Crucial Role of Fluorine in Fully Alkylated Ladder-Type Carbazole-Based Nonfullerene Organic Solar Cells

Abstract

Two fused ladder type non-fullerene acceptors, DTCCIC and DTCCIC-4F, based on an electron-donating alkylated dithienocyclopentacarbazole core flanked by electron-withdrawing non-fluorinated or fluorinated 1,1dicyanomethylene-3-indanone (IC or IC-4F), are prepared and utilized in organic solar cells (OSCs). The two new molecules reveal planar structures and strong aggregation behavior, and fluorination is shown to red shift the optical band gap and down shift energy levels. OSCs based on DTCCIC-4F exhibit a power conversion efficiency of 12.6 %, much higher than that of DTCCIC based devices (6.2 %). Microstructural studies reveal that while both acceptors are highly crystalline, bulk heterojunction blends based on the non-fluorinated DTCCIC result in overly coarse domains, while blends based on the fluorinated DTCCIC-4F exhibit a more optimal nanoscale morphology. These results highlight the importance of end group fluorination in controlling molecular aggregation and miscibility.

The Path to 20% Power Conversion Efficiencies in Nonfullerene Acceptor Organic Solar Cells

“…Several other studies have demonstrated that fluorinating the NFA, which is a commonly employed method to increase PCEs, can directly impact the BHJ morphology and consequently the recombination and extraction dynamics. [ 29,228–230 ] Favorable morphological features, such as increased atomic‐level local ordering and π–π interactions resolved via ssNMR analyses, were demonstrated in the PTB7‐Th:IOTIC‐4F blend over the nonfluorinated PTB7‐Th:IOTIC‐0F counterpart; consequently, the faster extraction in the PTB7‐Th:IOTIC‐4F blend could be attributed to the morphological effects induced by fluorination alone. [ 29 ] A similar conclusion was reached in a study by Tang et al.…”

Unifying Charge Generation, Recombination, and Extraction in Low‐Offset Non‐Fullerene Acceptor Organic Solar Cells

“…The effect of fluorinating NFAs is known to be a widely used method to enhance photovoltaic device performances in polymer:NFA blends. [5,[25][26][27][28][29][30] However, because of the structural similarity in the NFAs used, it becomes rather challenging to identify the molecular level origins of this subtle change in molecular structure and its consequent impact on device performances. In fact, while there are several studies in the literature that have utilized ssNMR to characterize blend morphologies for polymers or polymer:fullerene blends, [23,[31][32][33][34][35][36][37] the application of ssNMR techniques to characterize structurally similar polymer:NFA blends is limited.…”

Engineering the Core Units of Small‐Molecule Acceptors to Enhance the Performance of Organic Photovoltaics

“…Furthermore, introducing the nitrogen atoms form S,N‐heteroarene ladder‐type fused ring cores and replacing sp 3 carbon with sp 2 nitrogen units of A–D–A‐structured small‐molecule acceptors often enhance their possibility to undergo intramolecular charge transfer (ICT) as well as alter their packing orientation. [ 11 ] In this vein, chlorination of the conjugated backbone might be preferable because it is inexpensive, synthetically simple, and higher yielding. [ 12 ] At present, the two main methods to promote charge transport, by accelerating charge dissociation and decreasing charge density at the contact interface, involve 1) extending the fused‐ring conjugated system to enhance the electron‐donating ability and 2) adding a strong electron‐withdrawing group at both ends of the fused ring core to promote electron delocalization.…”