Investigating Ultrafast Charge Generation in Y6 Non-Fullerene Acceptor Using Ultrafast Spectroscopies

In recent years, the development of organic photovoltaics (OPV) has progressed rapidly due to the emergence of non-fullerene acceptors (NFAs), in particular Y6 (i.e., BTP-4F) [1]. This is mainly attributed to the molecular structure of the NFAs and their packing in the solid state. Typically, NFAs are composed of electron-accepting (A) and electron-donating (D) subunits arranged in planar and curved shapes, which leads to large exciton delocalization over one molecule. In addition, strong electronic coupling between two adjacent molecules gives rise to exciton delocalization with charge transfer (CT) character [2]. In fact, there has been evidence that Y6 generates charges immediately upon photoexcitation without the need of a donor/acceptor interface [3, 4]. There is, however, no consensus about the mechanism governing the ultrafast free charge generation yet limiting the fabrication of single-component OPVs based on Y-series NFAs.<br/><br/>This work explores excited-state dynamics from generation, dissociation, and recombination in planar and curved NFAs, namely IT4F and Y6, in diluted and solid states. We investigate these processes using a combination of transient absorption (TA), time-resolved photoluminescence (TRPL), and quasi-steady-state photoinduced absorption (PIA) spectroscopies, among others. We identified the excited state species contributing to the TA signals using a combination of spectroelectrochemistry, and triplet-sensitized NFAs TA and PIA for charges and triplets, respectively.<br/><br/>Our results show that, unlike IT4F, the initial photoexcitations in Y6 are mostly polaronic states upon near bandgap excitation. However, the appearance of these species within our ~120 fs temporal resolution demands further investigation into whether they are primary photoexcitations or generated from exciton dissociation. We show that upon increasing the excitation energy, a higher-laying hot exciton dissociates into free polarons within 0.2 ps. This is supported by the observation of the initial photoexcitation peak shift, which is assigned to the electroabsorption effect caused by the separation of the electron-hole pair. Excitation-density-dependent TA results show these polarons are mobile and recombine quickly via bimolecular recombination within a few hundred picoseconds. Eventually, the long-lived species relax on one molecule, creating intramolecular charge transfer, as reflected by the spectral shape's resemblance to the first derivative of the absorption spectrum. These results give insight into the correlation between the molecular structure and the feasibility of intrinsic charge generation in pristine organic semiconductors; hence, the possibility of fabricating single-component OPVs.<br/><br/>Y. Lin et al., Adv. Mater. 31, 1902965 (2019).<br/>G. Zhang et al. Nat Commun 11, 3943 (2020).<br/>L. Zhu et al., Angew. Chem. Int. Ed. 60, 15348 (2021).<br/>M.B. Price, et al. Nat Commun 13, 2827 (2022).