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

Maria Saladina,Safakath Karuthedath,Frédéric Laquai,Philippe Blanchard,Anastasia Markina,Clemens Göhler,Clément Cabanetos,Yue Chen,Pablo Simón Marqués,Magali Allain,Christopher Wöpke,Denis Andrienko,Carsten Deibel,Julien Gorenflot

doi:10.1002/adfm.202007479

Abstract

AbstractIn organic solar cells, photogenerated singlet excitons form charge transfer (CT) complexes, which subsequently split into free charge carriers. Here, the contributions of excess energy and molecular quadrupole moments to the charge separation process are considered. The charge photogeneration in two separate bulk heterojunction systems consisting of the polymer donor PTB7‐Th and two non‐fullerene acceptors, ITIC and h‐ITIC, is investigated. CT state dissociation in these donor–acceptor systems is monitored by charge density decay dynamics obtained from transient absorption experiments. The electric field dependence of charge carrier generation is studied 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. The average distance between electron–hole pairs at the donor–acceptor interface is determined from empirical fits to the TDCF data. The delocalization 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.

Highlights

We have investigated the influence of photon energy and molecular quadrupole moments on the yield of complete. Knowledge (CT) dissociation for two non-fullerene acceptor systems, PTB7Th:ITIC and PTB7-Th:h-ITIC
In combination with the larger effective van der Waals volume of the h-ITIC dimer, this lower molecular quadrupole moment leads to a weaker electrostatic potential at the donor–acceptor interface
The observed yield of free charge carrier generation in the PTB7-Th:h-ITIC blend is inferior to its counterpart with a large molecular quadrupole moment PTB7-Th:ITIC, as consistently shown by Transient absorption spectroscopy (TAS), time delayed collection field (TDCF), and sensitive EQE measurements

Summary

Introduction

The of the factors that influence photogeneracontributions of excess energy and molecular quadrupole moments to the tion in these systems is essential to move charge separation process are considered. Upon excitation below the optical gap, free charge carrier generation becomes less field dependent with increasing photon energy, which challenges the view of charge photogenera-. It undergoes thermalization in the CT manifold and results in the formation of a bound electron–hole pair with initial separation distance r0. The delocalization 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 ground state or dissociate into free charge carriers.[6,7]

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