Grain growth studies of thin Cu(In, Ga)Se 2 films

Abstract

In order to analyze grain growth on the basis of a kinetic growth model, this study reports on annealing experiments with thin Cu(In, Ga)Se 2 films. The substrate heater in an ultra-high vacuum chamber serves to anneal Cu(In, Ga)Se 2 films of 1 μm thickness for varying times t and temperatures T. An ultra-high resolution scanning electron microscope visualizes the cross-section of the film in order to determine the average grain radius r. The theory of normal grain growth explains the increase of the average grain radius r in dependence on annealing time t and temperature T. For an annealing time t=3 h at T=610°C the average grain size increases by a factor of 2.4. Grains with a (1 1 2)-surface orientation are slightly energetically preferred and grow at the expense of other orientations. Grain growth is thermally activated. The Cu-content of the film determines the activation energy Q for grain boundary motion. Increasing the Cu-content of the film from 17.9% to 25.7% lowers Q from 3.5 to 3.0 eV.For high-efficiency Cu(In, Ga)Se 2 solar cell absorbers with a Cu-content below 22% the temperature T has to exceed at least 550°C in order to obtain sufficient atomic diffusion and therefore significant grain growth.