Correlating Aggregation Ability of Polymer Donors with Film Formation Kinetics for Organic Solar Cells with Improved Efficiency and Processability.

Film formation kinetics significantly impact molecular processability and power conversion efficiency (PCE) of organic solar cells. Here, two ternary random copolymerization polymers are reported, D18─N-p and D18─N-m, to modulate the aggregation ability of D18 by introducing trifluoromethyl-substituted pyridine unit at para- and meta-positions, respectively. The introduction of pyridine unit significantly reduces material aggregation ability and adjusts the interactions with acceptor L8-BO, thereby leading to largely changed film formation kinetics with earlier phase separation and longer film formation times, which enlarge fiber sizes in blend films and improve carrier generation and transport. As a result, D18─N-p with moderate aggregation ability delivers a high PCE of 18.82% with L8-BO, which is further improved to 19.45% via interface engineering. Despite the slightly inferior small area device performances, D18─N-m shows improved solubility, which inspires to adjust the ratio of meta-trifluoromethyl pyridine carefully and obtain a polymer donor D18─N-m-10 with good solubility in nonhalogenated solvent o-xylene. High PCEs of 13.07% and 12.43% in 1 cm2 device and 43 cm2 module fabricated with slot-die coating method are achieved based on D18─N-m-10:L8-BO blends. This work emphasizes film formation kinetics optimization in device fabrication via aggregation ability modulation of polymer donors for efficient devices.

Single Crystal Seed Induced Epitaxial Growth Stabilizes α-FAPbI3 in Perovskite Solar Cells.

FAPbI3 stands out as an ideal candidate for the photoabsorbing layer of perovskite solar cells (PSCs), showcasing outstanding photovoltaic properties. Nonetheless, stabilizing photoactive α-FAPbI3 remains a challenge due to the lower formation energy of the competitive photoinactive δ-phase. In this study, we employ tetraethylphosphonium lead tribromide (TEPPbBr3) single crystals as templates for the epitaxial growth of PbI2. The strategic use of TEPPbBr3 optimizes the evolution of intermediates and the crystallization kinetics of perovskites, leading to high-quality and phase-stable α-FAPbI3 films. The TEPPbBr3-modified perovskite exhibits optimized carrier dynamics, yielding a champion efficiency of 25.13% with a small voltage loss of 0.34 V. Furthermore, the target device maintains 90% of its initial PCE under maximum power point (MPP) tracking over 1000 h. This work establishes a promising pathway through single crystal seed based epitaxial growth for achieving satisfactory crystallization regulation and phase stabilization of α-FAPbI3 perovskites toward high-efficiency and stable PSCs.

Bis(benzoselenadiazol)ethane: A π-Extended Acceptor-Dimeric Unit for Ambipolar Polymer Transistors with Hole and Electron Mobilities Exceeding 10†cm2 V-1 s-1.

The lack of ambipolar polymers with balanced hole (µh) and electron mobilities (µe) >10 cm2 V-1 s-1 is the main bottleneck for developing organic integrated circuits. Herein, we show the design and synthesis of a π-extended selenium-containing acceptor-dimeric unit, namely benzo[c][1,2,5]selenadiazol-4-yl)ethane (BBSeE), to address this dilemma. In comparison to its sulfur-counterpart, BBSeE demonstrates enlarged co-planarity, selective noncovalent interactions, polarized Se-N bond, and higher electron affinity. The successful stannylation of BBSeE offers a great opportunity to access acceptor-acceptor copolymer pN-BBSeE, which shows a narrower band gap, lower-lying lowest unoccupied molecular orbital level (-4.05 eV), and a higher degree of backbone planarity. Consequently, the pN-BBSeE-based organic transistors display an ideally balanced ambipolar transporting property with µh and µe of 10.65 and 10.72 cm2 V-1 s-1, respectively. To the best of our knowledge, the simultaneous µh/µe values >10.0 cm2 V-1 s-1 are the best performances ever reported for ambipolar polymers. In addition, pN-BBSeE shows an excellent shelf-storage stability, retaining over 85 % of the initial mobility values after two months storage. Our study demonstrates the π-extended acceptor-dimeric BBSeE is a promising acceptor building block for constructing high-performance ambipolar polymers applied in next-generation organic integrated circuit.

