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

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 (CuxS) 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 CuxS films were accomplished to correlate the photovoltaic behavior of the obtained PSCs. In particular, covellite (CuS) and digenite (Cu1.8S) crystal phases were identified in the resulting CuxS film. The p-type CuxS semitransparent electrode exhibited high 1094 S cm−1 electrical conductivity comparable to state-of-the-art transparent conductive electrodes (TCE). Although the CuxS 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 CuxS electrode induces an optimal band alignment in the perovskite surface while trap states were favored by using CuxS regarding ITO. These results suggest that photovoltaic parameters of CuxS-based PSCs can be still improved by a better control of perovskite growing. Finally, material cost per square meter (US$/m2) was estimated for CuxS electrodes and an impressive reduction of 77.6% was observed compared to commercial ITO. These results highlight the potential application of CuxS film as low-cost, non-toxic and scalable p-type semitransparent electrode in PSCs and other optoelectronic devices.