First-principles studies on the interface between light-absorbing layer and Mo back electrode in Cu(In,Ga)Se2, Cu2ZnSn(S,Se)4, and Cu2SnS3 solar cells

Abstract

We study the interface between absorbing layers such as Cu(In,Ga)Se2 (CIGS), Cu2ZnSn(S,Se)4 (CZTSSe), Cu2SnS3, and Mo back electrodes by first-principles calculation. The stabilities of the compounds formed at the interface were studied using density functional theory (DFT) methods. We evaluated the stabilities of MoSe2, Mo3Se4, Mo9Se11, and Mo15Se19 in the Mo–Se system and MoS2, Mo2S3, Mo3S4, and Mo15S19 in the Mo–S system from the estimated formation enthalpies, ΔHf, using a DFT-D2 approach based on van der Waals (vdW) forces. For CIGS solar cells, we calculated the enthalpy changes of reactions between the CuInSe2–Mo and CuInS2–Mo systems. The enthalpy changes of reactions expressed by CuInSe2 + 1/6Mo → 1/2Cu2Se + 1/6In6Se7 + 1/6MoSe2 and CuInS2 + 1/6Mo → 1/2Cu2S + 1/6In6S7 + 1/6MoS2 had positive values of +28 and +41 kJ/mol, respectively. The enthalpy changes of others reactions of the CuInSe2 (CISe)–Mo and CuInS2 (CIS)–Mo systems had positive values, too. These results suggest that CISe and CIS do not easily decompose even when Mo coexists. On the other hand, for Cu2ZnSnSe4 (CZTSe) solar cells, the enthalpy change of the reaction expressed by Cu2ZnSnSe4 + 1/2Mo → Cu2Se + ZnSe + SnSe + 1/2MoSe2 had a negative value of −11 kJ/mol. The mixed-phase system of Cu2Se + ZnSe + SnSe + 1/2MoSe2 is more stable than the Cu2ZnSnSe4 + 1/2Mo system, indicating that CZTSe readily decomposes when Mo coexists. The thickness of the MoSe2 layer at the interface between the CZTSe absorber and the Mo back contact tends to increase during the deposition of the CZTSe films. For Cu2SnS3 (CTS) solar cells, the enthalpy change of the reaction expressed by Cu2SnS3 + 1/2Mo → Cu2S + SnS + 1/2MoS2 had a small negative value of −4 kJ/mol. The mixed phase of the Cu2S + SnS + 1/2MoS2 system is slightly more stable than the Cu2SnS3 + 1/2Mo system. CTS decomposes slightly when Mo coexists. A thin MoS2 layer is formed at the interface between the CTS absorber and the Mo back contact.