Simple-Structured Acceptor with Highly Interconnected Electron-Transport Pathway Enables High-Efficiency Organic Solar Cells.

Achieving desirable charge-transport highway is of vital importance for high-performance organic solar cells (OSCs). Here, it is shown how molecular packing arrangements can be regulated via tuning the alkyl-chain topology, thus resulting in a 3D network stacking and highly interconnected pathway for electron transport in a simple-structured nonfused-ring electron acceptor (NFREA) with branched alkyl side-chains. As a result, a record-breaking power conversion efficiency of 17.38% (certificated 16.59%) is achieved for NFREA-based devices, thus providing an opportunity for constructing low-cost and high-efficiency OSCs.

Correlating the Photovoltaic Performance and Stability of the All-Small-Molecule Organic Solar Cells to Their Intermixed Phases Determined by Concentration-Dependent Ultraviolet-Visible Absorption Spectroscopy.

In addition to the donor-acceptor nano phases, the intermixed phase within the organic blends is crucial for the photovoltaic performance and stability of the bulk-heterojunction organic solar cells (OSCs). Here, the intermixed phase of a representative M-PhS:BTP-eC9 all-small-molecule organic solar cell was investigated by a concentration-dependent ultraviolet-visible (UV-vis) absorption spectroscopy method, where a shift of the absorption maximum wavelength was measured for the acceptor component with the increase of the acceptor concentration. The blend ratios of the acceptor to the donor in the intermixed phase, corresponding to the critical concentration for the formation of the acceptor nanophase (CAP), were determined to be 0.35, 0.20, and 0.15 for the as-cast, thermal annealing (TA), and the combined TA and solvent vapor annealing films. These results indicated that M-PhS and BTP-eC9 are kinetically well intermixed during spin coating, whereas TA and the following solvent annealing promote the crystallization of BTP-eC9 molecules out of the intermixed phase. The photovoltaic performance of the M-PhS:BTP-eC9 cells with different blend ratios was investigated. The formation of the BTP-eC9 nano phase in the blend film leads to stable VOC and fast increased JSC, which can be understood by the reduction of bimolecular charge recombination and the formation of electron transporting pathways within the photoactive layer. Similarly, the critical concentration for the formation of the donor phase was estimated to be 0.15 by measuring the stabilized VOC and increased JSC values of the cells with different donor blending ratios. More importantly, after a fast "burn-in" thermal degradation, the M-PhS:BTP-eC9 cell showed excellent thermal stability aging at 85 °C for over 1128 h, which is in good accordance with the unchanged intermixed phases measured by the UV-vis spectra of the annealed films. The current work demonstrates the feasibility of the spectroscopy method to investigate the intermixed phases for organic bulk-heterojunction solar cells and proves that all-small-molecule solar cells can be intrinsically very stable.

High-Performance Organic Narrow Dual-Band Circular Polarized Light Detection for Encrypted Communications and Color Imaging.

Conventional circularly polarized light (CPL) detectors necessitate several optical elements, posing difficulties in achieving miniature and integrated devices. Recently developed organic CPL detectors require no additional optical elements but usually suffer from low detectivity or low asymmetry factor (g-factor). Here, an organic CPL detector with excellent detectivity and a high g-factor is fabricated. By employing an inverted quasi-planar heterojunction (IPHJ) structure and incorporating an additional liquid crystal film, a CPL detector with an outstanding g-factor of 1.62 is developed. Unfavorable charge injection is effectively suppressed by the IPHJ structure, which reduces the dark current of the organic photodetector. Consequently, a left CPL detectivity of 6.16 × 1014 Jones at 640 nm is realized, surpassing all of the latest photodiode-type CPL detectors. Adopting a liquid crystal film with adjustable wavelengths of selectively reflected light, the hybrid device achieves narrow dual-band CPL detection, varying from 530 to 640 nm, with a half-maximum full width below 90 nm. Notably, the device achieves excellent stability of 260 000 on/off cycles without attenuation. To the best of the authors' knowledge, all these features have rarely been reported in previous work. The CPL detector arrays are also demonstrated for encrypted communications and color imaging.

