Effect of substrate temperature and depostion profile on evaporated Cu(InGa)Se2 films and devices

Abstract

This paper addresses the effect of substrate temperature (TSS) and deposition profile for Cu(InGa)Se2 films deposited by multisource elemental evaporation on film structure and solar cell performance. Different temporal Cu flux profiles are utilized to give either a graded deposition incorporating a Cu-rich growth step, with [Cu]>[In]+[Ga], before achieving the final film composition with [Cu]<[In]+[Ga], or a uniform deposition with [Cu]<[In]+[Ga] throughout. The Cu(InGa)Se2 morphology, including quantitative analysis of the grain size distributions, and the performance of completed solar cells are compared at TSS=400 and 550°C. The higher TSS gives larger grains and better device performance with the best devices obtained in this work having efficiencies of 16.4% for 550°C and 14.1% for 400°C. At 550°C, Cu-rich film growth gives bigger grains than a uniform flux process, but there is no difference in the device performance. With TSS=400°C, there is no significant difference in the grain size with the different flux profiles, but the Cu-rich growth is needed for improved devices.