Mechanistic Study of Charge Separation in a Nonfullerene Organic Donor–Acceptor Blend Using Multispectral Multidimensional Spectroscopy

Abstract

Organic photovoltaics (OPVs) based on nonfullerene acceptors are now approaching commercially viable efficiencies. One key to their success is efficient charge separation with low potential loss at the donor–acceptor heterojunction. Due to the lack of spectroscopic probes, open questions remain about the mechanisms of charge separation. Here, we study charge separation of a model system composed of the donor, poly­[(2,6-(4,8-bis­(5-(2-ethylhexyl)­thiophen-2-yl)-benzo­[1,2-b:4,5-b′]­dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis­(2-ethylhexyl)­benzo­[1′,2′-c:4′,5′-c′]­dithiophene-4,8-dione) (PBDB-T), and the nonfullerene acceptor, 3,9-bis­(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis­(4-hexylphenyl)-dithieno­[2,3-d:2′,3′-d′]-s-indaceno­[1,2-b:5,6-b′]­dithiophene (ITIC), using multidimensional spectroscopy spanning the visible to the mid-infrared. We find that bound polaron pairs (BPPs) generated within ITIC domains play a dominant role in efficient hole transfer, transitioning to delocalized polarons within 100 fs. The weak electron–hole binding within the BPPs and the resulting polaron delocalization are key factors for efficient charge separation at nearly zero driving force. Our work provides useful insight into how to further improve the power conversion efficiency in OPVs.

Coupling between Harmonic Vibrations Influences Quantum Beating Signatures in Two-Dimensional Electronic Spectra

“…The frequencies of these beating signatures match well with that in the existing literature for MB, 77 ITIC, 75,107 and NB, 108,109 and in each case are among the strongest lowfrequency vibrations coupled to their respective S 1 ← S 0 transitions. In each compound, the most intense quantum beating occurs within the region near the diagonal characteristic of a purely vibrational coherence, 53 accounting for systematic blueshifts from the S 1 ← S 0 transition energies 76,109,110 due to the overlap of the molecular absorption and pump spectra.…”

“…Here, we use a joint experimental-theoretical approach to test the ability of the single-DHO versus multiple-DHO formalisms to reproduce experimental 2DES results from an array of chromophores, namely terrylenediimide (TDI), ITIC, methylene blue (MB), and Nile blue A (NB). Both TDI and ITIC have applications in organic photovoltaics as singlet fission sensitizers − and nonfullerene electron acceptors, , respectively, while MB and NB are important dyes to the biomedical community. , We first focus on comparisons between simulated quantum beatmaps for TDI, using both single- and multiple-DHO Hamiltonians, and experimental 2DES results. We show unambiguously that accounting for multiple unique vibrations significantly increases the accuracy of the simulation in capturing patterns within the experimental beatmaps.…”

Spontaneous exciton dissociation enables spin state interconversion in delayed fluorescence organic semiconductors

“…For example, the generation of loosely bound charge transfer states, and even free charge carriers, in neat BF2 films without the inbuilt driving energy provided by offset molecular orbitals in D:A blends will facilitate the development of efficient OSCs that do not require separate electron D and A materials. In addition, our study of BF2 provides an insight into the mechanism of the ultra-low driving energy charge generation seen in nonfullerene acceptor (NFA) OSCs, where it has been proposed that hole transfer process from the NFA to the polymer occurs via an inter-CT type state similar to those we report in BF2 47,60,61 .…”

Direct Observation of Increased Free Carrier Generation Owing to Reduced Exciton Binding Energies in Polymerized Small-Molecule Acceptors

“…Upon 750 nm excitation, Y6 reaches its first locally excited state S1. As shown in Figure a, a broad band absorption appears, which originates from free carrier formation. ,− The ground-state bleach (GSB) of a series of vibrational modes attributed to conjugated skeleton vibrations and CO stretching mode overlay on the broad band absorption of free carriers, matching the steady-state FTIR spectrum (Figure c) with slight frequency shifts induced by the electronic excitation. By investigating the vibrational behavior of the excited state, we can understand the distribution of free carriers on the excited state of the Y6 molecule, because some vibrational modes have stronger coupling to the electron transition dipole moment than others, resulting in deeper GSBs in the spectra.…”