Study Reveals New Mechanism of Efficient Charge Generation in Low-Driving-Force Organic Solar Cells

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With the development of acceptor-donor-acceptor (A-D-A) structured non-fullerene molecular acceptors (NFAs), the performances of organic solar cells (OSCs) have dramatically improved. Especially, the power conversion efficiencies (PCEs) of OSCs using the state-of-the-art NFAs of Y6 and its derivatives reach 18% for the single-junction devices. The improvement can be attributed to the small or even near-zero interfacial energy offset in the highest occupied molecular orbital (HOMO) level demanded for efficient dissociation of excitons on NFAs through the hole transfer channel. However, the mechanisms of efficient charge generation in the NFA-based OSCs remain poorly understood. Basically, the demand of interfacial driving forces for exciton dissociation in OSCs is ascribed to the strong exciton binding energies (Eb) in organic active materials.
Previously, scientists found that the driving forces for dissociation of the NFAs into charge separated states are linearly correlated to the Eb and shed some light to reduce the energy loss of OSCs (J. Phys. Chem. C 2018, 122, 22309). To obtain an accurate estimate of the Eb in the solid state, a self-consistent quantum mechanics/embedded charge (QM/EC) method is employed to fully consider the polarization effects including both electrostatic and induction interactions. It was verified Eb of NFAs can be effectively reduced by optimizing the molecular packing structures. Remarkably, an extremely weak Eb of 0.04 eV is achieved in a three-dimensional packing crystal. This breaks the conventional understanding of the Eb of organic materials above 0.3 eV (J. Phys. Chem. Lett. 2019, 10, 4888). By regulating the polymorphs of non-fullerene receptors, it can also effectively reduce the Eb, providing an important way to reduce the energy losses for organic solar cells (J. Phys. Chem. Lett. 2020, 11, 10227).
Recently, a research team led by Profs. ZHU Lingyun and WEI Zhixiang from the National Center for Nanoscience and Technology in cooperation with Prof. YI Yuanping from the Institute of Chemistry of Chinese Academy of Sciences studied the Eb in Y6 by a joint theoretical and experimental study. This work was published in Angew. Chem. Int. Ed..
In this study, it is shown that owing to the very strong charge polarization effects resulting from the tight three-dimensional molecular packing, Y6 has remarkable small Eb, which is even lower than those of perovskites in many cases. Furthermore, the energy barrier for exciton dissociation into free charge carriers is evidently lower than the thermal energy at room temperature according to the temperature dependence of photoluminescence (PL). Especially, with increasing temperature, the PL intensity is enhanced. Accordingly, without the help of D/A interfaces, direct generation of charge carriers is enabled by the remarkable low Eb upon photoexcitation of Y6. This work underlines the importance of polarization effects on achieving small Eb in organic active materials and paves the way towards high-efficiency OSCs with low driving forces. 
The work was supported by National Natural Science Foundation of China, the Ministry of Science and Technology of China and Strategic Priority Research Program of Chinese Academy of Sciences. 
Figure 1. QM/EC calculated the exciton binding energy (Eb) in solid phase with consideration of the interplay of both polarization and delocalization effects.

Figure 2. Temperature-dependent photoluminescence spectra for and charge generation mechanisms for Y6 and ITIC.

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