Calculating Investigation On Surface State Passivation And Doping Properties Of Cu-?-?₂ Optoelectronic And Thermoelectric Materials With Chalcopyrite Structure

As new members of clean and green energy materials,both the photovoltaic and the thermoelectric materials,have great potential for development and application.Both of them can directly convert different forms of energy into environmentally friendly,efficient,and inexpensive electricity.The photovoltaic materials can convert the continuous sunlight into electrical energy,and the thermoelectric materials can directly convert the waste heat in environment?e.g.the waste heat from industry and automobile exhaust?into electrical energy.Solar cells are devices that convert light energy into electrical energy,where the Cu In_1-x-x Ga_x Se₂?CIGS?thin-film solar cells have the advantages of high conversion efficiency,low manufacturing cost,and relatively stable performance,and have attracted widespread attention at home and abroad.However,the presence of surface states in CIGS material,which is caused by a large number of surface defects,leads to the recombination of minority carriers at the back surface of CIGS,which severely decreases the photoelectric conversion efficiency of solar cells.Therefore,it is of great significance to investigate how to passivate or eliminate the surface states of CIGS materials for improving photovoltaic performance.The thermoelectric materials,which are commonly used in thermoel-ectric devices and can increase the energy efficiency,have great potential for application in solving energy crisis and environmental pollution problems.In recent years,it has been found in experiments that the Cu_0.714Ga Te₂?CGT?semiconductor materials have excellent thermoelectric properties.However,the current theoretical calculation researches lag behind the experimental researches.Therefore,in this thesis,theoretical calculation method was used to study the influence of Sb interstitial doping on the structure-property of CGT thermoelectric materials.It may provide theoretical basis and guidance for improving the Seebeck coefficient of CGT and increasing its thermoelectric performance.Using the first-principles calculation approaches,the lattice structure,band structure and electronic density of states?DOS?for the bulk Cu In_0.75Ga_0.25Se₂?CIGS?as well as the reconstruction behavior and surface states of the CIGS?112?surface were systematically studied.The passivation effect of Cl,F and H elements on the CIGS?112?surface has also been investagated.It is found that the Cu-In-Ga?CIG?terminated CIGS?112?surface has no DOS near the Fermi level,which is due to the large changes in atomic positions of surface layer and the partial self-passivation of the surface edge caused by the surface reconstruction.For the Se-terminated CIGS?112?surface,there is a new peak of DOS?surface state?near the Fermi level.This new peak mainly originates from the Cu-3d orbital electrons and Se-4p orbital electrons and may decrease the conversion efficiency of CIGS solar cell.In order to eliminate surface states and improve cell performance,we attempted to passivate Se-terminated CIGS?112?surfaces with Cl,F and H atoms.The calculated results show that when Cl,F and H atoms with a coverage of 0.5 ML are adsorbed at the top,bridge,hcp and fcc sites of CIGS?112?surface,the top sites of CIGS?112?surface are preferentially occupied.It indicates that the top sites are the most stable adsorption positions among the four kinds of adsorption positions.From the DOS of stable adsorption structures,it is seen that the DOS near the Fermi level are significantly diminished after the addition of Cl,F and H atoms on the surface,which means that Cl,F and H elements can effectively passivate the surface states.By comparison,H atom has the best passivation effect,Cl atom has the second passivation effect,and the passivation effect of F atom is the weakest.By calculating the lattice structure,band structure,and DOS of Cu_0.714Ga Te₂?CGT?thermoelectric material with different Sb doping concentrations,we found that the distortion degree surrounding the Sb atoms is larger when Sb atoms are doped at the intersial positions of CGT,while the lattice constants of CGT remain unchanged.The Sb-doped CGT has a relatively stable lattice structure since the Sb atoms can form stable bonds with their neighboring atoms in the lattice cells.The band gap of CGT after doping becomes narrower or even vanished,and a new sharp DOS appears at the Fermi level.The new DOS is mainly from Sb-5p orbital electrons.The results mentioned above show that the doping of Sb atom in CGT changes the electronic properties greatly,especially increases the DOS near the Fermi level,and also improves the Seebeck coefficient,which is beneficial to promote the thermoelectric properties of CGT.