Tailoring the Position of Ester Group on N-Alkyl Chains of Benzotriazole-based Small Molecule Acceptors for High-Performance Organic Solar Cells.

Side chain engineering plays a vital role in exploring high-performance small molecule acceptors (SMAs) for organic solar cells (OSCs). In this work, we designed and synthesized a series of A-DA'D-A type SMAs by introducing different N-substituted alkyl and ester alkyl side chains on benzotriazole (BZ) central unit and aimed to investigate the effect of different ester substitution positions on photovoltaic performances. All the new SMAs with ester groups exhibit lower the lowest unoccupied molecular orbital (LUMO) energy levels and more blue-shifted absorption, but relatively higher absorption coefficients than alkyl chain counterpart. After blending with the donor PM6, the ester side chain-based devices demonstrate enhanced charge mobility, reduced amorphous intermixing domain size and long-lived charge transfer state compared to the alkyl chain counterpart, which are beneficial to achieve higher short-circuit current density (Jsc ) and fill factor (FF), simultaneously. Thereinto, the PM6 : BZ-E31 based device achieves a higher power conversion efficiency (PCE) of 18.33 %, which is the highest PCE among the OSCs based on the SMAs with BZ-core. Our work demonstrated the strategy of ester substituted side chain is a feasible and effective approach to develop more efficient SMAs for OSCs.

Precisely Manipulating Molecular Packing via Tuning Alkyl Side-Chain Topology Enabling High-Performance Nonfused-Ring Electron Acceptors.

In the development of high-performance organic solar cells (OSCs), the self-organization of organic semiconductors plays a crucial role. This study focuses on the precisely manipulation of molecular assemble via tuning alkyl side-chain topology in a series of low-cost nonfused-ring electron acceptors (NFREAs). Among the three NFREAs investigated, DPA-4, which possesses an asymmetric alkyl side-chain length, exhibits a tight packing in the crystal and high crystallinity in the film, contributing to improved electron mobility and favorable film morphology for DPA-4. As a result, the OSC device based on DPA-4 achieves an excellent power conversion efficiency of 16.67 %, ranking among the highest efficiencies for NFREA-based OSCs.

Stereoisomeric Non-Fullerene Acceptors-Based Organic Solar Cells.

Chiral alkyl chains are ubiquitously observed in organic semiconductor materials and can regulate solution processability and active layer morphology, but the effect of stereoisomers on photovoltaic performance has rarely been investigated. For the racemic Y-type acceptors widely used in organic solar cells, it remains unknown if the individual chiral molecules separate into the conglomerate phase or if racemic phase prevails. Here, the photovoltaic performance of enantiomerically pure Y6 derivatives, (S,S)/(R,R)-BTP-4F, and their chiral mixtures are compared. It is found that (S,S) and (R,R)-BTP-4F molecule in the racemic mixtures tends to interact with its enantiomer. The racemic mixtures enable efficient light harvesting, fast hole transfer, and long polaron lifetime, which is conducive to charge generation and suppresses the recombination losses. Moreover, abundant charge diffusion pathways provided by the racemate contribute to efficient charge transport. As a result, the racemate system maximizes the power output and minimizes losses, leading to a higher efficiency of 18.16% and a reduced energy loss of 0.549 eV, as compared to the enantiomerically pure molecules. This study demonstrates that the chirality of non-fullerene acceptors should receive more attention and be designed rationally to enhance the efficiency of organic solar cells.

Strengthening the Hetero-Molecular Interactions in Giant Dimeric Acceptors Enables Efficient Organic Solar Cells.

