Elucidating the role of the hole-extracting electrode on the stability and efficiency of inverted CsSnI3/C60 perovskite photovoltaics

Abstract

The correlation between the stability of thin films of black (B)-γ CsSnI₃ 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. CsSnI₃ film stability is shown to depend strongly on the density of pinholes and grain boundaries, although not on the preferred CsSnI₃ crystallite orientation. Solution processed CuI is shown to be unsuitable as a hole-transport layer (HTL) for inverted CsSnI₃ PV devices because it is almost completely displaced by the CsSnI₃ precursor solution during the spin coating process, and its large ionisation potential is poorly matched to the valence band edge of CsSnI₃. 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 CsSnI₃ layer recovered from PV devices after 24 hours testing in ambient air (with no device encapsulation) shows that ≤11% of the CsSnI₃ film thickness is oxidised to Cs₂SnI₆ 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 CsSnI₃ oxidation. Additionally it is shown that SnCl₂ added during CsSnI₃ 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 C₆₀ electron transport layer.