  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.

Materials Science;Applied Physics