Hot-Casting Strategy Empowers High-Boiling Solvent-Processed Organic Solar Cells with Over 18.5% Efficiency

Engineering Surface Tension of Active Layer Solutions to form Uniform Films on Water Surface for Large-Area Flexible Organic Photovoltaic Modules

Fabricating large-area uniform thin (about 100 nm) active layer films via solution processing is still challenging to realize efficient scalable organic photovoltaic (OPV) modules. In this work, we report a method to fabricate large-area active layer films with the help of Marangoni force via engineering the surface tension of their solutions. Silicone oil was first adopted as an additive to substantially reduce surface tension of the active layer solutions from 34.8 to 20.6 mN/m. Large-area (up to 700 cm2) thin active layer films formed spontaneously on water by Marangoni force due to the increased surface tension difference between the active layer solution and water. The films were then transferred onto charge transporting layer to fabricate devices. The active layer films fabricated by Marangoni force-assisted coating (MAC) displayed power conversion efficiencies (PCE), 17.4 ±0.3 % for PM6:BTP-eC9, 17.9±0.7 % for D18:N3 and 16.4±0.3 % for PM6:QM-1. Furthermore, large-area (32.5 cm2) OPV modules were fabricated based on the MAC method with a PCE of 14.3 %. This is the first example that MAC method is used to successfully fabricate efficient OPV modules via the surface tension engineering of active layer films with silicone oil used as a low surface tension additive.

High Cell to Module Efficiency Remaining Ratio of ≈90% for the 100 cm2 Fully Roll-to-Roll Gravure Printed Flexible Organic Solar Cells From Non-Halogenated Solvent

The cell-to-module (CTM) efficiency remaining ratio from monolithic device to large-area module indicates the scalability potential for large-area organic solar cells (OSCs). Nowadays, the CTM value is still low as the area increases to larger than 100 cm2. In this work, the crucial role of solvent in CTM for printing, which on one side influenced the large area homogeneity due to the ink rheology property, and on the other side impacted phase separation dynamics because of vaporization and crystalline rate is highlighted. The films from TMB show excessive pure phase and printing line defects in vertical the printing direction due to slow volatilization speed and low adhesion, while Tol-based films present printing line defects along the printing direction due to large surface adhesion are demonstrated. In contrast, the films from non-halogenated solvent, o-XY exhibited a suitable phase separation size and excellent large-area homogeneity. Consequently, the fully printed 1 cm2 FOSCs exhibit an efficiency of 14.81%. Moreover, the FOSCs module with an area of 28-104 cm2 gives an efficiency of over 13%, with a CTM of 0.9. Selecting suitable non-halogenated solvents to achieve large-area uniformity and appropriate phase separation morphology in >100 cm2 modules is of great importance for the industrialization of FOSCs.

Improving the Efficiency of Layer-by-Layer Organic Photovoltaics to Exceed 19% by Establishing Effective Donor-Acceptor Interfacial Molecular Interactions

The power conversion efficiency of layer-by-layer organic solar cells (LOSCs) has reached an impressive level by utilizing sequential processing (SqP) for the individual deposition and regulation of both donor and acceptor materials. However, the fundamental understanding of phase separation in LOSCs remains contentious, hindering the rational design of LOSCs due to the ambiguous contribution of stratification or the beneficial vertical segregation morphology. Here, we systematically investigate the utility of solvent effects on drying kinetics to understand how the interaction between the upper and bottom layers affects the formation of the donor/acceptor (D/A) interface and its impact on the performance of LOSCs. Particularly emphasizing the substantial impact of the upper layer solvent on the establishment of the effective D/A interface rather than on the formation of significant stratification in LOSCs, this understanding facilitates the utilization of blend casting in the SqP, introducing an adequate D/A interface, which contributes to a superior performance of 19.05%. Ultimately, we provide three design rules for enhancing the performance in LOSCs: (1) appropriate selection of solvents for the acceptor material to ensure a desired crystalline orientation, (2) utilization of strongly polar and volatile solvents in the upper layer capable of dissolving the bottom layer to form effective D/A interfacial interaction, and (3) establishment of sufficient D/A interfaces.

