First-principles Study On Two Kinds Of Cu-based Semiconductor Materials As Solar Energy Absorber

Utilizing solar energy is regarded as the ultimate way to solving the energy crisis and keeping the sustainable development for human being.Solar cells have been paid much attention as a device transferring solar light to electricity directly without pollution and other secondary products during the application process.Solar cell adopted semiconductor material as the absorber,of which the band gap plays an important role on if this semiconductor is suitable as the absorber of solar cell.Therefore,searching the semiconductor with suitable band gap or adjusting the band gap of semiconductor to a reasonable value,which make the theoretical efficiency towards Shockly-Qussier limit 31%,are still the hot topics on the research of solar cells.In this dissertation,starting from two low-cost semiconductor materials CuAlZn₂(S,Se)₄ and Cu₂SnS₃,by using first-principles calculations with HSE hybrid density functional,the band structure,density of state,carrier effective masses,bonding/antibonding analysis,optical absorption property etc.have been studied.Based on the calculation results,we have accessed these two materials if they are suitable as the absorber for solar cell.And then we have supplied the guide to optimize their properties.The detailed contents as following:1.Although the experimental results have reported that CuAlZn₂(S,Se)₄ can be regarded as the solar cell absorber owing to its suitable optical band gap(?1.6e V)and just containing low cost and non-toxic elements,we found the band gap of CuAlZn₂S₄is?3 e V from our calculation,which is wide for the solar cell absorber.To verify our calculation results,we have carried out the bonding analysis CuAlZn₂S₄for and found that its valence band maximum comes from the antibonding state of Cu-d and S-p states and its conduction band minimum originates from the antibonding state of Al-s and S-p states,which are the same with those from Cu Al S₂(experimental band gap 3.5 e V).Based the calculation of band offset between CuAlZn₂S₄and Cu Al S₂,we found the band edge positions between them do not show a significant change,which is consistent with the bonding/antibonding analysis and the components at band edges.It demonstrated that CuAlZn₂S₄ should have a similar band gap with Cu Al S₂ and is not suitable as the absorber of solar cell owing to its wide band gap.Furthermore,we searched the solar light absorber with suitable band gap by considering element substitution on CuAlZn₂S_4.We have calculated the total energies and electronic properties such as band structure,density of state,carrier effective masses,optical absorption coefficient etc.of CuAlZn₂(S,Se)₄,Cu Ga Zn₂(S,Se)₄,Cu In Zn₂(S,Se)₄with kesterite,Cu Au and stannite-types crystal structure.We found that only Cu In Zn₂Se₄has a suitable band gap as the solar light absorber.The band gaps of 1.60 e V,1.51 e V and 1.67 e V from the three different phases of Cu In Zn₂Se₄ are close to the ideal band gap(?1.5 e V)for thin film solar cell.After the discussion on electronic structure,carrier effective masses,optical absorption coefficient and spectroscopic limited maximum efficiency(SLME),we found Cu In Zn₂Se₄ can be regard as a potential absorber for thin film solar cell.2.As a low cost and non-toxic semiconductor material,many experimental works have reported that the band gap of Cu₂SnS₃is 0.91e V,which is a little narrow for the absorber of solar cell.Therefore,we tried to increase the band gap of Cu₂SnS₃by substituting Li,Na,K and Ag at Cu site and substituting Si and Ge at Sn site to improve its property.Based on the bonding/antibonding analysis,we found that the valence band maximum of Cu₂SnS₃comes from the antibonding state of Cu-d and S-p states and its conduction band minimum originates from the antibonding state of Sn-s and S-p states.Based on the band offsets between pure and doped systems,we found the valence band maximum of Cu₂SnS₃moved down after Li,Na,K and Ag doping at Cu site while its conduction band minimum shifted up after Si and Ge doping at Sn site.All of them can widen the band gap of Cu₂SnS_3.We also found that the band gap can change to 1.31 e V and 1.39 e V after 25%Li and K doping at Cu site,which are close to the ideal band gap for solar cell absorber.According to the suitable band gap and high absorption coefficient,we suggested that Li or K doped Cu₂SnS₃can be regarded as the promising candidates as the absorber for the thin-film solar cells.