Investigation Of Defects In Newthin-film Solar Cells

Solar energy is an important energy source which is clean and renewable.Solar cells are the most important approach to convert solar energy into electric power.Up to now,the best efficiency of a single-junction thin-film solar cell has exceeded 25%,whereas it is still far below the theoretical limit.Therefore,it’s urgent to provide more guidance to improve the cell performance.It is well-known that defects could appear in the photo-absorber layer of solar cells inevitable,which strongly suffer the cell efficiency.So,it is of great significance to analyze the defects for benefiting the efficiency.In addition,the carrier transport and recombination mechanisms are also crucial for understanding possible limitations towards high efficiency for solar cells.Recent years,perovskite（CH₃NH₃PbI₃）,antimony selenide（Sb₂Se₃）,and antimony sulfide（Sb₂S₃）solar cells have attracted much interests due to their high absorption coefficients,suitable bandgap,variety prepration methods,low costs,and earth-abundant resource materials.However,few quantitative details have been performed for defect characterizations of these solar cells.In this paper,we mainly focused on the identification and characterization of defects in thin-film solar cells via admittance measurement.Besides,electronic transport mechanisms in thin-film solar cells were investigated.The main research contents of this thesis are listed as follows:1.Based on the basic principle of the admittance measurement,a system for admittance measurement of solar cells was established,and the compenents of the measurement system were explained.By measuring the admittance spectroscopy of solar cell,it is possible to extract the density of states,activation energy,capture cross section of the defects etc,which may lay the foundation for the in-depth study of the internal defects of solar cells.2.The defect properties of CH₃NH₃PbI₃ solar cells with same structure but different efficiencies were characterized using admittance spectroscopy measurements.Furthermore,the defect parameters,including defect density,the capture cross-section of holes and capture lifetime of holes,were extracted and the impact of the efficiencies on the cell efficiencies were investigated.Only one type of defect with activation energies ranging from 0.16 eV to 0.23 eV above the valence band were found for different cells and identified as an interface-type defect.The results indicated that,compared with other parameters,defect density is a critical factor for CH₃NH₃PbI₃ solar cell performance.3.Sb₂Se₃ thin films were treated by post annealing process,and the comparative studies between the post annealing treatment（PAT）and without the treatment were carried out.Electrical properties from SEM,XRD and Raman measurements were compared for the two cell samples.The grain size of the cell sample with PAT was a little larger than that of the cell sample without PAT.The defect properties of Sb₂Se₃solar cells were investigated using admittance spectroscopy measurements.Three defect levels（D1,D2 and D3）in the bandgap of Sb₂Se₃ were detected.Moreover,the defect parameters were extracted.It could be seen that the density Nt of each defect of the Sb₂Se₃ cell sample with PAT were lower than the one without PAT.The decreased defect density indicated that the process of PAT can passivate the defect and reduce the defect density,which reduces the carrier recombination rate and benefits the efficiency of the solar cell.4.Sb₂Se₃ thin films were deposited,and comparative studies were performed for Sb₂Se₃ films formed on substrates located at three different positions away from the furnace center.Electrical properties from SEM,XRD and Raman measurements were compared for the three cell samples.The grain size for the S2 was a little larger than that of the other cells.Temperature-dependent saturation current and open-circuit voltage(V_oc)measurements indicated that the dominant carrier recombination occurred in the CdS/Sb₂Se₃ interface region,which possibly influenced the V_oc for all cells.The highest V_oc for the S2 cell was possible due to it having the lowest CdS/Sb₂Se₃ interface recombination rate.Moreover,the reverse bias current relationship revealed that nonohmic shunt current（space-charge-limited current,SCLC）plays an important role in affecting the performance of solar cells,with lower-efficiency cells have higher non-ohmic shunt current.5.Sb₂S₃ thin films prepared with varied working pressure were fabricated into solar cells.The Sb₂S₃ cell samples prepared with appropriate working pressure（about 2 Pa）were found to own better crystal qualities,achieving a higher J_sc.The defect properties of Sb₂S₃ solar cells were investigated using admittance spectroscopy measurements.Defect levels of the same type with activation energies varied in the range of0.103⁰.146 eV above the valence band were found for different samples.Moreover,the defect parameters were extracted and their relationships with the cell efficiencies were investigated.The results indicated that compared with other parameters,activation energy E_A is a critical factor for Sb₂S₃ solar cell performance.We mainly focused on the identification and characterization of defects in thin-film solar cells via admittance measurement.The results indicated that,compared with other parameters,defect density is a critical factor for CH₃NH₃PbI₃ and Sb₂Se₃ solar cell performance,however,activation energy E_A is a critical factor for Sb₂S₃ solar cell performance.For Sb₂Se₃ solar cells,the dominant carrier recombination occurred in the CdS/Sb₂Se₃ interface region,the nonohmic shunt current（space-charge-limited current,SCLC）plays an important role in affecting the performance of Sb₂Se₃ solar cells.The investigation of the defect mechanisms that may decrease the efficiency of solar cells is very important.