Utilization of Polycyclic Aromatic Solid Additives for Morphology and Thermal Stability Enhancement in Photoactive Layers of Organic Solar Cells.

Volatile solid additives have emerged as a promising strategy for enhancing film morphology and promoting the power conversion efficiency (PCE) of organic solar cells (OSCs). Herein, a series of novel polycyclic aromatic additives with analogous chemical structures, including fluorene (FL), dibenzothiophene (DBT), and dibenzofuran (DBF) derived from crude oils, are presented and incorporated into OSCs. All these additives exhibit strong interactions with the electron-deficient terminal groups of L8-BO within the bulk-heterojunction OSCs. Moreover, they demonstrate significant sublimation during thermal annealing, leading to increase free volumes for the rearrangement and recrystallization of L8-BO. This phenomenon leads to an improved film morphology and an elevated glass-transition temperature of the photoactive layers. Consequently, the PCE of the PM6:L8-BO blend has been boosted from 16.60% to 18.60% with 40 wt% DBF additives, with a champion PCE of 19.11% achieved for ternary PM6:L8-BO:BTP-eC9 OSCs. Furthermore, the prolonged shelf and thermal stability have been observed in OSCs with these additives. This study emphasizes the synergic effect of volatile solid additives on the performance and thermal stability of OSCs, highlighting their potential for advancing the field of photovoltaics.

A Pyrene-Fused Dimerized Acceptor for Ternary Organic Solar Cells with 19% Efficiency and High Thermal Stability.

A pyrene-fused dimerized electron acceptor has been successfully synthesized and subsequently incorporated as the third component in ternary organic solar cells (OSCs). Diverging from the traditional dimerized acceptors with a linear configuration, this novel electron acceptor displays a distinctive "butterfly-like" structure, comprising two Y-acceptors as wings fused with a pyrene-based backbone. The extended π-conjugated backbone and the electron-donating nature of pyrene enable the new acceptor to show low solubility, elevated glass transition temperature (Tg ), and low-lying frontier energy levels. Consequently, the new dimerized acceptor seamlessly integrates as the third component into ternary OSCs, enhancing electron transporting properties, reducing non-radiative voltage loss, and elevating open-circuit voltage. These merits have enabled the ternary OSCs to show an exceptional efficiency of 19.07%, a marked improvement compared to the 17.6% attained in binary OSCs. More importantly, the high Tg exhibited by the pyrene-fused electron acceptor helps to stabilize the morphology of the photoactive layer thermal-treated at 70 °C, retaining 88.7% efficiency over 600 hours. For comparison, binary OSCs experience a decline to 73.7% efficiency after the same duration. These results indicate that the "butterfly-like" design and the incorporation of a pyrene unit is a promising strategy in the development of dimerized electron acceptors for OSCs.

Ethylenedioxythiophene-Based Small Molecular Donor with Multiple Conformation Locks for Organic Solar Cells with Efficiency of 19.3 .

Ternary organic solar cells (T-OSCs) represent an efficient strategy for enhancing the performance of OSCs. Presently, the majority of high-performance T-OSCs incorporates well-established Y-acceptors or donor polymers as the third component. In this study, a novel class of conjugated small molecules has been introduced as the third component, demonstrating exceptional photovoltaic performance in T-OSCs. This innovative molecule comprises ethylenedioxythiophene (EDOT) bridge and 3-ethylrhodanine as the end group, with the EDOT unit facilitating the creation of multiple conformation locks. Consequently, the EDOT-based molecule exhibits two-dimensional charge transport, distinguishing it from the thiophene-bridged small molecule, which displays fewer conformation locks and provides one-dimensional charge transport. Furthermore, the robust electron-donating nature of EDOT imparts the small molecule with cascade energy levels relative to the electron donor and acceptor. As a result, OSCs incorporating the EDOT-based small molecule as the third component demonstrate enhanced mobilities, yielding a remarkable efficiency of 19.3 %, surpassing the efficiency of 18.7 % observed for OSCs incorporating thiophene-based small molecule as the third component. The investigations in this study underscore the excellence of EDOT as a building block for constructing conjugated materials with multiple conformation locks and high charge carrier mobilities, thereby contributing to elevated photovoltaic performance in OSCs.

