Study Of Fundamental Problems In CdTe Thin Film Solar Cells

CdTe is an important semiconductor material with a direct band gap of 1.45 eV.It has a light absorption as high as 105 cm-1 in the visible range.2-?m-thick CdTe thin film can absorb 99%of photons whose energy is larger than the CdTe band gap.At the same time,CdTe is an ?-? binary compound.Therefore,CdTe is an excellent material for photovoltaic device fabrication.In recent years,CdTe thin film solar cells have been developed rapidly.At present,the world highest conversion efficiency of CdTe solar cell is 22.1%,and the highest module conversion efficiency for commercial application is 18.6%.In the photovoltaic industry,the market share of CdTe solar cells is increasing,and CdTe solar cell has been the leading type of all the thin film solar cells.The short-circuit current density of CdTe solar cell has reached the theoretical value.While the open-circuit voltage and fill factor are much smaller than the theoretical values.To remain competitive in the photovoltaic market,CdTe solar cell still has many critical scientific problems to be solved,for example,the carrier concentration of CdTe material is low,usually about 1013-1014 cm-3;CdTe has a strong self-compensation effect,it is difficult to realize effective p-type doping,resulting in high series resistance,low carrier concentration and open circuit voltage of CdTe solar cells;CdTe has a high work function,about 5.7 eV,and it is difficult to find matching metal to form ohmic contact with it.This leads to the formation of back contact potential barrier in a CdTe solar cell.This is not conducive to the transport and collection of minority holes;There are several contact interfaces inside the CdTe solar cell,the interface recombination will lead to a decrease of the minority carrier lifetime and the open circuit voltage;There are many micro-scale leakage channels inside a CdTe thin film solar cell,which decrease the output current of the cell.The doped elements would diffuse to the CdS/CdTe interface,reducing the cell's long-term stability.Considering the key scientific problems in CdTe thin film solar cells,this paper is mainly focused on CdTe thin film solar cell from the following aspects:In Chapter I,the general situation and future prospect of solar cells were introduced.The structure,formation principle and output characteristics of solar cells based on semiconductor p-n junction are described.At last,I summarized the development process,cell structure,material properties and key technical issues of CdTe solar cells.In Chapter ?,we studied the output characteristics of GaAs and CdTe thin film solar cells under weak light condition.Then,we analyzed the relationship between the performance parameters of the two kinds of cells and the light intensity.Next,by comparing the difference under weak light irradiation between the two types of solar cells and the characteristics of the two types of cells,such as the structures of the two cells,crystallization of materials,doping concentration of active layer and interface states,we analyzed the main limiting factors affecting the weak light performance of the CdTe solar cell.Finally,through analysis of the change of GaAs and CdTe cell efficiency with the light intensity,we established the ideal functional relationship between the solar cell efficiency and the light intensity.In Chapter III,the formation mechanism of space-charge limited current in CdTe thin film solar cell was investigated.A simplified FTO/CdTe/metal metal-semiconductor-metal structure was prepared,and we studied how this structure was formed in the CdTe solar cells.We analyzed the effect of space-charge limited current on leakage current of CdTe solar cells.Experimental results and theoretical analysis show that space-charge limited current greatly reduces the minority carrier lifetime and the open-circuit voltage of the CdTe solar cells.In Chapter IV,we employed CoO as a back contact buffer layer for CdTe solar cell fabrication.CoO buffer layer acts as back electric field and electron reflector at CdTe back contact,it can reduce the recombination of minority carrier at the back contact interface,increase the minority carrier lifetime and the open-circuit voltage;also it is conducive to the transport and collection of holes,thus increases the short-circuit current density.At the same time,the use of a CoO buffer layer can reduce the amount of Cu at the back contact of CdTe solar cell,and thus improve the long-term stability of CdTe solar cell devices.In Chapter V,the admittance spectrum(capacitance-frequency curve,C-F)and the impedance spectrum of CdTe cells with different back contact structures were studied.By analyzing the C-V curves,we obtained the distribution of carrier concentration in the cells and the built-in potential of the CdS/CdTe hetero-junction.Through studying the admittance spectrum of the CdTe cells,we obtained the distribution of defect density in the CdTe film and at the CdS/CdTe interface..In Chapter VI,the main research contents and conclusions of this paper were summarized,and the future development direction of CdTe solar cells was briefly introduced.