9,9'-Bifluorenylidene-Core Perylene Diimide Acceptors for As-Cast Non-Fullerene Organic Solar Cells: The Isomeric Effect on Optoelectronic Properties

Wavily Curved Perylene Diimides: Synthesis, Characterization, and Photovoltaic Properties

Solubility enhancement is a key issue for developing the perylene diimide-based functional materials. Introduction of curved structure proved an effective solubilizing method without employing steric repulsion. In this work, wavily curved perylene diimides were developed as a new family of highly soluble curved perylene diimides. Moreover, their conformational dynamics, aggregating properties, electronic properties, and photovoltaic performances were thoroughly examined in comparison to the previously reported isomer exhibiting an arched curvature. The waved isomer demonstrated heightened rigidity and a greater propensity for aggregation compared to the arched isomer, likely attributed to its more planar structure. Each benzoxepin unit played a role in cancelling out the curvature on the opposite side. While the difference in the molecular curvature did not cause significant alterations in the photophysical and electron-accepting properties, we identified that the modulation of the curved structure is effective in controlling the morphology of the photoelectric conversion layer.

Recent progress and prospects of dimer and multimer acceptors for efficient and stable polymer solar cells

High power conversion efficiency (PCE) and long-term stability are essential prerequisites for the commercialization of polymer solar cells (PSCs). Small-molecule acceptors (SMAs) are core materials that have led to recent, rapid increases in the PCEs of the PSCs. However, a critical limitation of the resulting PSCs is their poor long-term stability. Blend morphology degradation from rapid diffusion of SMAs with low glass transition temperatures (Tgs) is considered the main cause of the poor long-term stability of the PSCs. The recent emergence of oligomerized SMAs (OSMAs), composed of two or more repeating SMA units (i.e., dimerized and trimerized SMAs), has shown great promise in overcoming these challenges. This innovation in material design has enabled OSMA-based PSCs to reach impressive PCEs near 19% and exceptional long-term stability. In this review, we summarize the evolution of OSMAs, including their research background and recent progress in molecular design. In particular, we discuss the mechanisms for high PCE and stability of OSMA-based PSCs and suggest useful design guidelines for high-performance OSMAs. Furthermore, we reflect on the existing hurdles and future directions for OSMA materials towards achieving commercially viable PSCs with high PCEs and operational stabilities.

Synthesis of Ladder-Type 9,9'-Bifluorenylidene-Based Conjugated Oligomers via a Pd-Catalyzed Tandem Suzuki Coupling/Heck Cyclization Approach

For new ladder-type oligomers and polymers with versatile and robust synthetic strategies, in this study, four fully conjugated ladder-type overcrowded 9,9'-bifluorenylidene-based compounds and oligomers (BFY1, BFY2, BFY3, and BFY4) were synthesized via a Pd-catalyzed tandem Suzuki coupling/Heck cyclization reaction. By monomer screening and route optimization, the target products were obtained in high yields and characterized by ¹H and ¹³C NMR spectroscopy and high resolution mass spectroscopy.

Efficient Medium Bandgap Electron Acceptor Based on Diketopyrrolopyrrole and Furan for Efficient Ternary Organic Solar Cells

We report the design of novel medium bandgap nonfullerene small molecule acceptor NFSMA SPS-TDPP-2CNRh with A₂-π-A₁-π-A₂ architecture, with the molecular engineering of this material comprising a strong electron-accepting backbone unit DPP (A₁) as the acceptor, which is attached to the dicyanomethylene-3-hexylrhodanine (A₂) acceptor via a furan (π-spacer) linker. We systematically studied its structural and optoelectronic properties. The incorporation of dicyanomethylene-3-hexylrhodanine and furan enhance the light absorption and electrochemical properties by extending π-conjugation and is anticipated to improve V_OC by decreasing the LUMO level. The long alkyl chain units were responsible for the better solubility and aggregation of the resultant molecule. Binary BHJ-OSCs constructed with polymer P as the donor and SPS-TDPP-2CNRh as the acceptor resulted in a PCE of 11.49% with improved V_OC = 0.98 V, J_SC = 18.32 mA/cm², and FF = 0.64 for P:SPS-TDPP-2CNRh organic solar cells. A ternary solar cell device was also made using Y18-DMO and SPS-TDPP-2CNRh as acceptors having complementary absorption profiles and polymer P as the donor, resulting in a PCE of 15.50% with improved J_SC = 23.08 mA/cm², FF = 0.73, and V_OC = 0.92 V for the P:SPS-TDPP-2CNRh:Y18-DMO solar cell. The ternary OSCs with SPS-TDPP-2CNRh as the host acceptor in the P:Y18-DMO binary film were shown to have improved PCE values, which is mainly attributed to the effective photoinduced charge transfer through multiple networks and the use of excitons from SPS-TDPP-2CNRh and Y18-DMO. Moreover, in the ternary BHJ active layers, the superior stable charge transport that was observed compared to the binary counterparts may also lead to an increase in the fill factor. These results demonstrate that combining medium bandgap and narrow bandgap NFSMAs with a wide bandgap polymer donor is a successful route to increasing the overall PCE of the OSCs via the ternary BHJ concept.