Noncovalent π-stacked robust topological organic framework.

Organic frameworks (OFs) offer a novel strategy for assembling organic semiconductors into robust networks that facilitate transport, especially the covalent organic frameworks (COFs). However, poor electrical conductivity through covalent bonds and insolubility of COFs limit their practical applications in organic electronics. It is known that the two-dimensional intralayer π∙∙∙π transfer dominates transport in organic semiconductors. However, because of extremely labile inherent features of noncovalent π∙∙∙π interaction, direct construction of robust frameworks via noncovalent π∙∙∙π interaction is a difficult task. Toward this goal, we report a robust noncovalent π∙∙∙π interaction-stacked organic framework, namely πOF, consisting of a permanent three-dimensional porous structure that is held together by pure intralayer noncovalent π∙∙∙π interactions. The elaborate porous structure, with a 1.69-nm supramaximal micropore, is composed of fully conjugated rigid aromatic tetragonal-disphenoid-shaped molecules with four identical platforms. πOF shows excellent thermostability and high recyclability and exhibits self-healing properties by which the parent porosity is recovered upon solvent annealing at room temperature. Taking advantage of the long-range π∙∙∙π interaction, we demonstrate remarkable transport properties of πOF in an organic-field-effect transistor, and the mobility displays relative superiority over the traditional COFs. These promising results position πOF in a direction toward porous and yet conductive materials for high-performance organic electronics.

Non-Covalent Interactions between Polyvinyl Chloride and Conjugated Polymers Enable Excellent Mechanical Properties and High Stability in Organic Solar Cells.

The incorporation of insulating polymers into conjugated polymers has been widely explored as a strategy to improve mechanical properties of flexible organic electronics. However, phase separation due to the immiscibility of these polymers has limited their effectiveness. In this study, we report the discovery of multiple non-covalent interactions that enhances the miscibility between insulating and conjugated polymers, resulting in improved mechanical properties. Specifically, we have added polyvinyl chloride (PVC) into the conjugated polymer PM6 and observed a significant increase in solution viscosity, indicative of favorable miscibility between these two polymers. This phenomenon has been rarely observed in other insulating/conjugated polymer composites. Thin films of PM6/PVC exhibit a much-improved crack-onset strain of 19.35 %, compared to 10.12 % for pristine PM6 films. Analysis reveal that a "cyclohexyl-like" structure formed through dipole-dipole interactions and hydrogen bonding between PVC and PM6 acted as a cross-linking site in the thin films, leading to improved mechanical properties. Moreover, PM6/PVC blend films have demonstrated excellent thermal and bending stability when applied as an electron donor in organic solar cells. These findings provide new insights into non-covalent interactions that can be utilized to enhance the properties of conjugated polymers and may have potential applications in flexible organic electronics.

Double-Cable Conjugated Polymers with Pendant Rylene Diimides for Single-Component Organic Solar Cells.

