High efficiency solar cells based on copper indium gallium diselenide (CIGS) prepared by a simplified electrochemical process and innovative structures

Thin-film solar cells based on Cu(In,Ga)Se₂ obtained by a vacuum process achieve conversion efficiencies higher than 20%, but require the use of expensive apparatus. Electrodeposition is a low cost alternative deposition method that can be used to prepare Cu-In-Ga films. In this manuscript, the electrodeposition method consists of a one-step deposition of Cu-In-Ga oxide/hydroxide thin-films. This method is based on a local pH increase on the surface of the electrode, generated by the electrochemical reduction of nitrate ions. The reaction mechanism governing the deposition of the oxide thin-films was investigated by means of a ring-disc electrode to identify the generated reaction intermediates and products. The reducing heat treatment of the oxide layer was then studied under an atmosphere of diluted and pure hydrogen gas. The use of pure hydrogen gas reduces the temperature and the annealing time necessary for the complete reduction of the oxide layer into the corresponding metal alloy layer. The selenization of the metal alloy at 550 deg. C leads to the formation of CuInSe₂, and the formation of Cu(In,Ga)Se₂ near the back contact. Increasing the selenization temperature and gallium content and inserting sodium from an external source lead to the homogenization of gallium throughout the CIGS layer. Cells with 11.9% efficiency were obtained. In order to reduce the quantity of material used, localized electrodeposition of Cu-In-Ga oxide layer was also studied using microelectrodes array. After a reducing heat treatment and selenization of the microcells, an efficiency of 1.4% was obtained at 240 suns. (author)