Large-area Flexible Organic Solar Cells: Printing Technologies and Modular Design

Controlled Solution Flow via Patterned Meniscus Assist for Elongated Exciton Diffusion Length to Approaching 20% Efficiency in Pseudo-Planar Heterojunction Organic Solar Cells

Precisely controlling the ideal vertical phase morphology of blade-coated pseudo-planar heterojunction (PPHJ) organic photovoltaics presents a key challenge due to Marangoni flow and coffee-ring effect, which further limits large-area film uniformity and shortens exciton diffusion length. Here, the patterned meniscus assist (PMA) strategy is used to stretch polymer chains and construct regular micropatterns to facilitate donor/acceptor inter-penetration, resulting in a high-performance printable PPHJ device with extended exciton diffusion length (from ≈45 to ≈56 nm). More importantly, micropatterns can mitigate Marangoni flow and promote film uniformity by enhancing solution flow. Consequently, the PPHJ device via PMA strategy exhibits one of the highest power conversion efficiencies (PCE) of 19.91% (certified as 19.63%) for the D18/BO-4Cl:L8-BO ternary system. Furthermore, the enlarged-area (1 and 16.94 cm2) devices show competitive PCEs of 18.90%/17.05% with one of the minimum PCE losses (5.07%/14.36%) during area expansion. This PMA strategy provides a feasible guiding avenue for realizing ideal active layer morphology to obtain large-scale, high-efficiency PPHJ devices.

Volatile Imide Additives with Large Dipole and Special Film Formation Kinetics Enable High-Performance Organic Solar Cells

Large dipole moment additives have strong interactions with the host materials, which can optimize morphology and improve the photovoltaic performance of organic solar cells (OSCs). However, these additives are difficult to remove due to their strong intermolecular interactions, which may impair stability. Developing volatile additives with large dipole moments is challenging. Herein, we first report volatile imide additives that could effectively improve the performance of OSCs through morphology modification. Three additives N-(o-chlorophenyl)phthalimide (oClPA), N-(m-chlorophenyl)phthalimide (mClPA), and N-(p-chlorophenyl)phthalimide (pClPA) were screened to investigate the effort of positional isomerization on molecular configuration and interaction. These additives (ClPAs) have larger dipole moments (2.0664 Debye for oClPA, 4.2361 Debye for mClPA, and 4.7896 Debye for pClPA) compared to reported solid additives. In contrast to traditional simultaneous nucleation and crystal growth, ClPAs could induce the acceptor to nucleate first and then grow, which contributes to forming high-quality acceptor domains with better crystallinity. To our knowledge, this unique film formation kinetics was reported first. The power conversion efficiency (PCE) of OSCs based on PM6:BTP-eC9 treated with pClPA was improved from 16.13 % to 18.58 %. Additive pClPA also performed well in PM6:L8-BO, PM6:Y6, and D18:L8-BO systems, and a high PCE of 19.04 % was achieved. Our results indicate using imide unit to construct solid additives is a simple and effective strategy, and the positional isomerization of halogen atom also has a large effect on the photovoltaic performance.

A Strain Relaxation Modulation for Printing High-performance Flexible Pseudo-Planar Heterojunction Organic Solar Cells

The rational toughening of photosensitive films is crucial for the development of robust and flexible organic solar cells (F-OSCs), which are always influenced by mechanical strain and thermodynamic relaxation within the films. Nevertheless, the potential determinants of these properties and quantitative metrics modulating the overall performance of flexible devices have not been thoroughly defined. Herein, a fine-grain strengthening strategy is demonstrated for mitigating the excessive aggregation or crystallization in small-molecule acceptor films, the secondary thermal relaxation of side chains in polyethylene oxide (PEO) local motion restricts the free fluctuation volume through hydrogen-bonding interactions, thereby suppressing the non-ideal thermodynamic behavior and residual-enriched state. These contribute to an increase in yield strength and a reduction in microcracks while enhancing the fracture energy at the donor/acceptor interface. Finally, the optimal F-OSCs demonstrate champion PCEs of 19.12% (0.04 cm2) and 16.92% (1.00 cm2), and maintain 80% of their initial efficiency after heating at 85 °C for 2600 h. Besides, the flexibility and mechanical robustness of devices are also optimized, the elastic modulus and stiffness are decreased by 50.68% and 5.71%. This work provides interesting references for the synergistic enhancement of efficiency, mechanical and environmental stability in flexible organic photovoltaics.

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.

