Superstrate solar cells were prepared by thermal evaporation of Cu(In,Ga)Se2 onto ZnO coated glass substrates. For the first time, photo-conversion efficiencies above 11% were reached without the necessity of additional light soaking or forward biasing of the solar cell. This was achieved by modifying the deposition process as well as the sodium doping. Limitations of the superstrate device configuration and possible ways to overcome these were investigated by analyzing the hetero-interface with electron microscopy and X-ray photoemission spectroscopy measurements, combined with capacitance spectroscopy and device simulations. A device model was derived that explains how on the one hand the GaO.sub.x, which forms at the CIGSe/ZnO interface, reduces the interface recombination. On the other hand how it limits the efficiency by acting as an electron barrier at the hetero-interface presumably because of a high density of negatively charged acceptor states like Cu.sub.Ga. The addition of sodium enhances the p-type doping of the absorber but also increases the net doping within the GaO.sub.x. Hence, a trade-off between these two effects is required. The conversion efficiency was found to decrease over time, which can be explained in our model by field-induced diffusion of sodium cations out from the GaO.sub.x layer. The proposed device model is able to explain various effects frequently observed upon light soaking and forward biasing of superstrate devices. Copyright [c] 2014 John Wiley & Sons, Ltd.


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    Title :

    CuSe2 superstrate solar cells: prospects and limitations



    Published in:

    Publication date :

    2015




    Type of media :

    Article (Journal)


    Type of material :

    Print


    Language :

    English



    Classification :

    BKL:    53.36 Energiedirektumwandler, elektrische Energiespeicher