Giant dimeric acceptor (G-Dimer) is becoming one of the most promising organic solar cell (OSC) materials because of its definite structure, long-term stability, and high efficiency. Strengthening the hetero-molecular interactions by monomer modification greatly influences the morphology and thus the device performance, but lacks investigation. Herein, two novel quinoxaline core-based G-Dimers, Dimer-QX and Dimer-2CF, are synthesized. By comparing trifluoromethyl-substituted Dimer-2CF and non-substituted Dimer-QX, the trifluoromethylation effect on the G-Dimer is investigated and revealed. The trifluoromethyl with strong electronegativity increases electrostatic potential and reduces surface energy of the G-Dimer, weakening the homo-molecular ordered packing but reinforcing the hetero-molecular interaction with the donor. The strong hetero-molecular interaction suppresses the fast assembly during the film formation, facilitating small domains with ordered molecular packing in the blend, which is a trade-off in conventional morphology control. Together with favorable vertical phase separation, efficient charge generation, and reduced bimolecular recombination are concurrently obtained. Hence, the Dimer-2CF-based OSCs obtain a cutting-edge efficiency of 19.02% with fill factor surpassing 80%, and an averaged extrapolated T80 of ≈12 000 h under continuous 80 °C heating. This study emphasizes the importance of hetero-molecular interaction and trifluoromethylation strategy, providing a facile strategy for designing highly efficient and stable OSC materials.

Flexible Organic Photovoltaic-Powered Hydrogel Bioelectronic Dressing With Biomimetic Electrical Stimulation for Healing Infected Diabetic Wounds.

Electrical stimulation (ES) is proposed as a therapeutic solution for managing chronic wounds. However, its widespread clinical adoption is limited by the requirement of additional extracorporeal devices to power ES-based wound dressings. In this study, a novel sandwich-structured photovoltaic microcurrent hydrogel dressing (PMH dressing) is designed for treating diabetic wounds. This innovative dressing comprises flexible organic photovoltaic (OPV) cells, a flexible micro-electro-mechanical systems (MEMS) electrode, and a multifunctional hydrogel serving as an electrode-tissue interface. The PMH dressing is engineered to administer ES, mimicking the physiological injury current occurring naturally in wounds when exposed to light; thus, facilitating wound healing. In vitro experiments are performed to validate the PMH dressing's exceptional biocompatibility and robust antibacterial properties. In vivo experiments and proteomic analysis reveal that the proposed PMH dressing significantly accelerates the healing of infected diabetic wounds by enhancing extracellular matrix regeneration, eliminating bacteria, regulating inflammatory responses, and modulating vascular functions. Therefore, the PMH dressing is a potent, versatile, and effective solution for diabetic wound care, paving the way for advancements in wireless ES wound dressings.

Small Molecule Donors Design Rules for Non-Halogen Solvent Fabricated Organic Solar Cells.

Compared with all-small-molecule (ASM) and other types of organic solar cells (OSCs), the small molecule donor:polymer acceptor (SMD:PA) OSCs develop much slower due to the lack of material matching rules. Herein, by changing the end-cap substituent of the small molecule donor from ethyl (MPhS-C2) to benzyl (MPhS-Ph), the different selection rules of donor properties and thermal annealing (TA) treatment between the ASM and the SMD:PA system under tetrahydrofuran processing are thoroughly investigated. Therefore, MPhS-Ph exhibits more ordered molecular packing, leading to better adaptability in the SMD:PA system without TA; while the inferior molecular packing of MPhS-C2 after spin-coating performs better in the ASM system with TA. Whether spin-coating or TA process dominates morphological optimization also dominates their energy loss. Therefore, the MPhS-Ph:PYF-T-o and MPhS-C2:BTP-eC9 based devices achieve the highest power conversion efficiency (PCE) of 12.1% and 15.7%, respectively, both of which are cutting-edge PCEs in their own type of OSCs fabricated by non-halogen solvent. This result suggests that intrinsic strong crystallization independent of the thermal drive is hoped in SMD:PA-OSCs, while high miscibility after spin-coating and proper assembly under thermal drive is expected in ASM-OSCs, providing deep understanding and guidance in highly efficient materials design rules in both ASM-OSCs and SMD:PA-OSCs.

The central core size effect in quinoxaline-based non-fullerene acceptors for high VOC organic solar cells.

