Theory Investigation Of Surface Characteristics Of Window And Absorber And Its Influence On The Performance Of Cd Te Solar Cells

Cd Te solar cell is the best thin film solar cells in commercialization progress because of its high photoelectric conversion efficiency,good stability and low cost of mass production.However,the highest efficiency（22.1%）of single junction device is far away from the theoretical limit（about 32%）.It continues to be the research focus to cognize the factor influencing the device performance of Cd Te thin film solar cells and to improve the photoelectric conversion efficiency.At present,the short circuit current density(J_SC)of highest efficiency device exceeds 30 m A/cm²and is very close to the theoretical expectation,but the open-circuit voltage(V_OC)and fill factor（FF）are less than 80%and 90%of theoretical limit,respectively.The interface characterization is the critical factor that influences the V_OC and FF.This thesis is mainly investigating the surface and interface characterization of window and absorber layers of Cd Te thin film solar cells.Based on the premier experimental work,firstly,the surface and interface characterizations of Sn O₂,Mg_0.1875Zn_0.8125O（MZO18）,Cd Te,Cd Te_0.75Se_0.25（CST25）are investigated by first-principle calculation,and then the influence of the surface and interface characterizations of these materials on device performance is studied,finally,the strategy of preparation high efficiency Cd Te thin film solar cells is proposed.The detailed researches are as follows:1.The characterizations of Sn O₂（100）surface and Mg_0.1875Zn_0.8125O（MZO18）（001）surface which are used as the window layer in Cd Te solar cells are investigated.We get the surface characterizations of Sn O₂ and MZO18 and explain the formation reasons of surface states.This study is important for guiding the development of preparation technology of window layer,and reducing the density of surface and interface defects,and developing the device performance of solar cells:（1）The surface relaxation and charge transfer of three kind of Sn O₂（100）surface structure,S1,S2 and S3,determine the band structure and the surface defect states.The band gap of Sn O₂（100）surface is 0.22～0.37 e V larger than Sn O₂,and its electron affinity is 0.11～0.16 e V lower than Sn O₂.The defect states are related with the outer 10 monoatomic layers,and mainly with the outmost 2 layers.The defect states in S2 and S3 are close to valence band maximum,while the defect states in S1 are located at the middle of the band gap（1.25～2.94 e V）.The density of surface and interface defect states can be controlled by regulating the proportion of S1,S2 and S3.（2）The characterizations of MZO are related to the position and aggregation of doped Mg,and the band gaps of three kind of MZO18（B1,B2 and B3）are 3.32～3.92 e V,and the electron affinities of them are 4.38～4.53 e V.Compare to the bulk structure,the band gap of MZO（001）surface is 0.15 e V larger,and the electron affinity is 0.47 e V lower.The two sides of the MZO（001）surface structure form different defect states.One side with Zn（Mg）-O₃ forms no defect states in the band gap,while the other side with Zn（Mg）-O forms 4 defects which extend over 3 e V.Two of these defects are close to valence band maximum,while another two are located at the middle of the band gap.This study is important for guiding the interface technology of MZO/Cd Te,and reducing the interface defect states,and developint the device performance of solar cells.2.The band structures and defect states of Cd Te（100）surface and Cd Te_0.75Se_0.25（CST25）（111）surface which are used as the absorber layer in Cd Te solar cells are investigated,and the formation reasons of surface states are explained.This study is helpful in developint the thin film solar cell technology:（1）Compared to the bulk structure,the band gap of Cd Te（111）are 0.37 e V wider,and the electron affinity is 0.41 e V lower.When terminal atoms are Cd-Te,the positions of each atom layers are increased by 1.62 e V（Cd 4d）or 0.89 e V（Te 5s）,and it forms a defect band arrange from 0.29 e V to 1.40 e V.When terminal atoms are Cd-Te₃,the positions of each atom layers are decreased by 0.98 e V（Cd 4d）or 1.08e V（Te 5s）,and it forms a defect band arrange from 0.38 e V to 1.49 e V.（2）Compared to the bulk structure,the band gap of CST（111）is 0.47 e V wider,and the electron affinity is 0.13 e V higher.CST（111）forms only one defect states at middle of the band gap arrange from 0.45 e V to 1.05 e V.Hence,the surface states of CST can be regulated by control the content of Se in the terminal atomic layer.3.Based on investigation of surface characterization of window and absorber,the influence of band alignment and defect states of the front interface on device performance of Cd Te thin film solar cells is investigated systematically,and then the strategy of preparation high efficiency Cd Te thin film solar cells is proposed:（1）The band alignment of the front interface is very important to device performance of solar cells.The surface of window layer may form a barrier which increases the carrier recombination at absorber and the front interface,and reduces the performance of solar cell.The absorber surface increases the carrier recombination at the front interface and reduces the device performance.Whether the conduction band offset is very positive or negative at the front interface,the I-V characteristic curve illustrates a kink with different mechanisms.There exists a critical point when the conduction band minimum of absorber layer is around 0.7 e V at the front interface,and the mainly recombination progress of Cd Te solar cell is switched between the front interface recombination and the bulk recombination.（2）The defect states at the middle of the band gap of the front interface are very important to solar cell performance.Interface states of window layer reduce the band bending of absorber layer and form a barrier to discourage the separation and transportation of the photo-generated carriers,thus increase the carrier recombination,and the cell performance reduces as the energy level and density of interface states increasing,and the QE and J_SC of solar cells are reduced even to 0.The interface states of absorber layer increase the interface carrier recombination,and decreases the cell performance.Meanwhile,the band bending of absorber layer is increased as the energy level and density of interface states of absorber layer increasing,as a result,the QE,J_SC and FF of solar cell are improved significantly.（3）Apart from improving the performance of absorber layer,the characterizations of the front interface must be controlled properly when preparation of high efficiency Cd Te solar cells.The band structure around the front interface should meet a certain band matching condition and the conduction band offset should not be very positive or negative to avoid barrier of electron transportation at the front interface.The band bending of absorber must keep downward to the greatest extent and the conduction band minimum of absorber should keep under 0.7 e V at the front interface.The composition of some surface,such as S1 of Sn O₂ and Zn（Mg）-O side of MZO,should be increased to the greatest extent near the front interface by improving of preparation techniques,such as adjusting the ambience.It should be avoid to form some defect states in the middle of the band gap from window materials,and the density of these defects must keep under 10¹¹/cm².