First-principles Study On Photoelectronic Properties Of Semiconductor With Lone-pair Electrons

Semiconductor materials with lone-pair electrons exhibit excellent photoelectric properties,where solar cells and light-emitting diodes prepared by organic-inorganic hybrid halide perovskite materials with Pb 6s² lone-pair electrons have great commercial potential in application.However,the poor stability of perovskite solar cells and the environmental problem caused by the toxic heavy metal Pb limit their development in the future.This has prompted researchers to search for new photoelectronic materials that can replace hybrid perovskite materials.These new materials need to meet two conditions,which can not only inherit the characteristics of lone-pair electrons of perovskite materials,but also overcome the instability and Pb toxicity.In this paper,the first-principles calculations are performed to study the stable and environmentally friendly optoelectronic materials with lone-pair electrons,indicating they have superior optoelectronic characteristics,and our results provide theoretical guidance for the experiments to fabricate their optoelectronic devices.This paper is composed of six chapters.The first chapter introduces the development of hybrid perovskite solar cells,summarizes its advantages and shortcomings,points out the direction for finding new semiconductors to substitute hybrid perovskite materials,and briefly introduces current research status of potential materials with lone-pair electrons.The second chapter introduces the development of the first-principles calculation methods and the defect calculation theory involved in this thesis.In the third chapter,through first-principles calculations we find that two lattice-matched halide double-perovskites Cs₂NaBiBr₆ and Cs₂AgBiBr₆ have a type-I band alignment and can form highly miscible alloys in which the disordering makes the bandgaps become direct and activates the direct transition from the valence to conduction band edge,leading to a strong optical absorption and high radiative recombination rate.The bandgaps of the alloys are tunable in a wide range of 1.93-3.24eV,while the lattice constants keep unchanged.This advantage inspires us to design a coherent crystalline matrix based on the Cs₂?Na,Ag?BiBr₆ alloys,in which the Ag-rich and narrower-bandgap regions are embedded in the Na-rich and wide-bandgap region with lattice-matched and coherent interfaces.The type-I band alignment drives the photo-generated excitons into the narrower-bandgap Ag-rich regions,so the regions become light-emitting centers with a high photoluminescence quantum yield?PLQY?.The bandgaps of the Ag-rich regions are tunable,so the color of emitted light can be adjusted,making a broadband emission possible.Such kind of coherent crystalline matrix with high-PLQY and broadband emission can also be fabricated based on the alloys of other lattice-matched halide double-perovskites,demonstrating the flexibility of band structure engineering in the coherent heterostructures of various halide double-perovskites.In the fourth chapter,our first-principles calculations show that NaSbSe₂ has a quasi-direct band gap,which is beneficial for increasing the lifetime of minority carriers.The optical absorption coefficient is high(exceeding 10^-4 cm^-1 for visible light)because of the direct band-edge transition from the?Sb-5s/5p+Se-4p?valence band to?Sb-5p+Se-4p?conduction band.The formation of the dominant acceptor defects such as Na_Sb,V_Na and V_Sb makes it difficult to dope NaSbSe₂ to n-type and thus only the intrinsic p-type conductivity has been observed.Se-rich conditions are found to produce high concentration of hole carriers and low concentration of recombination-center defects,so we propose the Se-rich conditions should be adopted for fabricating high efficiency NaSbSe₂ solar cells.Furthermore,the mixed-anion NaSb?S,Se?₂ alloys are predicted to be highly miscible with a low formation enthalpy and a low miscibility temperature?below room temperature?,and their band gaps can be tuned almost linearly from 1.1 to 1.6 eV,covering the optimal band gap range for single-junction solar cells.Therefore,we propose that alloying provides a promising method for optimizing the performance of NaSbSe₂-based solar cells.In the fifth chapter,through first-principles calculations,it is found that GeSe is easily sublimated without decomposition,due to its layered crystal structure similar to black phosphorus.This feature highlights its great advantages in preparing thin films by rapid thermal sublimation?RTS?process.GeSe has an indirect bandgap?1.13 eV?,whose valence band maximum?VBM?is composed of bonding state of Ge 4p+Se 4p hybridization and antibonding state of Ge 4s and Se 4p hybridization.GeSe exhibits strong optical absorption capacity(the optical absorption coefficient exceeds 10⁴ cm^-1for visible light),owing to direct band-edge transition from the?Ge-4s/4p+Se-4p?valance band to?Ge-4p+Se-4p?conduction band.The acceptor V_Ge is the dominant defect,thus GeSe is an intrinsic p-type semiconductor,and its transition levels?-/0?and?2-/-?are very shallow,which is beneficial for carrier ionization.The deep donor V_Se is a potential non-radiative recombination center,but its concentration is very low,so its adverse effects can be ignored.The antibonding character of VBM is responsible for benign defect property of GeSe.In addition,the size effect on GeSe is very strong,the bandgap engineering can be tunable in a wide range of 1.13-1.56 eV.Considering that?001?surface is a chemically inert surface,which can suppress the formation of grain boundaries,and carriers can be efficiently transported along[010]direction,we recommend that a[010]-oriented grain consist of GeSe ribbons stacked vertically on the substrate is necessary to optimize the performance of GeSe photovoltaic devices.Finally,the bandgap of the single-layer GeSe is direct?1.56 eV?,which indicates that GeSe is a potential ultrathin photovoltaic material.The sixth chapter summarizes the main conclusions in this paper and the potential research directions are prospected.