Solution-processed metal-oxide based hole transport layers for organic and perovskite solar cell: A review

Metal oxides are an important class of materials with interesting electronic properties. Transparent conducting oxides has been key focus in display industry as well as in photovoltaic industry. However, metal oxides with semiconducting character are also gaining attention for their application potential in printable electronic devices. While n-type metal oxides with wide band gap (> 3 eV) are readily available, the p-type ones with matching conductivity and transparency are rare. In this review, we discuss the theoretical background of p-type oxides to account for their inferior conductivity and visible light transparency compared to their n-type counterpart. We review the current state-of-the-art solution processed p-type oxides and their applications in organic and perovskite solar cell devices as hole transport layer (HTL). Binary oxides of transition metals such as copper, nickel, ternary oxides of copper (delafossites) and transition metal oxides with n-type characteristics (MoO3, V2O5 and WO3) has been discussed in detail. Few emerging organic and inorganic HTL materials has been discussed along with their limitations in devices. Finally, we identify certain types of metal oxides that hold promise for development of future p-type metal oxides with comparable electrical conductivity and optical transparency as n-type ones.