Wild band edges: The role of bandgap grading and band-edge fluctuations in high-efficiency chalcogenide devices

Band-edge effects — including grading, electrostatic fluctuations, bandgap fluctuations, and band tails — affect chalcogenide device efficiency. These effects now require more careful consideration as efficiencies increase beyond 20%. Several aspects of the relationships between band-edge phenomena and device performance for NREL absorbers are examined. For Cu(In, Ga)Se2 devices, recent increases in diffusion length imply changes to the optimum bandgap profile. The origin, impact, and modification of electrostatic and bandgap fluctuations are also discussed. The application of the same principles to devices based on CdTe, kesterites, and emerging absorbers (Cu
2
SnS
3
, CuSbS
2
), considering differences in materials properties, is examined.