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The organic and hydrophobic polymer poly[N, N′-bis(4-butilphenyl)-N, N′-bis(phenyl)-benzidine] (polyTPD) represents a promising hole transport layer (HTL) for perovskite photovoltaics due to its suitable energy levels, whereby its highest occupied molecular orbital level matches well with the valence band level of methylammonium lead triiodide (CH3NH3PbI3, MAPbI3) perovskite. However, processing a perovskite layer from the solution on the surface of this organic material, is found to be difficult due to the surface properties of the latter. In this study, we evaluate efficient p-i-n type MAPbI3 perovskite solar cells employing differently processed polyTPD layers. We found that the surface coverage of the MAPbI3 perovskite layer strongly depends on the preparation method of the underlying polyTPD layer. By varying the solvents for the polyTPD precursor, its concentration, and by applying an optimised two-step perovskite deposition technique we increased both the surface coverage of the perovskite layer as well as the power conversion efficiency (PCE) of the corresponding solar cell devices. Our simple solvent-engineering approach demonstrates that no further interface modifications are needed for a successful preparation of efficient planar photovoltaic devices with PCEs in the range of 15 %–16 %.

Despite the intense research effort on metal halide perovskites, the fundamental correlation between the crystal structure and optoelectronic properties remains unclear. As many intriguing phenomena are expected to be based on the dipolar character of the rotating organic cations, an improved understanding of the material’s polarizability is of high relevance. Here, we study the orthorhombic–tetragonal phase transition of methylammonium lead iodide to gain insight into the polarization mechanism at low temperatures and the resulting effects on solar cell performances. Using thermally stimulated current measurements, we detect polarization currents when cooling or heating across the phase transition. These (de)polarization currents are found to correlate with a sudden change in rotational freedom of the organic cations, with a temperature range of 20 K separating polarizing and depolarizing processes. The nature of this cation polarization within the orthorhombic phase is investigated with respect to its i...

We tune the Fermi level alignment between the SnOx electron transport layer (ETL) and Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 and highlight that this parameter is interlinked with current–voltage hysteresis in perovskite solar cells (PSCs). Furthermore, thermally stimulated current measurements reveal that the depth of trap states in the ETL or at the ETL–perovskite interface correlates with Fermi level positions, ultimately linking it to the energy difference between the Fermi level and conduction band minimum. In the presence of deep trap states, charge accumulation and recombination at the interface are promoted, affecting the charge collection efficiency adversely, which increases the hysteresis of PSCs.

Our understanding of the crystallization process of hybrid halide perovskites has propelled the efficiency of state-of-the-art photovoltaic devices to over 22%. Further improvements to the performance will likely arise from reducing the number of grain boundaries. Here, current methods lead to grain sizes in the 1 micrometer range and the resulting optoelectronic properties suffer as compared to single-crystal materials. In this work, we introduce a new synthesis procedure with MAPbI3 leading to crystal sizes in the tens of microns range. This approach is based on the pre-crystallization of an intermediate phase (IP) based on the solid-state reaction of a lead-acetate trihydrate precursor mixture in a highly polar solvent. Beyond large grain sizes, the crystal orientation is also tightly controlled, leading to perovskite crystallites which remain perfectly aligned with the c-axis of the tetragonal structure parallel to the substrate as evidenced by grazing incidence wide angle X-ray scattering (GIWAXS). Furthermore, we demonstrate the high crystallinity and large grain size of the developed films via high-resolution transmission electron microscopy (HRTEM). The charge carrier mobilities are significantly improved with larger grain size and approach mobility values of about 40 cm2 V−1 s−1, moving toward the values observed for single crystals. We capitalize on the enhanced optoelectronic properties of the developed films by incorporating them into planar heterojunction solar cells which reach power conversion efficiencies of 18.5%, higher than MAPbI3-based device prepared from standard methods in a like-to-like comparison.

One of the key properties of lead-halide perovskites employed in solar cells is the defect tolerance of the materials, in particular regarding intrinsic point defects, which mainly form shallow traps. Considering that high luminescence yields and photovoltaic performance are obtained by simple solution processing from commercial chemicals, it is commonly anticipated that the defect tolerance-at least to a considerable degree-extends to grain boundaries and extrinsic defects, i.e. impurities, as well. However, the effect of impurities has hardly been investigated. Here, we intentionally introduce small quantities of bismuth (10 ppm to 2%) in solution to be incorporated in the perovskite films based on mixed cation mixed anion compositions. We observe that Bi impurities in the %-regime reduce charge carrier collection efficiency and, more importantly, that the open-circuit voltage decreases systematically with impurity concentration even in the ppm regime. This strong defect intolerance against Bi impurities comes along with reduced electroluminescence yields and charge carrier lifetimes obtained from transient photoluminescence experiments. Calculations based on molecular dynamics and density functional theory predict delocalized (≈0.16 eV) and localized deep (≈0.51 eV) trap states dependent on the structural arrangement of the surrounding atoms. Structural characterization supports the idea of Bi being present as a homogeneously spread point defect, which substitutes the Pb2+ by Bi3+ as seen from XPS and a reduction of the lattice parameter in XRD. Sensitive measurements of the photocurrent (by FTPS) and surface photovoltage (SPV) confirm the presence of tail states. Photoelectron spectroscopy measurements show evidence of a deep state. These results are consistent with the common idea of shallow traps being responsible for the reduced charge collection efficiency and the decreased fill factor, and deeper traps causing a substantial reduction of the open-circuit voltage. As Bi is only one potential impurity in the precursor salts used in perovskite solar cell fabrication, our findings open-up a research direction focusing on identifying and eliminating impurities that act as recombination centers-a topic that has so far not been fully considered in device optimization studies. Fil: Yavari, M.. École Polytechnique Fédérale de Lausanne; Suiza Fil: Ebadi, Firouzeh. École Polytechnique Fédérale de Lausanne; Suiza Fil: Meloni, Simone. Università di Roma; Italia Fil: Wang, Zishuai. École Polytechnique Fédérale de Lausanne; Suiza Fil: Yang, Terry Chien-Jen. École Polytechnique Fédérale de Lausanne; Suiza Fil: Sun, Shijing. Massachusetts Institute Of Technology; Estados Unidos Fil: Schwartz, Heidi. University Of Cologne; Alemania Fil: Wang, Zaiwei. École Polytechnique Fédérale de Lausanne; Suiza Fil: Niesen, Bjoern. École Polytechnique Fédérale de Lausanne; Suiza Fil: Durantini, Javier Esteban. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas Fisicoquímicas y Naturales. Instituto de Investigaciones en Tecnologías Energéticas y Materiales Avanzados. - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Tecnologías Energéticas y Materiales Avanzados; Argentina Fil: Rieder, Philipp. Julius Maximilian University Of Würzburg; Alemania Fil: Tvingstedt, Kristofer. Julius Maximilian University Of Würzburg; Alemania Fil: Buonassisi, Tonio. Massachusetts Institute Of Technology; Estados Unidos Fil: Choy, Wallace C.H.. The University Of Hong Kong; Hong Kong Fil: Filippetti, Alessio. Università Di Cagliari; Italia Fil: Dittrich, Thomas. Helmholtz Center Berlin For Materials And Energy; Alemania Fil: Olthof, Selina. University Of Cologne; Alemania Fil: Correa Baena, Juan Pablo. Massachusetts Institute Of Technology; Estados Unidos Fil: Tress, Wolfgang. École Polytechnique Fédérale de Lausanne; Suiza

