Improving Open Circuit Potential in Hybrid P3HT:CdSe Bulk Heterojunction Solar Cells <i>via</i> Colloidal <i>tert</i>-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.