Low-cost semi-transparent copper sulfide electrode for indium-tin-oxide-free perovskite solar cells

Abstract Development of transparent electronics has been hindered by the lack of stable p-type transparent conductive electrodes. Herein we report the fabrication of p-type copper sulfide thin films (Cu x S) used for the first time as semitransparent electrode used as an example in inverted-planar perovskite solar cells (PSCs). The resulting hole-transporting-material-free and indium-tin-oxide-free (ITO-free) device showed an unprecedented 5.9% efficiency (PCE). Morphological, structural and optoelectronic analysis of the Cu x S films were accomplished to correlate the photovoltaic behavior of the obtained PSCs. In particular, covellite (CuS) and digenite (Cu 1.8 S) crystal phases were identified in the resulting Cu x S film. The p-type Cu x S semitransparent electrode exhibited high 1094 S cm −1 electrical conductivity comparable to state-of-the-art transparent conductive electrodes (TCE). Although the Cu x S electrode exhibits a relatively low bandgap around 2.7 eV, the spectral response of the devices demonstrates that the related optical loss happens just in the blue and UV region of the visible spectrum leading to a reduction of the photocurrent below 10%. On the other hand, surface photovoltage analysis of the grown perovskite indicates that the Cu x S electrode induces an optimal band alignment in the perovskite surface while trap states were favored by using Cu x S regarding ITO. These results suggest that photovoltaic parameters of Cu x S-based PSCs can be still improved by a better control of perovskite growing. Finally, material cost per square meter (US$/m2) was estimated for Cu x S electrodes and an impressive reduction of 77.6% was observed compared to commercial ITO. These results highlight the potential application of Cu x S film as low-cost, non-toxic and scalable p-type semitransparent electrode in PSCs and other optoelectronic devices.