Thin-film solar cells suffer from various types of recombination, of which leakage current usually dominates at lower voltages. Herein, we demonstrate first a three-order reduction of the shunt loss mechanism in planar methylammonium lead iodide perovskite solar cells by replacing the commonly used hole transport layer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) with a better hole-selective polyarylamine. As a result, these cells exhibit superior operation under reduced light conditions, which we demonstrate for the extreme case of moonlight irradiance, at which open-circuit voltages of 530 mV can still be obtained. By the shunt removal we also observe the VOC to drop to zero after as long as 2 h after the light has been switched off. Second, at higher illumination intensities the dominant losses in the PEDOT:PSS-based cell are ascribed to surface recombination and are also proven to be substantially minimized by instead employing the polyarylamine. We attribute the reduced shunt and surface recombination to the far better suited semiconductor character of the polyarylamine, compared to that of PEDOT:PSS, efficiently blocking electrons from recombining at this electrode.

Adding cesium (Cs) and rubidium (Rb) cations to FA(0.83)MA(0.17)Pb(I0.83Br0.17)(3) hybrid lead halide perovskites results in a remarkable improvement in solar cell performance, but the origin of the enhancement has not been fully understood yet. In this work, time-of-flight, time-resolved microwave conductivity, and thermally stimulated current measurements are performed to elucidate the impact of the inorganic cation additives on the trap landscape and charge transport properties within perovskite solar cells. These complementary techniques allow for the assessment of both local features within the perovskite crystals and macroscopic properties of films and full devices. Strikingly, Csincorporation is shown to reduce the trap density and charge recombination rates in the perovskite layer. This is consistent with the significant improvements in the open-circuit voltage and fill factor of Cs-containing devices. By comparison, Rb-addition results in an increased charge carrier mobility, which is accompanied by a minor increase in device efficiency and reduced currentvoltage hysteresis. By mixing Cs and Rb in quadruple cation (Cs-Rb-FA-MA) perovskites, the advantages of both inorganic cations can be combined. This study provides valuable insights into the role of these additives in multiplecation perovskite solar cells, which are essential for the design of high-performance devices.

Thin-film solar cells suffer from various types of recombination, of which leakage current usually dominates at lower voltages. Herein, we demonstrate first a three-order reduction of the shunt loss mechanism in planar methylammonium lead iodide perovskite solar cells by replacing the commonly used hole transport layer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) with a better hole-selective polyarylamine. As a result, these cells exhibit superior operation under reduced light conditions, which we demonstrate for the extreme case of moonlight irradiance, at which open-circuit voltages of 530 mV can still be obtained. By the shunt removal we also observe the VOC to drop to zero after as long as 2 h after the light has been switched off. Second, at higher illumination intensities the dominant losses in the PEDOT:PSS-based cell are ascribed to surface recombination and are also proven to be substantially minimized by instead employing the polyarylamine. We attribute the reduced shunt and surface recombination to the far better suited semiconductor character of the polyarylamine, compared to that of PEDOT:PSS, efficiently blocking electrons from recombining at this electrode.

We studied charge carrier recombination in methylammonium lead iodide (MAPbI3) perovskite and the impact of interfaces on the charge carrier lifetime using time-resolved photoluminescence. Pristine films and those covered with organic electron and hole transport materials (TM) were investigated at various laser repetition rates ranging from 10 kHz to 10 MHz in order to separate the bulk and interface-affected recombination. We revealed two different components in the PL decay. The fast component (shorter than 300ns) is assigned to interfacial processes, and the slow one to bulk recombination. A high repetition pulse train was shown to shorten PL decay in pristine perovskite, while significantly prolonging the photocarrier lifetime in MAPbI3covered by TMs. This effect can be qualitatively explained with kinetic model taking interface traps into account. We demonstrate a significant influence of excitation repetition rate on photocarrier lifetime, which should be considered when studying charge carrier dynamics in perovskites.