Investigations of charge separation and migration in lead-free perovskite-based thin films and devices for photovoltaics using ultrafast broadband optical techniques and electrical characterization methods

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Over the last years, lead-based halide perovskite thin films have shown a phenomenal rise in photovoltaic performance, currently reaching efficiencies exceeding 22%. However, intrinsic stability problems related to, e.g., moisture and thermal decomposition as well as the toxicity of the resulting water-soluble lead salts might eventually hamper their widespread application. One of the main challenges in the field is therefore finding nontoxic and stable perovskites with good optoelectronic properties. This requires a deep understanding of the carrier dynamics and the photovoltaic properties of such systems. In the current proposal, which builds on previous studies by the Lenzer, Oum and Schlettwein groups on halide perovskites, we would therefore like to investigate the largely unexplored charge carrier dynamics and electrical properties of promising lead-free perovskite thin films and devices based on bismuth, antimony, palladium, tin and indium. Powerful time-resolved laser techniques based on pump - supercontinuum probe (PSCP) spectroscopy / microscopy covering the UV-Vis-NIR range over time scales from femtoseconds to milliseconds will be employed in combination with electrical characterization methods adapted to the analysis of thin photovoltaic active layers to investigate a well-defined set of perovskite thin films and devices based on double-perovskites and vacancy-ordered low-dimensional perovskite structures. This way we will obtain a microscopic understanding of the photoinduced charge carrier separation, transport, cooling and recombination dynamics. Electrical characterization will aim at the bulk properties and contact formation in the interfaces as well as at complete solar cells determining the current-voltage curves, power conversion efficiency and the spectral dependence of the external quantum efficiency of these systems. The lead-free perovskites will be prepared by wet-chemistry approaches involving spin-coating, physical vapor deposition and pulsed laser deposition methods. High-quality thin films will be deposited on a range of substrates to establish simple perovskite interface systems as well as complete photovoltaic devices employing n-i-p and p-i-n architectures, both mesoporous and planar. In addition to the spectroscopy / microscopy techniques used in the previous funding period, we will newly implement PSCP broadband transient reflection spectroscopy: For transparent samples, this will be simultaneously used with transient transmission spectroscopy to determine both, changes in reflectance and transmittance. In addition, this new setup will enable us to study the strongly absorbing and opaque perovskite thin films in solar cells.