Taketo Handa

Charge Injection Mechanism at Heterointerfaces in CH3NH3PbI3 Perovskite Solar Cells Revealed by Simultaneous Time-Resolved Photoluminescence and Photocurrent Measurements

Organic-inorganic hybrid perovskite solar cells are attracting much attention due to their excellent photovoltaic properties. In these multilayered structures, the device performance is determined by complicated carrier dynamics. Here, we studied photocarrier recombination and injection dynamics in CH3NH3PbI3 perovskite solar cells using time-resolved photoluminescence (PL) and photocurrent (PC) measurements. It is found that a peculiar slowdown in the PL decay time constants of the perovskite layer occurs for higher excitation powers, followed by a decrease of the external quantum efficiency for PC. This indicates that a carrier-injection bottleneck exists at the heterojunction interfaces, which limits the photovoltaic performance of the device in concentrator applications. We conclude that the carrier-injection rate is sensitive to the photogenerated carrier density, and the carrier-injection bottleneck strongly enhances recombination losses of photocarriers in the perovskite layer at high excitation conditions. The physical origin of the bottleneck is discussed based on the result of numerical simulations.

Charge Injection at the Heterointerface in Perovskite CH3NH3PbI3 Solar Cells Studied by Simultaneous Microscopic Photoluminescence and Photocurrent Imaging Spectroscopy

Charge carrier dynamics in perovskite CH3NH3PbI3 solar cells were studied by means of microscopic photoluminescence (PL) and photocurrent (PC) imaging spectroscopy. The PL intensity, PL lifetime, and PC intensity varied spatially on the order of several tens of micrometers. Simultaneous PL and PC image measurements revealed a positive correlation between the PL intensity and PL lifetime, and a negative correlation between PL and PC intensities. These correlations were due to the competition between photocarrier injection from the CH3NH3PbI3 layer into the charge transport layer and photocarrier recombination within the CH3NH3PbI3 layer. Furthermore, we found that the decrease in the carrier injection efficiency under prolonged light illumination leads to a reduction in PC, resulting in light-induced degradation of solar cell devices. Our findings provide important insights for understanding carrier injection at the interface and light-induced degradation in perovskite solar cells.

Optical characterization of voltage-accelerated degradation in CH 3 NH 3 PbI 3 perovskite solar cells

We investigate the performance degradation mechanism of CH3NH3PbI3 perovskite solar cells under bias voltage in air and nitrogen atmospheres using photoluminescence and electroluminescence techniques. When applying forward bias, the power conversion efficiency of the solar cells decreased significantly in air, but showed no degradation in nitrogen atmosphere. Time-resolved photoluminescence measurements on these devices revealed that the application of forward bias in air accelerates the generation of non-radiative recombination centers in the perovskite layer buried in the device. We found a negative correlation between the electroluminescence intensity and the injected current intensity in air. The irreversible change of the perovskite grain surface in air initiates the degradation of the perovskite solar cells.

Solvent-Coordinated Tin Halide Complexes as Purified Precursors for Tin-Based Perovskites

A series of solvent-coordinated tin halide complexes were prepared as impurity-free precursors for tin halide perovskites, and their structures were determined by single-crystal X-ray diffraction analysis. Using these precursors, the tin halide perovskites, MASnI3 and FASnI3, were prepared, and their electronic structures and photophysical properties were examined under inert conditions by means of photoelectron yield spectroscopy as well as absorption and fluorescence spectroscopies. Their valence bands (MASnI3: −5.02 eV; FASnI3: −5.16 eV) are significantly higher than those of MAPbI3 or the typical hole-transporting materials 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamino)-9,9′-spirobifluorene and poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine). These results suggest that to develop the solar cells using these tin halide perovskites with efficient hole-collection properties, hole-transporting materials should be chosen that have the highest occupied molecular orbital levels higher than −5.0 eV.

Photophysics of organic-inorganic hybrid perovskite solar cells

Organic-inorganic hybrid perovskites are attracting much attention as photoabsorbers for high-efficiency thin film solar cells. For further improvement of the device performance, the photophysics in the actual devices have to be clarified. Here, we studied carrier recombination and extraction dynamics in CH3NH3PbI3 perovskite solar cells using time-resolved photoluminescence (PL) and photocurrent (PC) measurements. It was found that the PL lifetime of the perovskite layer in the solar cell devices becomes longer with increasing excitation intensity, which is the exact opposite of the trend observed for the bare perovskite thin films. This slowdown of the PL decay reflects the bottleneck effect in the carrier extraction process at the interface, which also reduces the external quantum efficiency for PC. We clarified that the dynamical change of the carrier extraction efficiency plays an important role in the carrier recombination and transport in the perovskite solar cells. We successfully reproduced the experimental excitation intensity dependence by numerical simulations based on a simple model including carrier recombination and extraction processes. Furthermore, we performed microscopic imaging of PL and PC from CH3NH3PbI3 solar cells. The observed negative correlation between the PL and PC intensities reflects the competition between radiative recombination and carrier extraction processes. Our simultaneous measurement of PL and PC provides essential information to establish high-performance devices. This allows us to analyze the photocarrier dynamics in the lead-free CH3NH3SnI3 solar cells.

