet of things. The fabrication techniques are described and characterisation techniques common to both photovoltaic and electrochemical disciplines, with their recent results are discussed. The current understanding of the charge carrier dynamics and device structure under electrochemical cycling are discussed in depth followed by recent developments in device performance and characterisation. Given the potential commercial applicability of the device, ideas to enhance the device performance whilst reducing fabrication cost and increasing scale are used in conclusion.

Resume : Recently, the emergence of quasi two-dimensional (2D) layered perovskites, with their excellent and tunable optoelectronic behavior, has encouraged researchers to develop the next generation of optoelectronics based on these 2D materials. However, few studies on flexible optoelectronic devices based on quasi-2D perovskites have been reported. Here, a flexible photodetector is developed based on FA, Cs-doped (iBA)2(MA)3Pb4I13 (FA = Formamidinium; iBA = iso-butylamine; MA = methylamine) perovskites via a one-step solution processing method, in which both FA and Cs cations are introduced into the quasi-2D perovskites to modulate the qualities of films as well as improve the performance of devices. When incorporated in typical rigid photodetectors, the resulting FA, Cs-doped (iBA)2(MA)3Pb4I13 perovskites exhibit much enhanced photodetection properties compared to those of pristine (iBA)2(MA)3Pb4I13 under 532 nm illumination. Moreover, once fabricated as flexible photodetectors on polyimide, the FA, Cs-doped quasi 2D perovskites show further improved photodetection performance, demonstrating a high responsivity of 397.49 mA/W, a high detectivity up to 1.68 × 1012 Jones, large on-off ratio of 720.27 and fast response speed (rise and decay time of 43 and 22 ms, respectively). Importantly, the devices possess significant mechanical flexibility and durability. Their photocurrent maintains 82% of its initial value even after 9,000 bending cycles. This work provides a feasible design direction of quasi 2D perovskites to obtain high-performance flexible photodetectors for next-generation optoelectronic devices.

Resume : Organic–inorganic perovskites (OIP) are now regarded as strong candidate materials for light emission and photovoltaics applications. This is due to their outstanding electronic properties and their ability to be processed using cost effective fabrication techniques. In this work, we focus on one of the most crucial property of generated carriers in the context of applications: the diffusion of charges. Among the different techniques that have been employed to evaluate the mobility/diffusion coefficient in OIP materials, time resolved photoluminescence (TR-PL) microscopy is one of the most versatile. Here we explore the possibility of this technique to measure the diffusion properties on different perovskites materials (3d,2d, colloidal crystals). In particular, we will to separate the effect of purely electronic diffusion from the one of photonic effects (waveguiding, reabsorption). Additionally, we highlight the capabilities of time resolved two-photon (2P) photoluminescence tomography. Due to the much lower absorption coefficients of OIP at the 2P excitation wavelength, this technique has the ability to excite the materials up to a few microns inside the crystal (instead of a few hundreds of nanometres) and fully observe the diffusive behaviour on a large volume of the crystals. We apply this technique to methylammonium lead bromide (MAPBr) or iodide (MAPI) crystals as well as on PEA based 2d/3d perovskites. In these materials, we obtain a statistics of electronic diffusion coefficients, ranging between 0.01 to 0.5 cm2.s-2, in agreement with previous studies. Thanks to our microscopic resolution we are able to exhibit heterogeneities in the diffusive behaviours, in connection to the local concentration of defects. References: [1] Herz, Laura M. "Charge-carrier mobilities in metal halide perovskites: fundamental mechanisms and limits." ACS Energy Letters 2.7 (2017): 1539-1548. [2] Barnard, E. S., Hoke, E. T., Connor, S. T., Groves, J. R., Kuykendall, T., Yan, Z., ... & Peters, C. H. (2013). Probing carrier lifetimes in photovoltaic materials using subsurface two-photon microscopy. Scientific reports, 3, 2098. [3] Tian, Wenming, et al. "Visualizing carrier diffusion in individual single-crystal organolead halide perovskite nanowires and nanoplates." Journal of the American Chemical Society 137.39 (2015): 12458-12461.

Resume : Mixed cation perovskites based on methylammonium (MA) and formamidinium (FA) are strong candidates for fabrication of efficient and reproducible PSCs. However, the long-term stability of perovskite absorber against humidity and light has become a major concern due to intrinsic volatility and ionic migration of organic cations. [1,2,3] Here we report for the first time, the development of PSCs using MAFACsPbI3-xBrx/(CH3)3SPbI3 heterostructures. The XRPD of the composite 3D/1D perovskite layer fabricated by solution process shows no degradation of the perovskite components to PbI2 over more than one month in ambient conditions. The presence of 1D-wide band gap (CH3)3SPbI3 layer [4,5] at the interface between the primary perovskite absorber and spiro-OMeTAD hole transporting material (HTM) acts as a barrier for ionic migration and charge carrier recombination, resulting in significant improvement of stability and open circuit voltage Voc increase. Our results underscore the possibility for fabricating and developing multidimensional perovskite heterostructures as active layers in PSCs of high efficiency and increased stability. ACKNOWLEDGMENTS This work was supported by European Union’s Horizon 2020 Marie Curie Innovative Training Network 764787 “MAESTRO” project. M. Elsenety is financially supported by Science Achievement Scholarship of High Education Ministry of Egypt in cooperation with the Hellenic Ministry of Foreign Affairs for his PhD Scholarship. References [1] M. Saliba, T. Matsui, J. Y. Seo, K. Domanski, J. P. Correa-Baena, M. K. Nazeeruddin, S. M. Zakeeruddin, W. Tress, A. Abate, A. Hagfeldt, M. Gratzel, Energy Environ. Sci. 9 (2016) 1989. [2] M. M. Tavakoli, M. Saliba, P. Yadav, P. Holzhey, A. Hagfeldt, S. M. Zakeeruddin, M. Grätzel, Adv. Energy Mater. 9 (2019)1802646. [3] N. Balis, A.A. Zaky, D. Perganti, A. Kaltzoglou, L. Sygellou, F. Katsaros, T. Stergiopoulos, A.G. Kontos, P. Falaras, ACS Appl. Energy Mater., 1 (2018) 6161. [4] A. Kaltzoglou, M.M. Elsenety, I. Koutselas, A.G. Kontos, K. Papadopoulos, V. Psycharis, C.P. Raptopoulou, D. Perganti, T. Stergiopoulos, P. Falaras, Polyhedron, 140 (2018) 67. [5] M.M. Elsenety, A. Kaltzoglou, M. Antoniadou, I. Koutselas, A.G. Kontos, P. Falaras, Polyhedron 150 (2018) 83.

Resume : Solar energy is a promising solution in order to solve the energy issues our society is facing. In the 70’s, the appearance of Silicon-based solar panels has been the first step toward exploitation and commercialization of photovoltaic energy. Today, these solar cells have reached 26% yield efficiency after 35 years of development. Perovskite based solar cells have emerged in 2009 with 4% efficiency at that time. In 2018 they have already reached 23.7% and are on the way to industrialization to replace Silicon. But the best materials for this technology are made of organic molecules and while they give good photoelectric properties, they are very moisture-sensitive. In this context, we chose to focus our research on all inorganic based halogen perovskites. We are investigating on a “Green Chemistry” way to produce those materials by using a mixture of acid and water as a solvent for the synthesis. These experiments have demonstrated the possibility to synthesize Cesium and Lead based perovskites without the use of hazardous chemicals (DMF, GBL ...), but also Lead-free compounds. A mechanical synthesis has also been experimented and has shown very good results in terms of purity of the components synthesized. In addition, we are studying different stoichiometries and investigating the materials stabilities against light and certain solvents. Those studies are realized by using essentially X-Ray Diffraction and Scanning Electronic Microscopy. This preliminary study gives us the required knowledge to realize thin layers of these promising compounds in order to go forward and to build stable solar cells at a wider scale.

Resume : Since 2012, the hybrid organic-inorganic perovskite CH3NH3PbX3, with X a Halogen (I,Br,Cl), has emerged in the framework of photovoltaics and for light emitting devices such as electroluminescent diodes and lasers. We consider here one-dimensional planar microcavities containing two kinds of hybrid perovskites as optically active materials : the 3-dimensional (3D) halide perovskite CH3NH3PbBr3 and the 2-dimensional (2D) (C6H5-C2H4-NH3)2PbI4 also interesting for light-emitting devices. We choose hybrid perovskites emitting in the green range in order to address the problem of the green gap for lasers. We demonstrate, for both 2D and 3D perovskites, the strong coupling regime between the photon mode of the Fabry-Perot cavity and the excitonic mode of the optically active material at room temperature even with low quality factors [1-3], leading to the formation of the so-called polaritons, which are a linear and coherent superposition of the exciton and photon states. This work opens the route to fundamental studies on the Bose-Einstein condensations of polaritons and to new opto-electronic devices based on polaritonic effects, such as low threshold polariton lasers, working at room temperature. (1) Lanty, G.; Brehier, A.; Parashkov, R.; Lauret, J.-S.; Deleporte, E. New Journal of Physics 2008, 10, 065007. (2) Han, Z.; Nguyen, H.-S.; Boitier, F.; Wei, Y.; Abdel-Baki, K.; Lauret, J.-S.; Bloch, J.; Bouchoule, S.; Deleporte, E. Optics Letters 2012, 37, 5061-5063. (3) Arxiv : arXiv:1810.05720 This work has been financially supported by National French Agency for Research, in the framework of the projects PEROCAI, POPEYE and EMIPERO. The work of P. Bouteyre is supported by the Direction Générale de l’Armement (DGA).

Resume : A novel kind of solar cells which is based on halide organic?inorganic perovskites, are promising as an active layer to absorb sunlight in solar cells, attracting worldwide attention. Remarkably, perovskite solar cells have a series of advantages, such as low cost, solution-processability, flexibility, transparency and multi-layers, and their unique defect characteristics, so that the perovskite crystal materials can exhibit both N-type and P-type semiconductor properties, making their applications more diverse. The efficiency record of perovskite solar cells has remarkably reached over 23% in the past decade. However, the metal-halide perovskites contain toxic heavy metal lead in a soluble form in the absorption layer which is easy to pollute the environment and some hybrid compositions are unstable in humid air. Sn-based perovskite is a promising alternative due to the similar radius with that of Pd. However, instability is an important problem since Sn2+ is smaller than Pb2+ and Sn2+ is easily oxidized to a higher stable state of Sn4+. Recently, adjusting the composition of perovskite materials and constructing layered low-dimensional structures are effective to improve moisture resistance and photothermal stability. The interlayer separation and thickness of the inorganic layers can be tuned through the choice of organic amine protective group. In this work, we report our recent work identifying a novel class of less toxic layered low-dimensional Sn-based perovskite materials with suitable organic amines and the synthesis of the nanocrystals thereof, achieving the stability and then fabricating devices. The suitability of Sn-based layers in solar cells are studied via a range of optical spectroscopic methods (steady state and time-resolved optical spectroscopy) and device optoelectronic methods.

