Engineering Schottky Contacts in Open-Air Fabricated Heterojunction Solar Cells to Enable High Performance and Ohmic Charge Transport

The efficiencies of open-air processed Cu₂O/Zn_1–<i>x</i>Mg_<i>x</i>O heterojunction solar cells are doubled by reducing the effect of the Schottky barrier between Zn_1–<i>x</i>Mg_<i>x</i>O and the indium tin oxide (ITO) top contact. By depositing Zn_1–<i>x</i>Mg_<i>x</i>O with a long band-tail, charge flows through the Zn_1–<i>x</i>Mg_<i>x</i>O/ITO Schottky barrier without rectification by hopping between the sub-bandgap states. High current densities are obtained by controlling the Zn_1–<i>x</i>Mg_<i>x</i>O thickness to ensure that the Schottky barrier is spatially removed from the <i>p</i>–<i>n</i> junction, allowing the full built-in potential to form, in addition to taking advantage of the increased electrical conductivity of the Zn_1–<i>x</i>Mg_<i>x</i>O films with increasing thickness. This work therefore shows that the Zn_1–<i>x</i>Mg_<i>x</i>O window layer sub-bandgap state density and thickness are critical parameters that can be engineered to minimize the effect of Schottky barriers on device performance. More generally, these findings show how to improve the performance of other photovoltaic system reliant on transparent top contacts, e.g., CZTS and CIGS