Decay mechanisms in CdS‐buffered Cu(In,Ga)Se₂ thin‐film solar cells after exposure to thermal stress: Understanding the role of Na

Abstract

Due to their tunable bandgap energy, Cu(In,Ga)Se2 (CIGSe) thin‐film solar cells are an attractive option for use as bottom devices in tandem configurations. In monolithic tandem devices, the thermal stability of the bottom device is paramount for reliable application. Ideally, it will permit the processing of a top device at the required optimum process temperature. Here, we investigate the degradation behavior of chemical bath deposited (CBD) CdS‐buffered CIGSe thin‐film solar cells with and without Na incorporation under thermal stress in ambient air and vacuum with the aim to gain a more detailed understanding of their degradation mechanisms. For the devices studied, we observe severe degradation after annealing at 300°C independent of the atmosphere. The electrical and compositional properties of the samples before and after a defined application of thermal stress are studied. In good agreement with literature reports, we find pronounced Cd diffusion into the CIGS absorber layer. In addition, for Na‐containing samples, the observed degradation can be mainly explained by the formation of Na‐induced acceptor states in the TCO front contact and a back contact barrier formation due to the out‐diffusion of Na. Supported by numerical device simulation using SCAPS‐1D, various possible degradation models are discussed and correlated with our findings.