We show that adding ethylenediamine (EDA) to perovskite precursor solution improves the photovoltaic device performance and material stability of high-bromide-content, methylammonium-free, formamidinium cesium lead halide perovskites FA1-xCsxPb(I1-yBry)3 which are currently of interest for perovskite-on-Si tandem solar cells. Using spectroscopy and hyperspectral microscopy, we show that the additive improves film homogeneity and suppresses the phase instability that is ubiquitous in high-Br perovskite formulations, producing films that remain stable for over 100 days in ambient conditions. With the addition of 1 mol% EDA we demonstrate 1.69 eV-gap perovskite single-junction p-i-n devices with a VOC of 1.22 V, and a champion maximum power point tracked power conversion efficiency of 18.8%, comparable to the best reported methylammonium-free perovskites. Using nuclear magnetic resonance (NMR) spectroscopy and X-ray diffraction techniques, we show that EDA reacts with FA+ in solution, rapidly and quantitatively forming imidazolinium cations. It is the presence of imidazolinium during crystallization which drives the improved perovskite thin-film properties.
@article{arxiv.2209.15100,
title = {Ethylenediamine Addition Improves Performance and Suppresses Phase Instabilities in Mixed-Halide Perovskites},
author = {Margherita Taddei and Joel A. Smith and Benjamin M. Gallant and Suer Zhou and Robert J. E. Westbrook and Yangwei Shi and Jian Wang and James N. Drysdale and Declan P. McCarthy and Stephen Barlow and Seth R. Marder and Henry J. Snaith and David S. Ginger},
journal= {arXiv preprint arXiv:2209.15100},
year = {2022}
}