Fully unsaturated all-carbon bifluorenylidene-based polymeric frameworks: synthesis and efficient photocatalysis

Conjugated porous polymers with fully unsaturated all-carbon frameworks possess strong visible light-absorbing abilities, enabling efficient photodegradation of dye pollutants.

Panchromatic Organoboron Molecules with Tunable Absorption Spectra

Panchromatic molecules, e. g. organic small molecules with wide absorption spectra, are very desirable for solar energy-related applications. Here, we report the development of a series of organoboron compounds composed of an organoboron core unit, two π-bridging units and two electron-withdrawing end-capping units. All seven molecules have the HOMO localized on the core unit and the LUMO delocalized on the whole conjugated backbone. They exhibit wide absorption spectra consisting of two strong absorption bands with the full width at half maximum of ca. 280 nm. These panchromatic compounds can be used as electron acceptors in organic solar cells. We elucidate the relationship between the chemical structures and opto-electronic properties of these organoboron panchromatic compounds. Increasing the electron-withdrawing capability of the core units results in a downshifted HOMO level as well as blueshifted long-wavelength absorption band with increased extinction coefficient. Extending the π-bridging units causes an increased HOMO level and blueshifted long-wavelength absorption band with increased extinction coefficients. Weakening the electron-withdrawing capability of the end-capping units leads to an upshifted LUMO level and blueshifted long-wavelength absorption peak with decreased extinction coefficient. This work provides insight into the absorption spectrum manipulation of panchromatic molecules and would pave the way for the development of solar energy-related applications.

A "Rigid-Flexible" Approach for Processable Perylene Diimide-Based Polymers: Influence of the Specific Architecture on the Morphological, Dielectric, Optical, and Electronic Properties

Conjugation-break flexible spacers in-between π-conjugated segments were utilized herein toward processable perylene diimide (PDI)-based polymers. Aromatic-aliphatic PDI-based polymers were developed via the two-phase polyetherification of a phenol-difunctional PDI monomer and aliphatic dibromides. These polyethers showed excellent solubility and film-forming ability and deep lowest unoccupied molecular orbital (LUMO) levels (-4.0 to -3.85 eV), indicating the preservation of good electron-accepting character or characteristics, despite the non-conjugated segments. Their thermodynamic properties, local dynamics, and ionic conductivity demonstrate the suppression of PDI's inherent tendency for aggregation and crystallization, suggesting PDI-polyethers as versatile candidates for organic electronic applications. Their dynamics investigation using dielectric spectroscopy revealed weak dipole moments arising from the distortion of the planar perylene cores. Blends of the PDI-polyethers (as electron acceptors) with P3HT (as a potential electron donor component) showed UV-vis absorbances from 350 to 650 nm and a tendency of the PDI-polyethers to intertwine with rr-P3HT and restrain its high crystallization tendency.

Development of Perylene-Based Non-Fullerene Acceptors through Bay-Functionalization Strategy

Perylene has had a tremendous impact in the history of material research for the molecular semiconductors. Among numerous derivatives of this polyaromatic hydrocarbon, perylene diimide (PDI) represents a promising class of organic materials envisioned as non-fullerene acceptors (NFAs) for the practical organic photovoltaic (OPV) applications due to their enhanced photo- and thermal stability and remarkably high electron affinity, some of which realize band-like transport properties. The present review guides some of the representative achievements in the development of rationally designed PDI systems, highlighting synthetic methodologies based on bay-functionalization strategies for creating well-designed molecular nanostructures and structure-performance relationship of perylene-based small molecular acceptors (SMAs) for the photovoltaic outcomes.

