Volatilizable Solid Additive-Assisted Treatment Enables Organic Solar Cells with Efficiency over 18.8% and Fill Factor Exceeding 80

Optimizing Double-Fibril Network Morphology via Solid Additive Strategy Enables Binary All-Polymer Solar Cells with 19.50% Efficiency

Double-fibril network morphology (DFNM), in which the donor and the acceptor can self-assemble into a double-fibril structure, is beneficial for exciton dissociation and charge transport in organic solar cells. Herein, it is demonstrated that such DFNM can be constructed and optimized in all-polymer solar cells (all-PSCs) with the assistance of 2-alkoxynaphthalene volatile solid additives. It is revealed that the incorporation of 2-alkoxynaphthalene can induce a stepwise regulation in the aggregation of donor and acceptor molecules during film casting and thermal annealing processes. Through altering the alkoxy of 2-alkoxynaphthalene solid additives, both the intermolecular interactions and molecular miscibility with the host materials can be precisely tuned, which allows for the optimization of the molecular aggregation process and facilitation of molecular self-assembly, and thus leading to reinforced molecular packing and optimized DFNM. As a result, an unprecedented efficiency of 19.50% (certified as 19.1%) is obtained for 2-ethoxynaphthalene-processed PM6:PY-DT-X all-PSCs with excellent photostability (T80 = 1750 h). This work reveals that the optimization of DFNM via solid additive strategy is a promising avenue to boosting the performance of all-PSCs.

Tackling Energy Loss in Organic Solar Cells via Volatile Solid Additive Strategy

The energy loss induced open-circuit voltage (VOC) deficit hampers the rapid development of state-of-the-art organic solar cells (OSCs), therefore, it is extremely urgent to explore effective strategies to address this issue. Herein, a new volatile solid additive 1,4-bis(iodomethyl)cyclohexane (DIMCH) featured with concentrated electrostatic potential distribution is utilized to act as a morphology-directing guest to reduce energy loss in multiple state-of-art blend system, leading to one of highest efficiency (18.8%) at the forefront of reported binary OSCs. Volatile DIMCH decreases radiative/non-radiative recombination induced energy loss (ΔE2/ΔE3) by rationally balancing the crystallinity of donors and acceptors and realizing homogeneous network structure of crystal domain with reduced D-A phase separation during the film formation process and weakens energy disorder and trap density in OSCs. It is believed that this study brings not only a profound understanding of emerging volatile solid additives but also a new hope to further reduce energy loss and improve the performance of OSCs.

Processing the Interlayer and Optimizing the Active Layer by One-Step Dissolution Compensation in Organic Solar Cells

Optimized morphology of the active layer and electrode interface is critical for obtaining high-performance organic solar cells. However, achieving this typically involves a multifaceted, sequential process that renders outcomes unpredictable. Here, by exploiting the dissolution compensation, we propose a one-step method that integrates interlayer fabrication and a controllable morphology optimization. Taking an "out of the box" approach, we incorporate the good solvent of the active layer into the interlayer solution to act as dissolution compensation, breaking the orthogonal solvent principles to allow the morphology of the active layer to evolve to an optimized state while the interface layer is being processed. Using two commercially available material systems, D18:Y6 and D18:L8-BO, as examples, it was found that the JSC and fill factor (FF) device can be improved by using an appropriate ratio of the compensation solvent chloroform in the interlayer solution. As a result, the power conversion efficiency of the device based on the two state-of-the-art systems can be increased by about 7.5% (D18:Y6, from 17.04 to 18.31%; D18:L8-BO, from 17.97 to 19.31%). This one-step strategy has been shown to be universally applicable to other diverse systems and provides a simple yet reliable method for accurately depositing high-quality interlayers with an optimized active layer morphology in high-performance organic solar cells and other solution-processable organic electronics.

Crossbreeding Effect of Chalcogenation and Iodination on Benzene Additives Enables Optimized Morphology and 19.68% Efficiency of Organic Solar Cells

Volatile solid additives have attracted increasing attention in optimizing the morphology and improving the performance of currently dominated non-fullerene acceptor-based organic solar cells (OSCs). However, the underlying principles governing the rational design of volatile solid additives remain elusive. Herein, a series of efficient volatile solid additives are successfully developed by the crossbreeding effect of chalcogenation and iodination for optimizing the morphology and improving the photovoltaic performances of OSCs. Five benzene derivatives of 1,4-dimethoxybenzene (DOB), 1-iodo-4-methoxybenzene (OIB), 1-iodo-4-methylthiobenzene (SIB), 1,4-dimethylthiobenzene (DSB) and 1,4-diiodobenzene (DIB) are systematically studied, where the widely used DIB is used as the reference. The effect of chalcogenation and iodination on the overall property is comprehensively investigated, which indicates that the versatile functional groups provided various types of noncovalent interactions with the host materials for modulating the morphology. Among them, SIB with the combination of sulphuration and iodination enabled more appropriate interactions with the host blend, giving rise to a highly ordered molecular packing and more favorable morphology. As a result, the binary OSCs based on PM6:L8-BO and PBTz-F:L8-BO as well as the ternary OSCs based on PBTz-F:PM6:L8-BO achieved impressive high PCEs of 18.87%, 18.81% and 19.68%, respectively, which are among the highest values for OSCs.

Tight-binding model predicts exciton energetics and structure for photovoltaic molecules

Conjugated molecules and polymers are being designed as acceptor and donor materials for organic photovoltaic (OPV) cells. OPV performance depends on generation of free charge carriers through dissociation of excitons, which are electron-hole pairs created when a photon is absorbed. Here, we develop a tight-binding model to describe excitons on homo-oligomers, alternating co-oligomers, and a non-fullerene acceptor - IDTBR. We parameterize our model using density functional theory (DFT) energies of neutral, anion, cation, and excited states of constituent moieties. A symmetric molecule like IDTBR has two ends where an exciton can sit; but the product wavefunction approximation for the exciton breaks symmetry. So, we introduce a tight-binding model with full correlation between electron and hole, which allows the exciton to coherently explore both ends of the molecule. Our approach predicts optical singlet excitation energies for oligomers of varying length as well as IDTBR in good agreement with time-dependent DFT and spectroscopic results.

