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