Effects of Dispersion Forces on Structure and Photoinduced Charge Separation in Organic Photovoltaics

Abstract

We present a theoretical study on the role of van der Waals (vdW) interactions on the structure and, as a consequence, the photoinduced charge separation (CS) of a series of dimer complexes formed by the polymer P3HT and the fullerene derivative PCBM. CS rate constants for P3HT/PCBM dimer structures in which vdW interactions are taken into account agree well with experimental data. Without proper treatment of vdW interactions during geometry optimizations, the predicted CS rates can be too low by up to 3 orders of magnitude. These variations in computed CS rates are not due to changes in the Gibbs energy for CS. Instead, the electronic coupling increases by up to 2 orders of magnitude for structures obtained with dispersion-corrected density functionals that lead to deformations in the P3HT oligomer with pronounced π–π stacking interactions with PCBM.

Van der Waals effects on structure and optical properties in organic photovoltaics

“…Figure A shows two stable dimer isomers I1 and I2 of P3HT/PCBM dimer that were reported in our previous papers . Panels B and C of Figure show the isolated P3HT oligomer at the U‐shaped structures it adopts in the corresponding dimers.…”

“…Thus, it can be argued that the perturbation of the P3HT chain is more drastic for oligomer P3HT‐I2. Notice that the deformation (bending and thiophene ring rotations) of the P3HT in the P3HT/PCBM dimer is due to vdW interactions . GGA approximations or hybrid functionals without the Grimme correction or other means to include dispersion forces predict either idealized structures (similar to dimers I3‐I5 shown in Figure ) or nonstable dimers where PCBM and P3HT dissociate …”

“…The efficiency of each of these processes can change orders of magnitude depending on the structure formed between P3HT and PCBM, and the intermolecular distance between donor and acceptor . As in other research fields, computational studies can be used to understand and characterize OPVs . Yet there is not a complete understanding of the structure that P3HT and PCBM form at the interface and the precise effects of this structure on the electronic and optical properties of manufactured P3HT/PCBM films.…”

“…Recently, we reported the effects of dispersion forces on the structure and charge separation (CS) in the P3HT/PCBM dimer . We demonstrated that it is essential to properly account for van der Waals (vdW) forces when determining stable dimer structures, which can affect CS rate constants k CS by up to three orders of magnitude.…”

“…We demonstrated that it is essential to properly account for van der Waals (vdW) forces when determining stable dimer structures, which can affect CS rate constants k CS by up to three orders of magnitude. A key feature of such stable dimers is that the P3HT oligomer wraps around the PCBM, thus adopting a U‐shape conformation that maximizes attractive dispersion forces (see more details below). Time‐dependent density functional theory (TDDFT) calculations on these stable dimer isomers showed a reduced intensity in the red region and a blue‐shift of the absorption maximum of the dimer spectrum with respect to the spectrum of the isolated, planar P3HT oligomer, which is in agreement with experiment .…”