Recombination Effects of Urbach Tails as Trap States in Cu(In, Ga)Se2 Solar Cells Probed by Temperature-Dependent Junction-Transient Spectroscopies

Abstract

Recombination centers such as carrier traps due to point defects have long been known as one major limitation to the device efficiency of solar cells. Realizing the origin and the mechanism of the trapping and recombination processes enables a smart design strategy of high-performance photovoltaic (PV) technologies. Thin film Cu(In, Ga)Se₂ solar cells have been commercially used in the PV community due to their high power conversion efficiency, cost-effectiveness, and chemical stability. In this work, we explore the roles of subgap defect states in carrier trapping, in particular effects of Urbach tails in terms of the recombination mechanism, in Cu(In, Ga)Se₂ solar cells via junction-transient spectroscopic techniques. The temperature-dependent Urbach energy (E_U) was extracted from transient photocapacitance (TPC) and transient photocurrent (TPI) measurements. Thermal quenching behavior is observed at ~220 K for slightly different optimum Ga concentrations, with activation energies of 0.2–0.3 eV obtained from the thermal quenching model. The thermal and optical activation processes along the defect states are further interpreted using a 1D configuration coordinate model which takes the electron–phonon interaction into consideration.