First-Principles Studies On Low-Dimensional Materials Applied In Solar Energy Conversion

Energy is the material basis for the survival of human society.As a renewable and clean energy,solar energy has become an ideal energy source.Besides the conversion into heat energy,the utilization modes of the solar energy mainly include solar cells and photocatalytic reaction.Solar cells can convert solar energy into electric energy,while photocatalytic reaction can convert solar energy into chemical energy,which can effectively make up for the shortcomings of solar energy such as low energy density and intermittent distribution.These two methods need to use semiconductor materials,in which the photo-excitation processes are involved.As a typical type of low-dimensional materials,two-dimensional materials have large specific surface area,resulting in large light absorption area and considerable catalytic active sites.Meanwhile,the confinement effect caused by the reduction of dimension enhances light-matter interaction and exciton effect in low-dimensional materials.Thus,low-dimensional semiconductor materials have become suitable solar energy conversion materials.The construction of van der Waals（vdW）heterojunction is conducive to the separation of photo-generated carriers,moreover,the vdW heterojunction can combine the advantages of the two materials and make up for the shortcomings of one single material.Therefore,low-dimensional vdW heterojunctions are more suitable for light energy conversion.In this paper,we mainly study the application feasibility of low-dimensional materials in light energy conversion by using the first-principles ground-state and excited-state calculations,and the content is divided into the following six chapters.The first chapter introduces the research background,mainly including solar energy,light excitation and low-dimensional materials.Firstly,the energy and its classification are introduced,then the focus is on the solar energy,including its source,transmission process and distribution in China,further,a brief introduction of the development history and principle of its two main utilization modes,including solar cell and photocatalytic hydrogen production is elaborated.Because the above two methods must involve the photo-excitation process of electrons in semiconductor materials,then we introduce the photo-excitation in atoms and solids,as well as the exciton states in solids.Finally,low-dimensional nano-materials and two-dimensional vdW heterojunctions suitable for light energy conversion are introduced.The second chapter briefly introduces the computing tools and the related theoretical background,including density functional theory（DFT）and timedependent density functional theory（TDDFT）.In the first section,the DFT corresponding to the calculations of ground state is introduced,including BornOppenheimer approximation,Hartree-Fock method,Thomas-Fermi model,HohenbergKohn theorem and Kohn-Sham method.In the second section,the TDDFT corresponding to the calculation of excited state is introduced,including Runge-Gross theorem,linear-response time-dependent density functional theory（LR-TDDFT）and nonadiabatic molecular dynamics（NAMD）.At the end of this chapter,the software used in this paper is introduced,including VASP,KSSOLV and HefeiNAMD.In Chapter 3,heterojunction solar cells are constructed by using edge-modified one-dimensional tellurene nanoribbons（X-ZTNR）.Heterojunction solar cells with high energy conversion efficiency（PCE）are obtained by adjusting the band gap with different bandwidths and energy level positions with different functional groups.The calculated maximum PCE of Cl/Br-ZTNRs heterobilayer with tetragonal edge and bandwidth of 11 reaches 22.6%,which is a competitive data in low-dimensional material heterojunction solar cells benefiting from the conduction band offset of 0.016 eV and the appropriate band gap of donor semiconductor of 1.44 eV.In Chapter 4,a Z-scheme heteroj unction photocatalyst for overall water splitting is constructed by using edge-modified one-dimensional phosphorene nanoribbons（X-PNR）.The photo-generated charge distribution in OH/OCN-PNRs is calculated by LR-TDDFT,and the generation of interlayer excitons in heterojunction is proved.The ultrafast charge transfer at the interface is studied by NAMD simulation,indicating that the interlayer electron hole recombination of OH/OCN-PNRs heterojunction precedes the interlayer recombination,showing the characteristics of a Z-scheme heterojunction.The catalytic performance of the system for water decomposition is studied by Gibbs free energy step diagram.Therefore,a general strategy for designing and characterizing Z-scheme heterostructures is obtained.In Chapter 5,the p-type and n-type doping of two-dimensional palladium selenide（PdSe₂）are realized by organic molecular adsorption,which could be adjusted by the electric field,layer spacing and the number of substrate layers.TCNQ and TTF adsorption introduces acceptor and donor shallow impurity levels.The excited-state electronic structure calculations by using LR-TDDFT shows the charge distribution of the lowest-energy excited state,indicating that for molecule-adsorbed PdSe₂ monolayer system,photo-generated electrons（holes）are localized on TCNQ（TTF）,which indicates that the impurity energy level introduced by molecular adsorption can participate in the optical transition.This method of introducing shallow impurity level by doping of organic molecules adsorption is helpful to regulate the photoelectric properties of two-dimensional materials so that they can be applied in solar energy conversion.In Chapter 6,the full text is summarized,and the prospect for the related research field of excited-state calculation is put forward.