The Conjugated/Non-Conjugated Linked Dimer Acceptors Enable Efficient and Stable Flexible Organic Solar Cells

The fabrication of the flexible devices with excellent photovoltaic performance and stability is critical for the commercialization of organic solar cells (OSCs). Herein, the conjugated dimer acceptor DY-TVCl and the non-conjugated dimer acceptor DY-3T based on the monomer MY-BO are synthesized to regulate the molecular glass transition temperatures (Tg) for improving the morphology stability of active layer films. And the crack onset strain values for the blend films based on dimer acceptors are superior than that of small molecule, which are beneficial for the preparation of flexible devices. Accordingly, the binary device based on PM6:DY-TVCl achieves a maximum power conversion efficiency (PCE) of 18.01%. Meanwhile, the extrapolated T80 (time to reach 80% of initial PCE) lifetimes of the PM6:DY-TVCl-based device and PM6:DY-3T-based device are 3091 and 2227 h under 1-sun illumination, respectively, which are better than that of the PM6:MY-BO-based device (809 h). Furthermore, the flexible devices based on DY-TVCl and DY-3T exhibit the efficiencies of 15.23% and 14.34%, respectively. This work affords a valid approach to improve the stability and mechanical robustness of OSCs, as well as ensuring the reproducibility of organic semiconductors during mass production.

Preparation of Dual-Asymmetric Acceptors via Selenium Substitution Combined with Terminal Group Optimization Strategy for High Efficiency Organic Solar Cells

Improving both the open-circuit voltage (VOC) and short-circuit current density (JSC) through the development of photovoltaic materials to achieve high power conversion efficiency (PCE) is critical and a significant challenge for organic solar cells (OSCs). Here, we designed novel dual-asymmetric acceptors A-SSe-TCF and A-SSe-LSF by simultaneously asymmetrically regulating the backbone and terminal groups and investigated their synergistic effects on photovoltaic performance in comparison with the monoasymmetric acceptor A-SSe-4F. The dual-asymmetric acceptors exhibit broader spectral absorption and larger half-molecule dipole moment differences, which favored the enhancement of JSC and the reduction of energy loss (Eloss). Among the binary blends, PM6:A-SSe-TCF exhibits superior phase separation, vertical phase distribution morphology, and more ordered π-π stacking compared to PM6:A-SSe-LSF and PM6:A-SSe-4F. As a result, OSCs based on PM6:A-SSe-TCF achieved a higher PCE of 18.53% with both higher VOC and JSC due to the suppressed nonradiative recombination and enhanced charge extraction capabilities. Furthermore, by incorporating A-SSe-TCF as the third component, the PM6:L8-BO:A-SSe-TCF-based device achieves a champion PCE of 19.73% without VOC loss on account of the decrement of Eloss. The novel dual-asymmetric strategy provides new insights into the molecular design and the improvement of PCE for OSCs.

Synergistic Multimodal Energy Dissipation Enhances Certified Efficiency of Flexible Organic Photovoltaics beyond 19

All-polymer organic solar cells (OSCs) have shown unparalleled application potential in the field of flexible wearable electronics in recent years due to the excellent mechanical and photovoltaic properties. However, the small molecule acceptors after polymerization in still retain some mechanical and aggregation properties of the small molecule, falling short of the ductility requirements for flexible devices. Here, based on the multimodal energy dissipation theory, the mechanical and photovoltaic properties of flexible devices are co-enhanced by adding the thermoplastic elastomer material (polyurethane, PU) to the PM6:PBQx-TF:PY-IT-based active layer films. The construction of multi-fiber network structure and the decrease of films' residual stresses contribute to the enhancement of carrier transport properties and the decrease of defect state density. Eventually, the PCE (power conversion efficiency) of 19.40% is achieved on the flexible devices with an effective area of 0.102 cm2, and the third-party certified PCE reaches 19.07%, which is the highest PCE for flexible OSCs currently available. To further validate the potential of this strategy for large-area module applications, the 25-cm2-based flexible and super-flexible modules are prepared with the PCEs of 15.48% and 14.61%, respectively, and demonstration applications are implemented.

