Deposition of single-phase Cu(In,Ga)Se₂ thin films

M.Sc.Thin film solar cell devices based on chalcopyrite absorber layers have reached a high performance level over the last few years, especially on laboratory scale. Despite this progress, there is still an urgent need to develop an industrial easily scalable deposition technology for depositing chalcopyrite thin films on a large scale. In this study, homogeneous single-phase quaternary Cu(In1-xGax)Se2 thin films were prepared with a reproducible two-step growth technique. The growth process is based on the controlled selenization of sputtered metallic CuIn0.75Ga0.25 alloys in a H2Se/Ar gas mixture at atmospheric pressure. Attention was mainly focused on the optimization of the reaction parameters such as the temperature profiles, gas concentrations and reaction periods. In an optimal reaction process, the reaction velocities of the binary selenide phases were carefully controlled to prevent the formation of stable group I-III-VI2 ternary alloys during the initial selenization step. The composite alloys were subsequently annealed in an inert atmosphere, followed by a second selenization step to promote the homogeneous alloying of gallium with partially formed CuInSe2. Glancing incident angle x-ray diffraction (GIXRD) at incident angles between 0.2º and 10º revealed virtually no shift in d-spacing with sample depth, which confirmed the monophasic nature of the quaternary alloys. Optical measurements revealed an increase in the band gap value of the chalcopyrite alloy due to the homogeneous incorporation of gallium into the CuInSe2 structure. Solar cell devices were fabricated by depositing cadmium sulphide (CdS) buffer layers and zinc oxide (ZnO) window layers onto the CuIn0.75Ga0.25Se2 absorber films. These devices were measure under standard A.M. 1.5 conditions and favorable conversion efficiencies were demonstrated.Prof. V. Albert