(454d) Selenization of Automated, Ultra-Sonic Spray-Deposited Cu(In,Ga)Se2 Nanocrystal Films for Photovoltaics

Direct conversion of sunlight to electricity using photovoltaics (PVs) has the potential to be a source of energy that is both cost effective and clean.  Lower PV manufacturing cost is needed to realize this promise. One possible route to lower cost is to eliminate vacuum processing during the absorber layer deposition using copper indium gallium selenide (CIGS) nanocrystal inks. Efficiencies of 7.1% and 12% have been reached by annealing films of CuInGaSe₂ and GuInGaS₂ nanocrystals in a selenium atmosphere, also known as selenization. While promising, these results were achieved with small area devices using deposition techniques that are potentially scalable, but will require significant modification.

As the next step in developing this technology, we have used a highly scalable, automated, ultra-sonic spray coating system. Utilizing this technology leads to finer control in the fabrication process; however, significant variations were observed in film morphology. These variations were traced to small changes in the organic content of the nanocrystal ink. The organic content of the film was found to be highly dependent on the amount of anti-solvent used during the nanocrystal washing procedure. An anti-solvent/solvent centrifugation wash procedure is used in almost every nanocrystal synthesis, but the importance of this procedure on device performance has been overlooked. We trace small changes in the washing of the nanocrystals to morphology changes that then cause dramatic changes in device performance.

By implementing tighter controls on the entire nanocrystal PV device fabrication, we have demonstrated reproducible 5% efficiencies using a scalable deposition technique. We also have explored the effect of air pressure, nanocrystal concentration, and liquid flow rate on the ultrasonic spray deposition of nanocrystal solar inks.