Printing and Coating Techniques for Scalable Organic Photovoltaic Fabrication

Within recent years, there has been an increased interest towards organic photovoltaics (OPVs), especially with their significant device performance reaching beyond 19% since 2022. With these advances in the device performance of laboratory-scaled OPVs, there has also been more attention directed towards using printing and coating methods that are compatible with large-scale fabrication. Though large-area (>100 cm2) OPVs have reached an efficiency of 15%, this is still behind that of laboratory-scale OPVs. There also needs to be more focus on determining strategies for improving the lifetime of OPVs that are suitable for scalable manufacturing, as well as methods for reducing material and manufacturing costs. In this paper, we compare several printing and coating methods that are employed to fabricate OPVs, with the main focus towards the deposition of the active layer. This includes a comparison of performances at laboratory (<1 cm2), small (1-10 cm2), medium (10-100 cm2), and large (>100 cm2) active area fabrications, encompassing devices that use scalable printing and coating methods for only the active layer, as well as "fully printed/coated" devices. The article also compares the research focus of each of the printing and coating techniques and predicts the general direction that scalable and large-scale OPVs will head towards.

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.

Understanding Causalities in Organic Photovoltaics Device Degradation in a Machine-Learning-Driven High-Throughput Platform

Organic solar cells (OSCs) now approach power conversion efficiencies of 20%. However, in order to enter mass markets, problems in upscaling and operational lifetime have to be solved, both concerning the connection between processing conditions and active layer morphology. Morphological studies supporting the development of structure-process-property relations are time-consuming, complex, and expensive to undergo and for which statistics, needed to assess significance, are difficult to be collected. This work demonstrates that causal relationships between processing conditions, morphology, and stability can be obtained in a high-throughput method by combining low-cost automated experiments with data-driven analysis methods. An automatic spectral modeling feeds parametrized absorption data into a feature selection technique that is combined with Gaussian process regression to quantify deterministic relationships linking morphological features and processing conditions with device functionality. The effect of the active layer thickness and the morphological order is further modeled by drift-diffusion simulations and returns valuable insight into the underlying mechanisms for improving device stability by tuning the microstructure morphology with versatile approaches. Predicting microstructural features as a function of processing parameters is decisive know-how for the large-scale production of OSCs.

Impact of Electrostatic Interaction on Vertical Morphology and Energy Loss in Efficient Pseudo-Planar Heterojunction Organic Solar Cells

Although a suitable vertical phase separation (VPS) morphology is essential for improving charge transport efficiency, reducing charge recombination, and ultimately boosting the efficiency of organic solar cells (OSCs), there is a lack of theoretical guidance on how to achieve the ideal morphology. Herein, a relationship between the molecular structure and the VPS morphology of pseudo-planar heterojunction (PPHJ) OSCs is established by using molecular surface electrostatic potential (ESP) as a bridge. The morphological evolution mechanism is revealed by studying four binary systems with vary electrostatic potential difference (∆ESP) between donors (Ds) and acceptors (As). The findings manifest that as ∆ESP increases, the active layer is more likely to form a well-mixed phase, while a smaller ∆ESP favors VPS morphology. Interestingly, it is also observed that a larger ∆ESP leads to enhanced miscibility between Ds and As, resulting in higher non-radiative energy losses (ΔE3). Based on these discoveries, a ternary PPHJ device is meticulously designed with an appropriate ∆ESP to obtain better VPS morphology and lower ΔE3, and an impressive efficiency of 19.09% is achieved. This work demonstrates that by optimizing the ΔESP, not only the formation of VPS morphology can be controlled, but also energy losses can be reduced, paving the way to further boost OSC performance.

Tuning polymer-backbone coplanarity and conformational order to achieve high-performance printed all-polymer solar cells

All-polymer solar cells (all-PSCs) offer improved morphological and mechanical stability compared with those containing small-molecule-acceptors (SMAs). They can be processed with a broader range of conditions, making them desirable for printing techniques. In this study, we report a high-performance polymer acceptor design based on bithiazole linker (PY-BTz) that are on par with SMAs. We demonstrate that bithiazole induces a more coplanar and ordered conformation compared to bithiophene due to the synergistic effect of non-covalent backbone planarization and reduced steric encumbrances. As a result, PY-BTz shows a significantly higher efficiency of 16.4% in comparison to the polymer acceptors based on commonly used thiophene-based linkers (i.e., PY-2T, 9.8%). Detailed analyses reveal that this improvement is associated with enhanced conjugation along the backbone and closer interchain π-stacking, resulting in higher charge mobilities, suppressed charge recombination, and reduced energetic disorder. Remarkably, an efficiency of 14.7% is realized for all-PSCs that are solution-sheared in ambient conditions, which is among the highest for devices prepared under conditions relevant to scalable printing techniques. This work uncovers a strategy for promoting backbone conjugation and planarization in emerging polymer acceptors that can lead to superior all-PSCs.

Efficient all-small-molecule organic solar cells processed with non-halogen solvent

All-small-molecule organic solar cells with good batch-to-batch reproducibility combined with non-halogen solvent processing show great potential for commercialization. However, non-halogen solvent processing of all-small-molecule organic solar cells are rarely reported and its power conversion efficiencies are very difficult to improve. Herein, we designed and synthesized a small molecule donor BM-ClEH that can take advantage of strong aggregation property induced by intramolecular chlorine-sulfur non-covalent interaction to improve molecular pre-aggregation in tetrahydrofuran and corresponding micromorphology after film formation. Tetrahydrofuran-fabricated all-small-molecule organic solar cells based on BM-ClEH:BO-4Cl achieved high power conversion efficiencies of 15.0% in binary device and 16.1% in ternary device under thermal annealing treatment. In contrast, weakly aggregated BM-HEH without chlorine-sulfur non-covalent bond is almost inefficient under same processing conditions due to poor pre-aggregation induced disordered π-π stacking, indistinct phase separation and exciton dissociation. This work promotes the development of non-halogen solvent processing of all-small-molecule organic solar cells and provides further guidance.

