The study of below and above band edge imperfection states in In2S3 solar energy materials
Abstract b In 2 S 3 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 b In 2 S 3 has also been proposed to increase conversion efficiency in In 2 S 3 since multi photons absorption by intermediate band (IB) would happen in the sulfide. In this paper, single crystals of undoped and Nb doped b In 2 S 3 have been grown by the chemical vapor transport (CVT) method using ICl 3 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 b In 2 S 3 are direct semiconductors with energy gap of 1.935 eV for undoped b In 2 S 3 1.923 eV for b In 2 S 3 :Nb 0.005 and 1.901 eV for b In 2 S 3 :Nb 0.01 For undoped b In 2 S 3 PC and PL measurements are used to characterize defect transitions below band gap. The above band edge transitions of undoped b In 2 S 3 have also been evaluated using PL, PC, and SPR measurements. For the evaluation of Nb doped b In 2 S 3 an intermediate band with energy of ~0.4 eV below the conduction band edge has been detected in the TR measurements in both b In 2 S 3 :Nb 0.005 and b In 2 S 3 :Nb 0.01 The photo V I measurements also verified that the photoelectric conversion efficiency would be enhanced in the b In 2 S 3 with higher niobium content. Based on the experimental analyses, the optical behavior of the defects, surface states, and IB (formed by Nb) in the In 2 S 3 crystals is thus explored.