How antisolvent miscibility affects perovskite film wrinkling and photovoltaic properties

Charge carriers’ density, their lifetime, mobility, and the existence of trap states are strongly affected by the microscopic morphologies of perovskite films, and have a direct influence on the photovoltaic performance. Here, we report on micro-wrinkled perovskite layers to enhance photocarrier transport performances. By utilizing temperature-dependent miscibility of dimethyl sulfoxide with diethyl ether, the geometry of the microscopic wrinkles of the perovskite films are controlled. Wrinkling is pronounced as temperature of diethyl ether (T_DE) decreases due to the compressive stress relaxation of the thin rigid film-capped viscoelastic layer. Time-correlated single-photon counting reveals longer carrier lifetime at the hill sites than at the valley sites. The wrinkled morphology formed at T_DE = 5 °C shows higher power conversion efficiency (PCE) and better stability than the flat one formed at T_DE = 30 °C. Interfacial and additive engineering improve further PCE to 23.02%. This study provides important insight into correlation between lattice strain and carrier properties in perovskite photovoltaics.

Since the pioneering reports on the ~10% efficient solid-state perovskite solar cell (PSC) in 2012, demonstrating long-term stability by resolving the dissolution issue of organic–inorganic lead halide perovskite in photoelectrocehmical-type solar cell employing liquid electrolyte, the perovskite photovoltaics has surged swiftly. As a result, the power conversion efficiency (PCE) as high as 25.5% has been achieved in 2020. Although the composition of perovskite started with methylammonium lead iodide, abbreviated to MAPbI3, the recent excellent performing PSCs are based on formamidinium lead iodide, abbreviated to FAPbI3 or its derivatives with a certain amount of other cations in FA-site and/or bromide in I site. Along with compositional engineering for making progress toward higher PCE, methods for controlling crystal growth have significantly contributed to producing defect-less high-quality perovskite films. Forming Lewis acid–base adduct intermediate via antisolvent engineering was widely adapted to achieve large perovskite crystals with less grain boundaries, instead of a direct conversion of wet film to the perovskite phase. Despite the enlarged perovskite grains, the crystal growth by solvent engineering can hardly manipulate morphology of perovskite layer. Epitaxial growth, for example, could provide crystal growth normal to the substrate, which is expected to be beneficial to carrier transport. In addition, a flat perovskite surface induced by the solvent engineering may not be effective in optimizing light in- and out-coupling. Thus, it is still required to develop a methodology enabling an opto-electronically optimized perovskite layer.

Recently, an approach to control the perovskite morphology has been explored. For example, microscopic wrinkles have been observed for a certain composition of perovskite that suffers buckling of the perovskite thin film. In particular, the buckling was explained as a result of local compressive stress relaxation. However, detailed and comprehensive studies for effects of the microscopic wrinkles on the photovoltaic performances, as well as wrinkling mechanism have not been reported yet. Here, we report a simple and yet effective experimental approach to control and optimize the microscopic geometry of the wrinkles of perovskite thin films to maximize the photovoltaic performances, as well as long-time durability. We also suggest a theoretical model elucidating the wrinkling mechanism based on the detailed experimental data. To control the wrinkled morphology, we have designed an experimental method based on temperature-dependent miscibility of dimethyl sulfoxide (DMSO) with diethyl ether (DE) and composition optimization of perovskite materials (i.e., FA1–xMAxPb(BryI1–y)3, FA1-zCszPb(BryI1–y)3 and MA1–wCswPb(BryI1–y)3). To study the detailed mechanism of the wrinkling, we suggest a bilayer wrinkling model with a theoretical analysis supported by numerical simulations and experimental measurement of optical diffraction. We extend a scope of the study to the investigation of the effects of the microscopic wrinkles on the charge carrier dynamics with time-correlated single-photon counting (TCSPC) coupled with fluorescence lifetime imaging microscopy (FLIM) and photoconductive atomic force microscope (pc-AFM). From the combined experimental data, we have found that the wrinkled morphology notably facilitates the charge carriers transport inside the perovskite films.

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