Achievement of 17.9% efficiency in 30 × 30 cm² Cu(In,Ga)(Se,S)₂ solar cell sub-module by sulfurization after selenization with Cd-free buffer

We have achieved 17.9% efficiency in a 30 × 30 cm² Cu(In,Ga)(Se,S)₂ solar cell sub-module prepared by selenization and sulfurization processes with a Cd-free buffer. The development of an absorber layer, transparent conducting oxide window layer, and module design was the key focus. This permitted 1.8% higher efficiency than our last experimental result. The quantity and the injection time of the sodium were controlled, resulting in higher open circuit voltage (V_oc) and short circuit current (J_sc). In order to increase J_sc, we changed the thickness of the window layer. Boron-doped zinc oxide was optimized for higher transmittance without reducing the fill factor. The uniformity of each layer was improved, and patterns were optimized for each module. Therefore, V_oc, J_sc, and FF could be theoretically improved on the reported results of, respectively, 20 mV, 2 mA/cm², and 1.4%. The module's efficiency was measured at the Korea Test Laboratory to compare with the data obtained in-house. Various analyses were performed, including secondary ion mass spectroscopy, photoluminescence, quantum efficiency, solar simulator, and UV-vis spectrometry, to measure the cell's depth profile, carrier lifetime, external quantum efficiency, module efficiency, and transmittance, respectively.