High-Performance Wearable Organic Photodetectors by Molecular Design and Green Solvent Processing for Pulse Oximetry and Photoplethysmography

White-light detection from the visible to the near-infrared region is central to many applications such as high-speed cameras, autonomous vehicles, and wearable electronics. While organic photodetectors (OPDs) are being developed for such applications, several challenges must be overcome to produce scalable high-detectivity OPDs. This includes issues associated with low responsivity, narrow absorption range, and environmentally friendly device fabrication. Here, an OPD system processed from 2-methyltetrahydrofuran (2-MeTHF) sets a record in light detectivity, which is also comparable with commercially available silicon-based photodiodes is reported. The newly designed OPD is employed in wearable devices to monitor heart rate and blood oxygen saturation using a flexible OPD-based finger pulse oximeter. In achieving this, a framework for a detailed understanding of the structure-processing-property relationship in these OPDs is also developed. The bulk heterojunction (BHJ) thin films processed from 2-MeTHF are characterized at different length scales with advanced techniques. The BHJ morphology exhibits optimal intermixing and phase separation of donor and acceptor moieties, which facilitates the charge generation and collection process. Benefitting from high charge carrier mobilities and a low shunt leakage current, the newly developed OPD exhibits a specific detectivity of above 1012  Jones over 400-900 nm, which is higher than those of reference devices processed from chlorobenzene and ortho-xylene.

Nonfullerene Acceptor Featuring Unique Self-Regulation Effect for Organic Solar Cells with 19 % Efficiency

The blend nanomorphology of electron-donor (D) and -acceptor (A) materials is of vital importance to achieving highly efficient organic solar cells. Exogenous additives especially aromatic additives are always needed to further optimize the nanomorphology of blend films, which is hardly compatible with industrial manufacture. Herein, we proposed a unique approach to meticulously modulate the aggregation behavior of NFAs in both crystal and thin film nanomorphology via self-regulation effect. Nonfullerene acceptor Z9 was designed and synthesized by tethering phenyl groups on the inner side chains of the Y6 backbone. Compared with Y6, the tethered phenyl groups participated in the molecular aggregation via the π-π stacking of phenyl-phenyl and phenyl-2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC-2F) groups, which induced 3D charge transport with phenyl-mediated super-exchange electron coupling. Moreover, ordered molecular packing with suitable phase separation was observed in Z9-based blend films. High power conversion efficiencies (PCEs) of 19.0 % (certified PCE of 18.6 %) for Z9-based devices were achieved without additives, indicating the great potential of the self-regulation strategy in NFA design.

Molecular Stacking and Aggregation Optimization of Photoactive Layer through Solid Additive Enables High-Performance Organic Solar Cells

Regulating molecular packing and aggregation of photoactive layer is a critical but challenging issue in developing high-performance organic solar cells. Herein, two structurally similar analogues of anthra[2,3-b : 6,7-b']dithiophene (ADT) and naphtho[1,2-b : 5,6-b']dithiophene (NDT) are developed as solid additive to exploit their effect in regulating the molecular aggregation and π-stacking of photoactive layer. We clarify that the perpendicular arrangements of NDT can enlarge the molecular packing space and improve the face-on stacking of Y6 during the film formation, favoring a more compact and ordered long-range π-π stacking in the out-of-plane direction after the removal of NDT under thermal annealing. The edge-to-face stacked herringbone-arrangement of ADT along with its non-volatilization under thermal annealing can induce the coexistence of face-on and edge-on stacking of blend film. As a result, the NDT treatment shows encouraging effect in improving the photovoltaic performance of devices based on various systems. Particularly, a remarkable PCE of 18.85 % is achieved in the PM6 : L8-BO-based device treated by NDT additive, which is a significant improvement with regard to the PCE of 16.41 % for the control device. This work offers a promising strategy to regulate the molecular packing and aggregation of photoactive layer towards significantly improved performance and stability of organic solar cells.

Octafluoronaphthalene as a thermal-annealing-free volatile solid additive enables high-performance organic solar cells

A thermal annealing-free solid additive octafluoronaphthalene was developed for high-performance organic solar cells. In an additive-modified device, an impressive power conversion efficiency of 18.59% from 17.27% was achieved with simultaneously enhanced current density from 26.86 to 27.53 mA cm-2 and fill factor from 74.34% to 78.85%.

Electrostatic force promoted intermolecular stacking of polymer donors toward 19.4% efficiency binary organic solar cells

Conjugated polymers are generally featured with low structural order due to their aromatic and irregular structural units, which limits their light absorption and charge mobility in organic solar cells. In this work, we report a conjugated molecule INMB-F that can act as a molecular bridge via electrostatic force to enhance the intermolecular stacking of BDT-based polymer donors toward efficient and stable organic solar cells. Molecular dynamics simulations and synchrotron X-ray measurements reveal that the electronegative INMB-F adsorb on the electropositive main chain of polymer donors to increase the donor-donor interactions, leading to enhanced structural order with shortened π-π stacking distance and consequently enhanced charge transport ability. Casting the non-fullerene acceptor layer on top of the INMB-F modified donor layer to fabricate solar cells via layer-by-layer deposition evidences significant power conversion efficiency boosts in a range of photovoltaic systems. A power conversion efficiency of 19.4% (certified 18.96%) is realized in PM6/L8-BO binary devices, which is one of the highest reported efficiencies of this material system. The enhanced structural order of polymer donors by INMB-F also leads to a six-fold enhancement of the operational stability of PM6/L8-BO organic solar cells.

