Fabrication of Cu(In,Ga)Se2 thin films for high efficiency solar cells

Abstract

A deposition system for fabrication of high efficiency Cu(In,Ga)Se2(CIGS) thin films was designed and constructed. The structure of the CIGS solar cells consists of five different layers of materials, (Ni)Al/ZnO(Al)/CdS/CIGS/Mo/SLG, were the SLG is the soda-lime-glass substrate. The CIGS absorber layers of approximately 2 µm thick were co-evaporated from four elemental sources onto the Mo/SLG substrates with constant substrate temperatures. The temperature effect on CIGS films was also studied using different substrate temperatures of 475, 500, 525 and 550℃. The in situ monitoring technique was employed for process control and end point detection (EPD). Two temperature profiles were performed in the CIGS deposition process. First, Cu-Rich-Off (CURO) process was started with the Cu-rich stage such that the atomic ratio [Cu]/[In]+[Ga]) was greater than 1 (y > 1), then followed by the Cu-poor stage until y < 1 was reached, and the process was finished at y ≈ 0.9. Film analysis results using XRD and SEM showed that these films were typically (112) oriented chalcopyrite with large columnar grains and rough surfaces with deep crevices. From the current-voltage (I-V) and the quantum efficiency (QE) measurements, the CIGS thin film solar cells fabricated with this process yielded efficiencies up to 14%. In the second profile, Cu-Poor-Rich-Off (CUPRO) process started with the Cu-poor stage, y < 1, followed by the Cu-rich stage until y > 1, the finished with the Cu-poor stage at the same value (y ≈ 0.9). The results showed that these films were weakly (220)(204) oriented chalcopyrite with columnar grains and smooth surfaces with shallow crevices. These CIGS thin films solar cells showed their efficiencies at the level of 15%. The proposed growth models for these two processes implicated that the excess CuxSe in the Cu-rich films segregated and presented at the surfaces of the films and between the grain boundaries. The crevices resulted from the conversion of the CuxSe to CIGS. Furthermore, recrystallization was found to occur in the CUPRO growth process.