High-Performance Nonfused Electron Acceptor with Precisely Controlled Side Chain Fluorination

Tuning Active Layer Morphology via Ternary Copolymerization with an Asymmetric Benzothiazole Unit as the Third Component to Enhance Photovoltaic Performance

Controlling the morphology and phase separation of the active layer is of great significance for the development of efficient organic solar cells. This study employs ternary copolymerization to optimize the phase separation and surface morphology of the active layer. Three terpolymers (PMz-5, PMz-10, and PMz-20) are synthesized by incorporating an asymmetric benzo[d]thiazole (BTz) unit as the third component to suppress the strong aggregation of the PM6:L8-BO blend film. Compared to the PM6 blend film, terpolymer blend films exhibit improved surface morphology, an appropriate phase separation scale, and, thus, an enhanced device performance. Furthermore, the incorporation of a BTz unit can lower the highest occupied molecular orbital levels of three terpolymers, which is conducive to charge transport. The champion device PMz-5:L8-BO system achieves an efficiency of 16.57%, featuring a high open-circuit voltage of 0.886 V, a high short-circuit current density of 25.49 mA cm-2, and a remarkable fill factor of 73.35%. This work provides an efficient and straightforward ternary copolymerization strategy that can be used to enhance the device performance.

Fluorinated Benzothiadiazole-Based Polymers for Organic Solar Cells: Progress and Prospects

The integration of fluorinated benzothiadiazole (FBT) into donor-acceptor (D-A) copolymers represents a major advancement in the field of organic solar cells (OSCs). The fluorination process effectively fine-tunes the energy levels, reduces the highest occupied molecular orbital levels, and enhances the open-circuit voltages of the polymers. Furthermore, fluorination improves molecular packing and crystallinity, which significantly boosts the charge transport and overall device performance. This review provides a detailed analysis of the progress made with FBT-based polymers in OSCs, classifying these materials according to their copolymerization units. It discusses the design strategies and structure-property relationships that have emerged as well as the current challenges and future directions for optimizing these polymers. By offering a comprehensive overview of the existing research, this review aims to facilitate the development of high-performance FBT-based organic photovoltaic materials, ultimately contributing to the advancement of sustainable energy solutions.

Influence of alkyl chain fluorination proportion on the photovoltaic performance of non-fullerene acceptors

Three non-fullerene acceptors (NFAs) with varying alkyl chain sizes and lengths but constant fluorine atoms were synthesized. BO-17F and HD-17F, with optimal fluorination, exhibit improved molecular packing and morphology, achieving efficiencies of 14.2% and 14.4%. This work highlights the importance of fine-tuning alkyl chains and fluorination levels in organic solar cells.

Cyclization Engineering of Electron-Deficient Maleimide Unit for Nonfused Ring Electron Acceptors Enables Efficient Organic Solar Cells

Nonfused ring electron acceptors (NFREAs) have emerged as promising materials for commercial applications in organic solar cells due to their straightforward synthesis process and cost-effectiveness. The rational design of their structural frameworks is crucial for enhancing device efficiency. In this study, we explore the use of maleimide and thiophene as key building blocks, employing cyclization engineering techniques. Additionally, cyclopentanedithiophene was chosen as the bridging unit, coupled with fluorinated terminals, to fabricate NFREAs, namely, PI-DTS and DPI-DTS. DPI-DTS demonstrated superior molecular planarity and an upshifted lowest unoccupied molecular orbital energy level. Moreover, DPI-DTS-based blend films display enhanced π-π interactions and crystallinity, alongside a predominantly face-on orientation. Consequently, DPI-DTS-based devices displayed enhanced and more balanced carrier mobility, reduced bimolecular recombination, and trap-assisted recombination, leading to improved charge transfer efficiency. Ultimately, this led to an excellent efficiency of 10.48%, with an open-circuit voltage as high as 0.914 V. These findings highlight the significant promise of aromatic imides in constructing NFREAs, and the established structure-performance relationship provides a theoretical basis for the design of high performance NFREAs.

Advancing Integration of Direct C-H Arylation-Derived Star-Shaped Oligomers as Second Acceptors for Ternary Organic Solar Cells

Organic solar cells (OSCs) could benefit from the ternary bulk heterojunction (BHJ), a method that allows for fine-tuning of light capture, cascade energy levels, and film shape, in order to increase their power conversion efficiency (PCE). In this work, the third components of PM6:Y6 and PM6:BTP-eC9 BHJs are a set of four star-shaped unfused ring electron acceptors (SSUFREAs), i.e., BD-IC, BFD-IC, BD-2FIC, and BFD-2FIC, that are facilely synthesized by direct C-H arylation. The four SSUFREAs all show complete complementary absorption with PM6, Y6, and BTP-eC9, which facilitates light harvesting and exciton collection. When BFD-2FIC is added as a third component, the PCEs of PM6:Y6 and PM6:BTP-eC9 binary BHJs are able to be improved from 15.31% to 16.85%, and from 16.23% to 17.23%, respectively, showing that BFD-2FIC is useful for most effective ternary OSCs in general, and increasing short circuit current (JSC) and better film morphology are two additional benefits. The ternary PM6:Y6:BFD-2FIC exhibits a 9.7% percentage of increase in PCE compared to the PM6:Y6 binary BHJ, which is one of the highest percentage increases among the reported ternary BHJs, showing the huge potential of BFD-2FIC for ternary BHJ OSCs.

Over 19 % Efficiency Organic Solar Cells Enabled by Manipulating the Intermolecular Interactions through Side Chain Fluorine Functionalization

Fluorine side chain functionalization of non-fullerene acceptors (NFAs) represents an effective strategy for enhancing the performance of organic solar cells (OSCs). However, a knowledge gap persists regarding the relationship between structural changes induced by fluorine functionalization and the resultant impact on device performance. In this work, varying amounts of fluorine atoms were introduced into the outer side chains of Y-series NFAs to construct two acceptors named BTP-F0 and BTP-F5. Theoretical and experimental investigations reveal that side-chain fluorination significantly increase the overall average electrostatic potential (ESP) and charge balance factor, thereby effectively improving the ESP-induced intermolecular electrostatic interaction, and thus precisely tuning the molecular packing and bulk-heterojunction morphology. Therefore, the BTP-F5-based OSC exhibited enhanced crystallinity, domain purity, reduced domain spacing, and optimized phase distribution in the vertical direction. This facilitates exciton diffusion, suppresses charge recombination, and improves charge extraction. Consequently, the promising power conversion efficiency (PCE) of 17.3 % and 19.2 % were achieved in BTP-F5-based binary and ternary devices, respectively, surpassing the PCE of 16.1 % for BTP-F0-based OSCs. This work establishes a structure-performance relationship and demonstrates that fluorine functionalization of the outer side chains of Y-series NFAs is a compelling strategy for achieving ideal phase separation for highly efficient OSCs.