Scanning X-ray nanodiffraction unveils systematic strain gradient in 75 000 perovskite grains

Perovskite solar cells (PSCs) have become a major focus of photovoltaics research. Degradation and heterogeneity in performance, composition, and structure are among the pressing challenges on the pathway towards commercialization. Here, scanning X-ray diffraction (sXRD) measurements of a wide-bandgap triple-cation PSC with a Cs0.12FA0.8MA0.08PbI1.8Br1.2 absorber were performed over an of area 300 μm × 300 μm at the Hard X-ray Nano-/Microprobe P06 of PETRA III (Hamburg, Germany). The cumulated diffractogram was found to agree with the cubic perovskite α-phase at a lattice constant of a = 6.13 Å. In this contribution, we give insights into the development of a 4D clustering algorithm optimized for large datasets that enabled the mapping of 75 000 grains, obtaining the size of each grain, its azimuthal orientation χ, and spatial distribution of the lattice strain ε. Specifically, a systematic gradient in the strain is observed within most grains at an average of 0.1% per μm, indicating significant structural inhomogeneity. This strain gradient likely arose during perovskite growth, as it aligns with the radial axis of the spin-coater. Although it appears lateral in sXRD, it may indeed represent a depth gradient, unveiled by systematic tilting towards the outboard direction of the spin-coater due to centrifugal force. Simultaneously performed X-ray excited optical luminescence (XEOL) measurements indicate a subpar charge-carrier lifetime, and the formation of morphological wrinkles on the surface.