Impact of Thin CuGa Layers Added at the Rear Interface of Cu2ZnSnSe4 Solar Cells
Sergio Giraldo1, Zacharie Jehl Li-kao1, Robert Fonoll1, Lorenzo Calvo-Barrio2, Victor Izquierdo-Roca1, Alejandro Pérez-Rodríguez1,3, Edgardo Saucedo1
1Catalonia Institute for Energy Research (IREC), Sant Adrià de Besòs (Barcelona), Spain
/2Centres Científics i Tecnològics de la Universitat de Barcelona (CCiTUB), Barcelona, Spain
/3IN2UB, Departament d’Enginyeria Electrònica i Biomèdica, Universitat de Barcelona, Barcelona, Spain

This work, partly inspired by Cu(In,Ga)(S,Se)2 technology and the formation of the wide band gap Ga-rich rear region that naturally acts as an electron back reflector, investigates the effect of adding small amounts of Ga at the back region of kesterite Cu2ZnSnSe4 (CZTSe) absorbers. For this purpose, thin CuGa layers (from 0 to 50 nm) were deposited on Mo-coated substrates, before the sputtering deposition of standard Sn/Cu/Zn metallic stacks, and subjected to different short selenization treatments at different temperatures. The aim of this approach was to promote the formation of wider band gap CuGaSe2 compound at the back region of the absorber and study its effect as possible back electron reflector, as well as the effect on the rear interface morphology. In order to investigate the effect of CuGa addition on material and devices properties, absorber layers were characterized using X-ray fluorescence, scanning electron microscopy, Raman spectroscopy, Auger and X-ray photoelectron spectroscopy. Solar cell devices were characterized with a solar simulator to determine photovoltaic parameters, and spectral response to acquire the quantum efficiency. Main results of this study showed an improved performance of kesterite devices by the implementation of pre-selenized thin CuGa layers, with enhanced long-wavelength response, together with an improved interface morphology without voids at the Mo/CZTSe interface. Thus, preliminary results might indicate the formation of a back-surface field or a better passivation of the rear interface region.