Study On The Surface And Interface Of Copper-zinc-tin-sulfur Thin-film Solar Cells

With the rapid development of the global economy,the shortage of energy supply has gradually become one of the most pressing problems facing the development of contemporary human society.Among them,solar energy is an effective way to solve the shortage of energy supply in renewable energy.The solar cells with photoelectric conversion is the most promising form of solar energy utilization.Among them,Cu₂ZnSnS₄?CZTS?,as a new type of solar cell,has become one of the most commercial development prospect thin film solar cells because of its rich content,non-toxicity,suitable band gap and high absorption coefficient.In addition to the basic properties of CZTS materials,synthesis conditions and other important factors affecting the performance of solar cells,there is another aspect worth paying attention to:the microstructure and characteristics related to the surface and interface are also key factors determining the performance of CZTS thin film solar cells.The surface and interface of CZTS thin film solar cells are mainly derived from the following three aspects:1)CZTS thin film solar cells are thin film solar cells composed of polycrystalline grains,and there are interfaces?ie,grain boundary?between the crystal grains;2)CZTS thin film solar cells are composed of multiple layers of different functional materials,and the interfaces between different materials are inevitable component;3)Since CZTS is a complex quaternary compound semiconductor,it is easy to form binary or ternary heterophases during preparation,and there are interfaces between these heterophases and the matrix.Further improvements in the efficiency of CZTS thin film solar cells are currently limited by the interfacial structure and characteristics between various semiconductor materials.The establishment of the model with surface and interface in CZTS,the study of the formation mechanism of defects with surface and interface,the way to control the heterogeneous interface and clearer understanding of the heterogeneous interface will help to find the optimal conditions in the existing parameter space so as to reduce the recombination with surface and interface and improve the stability of CZTS material.Therefore,we used the first-principles calculation method to detailed research and analysis on the above three problems about surface sand interfaces.The main research contents and results are as follows:?1?The low-index CZTS surfaces was studied.The importance of surface with CZTS proformance were investigated in detail by analyzing the microstructure and electronic properties for the?112?,?100?,?110?,?001?and?101?surfaces with CZTS.The calculation results shows that the relaxation for the?001?,?100?,?101?and?112?surfaces with Cu as the temination surface are relatively large,larger surface relaxations result in larger surface additional dipole moments.Moreover,more surface states appear at the bottom of the conduction band?CB?and at the top of the valance band?VB?due to the presence of four dangling bonds at the interfaces.The variation of the band gap values with these five surfaces are small,the effect of surface relaxation on the band gap of different low index surfaces varies within 0.37 eV.The?112?surface has the smallest dangling bond density,resulting in minimal surface energy.The{101}surface is the dominant surface?72.6%?in the area of the equilibrium crystal morphology of CZTS crystal.?2?The interfaces of CZTS with hetero-phase junctions was studied.The possibility and characteristics with the phase transition were analyzed,and the influence of the hetero-phase junctions on the solar cell were clarified.The calculation results show the transition pressure exists at around 31 GPa and the phase transition is most likely to appear along the[100]crystal orientation.The hetero-phase junctions that composed by?101?,?110?,and?100?crystal planes present the type-II band alignment,which is beneficial for the photoelectric conversion.While,the hetero-phase juncitons that composed by?112?and?001?crystal planes present the type-I band alighment,which is disadvantageous for solar cells.So the hetero-phase junctions that composed by?112?and?001?crystal planes should be avoided as much as possible in the experiment.?3?The interface between solid solution ZnO_1-xS_x as buffer layer and CZTS was investigated.By analyzing the properties of ZnO_1-xS_x solid solution and selecting a suitable solid solution according to the conditions with buffer layer,the influence of the interface composed by CZTS and buffer layer on the solar cell were clarified.The calculation results indicate that the lattice constant is found to be linearly interpolated between ZnO and ZnS for the ZnO_1-xS_x solid solution,,and the band gap of the solid solution changes parabolically with the increase of solid solubility.The type of CZTS/ZnO_0.375S_0.625 hetero interface belongs to the Type II hetero interface,which is friendly to CZTS-based thin film solar cells.The ZnO_0.375S_0.625 solid solution can be used as a buffer layer for CZTS thin film solar cell devices.?4?The interface of the second phase Cu₂S,ZnS and CZTS was investigated.,The influence of the second phase on the solar cell was clarified by analyzing the interface electronic properties between the second phase and CZTS.The calculation results indiacate that the type of CZTS/Cu₂S heterojunction belongs to the type II hetero interface,which is beneficial to the CZTS-based solar cell.Even if the second phase with Cu₂S appears in the experimental preparation,it can not adversely affect the performance with solar cells.The type of CZTS/ZnS interface belong to type I hetero interface,which is detrimental to the performance of solar cells.Therefore,the second phase of ZnS should be avoided as much as possible in the experiment.In the paper,the first-principles calculation method was used to study the problems with surfaces and interfaces in CZTS thin film solar cells.The vital factors?microstructure,electronic states of surface and interface,band bending,influence on carrier transport,etc?of the problems with surfaces and interfaces were studied in detail.The micro-influence factors and intrinsic action laws of surface and interface and regulation performance of photovoltaic were summarized,which can provide effective basic data for exploring efficient,low-cost and stable solae cells.