Facile Strategy for Reducing Cell-to-Module Efficiency Gap in Organic Solar Cells by Controlling the Preaggregation of Photoactive Solutions

Organic solar cells (OSCs) have recently achieved efficiencies of >20% in single-junction unit cells owing to rapid advancements in materials and device technologies. Large-area OSCs face several challenges that adversely affect their efficiency compared to small unit cells. These challenges include increased resistance loads derived from their larger dimensions, as well as limitations related to morphology, miscibility, and crystallinity. In this study, a preaggregation control technique was employed to develop efficient OSC modules. This was achieved by incorporating a low-concentration D18 solution into a high-performance donor-acceptor combination of PM6 and L8-BO facilitating optimal chain entanglement. As a result, macroscopically clean films and microscopically phase-separated morphologies were obtained. For devices with a small area of 0.04 cm2, the device incorporating D18 exhibited a marginally higher power conversion efficiency (PCE) of 17.82% compared to 17.32% for the device without D18. For devices with significantly larger areas of 4.725 and 30.24 cm2, the PCEs increased significantly with the introduction of D18, rising from 14.85 to 15.31% and from 12.77 to 13.49%, respectively. In particular, the module with the largest area of 30.24 cm2 demonstrated a significant decrease in load resistance, leading to a substantial reduction in the cell-to-module efficiency gap, which decreased from 26.3 to 24.3%.

The Pseudo-Bilayer Bulk Heterojunction Active Layer of Polymer Solar Cells in Green Solvent with 18.48% Efficiency

Planar heterojunction (PHJ) is employed to obtain proper vertical phase separation for highly efficient polymer solar cells (PSCs). However, it heavily relies on the choice of orthogonal solvent in the production process. Here, we fabricated a pseudo-bilayer bulk heterojunction (PBHJ) PSC with cross-distribution in the vertical direction by preparing two layers of PM6 and BTP-eC9 blends in an o-XY solution with different dilution ratios to study the morphological evolution of PBHJ film. We found that the PBHJ film exhibits more uniform and suitable continuous interpenetrating network morphology and proper phase separation in the vertical direction for the formation of p-i-n structure. This provides an effective channel for exciton dissociation and charge transport, which is confirmed by both exciton generation simulations and charge dynamics measurements. The PBHJ devices can effectively inhibit trap recombination and accelerate charge separation and transfer. Based on good active layer morphology and balanced charge mobility, all-green solvent-processed PSCs with champion power conversion efficiencies (PCEs) of 18.48% and 16.83% are obtained in PM6:BTP-eC9 and PTQ10:BTP-eC9 systems, respectively. This work reveals the potential mechanism of morphological evolution induced by the PBHJ structure and provides an alternative approach for developing solution processing PSCs.

Combination of Transfer Learning and Chemprop Interpreter with Support of Deep Learning for the Energy Levels of Organic Photovoltaic Materials Prediction and Regulation

It is challenging to build a deep learning predictive model using traditional data mining methods due to the scarcity of available data, and the model's internal decision-making process is often nonintuitive and difficult to explain. In this work, a directed message passing neural network model with transfer learning (TL) and chemprop interpreter is proposed to improve energy levels prediction and visualization for organic photovoltaic materials. The established model shows the best performance, with coefficient of determination reaching 0.787 for HOMO and 0.822 for LUMO in a small testing set after TL, compared to the other four models. Then, the chemprop interpreter analyzes local and global effects of 12 molecular structures on the energy levels for organic materials. After a comprehensive analysis of the energy level effects of nonfullerene Y-series, IT-series, and other organic materials, 12 new IT-series derivatives are designed. 1,1-dicyano-methylene-3-indanone (IC) end group halogenation can reduce HOMO and LUMO energy levels to varying degrees, while IC end group modified by electron-withdrawing aromatic groups can increase HOMO and LUMO energy levels and obtain relatively smaller electrostatic potential (ESP) to reducing intermolecular interactions. The influence of side-chain modification on energy levels is limited. It is worth mentioning that the predicted results of IT-series derivatives match density functional theory calculations. The model also shows good generalization and transferability for predicting the energy levels of other organic electronic materials. This work not only provides a cost-effective model for predicting the energy levels of organic photovoltaic materials but also explains the potential bridge between molecular structure and electronic properties.

Highly Efficient and Stable Organic Solar Cells Enabled by a Commercialized Simple Thieno[3,2-b]thiophene Additive

Delicately manipulating nanomorphology is recognized as a vital and effective approach to enhancing the performance and stability of organic solar cells (OSCs). However, the complete removal of solvent additives with high boiling points is typically necessary to maintain the operational stability of the device. In this study, two commercially available organic intermediates, namely thieno[3,2-b]thiophene (TT) and 3,6-dibromothieno[3,2-b]thiophene (TTB) are introduced, as solid additives in OSCs. The theoretical simulations and experimental results indicate that TT and TTB may exhibit stronger intermolecular interactions with the acceptor Y6 and donor PM6, respectively. This suggests that the solid additives (SAs) can selectively intercalate between Y6 and PM6 molecules, thereby improving the packing order and crystallinity. As a result, the TT-treated PM6:Y6 system exhibits a favorable morphology, improved charge carrier mobility, and minimal charge recombination loss. These characteristics contribute to an impressive efficiency of 17.75%. Furthermore, the system demonstrates exceptional thermal stability (T80 > 2800 h at 65 °C) and outstanding photostability. The universal applicability of TT treatment is confirmed in OSCs employing D18:L8-BO, achieving a significantly higher PCE of 18.3%. These findings underscore the importance of using appropriate solid additives to optimize the blend morphology of OSCs, thereby improving photovoltaic performance and thermal stability.

