Hybrid lead halide perovskites such as MAPbI3 (MA = CH3NH3+) and their mixed halide analogues represent an emerging class of materials for solar energy conversion. Intriguing aspects include sizable carrier diffusion lengths, large optical absorption coefficients, and certified power conversion efficiencies that now exceed 22%. Halide-composition-tunable band gaps also make MAPb(I1–xBrx)3 systems ideal candidates for tandem solar cells. Unfortunately, preventing the effective integration of MAPb(I1–xBrx)3 into working devices are intrinsic instabilities due to light-induced halide phase segregation. Namely, under illumination, mixed halide perovskites reversibly segregate into low-band-gap I-rich and high-band-gap Br-rich domains. Under electrical bias, halide migration has also been proposed as the source of undesirable charge injection barriers that degrade photovoltaic performance. In this Perspective, we review the origin of light-induced halide phase segregation, its effects on photovoltaic response,...

Light-Induced Anion Phase Segregation in Mixed Halide Perovskites

Design and modification of interfaces, always a critical issue for semiconductor devices, has become a primary tool to harness the full potential of halide perovskite (HaP)-based optoelectronics, including photovoltaics and light-emitting diodes. In particular, the outstanding improvements in HaP solar cell performance and stability can be primarily ascribed to a careful choice of the interfacial layout in the layer stack. In this review, we describe the unique challenges and opportunities of these approaches (section 1). For this purpose, we first elucidate the basic physical and chemical properties of the exposed HaP thin film and crystal surfaces, including topics such as surface termination, surface reactivity, and electronic structure (section 2). This is followed by discussing experimental results on the energetic alignment processes at the interfaces between the HaP and transport and buffer layers. This section includes understandings reached as well as commonly proposed and applied models, especia...

Halide Perovskites: Is It All about the Interfaces?

This work provides a thorough overview of state of the art of stability of perovskite solar cells (PSCs) and covers important degradation issues involved in this technology. Degradation factors, which are reported in the literature affecting the stability of PSCs, are discussed. Several degradation mechanisms resulting from thermal and chemical instabilities, phase transformations, exposure to visible and UV light, moisture and oxygen and most importantly sealing issues are thoroughly analyzed. Methods are suggested to study most of these degradation mechanisms in a systematic way. In addition, environmental assessment of PSCs is briefly covered. Alternative materials and their preparation methods are screened with respect to stability of the device. Overall, this work contributes in developing better understanding of the degradation mechanisms and help in improving overall stability of the PSCs.

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Device stability of perovskite solar cells – A review

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 sta...

http://pubs.acs.org/doi/pdf/10.1021/acs.jpclett.6b00946

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

Mixed halide lead perovskites (e.g., CH3NH3PbBrxI3–x) undergo phase segregation creating iodide-rich and bromide-rich domains when subjected to visible irradiation. This intriguing aspect of halide ion movement in mixed halide films is now being tracked through excited-state behavior using emission and transient absorption spectroscopy tools. These transient experiments have allowed us to establish the time scale with which such separation occurs under laser irradiation (405 nm, 25 mW/cm2 to 1.7 W/cm2) as well as dark recovery. While the phase separation occurs with a rate constant of 0.1–0.3 s–1, the recovery occurs over a time period of several minutes to an hour. The relative photoluminescence quantum yield observed for Br-rich regions (em. max 530 nm) is nearly 2 orders of magnitude lower than that of I-rich regions (em. max 760 nm) and arises from the fact that I-rich regions serve as sinks for photogenerated charge carriers. Understanding such cascading charge transfer to localized sites could furth...

http://pubs.acs.org/doi/pdf/10.1021/acsenergylett.6b00158

Tracking Iodide and Bromide Ion Segregation in Mixed Halide Lead Perovskites during Photoirradiation

ConspectusHybrid alkylammonium lead halide perovskite solar cells have, in a very few years of research, exceeded a light-to-electricity conversion efficiency of 20%, not far behind crystalline silicon cells. These perovskites do not contain any rare element, the amount of toxic lead used is very small, and the cells can be made with a low energy input. They therefore already conform to two of the three requirements for viable, commercial solar cells–efficient and cheap. The potential deal-breaker is their long-term stability. While reasonable short-term (hours) and even medium term (months) stability has been demonstrated, there is concern whether they will be stable for the two decades or more expected from commercial cells in view of the intrinsically unstable nature of these materials. In particular, they have a tendency to be sensitive to various types of irradiation, including sunlight, under certain conditions.This Account focuses on the effect of irradiation on the hybrid (and to a small degree, a...

Effects of Light and Electron Beam Irradiation on Halide Perovskites and Their Solar Cells

TiO2/CdSe/CuSCN extremely thin absorber (ETA) solar cells are found to give relatively high values of open-circuit voltage (>0.8 V) but low currents upon annealing the cadmium selenide (CdSe) in air (500 oC). Annealing in N2 produces much lower photovoltages and slightly lower photocurrents. Band structure measurements show differences between the two annealing regimes that, however, appear to favor the N2-annealed CdSe. On the other hand, chemically resolved electrical measurements (CREM) of the cells reveal marked differences in photo-induced charge trapping, in particular at absorber grain boundaries of the air versus N2-annealed systems, correlated with the formation of Cd–O species at the CdSe surface. Using transient absorption and photovoltage decay, pronounced lifetime differences are also observed, in agreement with the strong suppression of charge recombination. The results point to a multiple role of grain surface-oxidation, which both impedes electron injection from the CdSe to the TiO2, but, much more significantly, enhances hole injection to the CuSCN via passivation of hole traps that act as efficient recombination centers.

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Nir Klein-Kedem

Papers

Effects of Light and Electron Beam Irradiation on Halide Perovskites and Their Solar Cells

Surface Oxidation as a Cause of High Open-Circuit Voltage in CdSe ETA Solar Cells