Design Of High Efficient Photovoltaic Materials Based On Strain And Doping

To exploit the solar energy effectively is the best avenue to solve the energy crisis and environmental problems.It is a focus of research to tune the band gap by strain and doping,so as to increase the absorb efficiency of solar cell material.First-principles method is employed to investigate the strain effect on band gap of SrTcO₃ and doping effect on band gap of BaTiO₃.The possibilities of using them as solar cell materials are explored.In Chapter 1,some studies of the development history of solar cell materials are reviewed.Chapter 2 gives a brief introduction of the theories and calculation methods used in the thesis,including density functional theory,local density approximation?LDA?and Heyd–Scuseria–Ernzerhof hybridized density functional approach?HSE?.In Chapter 3,the electronic structure of SrTcO₃ as a function of biaxial strain is investigated by using GGA+U method,and the tuning on the band gap by strain is discussed.In Chapter 4,HSE method is used to study the tuning of doping on the band gap of BaTiO₃.Two different methods are used in Chapter 3 and Chapter 4 due to the following reasons.The material studied in Chapter 3 is SrTcO₃,whose ground-state properties such as magnetic moments from GGA+U calculations are consistent with the experimental values,indicating that the GGA+U method is applicable for SrTcO₃.While in Chapter 4,the materials studied are related to BaTiO₃,the band gap of which cannot be obtained by using LDA/GGA?+U?method.The computing-time-consuming HSE method is therefore employed.The summary and outlook are given in Chapter 5.The mechanism of tuning the band gap of SrTcO₃ with the biaxial strain is explored by using GGA+U method.The calculated results indicate that the band gap decreases with increasing compressive/tensile strain.Particularly,upon a compressive strain of 1.3%/2.2%/2.4%/4.1%,which can be realized by growing SrTcO₃ on the common-used substrate Sr Ti O3?STO?/La_0.3Sr_0.7Al_0.35Ta_0.35O₉?LSAT?/NdGaO₃?NGO?/LaAlO₃?LAO?,the band gap becomes 1.56/1.47/1.43/1.12 eV,falling in the range for high-efficiency solar cell materials.The mechanism of tuning the band gap can be figured out by analyzing the electronic structure.During the compressive process,the fixed top of valence band?dxy?and the downward shifting bottom of conduction band?d₃z₂-r₂?lead to the decrease of the band gap.In Chapter 4,the band gap of BaTiO₃ calculated by HSE method is found to be 3.3 eV,which agrees with the experimental value 3.4 eV very well.The HSE method is employed to examine the Co-doping,Pd-doping and?Co,Pd?co-doping effect on the band gap of BaTiO₃.Four systems are found to meet the band gap expectation for high efficient solar cell material.The band gap of Ba Ti_0.875Co0.125O2.875,Ba Ti_0.875Pd_0.125O₃,BaTi_0.75Co——?0.125?Pd_0.125O_2.75,BaTi_0.926Co_0.037Pd_0.037O_2.926 are found to be 1.95 eV,1.85 eV,1.95 eV and 1.90 eV,respectively.The mechanism of band gap decreasing is explored by electronic structure calculations.The state induced by doping Pd provides a new bottom of conduction band,which is closer to Fermi level than the original Ti 3d one,resulting in a decreased band gap.In the case of Co-doping case,the energy band of Co 3d is found to cross Fermi level slightly and holes are therefore produced.When electrons originated from oxygen vacancy fill these holes,new top of valence band is produced and the band gap decreases.The?Co,Pd?co-doping case has both increasing top of valence band and decreasing bottom of conduction band,resulting a much smaller band gap.The thesis presents important theoretical base for developing highefficiency solar cell materials.