Thermodynamic Phase Transition of Three-Dimensional Solid Additives Guiding Molecular Assembly for Efficient Organic Solar Cells

Fine-tuning the thermodynamic self-assembly of molecules via volatile solid additives has emerged to be an effective way to construct high-performance organic solar cells. Here, three-dimensional structured solid molecules have been designed and applied to facilitate the formation of organized molecular assembly in the active layer. By means of systematic theory analyses and film-morphology characterizations based on four solid candidates, we preselected the optimal one, 4-fluoro-N,N-diphenylaniline (FPA), which possesses good volatility and strong charge polarization. The three-dimensional solids can induce molecular packing in active layers via strong intermolecular interactions and subsequently provide sufficient space for the self-reassembly of active layers during the thermodynamic transition process. Benefitting from the optimized morphology with improved charge transport and reduced energy disorder in the FPA-processed devices, high efficiencies of over 19 % were achieved. The strategy of three-dimensional additives inducing ordered self-assembly structure represents a practical approach for rational morphology control in highly efficient devices, contributing to deeper insights into the structural design of efficient volatile solid additives.

Phase Behavior and Role of Organic Additives for Self-Doped CsPbI3 Perovskite Semiconductor Thin Films

The phase change of all-inorganic cesium lead halide (CsPbI3) thin film from yellow δ-phase to black γ-/α-phase has been a topic of interest in the perovskite optoelectronics field. Here, the main focus is how to secure a black perovskite phase by avoiding a yellow one. In this work, we fabricated a self-doped CsPbI3 thin film by incorporating an excess cesium iodide (CsI) into the perovskite precursor solution. Then, we studied the effect of organic additive such as 1,8-diiodooctane (DIO), 1-chloronaphthalene (CN), and 1,8-octanedithiol (ODT) on the optical, structural, and morphological properties. Specifically, for elucidating the binary additive-solvent solution thermodynamics, we employed the Flory-Huggins theory based on the oligomer level of additives' molar mass. Resultantly, we found that the miscibility of additive-solvent displaying an upper critical solution temperature (UCST) behavior is in the sequence CN:DMF > ODT:DMF > DIO:DMF, the trends of which could be similarly applied to DMSO. Finally, the self-doping strategy with additive engineering should help fabricate a black γ-phase perovskite although the mixed phases of δ-CsPbI3, γ-CsPbI3, and Cs4PbI6 were observed under ambient conditions. However, the results may provide insight for the stability of metastable γ-phase CsPbI3 at room temperature.

Improving the efficiency of ternary organic solar cells by reducing energy loss

Effectively reducing the voltage loss in organic solar cells (OSCs) is critical to improving the power conversion efficiency (PCE) of OSCs. In this study, highly efficient ternary OSCs were constructed by adding a non-fullerene acceptor Qx2 with a high open-circuit voltage (VOC) and low energy loss (Eloss) into PM6:m-BTP-PhC6 based binary devices. The third component Qx2 shows slightly complementary absorption with m-BTP-PhC6 and also optimizes the molecular packing, orientation, and morphology of the active layer. Moreover, the incorporation of Qx2 reduced the energetic disorder and improved the electroluminescence quantum efficiency, which suppresses the Eloss and further leads to a higher VOC than the PM6:m-BTP-PhC6 binary blend. Consequently, synergetic enhancements of VOC, short circuit current (JSC), and fill factor (FF) are realized, resulting in the PCE of 18.60%. This work shows that the selection of the appropriate third component has positive implications for reducing Eloss and improving the PCE of OSCs.

Intrinsic Role of Volatile Solid Additive in High-Efficiency PM6:Y6 Series Nonfullerene Solar Cells

Organic nonfullerene solar cells (ONSCs) have made unprecedented progress; however, morphology optimization of ONSCs is proven to be particularly challenging relative to classical fullerene-based devices. Here, a novel volatile solid additive (VSA), 2-hydroxy-4-methoxybenzophenone (2-HM), is reported for achieving high-efficiency ONSCs. 2-HM functions as a universal morphology-directing agent for several well-known PM6:Y6 series nonfullerene blends, viz. PM6:Y6, PM6:BTP-eC9, PM6:L8-BO, leading to a best efficiency of 18.85% at the forefront of reported binary ONSCs. VSAs have recently emerged, while the intrinsic kinetics is still unclear. Herein, a set of in situ and ex situ characterizations is employed to first illustrate the molecule-aggregate-domain transition dynamic process assisted by the VSA. More specifically, the role of 2-HM in individual donor PM6 and acceptor Y6 systems is unlocked, and the function of 2-HM in altering the PM6:Y6 bulk heterojunction blends is further revealed for enhanced photovoltaic performance. It is believed that the achievement brings not only a deep insight into emerging volatile solid additive, but also a new hope to further improve the molecular ordering, film microstructure, and relevant performance of ONSCs.

An Isomeric Solid Additive Enables High-Efficiency Polymer Solar Cells Developed Using a Benzo-Difuran-Based Donor Polymer

Currently, nearly all high-efficiency organic photovoltaic devices use donor polymers based on the benzo-dithiophene (BDT) unit. To diversify the choices of building blocks for high-performance donor polymers, the use of benzo-difuran (BDF) units is explored, which can achieve reduced steric hindrance, stronger molecular packing, and tunable energy levels. In previous research, the performance of BDF-based devices lagged behind those of BDT-based devices. In this study, a high efficiency (18.4%) is achieved using a BDF-based polymer donor, which is the highest efficiency reported for BDF donor materials to date. The high efficiency is enabled by a donor polymer (D18-Fu) and the aid of a solid additive (2-chloronaphthalene), which is the isomer of the commonly used additive 1-chloronaphthalene. These results revealed the significant effect of 2-chloronaphthalene in optimizing the morphology and enhancing the device parameters. This work not only provides a new building block that can achieve an efficiency comparable to dominant BDT units but also proposes a new solid additive that can replace the widely used 1-chloronaphthalene additive.

