An electroabsorption perspective on charge-transfer excitations in non-fullerene acceptors

Using electroabsorption spectroscopy to investigate intra- and intermolecular charge transfer excitations in Y6 and ITIC non-fullerene acceptors.
Published in Chemistry and Materials

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Organic photovoltaic cells (OPVs) using Y6 non-fullerene acceptors (NFA) have recently achieved higher efficiencies, closer to 20%, with negligible voltage loss. The formation of strong Frenkel excitons is detrimental to exciton dissociation efficiency in organic semiconductors due to their high binding energy and low dielectric constant. A viable approach to overcome this issue is to use a ‘bulk heterojunction’ (BHJ) device structure composed of electron donor and acceptor materials, which leads to the formation and dissociation of interfacial charge transfer (CT) states. Since the mid-1990s, fullerene and its derivatives have been the commonly used acceptors, which faced a major setback afterward due to large voltage loss and limited optical absorption. In recent years, the promising development of non-fullerene acceptors such as Y6 and ITIC has sparked research interest by overcoming the limitations of fullerene-based acceptors.

At present, BHJ solar cells with Y6 or its molecular derivatives as acceptors can facilitate ultrafast charge transfer and exciton dissociation despite the negligible energy offsets at the donor/acceptor interface. It is reported that Y6 molecules can form extensive crystalline packing or aggregates due to their molecular conformation, high molecular rigidity, and the absence of out-of-plane side chains. However, it is still unclear whether the charge transfer property of Y6 is facilitated by its molecular packing or is inherent in the single molecule. Despite the great interest in the intra- and inter-molecular excitations in Y6, a direct experimental investigation of their CT character is also missing.

Electroabsorption (EA) spectroscopy is one such technique that has been used to probe the excitonic properties of organic semiconductor materials. Based on the Stark effect, it measures the change in the optical absorbance under the influence of an electrical field, which depends on the changes in dipole moment (Δµ) and polarizability (Δp) of the transition states. It is expected that an excited state with strong CT character should have a large increase in exciton radius, and its EA spectral characteristics would be mainly contributed by Δµ. However, the physical significance of these excitonic parameters and their correlation to the assessment of CT character remain elusive.

In this work, we used electroabsorption spectroscopy and electronic structure calculations to study the CT characteristics of electronic excitations of isolated (intramolecular) and aggregated (intermolecular) Y6 and ITIC molecules. The primary challenge was to find a suitable insulating polymer for the solid-solvation method to disperse these NFA molecules in thin films. In the optical absorption spectrum, as the loading ratios of Y6 in the PVK polymer matrix increased, the spectral redshift and broadening of the Y6 thin films continued to increase. Then, we used GIWAXS and Franck-Condon fitting to further determine the contributions from different aggregates and non-interacting molecules in NFA thin films. From the EA spectrum, it is found that the CT character already exists in isolated Y6 molecules, but it is largely increased when there is molecular packing. Surprisingly, it is found that the strong charge transfer in packed Y6 is due to a reduced polarizability change (Δp), rather than an increase in excited-state dipole moment, Δμ, as observed in other organic systems. In the case of ITIC, both diluted and pure films have similar Δµ and Δp values, indicating strong Frenkel excitons.

To further quantify the CT character of the Y6 excitations, we collaborated with Brédas group (University of Arizona) to perform electronic structure calculations. Interestingly, they have identified two different monomer configurations of Y6 based on conformations, which were overlooked in previous works. From density functional theory (DFT) and four-state model calculations, it is proposed that such a strong charge-transfer character is promoted by the stabilization of the charge-transfer energy upon aggregation.

In summary, Y6 molecules possess charge-transfer excitations in isolated molecules (intra-molecular) and aggregated states (intermolecular), as confirmed by electroabsorption spectroscopy. The four state model also validates that intermolecular CT excitations of Y6 lead to a substantial decrease in the ∆𝑝 value upon packing. However, for ITIC, due to the absence of CT contributions, both molecular and aggregate excited states in ITIC have a purely Frenkel excitonic nature. These findings bring insight into the correlation between inter-molecular charge transfer and molecular electronic configurations, which would facilitate the development of novel organic semiconductors with enhanced properties.

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Physical Sciences > Chemistry > Organic Chemistry > Photochemistry > Photovoltaics
Optical Spectroscopy
Physical Sciences > Materials Science > Materials Characterization Technique > Optical Spectroscopy

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