Elucidating the role of the hole-extracting electrode on the stability and efficiency of inverted CsSnI₃/C₆₀perovskite photovoltaics

Abstract

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.

Cs1−xRbxSnI3light harvesting semiconductors for perovskite photovoltaics

“…S2) View Article Online relative intensities of reflections, indeed we have previously shown that B-g CsSnI 3 films deposited in the same way as used in the current study can exhibit substrate specific preferred crystallite orientation. 9 Since the degree of orientation may also conceivably be affected by the compositions of the samples this analysis cannot be quantified.…”

“…We have recently shown that B-g CsSnI 3 PV devices with an inverted planar device architecture exhibit the best efficiently and stability when not using a hole-transport layer. 8,9 For this reason we have used this simplified device architecture as a test bed for these new perovskite materials that are closely related to B-g CsSnI 3 .…”

“…11 Additionally, for PV devices using B-g CsSnI 3 as the light harvesting layer, we have shown that SnCl 2 is an effective additive for reducing the density of tin vacancy defects in the band gap and reducing the reverse saturation current, 8 both of which help to improve V oc . However, V oc is ultimately limited by the relatively small ionisation potential (I p ) of B-g CsSnI 3 of B4.9 eV, 9,12 which is approximately half an electron volt smaller than that of lead halide perovskites. 13 Fortunately, similar to lead halide perovskites, B-g CsSnI 3 is amenable to substitution of iodide ions with bromide ions, which results in an increase in I p and band gap which translates to an increase in V oc in PV devices.…”

“…To date it has been used as the light harvesting semiconductor in PV devices based on mesoporous TiO 2 5,6 and planar [7][8][9] device architectures, demonstrating the potential to achieve very high short circuit current density ( J sc ) of 420 mA cm À2 under one sun illumination 6 and high device fill factor (FF). 8 The primary factor limiting the power conversion efficiency of B-g CsSnI 3 based PVs is the low open-circuit voltage (V oc ): to date the highest V oc reported is 0.55 V, 8 achieved for an inverted device architecture using phenyl-C 61 -butyric acid methylester (PCBM) as the electron transport layer (ETL), which is approximately half that attainable using the lead analogue CsPbI 3 .…”

“…10 When using C 60 in place of PCBM the highest V oc reported is reduced to 0.36 V due to the larger electron affinity of the former. 9 The V oc in inverted perovskite PV devices depends strongly on a number of factors including the energetics at the perovskite-ETL interface 7 and the degree of crystallinity in the ETL. 11 Additionally, for PV devices using B-g CsSnI 3 as the light harvesting layer, we have shown that SnCl 2 is an effective additive for reducing the density of tin vacancy defects in the band gap and reducing the reverse saturation current, 8 both of which help to improve V oc .…”