Cuprous oxide–indium–tin oxide thin film photovoltaic cells

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Abstract
We studied thin films of cuprous oxide deposited on glass coated with transparent conducting indium–tin oxide (ITO) films. The deposition of both Cu2O and ITO was made by rf sputtering in an Ar/O2 gas mixture. For the deposition of Cu2O a pure copper target was used and ITO films were deposited from a disk target, the halves of which were made of Sn and In, respectively. This allows variation of the stoichiometry of the deposited ITO film by changing the position of the substrate glass beneath the Sn/In target. X-ray diffraction of Cu2O films indicates the typical pattern of amorphous material. We were able to produce Cu2O films of different stoichiometry by varying the O2 to Ar ratio during rf sputtering. The maximum resistivity of the films corresponds to an ideal stoichiometry of Cu2O. An activation energy of 0.55 eV found from thermostimulated conductivity is related to excess Cu vacancies. The band gap found from the spectral dependence of the photovoltaic effect is 2.0 eV. The composition of ITO films was studied by Auger analysis and can be described as a variable composition mixture of SnO2+x and In2O3+y. To produce an Ohmic electrode, gold was evaporated on the top of the Cu2O film and hence the resulting structure of the photocell could be specified as ITO–Cu2O–Au, for which we propose a barrier band diagram. We studied the photovoltaic characteristics of the fabricated photocells under an incandescent lamp with ≂100 mW/cm2 output. The open-circuit voltage and short-circuit current of our cells were about 20–90 mV and 50 μA/cm2, respectively, and some dependence of the output characteristics on the composition of ITO film was observed. Conversion efficiency for thin films Cu2O/ITO cells was found to be substantially lower than for Cu2O/Cu Schottky barrier cells. This is tentatively attributed to small diffusion lengths and/or presence of interface recombination centers.