Conformation-Mediated Doping in P3HT:F4TCNQ Dimers from Density Functional Theory

Molecular doping of charge carriers in organic semiconductors is a complex process influenced by both single-molecule energetics and multiscale morphology. Here, we employ atomistic molecular dynamics simulations and density functional theory (DFT) calculations of disordered P3HT:F4TCNQ morphologies to understand how conformational disorder influences doping efficiency. We find that conformational variations in P3HT:F4TCNQ dimers can modulate the amount of ground-state charge transfer between P3HT and F4TCNQ by more than 0.5 C. The amount of charge transfer in P3HT:F4TCNQ dimers exhibits a linear correlation with the difference between the ionization potential (IP) of P3HT and the electron affinity (EA) of F4TCNQ, when using geometries extracted from DFT-optimized dimer aggregates. We find that most variation in IP - |EA| that governs the amount of ground-state charge transfer is due to conformational variations of P3HT's IP (∼0.35 eV) compared with F4TCNQ's EA (∼0.15 eV). Finally, we show that the free energy of charge generation (ΔG_rxn) of the dimer, when treated as the sum of IP - |EA| and electrostatic energies derived from atomic partial charges, correlates linearly with the amount of charge transfer in the dimer. These findings shed light on the energetics of the molecular doping process, justifying simple approximations employed in recent reactive Monte Carlo methods for molecular doping and providing an avenue for systematic DFT parametrization of multiscale doping methods.