Illuminating Structure-Property Relationships of Methylammonium-Free Lead Halide Perovskites Through Advanced Characterization Studies of Halide- and Phase- Segregation

Perovskite solar cells have rapidly emerged over the past 25 years as a formidable competitor to traditional thin film solar cell technologies, with low costs and high efficiencies. The typical perovskite structure is ABX3 with A = (Cs, methylammonium (MA), formamidinium (FA)); B = (Pb, Sn); and X = (Cl, Br, I). Lead halide perovskites are gaining traction as potential materials for pure perovskite solar cells or tandem solar cells due to their tunable composition and bandgap. However, these materials have issues with long term stability that must be resolved before commercialization, stemming from phase segregation, halide segregation, and chemical decomposition all resulting in the formation of photoinactive phases. Most lead halide perovskites are made with methylammonium (MA), which undergoes a phase transition at 57C and decomposes into PbI2 even in an inert environment. To overcome thermal stability issues, past work has begun exploring the formaminidium (FA) and cesium (Cs) compositional space, varying ratios of iodine and bromine to work towards matching the efficiencies of MA-based perovskites while harnessing the thermal stability of FA and Cs.<br/> However, the structure-property relationships of these CsFAPbIBr perovskites are not entirely understood, particularly regarding phase and halide segregation. Using a variety of advanced characterization methods, the mechanisms behind phase and halide segregation are being explored with the goal of understanding the formation of photoinactive phases to improve long term stability. Preliminary XRD data where perovskite films of varying compositions were annealed while carrying out in-situ measurements indicate both irreversible and reversible phase changes occurring while increasing temperatures as high as 300C. This data indicates that phase transformation mechanisms are more complex than simple degradation of the perovskite phase at elevated temperature. <br/>Further exploration of these phase changes is required, and additional experiments will be carried out at the Synchrotron at Lawrence Berkeley National Labs, pending proposal approval that is currently under review. X-ray microdiffraction in-situ while spin coating at changing temperatures will show phase transitions that occur during spincoating which cannot be understood without in-situ measurements and thus are currently entirely unexplored for these compounds. Through another pending proposal at Oak Ridge National Lab, further heating studies will be carried out using in-situ heating during AFM. As each film is heated by the AFM tip, changes in the microstructure will be immediately detected, providing much more specific data than gained from the preliminary in-situ heating during XRD. The in-situ heating during AFM will provide instantaneous feedback on the relationship between temperature and phase- and halide- segregation by tracking the changes in physical morphology of the film on the nanoscale while heating occurs. By harnessing these techniques, we will be able to further analyze the spatial distribution of the phase segregation of our promising mixed-cation (Cs, FA), mixed halide (Br, I) Lead perovskite compositions.<br/> In addition to stability issues stemming from phase segregation, new data indicates that halide segregation is probable in all films, regardless of cation choice. XRF and XBIC show that current is closely correlated with bromine segregation in solar cell devices. To further understand halide segregation and connect it with the temperature studies on phase segregation, these same compounds will be explored at Oak Ridge National Lab using SEM with correlated cathodoluminescence. This technique allows for the spatial correlation of data on optical properties with the structural properties of the film. Using this technique, final links can be drawn between structure and optical properties by correlating phase and halide segregation with optical properties spatially.