Alkali treatments of Cu(In,Ga)Se-2 thin-film absorbers and their impact on transport barriers

We study the impact of different alkali post-deposition treatments by thermal admittance spectroscopy and temperature-dependent current-voltage (IVT) characteristics of high-efficiency Cu(In,Ga)Se-2 thin-film solar cells fabricated from low-temperature and high-temperature co-evaporated absorbers. Capacitance steps observed by admittance spectroscopy for all samples agree with the widely observed N1 signature and show a clear correlation to a transport barrier evident from IVT characteristics measured in the dark, indicating that defects are likely not responsible for these capacitance steps. Activation energies extracted from capacitance spectra and IVT characteristics vary considerably between different samples but show no concise correlation to the alkali species used in the post-deposition treatments. Numerical device simulations show that the transport barrier in our devices might be related to conduction band offsets in the absorber/buffer/window stack.