ConspectusConjugated polymers for application in organic solar cells (OSCs) have been developed from poly(phenylenevinylene) to poly(3-hexylthiophene) and then to "donor-acceptor" structures, providing power conversion efficiencies (PCEs) over 18% when blending with the electron acceptor as a two-component photoactive layer. Besides, graft-structural double-cable conjugated polymers that use an electron donor as conjugated backbones and an electron acceptor as pendant side units are one kind of conjugated polymer, in which charge carriers are generated in a single polymer. Therefore, double-cable conjugated polymers can be used as a single photoactive layer in single-component OSCs (SCOSCs). The covalently linked electron donor and acceptor enable double-cable polymers to maintain stable microstructures during long-term operation compared to two-component systems, which is very important for OSCs toward large-area applications. However, SCOSCs based on double-cable conjugated polymers provided PCEs below 3% in a long period, which is lagging far behind PCEs of two-component OSCs. The key reason for this is the limited number of chemical structures and the difficulty to tune the morphology in these polymers.In this Account, we provide an overview about our efforts on developing new double-cable conjugated polymers with rylene diimides as side units, and how to realize high PCEs in SCOSC devices. The studies start from developing a "functionalization-polymerization" method to synthesize the polymers containing rylene diimide acceptors, so that large amounts of double-cable conjugated polymers with distinct physical and electrochemical properties were obtained. Then, we will discuss how to control the nanophase separation in the crystalline region and optimize the miscibility in the amorphous region of double-cable polymers, simultaneously facilitating exciton dissociation and charge transport. With these efforts, a high PCE of 8.4% has been obtained, representing the record PCE in SCOSCs. In addition, the physical process and the stability of SCOSCs will be discussed. We hope that this account will inspire many innovative studies in this field and push the PCEs of SCOSCs to a new stage.

Insulating Polymers as Additives to Bulk-Heterojunction Organic Solar Cells: The Effect of Miscibility.

Adding insulating polymers to conjugated polymers is an efficient strategy to tailor their mechanical properties for flexible organic electronics. In this work, we selected two insulating polymers as additives for high-performance photoactive layers and investigated the mechanical and photovoltaic properties in organic solar cells (OSCs). The insulating polymers were found to reduce the electron mobilities in the photoactive layers, and hence the power conversion efficiencies were significantly decreased. More importantly, we found that the insulating polymers exhibited negative effect on the mechanical properties of the photoactive layers, with reduced Young's modulus and low crack onset strains. Further studies revealed that the insulating polymers had poor miscibility with the photoactive layers, providing large domains and more cavities in blend thin films, which act as negative effect for the tensile test. The studies indicate that rational selection of insulating polymers, especially enhancing the non-covalent interaction with the photoactive layers, will be critically important for the stretchable OSCs.

An Organic-Inorganic Hybrid Electrolyte as a Cathode Interlayer for Efficient Organic Solar Cells.

An organic-inorganic hybrid electrolyte based on a cyclic Ti-oxo cluster as the inorganic core and naphthalene-based organic ammonium bromide salts as the electrolyte was developed with easy synthesis and low cost. The new hybrid electrolyte exhibits excellent solubility in methanol, aligned work function, good conductivity, and amorphous state in thin film, enabling its successful application as a cathode interlayer in organic solar cells with a high power conversion efficiency of 17.19 %. This work demonstrates that the hybrid electrolytes are a new kind of semiconductor, exhibiting promising applications in organic electronics.

Rational Functionalization of a C70 Buckybowl To Enable a C70:Buckybowl Cocrystal for Organic Semiconductor Applications.

Fullerene fragments, referred to as buckybowls, are garnering interest due to their distinctive molecular shapes and optoelectronic properties. Here, we report the synthesis and characterization of a novel C70 subunit, diindeno[4,3,2,1-fghi:4',3',2',1'-opqr]perylene, that is substituted with either triethylsilyl(TES)-ethynyl or 2,4,6-triisopropylphenyl groups at the meta-positions. The resulting compounds (1 and 2) display a bowl-to-bowl inversion at room temperature. Notably, the substituent groups on the meta-positions alter both the geometric and the electronic properties as well as the crystal packing of the buckybowls. In contrast to the 2,4,6-triisopropylphenyl groups in 2, the TES-ethynyl groups in 1 lead to enhanced bond length alternation, resulting in weaker aromaticity of the six-membered rings of the buckybowl skeleton. 1 forms one-dimensional (1D) concave-in-convex stacking columns, and when 1 is blended with C70, the buckybowls encapsulate C70 and result in two-dimensional cocrystals. Organic field-effect transistor (OFET) measurements demonstrate that 1 displays a hole mobility of 0.31 cm2 V-1 s-1, and the 1-C70 cocrystal exhibits ambipolar transport characteristics with electron and hole mobilities approaching 0.40 and 0.07 cm2 V-1 s-1, respectively. This work demonstrates the potential of buckybowls for the development of organic semiconductors.