Poly(dimethylsiloxane)-block-PM6 Polymer Donors for High-Performance and Mechanically Robust Polymer Solar Cells

High power conversion efficiency (PCE) and stretchability are the dual requirements for the wearable application of polymer solar cells (PSCs). However, most efficient photoactive films are mechanically brittle. In this work, highly efficient (PCE = 18%) and mechanically robust (crack-onset strain (COS) = 18%) PSCs are acheived by designing block copolymer (BCP) donors, PM6-b-PDMSx (x = 5k, 12k, and 19k). In these BCP donors, stretchable poly(dimethylsiloxane) (PDMS) blocks are covalently linked with the PM6 blocks to effectively increase the stretchability. The stretchability of the BCP donors increases with a longer PDMS block, and PM6-b-PDMS_19k :L8-BO PSC exhibits a high PCE (18%) and 9-times higher COS value (18%) compared to that (COS = 2%) of the PM6:L8-BO-based PSC. However, the PM6:L8-BO:PDMS_12k ternary blend shows inferior PCE (5%) and COS (1%) due to the macrophase separation between PDMS and active components. In the intrinsically stretchable PSC, the PM6-b-PDMS_19k :L8-BO blend exhibits significantly greater mechanical stability PCE_80% ((80% of the initial PCE) at 36% strain) than those of the PM6:L8-BO blend (PCE_80% at 12% strain) and the PM6:L8-BO:PDMS ternary blend (PCE_80% at 4% strain). This study suggests an effective design strategy of BCP P_D to achieve stretchable and efficient PSCs.

High-Performance Intrinsically Stretchable Polymer Solar Cell with Record Efficiency and Stretchability Enabled by Thymine-Functionalized Terpolymer

Designing new polymer semiconductors for intrinsically stretchable polymer solar cells (IS-PSCs) with high power conversion efficiency (PCE) and durability is critical for wearable electronics applications. Nearly all high-performance PSCs are constructed using fully conjugated polymer donors (P_D) and small-molecule acceptors (SMA). However, a successful molecular design of P_Ds for high-performance and mechanically durable IS-PSCs without sacrificing conjugation has not been realized. In this study, we design a novel thymine side chain terminated 6,7-difluoro-quinoxaline (Q-Thy) monomer and synthesize a series of fully conjugated P_Ds (PM7-Thy5, PM7-Thy10, PM7-Thy20) featuring Q-Thy. The Q-Thy units capable of inducing dimerizable hydrogen bonding enable strong intermolecular P_D assembly and highly efficient and mechanically robust PSCs. The PM7-Thy10:SMA blend demonstrates a combination of high PCE (>17%) in rigid devices and excellent stretchability (crack-onset value >13.5%). More importantly, PM7-Thy10-based IS-PSCs show an unprecedented combination of PCE (13.7%) and ultrahigh mechanical durability (maintaining 80% of initial PCE after 43% strain), illustrating the promising potential for commercialization in wearable applications.

Enhanced Efficiency and Stability of Novel Pseudo-ternary Polymer Solar Cells Enabled by a Conjugated Donor Block Copolymer

The recent breakthrough in power conversion efficiencies (PCEs) of polymer solar cells (PSCs) that contain an active layer of a ternary system has achieved values of 18-19%; this has sparked interest for further research. However, this system has difficulties in optimizing the composition and controlling the interaction between the three active materials. In this study, we investigated the use of a donor₁ (D₁)-donor₂ (D₂) conjugated block copolymer (CBP), PM6-b-TT, to replace the physical blend of two donors. PM6-b-TT, which exhibits an extended absorption range, was synthesized by covalently bonding PM6, a medium-band gap polymer, with PBDT-TT, a wide-band gap polymer. The blend films containing PM6-b-TT and Y6-BO acceptor, demonstrated excellent crystallinity and a film morphology favorable for PSCs. The corresponding pseudo-ternary PSC exhibited significantly higher PCE and thermal stability than the PM6:PBDT-TT-based ternary device. This study unambiguously demonstrates that the novel D₁-D₂ CBP strategy, combined with the conventional binary and ternary system advantages, is a promising material production strategy that can boost the performance of future PSCs.

Finely Tuned Molecular Packing Realized by a New Rhodanine-Based Acceptor Enabling Excellent Additive-Free Small- and Large-Area Organic Photovoltaic Devices Approaching 19 and 12.20% Efficiencies

A new nonfullerene acceptor (NFA), BTA-ERh, was synthesized and integrated into a PM6:Y7:PC₇₁BM ternary system to regulate the blend film morphology for enhanced device performance. Due to BTA-ERh's good miscibility with host active blend films, an optimized film morphology was obtained with appropriate phase separation and fine-tuning of film crystallinity, which ultimately resulted in efficient exciton dissociation, charge transport, lower recombination loss, and decreased trap-state density. The resulting additive-free quaternary devices achieved a remarkable efficiency of 18.90%, with a high voltage, fill factor, and current density of 0.87 V, 76.32%, and 28.60 mA cm^-2, respectively. By adding less of a new small molecule with high crystallinity, the favorable nanomorphology shape of blend films containing NFAs might be adjusted. Consequently, this strategy can enhance photovoltaic device performance for cutting-edge NFA-based organic solar cells (OSCs). In contrast, the additive-free OSCs exhibited good operational stability. More importantly, large-area modules with the quaternary device showed a remarkable efficiency of 12.20%, with an area as high as 55 cm² (substrate size, 100 cm²) in an air atmosphere via D-bar coating. These results highlight the enormous research potential for a multicomponent strategy for future additive-free OSC applications.