Resume : Low-dimensional hybrid perovskites are emerging as a promising class of materials for use in optoelectronic applications. A significant advantage of the low-dimensional hybrids over the currently more extensively studied 3D hybrid perovskites, is the greater compositional flexibility of the organic layer. The structural constraints on the organic cation are much more limited for low-dimensional hybrids compared to their 3D counterparts and a large variety of possible molecules remains unexplored. An additional way to introduce functionality is through the use of intercalation. In this way, molecules that do not contain an ammonium tethering group can be incorporated into the organic layer of the hybrids. Mitzi et al. intercalated hexafluorobenzene molecules into phenethylammonium based 2D layered perovskites using the fluoroaryl-aryl interactions. More recently, Smith et al. intercalated molecular iodine into hexylammonium based 2D layered perovskites in order to decrease the electronic confinement in the layered perovskite. We incorporate donor-acceptor charge-transfer complexes into the organic layer of low-dimensional hybrids. The organic layer of these hybrids contains donor molecules with an ammonium tethering group. Through self-assembly, small electron acceptor molecules intercalate into the organic layer. The structural and optical properties of the different hybrids are compared and the influence of the incorporation of the charge-transfer complexes is examined.

Resume : Despite the outstanding optoelectronic properties of lead halide perovskites and the impressive performances achieved in perovskite-based devices, several concerns still hinder the widespread application of these materials. Two of the main bottlenecks today are: (i) the toxicity of Pb2+ cations, and (ii) the instability of hybrid organic-inorganic perovskites coupled with the difficulty in synthesizing fully-inorganic ones due to poor solubility of cesium halide salts in common solvents. Here, we have used a fully-dry mechanical approach via ball-milling of different precursors to synthesize a wide range of phase-pure halide perovskites and related compounds with suitable optoelectronic properties for different applications from photovoltaics to light-emission. In particular, we have synthesized fully-inorganic CsPbX3 (X = I, Br, and Cl) compounds and investigated the structural, chemical, and optical effects of adding potassium halides (KX). We have also synthesized lead-free hybrid and inorganic perovskites based on Sn2+ as well as other related compounds such as A2Sn(IV)X6 “vacancy-ordered perovskites” and A3Bi2X9 (A = Cs, formamidinium, and methylammonium). Eventually, we have demonstrated that the so-formed high quality dry powders can be used to make thin films via single-source vacuum deposition, thus enabling their use in different devices such as solar cells or light-emitting diodes. References: (1) El Ajjouri, Y.; Palazon, F.; Sessolo, M.; Bolink, H. J. Single-Source Vacuum Deposition of Mechanosynthesized Inorganic Halide Perovskites. Chem. Mater. 2018, 30, 7423–7427. (2) El Ajjouri, Y.; Chirvony, V.; Sessolo, M.; Palazon, F.; Bolink, H. J. Incorporation of potassium halides in the mechanosynthesis of inorganic perovskites: feasibility and limitations of ion-replacement and trap passivation. RSC Advances. 2018.

Resume : With perovskite-based solar cells reaching efficiencies as high as 23.7%, photon management in these materials is becoming a key factor for pushing performance towards radiative limits. Recent studies have demonstrated internal photoluminescence quantum efficiencies exceeding 90% in passivated thin films of these materials leading to quasi-Fermi-level splitting 97% of the thermodynamic limit. These results suggest that power conversion efficiencies in perovskite-based solar cells can approach those of GaAs-based photovoltaics at 27.9%. Such improvements, however, require significant interface-loss reductions in full device stacks, as well as back-reflector optimization to reduce parasitic absorption and improve light trapping. Back-contacted architectures for perovskite solar cells both open up the possibility of top surface passivation and texturing, as well as the organization of back contacts into plasmonic structures for enhanced scattering and long-wavelength absorption. Here we demonstrate the fabrication of quasi-interdigitated back electrodes through a scalable photolithography-free approach. Using these structures, we demonstrate increased low-wavelength absorption and significantly enhanced open circuit voltages through active layer thickness reduction and top surface passivation. Moreover, we show that lateral carrier extraction over quasi-interdigitated electrodes can occur over length scales exceeding 10 um due to rapid separation of charge carrier populations over selective electrodes, considerably reducing fabrication constraints for the back electrodes.

Resume : Despite the fact that stability is a critical issue affecting halide perovskite after the materials have been developed, these materials continue to be studied due to their outstanding characteristics. We report the development of stable cesium lead halide (CsPbX3; X=Cl, Br, I) perovskite films using norland optical adhesive 63 (NOA 63) to generate white LEDs by placing films on the InGaN 450nm blue chip. The encapsulated perovskites have narrow full widths at half maximum of 18nm and 31nm for green and red, respectively. Unencapsulated perovskite is decomposed immediately at high humidity and temperature, but NOA 63 encapsulated perovskite maintained a PL emission property of 60% after four hours in artificial atmosphere, and the CIE color triangle reached ~119% of the NTSC standard. We confirm that the NOA 63 encapsulated halide perovskites are beneficial when applied in optoelectronic applications due to their improved stability and maintained characteristics.

Resume : Hybrid organic–inorganic perovskites have recently attracted overwhelming attention due to their excellent photovoltaic performance yielding efficiencies well exceeding 20%.This has been related to properties such as long charge carrier lifetime, the exceptionally large diffusion length, large absorption coefficient, high carrier mobilities, large opencircuit voltages, and direct band gap,The organo-lead trihalide perovskite compounds, CH3NH3PbX3, are the forerunners in efficiency. The organic methylammonium (MA) cation, CH3NH3 , occupies the cuboctahedron A-sites of the perovskite structure surrounded by 12 halogen anions (X = I, Br, Cl). Pb2 resides on octahedron B-sites surrounded by six anions. In this presentation dielectric and acoustic properties in wide temperature and broad frequency range and Monte Carlo simulations of organic – inorganic perovskites CH3NH3PbX3 (X = I, Br, Cl) will be shown. [1, 2] Result show low temperature anti-polar phase. [1] Irina Anusca, Sergejus Balčiūnas, Pascale Gemeiner, Šarūnas Svirskas, Mehmet Sanlialp, Gerhard Lackner, Christian Fettkenhauer, Jaroslavas Belovickis, Vytautas Samulionis, Maksim Ivanov, Brahim Dkhil, Juras Banys, Vladimir V. Shvartsman and Doru C. Lupascu, "Dielectric Response: Answer to Many Questions in the Methylammonium Lead Halide Solar Cell Absorbers", Advanced Energy Materials, 2017 [2] Mantas Šimėnas, Sergejus Balčiūnas,Mirosław Mączka, Jūras Banys and Evaldas E. Tornau „Exploring Antipolar Nature of Methylammonium Lead Halide Perovskites: a Monte Carlo and Pyrocurrent Study“, Physical chemistry letters, 2017, 8.19: 4906-4911.

Resume : The organic−inorganic hybrid perovskites have attracted tremendous attention due to their excellent light absorbing characteristics, easy solution process and low production cost. Among various perovskites, the mixed-cation pseudohalide perovskites were particularly successful with a large power-conversion efficiency (PCE) and satisfactory stability against moisture. In experiments, the mixed cation perovskite solar cells were fabricated with formamidinium (FA: HC(NH)2) and Cs cations to replace methylammonium. The PCE greater than 16 % and structural stability were improved after optimizing the dopant level of lead thiocyanate (Pb(SCN)2) in mixed-cation pseudohalide perovskite FA0.9Cs0.1PbI3 solar cells. We investigated the origin of the improvement of PCE and stability of mixed-cation pseudohalide perovskite solar cells by using synchrotron spectroscopy and microscopy. The results indicate that the interaction between mp-TiO2 and perovskite plays a key role in determining the PCE performance. The morphology variation of perovskite is correlated with the interaction between mp-TiO2 and Pb(SCN)2, which illustrates that the interaction between mp-TiO2 and perovskite is not favored to grow large and defect-free perovskite layers for charge transfer.

Resume : Exciton fine structure splitting in semiconductors reflects the underlying symmetry of the crystal and quantum confinement. Since the latter factor strongly enhances the exchange interaction, most work has focused on nanostructures. Here, we report on the first observation of the bright excitone structure splitting in a bulk semiconductor crystal, where the impact of quantum confinement can be specially excluded, giving access to the intrinsic properties of the material. Detailed investigation of the exciton photoluminescence and reflection spectra of a bulk methylammonium lead tribromide single crystal reveals a zero magnetic field splitting as large as 200 µeV. The observed splitting can be understood in the exciton picture combined with symmetry considerations. We show that, within this model, the observed splitting can be reasonably estimated based on band structure parameters derived from magneto-optical studies. Our work constitutes a firm base for further exciton fine structure studies in lead-halide perovskites. The presented results should provide valuable insight for future investigations of the different mechanisms that contribute to the exciton fine structure, such as confinement anisotropy or Rashba effect, in perovskite based nanostructures.

Resume : We propose and demonstrate a novel thin film deposition technique by transferring the principles of atomic layer deposition (ALD), known with gaseous precursors, towards precursors dissolved in a liquid. The technique can also be considered as a generalization of already established methods such as the ‘layer by layer’ growth or the ‘successive ion layer adsorption and reaction’ (SILAR). 'Solution ALD' (sALD) shares the fundamental properties of standard ‘gas ALD’ (gALD), specially the self-limiting growth and the ability to coat conformally deep pores. It has been already shown that it is possible to transfer standard reactions from gALD to sALD such as TiO2 deposition . However, sALD also offers novel opportunities such as overcoming the need for volatile and thermally robust precursors. In the following the targeted field is photovoltaic. A great deal of interest has appeared on a new generation of material for solar cells application. Among them the Perovskites are particularly interesting and the most studied one is CH3NH3PbI3 (MAPI). The existing deposition methods such as spin coating or vapor-deposition techniques do not allow a control at the atomic level. ALD has been used to deposit PbS but it needed a two-step conversion method to obtain a Perovskite . Therefore, a new process based on sALD has been developed to deposit PbI2 and PbS. It allows the use of inexpensive lead salt and it is easy to process. Then, the PbI2 and PbS are easily converted to MAPI by vapor annealing. The PbI2 deposition was achieved with Pb(NO3)2 and LiI via s-ALD on large samples (up to 10 cm*10 cm). The ALD behavior has been shown. The influence of the deposition parameters on the morphology, the crystalline structure and the chemical composition has been investigated by scanning electron microscopy, atomic force microscopy, grazing incidence x-ray diffraction and x-ray photoelectron spectroscopy.

Resume : The hybrid perovskite materials have attracted a lot of attention in recent years, specifically CH3NH3PbI3. This organic inorganic hybrid perovskite presents a low cost of synthesis and it is a promising material for many applications. However, the obstruction of that material refers to its high degradation which influence its performances in these applications. The understandings of the reason of this rapid degradation stay a significant challenge. In this work we investigate the stability of CH3NH3PbI3 under UV by studying a phonon dispersion and we determine the capacity of doping CH3NH3PbI3 by bromine can influence the stability of this system and induces change in the electronic band structure and optical absorption. The results show imaginary acoustic phonon modes for CH3NH3PbI3 that indicate its dynamical instability. The doping by bromine inhibits the degradation of the system and prohibits the diffusion of iodine this leads to the stability of the system.

