Charge carrier transport via defect states in Cu(In, Ga)Se2 thin films and Cu(In, Ga)Se2/CdS/ZnO heterojunctions

We present results of static and dynamic electrical conductance experiments on Cu(In, Ga)Se-2 thin films and Cu(In, Ga)Se-2/CdS/ZnO heterojunction solar cells prepared by rapid thermal processing. For the static conductance, we find a temperature dependence of the conductance according to Mott's variable range hopping model. For the dynamic conductance at temperatures below 30 K, we find a power-law frequency dependence over nearly five decades with an exponent of about 0.87. The temperature dependence of the dynamic conductance is found to be linear. Thus, the conductance in the static limit and the dynamic conductance point to tunneling between localized states as the dominant charge-carrier transport mechanism at low temperatures. In addition, both methods applied provide an independent determination of the density-of-defect states at the Fermi level. We further observe a metastability of the kind that in an illuminated state the conductance and the density-of-states are higher than they are in an annealed state. We can reversibly switch between these states by illuminating and annealing subsequently.