Phenalene—A New Ring-Locked Vinyl Bridge for Nonfullerene Acceptors With Enhanced Chemical and Photochemical Stabilities

Currently, the two exocyclic vinyl bridges in the acceptor–donor–acceptor (A–D–A)-type nonfullerene acceptors (NFAs) have been widely recognized as one of the most vulnerable sites under external stresses. Embedding the exocyclic vinyl bridges into an aromatic ring could be a feasible solution to stabilize them. Herein, we successfully develop a phenalene-locked vinyl bridge via a titanium tetrachloride—pyridine catalytic Knoevenagel condensation, to synthesize two new A–D–A-type unfused NFAs, EH-FPCN and O-CPCN, wherein malononitrile is used as the electron-deficient terminal group while fluorene and carbazole rings are used as the electron-rich cores, respectively. These two NFAs possess wide bandgaps associated with deep energy levels, and significantly enhanced chemical and photochemical stabilities compared to the analogue molecule O-CzCN with normal exocyclic vinyl bridges. When pairing with a narrow bandgap polymer donor PTB7-Th, the fabricated EH-FPCN- and O-CPCN-based organic solar cells achieved power conversion efficiencies of 0.91 and 1.62%, respectively. The higher efficiencies for O-CPCN is attributed to its better film morphology and higher electron mobility in the blend film. Overall, this work provides a new design strategy to stabilize the vulnerable vinyl bridges of A–D–A-type NFAs.


