Abstract:

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-donor- acceptor (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-ethylhexyl)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 cm²∙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 (V_oc) of 0.886 V, short-circuit current (J_sc) 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.

Key words: Non-fullerene, Polymer solar cell, Fused-ring electron acceptor, Conformational locking, Intramolecular charge transfer