Liquid assisted grain growth in solution processed Cu(In,Ga)(S,Se)2

Many solution based methods of fabricating thin film Cu(In,Ga)Se-2 solar cell absorbers can be described as a two step process, consisting of initial precursor deposition followed by high temperature post processing. This post processing step is termed selenization when the sample is treated in a selenium-rich environment, and is responsible for transforming the nanocrystalline or amorphous precursor layer into the large crystal grains that are necessary for high efficiency solar cells. Here we study the selenization of nanoparticle Cu(In,Ga)S-2 precursor layers by a rapid thermal processing method, and develop a liquid assisted growth model to explain how large grain Cu(In,Ga)(S,Se)(2) crystals nucleate and grow. We find that processing with saturated selenium, achieved by maintaining a higher temperature for the selenium source than the sample, is a necessary but not sufficient condition for grain growth of a 1-2 mu m thick Cu(In,Ga)(S,Se)(2) film. An additional necessary condition is an external supply of Na, without which grain growth of films on soda-lime glass stagnates at about 400 nm thickness, with the remainder of the film consisting of a bottom layer of fine grains. This stagnation of grain growth is hypothesized to be caused by diffusion limitations of Cu, In and Ga containing species from the coarsened selenized particles in the fine grain layer as well as accumulation of carbon left behind by the solution processing method which is rejected during grain growth. We find that deposition of a NaF layer on the nanoparticle precursor film followed by selenization at temperatures greater than 534 degrees C under a saturated selenium atmosphere results in large surface grains that span the thickness of the film, with only a thin bottom small grain layer primarily composed of carbon and selenium. Grain growth prior to stagnation is well described by a normal grain growth model, and our kinetic study yielded an estimated rate constant activation energy of 126.1 kJ/mol. Attempting to grow grains beyond the stagnation point results in abnormal grain growth. We develop a liquid assisted grain growth model to explain these observations.