Development of multinary chalcogenide nanocrystal inks for low cost solar cells
Abstract
Copper indium gallium diselenide (CIGSe) and related materials are some of the most promising candidates for thin film photovoltaic applications due to their unique structural and electrical properties. However, commercial scale adaptation of CIGSe based solar cells has yet been realized due to high costs associated with the fabrication processes. The manufacturing processes need to be able to adapt to a high throughput roll to roll process in order to reduce cost. The creation of a suitable inorganic colloidal nanocrystal ink for use in a scalable coating process is a key step in the development of low-cost solar cells. Here a facile solution synthesis of copper indium diselenide (CISe) and related nanocrystal inks has been developed. These CISe nanocrystals can be easily assembled into thin films and converted into active CISe absorber film under mild conditions. Proof of concept is demonstrated by the fabrication of functional solar cell devices using films based on CISe nanocrystal inks. Solar cells efficiencies based on the selenide nanocrystals were relatively low mainly because of poor film formation and the presence of voids within the film. To overcome this challenge, an innovative approach utilizing copper indium gallium disulfide nanocrystals (CIGS) as the precursors for the formation of CIGSSe via selenization is developed. The replacement of Se of S in the CIGS matrix creates ∼14.6% volume expansion and reproducibly leads to a dense and device quality CIGSSe thin film. Also, by using the CIGS nanocrystals, composition uniformity at all scales can be maintained. Furthermore, the ability to synthesize CIGS nanocrystals with various In/Ga ratios allows the unique capability to bandgap engineer the CIGSSe absorber. Using a combination of CIGS nanocrystal inks with different In/Ga ratios, graded bandgap CIGSSe films can be fabricated easily for improvements in the resulting solar cell efficiency. By optimizing processing conditions of the various layers in our devices, solar cell efficiency greater than 10% under AM1.5 illumination has been achieved. The processing methods developed for CIGSSe solar cells are very versatile and can be adapted for the fabrication of nanocrystals and solar cells based on copper zinc tin sulfide—a semiconductor made of naturally abundant materials.
Degree
Ph.D.
Advisors
Hillhouse, Purdue University.
Subject Area
Nanoscience|Energy
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