Synthesis and Optoelectronic Characterization of Perylene Diimide-Quinoline Based Small Molecules

Perylene diimide (PDI) is one of the most studied functional dyes due to their structural versatility and fine tuning of the materials properties. Core substituted PDIs are prominent n-type semiconductor materials that could be used as non-fullerene acceptors in organic photovoltaics. Herein, we develop versatile organic building blocks based on PDI by decorating the PDI core with quinoline groups. Styryl and hydroxy phenyl mono and difunctionalized molecules were prepared using mono-nitro and dibromo bay substituted PDIs by Suzuki coupling with the respective boronic acid derivatives. A novel methodology using nitro-PDI under Suzuki coupling conditions as an electrophile partner was successfully tested. Furthermore, the PDI derivatives were used for the synthesis of soluble, electron accepting small molecules combining PDI with weak electron withdrawing quinoline derivatives. The new molecules presented wide absorbance in the visible spectrum from 450 to almost 700 nm while their LUMO levels and their energy levels are in the range of −3.8 to −4.2 eV.

Star-Shaped Non-Fullerene Small Acceptors for Organic Solar Cells

Over the past decade, organic solar cells (OSCs) have received considerable attention from the scientific community and are considered one of the most important sources of low-cost electricity production. Recently, OSC-based on star-shaped small-molecule (SM) non-fullerene acceptors (NFAs) have developed rapidly, and the highest power conversion efficiency (PCE) has exceeded 10 %. The star-shaped SM NFAs not only have three-dimensional charge-transport characteristics similar to fullerenes but also have a strong light absorption capacities and easily tunable energy levels. They are potential candidates as outstanding acceptor materials. In this Review, research progress in of star-shaped SM NFAs OSCs is reviewed specifically. Moreover, the influence of molecular structure, central unit, and peripheral linking group on OSC performance has been evaluated systematically. This Review could stimulate inspiration for designing high-performance OSC acceptor materials in the future.

Self-assembly behaviors of perylene- and naphthalene-crown macrocycle conjugates in aqueous medium

The synthesis of conjugates of perylene diimide (PDI) and naphthalene diimide (NDI) modified with two benzo-21-crown-7 ethers (B21C7) are herein described. Their self-assembly behavior in various solvents was investigated particularly in aqueous medium, due to the recently discovered hydrophilic properties of B21C7 crown macrocycle. An unexpected fluorescence quenching phenomenon was observed in the PDI-B21C7 macrocycle conjugate in chloroform. The detailed UV-vis absorption and fluorescence spectra of these PDI/NDI derivatives in different solvents as well as their morphologies were investigated.

Tweaking the Molecular Geometry of a Tetraperylenediimide Acceptor

Partial flattening of the spatially extended molecular scaffold has been employed as an effective tactic to improve the device performance of a perylenediimide (PDI)-based small-molecule acceptor because the less twisted yet not completely planar molecular geometry is anticipated to improve the molecular packing and thereby attain a more suitable balance between the carrier transport ability and phase domain size. A small-molecule acceptor BF-PDI comprising four α-substituted PDI units attached around a 9,9'-bifluorenylidene (BF) central moiety is designed and studied in polymer solar cells. The BF group is deemed a ring-fused analogue of the tetraphenylethylene (TPE) unit. Due to the less twisted and better conjugated BF skeleton, BF-PDI displays more delocalized lowest unoccupied molecular orbital. By virtue of both the electronic and steric effects, BF-PDI is suggested to bring about superior intermolecular stacking and donor-acceptor phase separation morphology in blend films. Indeed, the experimental results show that BF-PDI displays improved charge transport ability and a higher power-conversion efficiency of 8.05% than that of TPE-PDI. Grazing-incidence wide-angle X-ray diffraction and resonant soft X-ray scattering confirm the more compact and ordered molecular packing as well as smaller domain sizes in the P3TEA/BF-PDI blend.

A 9,9′-bifluorenylidene derivative containing four 1,1-dicyanomethylene-3-indanone end-capped groups as an electron acceptor for organic photovoltaic cells

A non-fullerene receptor (BF-TDCI4) was successfully synthesized for organic photovoltaic cells, and the PCE of the device was 4.35%.

Diketopyrrolopyrrole-based acceptors with multi-arms for organic solar cells

Three diketopyrrolopyrrole-based small molecule acceptors consisting of a spirobifluorene (SBF) core unit and diketopyrrolopyrrole dicyanovinyl (DPPDCV) units as the arms have been synthesized for organic solar cells.