Dodecatwistarene Imides with Zigzag-Twisted Conformation for Organic Electronics.

1D nonplanar graphene nanoribbons generally have three possible conformers: helical, zigzag, and mixed conformations. Now, a kind of 1D nonplanar graphene nanoribbon, namely dodecatwistarene imides featuring twelve linearly fused benzene rings, was obtained by bottom-up synthesis of palladium-catalyzed Stille coupling and C-H activation. Single-crystal X-ray diffraction analyses revealed that it displays a zigzag-twisted conformation caused by steric hindrance between imide groups and neighboring annulated benzene rings with the pendulum angle of 53°. This conformation is very stable and could not convert into other conformations even when heated up to 250 °C for 6 h. Despite of the highly twisted topology, organic field-effect transistor based on it exhibits electron mobility up to 1.5 cm2 V-1 s-1 after annealing.

Miscibility-Controlled Phase Separation in Double-Cable Conjugated Polymers for Single-Component Organic Solar Cells with Efficiencies over 8 .

A record power conversion efficiency of 8.40 % was obtained in single-component organic solar cells (SCOSCs) based on double-cable conjugated polymers. This is realized based on exciton separation playing the same role as charge transport in SCOSCs. Two double-cable conjugated polymers were designed with almost identical conjugated backbones and electron-withdrawing side units, but extra Cl atoms had different positions on the conjugated backbones. When Cl atoms were positioned at the main chains, the polymer formed the twist backbones, enabling better miscibility with the naphthalene diimide side units. This improves the interface contact between conjugated backbones and side units, resulting in efficient conversion of excitons into free charges. These findings reveal the importance of charge generation process in SCOSCs and suggest a strategy to improve this process: controlling miscibility between conjugated backbones and aromatic side units in double-cable conjugated polymers.

Corannurylene Pentapetalae.

Despite the great advances in the synthesis of diverse nonplanar graphenoids, investigations into the relationship between structural features and intermolecular interactions still present significant challenges. Herein, the novel nonplanar graphenoid structure, corannurylene pentapetalae (CRP), obtained via bottom-up syntheses of hybridization between perylene diimide (PDI) planar fragments and a corannulene curved core, is presented. Single crystal studies reveal a D5-symmetric as well as a C2-symmetric graphenoid corannurylene pentapetalae. The D5-symmetric structure has a unique honeycomb lattice with two chiral honeycomb layers alternately stacked, whereas the C2-symmetric CRP forms dimer units via π-π stacking. Transistor devices demonstrate that, without any π-π stacking, the honeycomb lattice of the D5-symmetric CRP has the potential to also facilitate electron transport.

Hexacene Diimides.

Acene imides are expected to possess smaller band gaps than homologous acenes while maintaining good solubility and stability. However, the design and synthesis of large acene imides are still a big challenge. Herein, we report a one-pot synthesis of hexacene diimides (HDI) by double aromatic annulation between zirconabenzocyclopentene and tetrabrominated naphthalene diimides. HDIs with branched alkyl chains exhibit very good solubility, stability, and much smaller band gaps than hexacene. Organic field-effect transistors (OFETs) based on HDI microribbons exhibit excellent ambipolar transport behavior with the highest electron mobility of 2.17 cm2 V-1 s-1 and hole mobility of 0.30 cm2 V-1 s-1 under ambient conditions.

Electron-Transporting Bis(heterotetracenes) with Tunable Helical Packing.