Resume : During the past few years halide organic-inorganic perovskites (HOIPs) have attracted much interest as solution-processed semiconductors with high potentialities in optoelectronics and photovoltaics. On one hand, 2D layered perovskites such as (C6H5C2H4NH3)2PbI4 (PEPI) are promising materials for light emitting devices because of their strong emission at room temperature [1]. In other hand, 3D HOIPs such as CH3NH3PbI3 have shown their outstanding performances when incorporated in solar cells [2]. More recently, Ruddlesden-Popper phases of formula (RNH3)2(CH3NH3)n-1PbnI3n+1, a new class of layered perovskites containing methylammonium and a larger organic cation, have demonstrated their ability to increase the stability of solar cells despite of a moderate yield [3]. A key factor to enhance the optoelectronic properties of hybrid perovskites is to improve the crystallinity of the films, which generally suffer from a microscale grain structure [4]. In order to take advantage of the great potential of these materials for both photovoltaics and emitting devices, the synthesis of large monocrystalline films is a key issue. Here we propose a fast crystallization method for layered hybrid perovskites. A vapor-assisted process coupled with a capping of the precursor solution allows to grow 2-dimensional thin films with millimetric monocrystalline grains, a high aspect ratio and a good surface quality [5]. The choice of the best solvent and anti-solvent used in this method will be discussed. In particular, we highlight the benefits of using ?-butyrolactone (GBL) for the growth of layered perovskites monocrystalline grains. [1] K. Gauthron, J. Lauret, L. Doyennette, G. Lanty, A. Al Choueiry, S. J. Zhang, L. Largeau, O. Mauguin, J. Bloch, and E. Deleporte, ?Optical spectroscopy of two-dimensional layered (C6H5C2H4-NH3)2-PbI4 perovskite,? Opt. Express, vol. 18, no. 6, pp. 5912?5919, 2010. [2] ?Efficiency_chart NREL.? [Online]. Available: http://www.nrel.gov/ncpv/images/efficiency_chart.jpg. [Accessed: Sept-2018]. [3] H. Tsai, W. Nie, J.-C. Blancon, C.C. Stoumpos, R. Asadpour, B. Harutyunyan, A.J. Neukirch, R. Verduzco, J.J. Crochet, S. Tretiak, "High-efficiency two-dinensional Ruddlesde-Popper perovsite solar cells, Nature 536, pp312-316, 2016. [4] W. Nie, H. Tsai, R. Asadpour, A. J. Neukirch, J.-C. Blancon, G. Gupta, J. J. Crochet, M. Chhowalla, S. Tretiak, M. a Alam, H. Wang, and A. D. Mohite, ?High-efficiency solution-processed perovskite solar cells with millimeter-scale grains,? Science (80-. )., vol. 347, no. 6221, pp. 522?525, 2015 [5] F. Lédée, G. Trippé-Allard, H. Diab, P. Audebert, D. Garrot, J.-S. Lauret, and E. Deleporte, ?Fast growth of monocrystalline thin films of 2D layered hybrid perovskite,? Cryst. Eng. Comm, vol. 8, pp. 208?215, 2017.

Resume : Hybrid organic-inorganic perovskites (HOIPs) have attracted significant research attention over the past years, with perovskite solar cells seeing an impressive increase in power conversion efficiency up to 23.7%. Although progress has been made, the stability of the hybrid perovskite materials still remains an important hurdle towards the commercialization of perovskite solar cells. Research into 2D layered hybrid perovskites is on the rise due to the enhanced stability of these materials compared to 3D hybrid perovskites. Recently, interest towards the use of functional organic cations for these materials is increasing. However, a vast amount of the parameter space remains unexplored in multi-layered (n > 1) hybrid perovskites for solar cell applications. Here, we incorporate carbazole derivatives as a proof of concept towards the use of tailored functional molecules in multi-layered perovskites. Solar cells containing these materials have comparable efficiencies to our reference devices. Moisture stability tests were performed both at the material and device levels. In comparison to MAPI and PEA-based materials and solar cells, the addition of a small percentage of the carbazole derivative to the perovskite material significantly enhances the moisture stability.[1] [1] R. Herckens, W. T. M. Van Gompel, W. Song, M. C. Gélvez-Rueda, A. Maufort, B. Ruttens, J. D'Haen, F. Grozema, T. Aernouts, L. Lutsen, D. Vanderzande, J. Mater. Chem. A, 2018, 6, 22899.

Resume : Many research activities on solar cells have been conducted and huge efforts to raise the efficiency are having been exerted for decades. In this research, we focus on understanding fundamental properties, especially the relation between optical properties and structural changes happening in methylammonium(MA) lead halide perovskite (CH3NH3PbX3, X - I, Br, and Cl) materials. For example, CH3NH3PbBr3 shows a structural phase transition from cubic to tetragonal phase at ~ 235K and from tetragonal to orthorhombic phase at ~ 140K and other compounds show similar transition behavior. We measured temperature dependent Raman spectra and photoluminescence spectra from single crystalline MAPbX3 samples. We discuss possible microscopic origin for the structural phase transition and its further implications. Our results clearly show that optical spectroscopic techniques, especially Raman scattering spectroscopy, are very effective means to monitor different phases by observing abrupt and gradual changes in phonon spectra that depend on microscopic environment and configurations.

Resume : The all-inorganic perovskite nanocrystals (IP-NCs made of CsPbX3 with X= Cl, Br, I) are currently in the research spotlight owing to their superior optical properties (high photoluminescence quantum yields, narrow emission bands, tenability along the light spectrum) which make them interesting for optoelectronic and photovoltaic applications. Here, we report on the observation of highly efficient carrier multiplication in colloidal CsPbI3 nanocrystals (1.78eV) prepared by a hot-injection method. The carrier multiplication process counteracts thermalization of hot carriers and as such provides the potential to increase the conversion efficiency of solar cells. We demonstrate that carrier multiplication commences at the threshold excitation energy near the energy conservation limit of twice the band gap, and has step-like characteristics with an extremely high quantum yield of up to 98%. Using ultrahigh temporal resolution, we show that carrier multiplication induces a longer build-up of the free carrier concentration, thus providing important insights into the physical mechanism responsible for this phenomenon. The evidence is obtained using three independent experimental approaches, and is conclusive. The presented experimental results provide new and unique insights into the physical mechanism of the carrier multiplication process. This information can be found on our published work on Nature Communications 9, 4199 (2018).

Resume : Organic-inorganic halide perovskites have recently received attention in the field of thermoelectric materials due to their appealing properties and structural versatility. A material to exhibit good thermoelectric properties must be characterized by a low thermal conductivity and a tuned carrier concentration. In this respect, the 3D CH3NH3PbI3 (MAPI) perovskite has been recently reported for its intrinsically low thermal conductivity [1], and computational studies showed that with a proper carrier optimization MAPI can achieve good thermoelectric properties [2, 3]. Lower-dimensional perovskites based on MAPI are promising compounds with a useful structural flexibility that can be exploited to optimize the thermoelectric properties of the material. Accordingly, starting from MAPI as a reference material, we planned to synthesize by solution route several lower dimensional perovskites using organic cations with different hindrance as spacer (e.g. guanidinium, imidazolium and histaminium). To characterize the products, we employ different techniques such as X-ray diffraction, 13C solid state NMR, TGA and SEM. With the aim to extend the library of available thermoelectric materials we are now taking into consideration further organic cations which can be accommodated in the perovskitic structure. [1] Pisoni et al., J. Phys. Chem. Lett., 2014, 5, 2488−2492. [2] He & Galli, Chem. Mater., 2014, 26, 5394−5400. [3] Zhao et al., Synth. Met., 2017, 225, 108–114.

Resume : Encapsulation is an effective method to protect the Perovskite Solar Cells from extrinsic causes of degradation. In this work, we encapsulate the Triple cation perovskite solar cells with Al2O3 deposited by low-temperature thermal Atomic Layer Deposition (ALD) process. An increment in the performance of cells post-encapsulation with a significant decrease in hysteresis index is evident. Conductivity measurement shows that the conductivity of Spiro-OMeTAD has increased by an order of magnitude post-encapsulation. The decrease in low-frequency capacitance reflecting on the efficient charge extraction at the interfaces supports the result that the cells have indeed improved in performance. The cells were then measured in different stress conditions to test the effectivity of the encapsulation.

Resume : The quality of perovskite films is critical to the performance of perovskite solar cells. However, it is challenging to control the crystallinity and orientation of solution-processed perovskite films. Here, we report solution-phase Van der Waals epitaxy growth of MAPbI3 perovskite films on MoS2 flakes for the first time. Under transmission electron microscopy, in-plane coupling between perovskite and MoS2 crystal lattices is observed, leading to perovskite films with larger grain size, lower trap density and preferential growth orientation along (110) normal to the MoS2 surface. In perovskite solar cells, when perovskite active layers are grown on MoS2 flakes coated on hole transport layers, the power conversion efficiency (PCE) is substantially enhanced for 15 % relatively due to the increased crystallinity of the perovskite layer and the improved hole extraction and transfer rate at the interface. This work paves a way for preparing high-performance perovskite solar cells and other optoelectronic devices by introducing 2D materials as interfacial layers.

Resume : A detailed understanding of the carrier dynamics and emission characteristics of organic−inorganic lead halide perovskites is critical for their applications. In this work, we reveal the effect of the lattice disorder on the optical properties of triple cation films by means of PL and transmission measurements. We show that at very low temperatures (T <20 K) the PL possesses the characteristic features observed in inorganic disordered semiconductors. The power dependence of the Stokes shift, PL decay time dispersion, and the low-energy tail of the PL spectrum all point to a disorder-induced exponential tail density of states. A hopping model for excitons qualitatively explains all the characteristic features of the PL spectra at very low temperatures. Unlike conventional semiconductors, the temperature behavior of the PL indicates that the disorder and trapping state population evolve with increasing temperature. The character of disorder changes its nature from static to dynamic when lattice vibration couples with the carriers and probably leads to polaron formation. This is supported by a rapid decrease in Stokes shift and FWHM of PL, which cannot be explained by a simple carrier redistribution over the exponential tail in the density of states. The change of the disorder character is accompanied by a significant drop of the PL efficiency. We attribute this observation to the increased mobility of the excitons, which can reach nonradiative recombination centers more easily.

Resume : Organic-inorganic lead halide perovskite is a promising candidate for photovoltaic, LED, photodetector, Laser etc. applications, owing to its excellent optoelectronic properties. With the concerted research interest and efforts for its photovoltaic application, thus far the power conversion efficiency (PCE) of perovskite solar has been reached to 22.7%1, since its beginning with 3.8% in 20092. Despite achieving high efficiency three-dimensional (3D) metal halide perovskites (MHP) are prone to degrade when exposed to moisture, heat, and light, therefore achieving long-term stability of perovskite is the key challenge to make this new technology commercially successful. In this regards, mixed dimensional approach, that is, embedding two-dimensional (2D) perovskite with 3D perovskite to form mixed dimensional, layered or quasi 2D perovskites, emerged showing promising stability under ambient conditions3. Here, we have embedded an organic ammonium cation by two different approaches to evaluate the impact of perovskite fabrication process on thermal stability of the mixed dimensional perovskites. The mixed dimensional perovskites have demonstrated the enhanced thermal stability and charge transport properties with less recombination as compared to the 3D MA-based perovskite along with the power conversion efficiency~16%. The current results reveal the impact of methodologies of perovskite film formation on the overall stability of perovskite solar cells, which could further help to obtain an efficient and stable multidimensional (2D-3D) perovskites solar cell by using suitable approach. References: 1. W. S. Yang, B.-W. Park, E. H. Jung, N. J. Jeon, Y. C. Kim, D. U. Lee, S. S. Shin, J. Seo, E. K. Kim, J. H. Noh, Science 2017, 356, 1376. 2. Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Journal of the American Chemical Society 2009, 131, (17), 6050-6051. 3. Koh, T. M.; Thirumal, K.; Soo, H. S.; Mathews, N. ChemSusChem 2016, 9, (18), 2541-2558.

