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Abstract
AbstractDue 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.
Highlights
Cu(In,Ga)Se2 (CIGSe) thin-film solar cells have shown an efficiency boost up to 23.35% by alkali post-deposition treatments (PDTs).[1]
The bandgap energy (Eg), electron affinity (χ), and thickness of the CIGSe absorber are determined according to the Ga/(Ga + In) (GGI) ratio measured by glow discharge optical emission spectrometry (GD-OES) and integrated into the SCAPS simulation program using optical absorption parameters of experimentally deposited CuInSe2 and CuGaSe2 and further employing a corresponding interpolation algorithm within SCAPS based on Burgelman and Marlein.[21]
The electrical, optoelectronic, and compositional properties of the CIGSe devices have been characterized before and after thermal stress was applied in air and under vacuum conditions
Summary
Cu(In,Ga)Se2 (CIGSe) thin-film solar cells have shown an efficiency boost up to 23.35% by alkali post-deposition treatments (PDTs).[1]. Ramanathan et al have demonstrated that heat treatments in air (“aging”) at 250C of CIGSe/CdS/i-ZnO junctions led to the formation of a wide bandgap secondary phase as seen in the corresponding external quantum efficiency (EQE).[8] Kijima and Nakada have conducted vacuum annealing for 30 min on CdS and ZnS(O,OH)-buffered CIGSe solar cells They found that excess Cd and Zn diffuse into the CIGSe absorber layer leading to degradation of the device performance.[9] Similar observations were confirmed by other groups.[10,11,12] They have not reported the effect of Na on the cell degradation.
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