For organic solar cells (OSCs), obtaining a high open circuit voltage (VOC) is often accompanied by the sacrifice of the circuit current density (JSC) and filling factor (FF), and it is difficult to strike a balance between VOC and JSC × FF. The trade-off of these parameters is often the critical factor limiting the improvement of the power conversion efficiency (PCE). Extended backbone conjugation and side chain engineering of non-fullerene acceptors (NFAs) are effective strategies to optimize the performance of OSCs. Herein, based on the quinoxaline central core and branched alkyl chains at the ß position of the thiophene unit, we designed and synthesized three NFAs with different sized cores. Interestingly, Qx-BO-3 with a smaller central core showed better planarity and more appropriate crystallinity. As a result, PM6:Qx-BO-3-based devices obtained more suitable phase separation, more efficient exciton dissociation, and charge transport properties. Therefore, the OSCs based on PM6:Qx-BO-3 yielded an outstanding PCE of 17.03%, significantly higher than the devices based on PM6:Qx-BO-1 (10.57%) and PM6:Qx-BO-2 (11.34%) although the latter two devices have lower VOC losses. These results indicated that fine-tuning the central core size can effectively optimize the molecular geometry of NFAs and the film morphology of OSCs. This work provides an effective method for designing high-performance NFA-OSCs with high VOCs.

Optimizing the Structure and Electrochemical Properties of Benzoquinone-Embedded COF via Heat Treatment for a High-Energy Organic Cathode.

A benzoquinone-embedded aza-fused covalent organic framework (BQ COF) with the maximum loading of redox-active units per molecule was employed as a cathode for lithium-ion batteries (LIBs) to achieve high energy and power densities. The synthesis was optimized to obtain high crystallinity and improved electrochemical performance. Synthesis at moderate temperature followed by a solid-state reaction was found to be particularly useful for achieving good crystallinity and the activation of the COF. When used as a cathode for LIBs, very high discharge capacities of 513, 365, and 234 mAh g-1 were obtained at 0.1C, 1C, and 10C, respectively, showing a remarkable rate performance. More than 70% of the initial capacity was retained after 1000 cycles when the cathode was investigated for cyclic performance at 2.5C. We demonstrated that a straightforward heat treatment led to enhanced crystallinity, an optimized structure, and favorable morphology, resulting in enhanced electrode kinetics and an improved overall electrochemical behavior. A comparative study was conducted involving an aza-fused COF lacking carbonyl groups (TAB COF) and a small molecule containing phenazine and carbonyl (3BQ), providing useful insights into new material design. A full cell was assembled with graphite as the anode to assess the commercial feasibility of BQ COF, and a discharge capacity of 240 mAh g-1 was obtained at 0.5C. Furthermore, a pouch-type cell with a high discharge capacity and an excellent rate performance was assembled, demonstrating the practical applicability of our designed cathode. Considering the entire mass of the working electrode, a specific energy density of 492 Wh kg-1 and a power density of 492 W kg-1 were achieved at the high current density of 1C, which are comparable to those of commercially available cathodes. These results highlight the promise of organic electrode materials for next-generation lithium-ion batteries. Furthermore, this study provides a systematic approach for simultaneously designing organic materials with high power and energy densities.

Optimized Crystal Framework by Asymmetric Core Isomerization in Selenium-Substituted Acceptor for Efficient Binary Organic Solar Cells.

Both the regional isomerization and selenium-substitution of the small molecular acceptors (SMAs) play significant roles in developing efficient organic solar cells (OSCs), while their synergistic effects remain elusive. Herein, we developed three isomeric SMAs (S-CSeF, A-ISeF, and A-OSeF) via subtly manipulating the mono-selenium substituted position (central, inner, or outer) and type of heteroaromatic ring on the central core by synergistic strategies for efficient OSCs, respectively. Crystallography of asymmetric A-OSeF presents a closer intermolecular π-π stacking and more ordered 3-dimensional network packing and efficient charge-hopping pathways. With the successive out-shift of the mono-selenium substituted position, the neat films give a slightly wider band gap and gradually higher crystallinity and electron mobility. The PM1 : A-OSeF afford favourable fibrous phase separation morphology with more ordered molecular packing and efficient charge transportation compared to the other two counterparts. Consequently, the A-OSeF-based devices achieve a champion efficiency of 18.5 %, which represents the record value for the reported selenium-containing SMAs in binary OSCs. Our developed precise molecular engineering of the position and type of selenium-based heteroaromatic ring of SMAs provides a promising synergistic approach to optimizing crystal stacking and boosting top-ranked selenium-containing SMAs-based OSCs.