Resume : Lanthanide-doped perovskite films have exhibited near-infrared photoluminescence with significantly high quantum yields. In this study we investigate carrier transfer mechanism in this materials system using transient absorption spectroscopy. Large-bandgap quasi-2D perovskites and ytterbium (Yb3+), applied as a dopant, are used to generate efficient quantum-cutting effect. An infrared light emitting diode will be fabricated using the newly designed Yb3+-doped perovskites.

Resume : Recently, the fully inorganic perovskite nanocrystals (NCs) with luminous characteristics, CsPbX3, identical to those of CH3NH3PbX3, have been developed to address the stability issues. The use of colloidal perovskite CsPbX3 NCs have been widely studied because of their important properties, such as high photoluminescence quantum yield (PLQY), narrow size distribution, low cost, simple preparation, high reproducibility, narrow emission width as well as their easy color tenability via control of their halide composition [1,2]. Also, the perovskite NCs are applied to the various optoelectronic device such as light emitting diode, solar cell, photodetector, and so on [3]. Here, we report the various luminescence perovskite NCs using the exchange of halide ions which include the encapsulation for stability properties. The perovskite NCs take the place of the top of optoelectronic device. We also optimized the optical properties. Our results that perovskite NCs can be a potential candidate for optoelectronic device for next generation display applications.

Resume : Methylammonium lead halide perovskite solar cells have rapidly advanced to forefront of solution-processable photovoltaic devices, but the organic-inorganic halide perovskite material decomposes quickly in the ambient condition, limiting their commercial utility. In this work, we report an interface engineering of hole transport layer plays a role in achieving high device performance and stability. Here, the MoS2 flake as hole transport layer in planar perovskite devices achieved higher power conversion efficiency than PEDOT:PSS and PTAA layer. The electrical properties at the interface were investigated by using KPFM and C-AFM. The analysis allowed us to quantify the carrier transport and the decompose state at interface between hole transport layer and perovskite.

Resume : Imitating signal process of the human brain has great potential for future devices beyond conventional von Neumann architecture. Synaptic behavior, especially its synaptic plasticity and non-volatility, is a key parameter for memory and learning. Time-dependent memristor is considered to be the appropriate platform to mimic synaptic system due to their similarity. We investigated synaptic behaviors in electrical and/or light-tunable memristive organic-inorganic halide perovskite (OHP), which is promising material for solar cell, light-emitting diodes and photo-detector. OHP films were fabricated with various halogen anion compositions using spin-coating method. Electrochemically inactive (e.g. gold) and active metals (e.g. silver) were used for top electrodes. Measurement of synaptic behaviors was performed in both in-plane and out-of-plane directions. OHP films were demonstrated to imitate the synaptic functionalities efficaciously, such as repeated potentiation-depression, short-term and long-term plasticity. Also, electrical response of the device with respect to external optical signals was observed.

Resume : All-inorganic perovskite quantum dots have attracted substantial attention due to their excellent optoelectronic properties, such as large absorption coefficient, long diffusion length and tunable direct bandgap. However, CsPbBr3 QDs, the same as conventional colloidal quantum dots, suffer from the surface states and insufficient carrier transport in a quantum dot film, which hinder their further development. In this work, solution-processed CsPbBr3/ZnO quantum dot/nanoparticle nanocomposites are used to lessen the impact of surface states as well as facilitate charge transport. The cooperation of ZnO nanoparticles during CsPbBr3 quantum dot synthesis leads to improved optical properties as well as film formation that enhances carrier transportation. A photodetector based on the CsPbBr3/ZnO/glassy-graphene heterostructure is fabricated, which exhibits an enhanced photoresponse with improved photocurrent and suppressed dark current, and distinct self-powered operation with an open-circuit voltage as large as 150 mV. Most importantly, an excellent stability of the hybrid nanoparticle/quantum dot photodetector is demonstrated and consistent high performance with negligible degradation is achieved for more than 7 months. Acknowledgements: The authors acknowledge the support of the EPSRC grants (EP/L018330/1 and EP/P006973/1) and the financial support of Innovate UK under High-Prospect project (102470).

Resume : Metal halide perovskites are promising candidates for use in light emitting diodes (LEDs), due to their potential for color tunable and high luminescence efficiency. While recent advances in perovskite-based light emitting diodes have resulted in external quantum efficiencies exceeding 12.4% for the green emitters, and infrared emitters based on 3D/2D mixed dimensional perovskites have exceeded 20%, the external quantum efficiencies of the red and blue emitters still lag behind. A critical issue to date is creating highly emissive and stable perovskite emitters with the desirable emission band gap to achieve full-color displays and white LEDs. Herein, we report the preparation and characterization of a highly luminescent and stable suspension of cubic-shaped methylammonium lead triiodide (CH3NH3PbI3) perovskite nanocrystals, where we synthesize the nanocrystals via a ligand-assisted reprecipitation technique, using an acetonitrile/methylamine compound solvent system to solvate the ions and toluene as the anti-solvent to induce crystallization. Through tuning the ratio of the ligands, the ligand to toluene ratio, and the temperature of the toluene, we obtain a solution of CH3NH3PbI3 nanocrystals with a photoluminescence quantum yield exceeding 93% and tunable emission between 660 and 705 nm. We also achieved red emission at 635 nm by blending the nanocrystals with bromide salt and obtained perovskite-based light emitting diodes with maximum electroluminescent external quantum efficiency of 2.75%.

Resume : Over the last years we have witnessed the explosive rise of lead-halide perovskites. In an effort towards new materials that do not contain Pb, another class of compounds has also emerged; the so-called halide double perovskites. Today, four inorganic crystals have been successfully synthesized and characterized; Cs2BiAgCl6, Cs2BiAgBr6, Cs2SbAgCl6, and Cs2InAgCl6. Among these, Cs2BiAgBr6 has the narrower indirect band gap of 1.9 eV, and Cs2InAgCl6 is the only direct band gap semiconductor, yet with a large gap of 3.3 eV. All of them exhibit low carrier effective masses and consequently, are prominent candidates for opto-electronic applications such as photovoltaics, light-emitting devices, sensors, and photo-catalysts. In this talk, we will first outline the computational design strategy that lead to the synthesis of such compounds [1]. We will focus on the insights we can get from first-principles calculations in order to facilitate the synthesis, improve their opto-electronic properties and the in-silico identification of compounds with properties that are similar to the lead-halide perovskites [2]. We will also show how to employ ab initio calculations and explore more fundamental properties of the materials such as their surface properties, and identify compounds that might be suitable for photo-catalysis like Cs2BiAgCl6 and Cs2BiAgBr6 [3]. Finally, we will use this developed computational design strategy in an attempt to broaden the search-space for novel low-band gap semiconductors. [1] Patent WO 2017/037448 Al (2015); JPCL 7 1254 (2016); JPCL 8 772 (2017) [2] JPCL 8 3917 (2017) [3] APL 112 243901 (2018)

Resume : The spatial localization of charge carriers to promote the formation of bound excitons and concomitantly enhance radiative recombination has long been a goal for luminescent semiconductors. Zero-dimensional materials structurally impose carrier localization and result in the formation of localized Frenkel excitons. We present fully inorganic, perovskite-derived zero-dimensional SnII material Cs4SnBr6 that exhibits room-temperature broad?band photoluminescence centered at 540 nm with a quantum yield (QY) of 10-20?% [1]. A series of analogous compositions following the general formula Cs4-xAxSn(Br1-yIy)6 (A=Rb, K; x?1, y?1) can be prepared. The emission of these materials ranges from 500 nm to 620 nm with the possibility to compositionally tune the Stokes shift and the self-trapped exciton emission bands. We also present the synthesis, the structure as well as electronic and optical properties of a family of hybrid tin (II) bromide compounds comprising guanidinium [G, C(NH2)3 ] and mixed cesium-guanidinium cations: G2SnBr4, CsGSnBr4, and Cs2GSn2Br7. G2SnBr4 has a one-dimensional structure that consists of chains of corner-shared [SnBr5]2- square pyramids and G cations situated in-between the chains. G2SnBr4 is a luminescent phase with a broad emission band resulting from trapped excitonic states. Cs exhibits pronounced structure-directing effect: with a mixture of Cs and G cations mono and bilayer two-dimensional perovskites, CsGSnBr4 and Cs2GSn2Br7 are formed. The dimensionalities of the crystallographic structures have a direct impact on the electronic structures and the experimental optical band gaps are consistent with quantum confinement effects predicted by first principle simulations. Moreover, the flat shape of guanidinium cations induces anisotropic out-of-plane tilts of the [SnBr6]4- octahedra in the CsGSnBr4 and Cs2GSn2Br7 compounds. The related strong anisotropies of the halide perovskite lattice distortions have in turn a direct influence on their electronic and optical properties. We will also present several unconventional applications of such metal halide luminophores, which harness molecular-like, moderately fast and single exponential radiative recombination kinetics. In addition, we will discuss similar antimony-based compounds, which benefit from higher chemical robustness as compared to 0D Sn halides. References: 1. B.M. Benin, D.N. Dirin, V. Morad, M. Wörle, S. Yakunin, G. Rainò,O. Nazarenko, M. Fischer, I. Infante, M.V. Kovalenko et al. Angew. Chem. 2018, 57, 11329?11333 2. O. Nazarenko, M. R. Kotyrba, S. Yakunin, M. Wörle, B. M. Benin, G. Raino?, F. Krumeich, M. Kepenekian, J. Even,| C. Katan, M. V. Kovalenko et al. submitted

Resume : Halide perovskite have attracted enormous interest during the last decade due to their outstanding electronic properties. However, there are still unsolved issues that keep halide perovskites out of large-scale market: their instability and lead toxicity. To tackle the second issue, we have investigated halide perovskites based on Ge and Sn and silver bismuth iodides. We identified few lead-free halide perovskite compounds that can replace Pb-based ones and we discuss their stability with respect to decomposition and oxidation. We have additionally found that silver bismuth iodides, despite having band gaps suitable for solar cell application, have few intrinsic drawbacks which limit their solar cell efficiency.

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 : Layered 2D lead-halide hybrid perovskites feature a set of properties, as large exciton binding energies, sharp emission, multiexciton resonances, which make them very intriguing for opto-electronics. In this sense, a robust description of the nature of the photogenerated species and of their relaxation pathways is highly coveted, to envisage an effective use of these materials in real devices. To this task, we performed resonance impulsive stimulated Raman spectroscopy on two widely studied 2D perovskites, phenylammonium and buthylamminium lead iodide, observing specific resonances associated to photoexcitation, in the frequency range below 50 cm-1. Selective photoexcitation demonstrated specific coupling for two (previously identified) distinct excitonic transitions and for free carriers, suggesting complex relaxation pathways for photogenerated species. These data are well described in the context of theory of polarons. Indeed, vibrational frequencies from DFT simulations well reproduce the experimental oscillations and provide a fingerprint of the lattice motion, and of the corresponding electron-phonon couplings (in the range of 5-20 meV). Notably, related Huang-Rhys factors are comparable to those estimated in organic materials (few units), but with both relaxation energies and lattice motions lying on much smaller energy scale. Thermal effects and structural order are also studied, by means of Raman spectroscopy and molecular dynamics simulations.