INTRODUCTION
As one of the third-generation photovoltaic techniques, organic solar cells (OSCs) have demonstrated outstanding potentials in the fields of flexible devices, semi-transparent devices, indoor photovoltaics, and so on (Liu et al., 2021c;Dauzon et al., 2021;Kini et al., 2021;Xie et al., 2021). Excitingly, the record power conversion efficiency (PCE) of OSCs has been continuously rising in recent years, and a value exceeding 19% has been achieved already Wang et al., 2021), attributed to the innovative design and exploitation of photovoltaic materials and device architecture Cui and Li, 2021;Liu W. et al., 2021;Gao et al., 2021;Ren et al., 2021;Yin et al., 2021;Zheng et al., 2021). In particular, A-D-A-type nonfullerene acceptors (NFAs) play the most critical role in achieving high-performance OSCs since the emergence of the representative NFA molecule, namely ITIC, reported by Zhan et al., in 2015(Lin et al., 2015, where "D" and "A" represent the electron-donating and electron-accepting units, respectively. Currently, most of ITIC-like NFAs are constructed by using a largely fused ring as the D core such as indacenodithieno [3,2-b]thiophene (IDTT) endcapped by 2 A units such as 2-(3-oxo-2,3-dihydroinden-1ylidene)malononitrile (INCN). However, such design always involves high synthetic complexity, thereby limiting their further practical applications. In this context, many researchers have paid attention to explore new A-D-A-type NFAs with semi-fused (Li et al., 2018;Huang et al., 2019;Liu X. et al., 2020;Luo et al., 2021) and even fully unfused electronrich cores Ma et al., 2021;Wen et al., 2021;Zhou et al., 2021).
In addition to simplifying the materials synthesis, improving the stability is another critical factor for NFAs to be commercialized. From the structure point of view, A-D-A-type NFAs always contain two exocyclic vinyl groups as the conjugation bridges between D and A units as a result of the condensation reaction. Although the vinyl bridges improve the planarity and enlarge conjugation length, leading to a smaller bandgap and enhanced charge transport properties, they encounter unsatisfactory intrinsic chemical and photochemical stabilities due to the strong push-pull effect that makes them very electrophilic (Li W. et al., 2021;Liu et al., 2021a). In this regard, the nucleophiles such as primary and secondary amines in the commonly used cathode interfacial materials (PEI and PEIE) would easily engender Michael addition reactions with the β-carbon atoms of these electrophilic vinyl bridges (Hu et al., 2018;Zeng et al., 2021). In the meanwhile, photooxidation and/or photodegradation of vinyl bridges also occur easily due to the generation of reactive oxygen species (ROS) under light irradiation Jiang et al., 2019;Park and Son, 2019;Liu B. et al., 2020). To improve the chemical and photochemical stabilities of NFAs, scientists have proposed interfacial engineering approaches. For example, protonation of PEIE (Xiong et al., 2019) or formation of Zn 2+ -chelated PEI (Qin et al., 2020) has shown to significantly reduce its reactivity with the NFAs as reported by Zhou et al. More recently, Forrest et al. improved the chemical and morphological stabilities at organic active layer/inorganic interfacial layers by inserting buffer layers between them . Besides, development of new interfacial and charge transport materials with low reactivity and/or low defects (Cui et al., 2020;Kyeong et al., 2021;Prasetio et al., 2021) as well as additive engineering such as doping a stabilizer to scavenge ROS (Qin et al., 2017;Du et al., 2019;Lu et al., 2021) have also been used as alternative approaches.
Despite these successes, it would be of greater importance to develop efficient molecular strategies that can improve the intrinsic stability of NFAs at the molecular level; however, the related progress remains tardy. For example, Li et al. have demonstrated that the undesired photoisomerization of vinyl bridges of NFAs can be effectively suppressed by enhancing the intermolecular packing via the subtle side-chain modification (Yu et al., 2019;. Moreover, construction of all-fused NFAs is also a feasible design strategy for enhancing the stability of NFAs . Recently, we reported a new ring-locked strategy by incorporating the vinyl bridge of NFAs into a cyclohexane ring, which significantly improves the chemical and photochemical stabilities of vinyl bridges due to enhanced steric hindrance of nucleophilic attaching and suppressed photoisomerization induced by the noncovalent interactions . Therefore, it would be reasonable to embed the exocyclic vinyl bridges into an aromatic ring for enhancing their stabilities. Phenalene fused by three benzenes arranging in a triangle has a simple and symmetrical molecular structure. Since Haddon pointed out the potential of phenalene to construct molecular conductors in 1975 (Haddon, 1975), researchers have designed and synthesized lots of phenalene-based materials for broad applications such as organic electronics, energy storage, information processing system, etc (Mandal et al., 2005;Morita et al., 2011;Raman et al., 2013). Herein, based on our ring-locked strategy, we successfully develop a feasible synthetic route to embed the exocyclic vinyl bridges into the phenalene unit as a conjugation bridge of A-D-A-type NFAs ( Figure 1A). To reduce the synthetic complexity, carbazole and fluorene are used as the central core for constructing phenalene-locked NFAs with an unfused structure, EH-FPCN and O-CPCN ( Figure 1B), respectively. Both of them show good thermal/chemical/ photochemical stabilities, broadened and red-shifted absorption, and decreased lowest unoccupied molecular orbital (LUMO) energy levels compared to the reference O-CzCN ( Figure 1B) without a phenalene bridge. When pairing with a polymer donor PTB7-Th, the resulting EH-FPCN-and O-CPCN-based OSCs can achieve PCEs of 0.91 and 1.62%, respectively. The better performance of O-CPCN is attributed to its enhanced electron mobility and more uniform blend film morphology.

Synthesis and Structural Characterization
Scheme 1 displays the synthetic route of the targeting molecules.1-Oxo-1H-phenalen-3-yl trifluoromethanesulfonate (OTf-P) was synthesized according to the literature method (Ospina et al., 2017). By employing an unusual titanium tetrachloride-pyridine catalytic system , the cyano-substituted phenalene unit (OTf-PCN) was obtained via a Knoevenagel condensation between OTf-P and malononitrile. Then, OTf-PCN reacted with fluorene and carbazole borate esters such as EH-Fl-Bpin and O-Cz-Bpin to produce EH-FPCN and O-CPCN via a Pd-catalyzed Suzuki coupling, respectively. Finally, O-CzCN as the control molecule without the ringlocked bridge was synthesized based on the trimethylaminecatalyzed Knoevenagel condensation between O-CzCHO (Supplementary Scheme S1) and malononitrile. The detailed syntheses and structural characterizations are provided in the Supporting Information.