A novel kind of electron-deficient bis(heterotetracenes) namely perylenotetrathiophenediimides (PTTIs) involving double S-hetero[5]helicene diimides, is developed by a fourfold thienannulation route via ortho-functionalization of perylene diimides (PDIs). PTTIs exhibit significantly red-shifted absorption capacity with lowest-energy transition maxima beyond 700 nm and narrowed HOMO-LUMO energy gaps. Through delicately tuning the side-chain substitution, the distorted propeller-like framework could self-assemble into unprecedented 1D helical π-stacking structures with short π-π contacts and rich nonbonding interactions from alternating arrangements of P/M enantiomeric couples or tetrads. Excellent electron transporting efficiency in racemate PTTI crystals with 0.40 cm2 V-1 s-1 for 5 a and 0.90 cm2 V-1 s-1 for 5 b, was witnessed in single-crystalline transistors, signifying the prospects of the chiral π-helix in optoelectronic applications.

Three-Bladed Rylene Propellers with Three-Dimensional Network Assembly for Organic Electronics.

Two kinds of conjugated C3-symmetric perylene dyes, namely, triperylene hexaimides (TPH) and selenium-annulated triperylene hexaimides (TPH-Se), are efficiently synthesized. Both TPH and TPH-Se have broad and strong absorption in the region 300-600 nm together with suitable LUMO levels of about -3.8 eV. Single-crystal X-ray diffraction studies show that TPH displays an extremely twisted three-bladed propeller configuration and a unique 3D network assembly in which three PBI subunits in one TPH molecule have strong π-π intermolecular interactions with PBI subunits in neighboring molecules. The integration of selenophene to TPH endows TPH-Se with a more distorted propeller configuration and a more compact 3D network assembly due to the Se···O interactions. A single-crystal transistor confirms that both TPH and TPH-Se possess good electron-transport ability. TPH and TPH-Se acceptor-based solar cells show high power conversion efficiency of 8.28% and 9.28%, respectively, which mainly results from the combined properties of broad and strong absorption ability, appropriate LUMO level, desirable aggregation, high electron mobility, and good film morphology with the polymer donor.

Thermally sensitive self-assembly of glucose-functionalized tetrachloro-perylene bisimides: from twisted ribbons to microplates.

Chiral supramolecular structures are becoming increasingly attractive for their specific molecular arrangements, exceptional properties, and promising applications in chiral sensing and separation. However, constructing responsive chiral supramolecular structures remains a great challenge. Here, glucose-functionalized tetrachloro-perylene bisimides (GTPBIs) with thermally sensitive self-assembly behaviors are designed and synthesized. In a methanol/water mixture, GTPBIs self-assembled into twisted ribbons and microplates at 4 and 25 °C, respectively. Furthermore, the ribbon structure was metastable and could transform into microplates when the temperature was increased from 4 to 25 °C. Transmission electron microscopy (TEM) was used to track the evolution of morphology and study the assembly mechanisms of correponding nanostructures at different time intervals. The supramolecular structures were characterized with various techniques, including circular dichroism, TEM, scanning electron microscopy, atomic force microscopy, ultraviolet-visible absorption, and fluorescence spectra. This study provides insight into controlling molecular parameters and assembly conditions to construct chiral supramolecular structures.

NiOx Nanoparticles Hole-Transporting Layer Regulated by Ionic Radius-Controlled Doping and Reductive Agent for Organic Solar Cells with Efficiency of 19.18.

Nickel oxide (NiOx ) has garnered considerable attention as a prospective hole-transporting layer (HTL) in organic solar cells (OSCs), offering a potential solution to the stability challenges posed by traditional HTL, PEDOT:PSS, arising from acidity and hygroscopicity. Nevertheless, the lower work function (WF) of NiOx relative to donor polymers reduces charge injection efficiency in OSCs. Herein, NiOx nanoparticles are tailored through rare earth doping to optimize WF and the impact of ionic radius on their electronic properties is explored. Lanthanum (La3+ ) and yttrium (Y3+ ) ions, with larger ionic radii, are effectively doped at 1 and 3%, respectively, while scandium (Sc3+ ), with a smaller ion radius, allows enhanced 5% doping. Higher doping ratios significantly enhance WF of NiOx . A 5% Sc3+ doping raises WF to 4.99 eV from 4.77 eV for neat NiOx while maintaining high conductivity. Consequently, using 5% Sc-doped NiOx as HTL improves the power conversion efficiency (PCE) of OSCs to 17.13%, surpassing the 15.64% with the neat NiOx . Further enhancement to 18.42% is achieved by introducing the reductant catechol, outperforming the PEDOT:PSS-based devices. Additionally, when employed in a ternary blend system (D18:N3:F-BTA3), an impressive PCE of 19.18 % is realized, top-performing among reported OSCs utilizing solution-processed inorganic nanoparticles.

