Kyle Frohna, University of Cambridge
Late News: Spatial Chemical Heterogeneity Circumvents Power Losses from Local Electronic Disorder in High-Performance Halide Perovskite Solar Cells
Written by Jessalyn Hui Ying Low
Halide perovskites are a class of materials commonly used in optoelectronic devices. They typically exhibit compositional, structural, and optoelectronical heterogeneity; however, understanding of heterogeneity in perovskites is still lacking and necessitates a deeper understanding of their mechanisms to improve performance. In particular, Kyle Frohna and his research team had previously observed in FA0.79MA0.16Cs0.05Pb(I0.83Br0.17)3 (FA=formamidinium, MA=methylammonium, Cs=caesium, Pb=lead, I=iodine, Br=bromine) perovskite samples the localization of deep trap states at interfaces.
To further gain insights into this phenomenon, a series of microscopy techniques were used - hyperspectral microscopy, synchrotron nanoprobe x-ray microscopy, and transient absorption microscopy (TAM), as Frohna reports in this talk. Interestingly, hyperspectral data indicated that the region with highest photoluminescence quantum efficiency (PLQE) showed a red shoulder in emission, suggesting phase segregation between Br and I.
To understand this, correlation between data from the various techniques was studied. From hyperspectral-synchrotron correlated data, it was found that regions with highest Br:Pb corresponded to regions with lowest Urbach energy (EU) and highest PLQE. This meant that regions that are most emissive are Br-rich regions, but also have a red shoulder, which seemed counter-intuitive. TAM-synchroton correlated results was then studied, which showed that at Br-rich regions, ground state bleach (GSB) energy shifted from a high to low energy state, while in I-rich regions, no energetic shift was observed.
With these interesting results, Frohna explains that they believe that Br-rich regions exhibit local phase segregations, but not in Br-poor regions. Small I-rich inclusions possess a lower bandgap, thus allowing in Br-rich regions the funneling of carriers energetically from high to low band regions. This locally increases carrier density and overwhelms the trap density, allowing for increased emission. Br-poor regions, however, lack such carrier funnels to avoid traps, resulting in poorer performance. This study therefore reveals how intrinsic disorder can bring about a carrier funneling mechanism, an important contributor to perovskite performance, thereby opening up new possibilities for designing high-performance perovskites.