通过非共价构象锁定和端基工程策略设计高效率的A-D-A型稠环电子受体

Designing a High-Performance A-D-A Fused-Ring Electron Acceptor via Noncovalently Conformational Locking and Tailoring Its End Groups

近年来,非富勒烯太阳能电池的发展迅猛。目前报道的高效率的非富勒烯稠环电子受体主要采用受体-给体-受体(A-D-A)型结构。本工作中,我们在给受体间引入3,4-二己氧基噻吩作桥,用5,6-二氯-3-(二氰基亚甲基)靛酮作端基设计合成了一种新的稠环电子受体(ITOIC-2Cl)。一方面,可以通过S···O和O···H等作用在分子内形成非共价键构象锁促进分子的平面性;另一方面,通过增加端基的缺电子性可以增强分子内的电荷迁移。在两者的协同作用下,ITOIC-2Cl的光谱吸收拓宽到近红外区,这有利于获得宽的光谱响应。将ITOIC-2Cl与一种吸收互补的给体聚合物(PBDB-T)共混制备活性层,我们用原子力显微镜(AFM)和透射电子显微镜(TEM)表征其形貌,发现共混薄膜可以形成纤维状的互传网络结构和合适纳米尺寸的相分离,这有利于电荷的分离和传输,从而获得高的短路电流(J_(sc))和填充因子(FF)。最终,基于PBDB-T:ITOIC的电池,我们获得了9.37%的光电转换效率,其开路电压(V_(oc))为0.886 V,J_(sc)为17.09 mA·cm^(-2),FF为61.8%。这些研究结果为我们提供了一种设计高效率的非富勒烯稠环电子受体的有效的策略。

Recently, non-fullerene polymer solar cells(NPSCs) have been developed rapidly because of the flexible energy-level variability and excellent optical absorption properties of non-fullerene electron acceptors. Among them, fused-ring electron acceptors(FREAs) with acceptor-donoracceptor(A-D-A) structures have been extensively exploited in high-performance NPSCs. These FREAs often employ central aromatic fused rings attached to several rigid side-chains and flanked by two electron-deficient terminals. Many efforts have focused on the modification of the central flat conjugated backbone in order to gain broad and strong absorption and dense stacking. However, the preparation of such FREAs is relatively complex, especially for large fused-ring structures. In a previous work, we provided a simple and useful method to extend the effective conjugation length and broaden the absorption spectrum of the acceptor by noncovalent intramolecular interactions. On this basis, in this work, we have designed and synthesized a new A-D-A-type FREA(ITOIC-2Cl) that uses 4,9-dihydro-s-indaceno[1,2-b:5,6-b’]dithiophene(IDT) as a central donor unit, bis(alkoxy)-substituted thiophene rings as conformational locking π-bridges between the donor and acceptor units, and cyanoindanones modified with two high-electron-affinity chlorine atoms as end-capping acceptor units. On one hand, we can attain good backbone planarity in the solid state via the noncovalent conformational locking induced by sulfur-oxygen(S···O) and oxygen-hydrogen(CH···O) interactions, which are not strong enough to lock the coplanar conformation in solution, thus simultaneously endowing ITOIC-2Cl with good solubility. On the other hand, we can enhance the intramolecular charge transfer by enhancing the electron deficiency of the terminal groups. The optical and electrochemical properties of ITOIC-2Cl were systematically explored. Moreover, in combination with the donor polymer of [(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-ethylhex yl)benzo[1’,2’-c:4’,5’-c’]dithiophene-4,8-dione))](PBDB-T), the photovoltaic performances of the devices and the corresponding blend morphologies were studied. ITOIC-2Cl exhibited a broad absorption spectrum up to 900 nm, which is beneficial for broad harvesting of photons across the visible and NIR region. The PBDB-T:ITOIC-2Cl-based blend films exhibited favorable fibrous nanostructures with appropriate nanoscale phase separation, verified by atomic force microscopy and transmission electron microscopy characterizations. This morphology is beneficial for charge transport. Through the space-charge-limited current measurement, the PBDB-T:ITOIC-2Cl-based device exhibited the high hole/electron mobility of 1.85 × 10^-4/1.19 × 10^-4 cm^2·V^-1·s^-1. The PBDB-T:ITOIC-2Cl-based devices obtained a high power conversion efficiency of 9.37%, with an open-circuit voltage(Voc) of 0.886 V, short-circuit current(Jsc) of 17.09 mA cm-2, and a fill factor(FF) of 61.8%. These results thus demonstrate the efficacy of the proposed strategy for designing high-performance non-fullerene FREAs.