Abstract
The experimentally observed e ffect of selective deuterium substitution on the open
circuit voltage for a blend of poly(3-hexylthiophene)(P3HT) and [6,6]-phenyl-C61-
butyric acid methyl ester (PCBM) (Nat. Commun. 5:3180, 2014) is explored using a
221-atom model of a polymer-wrapped PCBM molecule. The protonic and deuteronic
wavefunctions for the H/D isotopologues of the hexyl side chains are described within a
Quantum Trajectory/Electronic Structure approach where the dynamics is performed
with newly developed nonlinear corrections to the quantum forces, necessary to describe the nuclear wavefunctions; the classical forces are generated with a Density Functional Tight Binding method. The resulting protonic and deuteronic time-dependent wavefunctions are used to assess the e ffects of isotopic substitution (deuteration) on the energy gaps relevant to the charge transfer for the donor and acceptor electronic states. While the isotope eff ect on the electronic energy levels is found negligible, the
quantum-induced fluctuations of the energy gap between the charge transfer and charge
separated states due to nuclear wavefunctions may account for experimental trends by
promoting charge transfer in P3HT/PCBM and increasing charge recombination on
the donor in the deuterium substituted P3HT/PCBM.
