Study On The Band Alignment And Fabrication Of New Type CdTe Thin Film Solar Cells

Cadmium telluride(CdTe)thin-film solar cell is one of the most promising thin-film solar cells.It has the advantages of low cost,small temperature coefficient and excellent weak light performance.In recent years,the cell efficiency has been increased rapidly.The band gap of CdTe thin films is 1.45 eV,which is within the range of the ideal band gap of solar cell materials.CdTe is a direct bandgap semiconductor material,and its absorption coefficient for visible light is as high as 105 cm-1.2-?m-thick CdTe film can absorb 99%of the photons with energy higher than its band gap.CdTe is one of the excellent solar cell materials.The highest laboratory efficiency of CdTe solar cells has been reached 22.1%,and the efficiency of large-area modules is as high as 19%.However,the theoretical limit efficiency of single-junction CdTe solar cell is 29%,and there are still many key issues that need to be studied and solved.The traditional cadmium sulfide(CdS)window layer has photon loss of wavelengths below 510 nm,which leads to a reduction in the short-circuit current of CdS/CdTe solar cell.Magnesium-doped zinc oxide(MZO)window layer has band mismatch with CdTe,and causes serious "S-kink" in the solar cell J-V curves.In this thesis,we studied the improvement of the performance of traditional CdS/CdTe solar cells and the band alignment at the MZO/CdTe interface.In Chapter ?,we introduced the background and history of solar cells,briefly described the structure and working principle of solar cells.We also summarized the preparation and development of CdTe thin film solar cells.In Chapter ?,we optimized the performance of traditional CdS/CdTe thin film solar cells.CdS thin film can strongly absorb the visible light below 510 nm,due to its band gap of 2.42 eV.By reducing the thickness of the CdS window layer,the loss of visible light in the window layer was reduced.More photons can reach the CdTe layer.From the transmission spectrum of the CdS films with different thicknesses,it can be seen that the transmittance of wavelengths below 510 nm was increased significantly as the thickness of CdS was decreased.The experimental results show that as the thickness of CdS was decreased,the short-circuit current of cells was increased.It can be seen from the EQE curves that the EQE response were increasesd significantly below the 510 nm wavelength.However,as the thickness of the CdS film becomes thinner,cell current leakage became more serious,and the cell efficiency was no longer increased.The band gap of the CdS film can be increased as the content of doped oxygen is increased.Doping the CdS with oxygen can increase the band gap of the CdS films,thereby reducing the absorption of short-wave photons in the visible light range.By increasing the amount of oxygen to 7%,we got the highly efficient CdS/CdTe solar cell with an efficiency as high as 16.4%.In Chapter ?,we studied the band alignment at MZO/CdTe interface.Time-resolved photoluminescence spectroscopy spectra show that the minority carrier lifetime of the MZO/Al2O3/CdTe structure is much longer than the MZO/CdTe structure,which indicates that Al2O3 has strong passivation effect on CdTe.However,CdTe solar cells with Al2O3 as the passivation layer exhibited serious "S-kink" in the J-V curves.The large conduction band offset as high as 2.83 eV between Al2O3 and CdTe was found to be the main reason of the "S-kink" for the solar cells with Al2O3 as passivation layer.The conduction band offset between SnO2 and CdTe is only 0.21 eV.The conduction band offset between SnO2 and CdTe can be adjusted by controlling the thickness of the SnO2 films.With the increase of SnO2 film thickness,the VOC was decreased and FF was increased.The "S-kink" was weakened by increasing the thickness of the SnO2 films.In Chapter ?,we fabricated CdTe solar cells by using high temperature substrate method.By increasing the substrate temperature during the CdTe film growth,the crystal quality of the CdTe film was improved.Experiments showed that large CdTe grains can be prepared by high temperature substrate method.The results show that the MZO/CdTe cell prepared by using the high temperature substrate method has higher efficiency.The EQE curve also shows that the crystalline quality of the CdTe thin film becomes better.The high-temperature substrate method also improves the performance of MZO/CdSe/CdTe and MZO/CdS/CdSe/CdTe solar cells.In Chapter V,we summarized the research work and the main conclusions of this thesis,and prospected the future and development of CdTe thin film solar cells.