Optimizing Absorber Crystallinity And Surface Energy Band For High Efficiency Solution Processed CIGS Solar Cells

Cu(In,Ga)Se2(CIGS)has attracted much attention in the photovoltaic field due to its advantages such as adjustable band gap,high absorption coefficient and good stability.Currently,the efficiency of CIGS solar cell produced by vacuum has arrived 23.6%.However,these processes are high initial capital cost which is the primary roadblock to the commercialization of this technology.In contrast,CIGS thin film prepared by solution-processing routes can reduce the costs of equipment and processing.Meanwhile,it has many advantages such as high material utilization,high throughputs,and roll to roll.However,the efficiency of CIGS cells prepared by solution-processed is still far from that of devices prepared by vacuum method.Poor crystallizability,high concentration of bulk defects and unsatisfactory surface band structure are considered to be one of the main reasons for the low performance of CIGS devices prepared by solution process.Learning from the successful experience of vacuum method is an effective way to improve the performance of CIGS devices by solution-processed.The Cu-rich phase during the preparation of CIGS thin film by vacuum method can promote crystallization and reduce the defects of the absorber.However,the solution-processed is a one-step selenization,which lack of the Cu-rich phase process.Next,alkali metals(K,Rb and Cs)can passivate the defects of the CIGS absorber and regulate the surface band,which play an important role in the improvement of the performance of CIGS cells prepared by vacuum method.However,in the solution-processed,the work of alkali metal treatment is still in its initial stage,and the effective alkali metal treatment method and treatment principle still need to be explored.In view of the problems existing in the preparation of CIGS by solution-processed,this paper focus on improving the quality of CIGS absorpber and optimizing its surface energy band structure.The detailed work is as follows:(1)In this work,we demonstrated a simple and effective strategy to prepare high-performance solution-processed CIGS solar cells by incorporating a Cu-rich layer in the bottom of CIGS precursor films.The crystallization of the absorber is improved and the defect density is reduced by the introduction of Cu-rich layer.The growth process of CIGS grains was clearly revealed by comparing the SEM images of samples with different selenization stages.It is found that the Cu-rich bottom layer can distinctly boost grain growth and reduce the fine-grain layer in the CIGS film.Furthermore,the introduction of Cu-rich bottom layer widens the depletion region,reduces defect density,and increases carrier collecting efficiency,all of which serve to diminish carrier recombination.These beneficial impacts simultaneously led to a substantial increase in VOC,JSC,and FF,which significantly boost the efficiency from 12.83%to 16.05%.(2)In this work,we further optimized the process of selenization based on the work of the improved crystallinity.The doping of alkali metal K is achieved by co-selenization of Se&K2S by placing K2S and Se into the graphite box at the same time.The results suggest the introduction of K leads to the formation of K-In-Se phase on the surface of the absorber and the valence band on the surface of the absorber is bent downward,which forms a hole barrier and effectively inhibits carrier recombination at the heterojunction interface.Deep level transient spectra results show that the density of defects is significantly reduced,and the best device efficiency has achieved 16.61%.