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

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.