No abstract

A–D–A′–D–A-type nonfused-ring acceptors (NFRAs) have recently received extensive attention because of their suitable tuning of absorption spectra, frontier energy levels, and promising sunlight harvesting capability. However, no attention has yet been paid to the effects of A′ core substitutions on optoelectronic, morphological, and photovoltaic properties in high-performance organic solar cells (OSCs). In this work, to deeply understand the effects of electron-donating and electron accepting substitutions on the A′ core, we designed and synthesized three A–D–A′–D–A-type NFRAs, BTCPDT, BTCPDTO4, and BTCPDTF, with different substitutions on the benzo­[c]­[1,2,5]­thiadiazole core, such as hydrogen, alkoxy, and fluorine groups, respectively. Based on the characterization results, BTCPDT, BTCPDTO4, and BTCPDTF showed considerable variations in optical, electrochemical, and morphological properties because of their different electronegativity, steric hindrance, and intramolecular charge transfer effects. Among the NFRAs in this study, BTCPDTO4 showed the highest absorption coefficient, a high-lying frontier energy level, well-mixed and interpenetrating network morphology with enhanced charge transfer/transport, and suppressed charge recombination, which result in an impressive efficiency of 11.85% in an NFRA-OSC.

The advance of nonfullerene acceptors (NFAs) has accelerated the improvement of photovoltaic performance of organic solar cells (OSCs). However, most NFAs are suggested as a multisynthesized planar molecular backbone structure containing fused conjugated rings. Therefore, nonfused NFAs (n-FNFAs) have recently been extensively studied and reported as an alternative strategy of fused-NFAs (FNFAs) by providing synthetic simplicity and fine-tuning optical properties and energy levels. In addition, the highly planar and fused NFAs have strong selfaggregation behaviors and reduced mixing properties between the polymer donor and FNFAs. To obtain excellent morphology and intermixing behavior, a same-accepting strategy on both of the polymer donor and n-FNFAs is adopted. In this study, three acceptors are designed and synthesized, BDD-IC4F, BTO4-IC4F, and TPD-IC4F, using a similar A−D−A′−D−A skeleton composed of two accepting center units linking cyclopentathiophene bridges that assure noncovalent conformational geometry. By inserting the A′ unit in the center, a full width at half-maximum of that material exhibits a broad absorption spectrum as above 200 nm in the near-infrared region and small band gaps. Among the three n-FNFAs, the optimized PM6:BDD-IC4F device achieved the best photovoltaic performance with smooth film morphology and well-miscible crystalline behavior between PM6 and BDD-IC4F. The effect of the same-accepting strategy improves formation of the intermixed blend domain and structure compatibility leading to efficient carrier transport and mobility. More importantly, these results demonstrate the possibility of nonfused acceptors for lowcost and high-efficient OSCs.