Origin of Charge Separation at Organic Photovoltaic Heterojunctions: A Mesoscale Quantum Mechanical View

TL;DR

This study uses mesoscale quantum-chemical modeling to investigate charge separation mechanisms in organic photovoltaic heterojunctions, revealing that quantum effects significantly reduce the binding energy of electron-hole pairs compared to classical predictions.

Contribution

It introduces a mesoscale quantum-chemical model to unbiasedly assess factors influencing charge dissociation in organic photovoltaics, highlighting the importance of quantum effects.

Findings

Classical models overestimate electron-hole binding energy.

Quantum mechanical energy differences are comparable to thermal energy.

Quantum effects facilitate charge separation at interfaces.

Abstract

The high efficiency of charge generation within organic photovoltaic blends apparently contrasts with the strong "classical" attraction between newly formed electron-hole pairs. Several factors have been identified as possible facilitators of charge dissociation, such as quantum mechanical coherence and delocalization, structural and energetic disorder, built-in electric fields, nanoscale intermixing of the donor and acceptor components of the blends. Our mesoscale quantum-chemical model allows an unbiased assessment of their relative importance, through excited-state calculations on systems containing thousands of donor and acceptor sites. The results on several model heterojunctions confirm that the classical model severely overestimates the binding energy of the electron-hole pairs, produced by vertical excitation from the electronic ground state. Using physically sensible parameters for the individual materials, we find that the quantum mechanical energy difference between the lowest interfacial charge transfer states and the fully separated electron and hole is of the order of the thermal energy.