In the confines of Cu(In,Ga)Se2 thin film solar cells with rear surface passivating oxide layers

Abstract: The material supply to build renewable energy conversion systems needs to be considered from both a cost and an energy security perspective. For Cu(In,Ga)Se2 (CIGS) thin film solar cells the use of indium in the absorber layer is most problematic. The material input per service unit can be reduced, if the absorber layers are thinned down without a loss in power conversion efficiency.Thinning down absorber layers can increase the conversion efficiency. However, for real CIGS solar cells absorption losses and recombination rates at the rear surface between the CIGS absorber and the Mo rear contact as well as shunt-like behavior increase. Thus, both rear surface passivation and optical management are essential for maintaining high power conversion efficiencies.In this work, thin oxide layers, so-called passivation layers, are introduced between the CIGS absorber layer and the Mo contact. They can passivate the CIGS surface, if the CIGS-oxide interface has a lower defect density than the CIGS-Mo interface and/or if they contain a negative fixed oxide charge, which increases the hole concentration and reduces the electron concentration in the CIGS in the vicinity of the oxide.As these oxides are insulators, electrical conduction through the passivation layer has to be ensured. In this work, nanopoint contacts were etched into ALD-Al2O3 passivation layers in CIGS solar cells. These solar cells had 0.5 -1.5 µm thin absorber layers with a low In content and a high band gap. Ga grading was not used. Although absorber layers with a high Ga content have a short minority carrier diffusion length, a passivation effect could be discerned with the help of external quantum efficiency measurements and current-voltage measurements under varying temperatures in combination with optical and electrical modeling with a two-diode model. Moreover, the possibility of leaving out the additional fabrication step has been explored for ALD-Al2O3 and HfO2 as passivation layers. The results suggest that the passivation layer does not necessarily need to be opened for electrical conduction in an additional fabrication step, if sodium fluoride (NaF) is deposited onto Al2O3 layers prior to CIGS evaporation. In this case solar cells with 215 nm absorber layers and 6 nm thin passivation layers have a power conversion efficiency of 8.6 %, which is 3 % (absolute) higher than the conversion efficiency on a reference. Shunt-like behavior is additionally reduced. For the HfO2 layers photoluminescence data indicate a good passivation effect, but the layers need to be opened up to ensure conduction.