Highly Efficient Organic Solar Cells with the Highly Crystalline Third Component as a Morphology Regulator

The morphology of the active layer is crucial for highly efficient organic solar cells (OSCs), which can be regulated by selecting a rational third component. In this work, the highly crystalline nonfullerene acceptor BTP-eC9 is selected as the morphology regulator in OSCs with PM6:BTP-BO-4Cl as the main system. The addition of BTP-eC9 can prolong the nucleation and crystallization progress of acceptor and donor molecules, thereby enhancing the order of molecular arrangement. Meanwhile, the nucleation and crystallization time of the donor is earlier than that of the acceptors after introducing BTP-eC9, which is beneficial for obtaining a better vertical structural phase separation. The exciton dissociation, charge transport, and charge collection are promoted effectively by the optimized morphology of the active layer, which improves the short-circuit current density and filling factor. After introducing BTP-eC9, the power conversion efficiencies (PCEs) of the ternary OSCs are improved from 17.31% to 18.15%. The PCE is further improved to 18.39% by introducing gold nanopyramid (Au NBPs) into the hole transport layer to improve photon utilization efficiency. This work indicates that the morphology can be optimized by selecting a highly crystalline third component to regulate the nucleation and crystallization progress of the acceptor and donor molecules.

Tetrahydrofuran Processable Organic Solar Cells with 19.45% Efficiency Realized by Introducing High Molecular Dipole Unit Into the Terpolymer

Developing organic solar cells (OSCs) processable with halogen-free, non-aromatic solvents is crucial for practical applications, yet challenging due to the limited solubility of most photoactive materials. This study introduces high-performance terpolymers processable in tetrahydrofuran (THF) by incorporating dithienophthalimide (DPI) into the PM6 backbone. DPI extends the absorption band, lowers HOMO levels, and improves THF solubility and film crystallinity through its large dipole moment effect. Optimal PBD-10:L8-BO devices processed with THF achieved a competitive power conversion efficiency (PCE) of 18.79%, approaching chloroform-processed devices (19.04%). By introducing PBTz-F as a second donor, ternary OSCs reached an impressive 19.45% PCE when processed with THF. This improvement stems from enhanced photon generation, improved morphology, better charge transport, longer exciton lifetimes, efficient charge dissociation and collection, and suppressed recombination. These PCEs of 18.79% and 19.45% for binary and ternary blend OSCs, respectively, represent the highest reported efficiencies for OSCs processed with halogen-free, non-aromatic solvents. This work demonstrates significant progress in eco-friendly OSC fabrication, paving the way for more sustainable and commercially viable organic photovoltaic technologies.

Constraining the Excessive Aggregation of Non-Fullerene Acceptor Molecules Enables Organic Solar Modules with the Efficiency >16

Translating high-performance organic solar cell (OSC) materials from spin-coating to scalable processing is imperative for advancing organic photovoltaics. For bridging the gap between laboratory research and industrialization, it is essential to understand the structural formation dynamics within the photoactive layer during printing processes. In this study, two typical printing-compatible solvents in the doctor-blading process are employed to explore the intricate mechanisms governing the thin-film formation in the state-of-the-art photovoltaic system PM6:L8-BO. Our findings highlight the synergistic influence of both the donor polymer PM6 and the solvent with a high boiling point on the structural dynamics of L8-BO within the photoactive layer, significantly influencing its morphological properties. The optimized processing strategy effectively suppresses the excessive aggregation of L8-BO during the slow drying process in doctor-blading, enhancing thin-film crystallization with preferential molecular orientation. These improvements facilitate more efficient charge transport, suppress thin-film defects and charge recombination, and finally enhance the upscaling potential. Consequently, the optimized PM6:L8-BO OSCs demonstrate power conversion efficiencies of 18.42% in small-area devices (0.064 cm2) and 16.02% in modules (11.70 cm2), respectively. Overall, this research provides valuable insights into the interplay among thin-film formation kinetics, structure dynamics, and device performance in scalable processing.

