In Operando, Photovoltaic, and Microscopic Evaluation of Recombination Centers in Halide Perovskite-Based Solar Cells

The origin of the low densities of electrically active defects in Pb halide perovskite (HaP), a crucial factor for their use in photovoltaics, light emission, and radiation detection, remains a matter of discussion, in part because of the difficulty in determining these densities. Here, we present a powerful approach to assess the defect densities, based on electric field mapping in working HaP-based solar cells. The minority carrier diffusion lengths were deduced from the electric field profile, measured by electron beam-induced current (EBIC). The EBIC method was used earlier to get the first direct evidence for the n-i-p junction structure, at the heart of efficient HaP-based PV cells, and later by us and others for further HaP studies. This manuscript includes EBIC results on illuminated cell cross sections (in operando) at several light intensities to compare optoelectronic characteristics of different cells made by different groups in several laboratories. We then apply a simple, effective single-level defect model that allows deriving the densities (Nr) of the defect acting as recombination center. We find Nr ≈ 1 × 1013 cm–3 for mixed A cation lead bromide-based HaP films and ∼1 × 1014 cm–3 for MAPbBr3(Cl). As EBIC photocurrents are similar at the grain bulk and boundaries, we suggest that the defects are at the interfaces with selective contacts rather than in the HaP film. These results are relevant for photovoltaic devices as the EBIC responses distinguish clearly between high- and low-efficiency devices. The most efficient devices have n-i-p structures with a close-to-intrinsic HaP film, and the selective contacts then dictate the electric field strength throughout the HaP absorber.We thank the Yotam project, Ullmann Family Foundation, Dears Foundation, the WIS’ Sustainability And Energy Research Initiative, SAERI, and the Minerva Centre for Self-Repairing Systems for Energy & Sustainability for support at the Weizmann Institute and the Israel Ministry of Energy and Infrastructure for the work at Bar-Ilan University. A.Z. thanks Katya Rechav for the FIB sample preparation, Ifat Kaplan-Asheri for assisting with EBIC operation, and Isaac Balberg (Hebrew University of Jerusalem) for fruitful discussions

How Important Is the Organic Part of Lead Halide Perovskite Photovoltaic Cells? Efficient CsPbBr₃ Cells

Hybrid organic–inorganic lead halide perovskite photovoltaic cells have already surpassed 20% conversion efficiency in the few years that they have been seriously studied. However, many fundamental questions still remain unanswered as to why they are so good. One of these is “Is the organic cation really necessary to obtain high quality cells?” In this study, we show that an all-inorganic version of the lead bromide perovskite material works equally well as the organic one, in particular generating the high open circuit voltages that are an important feature of these cells

Imaging Modalities to Identity Inflammation in an Atherosclerotic Plaque

Atherosclerosis is a chronic, progressive, multifocal arterial wall disease caused by local and systemic inflammation responsible for major cardiovascular complications such as myocardial infarction and stroke. With the recent understanding that vulnerable plaque erosion and rupture, with subsequent thrombosis, rather than luminal stenosis, is the underlying cause of acute ischemic events, there has been a shift of focus to understand the mechanisms that make an atherosclerotic plaque unstable or vulnerable to rupture. The presence of inflammation in the atherosclerotic plaque has been considered as one of the initial events which convert a stable plaque into an unstable and vulnerable plaque. This paper systemically reviews the noninvasive and invasive imaging modalities that are currently available to detect this inflammatory process, at least in the intermediate stages, and discusses the ongoing studies that will help us to better understand and identify it at the molecular level

Deleterious Effect of Negative Capacitance on the Performance of Halide Perovskite Solar Cells

Negative capacitance in photovoltaic devices has been observed and reported in several cases, but its origin, at low or intermediate frequencies, is under debate. Here we unambiguously demonstrate a direct correlation between the observation of this capacitance and a corresponding decrease in performance of a halide perovskite (HaP; CsPbBr3)-based device, expressed as reduction of open-circuit voltage and fill factor. We have prepared highly stable CsPbBr3 HaPs that do not exhibit any degradation over the duration of the impedance spectroscopy measurements, ruling out degradation as the origin of the observed phenomena. Reconstruction of current-voltage curves from the impedance spectroscopy provided further evidence of the deleterious role of negative capacitance on photoconversion performance

