Electrodeposited CuInSe2 Thin Film Junctions
We have investigated thin films and junctions based on copper indium diselenide (CIS) which have been grown by electrochemical deposition. CIS is a leading candidate for use in polycrystalline thin film photovoltaic solar cells. Electrodeposition is a cost-effective method for producing thin-film CIS. We have produced both p and n type CIS thin films from the same aqueous solution by simply varying the deposition potential. A CIS pn junction was deposited using a step-function potential. Stoichiometry of the single layer films was determined by energy dispersive spectroscopy. Carrier densities of these films increased with deviation from stoichiometry, as determined by the capacitance versus voltage dependence of Schottky contacts. Optical bandgaps for the single layer films as determined by transmission spectroscopy were also found to increase with deviation from stoichiometry. Rectifying current versus voltage characteristics were demonstrated for the Schottky barriers and for the pn junction.
CuInSe2 for photovoltaic applications
The properties and most successful methods for producing CuInSe2 films for solar‐cell applications are reviewed and the production, analysis, and performance of photovoltaic devices based on CuInSe2 are discussed. The most successful methods for depositing thin CuInSe2 films for high‐efficiency solar cells are three‐source elemental evaporation and selenization of Cu/In layers in H2Se atmospheres. Devices based on CuInSe2 have achieved the highest conversion efficiencies for any nonepitaxial thin‐film solar cell, 14.1% for a small cell and 10.4% (aperture efficiency) for a 3916‐cm2 (4 sq. ft) device. Furthermore, high‐efficiency devices have been produced by several groups and have shown no evidence of degradation of performance with time. The internal quantum efficiency is remarkably close to 100%, although various losses prevent making use of all of the generated carriers. The high performance results, in part, from the very‐high‐absorption coefficient of CuInSe2, which is of the order of 105 cm−1 for photons with energies slightly above 1 eV. Models of the operation of CuInSe2/CdS heterojunctions have begun to explain the processes limiting the device performance. The success of the models is based, in part, on the large amount of data which has accumulated on CuInSe2 in spite of the relatively short time it has been extensively studied.
Homojunction fabrication in CuInSe2 by copper diffusion
Rectifying p−n junctions have been fabricated in single crystals of CuInSe2 by diffusion of copper at relatively low temperatures. Some evidence of device instability at room temperature has been obtained.