What is the bandgap of kesterite?

- Determines role of tail states for optical characterization of semiconductors, in particular kesterite.
- Discusses influence of tail states on transport and quantum efficiency.
- Demonstrates role of tails states in recombination process.
- Clarifies interpretation of VOC extrapolation in the presence of tail states.

There are different ways to determine the bandgap of a semiconductor. In the case of strong tailing they lead to different results. Various versions of Tauc's plot give the gap of extended states, whereas the photoluminescence and the quantum efficiency extend into the tail states. The absorption edge in kesterite is determined by tail states therefore different methods to determine the band gap lead to different results. To decide whether the main recombination path is in the bulk or at the interface, the activation energy of the recombination rate should be compared to the energy of the radiative recombination in the bulk. This is the energy of the photoluminescence maximum and can be approximated by the linear extrapolation of the low energy edge of the quantum efficiency spectrum.