Perspective on Flexible Organic Solar Cells for Self-Powered Wearable Applications

The growing advancement of wearable technologies and sophisticated sensors has driven the need for environmentally friendly and reliable energy sources with robust mechanical stability. Flexible organic solar cells (OSCs) have become promising substitutes for traditional energy solutions thanks to their remarkable mechanical flexibility and high power conversion efficiency (PCE). These unique properties allow flexible OSCs to seamlessly integrate with diverse devices and substrates, making them an excellent choice for powering various electronic devices by efficiently harvesting solar energy. This review summarizes recent achievements in flexible OSCs from the perspective of self-powered wearable applications. It discusses advancements in materials, including substrates and transparent electrodes, evaluates performance criteria, and compares the PCEs of flexible OSCs to their rigid counterparts. Subsequently, novel applications of flexible OSCs in self-powered wearable applications are explored. Finally, a summary and perspectives on the current challenges and obstacles facing flexible OSCs and their applications in self-powered wearables are provided, aiming to inspire further research toward practical implementations.

Managing Solvent Complexes to Amplify Ripening Process by Covalent Interaction Driving Force Under External Field for Perovskite Photovoltaic

Up to now, post-annealing is most commonly used to post treat the perovskite film to accelerate the ripening process. Nonetheless, the top-down crystallization mechanism impedes the efficient desolvation of solvent complexes. Thus, residual solvent complexes tend to accumulate at the bottom of the film during the ripening process and deteriorate the device. Here, a new strategy with unique concept is promoted to amplify ripening process of perovskite film, in which a nematic thermotropic liquid crystal (LC) molecular is introduced to facilitate the conversion of solvent complexes by utilizing the liquid crystalline behavior under external field. Upon the concurrent application of thermal and force fields, the covalent interaction between LC and solvent complexes generates a driving force, which promotes upward migration of solvent complexes, thereby facilitating their engagement in the ripening process. In addition, the driving force under external fields assists the flattening of grain boundary grooves. Therefore, film quality is improved efficiently with amplified ripening process and adequately handled buried interface. Based on the positive effects, the devices achieve a champion efficiency of 25.24%, and sustained ≈75% of its initial efficiency level even after undergoing a damp heat test (85 °C/85% RH) for 1400 h.

Modulating Aggregation Behavior by Ternary Strategies for Efficient and Stable Thick-Film Organic Solar Cells

Functional third components targeted to improve a specific property of organic solar cells is an effective strategy. However, introducing a third component to simultaneously improve efficiency and stability and achieve good performance in thick-film devices has rarely been reported. Herein, low diffusion third components IDCN and ID2CN are reported to achieve a power conversion efficiency (PCE) of 18.08% and a high short-circuit current (J SC) of 27.82 mA cm-2, one of the highest values based on PM6:Y6. They increase light harvesting in the range of 400-500 nm while enhancing energy transfer via Förster resonance energy transfer (FRET). A tightly ordered molecular arrangement is achieved by modulating the preaggregation and film formation kinetics of Y6, which enhance exciton dissociation and charge transport. Moreover, the low-diffusion third component can effectively restrict the diffusion of Y6 to improve the morphology stability, and the T90 lifetime is increased from 689 to 1545 h. In 300 nm thick-film devices, PM6:ID2CN:Y6 achieves a PCE of 15.01%, much higher than PM6:Y6's 12.83%, demonstrating the great potential of ID2CN in thick-film devices.

Green Printing for Scalable Organic Photovoltaic Modules by Controlling the Gradient Marangoni Flow

Despite the rapid development in the performances of organic solar cells (OSCs), high-performance OSC modules based on green printing are still limited. The severe Coffee-ring effect (CRE) is considered to be the primary reason for the nonuniform distribution of active layer films. To solve this key printing problem, the cosolvent strategy is presented to deposit the active layer films. The guest solvent Mesitylene with a higher boiling point and a lower surface tension is incorporated into the host solvent o-XY to optimize the rheological properties, such as surface tension and viscosity of the active layer solutions. And the synergistic effect of inward Marangoni flow generation and solution thickening caused by the cosolvent strategy can effectively restrain CRE, resulting in highly homogeneous large-area active layer films. In addition, the optimized crystallization and phase separation of active layer films effectively accelerate the charge transport and exciton dissociation of devices. Consequently, based on PM6:BTP-eC9 system, the device prepared with the co-solvent strategy shows the a power conversion efficiency of 17.80%. Moreover, as the effective area scales to 1 and 16.94 cm2, the recorded performances are altered to 16.71% and 14.58%. This study provides a universal pathway for the development of green-printed high-efficiency organic photovoltaics.