Concretized structural evolution supported assembly-controlled film-forming kinetics in slot-die coated organic photovoltaics.

Bulk-heterojunction structured small-area organic solar cells are approaching 20% power conversion efficiency, but the blurred film-forming kinetics in the fabrication of large-area devices causes significant PCE loss and restrains the potential of commercialization. Such blurring came from insufficient knowledge of structural evolution during the film-forming process. Here, we concretize the evolution process with structures detailed to the submolecular level by comprehensive investigations of in-situ UV-vis spectroscopy, Atomic Force Microscope, Grazing Incident Wide Angle X-ray Scattering, and molecular dynamic simulation. With such hierarchical structural knowledge, assembly-controlled film-forming kinetics is proposed to explain the whole picture. Such assembly is determined by molecule configuration and can be tuned via external conditions. Understanding this kinetics will contribute to screening large-area device fabrication conditions, and the detailed structural knowledge could inspire the future design of novel photovoltaic materials that are intrinsically excellent in large-area device fabrications.

Central Core Substitutions and Film-Formation Process Optimization Enable Approaching 19% Efficiency All-Polymer Solar Cells.

Molecular interactions and film-formation processes greatly impact the blend film morphology and device performances of all-polymer solar cells (all-PSCs). Molecular structure, such as the central cores of polymer acceptors, would significantly influence this process. Herein, the central core substitutions of polymer acceptors are adjusted and three quinoxaline (Qx)-fused-core-based materials, PQx1, PQx2, and PQx3 are synthesized. The molecular aggregation ability and intermolecular interaction are systematically regulated, which subsequently influence the film-formation process and determine the resulting blend film morphology. As a result, PQx3, with favorable aggregation ability and moderate interaction with polymer donor PM6, achieves efficient all-PSCs with a high power conversion efficiency (PCE) of 17.60%, which could be further improved to 18.06% after carefully optimizing device annealing and interface layer. This impressive PCE is one of the highest values for binary all-PSCs based on the classical polymer donor PM6. PYF-T-o is also involved in promoting light utilization, and the resulting ternary device shows an impressive PCE of 18.82%. In addition, PM6:PQx3-based devices exhibit high film-thickness tolerance, superior stability, and considerable potential for large-scale devices (16.23% in 1 cm2 device). These results highlight the importance of structure optimization of polymer acceptors and film-formation process control for obtaining efficient and stable all-PSCs.

Co-Intercalation-Free Ether-Based Weakly Solvating Electrolytes Enable Fast-Charging and Wide-Temperature Lithium-Ion Batteries.

Ether-based electrolytes are competitive choices to meet the growing requirements for fast-charging and low-temperature lithium-ion batteries (LIBs) due to the low viscosity and low melting point of ether solvents. Unfortunately, the graphite (Gr) electrode is incompatible with commonly used ether solvents due to their irreversible co-intercalation into Gr interlayers. Here, we propose cyclopentyl methyl ether (CPME) as a co-intercalation-free ether solvent, which contains a cyclopentane group with large steric hindrance to obtain weakly solvating power with Li+ and a wide liquid-phase temperature range (-140 to +106 °C). A weakly solvating electrolyte (WSE) based on CPME and fluoroethylene carbonate (FEC) cosolvents can simultaneously achieve fast desolvation ability and high ionic conductivity, which also induces a LiF-rich solid electrolyte interphase (SEI) on the Gr anode. Therefore, the Gr/Li half-cell with this WSE can deliver outstanding rate capability, stable cycling performance, and high specific capacity (319 mAh g-1) at an ultralow temperature of -60 °C. Furthermore, a practical LiFePO4 (loading ≈25 mg cm-2)/Gr (loading ≈12 mg cm-2) pouch cell with this WSE also reveals outstanding rate capability and stable long-term cycling performance above 1000 cycles with a high Coulombic efficiency (≈99.9%) and achieves an impressive low-temperature application potential at -60 °C.