Preparation And Properties Of Metal Oxide Doped Cadmium Telluride Films

Cadmium telluride（CdTe）is the most prospective and leading category among all of the absorption materials for thin film solar cells.In order to reduce the instability related to conventional Cu-doping and the possibility of device degradation,and to adjust the required band gap,to decrease the open-circuit voltage loss of solar cell devices,certain other metals（such as titanium,vanadium and indium）are selected to dope CdTe films,which has resulted in considerable variation in the optical,electrical and mechanical properties of the films.For purpose of further boosting the performance of CdTe films as solar cell absorption layer,the CdTe films are doped with Bi₂O₃and ZnO respectively,and the structural,optical and electrical properties of the films are studied.In this thesis,the films are prepared by RF magnetron sputtering,and characterized by X-ray diffraction（XRD）,ultraviolet visible near infrared（UV-Vis-NIR）spectrophotometer,field Emission scanning electron microscope（FESEM）and Hall effect measurement system.Firstly,undoped CdTe films are prepared by using radio frequency magnetron sputtering method.It is indicated by the XRD result that the CdTe films have good crystallinity and cubic sphalerite polycrystalline structure,and grows preferentially along the（111）plane.UV-Vis-NIR spectrum analysis shows that the band gap decreased with increasing RF power,and CdTe films deposited at higher RF power have the optimal bandgap values（1.44-1.60 e V）for visible light,which can be applied as the absorbing material in Cd S/CdTe and ZnO/CdTe solar cells.Furthermore,it has been revealed by the FESEM patterns that the CdTe films have a continuous and dense surface morphology,and the grain size increases gradually with the increase of deposition power,which is consistent with the results of XRD.In addition,the ratio of the atomic percentage of Te and Cd is close to 1:1 in all samples which has been demonstrated in the EDAX study.The CdTe film deposited at the RF sputtering power of 100W and the pressure of 1Pa have the resistivity of 57.1Ω.cm,the carrier concentration of 1.63×10¹³cm^-3and the Hall mobility of 1.33×10⁴cm²/Vs.At this time,the CdTe film has the optimal bandgap of 1.45 e V and the relatively higher infrared transmittance of 58%as the absorption material of solar cells.Then,Bi₂O₃-doped CdTe（Bi₂O₃:CdTe）films are prepared by RF magnetron co-sputtering technology.XRD analysis shows that the Bi₂O₃:CdTe films are preferentially oriented on the CdTe（111）plane.No diffraction peaks related to Bi and O are found,but the existence of Bi and O element in the Bi₂O₃:CdTe films has been proved by the EDAX,indicating that O and Bi atoms can replace Te and Cd atoms,or be doped into the grain boundaries of CdTe films.The transmission spectrum study shows that with the increase of Bi₂O₃doping power,the visible light transmittance and near-infrared transmittance of the film descend constantly.When the doping power is 5W,the Bi₂O₃:CdTe film with the optical band gap of 1.48e V is most suitable to be the absorption layer of the solar cell device.At this time,the Bi₂O₃:CdTe film has the resistivity of 120Ω.cm,the carrier concentration of8.21×10¹⁴cm^-3and the Hall mobility of 49.5 cm²/Vs.Finally,ZnO-doped CdTe（ZnO:CdTe）films are prepared by RF magnetron co-sputtering technology.XRD analysis shows that the ZnO:CdTe films films are preferentially oriented on the CdTe（111）plane.No diffraction peaks related to Zn and O are found,the existence of Zn and O elements confirmed from EDAX indicates that O and Zn atoms may replace Te and Cd atoms or be doped into CdTe grain boundaries.When the doping power is 15W,the diffraction peak is the strongest,indicating the highest crystallinity.Under this condition,the resistivity of ZnO:CdTe film is 351Ω.cm,the carrier concentration is 8.76×10¹²cm^-3,Hall mobility 2.13×10³cm²/Vs.At this time,the film has good absorption performance in the visible light range（the optical band gap is 1.46e V）,so it is most suitable for the absorption layer of solar cell devices.The prepared Bi₂O₃:CdTe and ZnO:CdTe films may reduce the instability related to traditional Cu doping and and the possibility of device degradation.The films with optimized physical properties may play an important role in improving the performance of solar cell devices.