Thermal Properties
The thermal properties of NFAs were studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) in nitrogen. As shown in Supplementary Figure S1A, the thermal decomposition temperatures for EH-FPCN, O-CPCN, and O-CzCN, corresponding to 5% weight loss, were estimated to be 389, 396, and 340°C, respectively. This result clearly shows that the introduction of phenalene-locked bridge improves the thermal stability of resulting NFAs. Notably, there is no glass transition temperature (T g ) found for all molecules (Supplementary Figure S1B). Both EH-FPCN and O-CzCN exhibit an amorphous nature because no endothermic and exothermic peaks are observed in both heating and cooling processes. Differently, O-CPCN possesses an obvious endothermic peak at 217°C in the heating process assigned as the melting point, but no exothermic peak is found in the cooling process. This phenomenon could be attributed to the effect of alkyl chains on the molecular stacking. As compared to the branched alkyl chains, the linear alkyl chains are conducive to enhance intermolecular stacking (Holliday et al., 2016).      0.1 mol L −1 of Bu 4 NPF 6 . As shown in Figure 2C and Table 1, both EH-FPCN and O-CPCN have a quasi-reversible redox peak in the reduction process and an irreversible redox peak in the oxidation process. Compared to the reference O-CzCN, the introduction of phenalene-bridge in O-CPCN elevates the HOMO energy level and decreases the LUMO energy level. Moreover, replacing carbazole with fluorene unit induces a decreased HOMO level by~0.2 eV, associated with a slight change on the LUMO level.

Theory Calculations
To investigate the molecular conformation of EH-FPCN and O-CPCN, the density functional theory (DFT) calculations were conducted, which show that both NFAs have over 50°dihedral angles between the phenalene and fluorene/carbazole cores, as shown in Figures 3A,B. Such nonplanar molecular conformation may cause poor molecular stacking and thus weaken the intra-/intermolecular charge transfer. Figures 3C,D present the electron distribution of Frontier molecular orbitals of EH-FPCN and O-CPCN performed at the B3LYP/6-31G(d) level of theory. It is clearly shown that the wave function of HOMO delocalizes on the whole conjugated skeleton, and the wave function of LUMO mainly delocalizes on the phenalene-malononitrile units. The overlap between the wave function of HOMO and LUMO is favorable to exciton generation and charge transfer (Krishna et al., 2014).

Chemical and Photochemical Stabilities
The effects of phenalene-locked bridge on the chemical stability of three NFAs were then studied by testing the solution absorption change with or without adding a strong nucleophile (ethanol amine, EA) under different time. Figures 4A,B show that the absorption spectra have no significant changes after adding 100 equiv. molar amount of EA into the 1 × 10 −5 M EH-FPCN and O-CPCN solutions for 1 h. In sharp contrast, under the same conditions, the absorption intensity of O-CzCN solution immediately decreased by 21% after adding EA, and further decreased by more than 80% after 0.5 h, as shown in Figure 4C. The photochemical stability was then investigated by monitoring the solution absorption change under different light irradiation time (Xenon lamp, 100 mW cm −2 ). After light irradiation for 10 min, the absorption intensities of EH-FPCN and O-CPCN only decreased by~18 and~20%, respectively ( Figures 4D,E), while that of O-CzCN exhibited a 16% decrease only after light irradiation for 5 min ( Figure 4F), followed by a complete degradation after 10 min. These results therefore strongly illustrate that the phenalene-locked strategy can effectively enhance the chemical and photochemical stabilities of A-D-A-type NFAs by reducing the reactivity of vinyl bridges. Moreover, the film photostabilities of phenalene-locked NFAs were also measured under light irradiation (Supplementary Figure S2). After 2 h of continuous irradiation, the maximum absorption peak intensity of O-CPCN film retained 73% of its initial value, while only 34% was found for the EH-FPCN film, which could be attributed to the poor antioxidative ability of fluorene core in EH-FPCN.