Resume : Metal halide perovskites (MHPs) have emerged as a very high promising materials for optoelectronics and photonics, mostly due to their large absorption coefficient and excellent photoluminescence quantum yield (PLQY) at room temperature. Particularly, PLQY will depend on carrier recombination dynamics that can be mostly explained on the basis on the existence of shallow non-quenching traps under relatively low excitation powers [1], even if surface recombination and diffusion can play a relevant role in polycrystalline thin films [2]. Under high excitation power these traps are filled and bimolecular recombination is the main radiative recombination channel, which makes possible the observation of stimulated emission with very low thresholds in MHP films integrated on polymer waveguides both on rigid [3] and flexible substrates [4]. Finally, our most recent research activity in the study of optical properties of 2D/3D-MHP materials will be summarized, given their importance for more efficient emitting devices and solar cells with higher stability [5]. [1] V. S. Chirvony et al., J. Phys. Chem. C 121, 13381-13390 (2017). [2] Unpublished results. [3] I. Suárez et al., Advanced Materials 27, 6157–6162 (2015). [4] I. Suarez et al., Advanced Optical Materials 6, 1800201 (8 pp) (2018). [5] J. Rodríguez-Romero et al., Phys. Chem. Chem. Phys. 20, 30189-30199 (2018).

Resume : Organic-inorganic perovskites (HOP) have attracted a growing interest in the past decade due to their remarkable photophysics properties. One of the most promising perovskite type are the Ruddlesden-Popper (RP) 2D hybrid perovskites (2D-HOP) due to their chemical versatility and their remarkable self-assembled quantum well structures. Recently, solar cells and LED devices based on 2D-HOP have demonstrated high efficiencies and improved stabilities. [1,4] The study of large crystals of HOP rather than thin films has often proven necessary to unveil some of the intrinsic properties of hybrid perovskites. [2,3] In this work, we have studied the optical properties of single crystals of Phenylethylammonium based RP phases with n=1,2 and 3, using a confocal time-resolved photoluminescence microscope. For each n values, we unveil some remarkable time resolved photoluminescence dynamics in these materials, with the presence of a long nanosecond component. Fluence dependent measurements prove that these long dynamics are linked to an efficient conversion of excitons into free carriers despite large exciton binding energies in these materials. This discovery shows that it is possible to control the exciton/free carriers ratio in the RP perovskites, opening new perspectives for devices applications. 1) H. Tsai, et al., Nature 536, 312 (2016). 2) H. Diab et al., J. Phys. Chem. Lett. 7, 5093 (2016). 3) J. C. Blancon, et al., Science 355, 1288 (2017). 4) H.Tsai et al., Advanced Materials 30, 6 (2018). The project leading to this application has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 687008 (GOTSolar).

Resume : 2D family of perovskites have recently attracted increasing attention due to their great environmental stability. The research of this natural perovskite quantum wells is in infancy stage and their structural, dielectric, optical, and excitonic properties remain to be explored especially influence of organic spacers. The optical properties of these materials are dominated by strong electron-hole attraction resulting from dielectric confinement. Very often this materials exhibits complex absorption and emission spectrum with many sidebands which origin rises controversy since they are attributed to exciton-phonon coupling or bound excitonic states. In this work we address this issue for (CnH2n+1NH3)2PbI4 (with n=4,6,8,10,12) by means of optical spectroscopy in magnetic field up to 67T. We show that complex absorption features shift parallel in magnetic field indicating phonons related nature of the observed side bands. Moreover we found that the diamagnetic shifts of high and low temperature crystal phase of investigated structures are significantly different pointing on notable modification of carrier effective mass and/or dielectric screening upon phase transition and related octahedral distortion. Since the phase transition occurs close to room temperature this founding can provide additional way for 2D perovskite properties engineering.

Resume : Quasi two-dimensional or layered perovskites have made tremendous progress in a range of optoelectronic devices including light emitting diodes (LEDs). Perovskite based LEDs (PeLEDs) now demonstrate external quantum efficiency (EQE) higher than those of best organic and quantum-dot based LEDs without additional light outcoupling. Despite the remarkable performance shown by PeLEDs, a widespread observation is that their EQE drops with increasing injection current density, J – a process known as efficiency roll-off. Typical values of threshold current density (J0, J at 50% EQE drop) is between 5 – 30 mA/cm2. Various mechanisms such as light trapping in a multi-layer device design, Auger recombination and Joule heating are reported to be among possible mechanism causing EQE drop. Herein, we provide detailed insights into another plausible mechanism – charge injection balance using two most common perovskites, e.g., those employing a three-dimensional crystal structure as well as those employing a layered architecture (the so-called quasi two-dimensional perovskites). From an initially electron dominant device, we systematically tune electron injection into the device via doping of organic small molecules in electron transport layer to create charge balance. We report an EQE >16% in optimized devices which double from a reference device with imbalanced charge injection. We also note an exponential improvement in the device lifetime upon charge balance, and nearly 100% increase in J0 for optimized devices.

Resume : Fine control over the crystallization is the key of making any technologies based on a given novel crystalline material reliable. In this work a common mechanism underlying of hybrid perovskite nanowire formation will be discussed in detail [1]. The central role of the solvatomorph phase as the intermediate phase in crystallization will be highlighted. Next, our latest findings on the guided growth of perovskite nanowires by ‘solvatomorph-graphoepitaxy’ will be presented [2]. This method turned out to be a fairly simple approach to overcome the spatially random surface nucleation. The process allows the synthesis of extremely long (centimeters) and thin (a few nanometers) nanowires with a morphology defined by the shape of nanostructured open fluidic channels. This method might allow the integration of perovskite nanowires into advanced CMOS technologies. Solvatomorph-graphoepitaxy method could open up an entirely new spectrum of architectural designs of organometal-halide-perovskite-based heterojunctions -and tandem solar cells, LEDs, photodetectors and new type of magneto-optical data storage devices [5]. 1. Horváth et al. Nano Letters, 6761-6766 (12), (2014) 2. Spina et al. Scientific Reports, 6 (2016) 3. Spina et al. Small, 4824-4828, 11 (2015) 4. Spina et al. Nanoscale, 4888 (8) (2016) 5. Náfrádi et al. Nature Communications 7, 13406 (2016) Acknowledgement: This work was supported by the ERC Advanced Grant (PICOPROP#670918).

Resume : Research of perovskite thin film PV technology has resulted in power conversion efficiency above 23 % for a cell and 17 % for a module within less than a decade. To become commercially viable perovskite TFPV devices still needs to achieve long term stability and scalability with minimal performance losses. Upscaling from cell to module with monolithic serially interconnected cells introduces several losses. In our devices, layer inhomogeneities represent a loss mechanism that scales up with module size and represent up to 40 % of the total loss in aperture area efficiency going from a 0.1 cm2 cell to a 4 cm2 module. We use dark lock-in thermography, electroluminescence and micro photoluminescence for post-processing analysis of losses in solution processed perovskite modules, focusing on layer inhomogeneities. Analyzed 4 cm2 n-i-p modules are processed using either spin coating or blade coating. We identify four different types of particle inhomogeneities grouped based on the identification method and their effect on module performance. Moreover, electron microscopy is used to further analyze origin of identified inhomogeneities and extract lessons learned for further module optimization. The applicability of the method is demonstrated using 100 cm2 blade coated modules. Finally, we present unique aspects of perovskite module characterization using luminescence techniques. Presented methods can be used both in research and industrial production of perovskite upscaled devices.

Resume : Recently, the wide color gamut (WCG) of the white LEDs is an important issue for more realistic colors in displays. Therefore, lead halide perovskite (LHP) nanocrystals (NCs) have attained great attention as the candidates for optoelectronic devices due to their outstanding optical properties. Although, the synthesis and optical characteristics of Cs4PbBr6 solid have been reported, to the best of our knowledge, there has been no report on the large-scale production of the Cs4PbBr6 solid. In this study, we report for the first time the mass production of Cs4PbBr6 perovskite microcrystal with a Couette-Taylor flow reactor in order to enhance the efficiency of the synthesis reaction. Through this continuous method, we improved the synthesis scale over 500 times compared with the stirring method and obtained a pure Cs4PbBr6 perovskite solid within 3 hrs that then realized a high photoluminescence quantum yield (PLQY) of 46%. Furthermore, the Cs4PbBr6 perovskite microcrystal is applied with red emitting K2SiF6:Mn4+ phosphor on a blue-emitting InGaN chip, achieving high-performance luminescence characteristics which encompass 118% compared with National Television System Committee (NTSC) value. Therefore, this perovskite is expected to be a promising candidate material for applications in optoelectronic devices.

Resume : Colloidal quantum dots (CQDs) may have strong light absorption, allowing use of very thin CQD solid films for CQD solar cells with high power conversion efficiency (PCE) and a light conversion spectrum covering the ultraviolet-visible-near infrared region. We have prepared flexible and ultra-lightweight CQD solar cells with a solution-processed Ag nanowire network with good mechanical properties as the front transparent and conductive electrode. The thickness of the complete CQD solar cell is less than 2 µm, and ~10 % PCE with a weight of 6.5 g /m2 is achieved. The flexible solar cell shows good mechanical properties and maintains the photovoltaic performance under extreme deformation. We have also prepared semitransparent CQD solar cells with a combination of high light transmission and high solar cell efficiency, which also can be interesting in many applications, for example windows in buildings. In the presentation we also show how the surface defects of the CQDs can be minimized using different surface shells of the QDs, yielding highly efficient solar cells.

Resume : (no abstract; accepted last minute invited talk)

Resume : This presentation will be about doped Halide perovskite nanocrystals that hold promise for printable optoelectronic and photonic applications. Doping enhances their functionalities and is being investigated for substituting lead with environmentally friendlier elements. The most investigated dopant is Mn2 that acts as a color center sensitized by the host excitons. The sensitization mechanism is far from understood and no comprehensive picture of the energy-transfer process has been proposed. Similarly, the role of shallow states, particularly abundant in defect tolerant materials, is still unknown. Here, we address this problem via spectroscopic studies at controlled excitation density and temperature on Mn:CsPbCl3 nanocrystals. Our results indicate a two-step process involving exciton localization in a shallow metastable state that mediates the thermally assisted sensitization of the Mn2 emission, which is completely quenched for T < 200 K. At T ≤ 60 K, however, such emission surprisingly reappears, suggesting direct energy transfer from band-edge states. Electron spin resonance supports this picture, revealing the signatures of conformational rearrangements below 70 K, possibly removing the potential barrier for sensitization. Our results demystify anomalous behaviors of the exciton-to-Mn2 energy-transfer mechanism and highlight the role of shallow defects in the photophysics of doped perovskite nanostructures.

Resume : Perovskite-based devices have sprung to the forefront of photovoltaic research activity due to efficiency records up to 23% in single junction devices in less than a decade. Despite this rapid progress in device performance, one issue still puzzling the community is ionic conductivity and its effect on electronic conductivity. Mobile ions have the potential to profoundly disturb the device performance as they can shield the bulk material from externally applied voltages, and consensus has been reached that mobile ions play a major role in the origin of hysteresis. To fulfil the need of a complete picture of ionic effects on electronic transport mechanisms, we focus on methylammonium lead bromide single crystal (SC) perovskites, as they stand out as an ideal platform to study intrinsic properties of the bulk material. Contrary to previous reports on SCs, we highlight how mobile ions affect the shape and magnitude of the current-density voltage (JV) characteristics by localizing ions in both time and space, during the intentioned ‘space-charge limited current’ measurements. We introduce a well-defined voltage scanning protocol to achieve reproducible JV curves without hysteresis, and identify that the onset of conductivity, which has been repeatedly reported to be due to defect states in the band gap, is rather an artefact due to the presence of mobile ions. From our revised ‘pulsed’ JV sweep, we find it possible to determine the true electronic charge carrier mobility in the SCs.

