Role of Mo:Na layer on the formation of MoSe2 phase in Cu(In,Ga)Se2 thin film solar cells

• Thickness of MoSe2 phase decreases with an increase in the thickness of the Mo:Na layer.
• Ga distribution in the CIGS layer may vary with a change in the thickness of the Mo:Na layer.
• Reducing the thickness of the MoSe2 layer enhanced cell efficiency from 6.58% to 10.53%.
• Na2Sex remaining at grain boundaries is the dominant factor for the formation of MoSe2.

This study investigated the influence of Mo:Na on the formation of MoSe2 in Cu(In, Ga)Se2 (CIGS) thin film solar cells and its overall effect on the performance of cells with a structure of Ti/Mo:Na/Mo/CIGS/CdS/i-ZnO/ZnOAl/Al. Varying the thickness of the Mo:Na layer enabled the systematic control of the diffusion of Na into the CIGS layer. Experimental results demonstrate that the thickness of MoSe2 phase decreases with an increase in the thickness of the Mo:Na layer. Reducing the thickness of the MoSe2 layer enhanced cell efficiency. Additionally, Ga distribution in the CIGS layer may vary with a change in the thickness of the Mo:Na layer. When Mo:Na/Mo=500/500 nm, the Ga content and the thickness of the MoSe2 layer were close to optimal with regard to cell performance. This paper proposes a model to explain the impact of Na doping on the formation of MoSe2 phase formation in polycrystalline CIGS. It appears that the addition of Na leads to the formation of Na2Sex at the grain boundaries in the absorber layer, which reduces the diffusion of Se to form MoSe2.

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