(Invited) Probing Local, Hybrid Perovskite Photophysics through Spatially- and Temporally-Resolved Absorption/Emission Microscopy

Monday, 2 October 2017: 14:00
National Harbor 6 (Gaylord National Resort and Convention Center)
M. Kuno and S. Draguta (university of notre dame)
The presentation will describe recent work conducted to investigate local hybrid perovskite photophysics through spatially-resolved optical measurements. In particualar, by conducting local emission intensity (Iem) versus excitation intensity (Iexc) measurements, we have established the existence of optical response variations due to electronic disorder stemming from the presence of nonuniform trap densities. Regions of solution-processed MAPbI3 films exhibit effective free carrier recombination while others exhibit emission dynamics strongly influenced by trap states. Through appropriate kinetic modeling of the data, quantitative estimates of local trap densities (Nt) have been made. What results are Nt values between Nt~1015-1017 cm-3. We have additionally found that spatially-varying trap densities impact MAPbI3 radiative recombination efficiencies, resulting in local emission quantum yields that range from 5% to ~7% at 1 sun. To link the above conclusions with corresponding solar cell performance, we have begun to correlate optical response to local solar cell efficiencies. These measurements have entailed Iem versus Iexc, emission lifetime, and photocurrent measurements on planar (FTO/compact-TiO2/MAPbI3/Spiro-MeOTAD/Au) ~17% efficiency solar cells. Obtained results confirm that photogenerated electrons are the charge species being trapped in MAPbI3. The fraction of trapped electrons is small and is on the order of 2-3%. An even smaller (~0.2%) fraction undergoes radiative recombination. What remains available for current generation is ~97-98% of the initially photogenerated carriers. Unfortunately, interface recombination at electron/hole transporting layers significantly suppresses this value, resulting in net power conversion efficiencies below the Shockley limit. Subsequent spatially-resolved measurements (conducted on planar as well as on analogous planar-inverted and mesoporous solar cells) link local power conversion efficiencies to underlying charge trapping/charge injection efficiencies.