Quantum Dot Bulk Heterojunction Solar Cell Using Mesoporous Lead Sulfide Nanocrystal Thin Films
Authors:Justin Ondry, Shauna Robbennolt
Mentor:Sarah Tolbert, Professor of Chemistry , UCLA
Lead sulfide (PbS) quantum dots are attractive for photovoltaic applications due to their colloidal synthesis, efficient multiple exciton generation, and relative air stability. Coupling the PbS nanocrystals with titania as the electron acceptor in a heterojunction solar cell has led to devices that have achieved power conversion efficiencies in excess of 7%. However, very few studies have explored how mesoscale architecture affects device performance in these devices. Here we first attempted to use block copolymer templating of ligand stripped PbS nanocrystals to form mesoporous thin films of PbS nanocrystals. We have recently determined that the surface chemistry of PbS nanocrystals is not amiable to forming stable colloidal dispersions, which are necessary for block copolymer templating. Therefore we have now switched our approach to making mesoporous PbS thin films, and are using an indirect method that utilizes cation exchange. Cadmium sulfide is a very well studied nanocrystal system known to make stable ligand stripped colloidal dispersions. Recently developed cation exchange on mesoporous cadmium chalcogenide films show retention of mesoporous architecture. We are currently working to use cation exchange to turn mesoporous cadmium sulfide nanocrystal thin films into lead sulfide thin films via a copper sulfide intermediate. Once we synthesize mesoporous thin films of PbS nanocrystals via cation exchange, atomic layer deposition of TiO2 will be used to form a bulk heterojunction. The open porosity of mesoporous thin films should allow the TiO2 layer to be grown throughout the film. This should increase the surface contact between the PbS and TiO¬2 and thus increase efficiencies by limiting the distance an exciton has to migrate before being separated.