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

Abstract

β-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 In_1.99S₃:Nb_0.01, In_1.995S₃: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 β-phase crystals with tetragonal structure. Lattice constants of the β-In₂S₃:Nb show shrinkage with the increase of the Nb content in the In₂S₃. Experimental TR spectra reveal four transition features including direct gap (E₀), 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 (E₀) shows a reduction and the crystal color changes from fresh red, to red, to dark red with increasing Nb content in the undoped β-In₂S₃, β-In_1.995S₃:Nb_0.005, and β-In_1.99S₃:Nb_0.01. The PC and Photo V–I measurements verified that high photoelectric-conversion efficiency occurred in the β-In₂S₃ with higher niobium content. The intermediate band (IB) was formed by Nb substitution with indium in the β-In₂S₃ [i.e. detected by E_IB≈1.52 eV and E_d(IB) ≈ 1.42 eV]. The IB state should mainly dominate the multi-photon absorption capacity and enhance the photoelectric-conversion yield in the indium sulfide.