Enhanced photoelectric-conversion yield in niobium-incorporated In2S3 with intermediate band

beta-In2S3 is a nontoxic window layer material usually used in a thin-film solar cell. Transition metal (TM)-incorporated In2S3 has been proposed to promote conversion efficiency in In2S3 because multi-photon absorption by an intermediate band (IB) would happen in the sulfide. In this paper, band-edge and photoelectric-conversion properties of Nb-substituted In2S3 have been probed by thermoreflectance (TR), photoconductivity (PC), and photo-voltage-current (Photo V-I) measurements. The crystals of niobium-incorporated In2S3 with different Nb contents of In1.99S3: Nb-0.01, In1.995S3:Nb-0.005, and undoped In2S3 were grown by chemical vapor transport (CVT) method using ICl3 as a transport agent. X-ray diffraction measurements showed that the as-grown In2S3:Nb compounds are beta-phase crystals with tetragonal structure. Lattice constants of the beta-In2S3:Nb show shrinkage with the increase of the Nb content in the In2S3. Experimental TR spectra reveal four transition features including direct gap (E-0), valence band to donor (E-VS), valence band to IB (E-IB), and acceptor to IB [E-d(IB)] transitions detected in the Nb-substituted In2S3 compounds. The band gap (E-0) shows a reduction and the crystal color changes from fresh red, to red, to dark red with increasing Nb content in the undoped beta-In2S3, beta-In1.995S3:Nb0.005, and beta-In1.99S3: Nb0.01. The PC and Photo V-I measurements verified that high photoelectric-conversion efficiency occurred in the beta-In2S3 with higher niobium content. The intermediate band (IB) was formed by Nb substitution with indium in the beta-In2S3 [i.e. detected by E-IB approximate to 1.52 eV and E-d(IB) approximate to 1.42 eV]. The IB state should mainly dominate the multi-photon absorption capacity and enhance the photoelectric-conversion yield in the indium sulfide.