Spatially resolved ultrafast phenomena in perovskite solar films (Keshav Dani, Okinawa Institute of Science and Technology)

Spatially resolved ultrafast phenomena in perovskite solar films

Prof. Keshav Dani
Femtosecond Spectroscopy Unit
Okinawa Institute of Science and Technology Graduate University

Hybrid organic-inorganic perovskite (HOIP) materials for photovoltaic applications have seen tremendous development over the last several years due to their desirable properties, such as long carrier lifetime, strong absorption across the visible to the NIR, and ease of production using established low-cost solution based processing techniques [1]. Despite solar conversion efficiencies rapidly approaching that of traditional single crystal silicon, there are still many ongoing challenges and issues for HOIP materials. One of the significant issues for HOIP solar cell devices is the spatial inhomogeneity in quantum efficiency or photoluminescence seen across films. Previous studies have seen indirect evidence pointing to the presence of a high density of trap states. Other studies have pointed to spatial inhomogeneity in the diffusion of carriers. The precise nature and cause of the spatial inhomogeneity remains a topic of hot debate in the community.

Here, for the first time, we use time resolved photoemission electron microscopy (TR-PEEM) to directly image and visualize the presence of trap states on the surface of perovskite solar films. Using time-resolved photoemission electron microscopy (TR-PEEM), we then make a movie of the trapping of photoexcited charges at the individual trap sites with 10-nm scale spatial resolution and 100-femtosecond scale temporal resolution. We extract a number of useful parameters from the images of the trap states and the movies of trapping dynamics. 

Time permitting, I will end with a brief discussion about our other work in black phosphorous at OIST.