Direct Optoelectronic Imaging of Nanostructured Halide Perovskites

University Of California-Davis, Davis CA

Investigators

Abstract

Nontechnical Description: This project aims to investigate the charge transport mechanisms in a promising photovoltaic material, halide perovskites. Halide perovskite compounds, including methylammonium lead halide, have recently demonstrated great potentials for solar energy conversion, with a power conversion efficiency above 20%. However, the fundamental understanding of the material physics is lacking. The project aims to extract fundamental science and provides key insights on the charge transport mechanisms and guidelines for identifying more stable perovskite compounds for cost-effective photovoltaics. In particular, the principal investigator uses a novel experimental technique, which maps the photogenerated current distribution by scanning a focused laser beam. This direct imaging technique allows extraction of key information on how light is converted into charge and how charge transports in these materials. The fundamental understanding of the origins of the high conversion efficiency is not only intriguing to fundamental science, but is also critical to developing better perovskite materials with reduced toxicity and increased stability. The study may lead to earth abundant materials that promise to open a new paradigm for photovoltaics. This project educates and trains undergraduate and graduate students in the rapidly advancing nanoscale and energy sciences, and offers outreach activities targeting K-12 students, underrepresented minorities. Technical Description: The project investigates the charge transport in single-crystal halide perovskite nanostructures and distinguishes the dominant charge transport mechanism among ferroelectricity, ion migration, and charge traps. Despite the demonstration of efficient power conversion capability in these materials, material physics such as the origins of long carrier lifetime and hysteretic photocurrent is not understood. Nanowires and nanoplates composed of single-crystal halide perovskites can remove the convolution of grain boundaries and allow better understanding of the intrinsic properties. Devices consisting of individual nanostructures are investigated by wavelength-dependent scanning photocurrent microscopy to determine how minority carrier diffusion lengths depend on surface effects and carrier concentration. Direct optoelectronic imaging of these nanostructures under external electric field at various temperatures provides key insights for distinguishing among different charge transport mechanisms. Piezoelectric effects are studied in single-crystal nanostructures under an external strain. The transport of different ions such as hydrogen and lithium in halide perovskites is investigated by using a novel liquid gating method. The research deepens the fundamental understanding of the origins of the high conversion efficiency and allows formulating guidelines for choosing better molecules or atoms to substitute the cations or anions in halide perovskites.