Manipulating Alkyl Inner Side Chain of Acceptor for Efficient As-Cast Organic Solar Cells

As-cast organic solar cells (OSCs) possess tremendous potential for low-cost commercial applications. Herein, five small-molecule acceptors (A1-A5) are designed and synthesized by selectively and elaborately extending the alkyl inner side chain flanking on the pyrrole motif to prepare efficient as-cast devices. As the extension of the alkyl chain, the absorption spectra of the films are gradually blue-shifted from A1 to A5 along with slightly uplifted lowest unoccupied molecular orbital energy levels, which is conducive for optimizing the trade-off between short-circuit current density and open-circuit voltage of the devices. Moreover, a longer alkyl chain improves compatibility between the acceptor and donor. The in situ technique clarifies that good compatibility will prolong molecular assembly time and assist in the preferential formation of the donor phase, where the acceptor precipitates in the framework formed by the donor. The corresponding film-formation dynamics facilitate the realization of favorable film morphology with a suitable fibrillar structure, molecular stacking, and vertical phase separation, resulting in an incremental fill factor from A1 to A5-based devices. Consequently, the A3-based as-cast OSCs achieve a top-ranked efficiency of 18.29%. This work proposes an ingenious strategy to manipulate intermolecular interactions and control the film-formation process for constructing high-performance as-cast devices.

Improving Miscibility of Polymer Donor and Polymer Acceptor by Reducing Chain Entanglement for Realizing 18.64 % Efficiency All Polymer Solar Cells

All-polymer solar cells have experienced rapid development in recent years by the emergence of polymerized small molecular acceptors (PSMAs). However, the strong chain entanglements of polymer donors (PDs) and polymer acceptors (PAs) decrease the miscibility of the resulting polymer mixtures, making it challenging to optimize the blend morphology. Herein, we designed three PAs, namely PBTPICm-BDD, PBTPICγ-BDD and PBTPICF-BDD, by smartly using a BDD unit as the polymerized unit to copolymerize with different Y-typed non-fullerene small molecular acceptors (NF-SMAs), thus achieving a certain degree of distortion and giving the polymer system enough internal space to reduce the entanglements of the polymer chains. Such effects increase the chances of the PD being interspersed into the acceptor material, which improve the solubility between the PD and PA. The PBTPICγ-BDD and PBTPICF-BDD displayed better miscibility with PBQx-TCl, leading to a well optimized morphology. As a result, high power conversion efficiencies (PCEs) of 17.50 % and 17.17 % were achieved for PBQx-TCl : PBTPICγ-BDD and PBQx-TCl : PBTPICF-BDD devices, respectively. With the addition of PYFT-o as the third component into PBQx-TCl : PBTPICγ-BDD blend to further extend the absorption spectral coverage and finely tune microstructures of the blend morphology, a remarkable PCE of 18.64 % was realized finally.

Zirconium Oxide Doped Organosilica Nanodots as Light- and Charge-Management Cathode Interlayer for Highly Efficient and Stable Inverted Organic Solar Cells

In this work, it is reported that zirconium oxide (ZrO2) doped organosilica nanodots (OSiNDs: ZrO2) with light- and charge-management properties serve as efficient cathode interlayers for high-efficiency inverted organic solar cells (i-OSCs). ZrO2 doping effectively improves the light harvesting of the active layer, the physical contact between the active layer, as well as the electron collection property by habiting charge recombination loss. Consequently, all devices utilizing the OSiNDs: ZrO2 cathode interlayer exhibit enhanced power conversion efficiency (PCE). Specifically, i-OSCs based on PM6:Y6 and PM6:BTP-eC9 achieve remarkable PCEs of 17.16% and 18.43%, respectively. Furthermore, the PCE of device based on PM6:Y6 maintains over 97.2% of its original value following AM 1.5G illumination (including UV light) at 100 mW cm-2 for 600 min.

Halogenated Dibenzo[f,h]quinoxaline Units Constructed 2D-Conjugated Guest Acceptors for 19% Efficiency Organic Solar Cells

