The effects of annealing temperature on CIGS solar cells by sputtering from quaternary target with Se-free post annealing

For the two-step method of Cu(In,Ga)Se2 (CIGS) fabrication process in which the selenization step follows the CIGS precursors deposition step, H2Se gas is employed as the selenium source, which may lead to an inhomogeneous gas field and potentially hazardous working condition. In this paper, a method based on sputtering a Se-rich target to deposit CIGS precursor with subsequent annealing in selenium-free atmosphere to obtain CIGS absorber has been proposed. The influence of the annealing temperature on CIGS films and devices has been investigated. It is found that the grain growth of CIGS films is enhanced by the increase of the annealing temperature ranged from 450 °C to 525 °C, which results in the increase of the short circuit current and efficiency of the solar cell. The devices with absorber layers annealed at 525 °C exhibited the best efficiency of 11.8%. However, when the temperature increases further, amount of selenium vacancies appear which might result in the enhancement of recombination rate of free carriers and reduction of the conversion efficiency. Finally, Ohmic contact between Mo and CIGS is found to form for the devices by this method.