Written by Tyler W. Farnsworth
Marina S. Leite, University of Maryland, College Park
Nanoimaging of Perovskite Solar Cell Degradation
Marina S. Leite and co-workers at the University of Maryland tackled the problem of perovskite degradation using Kelvin probe force microscopy (KPFM). Her work is a great visual demonstration of the degradation process and offered important insight into the mechanism behind this occurrence. Leite’s technique employed ultrafast scans (16 seconds) to measure the
spatial variation (resolution = 100 nm) of photogenerated voltage within perovskite films. By first measuring a dark film and then illuminating it, she was able to calculate an offset voltage (post-illumination voltage variation) and map the perovskite dynamics in real-time. All measurements were taken in a constant temperature environment with less than 15% humidity. Her results show that KPFM is a four-dimensional universal tool for mapping Voc in photovoltaic materials and gives promise for the development of this instrument for future applications.
Sam Stranks, Massachusetts Institute of Technology (MIT)
Photo-induced Halide Redistribution in Hybride Perovskite Films
The stability of iodine-containing perovskites has been under review and marked a hot topic during the perovskite rump session. Sam Stranks (MIT) and co-workers provided insight into this discussion by showing direct experimental observation of halide migration in MAPbI3 after illumination. He used photoluminescence (PL) lifetime measurements to map the effect of non-radiative decay on device performance. Upon illumination, PL mapping shows a distribution of light and dark states; however, over time, the light states move throughout the film. Stranks used time-of-flight secondary ion mass spectrometry to investigate the local chemical environment and attributed the light movement to iodide migration away from the initial illuminated area. In addition, SIMS depth profiling showed a reduction of the iodide and an enrichment of iodide at the surface of the film. He proposed a mechanism of trap filling that would perturb the system and create fields to drive halide migration, giving important insight into the stability of MAPbI3 perovskites. His research is a helpful look at how macroscopic measurements can give detailed information about microscopic influences on device performance.
James Endres, Princeton University
PES Study of the Inorganic Perovskite CsPbBr3 and Complementary Hole Transporting Polymer
James Endres (Princeton) and co-workers presented CsPbBr3 as an alternative to the methyl ammonium-based perovskite MAPbI3. The optical gap of CsPbBr3 (2.3 eV) is similar to the more traditional perovskite and, with solar cell efficiencies similar to that of MAPbI3, the completely inorganic CsPbBr3 may very well be a promising alternative to MAPbI3 as a solar cell material. Endres focused his talk on measurements of the CsPbBr3 electronic structure, using both ultra-violet photoemission spectroscopy (UPS) to measure the valence band and inverse photoemission spectroscopy (IPES) to gather information on the conducting band. His results offered insight into band alignments with different stacking arrangements and doping profiles of poly-triarylamine (PTAA). His final device included CsPbBr3 sandwiched between a bottom layer of TiO2 and a 15 nm top layer of doped PTAA. His preliminary UPS and IPES results indicate good hole conduction, but a potential electron blocking effect. Future work will investigate the observed band bending and tweak device parameters for better performance.