Study Of Ultra-thin CIGS Solar Cells And Modules

Cu(In,Ga)Se2(CIGS) thin film solar cell is considered as the most promising photovoltaic device. This is due to the excellent properties of CIGS thin films, such as high absorption coefficient, suitable and tunable band gap, high stability and strong anti-radiation. At present, CIGS solar cells have been steadily progressing and transfered for mass production. How to reduce the cost of production has became the common goal of researchers around the world.If the thickness of the CIGS absorber layer is reduced with no, or minor, loss in solar cell performance, the deposition time will be shorter at the same deposition rate, then the labor and power costs will be cut down. In addition, a ultra-thin absorber layer will also reduce the materials usage and thereby materials costs. Furthermore, more CIGS solar cells can be fabricated using the limited In, Ga and Se resources. So it is important to study the ultra-thin CIGS absorber layer and solar cell. So far, the study of the ultra-thin CIGS solar cells has became a hot research topic around the world. In this paper, a series intensive studies on the properties of ultra-thin CIGS absorber layer material and solar cells has been carried based on the previous research results. And then, CIGS modules are also studied based on results of the small area solar cells. The innovative researches are as following.The impact of CIGS absorber layer thickness on the properties of CIGS material and solar cell is studied by theoretical calculations and actual experiments. It is found that the best critical thickness of the ultra-thin CIGS absorber is between0.8-1.0μm.CIGS films are prepared respectively by the metal elements sequential evaporation on the constant substrate temperature and three-stage evaporation process, it is comparatively studied the influence of the substrate temperature on the ultra-thin CIGS solar cells performance. It is found that the short-circuit current and conversion efficiency of the solar cell have been improved when the substrate temperature is350℃using metal elements sequential evaporation process. This may could be a reference for the development of high efficiency ultra-thin solar cell prepared on a low substrate temperature. However, it is not found this phenomenon in the three-stage process.The difference of influence of Ga content on the CIGS material and solar cell performance with ultra-thin and common thickness absorber is studied. When Ga/(Ga+In) changes between the0.20-0.36, little change of the crystal quality of the1μm CIGS film is found, while the grain size of2μm film decreases significantly with increasing Ga content. As Ga content increases, the intensities of (112) peaks of1μm films decrease, while that of2μm films increase, and the preferred orientation of both films decrease. In the case of the same Ga content, the value of the minimum band gap of1μm film is larger than that of2μm film. With increasing Ga content, the minimum band gap value of1μfilm grow faster than2μm film, which denotes that the band gap value of lμm film is more sensitive to the Ga content. As the Ga content increases, the resistivity of1μm film decreases, while2μm film gradually increases. By comparing the characteristics of solar cells with the different thickness absorber, it is found that the performance of ultra-thin solar cell is more strongly influenced by Ga content. The enhancement of Ga content results in the rapidly increase of the open circuit voltage of solar cells with1μabsorber, thereby the efficiency is greatly improved. When Ga/(Ga+In) is about0.37, the highest conversion efficiency of solar cells with1μm absorber is achieved. Ga back gradient are prepared using different Ga-In interval time in the first stage. As the interval time increasing, Ga back gradient is improved. However, the performance of ultra-thin solar cells is improved weakly due to the deterioration of CIGS films crystal quality. In the third stage, the deposition process of In/Ga is used, Ga distribution with V-type of CIGS films can be improved, then the open circuit voltage and short circuit current density of the ultra-thin solar cells increase, and the efficiency is improved, the efficiency of solar cells with1μm absorber has reached13.2%.Based on the optimization of ultra-thin CIGS materials preparation process, some other processes are tried to improve the performance of ultra-thin solar cells:①The process of deposited a CuInSe2(CIS) or CuGaSe2(CGS) buffer layer in CIGS/Mo interface in ultra-thin solar cells is first proposed. It is found that this process can improve the film crystallization, and the film surface is more compact and uniform. The shunt conductance of ultra-thin solar cells decreases using this process, thereby the open circuit voltage and efficiency are improved.②The characteristics and formation process of triangle island and cavities on the evaporated CIGS thin film surface is studied for the first time. The formation of triangle islands is strongly impacted by Cu content in the CIGS films. The triangle cavities are formed due to the insufficient coalescence of triangle islands. By compareing the performance of ultra-thin solar cells with and without triangle islands, it is found that efficiency of ultra-thin solar cells without triangle islands and cavities is improved.③Before CIGS films is deposited, NaF is evaporated. It is found that Na diffusion from soda line glass (SLG) substrate is insufficient to improve the performance of ultra-thin CIGS solar cells. Due to the passivation of defects by Na and the NaF residual at CIGS/Mo interface, shunt conductance of ultra-thin solar cells decreases and the photo-generated carrier collection in long wavelength increases, therefor, the performance of ultra-thin solar cells is improved. The depth profiles of elements are measured using a secondary ion mass spectroscopy (SIMS). It is found that the distributions of Na show a double grading as the depth profiles of Ga. It is believed that the formation mechanism of Na double grading is related to the growth process of CIGS films.Finally, the influence of scribing process, width of the cells, Ga and Cu content of CIGS basorber, thickness of CdS buffer layer, i-ZnO and ZnO:Al window layer on the performance of moudles with common thickness and ultra-thin absorber is studied based on the previous research of the CIGS material and small area soalr cells, which could provide a reliable basis data for the development of a large area CIGS modules.