Understanding Photoluminescence Heterogeneity in Mixed-Cation Mixed-Halide Perovskites

Abstract

The byproducts emitted from the heavy use of fossil fuels in our modern world result in high atmospheric concentrations of greenhouse gases which contribute to global warming and a variety of other detrimental environmental effects. To address this issue, the International Energy Agency has urged nations to transition to renewable energy sources and achieve zero-net emissions by the year 2050. In the last decade, the use of solar photovoltaic electricity (PV) has grown by over 30% in the United States and crystalline silicon solar cells account for more than 95% of the total solar PV market share. Despite this, commercially available silicon solar panels have reached an efficiency saturation at ~26%. To increase the performance of these silicon solar cells to over 45% an extra absorber layer with a wider bandgap can be added on top, creating a multijunction solar cell. In recent years, metal halide perovskites (MHPs) have gained attention as promising materials for multijunction solar cells due to their solution processability and tunable bandgap facilitated by simple substitutions of chemical elements at the A, B, and X sites of the ABX3 cubic crystal structure. Optimal performance of a perovskite-on-silicon multijunction solar cell requires the MHP layer to have a bandgap (Eg) of approximately 1.7 eV which can be achieved by increasingthe Cs+ -to-formamidinium (FA) ratio in the A-site or the Br- to I- ratio in the X-site of a lead-based halide perovskite. Increased Cs:FA and Br:I ratios, however, lead to the photoinduced formation of sub-bandgap regions and photoinactive domains which negatively impact the performance and the stability of the MHP active layer. In the first work described herein, we examine the effect of adding ethylenediamine (EDA) during film formation to ~1.7 eV FA0.83Cs0.17Pb(I0.75Br0.25)3 MHP. Our results demonstrate that films made with an optimal concentration of EDA (1 mol%) have enhanced phase stability over a period of 100 days in air and improved solar cell device performance. Using widefield photoluminescence (PL) microscopy, we demonstrate that films made with EDA display a greater degree of spatially resolved photoluminescence homogeneity at the microscale. In the second work, we further showed that the PL heterogeneity of these MHPs is a result of halide heterogeneity by correlating hyperspectral PL imaging and time-of-flight secondary ion mass spectrometry (ToF-SIMS). To further characterize charge recombination dynamics in these heterogeneous MHP films, we investigate best practices for analysis of PL decay curves. Via simulations and experiments, we demonstrate that a stretched exponential fitting better describes the physical heterogenous distribution of charge carriers recombining in polycrystalline FA0.83Cs0.17Pb(I0.75Br0.25)3 MHP films both horizontally and vertically. In the third work, we propose the use of multiple excitation wavelengths in fluorescence lifetime imaging to map depth-dependent non-radiative losses of 1.7 eV Eg MHP at the interface with charge transport layers (CTLs) which are a major pathway for charge recombination reducing the open circuit potential in the final solar cell device below its theoretical maximum value. Finally, we show that combining these approaches we were able to fabricate a Silicon-perovskite tandem with an amine-based interlayer between the perovskite and the electron-transport layer to achieve better performance and stability.