Photovoltaic Performance
To investigate the photovoltaic performance of EH-FPCN and O-CPCN, a representative low-bandgap polymer, PTB7-Th, was chosen as the donor to form a bulk heterojunction (BHJ) blend due to the compatible energy levels ( Figure 5A) and bandgaps (Supplementary Figure S3), and the donor:NFA weight ratio is optimized to 1:0.8. The devices were fabricated based on an inverted architecture: ITO/ZnO/PTB7-Th:NFAs/MoO 3 /Ag ( Figure 5B). The resultant current density-voltage (J-V) curves are shown in Figure 5C and the photovoltaic parameters summarized in Table 2. As shown, the optimal PCEs for EH-FPCN and O-CPCN-based devices are 0.91 and 1.62%, respectively. The low PCEs may be attributed to the inferior exciton diffusion and separation efficiencies, which caused by the poor molecular planarity of phenalene-locked acceptors and the weak electron-withdrawing ability of malononitrile. This thus suggests that more structural optimizations need to be done for the phenalene-locked NFAs. Compared to the EH-FPCN-based device, the higher PCE for the O-CPCN-based device is benefited from its enhanced open-circuit voltage (V oc ) and short-circuit current density (J sc ). The external quantum efficiency (EQE) curves ( Figure 5D) confirm the stronger light response for the O-CPCN-based device, thereby agreeing with the enhanced J sc .

Film Morphology and Electron Mobility
Atomic force microscope (AFM) was then conducted to probe the surface morphology of pristine and blended films. Figures 6A,B show the AFM images of pristine EH-FPCN and O-CPCN films, respectively. EH-FPCN has a rough and inhomogeneous morphology with a high root mean square roughness (R q ) of 45.80 nm, indicating its poor film-forming property. In contrary, O-CPCN has a smooth morphology with a low R q of 0.52 nm. Nonetheless, both NFAs exhibit good compatibility with the PTB7-Th donor, and no phase separation is observed for the resulting BHJ films (Figures 6C,D). Moreover, the PTB7-Th:O-CPCN film possesses a smother surface morphology with a lower R q of 1.44 nm, which is conducive to exciton separation and charge transfer. The electron mobility (μ e ) of active layers were also measured by the space-charge limited current (SCLC) method (Supplementary Figure S4) and the calculated μ e for the EH-FPCN blend (1.01 × 10 -3 cm 2 V −1 s −1 ) is lower than that for the O-CPCN blend (3.61 × 10 -3 cm 2 V −1 s −1 ). These morphology and electron mobility results thus well explains the superior performance for the PTB7-Th:O-CPCN-based device.

CONCLUSION
In summary, we have successfully developed a phenalene-locked strategy to stabilize the vulnerable vinyl bridges in the A-D-A-type NFAs. As a model design, two phenalene ringlocked unfused NFAs, EH-FPCN and O-CPCN, were synthesized via a facile route, showing enhanced thermal/chemical/ photochemical stabilities in comparison with the reference O-CzCN without phenalene bridge. Moreover, the introduction of phenalene bridge is found to broaden and red-shift the absorption spectra, elevate the HOMO energy levels, and decrease the LUMO energy levels. The nonplanar molecular conformations were observed for both NFAs by DFT calculations, which could negatively affect the ICT process. Due to their wide bandgap feature, a low bandgap polymer donor PTB7-Th was paired to fabricate invert OSCs, and the EH-FPCN-and O-CPCN-based devices achieved PCEs of 0.91 and 1.62%, respectively. SCLC and AFM results suggest that the higher PCE of O-CPCN-based device is benefited from its enhanced electron mobility and smoother BHJ film morphology. Obviously, this work provides a new strategy to enhance the intrinsic stabilities of A-D-A-type NFAs. Further works to improve the photovoltaic performance of phenalene-locked A-D-A-type NFAs still need collaboratively optimize the molecular structures of "D" and "A" units, which is ongoing in our lab.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.