Direct Arylation Synthesis of Small Molecular Acceptors for Organic Solar Cells

In recent years, small molecular acceptors (SMAs) have extensively promoted the progress of organic solar cells (OSCs). The facile tuning of chemical structures affords SMAs excellent tunability of their absorption and energy levels, and it gives SMA-based OSCs slight energy loss, enabling OSCs to achieve high power conversion efficiencies (e.g., >18%). However, SMAs always suffer complicated chemical structures requiring multiple-step synthesis and cumbersome purification, which is unfavorable to the large-scale production of SMAs and OSC devices for industrialization. Direct arylation coupling reaction via aromatic C-H bonds activation allows for the synthesis of SMAs under mild conditions, and it simultaneously reduces synthetic steps, synthetic difficulty, and toxic by-products. This review provides an overview of the progress of SMA synthesis through direct arylation and summarizes the typical reaction conditions to highlight the field's challenges. Significantly, the impacts of direct arylation conditions on reaction activity and reaction yield of the different reactants' structures are discussed and highlighted. This review gives a comprehensive view of preparing SMAs by direct arylation reactions to cause attention to the facile and low-cost synthesis of photovoltaic materials for OSCs.

Enhancing Photon Utilization Efficiency for High-Performance Organic Photovoltaic Cells via Regulating Phase-Transition Kinetics

Efficient photon utilization is key to achieving high-performance organic photovoltaic (OPV) cells. In this study, a multiscale fibril network morphology in a PBQx-TCl:PBDB-TF:eC9-2Cl-based system is constructed by regulating donor and acceptor phase-transition kinetics. The distinctive phase-transition process and crystal size are systematically investigated. PBQx-TCl and eC9-2Cl form fibril structures with diameters of ≈25 nm in ternary films. Additionally, fine fibrils assembled by PBDB-TF are uniformly distributed over the fibril networks of PBQx-TCl and eC9-2Cl. The ideal multiscale fibril network morphology enables the ternary system to achieve superior charge transfer and transport processes compared to binary systems; these improvements promote enhanced photon utilization efficiency. Finally, a high power conversion efficiency of 19.51% in a single-junction OPV cell is achieved. The external quantum efficiency of the optimized ternary cell exceeds 85% over a wide range of 500-800 nm. A tandem OPV cell is also fabricated to increase solar photon absorption. The tandem cell has an excellent PCE of more than 20%. This study provides guidance for constructing an ideal multiscale fibril network morphology and improving the photon utilization efficiency of OPV cells.

The Multiplicity of π-π Interactions of Fused-Ring Electron Acceptor Polymorphs on the Exciton Migration and Charge Transport

Efficient long-range exciton migration and charge transport are the key parameters for organic photovoltaic materials, which strongly depend on the molecular stacking modes. Herein, we extracted the stacked structures of the archetype fused-ring electron acceptor molecule, ITIC, based on the information on four polymorphic crystals and investigated the relationship between molecular stacking modes and exciton migration/charge transport properties through the intermolecular Coulomb coupling and charge transfer integral calculation. Experimentally, the thin film texture is crystallized through a post-annealing treatment through grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements, which lead to the enhanced exciton migration through exciton-exciton annihilation in the femtosecond transient absorption (fs-TA) measurements. This work demonstrates the relationship between the molecular arrangement and the exciton migration and electron transport and highlights the significance of optimizing molecular stacking for the development of high-performance electron acceptor materials.

Morphology Optimization of the Photoactive Layer through Crystallinity and Miscibility Regulation for High-performance Polymer Solar Cells

On the premise of strongly crystalline materials involved, it is a challenge to control the phase separation of bulk-heterojunction donor/acceptor active layer to fabricate high-performance polymer solar cells (PSCs). Herein, we develop a molecular design strategy of the third component to synthesize three guest materials (namely BTPT, BTP-Th, and BTP-2Th) to address this issue. We investigate and reveal the effect of crystallinity and miscibility of the third component in controlling the phase separation of Y6-derivatives-based blend film. As a result, a remarkable power-conversion efficiency of 18.53 % is obtained in the ternary PSC based on PTQ10 : m-BTP-PhC6 with BTP-Th as the third component, which is a significant improvement with regard to the efficiency of 17.22 % for the control binary device. Our study offers a molecular design strategy to develop a third component for building ternary PSCs in terms of crystallinity and miscibility regulation.

Non-Embedded Silver Nanowires/Antimony-Doped Tin Oxide/Polyethylenimine Transparent Electrode for Non-Fullerene Acceptor ITO-Free Inverted Organic Photovoltaics

Indium tin oxide (ITO)-free solution-processed transparent electrodes are an essential component for the low-cost fabrication of organic optoelectronic devices. High-performance silver nanowires (AgNWs) ITO-free inverted organic photovoltaics (OPVs) usually require a AgNWs-embedded process. A simple cost-effective roll-to-roll production process of inverted ITO-free OPVs with AgNWs as a bottom transparent electrode requires solution-based thick metal oxides as carrier-selective contacts. In this reported study, we show that a solution-processed antimony-doped tin oxide (ATO)/polyethylenimine (PEI) electron-selective contact incorporated on the top of non-embedded AgNWs provides a high-performance ITO-free bottom electrode for non-fullerene acceptor (NFA) inverted OPVs.

