Studies On Optical Constants Of Nano-Films And Fabrication Of Cu?In,Ga?Se₂ Thin Film Solar Cells With High Efficiency

Since the twentieth century,the development of thin film technology has brought many technological advances,such as electronic semiconductor technology,magnetic storage technology,optical coating,light-emitting diodes,thin film photovoltaic tech-nology and so on.One of the main applications of thin film technology is in optical field,and the applications in optical field can not be separated from the understanding of optical constants of the film,when film thickness is at the nanometer level,the opti-cal constants will be markedly different from the conventional bulk materials.It is the very unique optical and electromagnetic properties make it occupies an important posi-tion in the field of modern optoelectronics industry.the study of nano-thin film optical constants not only help us to improve the quality of films growth,but also help us to further tap the application potential of films.Based on this,we developed an Ellipsom-etry iterated with transmittance method to accurately determine the optical constants of nano-films with different materials and study the relationship between optical constants and thickness or temperature.Another important application of thin film technology is in the field of solar cells,CuIn1-xGaxSe2?CIGS?thin film solar cells,as the representative of the second gener-ation of thin film solar cells,with low cost,high efficiency,high stability and a series of advantages,has become one of the most promising solar cells.However,the main factor restricting its application at this stage is that its photoelectric conversion effi-ciency is not high enough to make the economic benefits of solar cells comparable to traditional energy sources.Thus,in the second half of this paper,we are committed to how to improve the process of preparing high efficiency CuIn1-xGaxSe2 photovoltaic devices.This paper mainly includes two aspects:the optical constants of different nano-thin films based on ellipsometry and the preparation of high efficiency CIGS solar cells.This thesis contains seven chapters,the specific contents are as follows:In the earlier part of the first chapter,we first introduced the propagation process of light in the material which begin from the Maxwell equations and then introduce the working principle of the ellipsometer and several main measurement methods of the optical constant of thin films.In the latter part,Begin with the current world en-ergy crisis,we introduced the birth of the photovoltaic background and the principle of photovoltaic,and focuses on the CIGS solar cell development history and its technical advantages brought by its unique nature nature.In Chapter 2,Ellipsometry iterated with transmittance method was developed on the basis of traditional ellipsometry.The optical constants of silver films with different thicknesses were obtained.On the basis of their optical constants,the surface and bulk energy loss functions of thin silver films were introduced.The relationship between surface plasmon and film thickness was analyzed.In Chapter 3,the effective optical constants of copper films with different thick-ness in the air are obtained by ellipsometry.The surface structures were analyzed by ellipsometry.The thickness of rough surface layer,oxide layer and the copper layer were obtained,respectively,and the mechanism of the natural oxidised copper film was analyzed according to the experimental results.In Chapter 4,In situ ellipsometry measurements in the spectra range from 246 to 1000nm were performed on Si02/Si system at temperatures varying from 25 to 600?.By using Cauchy dispersion model to describe Si02 layer and B-spline to param-eterize Si substrate,the temperature dependent optical constants of SiO₂ film and Si substrate were precisely determined and analyzed which is helpful to understand the temperature effect of substrate for ellipsometry measurement.The temperature coeffi-cient of refractive index of SiO₂ at high temperature was obtained by ellipsometry.The results indicate that the temperature coefficient increases with increasing wavelength and decreasing temperature.In chapter 5,the relationship between growth process,Ga gradient shape and the solar cell performance was studied and analyzed by controlling the temperature of evaporator source in the growth process.In the end,by making the temperature of In furnace increase gradually at the first stage and decrease gradually at the third stage while the temperature of Ga furnace varied in contrast to In furnace,the optimized V-shaped band gap formed which made a good open-circuit voltage,short-circuit current and FF pre-sented in one solar cell at the same time.Finally,the CIGS solar cell with a efficiency as high as 17.5%without antireflection layer was fabricated.In chapter 6,by adopting Al₂O₃ barrier layer with different thickness,it was proved that the amount of sodium diffusion from glass substrate into CIGS absorption layer would have an adverse impact on solar cell performance,especially on open-circuit voltage.Under our experimental condition,1 nm of Al₂O₃ can make the amount of diffused Na at the best level.Then,we tried to completely interdict the diffusion channel of sodium from glass to CIGS absorber layer with a sufficiently thick Al₂O₃ barrier layer,and Na was added by NaF post-deposition treatment.The results showed that solar cell fabricated at this condition is superior to the case of no Na at all,while inferior to the addition of Na diffused from glass.After that,Al₂O₃ barrier layer combined with NaF post-deposition treatment was used,and the solar cell performance is significantly improved.Finally,by using 3 nm Al₂O₃ barrier layer with 33 nm NaF post-deposition treatment,a solar cell with an efficiency of 16.5%was obtained?without antireflection layer?.Chapter 7 summarizes the research contents of this thesis.