First Principles Study On Defects In Inorganic Photovoltaic Materials

Environment pollution and energy source shortage are increasing serious in currentsociety. The development and utilization of renewable energy is imperative. Therefore,the research on new and renewable clean energy has become one of the dominatingfields for the world economy in the future. The solar energy photovoltaic technology isplaying an important role in the field of renewable energy. In the current work, we haveperformed the first principles calculations based on density functional theory to studythe defects in inorganic photovoltaic materials. The main results are as follows:1. We have studied the structure and effects of vacancies in Σ3（112） grain boundarywith the coincident-site lattice structure in Si. We find that the formation energy for a Sivacancy in the grain boundary is significantly lower than that in Si perfect region,indicating strong segregation of Si vacancy in grain boundary regions. The formation ofSi vacancies in grain boundaries either cleans up the deep levels by eliminating the extrabonds or facilitates complete passivation by H atoms if there are also dangling bondswhen Si vacancies in grain boundaries. Our results indicate that H still providesexcellent passivation on GBs in polycrystalline Si.2. We have presented our study of the effects of O in Te-rich grain boundaries inCdTe. We find that O prefers to segregate into Te-rich grain boundaries and substitutesTe atoms with dangling bonds. The Te atoms with dangling bonds in a Te-rich Σ3（112）grain boundary create deep gap states due to strong interaction between Te atoms.However, when such a Te atom is substituted by an O atom, the deep gap states can beshifted toward the valence band. We find that O atoms prefer energetically substitutingthese Te atoms and induce significant lattice relaxation due to their smaller atomic sizeand stronger electronegativity, which effectively reduces the anion-anion interaction.Consequently, the deep gap states are shifted to lower energy regions close to or evenbelow the top of the valence band. We also find similar effects for O in anion-rich grainboundaries in CuInSe₂, CuInS₂, CuGaSe₂, CuGaS₂, Cu₂ZnSnSe₄, and Cu₂ZnSnS₄.3. We have studied the doping properties of intrinsic and extrinsic defects inCoAl₂O₄by calculating the transition energies and formation energies of intrinsic andextrinsic impurity defects. We find that for intrinsic defects, AlCois a shallow donor, whereas both Co and Al vacancies are shallow acceptors. We find that due to strongcompensation, weak p-type CoAl2O4may be achievable at O-rich growth condition.With Li and Na doping at O-rich growth condition, excellent p-type conductivity couldbe realized. However, good n-type doping of CoAl2O4is not possible with all thedopants considered and any growth conditions because of the formation of intrinsicdefects, particularly AlCo, which has very low formation energy at all growth conditions.This strong asymmetrical doping behavior suggests that suitable application of CoAl2O4should be based on the p-type conductivity. For electronic and optoelectronicapplications that require p-n junctions, our results suggest that CoAl2O4should be usedwith other n-type semiconductors for heterojunctions.4. We have calculated the electronic and optical properties of Co based spineloxide CoX₂O₄（X=Al, Ga, In） alloys. We have shown that the bandgap, the electronmobility, and optical absorption coefficient can be tuned by controlling the alloycompositions based on different s-orbital energies and ionic radii of X （X=Al, Ga, In）.Our calculated bandgap map as a function of composition provides detailed practicalguidance for synthesizing Co based spinel oxide semiconductors with electronic andoptical properties suitable for the application to the hydrogen production by sunlightthrough PEC decomposition of water. Furthermore, our results also suggest theappropriate compositions for designing tandem configurations to achieve high solar tohydrogen conversion efficiency.