Resume : The device performance of perovskite solar cells is known to increase when devices are stored in dry and dark conditions at room temperature. In a previous work we showed that this is largely due to coalescence of small perovskite crystals, resulting in bigger crystals with less defects.1 In this follow-up study we investigate what drives the grain boundary migration that causes coalescence. By understanding what drives coalescence we can exploit the phenomenon to our advantage to fabricate better devices. We find that the key factors governing coalescence are composition and residual solvent. Iodine rich compositions exhibit more pronounced coalescence than iodine poor compositions and coalescence is not observed at all in pure bromide compositions. This indicates that the high migration mobility of iodide ions is key to coalescence. Secondly, we find that residual solvents such as DMSO, or moisture from humid air, can retard coalescence by complexing PbI2, which immobilizes the grain boundaries. In conclusion, this work shows that in order to optimally benefit from coalescence, perovskite films should contain a high fraction of iodine, solvents have to be fully removed during synthesis and contact with humid air should be avoided at all times. 1. B. Roose et al. Nano Energy 2017, 39, 24-29

Resume : Recently, halide perovskite families have been studied for light-emitting applications after their success in photovoltaics. Beyond regular three-dimensional perovskites based on a corner-sharing octahedral network, Cs4PbBr6, which is called as zero-dimensional perovskite, is getting attention because of its highly-efficient green light luminescence. However, green emission in Cs4PbBr6 is not due to its intrinsic band gap as band gap of this material is 3.9 eV. Two hypotheses have been suggested to explain this phenomenon: (i) CsPbBr3 phase impurities assisted green luminescence and (ii) point defects assisted green luminescence. By using state-of-the-art first-principles simulations we investigate the origin of green luminescence in Cs4PbBr6 and show the formation of polybromide Br3 defects can cause green light emission.

Resume : Colloidal organic/inorganic lead halide perovskite nanocrystals (NCs) are intensely pursued as versatile classical light sources and quantum emitters. These NCs exhibit unprecedented luminescent properties – narrow-band emission with high quantum efficiency, covering the whole visible spectral range and extending into near-infrared, all obtained without epitaxial overcoating of the NC surfaces for electronic passivation of the surface states [1]. Their processing and luminescent properties are challenged by the lability of their surfaces, i.e. the interface of the NC core and the ligand shell. Surface and sub-surface atoms are likely directly involved in all possible chemistry equilibria and transformations. Controlling NC surface structure is therefore paramount for mitigating these instabilities. On the example of CsPbBr3 NCs, we rationalize the typical observation of a degraded luminescence upon aging or the luminescence recovery upon post-synthesis surface treatments using a simple surface-structure model, supported by DFT calculations [2]. Healing of the surface trap states re-quires restoration of all damaged PbX6 octahedra and establishing a stable outer ligand shell. Restoration of such a structure, seen as an increase in the luminescence quantum efficiency to 90-100% and improvement in the overall robustness of CsPbBr3 NCs, was attained using a facile post-synthetic treatment with a PbBr2+DDAB (didodecyldimethylammonium bro-mimde) mixture. In practical terms, we demonstrate that such an approach is useful to obtain purified CsPbBr3 NCs samples, washed up to four times in several solvents, with near unity photoluminescence quantum yields and long-term colloidal stability. We also demonstrate that these surface chemistries led to an improved performance of CsPbX3 NCs-based light emitting diodes. We will also comment on the relationship between the surface chemistry and photophysics of formamidinium-based NC counterparts. 1. M. V. Kovalenko, L Protesescu, M. I. Bodnarchuk. Science 2017, 358, 745-750 2. M. I. Bodnarchuk,S. C. Boehme, S. ten Brinck, C. Bernasconi, Y. Shynkarenko, F. Krieg, R. Widmer, B. Aechlimann, D. Günther, M. V. Kovalenko, I. Infante. ACS Energy Letters 2018, 4, 63–74

Resume : Organic-inorganic (hybrid) perovskites have become one of the most studied photovoltaic materials thanks to their unique properties, such as the high absorption coefficient, high carrier mobility and diffusion length, which allow an efficient photocurrent generation with minimal non-radiative losses. Recently, perovskites have been also investigated for applications in lasers and light-emitting diodes. Their application in LEDs requires a precise control over their morphology, since this strongly influences the perovskite optical and electronic properties. In particular, perovskites with high photoluminescence quantum yield are desirable for the preparation of efficient LEDs, as they would lead to enhanced external quantum efficiency for electroluminescence. One of the most successful strategies to control the morphology and enhance the photoluminescence of perovskites is the preparation of nanostructured materials, such as colloidal nanocrystals. However, the current preparation methods for perovskite nanocrystals are rather complex. In this work, we will describe a simple method to prepare red-emitting hybrid perovskite nanocrystals, and discuss the structural and optical properties of the materials as a function of the synthesis conditions, such as some precursors ratio, use of anti-solvent or the synthesis mixture preparation sequence. Upon dispersion in a suitable solvent, these perovskite nanocrystals can be easily processed into homogeneous thin films, which is promising for the preparation of simple and inexpensive light-emitting devices.

Resume : All-inorganic perovskite nanocrystals (NCs) CsPbX3 (X = Cl, Br, I) have great potential for optoelectronic applications, such as in LEDs and solar cells.1 However, the poor stability and deteriorating photoluminescence quantum yield (PLQY) of deposited NC layers represent still great challenges which need to be overcome before practical devices can be considered. One possible approach to achieve that could be by encapsulation of the perovskite NCs with a protective and robust layer, for example, of silica or Cs4PbX6.2-4 TEM images of CsPbBr3 encapsulated by Cs4PbBr6 layers reveal uniform NCs which feature a round edge, which is a transitional morphology between nanocubes for CsPbBr3 and the hexagonal morphology for Cs4PbBr6. The absorption spectra of these structures feature a pronounced peak at ~ 310 nm, characteristic of Cs4PbBr6 as well as the excitonic peak at ~ 510 nm from CsPbBr3. Furthermore, the characteristic lattice fringe for CsPbBr3 can be indexed to (200) and (122) for the internal part of the NCs. Combining these data with the uniform morphology of the structure, we suggest that a Cs4PbBr6 layer was formed on the surface of the CsPbBr3 perovskite NCs making a core-shell system. We further investigate how tuning of the thickness of the surface layer will influence their optical and electronic properties. For NCs with different core- diameter-to-shell-thickness ratios we measure PLQY and PL lifetime through time-resolved photofluorescence (TRPL) spectroscopy over a wide temperature range. The conductivity of deposited thin layers of core-shell NCs, which is a very important parameter for device development, is also studied by time-resolved microwave conductivity (TRMC), and their stability versus light illumination and humidity. In that way, the practical potential of perovskite NCs for future optoelectronic applications is evaluated. Reference 1. E. M. Sanehira et al, Sci. Adv., 2017, 3, eaao4204. 2. H. C. Hu et al, J. Am. Chem. Soc., 2018, 140, 406-412. 3. B. Wang et al, ACS Appl. Mater. Interfaces, 2018, 10, 23303-23310. 4. C. Sun et al, Adv. Mater., 2016, 28, 10088-10094.

Resume : Near-infrared (NIR) light sources have a wide range of applications such as night-vision devices, optical communications, etc. The attempts of using thin film materials, e.g. organic semiconductors and lead chalcogenide quantum dots, have faced fundamental challenges that limit the improvement of external quantum efficiency (EQE). This makes the search of alternative NIR emitters important for the community.In this work, we exploit mixed Pb-Sn perovskites for efficient NIR LEDs with tunable emission peaks between 850 nm and 950 nm. We have demonstrated a high-efficiency NIR LED with an EQE of 5.0%, a low turn-on voltage of 1.65 V, and an emission peak of 917 nm. The EQE value is amongst the highest values reported for thin film NIR LEDs, and is achieved by the fabrication of a facile solution-processed nanocrystalline MAPb0.6Sn0.4I3 film with the aid of FPMAI additives. The emission spectra of mixed Pb-Sn perovskites are tuned either by changing the Pb:Sn ratio or by incorporating bromide, and notably exhibit no phase separation during device operation. The work demonstrates that mixed Pb-Sn perovskites are promising next generation NIR emitters. Reference: W. Qiu, Z. Xiao, K. Roh, N. K. Noel, A. Shapiro, P. Heremans, B. P. Rand, Advanced Materials, 2018, https://doi.org/10.1002/adma.201806105.

Resume : The easily tunable emission of halide perovskite nanocrystals throughout the visible spectrum makes them an extremely promising material for light-emitting applications. Whereas high quantum yields and long-term colloidal stability have already been achieved for nanocrystals emitting in the red and green spectral range, the blue region currently lags behind with low quantum yields, broad emission profiles, and insufficient colloidal stability. In this work, we present a facile synthetic approach for obtaining two-dimensional CsPbBr3 nanoplatelets with monolayer-precise control over their thickness, resulting in sharp photoluminescence and electroluminescence peaks with a tunable emission wavelength between 432 and 497 nm due to quantum confinement. Subsequent addition of a PbBr2-ligand solution repairs surface defects likely stemming from bromide and lead vacancies in a subensemble of weakly emissive nanoplatelets. The overall photoluminescence quantum yield of the blue-emissive colloidal dispersions is consequently enhanced up to a value of 73 ± 2 %. Transient optical spectroscopy measurements focusing on the excitonic resonances further confirm the proposed repair process. Additionally, the high stability of these nanoplatelets in films and to prolonged ultraviolet light exposure is shown.

Resume : Fully inorganic lead halide perovskite nanocrystals (LHP NCs) have shown very promising optical and electrical properties and also solved the stability issue of their organic-inorganic hybrid perovskite predecessors. The facile synthesis of LHP NCs and tunabilty in composition and size makes it a very versatile and exciting material. The mechanism behind the superior optical properties has been at the focal point of much research. In this work we explore several of the suggested explanations, including the formation of polarons, and energetically favorable defect states. For that the optical properties of CsPbBr3 and CsPbI3 perovskite nanocrystal films are studied from 5 K to room temperature via time-integrated and time-resolved photoluminescence and absorption measurements. Then, these results examined within the frameworks layed out in literature for both polaron as well as defect formation. In particular polaron formation has been found unlikely, in line with the results of PL investigations of single NCs.

Resume : It has been long known that numerous halide perovskites, as well as some oxide perovskites have non-ideal octahedra, with tilting, rotation, and metal atom displacements appearing as a single structural motif (“monomorphous structures”). It has also been known that even compounds that have similar octahedra at low temperatures would become at higher temperatures disordered, showing such non-ideal octahedra as an entropy effect. What is shown here is that structures having a distribution of different octahedra (“polymorphous networks”) emerge from the minimization of the systems internal energy, not as a thermal effect. Compared with the monomorphous counterpart, the polymorphous networks have lower total energy, significantly larger band gaps, and agrees much more closely with the observed pair distribution functions. The nominal cubic perovskites (Pm-3m) structure deduced from X-Ray diffraction is actually a macroscopically averaged configuration, which should not be used to model electronic properties, given that the latter reflect local configurations.  