Halogenation of Y-series small-molecule acceptors (Y-SMAs) is identified as an effective strategy to optimize photoelectric properties for achieving improved power-conversion-efficiencies (PCEs) in binary organic solar cells (OSCs). However, the effect of different halogenation in the 2D-structured large π-fused core of guest Y-SMAs on ternary OSCs has not yet been systematically studied. Herein, four 2D-conjugated Y-SMAs (X-QTP-4F, including halogen-free H-QTP-4F, chlorinated Cl-QTP-4F, brominated Br-QTP-4F, and iodinated I-QTP-4F) by attaching different halogens into 2D-conjugation extended dibenzo[f,h]quinoxaline core are developed. Among these X-QTP-4F, Cl-QTP-4F has a higher absorption coefficient, optimized molecular crystallinity and packing, suitable cascade energy levels, and complementary absorption with PM6:L8-BO host. Moreover, among ternary PM6:L8-BO:X-QTP-4F blends, PM6:L8-BO:Cl-QTP-4F obtains a more uniform and size-suitable fibrillary network morphology, improved molecular crystallinity and packing, as well as optimized vertical phase distribution, thus boosting charge generation, transport, extraction, and suppressing energy loss of OSCs. Consequently, the PM6:L8-BO:Cl-QTP-4F-based OSCs achieve a 19.0% efficiency, which is among the state-of-the-art OSCs based on 2D-conjugated Y-SMAs and superior to these devices based on PM6:L8-BO host (17.70%) and with guests of H-QTP-4F (18.23%), Br-QTP-4F (18.39%), and I-QTP-4F (17.62%). The work indicates that halogenation in 2D-structured dibenzo[f,h]quinoxaline core of Y-SMAs guests is a promising strategy to gain efficient ternary OSCs.

Revealing the Role of Dynamic and Static Disorder on Charge-Transfer-State Absorption in Polymer Solar Cells

In polymer solar cells (PSCs), charge-transfer (CT) state absorption plays an important role in evaluating the CT-state energy and energy loss. However, due to the disordered nature of polymers, a comprehensive understanding of CT absorption properties remains elusive. Especially, the dominant role of dynamic and static disorder in determining CT absorption is frequently debated. Herein, we theoretically constructed an organic donor-acceptor model to investigate the impact of these two types of disorders on CT absorption properties. It is demonstrated that the CT absorption properties depend significantly on the type of disorder. Specifically, it is found that dynamic disorder has a more significant impact on the peak and position of CT absorption as well as the broadening properties, compared to static disorder. The study indicates that minimizing dynamic disorder can lead to a reduction in overall disorder, which is beneficial for improving the performance of PSCs.

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.

Nitrogen-Blowing Assisted Strategy for Fabricating Large-Area Organic Solar Modules with an Efficiency of 15.6

Organic solar cells (OSCs) are one of the most promising photovoltaic technologies due to their affordability and adaptability. However, upscaling is a critical issue that hinders the commercialization of OSCs. A significant challenge is the lack of cost-effective and facile techniques to modulate the morphology of the active layers. The slow solvent evaporation leads to an unfavorable phase separation, thus resulting in a low power conversion efficiency (PCE) of organic solar modules. Here, a nitrogen-blowing assisted method is developed to fabricate a large-area organic solar module (active area = 12 cm2) utilizing high-boiling-point solvents, achieving a PCE of 15.6%. The device fabricated with a high-boiling-point solvent produces a more uniform and smoother large-area film, and the assistance of nitrogen-blowing accelerates solvent evaporation, resulting in an optimized morphology with proper phase separation and finer aggregates. Moreover, the device fabricated by the nitrogen-blowing assisted method exhibits improved exciton dissociation, balanced carrier mobility, and reduced charge recombination. This work proposes a universal and cost-effective technique for the fabrication of high-efficiency organic solar modules.

PC71BM as Morphology Regulator for Highly Efficient Ternary Organic Solar Cells with Bulk Heterojunction or Layer-by-Layer Configuration

The ternary strategy is one of the effective methods to regulate the morphology of the active layer in organic solar cells (OSCs). In this work, the ternary OSCs with bulk heterojunction (BHJ) or layer-by-layer (LbL) active layers are prepared by using the polymer donor PM6 and the non-fullerene acceptor L8-BO as the main system and the fullerene acceptor PC71BM as the third component. The power conversion efficiencies (PCEs) of BHJ OSCs and LbL OSCs are increased from 17.10% to 18.02% and from 17.20% to 18.20% by introducing PC71BM into the binary active layer, respectively. The in situ UV-vis absorption spectra indicate that the molecular aggregation and crystallization process can be prolonged by introducing PC71BM into the PM6:L8-BO or PM6/L8-BO active layer. The molecular orientation and molecular crystallinity in the active layer are optimized by introducing the PC71BM into the binary BHJ or LbL active layers, which can be confirmed by the experimental results of grazing incidence wide-angle X-ray scattering. This study demonstrates that the third component PC71BM can be used as a morphology regulator to regulate the morphology of BHJ or LbL active layers, thus effectively improving the performance of BHJ and LbL OSCs.

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.