Sintering of Cu(In,Ga)Se2 nanocrystal films for photovoltaics
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Low cost solar cells are needed to increase availability and reliability of electricity throughout the world. Spray deposition of Cu(In,Ga)Se₂ (CIGS) nanocrystal inks is a promising route to low cost photovoltaics(PVs). CIGS nanocrystal inks have been used to fabricate solar cells with 3.1% power conversion efficiency (PCE) without any heat treatment, but are limited by poor charge transport. Sintering the nanocrystals into a polycrystalline film improves charge transport and device performance. Two sintering methods are investigated here: selenization and photonic curing. The nanocrystal films can be sintered by annealing the films in a Se environment, also known as selenization. The selenized device morphology and efficiency is influenced by the starting nanocrystal composition. A Se/Carbon layer deriving from the organic ligand decreases device efficiency, but is eliminated by annealing the nanocrystal films in Ar before the selenization treatment. In addition to eliminating the Se/Carbon layer, the pre-selenization anneal drives Na from the soda-lime glass substrate into the film and improves grain growth. Devices with efficiencies above 7.0% are fabricated using a multi-step deposition and sintering process. To simplify device fabrication, a single-step, scalable deposition is demonstrated using fully automated, ultrasonic spray coating. The ultrasonic spray deposition is highly sensitive to the nanocrystal ink organic content, and optimization of the nanocrystal synthesis wash procedure leads to highly uniform, reflective nanocrystal films. Devices fabricated from these films achieve 6.6% efficiency after selenization. The use of rapid pulses of broadband light, or photonic curing, is an alternative sintering method that is compatible with roll-to-roll fabrication and does not use high temperature processing. A wide range of pulse energy is used to treat nanocrystal films after spray deposition on three different back contact materials. Nanocrystal dewetting and agglomeration was observed after photonic curing of the films on a Mo contact, but is reduced significantly by using Au or MoSe₂/Mo back contacts. Low energy pulses remove the organic capping ligand and bring the nanocrystals into close electrical contact, leading to devices exhibiting multiple exciton generation and extraction. Higher energy pulses sinter the nanocrystal layer and sintered nanocrystal photovoltaic devices are demonstrated.