Investigation Of CdTe Solar Cells And Related Thin Film Materials

CdTe is a direct band gap semiconductor with a high absorption coefficient of?104-106 cm-1 in the visible range.As a II-VI binary compound,CdTe can be easily fabricated into high quality single-phase films.The band gap of CdTe is?1.45 eV,which matches with the sun light spectrum very well.Due to the characteristics discussed above,CdTe has received wide attention as an ideal photovoltaic material,the CdTe thin film solar cell has became one of the most successful thin film solar cells in the photovoltaic market.At present,the world-record efficiency of CdTe solar cell is 22.1%and the highest efficiency of CdTe large-area module is 18.6%.Although CdTe thin film solar cell achieved great success,we should recognize that it still has some basic scientific problems to be solved,for example,effects of band alignment on carrier transport at heterojunctions interface,fabrication of stable low-resistive ohmic back contact,interaction between carriers and phonons in CdTe thin films and its effects on solar cell devices,interdiffusion between the thin film materials in CdTe solar cells,etc.Considering the problems above,this thesis mainly focuses on the CdTe solar cell from the following aspects:In Chapter ?,the background and history of solar cells were briefly reviewed.We also discussed the principle and the basic characteristics of solar cells,and briefly introduced different types of solar cells.In Chapter ?,the fabrication processes of high-efficient CdTe thin film solar cells were discussed.We optimized the fabrication process of CdS thin film by using magnetron sputtering.The band gap of CdS window layer was adjusted by controlling the oxygen content in the sputtering atmosphere,and the short-circuit current density of the CdTe solar cells was increased by using a CdS:O window layer.We deposited CdTe thin films by using close-space sublimation(CSS)technique,and optimized the heat treatment time of CdTe film.A good ohmic contact was fabricated at back electrode by employing different back contact buffer layer.Based on the optimization discussed above,high-efficient CdTe solar cells were fabricated.In Chapter ?,the CdTe solar cells with new back contact buffer layers were fabricated and investigated.We investigated the effects of Cul and ZnTe:Cu back contact buffer layers on the performance of CdTe solar cells.The band alignment at CuI/CdTe interface was analyzed,which demonstrated that the valence band maxima were well matched at CuI/CdTe interface,and the photo-generated holes can transport from CdTe to CuI layer without overcoming any energy barrier.The large conduction band offset at CuI/CdTe interface can act as an electron reflector,which helps repel photo-generated electrons from the back contact and reduce carrier recombination,and therefore,increase the open-circuit voltage(Voc)of solar cells.Compound CuI was used to replace metallic element Cu at the back contact,which avoided the rapid diffusion of Cu into the devices and improved the stability of CdTe solar cells.We also fabricated CdTe solar cells with ZnTe:Cu back contact buffer layer,and investigated the effects of rapid thermal processing(RTP)on the CdTe solar cells.By optimizing the RTP technique,the efficiency of a CdTe solar cell was improved from 13.3%to 16.2%.The VOC of this solar cell was improved from 0.744 to 0.841 V,namely,the relative VOC improvement was 13%.This work was completed in cooperation with the students in our laboratory.In Chapter ?,the resonant Raman scattering and the phonon anharmonic effect in polycrystalline CdTe thin films were investigated.Resonant Raman scattering of polycrystalline CdTe thin film was observed by using a laser light with a wavelength of 785 nm.The anharmonic effect and the effect of band gap change on the resonant Raman scattering of CdTe film were investigated by using temperature depedent Raman scattering.The lattice dynamics of CdTe thin film under different temperatures was analyzed,the results demonstrated that due to the anharmonic effect,the longitudinal optical phonon(LO)of CdTe decayed asymmetrically into a transverse optical(TO)and a transverse acoustic(TA)phonon.By fitting the measured Raman scattering peaks,the surface optical(SO)mode,which comes from the vibration of atoms located in the near-surface region of a polar solid material,was identified.The effects of CdTe grain size on exciton-phonon coupling were investigated by using resonant Raman scattering.In Chapter V,the CdTe solar cells with new structures were fabricated and the interdiffusion between the thin film materials was investigated by using Raman scattering.We fabricated CdTe solar cells with different window layer and analyzed the effects of window layer materials on the external quantum efficiency(EQE)of CdTe solar cells..We also studied the interdiffusion between CdSe and CdTe thin films by using Raman scattering.The results demonstrated that strong interdiffusion occurs between CdSe and CdTe when the temperature is higher than 300 ?.In Chapter ?,the main works in this thesis were summarized.The main problems faced by CdTe solar cells at the present stage and the future development trend were analyzed.