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The preparation of CsSnI3 films at room temperature with defect densities low enough for light harvesting in photovoltaic devices is reported, along with a new strategy for improving device stability and evidence that the energetics at the perovskite/fullerene interface is a key determinant of the open-circuit voltage.

The correlation between the stability of thin films of black (B)-g CsSnI 3 perovskite in ambient air and the choice of supporting substrate is examined for the substrates: (i) soda-lime glass; (ii) indium tin oxide (ITO) glass; (iii) copper iodide (solution processed)/ITO glass; (iv) poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/ITO glass; (v) and an optically thin (8 nm) gold film electrode. The performance of (ii)-(v) as the hole-extracting electrode in inverted photovoltaic (PV) devices with a simple bilayer architecture is compared for a test condition of 1 sun continuous solar illumination in air. CsSnI 3 film stability is shown to depend strongly on the density of pinholes and grain boundaries, although not on the preferred CsSnI 3 crystallite orientation. Solution processed CuI is shown to be unsuitable as a hole-transport layer (HTL) for inverted CsSnI 3 PV devices because it is almost completely displaced by the CsSnI 3 precursor solution during the spin coating process, and its large ionisation potential is poorly matched to the valence band edge of CsSnI 3 . Devices using an ITO (or Au) hole-extracting electrode with no HTL are found to be more stable than those using the archetypal HTL; PEDOT:PSS. Spectroscopic analysis of the CsSnI 3 layer recovered from PV devices after 24 hours testing in ambient air (with no device encapsulation) shows that #11% of the CsSnI 3 film thickness is oxidised to Cs 2 SnI 6 due to air ingress, which shows that the deterioration in device efficiency under continuous illumination does not primarily result from a reduction in the light absorption capability of the perovskite film due to CsSnI 3 oxidation. Additionally it is shown that SnCl 2 added during CsSnI 3 film preparation reduces the extent of p-type self-doping of the perovskite film and serves as an n-type dopant for the adjacent evaporated C 60 electron transport layer.