Charge Photogeneration in Non-Fullerene Organic Solar Cells: Influence of Excess Energy and Electrostatic Interactions

TL;DR

This study investigates how excess energy and electrostatic interactions influence charge separation in non-fullerene organic solar cells, revealing that higher photon energy reduces field dependence and that electrostatic potential affects charge dissociation.

Contribution

It demonstrates the role of molecular quadrupole moments and excess energy in charge photogeneration, providing new insights into the dissociation mechanisms in non-fullerene organic solar cells.

Findings

Charge generation becomes less field-dependent at higher photon energies.

Larger CT state delocalization correlates with higher quadrupole moments.

Electrostatic potential significantly influences charge dissociation efficiency.

Abstract

In organic solar cells, photogenerated singlet excitons form charge transfer (CT) complexes, which subsequently split into free charge carriers. Here, we consider the contributions of excess energy and molecular quadrupole moments to the charge separation process. We investigate charge photogeneration in two separate bulk heterojunction systems consisting of the polymer donor PTB7-Th and two non-fullerene acceptors, ITIC and h-ITIC. CT state dissociation in these donor-acceptor systems is monitored by charge density decay dynamics obtained from transient absorption experiments. We study the electric field dependence of charge carrier generation at different excitation energies by time delayed collection field (TDCF) and sensitive steady-state photocurrent measurements. Upon excitation below the optical gap free charge carrier generation becomes less field dependent with increasing photon energy, which challenges the view of charge photogeneration proceeding through energetically lowest CT states. We determine the average distance between electron-hole pairs at the donor-acceptor interface from empirical fits to the TDCF data. The delocalisation of CT states is larger in PTB7-Th:ITIC, the system with larger molecular quadrupole moment, indicating the sizeable effect of the electrostatic potential at the donor-acceptor interface on the dissociation of CT complexes.