Medium Bandgap Nonfullerene Acceptor for Efficient Ternary Polymer Solar Cells with High Open-Circuit Voltage

We have designed a new medium bandgap non-fullerene small-molecule acceptor consisting of an IDT donor core flanked with 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c]-thiophene-4-ylidene) malononitrile (TC) acceptor terminal groups (IDT-TC) and compared its optical and electrochemical properties with the IDT-IC acceptor. IDT-TC showed an absorption profile from 300 to 760 nm, and it has an optical bandgap of 1.65 eV and HOMO and LUMO energy levels of -5.55 and -3.83 eV, respectively. In contrast to IDT-IC, IDT-TC has an upshifted LUMO energy level, which is advantageous for achieving high open-circuit voltage. Moreover, IDT-TC showed higher crystallinity and high electron mobility than IDT-IC. Using a wide bandgap D-A copolymer P as the donor, we compared the photovoltaic performance of IDT-TC, IDT-IC, and IDT-IC-Cl nonfullerene acceptors (NFAs). Polymer solar cells (PSCs) using P: IDT-TC, P: IDT-IC, and P:IDT-IC-Cl active layers achieved a power conversion efficiency (PCE) of 14.26, 11.56, and 13.34%, respectively. As the absorption profiles of IDT-IC-Cl and IDT-TC are complementary to each other, we have incorporated IDT-TC as the guest acceptor in the P: IDT-IC-Cl active layer to fabricate the ternary (P:IDT-TC: IDT-IC-Cl) PSC, demonstrating a PCE of 16.44%, which is significantly higher than that of the binary BHJ devices. The improvement in PCE for ternary PSCs is attributed to the efficient exploitation of excitons via energy transfer from IDT-TC to IDT-IC-Cl, suitable nanoscale phase separation, compact stacking distance, and more evenly distributed charge transport.

End-Group Engineering of Chlorine-Trialkylsiylthienyl Chain-Substituted Small-Molecule Donors for High-Efficiency Ternary Solar Cells

Ternary architecture has been widely demonstrated as a facile and efficient strategy to boost the performance of organic solar cells (OSCs). However, the rational design of the third component with suitable core and end-group modification is still a challenge. Herein, two new small-molecule (SM) donors BT-CN and BT-ER, featuring the identical conjugated backbone with distinct end group, have been designed, synthesized, and introduced into the PM6:Y6 binary system as the second donor. Both molecules exhibit complementary absorption and good miscibility with PM6, contributing to the nanofibrous phases and strong face-on molecular packing. Importantly, the incorporation of BT-CN/BT-ER has significantly facilitated charge collection and transportation with remarkable suppression of carrier recombination. As a result, ternary OSCs with 20 wt% BT-CN/BT-ER achieved a PCE of 16.8%/17.22% with synchronously increased open-circuit voltage (V_OC ), short-circuit current density (J_SC ) and fill factor (FF). Moreover, replacing Y6 with L8-BO further improves the PCE to 18.05%/18.11%, indicating the universality of both molecules as the third component. This work demonstrates not only two efficient SM donors with 4,8-bis(4-chloro-5-(tripropylsilyl)thiophen-2-yl) benzo[1,2-b:4,5-b']dithiophene (BDTT-SiCl) as the core but also end group modification strategy to fine-tune the absorption spectrum, molecular packing, and energy levels of SM donors to construct high-performance ternary OSCs.

Quasi-Homojunction Organic Nonfullerene Photovoltaics Featuring Fundamentals Distinct from Bulk Heterojunctions

In contrast to classical bulk heterojunction (BHJ) in organic solar cells (OSCs), the quasi-homojunction (QHJ) with extremely low donor content (≤10 wt.%) is unusual and generally yields much lower device efficiency. Here, representative polymer donors and nonfullerene acceptors are selected to fabricate QHJ OSCs, and a complete picture for the operation mechanisms of high-efficiency QHJ devices is illustrated. PTB7-Th:Y6 QHJ devices at donor:acceptor (D:A) ratios of 1:8 or 1:20 can achieve 95% or 64% of the efficiency obtained from its BHJ counterpart at the optimal D:A ratio of 1:1.2, respectively, whereas QHJ devices with other donors or acceptors suffer from rapid roll-off of efficiency when the donors are diluted. Through device physics and photophysics analyses, it is observed that a large portion of free charges can be intrinsically generated in the neat Y6 domains rather than at the D/A interface. Y6 also serves as an ambipolar transport channel, so that hole transport as also mainly through Y6 phase. The key role of PTB7-Th is primarily to reduce charge recombination, likely assisted by enhancing quadrupolar fields within Y6 itself, rather than the previously thought principal roles of light absorption, exciton splitting, and hole transport.

Self-Aggregated Light-Trapping Nanodots for Highly Efficient Organic Solar Cells

The typical thickness of the photoactive layer in organic solar cells (OSCs) is around 100 nm, which limits the absorption efficiency of the incident light and the power conversion efficiency (PCE) of OSCs. Therefore, light-trapping schemes to reduce the optical losses in the thin photoactive layers are critically important for efficient OSCs. Herein, light-trapping and electron-collection dual-functional small organic molecules, N,N,N',N'-tetraphenyloxalamide (TPEA) and N,N,N',N'-tetraphenylmalonamide (TPMA), are designed and synthesized by a one-step acylation reaction. Driven by strong intermolecular force, TPEA and TPMA tend to self-aggregate into hemispherical light-trapping nanodots on the photoactive layer, resulting in enhanced light harvesting. Meanwhile, TPEA and TPMA demonstrate high electron mobility and excellent electron-collection ability.  Compared with the device without cathode buffer layer (CBL, PCE = 14.09%), PM6:BTP-eC9 based OSCs with TPEA and TPMA light-trapping CBLs demonstrate greatly enhanced PCE of 16.21% and 17.85%, respectively. Furthermore, a record PCE of 19.02% can be achieved for PM6:BTP-eC9:PC₇₁ BM based ternary OSC with TPMA light-trapping CBL. Moreover, TPMA exhibits a low synthesis cost of only 0.61 $ g^-1 with high yield. These findings could open a window for the rational design of multifunctional CBLs for efficient and stable OSCs.

