Luminescence Analysis of Charge-Carrier Separation and Internal Series-Resistance Losses in Cu(In,Ga)Se2 Solar Cells

Cu(In,Ga)Se-2 solar cells are investigated by luminescence measurements. We construct the current versus internal voltage characteristics of these devices from the luminescence intensity at different voltage and light bias conditions. A comparison of these characteristics to electrically measured current versus voltage curves unveils an internal resistance loss that is strongly dependent on voltage bias and illumination. In particular, we find significant residual luminescence for the device under short-circuit conditions. Numerical device simulations reveal that this effect is caused by a drop of the electron quasi-Fermi-level within the space charge region of the absorber material. We use a modified equivalent circuit model to describe the observed behavior in terms of simple equations. We show that such a voltage-dependent series resistance leads to a violation of a linear-network theorem, which under standard circumstances provides a useful method for the determination of the photocurrent collection efficiency. An analysis of resistive and recombination losses in the devices demonstrates that the internal voltage-dependent series resistance causes an efficiency loss of about 1.3% (absolute) for a device with an efficiency 13.4%. Finally, we show that the observed behavior is a general feature of charge-carrier separation in solar cells with finite charge-carrier mobility and that the intensity of the residual short-circuit luminescence provides valuable information on the efficiency of this process.