Investigation Of Window Layer And Buffer Layer Materials For Applications In Cu（In,Ga）Se₂ Thin Film Solar Cells

The energy shortage and environmental pollution have become serious problems and challenge with the rapid developments of industry and economic.The seeking and utilization of cheap renewable clean energy(green energy)are becoming more and more important.Solar enegy is a kind of clean,abundant and renewable energy and solar cells are effect devices to convert solar energy directly into electrical energy by photovoltaic effect.Cu(In,Ga)Se2(CIGS)thin film solar cells have attracted more and more attention due to their high conversion efficiency,outstanding stability and wide application in different fields etc.The photoelectric conversion efficiency of the solar cells is a very important parameter to identify the performance of the solar cells.In order to achieve a higher efficiency,the CIGS absorber layer must be optimized,and the optimization of other functional layers,such as the TCO layer and the buffer layer,also has a vital role to improve the efficiency.The transparent conductive oxide(TCO)materials are commonly applied in CIGS thin film solar cells as window layers,which could allow the light to enter into the solar cells and extract the photogenrated electrons at the same time.In2O3:SnO2(ITO)and doped ZnO are both kinds of important window layer materials in CIGS thin film solar cells due to their excellent optical and electrical properties(high transmission and conductive).ITO and ZnO:Al(AZO)films are usually deposited by magnetron sputtering method with a stabilible and controllable process.Another important doped ZnO material i.e.ZnO:B(BZO)film is usually deposited by low pressure chemical vapor deposition(LPCVD)technology.The as-deposited BZO films could be obtained textured surface and good optical and electrical properties with proper process parameters.The above TCO window layers are important to achieve high efficiency for CIGS thin film solar cells and need investigation deeply.Cadmium sulfide(CdS)as one of very important functional buffer layers in CIGS thin film solar cells has attracted more attentations.Recorded efficiency CIGS thin film solar cells are usually employing CdS thin films as buffer layer prepared by chemical bath deposition(CBD)technology.However,the effects of CBD CdS buffer layer on CIGS thin films are not very clear so far.The study and optimization of CBD CdS would benefit to the performance enhancement of CIGS thin film solar cells.In thin thesis,BZO thin films are prepared at different substrate temperatures and the corresponding material properties are investigated systematically.At the same time,the BZO are applied in CIGS thin film solar cells in window layer and the influence of the substrate temperature on BZO and CIGS thin film solar cells are studied.Moreover,the comparisons of sputtered ITO and sputtered AZO as well as LPCVD BZO window layer are carried out in term of their effect on the performance of CIGS thin film solar cells.It is found that the deposition rate of BZO thin films is increased with the increasing substrate temperature,and the preferential orientation is changed from single orientation to multiple orientations.The BZO thin films show lowest resistivity at 153 °C which is attributed to relatively carrier density and mobility.The high carrier mobility of BZO thin film at 153 °C is related with the large grain size pyramid-shape crystallites.CIGS thin film solar with LPCVD BZO window deposited at 119 °C showed a highest CIGS solar cell efficiency of 11.7%.Compared to the AZO and ITO window layers,the BZO film shows higher transmission especially in near infrared wavelength region.As a result,the CIGS thin film solar cells with BZO window layer show a relatively high Jsc.However,the CIGS thin film solar cells with AZO and ITO window layers display higher Voc and FF and a resulting higher efficiency.The highest efficiency CIGS solar cell of 12.2% has been achieved when optimized AZO window layers is used.The CBD CdS layers were deposited by using cadmium sulfate and cadmium acetate,for applications in CIGS thin film solar cells.When cadmium sulfate is used to deposit CdS,the deposition rate is slower.The CdS particles are smaller,which could be a better coverage on the surface of CIGS absorber and lead to a high Voc and FF.As a result,an improved efficiency is obtained for CIGS thin film solar cell(12.86%).Moreover,CdS thin films with different thicknesses are deposited by adjusting the deposition duration.It is found that the transmittance of CdS thin films decreased with the increasing CdS thickness.As a result,the spectrum responses of corresponding CIGS thin film solar cells between the wavelengths from 350 nm to 530 nm is decreased with the increasing CdS thickness.However,they show an enhanced Voc and FF when a thicker CdS is used.The best efficiency CIGS thin film solar cells of 13.5% is achieved when CdS thickness is 113 nm.Further investigation on the influence of CdS on CIGS is need.