Resume : Full inorganic perovskites display their potential to function as stable photovoltaic materials better than the hybrid organic−inorganic perovskites. However, to date, the cesium lead iodide perovskite, which displays a promising absorbance range, has suffered from low stability, which degrades to a non-active photovoltaic phase rapidly. We have recently showed that the black phase of cesium lead iodide can be stabilized when the perovskite dimensionality is reduced [A. S. Dayan et al., Chem. Mat., 30, 8017 (2018)]. X-ray diffraction (XRD), absorbance, and SEM were used to follow the degradation process of various dimensionalities under room conditions and 1 sun illumination. The aromatic barrier molecule displays better photostability for over 700 h. Theoretical calculations show that the addition of the barrier molecule makes a different charge distribution over the perovskite structure, which can make the CsPbI3 black phase perovskite as a stable photovoltaic material in solar cells. Note that the solvent used in the sample preparation (DMF, for instance) can also influence the sample stability if inserted in the composition [W. Ke et al., Nature Comm., 9, 4785 (2018)]. CsPbI3 (3D and 2D films) and CsPbBr3 (nanoparticles and thin films) were also studied in detail by in-situ XRD as a function of temperature. In the latter case, the CsPb2Br5 secondary phase disappears completely above 580K and pure cubic CsPbBr3 is observed up to 623K [C. Tenailleau et al., Nanoscale Adv. (2019)]. The CsPbBr3 phase is then kept upon cooling down to RT. This study provides detailed understanding of the phase behaviour vs T of CsPbBr3, which opens the way through pure CsPbBr3 phase, another interesting material for optoelectronic applications.

Resume : The tunability of the bandgap of perovskites makes them promising candidates for multijunction photovoltaics in combination with wide bandgap Si or CIGS. In addition, perovskite-perovskite tandem solar cells are a promising route to surpass the Shockley–Queisser limit of the single junction solar cell. While wide bandgap perovskite materials with bandgaps ranging from 1.6-1.8eV have been explored extensively in research, low bandgap perovskites (LBG) with bandgap < 1.3eV are much less investigated. The most established LBG perovskites explores the composition of MAyFA1-yPbxSn1-xI3, where most attention is given to tune the ratio of Sn to Pb, which determines the bandgap. In this work, we focus on the composition of the organic cation and vary the MA and FA ratio of the LBG perovskite solar cells in order to optimize the PCE and demonstrate stable power output. By keeping the stoichiometric ratio of Sn and Pb fixed (1:1), the optoelectronic properties of the LBG perovskite solar cells with different cation ratio of the MA and FA in the MAyFA1-yPbxSn1-xI3 is investigated. By incorporation of the 75% FA in the structure MA0.25FA0.75Pb0.5Sn0.5I3, the PCE of the devices increases up to a stable power output efficiency of 17.1% (5 min MPP tracking). The fabricated four-terminal Perovskite-Perovskite tandem solar cell with MA0.25FA0.75Pb0.5Sn0.5I3 composition of the LBG perovskite (with EG ~1.25eV) and semitransparent WBG perovskite (with EG ~1.63eV), presents 21.8% PCE under one sun illumination.

Resume : Transition-metal dichalcogenide(TMDC) in heterojunction with perovskite materials is promising for improving the performance in optoelectronic devices with assisting the charge transport. Investigation of the charge transport mechanism in TMDC/perovskite heterostructure is important for providing the possibility to assist the charge separation process which aids the advanced device design. Also, those interface properties of TMDC and perovskite materials influence on the performance of optoelectronic devices significantly. At this point, we fabricated the several heterostructures, and conductive atomic force microscopy was used to display the dark current and photocurrent generated in the heterostructure and electrical contacts of the heterojunctions. Analysis of the current maps reveal that the heterojunctions play a dominant role in the charge transport. Furthermore, Kelvin probe force microscopy was conducted to demonstrate the band structure of the heterostructure. Those results provide the local electrical properties in nanoscale region which have the direct correlation with the efficient heterojunction.

Resume : In this work, we introduce the use of porous metal oxide (MOx) thin films as scaffold of controlled nanopore size distribution to synthesize nc-ABX3 through the infiltration of perovskite liquid precursors. We demonstrated that the reaction volume imposed by the nanoporous scaffold leads to the strict control of the nanocrystal size, which allows us to observe well-defined quantum confinement effects on the photo-emission, being the luminescence maximum tunable with precision. This hybrid nc-MAPbBr3/MOx films presents high photoluminescence quantum yield (>50%), high optical quality, mechanical stability and permits subsequent elastomer infiltration to achieve a self-standing flexible film, which maintains the photo-emission efficiency of the nc-AMX3 unaltered and prevents the degradation from the external environment (water, humidity). Applications as adaptable color-converting layers for light-emitting devices are envisaged and demonstrated.

Resume : Low-temperature solution-processed metal halide perovskites have demonstrated great advances in the applications of high-performance electroluminescence (EL) devices, which has pushed the external quantum efficiencies (EQEs) of perovskite LEDs over to 20%, showing a great application potential. However, systematic investigations on how to tune film quality and device performance are still missing. Here, we demonstrate that through engineering the precursor stoichiometry of the perovskite precursor solution and the interfacial reaction, black-phase and high-luminescence MAPbI3 perovskite nanocrystals can be obtained in vacuum condition at room temperature. With the removal of the organic components during the vacuum-annealing process, only the films processed on ZnO exhibit good crystallinity while the crystallization of MAPbI3 on TiOx and SnO2 is greatly retarded. The deprotonation of the excess organic cations induced by a basic ZnO interface is critical to eliminate the undesired precursor components and promote the transition of intermediate phases into high-quality perovskite nanocrystals. Our results reveal the critical effect of interaction at ZnO/perovskite interface on the perovskite crystallization and device performance, which further provide new insights into the design of ZnO-based Qusi-2D, 3D and quantum dots perovskite optoelectronic devices.

Resume : In this research, we demonstrate regular-type perovskite solar cells based on tungsten trioxide nanoparticles layer (WO3 NL) as the electron transport layer and methylammonium lead iodide nanowires (MAPbI3 NWs) as the absorbing layer. WO3 NL was prepared via the hydrothermal method with high transmittance in the visible range and good opto-electrical properties. By adjust the amount of the anti-solvent DMF, MAPbI3 NWs with different diameters ranging from 50 to 100 nm and lengths extending to several micrometers were obtained. The characterization of perovskite NWs including morphological investigation (SEM and AFM), crystallinity (XRD) and optical study (absorption, steady-state and time-resolved photoluminescence decay experiments) were carried out. To fabricate photovoltaic devices, [6,6]-Phenyl-C61- butyric acid methyl ester (PCBM) was introduced between WO3 NL and MAPbI3 NWs to improve charge transfer ability and device performance. Meanwhile, Spiro-OMeTAD doped with bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI) was used as the hole transport layer. Finally, the perovskite solar cells with the configuration of ITO/WO3 NL/PCBM/MAPbI3 NWs/spiro-OMeTAD+Li-TFSI/Au were fabricated and evaluated.

Resume : Manifested by high absorption coefficient, large carrier diffusion length, ambipolar charge transport and moderate mobility of charge carriers, organo-lead halide perovskites recently emerging as a promising candidate for high efficiency and low-cost solution processed solar cells, needs no introduction. Perovskite Solar Cells (PSCs) have already gathered considerable attention, showing a tremendous hike in efficiency from 3.8% [1] by Kojima et al. (2009) to 22.1% [2] by Yang et al. (2017). A lot of research has been done on the efficiency improvement of PSCs by optimization of the film morphology at the interfaces by various characterization techniques including electrochemical impedance spectroscopy (EIS) in a solid-state active device geometry.[3] Recently, the optimization of the film morphology at liquid electrolyte interface by EIS is trending as a more simplified approach. Li et. al. have measured the flat band potential, density and type of charge carrier at the perovskite-liquid interface from Mott-Schottky plot for spin coated and spray coated films of methylammonium lead tri-iodide (MAPbI3 ) perovskite. [4] Srivastava et.al have optimized the morphology of MAPbI3 perovskite thin films at the liquid interface by controlling nucleation and growth during film fabrication by spin coating the films on preheated substrates. [5] However, there is a need for understanding the charge transfer at these interfaces in more detail. Here, we have studied the kinetics of charge transfer and diffusion at the hybrid organic-inorganic perovskite liquid electrolyte interface under the effect of applied bias. By applying the different dc bias from 0 to 1V and 0 to -1V, it was found that the ion diffusion at low frequency regime gets modulated due to charge accumulation. The charge transport resistance is initially increased to a maximum at around 0.4 V applied bias along with the decrease in the space charge capacitance. A transition state is observed at around 0.4 V to 0.6 V due to the strong electronic-ion interaction where charge transport resistance decreases and capacitance increases. However at higher applied bias voltage charge transport resistance increases again and capacitance starts to decrease due to excess ion accumulation. The perovskite films show a similar trend of change in impedance under both positive and negative bias. The Mott-Schottky Plot for forward and reverse voltage scan shows n-type and p-type behaviour which indicates the ambipolar nature of perovskite semiconductor. The significant difference in the impedance spectra can be seen in dark and light which is attributed to the high absorption coefficient of the perovskite material. We have proposed a model to explain the charge kinetics across the perovskite-liquid electrolyte interface. References (1) Akihiro Kojima Yasuo Shirai, and Tsutomu Miyasaka, K. T. J. Am. Chem. Soc. 2009, 131, 6050–6051. (2) Yang, W. S.; Park, B. W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C.; Lee, D. U.; Shin, S. S.; Seo, J.; Kim, E. K.; Noh, J. H.; Seok, S.I. Science 2017, 356 (6345), 1376–1379. (3) Bag, M.; Renna, L. A.; Adhikari, R. Y.; Karak, S.; Liu, F.; Lahti, P. M.; Russell, T. P.; Tuominen, M. T.;Venkataraman, D. J. Am. Chem. Soc. 2015, 137 (40), 13130–13137. (4) Li, Z.; Mercado, C. C.; Yang, M.; Palay, E.; Zhu, K. Chem. Commun. 2017, 53 (16), 2467–2470. (5) Srivastava, P.; Parhi, A. P.; Ranjan, R.; Satapathi, S.; Bag, M. ACS Appl. Energy Mater. 2018, 1, 4420–4425.

Resume : Resistive switching (RS) memories have been actively researched as a promising candidate for substituting current non-volatile memories owing to their simple structure, high operation speed, scale-down ability, and low energy consumption. They also have shown their possibilities to be applied for neuro-computing as an artificial synapse. CsPbI3, all-inorganic halide perovskite, is considered as a strong candidate material for RS memories because of its remarkable on-off ratio and higher stability compared to organic-inorganic hybrid perovskites. However, the desired cubic perovskite black phase (α-CsPbI3) is not stable at room temperature and quickly degraded to orthorhombic yellow phase (δ-CsPbI3). Herein, RS characteristics of bismuth-doped CsPbI3 (CsPb(1-x)BixI3) are reported with high air-stability at room temperature. As bismuth has small ion radius (1.03Å) than lead (1.19Å), the partial substitution of Pb2 to Bi3 causes lattice distortion and generates micro-strain in the crystal lattice constructing a 2D-like structure. It makes CsPb(1-x)BixI3 more stable at room temperature. The RS devices of Ag/CsPb(1-x)BixI3/Pt achieve bipolar switching characteristics with high on/off ratio (∼108), multilevel data storage, remarkable stability (>1month), and on-off resistive switching by pulse voltage operation (pulse duration < 50ms). They also show synaptic behaviors, spike rate dependent plasticity, and spike timing dependent plasticity, generated by RS properties. This study provides the optimal bismuth doping amount and its effects on the CsPbI3 perovskites crystal structure related to their resistive switching mechanism and air-stability. By demonstrating their notable stability, synaptic behaviors, and resistive switching performances, it is proved that CsPb(1-x)BixI3 perovskites have high potential to be applied as next-generation non-volatile data storages and artificial synapses.

