Effect of selenium effusion rate on CIGS thin films deposited at low substrate temperature

To study the issues of film growth at low substrate temperature, Cu(In,Ga)Se₂ (CIGS) films have been deposited at substrate temperatures of 350°C under variation of the selenium effusion rate. The properties of the resulting thin films were studied by X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Solar cells were fabricated from selected films and characterized by current-voltage and quantum efficiency measurements. While the medium-and high-Se flux samples showed only subtle differences in composition, structure, morphology and device performance, the low-Se samples exhibited very low Cu content, additional chalcopyrite phases, very small grain size, and poor solar cell performance. A minimum selenium/metal flux ratio of 3 is suggested for the low-temperature CIGS deposition process. Higher selenium rates yield modest improvements to solar cell performance at low substrate temperature. The highest cell efficiency, 7.8% at AM1.5 global light, was achieved with a selenium/metal flux ratio of 8.