Cu(In,Ga)Se2 thin film solar cells with a combined ALD-Zn(O,S) buffer and MOCVD-ZnO:B window layers

A Zn(O,S)/ZnO double buffer layer deposited using an atomic layer deposition (ALD) system and boron-doped ZnO window layer deposited using a metal organic chemical vapor deposition (MOCVD) system were applied to Cd-free Cu(In,Ga)Se2(CIGS) thin film solar cells. The bandgap energy of the Zn(O,S) buffer layer was varied from 2.8 to 3.6 eV by controlling the H2S/(H2O+H2S) pulse ratio. The solar cells with sulfur (S)-poor buffer layers showed a low open-circuit voltage (VOC) owing to the cliff nature of the conduction band offset (CBO) while in contrast, the solar cells with S-rich buffer layers showed a low short-circuit current density (JSC) owing to the spike nature of CBO. The CBO was optimized to a value of 0.36 eV which is a moderate “spike”, when Zn(O,S)(Eg=3.11 eV) was deposited at a pulse ratio of 0.27. The CIGS thin film solar cells with the optimized Zn(O,S) buffer layer showed a conversion efficiency of 18.3% after heat-light soaking at a temperature of 130 °C under AM1.5, 100 mW/cm2 illumination.