| Copper indium gallium selenide (Cu (In, Ga) Se2, CIGS) thin film solar cell with advantages of high conversion efficiency, good stability and anti-radiation ability, is one of the most promising thin-film solar cells. Owing to the advantages of high power to weight ratio, suitable to roll-to-roll deposition process, monolithic integration and flexible foldable, commercial development prospects of CIGS thin film solar cell deposited on flexible polyimide substrate get extensive attentions. At present, the conversion efficiency of CIGS thin film solar cell on PI foil has been improved to20.4%by Swiss Federal Laboratories for Material Science and Technology (EMPA), which is also the conversion efficiency world record of CIGS thin film solar cells.In this paper, the influence of composition control on CIGS thin film properties and device performance are investigated, including deposition rate and gradient distribution. For large area CIGS thin film solar cells on PI substrates, studies of Mo back contact on PI foils, metal grid, film uniformity and other aspects are also carried out.Since the maximum temperature for PI is typically below450℃, CIGS deposition has to use the so called low-temperature three-stage co-evaporation process. Because of the restriction of maximum substrate temperatures, CIGS thin films have the disadvantages of poor crystallization quality, rough surface and steep composition gradient, which result in poor device performance. Deposition rate and element gradient distribution are key issues of high quality CIGS thin films and corresponding devices.In the second stage of three-stage process, CIGS films experience a series of phase transitions, deposition rate in this stage has a significant impact on the growth mechanism of CIGS thin films. During the second stage the crystallinity and grain compactness of CIGS thin films are promoted with increasing deposition rates, double-peak reflection pattern is reduced obviously, which can reduce the recombination in the grain boundary and help to improve the conversion efficiency of the CIGS solar cell significantly. However, according to the experimental results, higher growth rates during the second stage lead to rougher surface and lower carrier concentration. Higher surface roughness can be attributed to the larger grain size of secondary-phase Cu2-xSe, and lower carrier concentration results from the reduction of passivation donor defects effects which induced by the hindrance of Na diffusion from the glass substrate. Application of high growth rates in the second stage is found to increase the interface recombination and induce shunt paths in the solar cells, and then the open circuit voltage and the cell parameters are deteriorated. Conversion efficiencies of CIGS thin film solar cells with low-temperature high-speed deposition process are found to deteriorate by about6%-10%, but the deposition time of second stage is shortened by86.3%, which can reduce preparation cycle and increase output efficiency, this is of great significance for industrialization of CIGS thin film solar cells.By changing Ga/In evaporation ratios and controlling the composition gradient of CIGS thin films prepared with low-temperature three-stage process, element diffusions and combinations are promoted effectively, two-phase separation phenomenon is eliminated, and film crystalline quality is improved significantly, which help to enhance the back electric-field driving effect for photo-induced electrons, and the carriers collection efficiency. At the same time, studies show that gradual distribution of band-gap gradient in CIGS thin film can decrease transport barrier for photo-induced electrons, reduce probability of recombination, and then improve open circuit voltage and fill factor significantly. Finally, a high conversion efficiency of10.99%has been achieved for CIGS thin-film solar cell on PI substrate.In addition, the influence of sputtering pressure and thickness on Mo contract, relationship between metal grid structure and light collecting area as well as electrical losses for large area single cell are also studied. By optimizing the preparation process of CIGS thin film,4cm×4cm and2cm x2cm CIGS thin-film solar cells on PI substrates are prepared, the highest cell efficiency have been reached5.30%and8.08%, respectively. |
