Multistep Selenization of Copper Indium Gallium Selenide (CIGS) Nanocrystal Photovoltaics
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abstract
Copper Indium Gallium Selenide (CIGS) nanocrystal inks have been used to fabricate photovoltaic devices without post-deposition thermal treatment, but have been limited to relatively low efficiency (~3%) by poor electron transport. The insulating organic nanocrystal capping ligands are largely responsible for this. Annealing the nanocrystal films at high temperature under selenium atmospherei.e., selenizationcan be used to improve device performance by removing ligands and increasing grain size in the film. The selenization process, however, is difficult to control due to non-uniform film sintering, the formation of insulating layers from residual carbon in the film, and film delamination with complete conversion of the Mo back contact to MoSe2. We have found that a one hour thermal anneal under argon at 525 C before selenization of the nanocrystal film can significantly improve device efficiency. This pre-selenization anneal removes organic ligands, which appears to be important for uniform nanocrystal sintering. Lowering the anneal temperature from 525 C to 425 C drastically lowers device efficiency although the capping ligands are still removed. Sodium has been shown to improve CIGS grain growth in co-evaporated films and a similar connection is found in selenized nanocrystal film performance. X-ray photoelectron spectroscopy (XPS) measurements taken after the pre-selenization anneal show increased Na presence at the Mo surface when the annealing temperature is increased due to greater diffusion from the soda-lime glass substrate. Additionally, this pre-selenization anneal accelerates grain growth in the film, shortening the necessary selenization time. Shortened selenization length reduces MoSe2 formation at the back contact, enabling the use of repeated selenization treatments without film delamination. Devices with over 7% efficiency have been fabricated by depositing and selenizing successive CIGS nanocrystal layers.