Polymer-Entangled Spontaneous Pseudo-Planar Heterojunction for Constructing Efficient Flexible Organic Solar Cells

Flexible organic solar cells (FOSCs) have attracted considerable attention from researchers as promising portable power sources for wearable electronic devices. However, insufficient power conversion efficiency (PCE), intrinsic stretchability, and mechanical stability of FOSCs remain severe obstacles to their application. Herein, an entangled strategy is proposed for the synergistic optimization of PCE and mechanical properties of FOSCs through green sequential printing combined with polymer-induced spontaneous gradient heterojunction phase separation morphology. Impressively, the toughened-pseudo-planar heterojunction (Toughened-PPHJ) film exhibits excellent tensile properties with a crack onset strain (COS) of 11.0%, twice that of the reference bulk heterojunction (BHJ) film (5.5%), which is among the highest values reported for the state-of-the-art polymer/small molecule-based systems. Finite element simulation of stress distribution during film bending confirms that Toughened-PPHJ film can release residual stress well. Therefore, this optimal device shows a high PCE (18.16%) with enhanced (short-circuit current density) JSC and suppressed energy loss, which is a significant improvement over the conventional BHJ device (16.99%). Finally, the 1 cm2 flexible Toughened-PPHJ device retains more than 92% of its initial PCE (13.3%) after 1000 bending cycles. This work provides a feasible guiding idea for future flexible portable power supplies.

Achieving Desired Pseudo-Planar Heterojunction Organic Solar Cells via Binary-Dilution Strategy

The sequential deposition process has demonstrated the great possibility to achieve a photolayer architecture with an ideal gradient phase separation morphology, which has a vital influence on the physical processes that determine the performance of organic solar cells (OSCs). However, the controllable preparation of pseudo-planar heterojunction (P-PHJ) with gradient distribution has not been effectively elucidated. Herein, a binary-dilution strategy is proposed, the PM6 solution with micro acceptor BO-4Cl and the L8-BO solution with micro donor PM6 respectively, to form P-PHJ film. This architecture exists good donor (D) and acceptor (A) vertical gradient distribution and larger D/A interpenetrating regions, which promotes exciton generation and dissociation, shortens charge transport distance and optimizes carrier dynamics. Moreover, the dilution of PM6 by BO-4Cl promotes the regulation of active layer aggregation size and phase purity, thus alleviating energy disorder and voltage loss. As a result, the P-PHJ device exhibits an outstanding power conversion efficiency of 19.32% with an excellent short-circuit current density of 26.92 mA cm-2 , much higher than planar binary heterojunction (17.67%) and ternary bulk heterojunction (18.49%) devices. This research proves a simple but effective method to provide an avenue for constructing desirable active layer morphology and high-performance OSCs.

Suppressed Ion Migration in FA-Rich Perovskite Photovoltaics through Enhanced Nucleation of Encapsulation Interface

With excellent homogeneity, compactness and controllable thickness, atomic layer deposition (ALD) technology is widely used in perovskite solar cells (PSCs). However, residual organic sources and undesired reactions pose serious challenges to device performance as well as stability. Here, ester groups of poly(ethylene-co-vinyl acetate) are introduced as a reaction medium to promote the nucleation and complete conversion of tetrakis(dimethylamino)tin(IV) (TDMA-Sn). Through simulations and experiments, it is verified that ester groups as Lewis bases can coordinate with TDMA-Sn to facilitate homogeneous deposition of ALD-SnOx , which acts as self-encapsulated interface with blocking properties against external moisture as well as internal ion migration. Meanwhile, a comprehensive evaluation of the self-encapsulated interface reveals that the energy level alignment is optimized to improve the carrier transport. Finally, the self-encapsulated device obtains a champion photovoltaic conversion efficiency (PCE) of 22.06% and retains 85% of the initial PCE after being stored at 85 °C with relative humidity of 85% for more than 800 h.

Delayed Crystallization Kinetics Allowing High-Efficiency All-Polymer Photovoltaics with Superior Upscaled Manufacturing

Though encouraging performance is achieved in small-area organic photovoltaics (OPVs), reducing efficiency loss when evoluted to large-area modules is an important but unsolved issue. Considering that polymer materials show benefits in film-forming processability and mechanical robustness, a high-efficiency all-polymer OPV module is demonstrated in this work. First, a ternary blend consisting of two polymer donors, PM6 and PBQx-TCl, and one polymer acceptor, PY-IT, is developed, with which triplet state recombination is suppressed for a reduced energy loss, thus allowing a higher voltage; and donor-acceptor miscibility is compromised for enhanced charge transport, thus resulting in improved photocurrent and fill factor; all these contribute to a champion efficiency of 19% for all-polymer OPVs. Second, the delayed crystallization kinetics from solution to film solidification is achieved that gives a longer operation time window for optimized blend morphology in large-area module, thus relieving the loss of fill factor and allowing a record efficiency of 16.26% on an upscaled module with an area of 19.3 cm2 . Besides, this all-polymer system also shows excellent mechanical stability. This work demonstrates that all-polymer ternary systems are capable of solving the upscaled manufacturing issue, thereby enabling high-efficiency OPV modules.