Carboxylating Elastomer via Thiol-Ene Click Reaction to Improve Miscibility with Conjugated Polymers for Mechanically Robust Organic Solar Cells with Efficiency of 19.

Incorporating flexible insulating polymers is a straightforward strategy to enhance the mechanical properties of rigid conjugated polymers, enabling their use in flexible electronic devices. However, maintaining electronic characteristics simultaneously is challenging due to the poor miscibility between insulating polymers and conjugated polymers. This study introduces the carboxylation of insulating polymers as an effective strategy to enhance miscibility with conjugated polymers via surface energy modulation and hydrogen bonding. The carboxylated elastomer, synthesized via a thiol-ene click reaction, closely matches the surface energy of the conjugated polymer. This significantly improves the mechanical properties, achieving a high crack-onset strain of 21.48%, surpassing that (5.93%) of the unmodified elastomer:conjugated polymer blend. Upon incorporating the carboxylated elastomer into PM6:L8-BO-based organic solar cells, an impressive power conversion efficiency of 19.04% is attained, which top-performs among insulating polymer-incorporated devices and outperforms devices with unmodified elastomer or neat PM6:L8-BO. The superior efficiency is attributed to the optimized microstructures and enhanced crystallinity for efficient and balanced charge transport, and suppressed charge recombination. Furthermore, flexible devices with 5% carboxylated elastomer exhibit superior mechanical stability, retaining ≈88.9% of the initial efficiency after 40 000 bending cycles at a 1 mm radius, surpassing ≈83.5% for devices with 5% unmodified elastomer.

13% Single-Component Organic Solar Cells based on Double-Cable Conjugated Polymers with Pendent Y-Series Acceptors.

Double-cable conjugated polymers with pendent electron acceptors, including fullerene, rylene diimides, and nonfused acceptors, have been developed for application in single-component organic solar cells (SCOSCs) with efficiencies approaching 10%. In this work, Y-series electron acceptors have been firstly incorporated into double-cable polymers in order to further improve the efficiencies of SCOSCs. A highly crystalline Y-series acceptor based on quinoxaline core and the random copolymerized strategy are used to optimize the ambipolar charge transport and the nanophase separation of the double-cable polymers. As a result, an efficiency of 13.02% is obtained in the random double-cable polymer, representing the highest performance in SCOSCs, while the regular double-cable polymer only provides a low efficiency of 2.75%. The significantly enhanced efficiencies are attributed to higher charge carrier mobilities, better ordering conjugated backbones and Y-series acceptors in random double-cable polymers.

Non-fused Polymerized Small-Molecule Acceptors with a Benzothiadiazole Core for All-Polymer Solar Cells.

Polymerized small-molecule acceptors (PSMAs) have made significant progress as the application in all-polymer solar cells (all-PSCs). Most PSMAs are constructed by near-infrared fused-ring electron acceptors via Stille polymerization, such as Y-series acceptors. However, very limited non-fused electron acceptors with simplified synthetic complexity have been used in PSMAs. In this work, two non-fused PSMAs with a benzothiadiazole (BT) core are developed for application in all-PSCs. The S-O non-covalent interaction and the regioregularity have been introduced to improve the crystallinity and charge transport properties. As a result, a high-power conversion efficiency (PCE) of 11.42% with an enhanced photocurrent has been obtained in these regioregular PBTO-γ-based solar cells, representing the highest PCEs based on non-fused PSMAs. The encouraging results will intrigue more design of non-fused PSMAs toward high-performance all-PSCs.