Fabrication Of CdTe Thin Film Solar Cell And Study Of Carrier Recombination In It

CdTe is a stable Ⅱ-Ⅵ binary compound with a band gap of 1.45 eV at room temperature and a high absorption coefficient of 105cm-1 in the visible range,making it an ideal photovoltaic material.CdTe solar cells have become one of the most competitive thin film photovoltaic because of its high conversion efficiency,excellent long-term stability and low cost.Over the past few decades,CdTe solar cells have been successfully commercialized.The theoretically predicted efficiency for CdTe thin film solar cell is 32.1%.The record power conversion efficiency(PCE)of CdTe solar cell and module have reached 22.1%and 18.6%,respectively.Although CdTe solar cells have been a great success,there are still some problems to be solved,for example,the effects of band alignment on carrier transport at the interface of heterojunction,the carrier recombination at heterojunction interface and in CdTe bulk region,the influence of low minority carrier lifetime on open circuit voltage in a cell,preparation of stable back contact with low resistance,etc.This thesis focuses on the following aspects of CdTe solar cells:In Chapter Ⅰ,we briefly introduced the background and development history of solar cells.And the working principle and output characteristics of solar cells based on semiconductor PN junction were also discussed.Finally,the development process,cell structure,material properties and carrier recombinantion processes in the CdTe thin film solar cells were discussed.In Chapter II,the traditional CdS/CdTe solar cells,new type MgxZn1-xO/CdTe(MZO/CdTe)and MZO/CdSe/CdTe solar cells were fabricated.During the sputtering of CdS films,O2 was introduced.And therefore,a CdS:O with larger band gap was obtained,which improved the blue light response of the cells.The CdTe thin films were deposited on the MZO substrates by using close-space sublimation(CSS)in oxygenfree ambinent.The substrate and source temperature were optimized to obtain CdTe thin film with grain size of 1～2 μm.And MZO/CdTe cells with higher efficiency were obtained.The CdSe layer was also introduced to increase the light absorption of the cell in the long-wave region.By optimizing the CdSe layer thickness,an MZO/CdSe/CdTe solar cell with a maximum short-circuit current of 27.1 mA/cm2 and a conversion efficiency of 14.00%was obtained.In addition,ZnTe:Cu films were used as a buffer layer and were prepared by thermal evaporation and rapid thermal processing.The preparation process was optimized,and it was found that when the thickness of ZnTe was more than 100 nm,the series resistance of the solar cell would increase and the short-circuit current would decrease.A sandwich structure of 2 nm Cu/100 nm ZnTe/3 nm Cu could ensure moderate doping of Cu to CdTe and avoid excessive Cu diffusion into CdTe.Finally,the CdS/CdTe solar cell prepared with 100 nm ZnTe showed low reverse saturation current,resulting in high open circuit voltage,fill factor and a maximum conversion efficiency of 16.02%.In Chapter Ⅲ,the performance of MZO/CdTe and CdS/CdTe solar cells was compared.It is found that the efficiency loss of MZO/CdTe caused by voltagedependent photocurrent collection was more serious.In this study,the efficiency loss in CdS/CdTe cell with conversion efficiency of 13.80%was 1.53%,while that in MZO/CdTe cell with conversion efficiency of 12.67%was as high as 3.89%.In order to explore the reason of the serious loss of efficiency,the minority carrier lifetime of the two solar cells was detected by time-resolved Photoluminescence spectrum(TRPL).It is found that the minority carrier lifetime of CdTe in MZO/CdTe structure is low.The low minority carrier lifetime is due to serious recombination of carriers at the interface of MZO/CdTe PN junction,which leads to a high open-circuit voltage loss.According to the relationship between the open-circuit voltage and the J-V test temperature T,the activation energy of MZO/CdTe and CdS/CdTe solar cells was 1.08 eV and 1.36 eV,respectively,indicating that bulk Shockley-Read-hall(SRH)recombination in CdTe films in the CdS/CdTe played a dominant role.However,the MZO/CdTe cell was dominated by PN junction interface recombination.In addition,the relationship between cell ideal factor and temperature indicated that the tunneling enhancement interface recombination existed in MZO/CdTe solar cells even at room temperature,while it only affected the performance of CdS/CdTe solar cells when the temperature was below 240 K.The results of capacitance frequency(C-F)test and solar cell capacitance simulator(SCAPS)simulation showed that the deep level defect located in the center of the forbidden band played the main role in the bulk recombination of CdTe cells.The simulated defect concentration and interface recombination velocity in MZO/CdTe solar cells were higher than those in CdS/CdTe cells,which were consistent with the experimental results.And the serious efficiency loss of MZO/CdTe cells was caused by the serious recombination in the cells that hindered electron transport.In Chapter Ⅳ,the metal oxides were used to passivate the CdTe surface,decrease the surface recombination and increase the minority carrier lifetime.The mechanism of the passivation was studied.The minority carrier lifetimes in CdTe thin films coated with NiO,V2O5 and Al2O3 were measured by TRPL.In CdTe thin films coated with a 3-nm-thick V2O5 or Al2O3 layer,the TRPL lifetime was increased compared to that of the bare CdTe or NiO-coated CdTe thin films.The CdTe on the surface was oxidized or reduced after metal oxides deposited as the XPS results indicated.Chemical passivation may be the reason for the increase of CdTe minority carrier lifetime.The C-V measurements showed fixed negative charges were exsited in V2O5 and Al2O3 films.These charges could induce electric filed in the CdTe films which helped accelerate carrier separation,reduce recombination and increase minority carrier lifetime.According to the TRPL simulation results,the mechanism for surface passivation of CdTe by V2O5 and Al2O3 was investigated.And the results revealed that the increased carrier lifetime was caused by field effect rather than the chemical passivation.The conclusion was verified by changing the incident condition of TRPL measurements and the thickness of Al2O3 deposited on the CdTe surface.In Chapter Ⅴ,the main resarches of this thesis were summarized.The future research direction of CdTe solar cells were prospected.