Impact of Electrostatic Interaction on Non-radiative Recombination Energy Losses in Organic Solar Cells Based on Asymmetric Acceptors

Reducing non-radiative recombination energy loss (ΔE3 ) is one key to boosting the efficiency of organic solar cells. Although the recent studies have indicated that the Y-series asymmetric acceptors-based devices featured relatively low ΔE3 , the understanding of the energy loss mechanism derived from molecular structure change is still lagging behind. Herein, two asymmetric acceptors named BTP-Cl and BTP-2Cl with different terminals were synthesized to make a clear comparative study with the symmetric acceptor BTP-0Cl. Our results suggest that asymmetric acceptors exhibit a larger difference of electrostatic potential (ESP) in terminals and semi-molecular dipole moment, which contributes to form a stronger π-π interaction. Besides, the experimental and theoretical studies reveal that a lower ESP-induced intermolecular interaction can reduce the distribution of PM6 near the interface to enhance the built-in potential and decrease the charge transfer state ratio for asymmetric acceptors. Therefore, the devices achieve a higher exciton dissociation efficiency and lower ΔE3 . This work establishes a structure-performance relationship and provides a new perspective to understand the state-of-the-art asymmetric acceptors.

Self-encapsulated wearable perovskite photovoltaics via lamination process and its biomedical application

Flexible perovskite solar cells (PSCs) are highly promising photovoltaic technologies due to the prospect of integration with wearable devices. However, conventional encapsulation strategies for flexible devices often cause secondary damage to the perovskite crystals, which affects device performance. Here, we present self-encapsulated flexible PSCs realized by lamination technology. The conversion of perovskite crystals is achieved by the diffusion of lead iodide and ammonium halide under the effect of temperature and pressure. In addition, the hydrogen bonding of the introduced polyacrylamide enhances the connections of the integral device while improving the crystal quality. The self-encapsulated flexible PSCs achieve an outstanding photovoltaic conversion efficiency of 22.33%, and comprehensive stability tests are conducted based on wearable device application scenarios to verify the feasibility. Finally, 25 cm² wearable perovskite modules are successfully applied into the neuro-assisted wearable devices.

The solvent effect on the morphology and molecular ordering of benzothiadiazole-based small molecule for inkjet-printed thin-film transistors

A small molecule organic semiconductor, D(D'-A-D')₂ comprising benzothiadiazole as an acceptor, 3-hexylthiophene, and thiophene as donors, was successfully synthesized. X-ray diffraction and atomic force microscopy were used to investigate the effect of a dual solvent system with varying ratios of chloroform and toluene on film crystallinity and film morphology via inkjet printing. The film prepared with a chloroform to toluene ratio of 1.5 : 1 showed better performance with improved crystallinity and morphology due to having enough time to control the arrangement of molecules. In addition, by optimizing ratios of CHCl₃ to toluene, the inkjet-printed TFT based on 3HTBTT using a CHCl₃ and toluene ratio of 1.5 : 1 was successfully fabricated and exhibited a hole mobility of 0.01 cm² V^-1 s^-1 due to the improved molecular ordering of the 3HTBTT film.

Ionic Liquid Assisted Imprint for Efficient and Stable Quasi-2D Perovskite Solar Cells with Controlled Phase Distribution

Although two-dimensional perovskite devices are highly stable, they also lead to a number of challenges. For instance, the introduction of large organic amines makes crystallization process complicated, causing problems such as generally small grain size and blocked charge transfer. In this work, imprint assisted with methylamine acetate were used to improve the morphology of the film, optimize the internal phase distribution, and enhance the charge transfer of the perovskite film. Specifically, imprint promoted the dispersion of spacer cations in the recrystallization process with the assistance of methylamine acetate, thus inhibited the formation of low-n phase induced by the aggregation of spacer cations and facilitated the formation of 3D-like phase. In this case, the corresponding quasi-2D perovskite solar cells delivered improved efficiency and exhibited superior stability. Our work provides an effective strategy to obtain uniform phase distribution for quasi-2D perovskite.