CulnSe2 and Other Chalcopyrite-Based Solar Cells

Abstract

CuInSe2 and related chalcopyrite semiconductors are among the compound semiconductors that have been considered for thin solar cells for about the past 20 years. Recently, high efficiencies close to 17% have been achieved. This result could be the starting point for a new category of solar cells—high-performance thin-film cells—that would combine the high performance of single-crystal cells with possible low-cost thin-film processing.The development of CuInSe2 cells started in 1974, when single-crystal cells with an efficiency of 12% were reported by a group at Bell Laboratories. Soon after, thin-film solar cells were demonstrated by Kazmerski et al. CuInSe2 thin films have been deposited by evaporating the CuInSe2 source material to completion and adding Se from a separate source. It was found that straight-forward evaporation of the compound does not generally lead to films with stoichiometric composition. By coevaporation of the elements, films with any desired composition can be obtained, provided there is appropriate process control. A “bilayer” recipe developed by Boeing, namely combining Cu-rich films and In-rich films, solved the problem of combining larger grains with suitable electronic properties. By this method, the first CuInSe2 thin-film solar cells with an efficiency exceeding 10% conversion efficiency were fabricated.Alloying CuInSe2 with CuGaSe2 and CuInS2 considerably increases the potential for the innovative development of solar cells from these materials. The energy gaps covered by these alloys range from about 1 eV to about 2.4 eV The possibility of increasing the energy gap and achieving absorber layers with graded bandgaps has many advantages for the application of these materials in thin-film solar cell modules.