Resume : Resistive random-access-memory (RRAM) has emerged as a promising candidate as the next-generation memory technology because of their advantages such as non-volatility, low-operational energy, and simple operation principles based on resistive switching effect. During the last decade, RRAM devices based on solution-processed materials have been extensively explored owing to their easy and low-cost fabrication processes. Recently, resistive memory devices based on organo-metal halide perovskite materials have shown outstanding performances; a low-voltage operation and a high ON/OFF ratio which are essential for realizing low-power consumption memory. In this presentation, we report the results on unipolar resistive memory devices in a cross-point array architecture made by using a non-halide lead source to deposit perovskite films via a simple single-step spin-coating method. Our perovskite memory devices achieved a high ON/OFF ratio up to 10^8 with a relatively low operation, a large endurance, and long retention times. In addition, we discuss a potential resistive switching mechanism for our perovskite memory devices which exhibit a unique unipolar (non-polar) resistive switching unlike the previously reported perovskite memory devices. Furthermore, a direct demonstration of one-diode-one-resistor scheme using our cross-point perovskite memory devices demonstrated a selective operation of memory cells connected via external diodes. These results, combined with a high-yield device fabrication based on solution-process demonstrated here, will contribute towards developing low-cost and high-density practical perovskite memory devices. Reference 1. K. Kang et al. manuscript under revision.

Resume : Because of its fast operation speed, resistive random-access memory is widely anticipated to dominate future’s non-volatile memory (NVM) device technologies. However, exploiting similar device concepts for emerging forms of electronics, such as printed large-area electronics, presents numerous technical and economic challenges. Here, we describe the development of hybrid blend semiconductor system consisting of the 2D layered organic–inorganic hybrid Ruddlesden−Popper perovskite (C6H5C2H4NH3)2PbBr4 and a conjugated organic molecule, and its application in high performance NVM transistors. We show that carefully engineered blends yields p-channel transistors with remarkably large and reprogrammable operating hysteresis. This feature results to a reprogrammable memory window of >100 V, write/erase channel current ratio of >10^4, excellent data retention (>10^4 s) and superior memory endurance (>15000 write/erase cycles). Using a complementary suite of characterization techniques (AFM, XPS, TRPL, PL mapping), key interactions between the organic and perovskite components in the solid state, are elucidated providing vital clues on the physical principles that underpin the operation of this new type of memory transistors. A further unique feature of these devices is the lack of temperature-dependence transport with the devices remaining operational down to 5 K. A novel transport mechanism, which could potentially account for the temperature-independent reprogrammable channel conductance, is proposed.

Resume : In recent years, organic-inorganic hybrid perovskites (OIHPs) have emerged as a promising material in organic and hybrid electronics, attributing to their excellent electrical and optical properties, ease of synthesis and solution processability. Even though there has been a tremendous achievement in the area of solar cell, very few efforts have been dedicated to use OIHPs towards thin film transistor (TFT) for circuit applications. To this end, it is essential to choose a suitable process technology that has scalability and is compatible with circuit application. Unlike the traditional trend of fabricating top contact TFT using shadow mask, we have successfully fabricated an array of photolithographically patterned bottom contact TFTs, based on solution processed methylammonium lead halide perovskite as a semiconductor on top of either silicon dioxide or polymers as the dielectric medium. In this work, we demonstrate the performance of the devices with channel length as low as 10 µm, showing ambipolar transistor characteristics at room temperature. As lithography compatible small channel devices are crucial for the development of circuits, this work will pave the way for further developing OIHPs as a potential player for flexible and transparent electronic circuits.

Resume : Although known for more than a century, hybrid and related inorganic halide-based perovskites have received extraordinary attention recently, because of the unique physical properties of the three-dimensional (3-D) lead-based systems, which make them outstanding candidates for application in photovoltaic and related optoelectronic devices. This talk will explore beyond the current focus on 3-D systems, to highlight the outstanding chemical flexibility of the more diverse 2-D perovskite family.[1] As part of the discussion, the importance of structural dimensionality for determining semiconducting character, along with opportunity for both organic/inorganic structural components to play an active role in determining hybrid properties, will be emphasized. In addition to structural/electronic flexibility of the 2-D perovskites, the talk will explore the unique challenges for preparing high-quality films of these systems, and will present recent progress along several fronts: two-step vacuum processing,[2] melt-processing,[3] and resonant infrared matrix-assisted pulse laser evaporation (RIR-MAPLE).[4] Outstanding functionality combined with versatile and facile processing provide two pillars for future application and study of this family. [1] B. Saparov, D. B. Mitzi, Chem. Rev. 116, 4558 (2016); [2] M. Khazaee et. al., Chem. Mater. 30, 3538 (2018); [3] T. Li et. al., Chem. Sci. DOI: 10.1039/C8SC03863E (2019); [4] W. A. Dunlap-Shohl et. al., ACS Energy Lett. 3, 270 (2018).

Resume : Ruddlesden-Popper phase metal-halide perovskites (RPPs) have attracted significant attention in recent years due to their promising light harvesting and emissive properties. In this study, we investigate how RPPs are formed; how their kinetics of formation determine their composition; and how they orient relative to their substrate.[1] We find that layered intermediate complexes formed with the solvent provide a scaffold that facilitates the nucleation and growth of RPPs during annealing. We observe the transformation via in-situ X-ray scattering throughout the film formation process. Ultrafast transient absorption spectra of both RPP single crystals and films enable the identification of the distribution of quantum well (QW) thicknesses. These insights allow us to develop a kinetic model of RPP formation that accounts for the experimentally-observed size distribution of quantum wells. RPP quantum wells exhibit a spatially uncorrelated distribution of thicknesses determined largely by the stoichiometric proportion between the large cation and the solvent complexes. The quantum well orientation is influenced by the thickness of the wells, but our results indicate a means of controlling the distribution, composition and orientation of RPPs via the selection of the large intercalating cation; the solvent; their relative proportion; and the film deposition technique. This fine-tuning of RPPs thereby offers new avenues to efficient and stable perovskite-based optoelectronic devices. [1] R Quintero-Bermudez, A Gold-Parker, A Proppe, ... E H Sargent (2018). Compositional and orientational control in metal halide perovskites of reduced dimensionality. Nature materials, 17(10), 900.

Resume : The advent of layered and reduced dimensional lead halide materials, for instance based upon the Ruddlesden-Popper and Dion-Jacobson compositions, has significantly widened the material ecosystem of hybrid lead halide materials beyond the 3D perovskite by relaxing the Goldschmidt tolerance factor and enabling the incorporation of larger and more sophisticate ligands and cations. Consequently, the formation of reduced dimensional lead halide thin films during solution-casting can be significantly more complex and challenging than their 3D perovskite counterparts owing to the requirement to control the size distribution, orientation/texture of grains and quantum wells in addition to thin film morphology and pinhole formation. Common processing routes involving spin-coating with anti-solvent dripping or hot-casting have been borrowed from the 3D perovskites to make controlled reduced dimensional perovskite films, but the actual phase transformation mechanism from solution to final solid state semiconductor were not well understood until recently. We have utilized a combination of in situ grazing incidence x-ray scattering (GIWAXS) during spin-coating and complementary ex situ transient absorption spectroscopy (TAS) to investigate the crystallization behaviors of PEA2MAn-1PbnI3n+1 [1], GAMAnPbnI3n+1 [2], BA2MA3Pb4I13 [3,4], and will report on our findings with regards to the complex solidification pathway of these materials. References: [1] Quintero-Bermudez et al., “Compositional and Orientational Control in Metal Halide Perovskites of Reduced Dimensionality”, Nat. Mater. 17 (2018) 900. [2] Zhang et al., “Dynamical Transformation of 2D Perovskites with Alternating Cations in the Interlayer Space for High-Performance Photovoltaics”, J. Amer. Chem. Soc. 141 (2019) 2684. [3] Zhang et al., “Phase Transition Control for High Performance Ruddlesden–Popper Perovskite Solar Cells”, Adv. Mater. 30 (2018) 1707166. [4] Zhang et al., “Stable high efficiency two-dimensional perovskite solar cells via cesium doping”, Energy Environ. Sci. 10 (2017) 2095.

Resume : Organic-inorganic hybrid perovskites allow for creating a wide variety of structures: from a 3D structure using small organic building blocks to essentially 2D layered structures using larger organic building blocks. This opens an avenue towards a quite new class of organic-inorganic nano-composites in which the inorganic perovskite sheet acts as a template for the self-assembly of organic chromophores confined between the sheets of the inorganic layer. Consequently, an organization can be obtained for the organic chromophores that resemble the order observed in a single crystal. The number of inorganic sheets can easily be tuned by changing the stoichiometry of the organic small and large building blocks. In this way, a fluent transition of electro-optical properties can be achieved of the inorganic part from confined 2D structures to strongly delocalized quasi-3D structures. We will discuss the structures obtained so far. The use of carbazole ammonium salts in 2D hybrid perovskites leads to materials for solar cells with enhanced photoconductivity and stronger resistance toward moisture yielding solar cells with enhanced stability. Also, the use of pyrene ammonium salts to synthesize 2D hybrid perovskites has been explored and initial results on the structure and optoelectronic properties will be discussed. Transitions between 1D and 2D structures were observed by specific modification of the stoichiometry of the components and also in function of temperature. In combination with the introduction of extra secondary interactions in the organic layer, a material is obtained with an exceptionally low bandgap. In this way, a new group of truly organic-inorganic hybrid materials is disclosed with possibly new applications for thin film electronics.

Resume : "The role of spacer molecules in designing 2D Ruddlesden-Popper perovskites" F.Jahanbakhshi, M.Mladenovic, N.AshariAstani and U.Roethlisberger Ruddlesden-Popper (RP) 2D perovskites have the generic chemical formula (RNH3)2MAn-1MnX3n+1 where RNH3 is an organic ammonium ion that acts as spacer between the 3D perovskite layers and methyl ammonium (MA) mostly serves as a monovalent cation, with M being a bivalent cation (e.g. Pb, Sn, …), and X representing a halogen. Basically, any aliphatic ammonium salt with a larger organic part can be used as spacer, however, butyl ammonium (BA), phenethylammonium (PEA), hexylammonium (Hex), 5-ammonium valeric acid (5-AVA), anilinium (Anyl), and benzyl ammonium have so far been incorporated. Despite the general agreement on the positive effect of the spacers on either the efficiency or the stability, it is not yet clear, whether the overall success is due to the formation of RP phases regardless of the type of spacer, or the spacer chemical composition also matters in cross-linking and determining RP phase properties. We have performed a comparative study to characterise structural and electronic properties of 2D RP phases with 5-AVA spacer and to compare them to analogous structures of other ligands such as BA. We have found that aliphatic spacers affect electronic properties of the 2D perovskites by distortions imposed on the perovskite frame, while the chemical composition of such spacer molecules is not as important as their geometrical arrangement in the isolating layer.