Photoelectronic properties of lead-free CH3NH3SnI3 perovskite solar cell materials and devices

Intensive research on lead halide perovskites clarified that these materials are indeed suitable candidates for photovoltaic applications due to their excellent photoelectronic properties. Yet, lead is considered a major issue for commercialization. Concurrently, in the last five years, increasing research efforts have been made to replace lead with tin. Although partially successful, the present conversion efficiencies of tin halide perovskite solar cells are limited. Further performance improvements should be possible, if the underlying energy loss mechanisms in these devices can be clarified. Here, we investigated the energy loss mechanisms in lead-free CH3NH3SnI3 (MASnI3) solar cells as well as intrinsic photoelectronic properties of MASnI3 to assess its potential for photovoltaics. Time-resolved photoluminescence (PL) measurements reveal that the short-circuit current (Jsc) in the MASnI3 solar cell deviates from an ideal value as a result of fast recombination of photogenerated carriers in the perovskite layer. Consequently, a larger Jsc should be possible with longer carrier lifetimes. Furthermore, resonantly excited PL and temperature-dependent PL data clearly reveal that the intrinsic electron–longitudinal optical phonon coupling governs the broadening of optical transitions at around 300 K. By performing a detailed comparison of the data of MASnI3 and MAPbI3, it is shown that the intrinsic optical properties of tin and lead perovskites are similar to each other. Our results suggest that solar cells based on tin halide perovskites can compete with lead halide perovskite solar cells, if the carrier lifetimes can be improved.

Lead‐Free Solar Cells based on Tin Halide Perovskite Films with High Coverage and Improved Aggregation

Two simple methods to improve tin halide perovskite film structure are introduced, aimed at increasing the power conversion efficiency of lead free perovskite solar cells. First, a hot antisolvent treatment (HAT) was found to increase the film coverage and prevent electrical shunting in the photovoltaic device. Second, it was discovered that annealing under a low partial pressure of dimethyl sulfoxide vapor increased the average crystallite size. The topographical and electrical qualities of the perovskite films are substantively improved as a result of the combined treatments, facilitating the fabrication of tin-based perovskite solar cell devices with power conversion efficiencies of over 7 %.

Charge carrier injection at the heterointerface in CH 3 NH 3 PbI 3 perovskite solar cells studied by time-resolved photoluminescence and photocurrent imaging spectroscopy

Organic-inorganic halide perovskite solar cells are attracting much attention from the photovoltaic community because of their high conversion efficiencies exceeding 20%. So far, intrinsic superior optoelectronic properties of this material class have been revealed through comprehensive studies on the thin films and single crystals [1,2]. For further improvement of the device architecture and conversion efficiency, the carrier recombination and transport dynamics in actual solar cell devices have to be clarified. The perovskite solar cell is usually implemented as a heterojunction structure consisting of a perovskite absorber layer and charge transport layers as selective contacts, and the carrier-injection properties at these heterointerfaces play a crucial role for the device performance. Time-resolved photoluminescence (PL) techniques are usually adopted to investigate carrier injection and transport properties [3]. However, PL is also additionally affected by traps and defects within the perovskite layer and also at the heterointerface. On the other hand, the photocurrent (PC) measurement can directly assess the net charge-carrier flow through the whole device. Therefore a combination of PL and PC enables us to investigate the details of the carrier injection. In addition, perovskite solar cells are prepared by a fast and cost-effective low-temperature solution-process, but this simple preparation method also causes a spatial nonuniformity in the optical and electrical properties [4]. Thus, the spatial imaging is invaluable for statistical evaluation of the solar cell characteristics.

Carrier injection and recombination processes in perovskite CH3NH3PbI3 solar cells studied by electroluminescence spectroscopy

Organic-inorganic hybrid perovskite materials, CH3NH3PbX3 (X = I and Br), are considered as promising candidates for emerging thin-film photovoltaics. For practical implementation, the degradation mechanism and the carrier dynamics during operation have to be clarified. We investigated the degradation mechanism and the carrier injection and recombination processes in perovskite CH3NH3PbI3 solar cells using photoluminescence (PL) and electroluminescence (EL) imaging spectroscopies. By applying forward bias-voltage, an inhomogeneous distribution of the EL intensity was clearly observed from the CH3NH3PbI3 solar cells. By comparing the PL- and EL-images, we revealed that the spatial inhomogeneity of the EL intensity is a result of the inhomogeneous luminescence efficiency in the perovskite layer. An application of bias-voltage for several tens of minutes in air caused a decrease in the EL intensity and the conversion efficiency of the perovskite solar cells. The degradation mechanism of perovskite solar cells under bias-voltage in air is discussed.