Sub-10 nm copper chromium oxide nanocrystals as a solution processed p-type hole transport layer for organic photovoltaics

Abstract

We report the synthesis of CuCrO₂ nanocrystals, a p-type transparent conducting oxide, and their application as an efficient hole transport layer (HTL) for organic photovoltaic (OPV) devices. A nanometer-sized mixture of Cu and Cr oxide/hydroxide is synthesized using microwave-assisted heating. With a 550 °C post-annealing treatment in N₂, <10 nm CuCrO₂ nanocrystals are successfully synthesized. XRD, XPS, EDAX, PESA, UV-vis spectrometry, and Kelvin probe technique are applied to confirm the delafossite phase, optical transmission, and p-type characteristics. Methanol is found to be a good solvent to disperse these nanocrystals for forming a smooth and transparent film. In comparison with the previously reported CuGaO₂ HTL, the reduced film roughness enables the CuCrO₂ HTL to produce highly efficient thin active layer OPV devices. UV-ozone treatment on the CuCrO₂ HTL is found to increase the fill factor. Drift-diffusion modeling, energy level measurements, and XPS results reveal that the device improvement is not due to the reduced injection barrier, but due to an improved CuCrO₂ conductivity arising from the formation of Cu²⁺ species.