CPM Seminar

The excitement and frustration of persistent photoconductivity in lead-halide perovskites

John A. Marohn

Department of Chemistry and Chemical Biology
Cornell University

Films of lead-halide perovskites, precipitated from solution at room temperature, exhibit high power-conversion efficiency in solar-cell applications. Yet these materials have a low defect-formation energy, have poor operational stability, and are difficult to fabricate reproducibly. In this talk I will describe my team’s work using electrical scanning probe microscopy to make non-contact measurements of the electronic and ionic conductivity of lead-halide perovskites. Measurements of non-contact friction and cantilever frequency shift in a wide variety of perovskites, both in the dark and under illumination, indicate that (1) conductivity is substrate dependent and (2) in most perovskites, light creates persistent ionic conductivity lasting tens of seconds at room temperature. This conductivity shows an activated recovery with an activation energy of 0.5 eV, consistent with the recombination of photogenerated halide vacancies and interstitials. From these measurements we conclude that light creates defects in lead-halide perovskites. Our experiments have forced us to rethink, from first principles, what sample properties are being measured in widely used semiconductor measurements like scanning Kelvin probe force microscopy. Thus while the measurement, as hoped, has taught us something new about the material, here the material has taught us something new about the measurement as well.