Effect of ZnO and PEDOT:PSS charge selective layers on photovoltage of cuprous oxide (Cu2O) heterojunction solar cells

Abstract

Electrochemical deposition (ECD) of Cu₂O provides a scalable and low temperature pathway to solar cells with a theoretical high energy conversion efficiency of 23%, based on the 2.0–2.2 eV band gap of Cu₂O. However, existing ECD-Cu₂O devices are plagued by poor crystallinity and low selectivity of the electron and hole transport layers. Here we use Vibrating Kelvin Probe Surface Photovoltage Spectroscopy (VKP-SPV) to probe the charge transfer selectivity in FTO/ZnO/Cu₂O/PEDOT:PSS/Ni heterojunction solar cells. Selective electron extraction is achieved at the ZnO back contact to Cu₂O, as confirmed by a negative surface photovoltage signal. While the uncoated Cu₂O surface is electron-selective due to the formation of a hole depletion layer, spin coating of a PEDOT:PSS film turns it into a hole-selective interface. After adding nickel metal ink top electrodes, functional 1.0 cm² solar cells with power conversion efficiency (PCE) of up to 0.07%, 195 mV open-circuit voltage, and 1.28 mA cm⁻² short-circuit photocurrent are achieved. The photovoltaic performance is higher for aluminum doped zinc oxide (AZO) substrates than for fluorine doped tin oxide (FTO) due to the presence of a n-/p-junction that further increases the electron selectivity of the AZO/ZnO/Cu₂O contacts. Overall, this work demonstrates the first application of PEDOT:PSS as a hole transport layer (HTL) for Cu₂O and the use of VKP-SPV to measure the photovoltage contributions of the Cu₂O interfaces. The ability to fabricate Cu₂O solar cells at near room temperature without the use of vacuum methods or rare elements is an important step towards a scalable Cu₂O PV technology.