Jariwala, S., Kumar, R., Eperon, G., Shi, Y., Fenning, D., Ginger, D.,
ACS Energy Letters
Adding a large A-site cation, such as dimethylammonium (DMA), to the perovskite growth solution has been shown to improve the performance and long-term operational stability of halide perovskite solar cells. To better understand the origins of these improvements, we explore the changes in film structure, composition, and optical properties of a formamidinium (FA), Cs, Pb, and mixed halide perovskite following the addition of DMA to the perovskite growth solution in the ratio of DMA0.1FA0.6Cs0.3Pb(I0.8Br0.2)3. Using time-of-flight secondary-ion mass spectrometry (TOF-SIMS), we show that DMA is indeed incorporated into the perovskite, with a higher DMA concentration at the surface. Using a combination of photoluminescence (PL) microscopy and photoinduced force microscopy-based (PiFM) nanoinfrared (nanoIR), we demonstrate that incorporating DMA into the film leads to increased local heterogeneity in the local bandgap and clustering of the local formamidinium (CH5N2+) composition. In addition, using nano-X-ray diffraction, we demonstrate that DMA incorporation also alters the local structural composition by changing the local d-spacing distribution and grain size. Our results suggest that compositional variations in the organic cations at the A-site drive the structural heterogeneity observed in the case of DMA-incorporated films. Our results also suggest that while current DMA-additive-based approaches do have benefits to operational stability and device performance, process optimization to achieve local compositional and structural homogeneity could further boost both of these gains in performance, bringing further gains to solar cells using DMA additives.