The study of below and above band-edge imperfection states in In2S3 solar energy materials

β-In2S3 is a nontoxic buffer layer material usually used in a thin-film solar cell due to a lot of vacancies and surface states naturally existing in the crystal to assist in photoelectric conversion. Transition metal (TM)-incorporated β-In2S3 has also been proposed to increase conversion efficiency in In2S3 since multi-photons absorption by intermediate band (IB) would happen in the sulfide. In this paper, single crystals of undoped and Nb-doped β-In2S3 have been grown by the chemical vapor transport (CVT) method using ICl3 as a transport agent. Optical properties of the imperfection states of the crystals are probed by thermoreflectance (TR), photoconductivity (PC), photoluminescence (PL), surface photoconductive response (SPR), optical absorption and photo–voltage–current (photo V–I) measurements. The TR and optical-absorption measurements confirmed that the undoped and Nb-doped β-In2S3 are direct semiconductors with energy gap of 1.935 eV for undoped β-In2S3, 1.923 eV for β-In2S3:Nb0.005, and 1.901 eV for β-In2S3:Nb0.01. For undoped β-In2S3, PC and PL measurements are used to characterize defect transitions below band gap. The above band-edge transitions of undoped β-In2S3 have also been evaluated using PL, PC, and SPR measurements. For the evaluation of Nb-doped β-In2S3, an intermediate band with energy of ∼0.4 eV below the conduction band edge has been detected in the TR measurements in both β-In2S3:Nb0.005 and β-In2S3:Nb0.01. The photo V–I measurements also verified that the photoelectric-conversion efficiency would be enhanced in the β-In2S3 with higher niobium content. Based on the experimental analyses, the optical behavior of the defects, surface states, and IB (formed by Nb) in the In2S3 crystals is thus explored.