Optoelectrical improvement of ultra-thin Cu(In,Ga)Se2 solar cells through microstructured MgF2 and Al2O3 back contact passivation layer

Decreasing the absorber layer thickness of thin-film solar cells can be an effective solution for cost reduction of photovoltaic electricity generation. Unfortunately, this reduction leads to detrimental effects such as incomplete photon absorption and increased charge carrier recombination at the rear electrode. To tackle these losses in ultra-thin 0.5 mu m Cu(In,Ga)Se-2 (CIGS) solar cells, we developed different passivation structures made of MgF2 and Al2O3 at the molybdenum-CIGS interface, leading to localized back contacts. The influence of the distance between those contacts on the cell performance was studied by varying the periodicity of the applied 1D patterns from 6 mu m to 30 mu m. Thus, an increase in performance was measured for microstructured layers with a periodicity of up to 12 mu m. More precisely, a MgF2 layer yielded an increase in power conversion efficiency (PCE) of up to 9%(rel) compared to an unpassivated cell design, and a passivation layer comprising Al2O3 led to up to a 5%(rel) increase in PCE. The gains were primarily attributed to an increased reflectivity of the back contact, while the formation of a negative backside field in the case of Al2O3 might have contributed to this increase by preventing electrons from recombining at the backside interface. Our findings indicate a high lateral conductivity for holes inside the multicrystalline CIGS compound over few tens of micrometres, which allows an independent design of future back contacts and light-trapping schemes. [GRAPHICS] False-colour scanning electron microscopy cross-section picture of a passivated solar cell, with the front contact layers coloured in green, the 0.5 mu m CIGS absorber in dark red, the MgF2 passivation layer in blue, and the Mo back contact in grey. (C) 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim