Defect Tuning To The Band Gap Of Solar Cell Semiconductor CuInS₂

CuInS2with chalcopyrite structure, which has high absorption, non-toxic, lowprice and easily industrialization properties, is a perfect material of thin film solar cell.In recent years it is always one of the hot topics in solar cell field. In the experiment,it is found that the wavelength of the light emitted can be tuned by controlling the[Cu]/[In] molar ratio. This property can be used to fabricate the continuouslywavelength-tunable laser.In this paper, using the first principle method, we study the formation of pointdefects and defect-complexes. The formation energy and transition level are given.We find that in the Cu-rich and In-poor conditionCu_In+2Cu_ithe maindefect-complex,2V_Cu+In_Cu；is the main defect-complex when In-rich and Cu-poor.In order to study the effect of defects on electronic structure and optic properties,we compare four defect configurations, Cu₁₁In₇S₁₆、Cu₁₄In₆S₁₆、Cu₅In₉S₁₆andCu₂In₁₀S₁₆。, with ideal CuInS₂. We do the calculation by PBE pseudo potential andhybrid functional, respectively. Both the results show that these five compounds aresemiconductors. The band gaps of these five compounds decrease with the increasingof [Cu]/[In] molar ratio, which is consistent with those experimental results. We studythe electronic structures and present a physical mechanism. The valence bands ofthese compounds are consisted of the hybrid band of Cu-d and S-p. When theconcentration of Cu increase the p-d repulsion become stronger therefore the valenceband are widened. The valence band maximum will rise and the band gap will narrow.Our results show the physics mechanism and give a theoretical guide of tuningwave length with [Cu]/[In] molar ration in chalcopyrite CuInS2.