Hole drift mobility measurements in polycrystalline CuIn _1-xGa_xSe₂

We present temperature-dependent hole drift mobility measurements on polycrystalline CuIn1-x Gax Se2 (CIGS) thin films incorporated into solar-cell structures. The drift mobilities were determined from photocarrier time-of-flight measurements in a depletion region at the top interface with cadmium sulfide. 12 cells, originating in two laboratories, were examined. The drift mobilities ranged from 0.02 to 0.7 cm2 /Vs at room temperature and were weakly temperature dependent in the range of 100-300 K. These drift mobilities are at the low end of the range of hole mobilities reported from previous Hall effect and admittance measurements for varying CIGS materials. We found approximately a square-root correlation between the width of the depletion layer in our samples and the magnitude of the drift mobility. Both the magnitude and the temperature dependence of the drift mobility are consistent with results in amorphous and nanocrystalline silicon that have been modeled using a disorder-induced transport edge. The source of nanometer-scale disorder in these CIGS materials is not noncrystallinity; chemical composition fluctuations are one alternative source of disorder. The correlation of the depletion-width and drift mobility measurements in CIGS may be evidence for a broader effect of disorder in these materials in both reducing the carrier drift mobility and generating acceptor defects near the valence bandedge. Hole drift mobilities are sensitive to disorder-induced traps near the valence bandedge. Our temperature-dependence measurements indicate that the width of the corresponding valence bandtail is less than 20 meV. Previous optical-absorption spectroscopy showed that Urbach tails in similar CIGS samples are generally 20 meV or wider, which indicates that the valence bandtail does not typically determine the Urbach tails.