Cesium Enhances Long-Term Stability of Lead Bromide Perovskite-Based Solar Cells

Direct comparison between perovskite-structured hybrid organic–inorganic methylammonium lead bromide (MAPbBr₃) and all-inorganic cesium lead bromide (CsPbBr₃), allows identifying possible fundamental differences in their structural, thermal and electronic characteristics. Both materials possess a similar direct optical band gap, but CsPbBr₃ demonstrates a higher thermal stability than MAPbBr₃. In order to compare device properties, we fabricated solar cells, with similarly synthesized MAPbBr₃ or CsPbBr₃, over mesoporous titania scaffolds. Both cell types demonstrated comparable photovoltaic performances under AM1.5 illumination, reaching power conversion efficiencies of ∼6% with a poly aryl amine-based derivative as hole transport material. Further analysis shows that Cs-based devices are as efficient as, and more stable than methylammonium-based ones, after aging (storing the cells for 2 weeks in a dry (relative humidity 15–20%) air atmosphere in the dark) for 2 weeks, under constant illumination (at maximum power), and under electron beam irradiation

Light-Induced Increase of Electron Diffusion Length in a p–n Junction Type CH₃NH₃PbBr₃ Perovskite Solar Cell

High band gap, high open-circuit voltage solar cells with methylammonium lead tribromide (MAPbBr₃) perovskite absorbers are of interest for spectral splitting and photoelectrochemical applications, because of their good performance and ease of processing. The physical origin of high performance in these and similar perovskite-based devices remains only partially understood. Using cross-sectional electron-beam-induced current (EBIC) measurements, we find an increase in carrier diffusion length in MAPbBr₃(Cl)-based solar cells upon low intensity (a few percent of 1 sun intensity) blue laser illumination. Comparing dark and illuminated conditions, the minority carrier (electron) diffusion length increases about 3.5 times from <i>L</i>_n = 100 ± 50 nm to 360 ± 22 nm. The EBIC cross section profile indicates a p–n structure between the n-FTO/TiO₂ and p-perovskite, rather than the p–i–n structure, reported for the iodide derivative. On the basis of the variation in space-charge region width with varying bias, measured by EBIC and capacitance–voltage measurements, we estimate the net-doping concentration in MAPbBr₃(Cl) to be 3–6 × 10¹⁷ cm^–3

Valence and Conduction Band Densities of States of Metal Halide Perovskites: A Combined Experimental–Theoretical Study

We report valence and conduction band densities of states measured via ultraviolet and inverse photoemission spectroscopies on three metal halide perovskites, specifically methylammonium lead iodide and bromide and cesium lead bromide (MAPbI₃, MAPbBr₃, CsPbBr₃), grown at two different institutions on different substrates. These are compared with theoretical densities of states (DOS) calculated via density functional theory. The qualitative agreement achieved between experiment and theory leads to the identification of valence and conduction band spectral features, and allows a precise determination of the position of the band edges, ionization energy and electron affinity of the materials. The comparison reveals an unusually low DOS at the valence band maximum (VBM) of these compounds, which confirms and generalizes previous predictions of strong band dispersion and low DOS at the MAPbI₃ VBM. This low DOS calls for special attention when using electron spectroscopy to determine the frontier electronic states of lead halide perovskites

Low-Temperature Solution-Grown CsPbBr₃ Single Crystals and Their Characterization

Cesium lead bromide (CsPbBr₃) was recently introduced as a potentially high performance thin-film halide perovskite (HaP) material for optoelectronics, including photovoltaics, significantly more stable than MAPbBr₃ (MA = CH₃NH₃⁺). Because of the importance of single crystals to study relevant material properties per se, crystals grown under conditions comparable to those used for preparing thin films, i.e., low-temperature solution-based growth, are needed. We show here two simple ways, antisolvent-vapor saturation or heating a solution containing retrograde soluble CsPbBr₃, to grow single crystals of CsPbBr₃ from a precursor solution, treated with acetonitrile (MeCN) or methanol (MeOH). The precursor solutions are stable for at least several months. Millimeter-sized crystals are grown without crystal-seeding and can provide a 100% yield of CsPbBr₃ perovskite crystals, avoiding a CsBr-rich (or PbBr₂-rich) composition, which is often present alongside the perovskite phase. Further growth is demonstrated to be possible with crystal seeding. The crystals are characterized in several ways, including first results of charge carrier lifetime (30 ns) and an upper-limit of the Urbach energy (19 meV). As the crystals are grown from a polar aprotic solvent (DMSO), which is similar to those used to grow hybrid organic–inorganic HaP crystals, this may allow growing mixed (organic and inorganic) monovalent cation HaP crystals