Charge and doping distributions by capacitance profiling in Cu(In,Ga)Se2 solar cells

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Abstract
Doping distributions in the Cu(In,Ga)Se2 solar cells with various gallium contents are analyzed by the use of capacitance-voltage and drive-level capacitance profiling. The influence of deep traps on the evaluation of the spatial-doping distribution in the bulk of Cu(In,Ga)Se2 absorbers is discussed. An analysis is presented, which shows that traps labeled N1, commonly observed in these devices, are interface states or compensating donors and their input to the capacitance is related only to the width of the depleted n-type insulating layer. We attribute the apparent increase of doping density toward the back electrode to the accumulation of the electrostatic charge in deep bulk acceptors with a concentration at an order of magnitude higher than net shallow doping. The metastable changes of doping distributions induced by light or reverse bias are also investigated and interpreted in terms of the Lany–Zunger model of VSe-VCu divacancies with negative-U property. All conclusions have been tested by numerical modeling. Conductivity of thin films prepared in the same process as absorbers of investigated cells in relaxed and light-soaked states have also been measured. The results provide additional arguments that capacitance methods, if interpreted with care, give credible estimation of doping level in the absorber of Cu(In,Ga)Se2 devices.