Dimethylammonium additives alter both vertical and lateral composition in halide perovskite semiconductors

TL;DR

This study investigates how dimethylammonium (DMA) additives influence the local composition, structure, and optical properties of halide perovskite films, revealing heterogeneity and potential pathways for performance enhancement.

Contribution

It provides detailed insights into the structural and compositional changes caused by DMA additives in halide perovskites using advanced microscopy and diffraction techniques.

Findings

DMA is incorporated mainly at the surface of the perovskite film.

DMA incorporation increases local heterogeneity in bandgap and composition.

Structural changes include altered grain size and d-spacing distribution.

Abstract

Adding a large A-site cation, such as dimethylammonium (DMA), to the perovskite growth solution has been shown to improve 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, mixed halide perovskite following addition of DMA to the perovskite growth solution in the ratio 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 PL microscopy and photo-induced 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 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.