title: Improving open circuit potential in hybrid P3HT:CdSe bulk heterojunction solar cells via colloidal tert-butylthiol ligand exchange.
abstract: Organic ligands have the potential to contribute to the reduction potential, or lowest unoccupied molecular orbital (LUMO) energy, of semiconductor nanocrystals. Rationally introducing small, strongly binding, electron-donating ligands should enable improvement in the open circuit potential of hybrid organic/inorganic solar cells by raising the LUMO energy level of the nanocrystal acceptor phase and thereby increasing the energy offset from the polymer highest occupied molecular orbital (HOMO). Hybrid organic/inorganic solar cells fabricated from blends of tert-butylthiol-treated CdSe nanocrystals and poly(3-hexylthiophene) (P3HT) achieved power conversion efficiencies of 1.9%. Compared to devices made from pyridine-treated and nonligand exchanged CdSe, the thiol-treated CdSe nanocrystals are found to consistently exhibit the highest open circuit potentials with V(OC) = 0.80 V. Electrochemical determination of LUMO levels using cyclic voltammetry and spectroelectrochemistry suggest that the thiol-treated CdSe nanocrystals possess the highest lying LUMO of the three, which translates to the highest open circuit potential. Steady-state and time-resolved photoluminescence quenching experiments on P3HT:CdSe films provide insight into how the thiol-treated CdSe nanocrystals also achieve greater current densities in devices relative to pyridine-treated nanocrystals, which are thought to contain a higher density of surface traps.
authors.affiliation: Department of Chemistry and the Center for Energy Nanoscience, University of Southern California, Los Angeles, California 90089-0744, USA.
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Greaney
authors.forename: Saptaparna
authors.initials: S
authors.lastname: Das
authors.forename: David H
authors.initials: DH
authors.lastname: Webber
authors.forename: Stephen E
authors.initials: SE
authors.lastname: Bradforth
authors.forename: Richard L
authors.initials: RL
authors.lastname: Brutchey