The determination of carrier mobilities in CIGS photovoltaic devices using high-frequency admittance measurements

We apply a new method to deduce the hole carrier mobilities of Cu(InGa)Se2 (CIGS) polycrystalline films that have been incorporated into working solar cell devices. Our approach extends admittance measurements to frequencies of nearly 100 MHz in order to observe the characteristic dielectric carrier freeze-out and thus deduce the resistivity of the undepleted bulk absorber region in these devices. This resistivity, together with carrier densities deduced using drive-level capacitance profiling have allowed us to obtain the hole mobilities in the temperature regime 125 to 200 K. Values in the range 3–22 cm2 V−1 s−1 are obtained. In addition, we have examined the changes in mobility that occur as a result of light-soaking these devices, after which the carrier density is roughly doubled. Implications of these results toward understanding carrier mobility in CIGS polycrystalline films are discussed.