Efficient Cathode Buffer Material Based on Dibenzothiophene-S,S-dioxide for Both Conventional and Inverted Organic Solar Cells

A novel conjugated molecule (PBSON) based on a main chain composed of bis(dibenzothiophene-S,S-dioxide) fused cyclopentadiene and side chains containing amino groups is presented as an efficient cathode buffer material (CBM) for organic solar cells (OSCs). PBSON showed a deep highest occupied molecular orbital (HOMO) energy level of -6.01 eV, which was beneficial for building hole-blocking layers at the cathodes of OSCs. The energy bandgap of PBSON reached 3.17 eV, implying high transmittance to visible and near-infrared light, which meant PBSON should be suitable for the applications to most inverted OSCs. The scanning Kelvin probe microscopy measurement and theoretical calculation on the PBSON/cathode interfacial interaction proved the excellent work function-regulating abilities of PBSON for various cathodes, suggesting that PBSON could promote the formation of Ohmic contacts at the cathodes and thus improve the transport and collection of electron carriers for OSCs. The characterization of electron-only devices demonstrated the good electron-transporting performance of PBSON at the cathodes. In the conventional OSCs, it was hinted that PBSON might restrain the infiltrations of evaporated cathode atoms into the active films, consequently reducing the reverse leakage currents. As a result, PBSON was able to boost the power conversion efficiencies (PCEs) by 58.2 and 56.4% for both conventional and inverted OSCs of the typical PTB7:PC₇₁BM system, respectively, as compared to the unadorned devices. In terms of the classical PTB7-Th:PC₇₁BM system, substantial increases in PCEs could also be found with PBSON interlayers, which were 54.7 and 59.8% for the conventional device and inverted device, respectively. Therefore, PBSON is a kind of promising CBM for realizing both conventional and inverted OSCs of high performance.

High performance polymerized small molecule acceptor by synergistic optimization on π-bridge linker and side chain

The polymerized small-molecule acceptors have attracted great attention for application as polymer acceptor in all-polymer solar cells recently. The modification of small molecule acceptor building block and the π-bridge linker is an effective strategy to improve the photovoltaic performance of the polymer acceptors. In this work, we synthesized a new polymer acceptor PG-IT2F which is a modification of the representative polymer acceptor PY-IT by replacing its upper linear alkyl side chains on the small molecule building block with branched alkyl chains and attaching difluorene substituents on its thiophene π-bridge linker. Through this synergistic optimization, PG-IT2F possesses more suitable phase separation, increased charge transportation, better exciton dissociation, lower bimolecular recombination, and longer charge transfer state lifetime than PY-IT in their polymer solar cells with PM6 as polymer donor. Therefore, the devices based on PM6:PG-IT2F demonstrated a high power conversion efficiency of 17.24%, which is one of the highest efficiency reported for the binary all polymer solar cells to date. This work indicates that the synergistic regulation of small molecule acceptor building block and π-bridge linker plays a key role in designing and developing highly efficient polymer acceptors.

The modification of small molecule acceptor building block and π−bridge linker is effective to improve photovoltaic performance. Here, the authors replace linear with branched alkyl chains and introduce difluorene-substituted linker to realise all-polymer solar cells with efficiency of 17.24%.

Controlling the Treatment Time for Ideal Morphology towards Efficient Organic Solar Cells

In this work, we performed a systematic comparison of different duration of solvent vapor annealing (SVA) treatment upon state-of-the-art PM6:SY1 blend film, which is to say for the first time, the insufficient, appropriate, and over-treatment’s effect on the active layer is investigated. The power conversion efficiency (PCE) of corresponding organic solar cell (OSC) devices is up to 17.57% for the optimized system, surpassing the two counterparts. The properly tuned phase separation and formed interpenetrating network plays an important role in achieving high efficiency, which is also well-discussed by the morphological characterizations and understanding of device physics. Specifically, these improvements result in enhanced charge generation, transport, and collection. This work is of importance due to correlating post-treatment delicacy, thin-film morphology, and device performance in a decent way.

Achieving and Understanding of Highly Efficient Ternary Organic Photovoltaics: From Morphology and Energy Loss to Working Mechanism

Ternary strategy, adding an additional donor (D) or acceptor (A) into conventional binary D:A blend, has shown great potential in improving photovoltaic performances of organic photovoltaics (OPVs) for practical applications. Herein, this review is presented on how efficient ternary OPVs are realized from the aspects of morphology, energy loss, and working mechanism. As to morphology, the role of third component on the formation of preferred alloy-like-phase and vertical-phase, which are driven by the miscibility tuning, is discussed. For energy loss, the effect of the third component on the luminescence enhancement and energetic disordering suppression, which lead to favorable increase of voltage, is presented. Regarding working mechanism, dilution effect and relationships between two acceptors or donor/acceptor, which explain the observed device parameters variations, are analyzed. Finally, some future directions concerning ternary OPVs are pointed out. Therefore, this review can provide a comprehensive understanding of working principles and effective routes for high-efficiency ternary systems, advancing the commercialization of OPVs.

High Performance and Stable Organic Solar Cells Fabricated by Y-Series Small Molecular Materials as the Interfacial Modified Layer

The organic solar cell (OSC) has received tremendous consideration for the impressive increased power conversion efficiency (PCE) from 11% to over 18% in the last decade, but another main parameter, the stability, still needs further study to meet the requirements of commercialization. Generally, the inverted structure device shows more stability than the conventional one owing to the structure characteristics, but even so, the performance and stability of the OSC device still need further improvement because of some undesirable contact between the electron transport layer (typically transition metal oxide like ZnO) and the active layer. Here, three Y-series small molecular acceptor materials (Y6, BTP-eC9, and L8-BO) are used as an interfacial modified layer (IML), which could optimize the interfacial characterization of the devices and thus enhance both the performance and stability. As a result, the insertion of the IML improved the interlayer charge transport capacity by passivating the surface of ZnO, leading to the enhancement of short circuit current density (J_SC), fill factor, and PCE of the OSCs. Furthermore, because of the protection of the IML, the OSCs show outstanding stability compared to the control device (without IML), which could maintain 80% performance of the device over 150 h.