Solid Additive-Assisted Layer-by-Layer Processing for 19% Efficiency Binary Organic Solar Cells

Minimizing Energy Loss by Designing Multifunctional Solid Additives to Independent Regulation of Donor and Acceptor Layers for Efficient LBL Polymer Solar Cells

Solid additives are crucial in layer-by-layer (LBL) polymer solar cells (PSCs). Despite its importance, the simultaneous application of solid additives into both donor and acceptor layers has been largely overlooked. In this work, two multifunctional solid additives are actively designed, and investigated the synergistic effect on both donor and acceptor layers. Incorporating the multifunctional solid additives into the donor layer could effectively enhance the aggregation and molecular stacking of the donor polymer, leading to reduced energy disorder and minimizing ΔE2. When the multifunctional solid additives are introduced into the acceptor layer, they just play a role in optimizing the morphology, thereby reducing the ΔE3. Excitedly, the simultaneous addition of the multifunctional solid additives into both donor and acceptor layers produced a synergistic effect for decreasing ΔE2 and ΔE3 simultaneously, especially adding SA2, thus enabling an excellent power conversion efficiency (PCE) of 19.95% (certified as 19.68%) with an open-circuit voltage (Voc) of 0.921 V, a short circuit current density (Jsc) of 27.08 mA cm-2 and a fill factor (FF) of 79.98%. The work highlights the potential of multifunctional solid additives in independently regulating the properties of donor and acceptor layers, which is expected as a promising approach for further developing higher performance PSCs.

An n-Doped Organic Cross-Linked Electron Transport Layer with High Electrical Conductivity for High-Efficiency Tandem Organic Photovoltaics

With merits of good solution processability, intrinsic flexibility, etc, organic/organic interconnecting layers (ICLs) are highly desirable for tandem organic photovoltaics (OPVs). Herein, an n-doped cross-linked organic electron transport layer (ETL), named c-NDI-Br:PEI is developed, via a simple in situ quaternization reaction between bromopentyl-substituted naphthalene diimide derivative (NDI-Br) and polyethylenimine (PEI). Due to strong self-doping, c-NDI-Br:PEI films exhibit a high electrical conductivity (0.06 S cm-1), which is important for efficient hole and electron reombination in ICL of tandem OPVs. In addition, the cross-linked ETLs show strong work function modulation ability, and good solvent-resistance. The above features enable c-NDI-Br:PEI to function as an efficient ETL not only for single-junction OPVs, but also for tandem devices without any metal layer in ICL. Under solar radiation, the single-junction device with c-NDI-Br:PEI as ETL achieves a power conversion efficiency (PCE) of 18.18%, surpassing the ZnO-based device (17.09%). The homo- and hetero-tandem devices with m-PEDOT:PSS:c-NDI-Br:PEI as ICL exhibit remarkable PCEs of 19.06% and 20.06%, respectively. Under 808 nm laser radiation with a photon flux of 57 mW cm-2, the homo-tandem device presents a superior PCE of 38.5%. This study provides a new ETL for constructing all-solution-processed organic/organic ICL, which can be integrated in flexible and wearable devices.

Different Sized Cycloalkyl Chains on Non-Fullerene Acceptors Enhance Molecular Packing, Film Morphology and Charge Transport for 19.62% Efficiency Organic Solar Cells

This work addresses the challenge of achieving advanced fibril morphology of non-fullerene acceptors (NFAs) in layer-by-layer organic solar cels (LBL-OSCs) by cycloalkyl chain strategy, focusing on ta series of Y6-type NFAs, namely BTP-C6, BTP-C8 and BTP-C12, featured with cyclohexyl, cyclooctyl and cyclododecyl chains with increasing steric hindrance. These side chains influenced significantly molecular planarity, packing and film morphology, which are critical for device performance. BTP-C6 exhibits optimal molecular packing and fibril network morphology, enabling efficient exciton dissociation, charge transport and balanced carrier mobilities, finally achieving PCEs of 19.28% and 19.62% with chloroform- and toluene-cast acceptor layers, respectively. BTP-C8 featuring enhanced planarity (dihedral angle 8.27°) showed the loosest packing (packing coefficient 49.6%) due to the increased steric hindrance of side chains, limiting intermolecular charge transport. Conversely, BTP-C12 formed a high crystalline and tightly packed 3D network but suffered from reduced intramolecular charge transfer caused by severe molecular distortion (dihedral angle 27.27°). The findings in this work underscore the critical role of side-chain engineering in governing molecular packing and morphology, offering a systematic understanding of the relationships between steric hindrance, crystallinity and device performance, while providing a rational design strategy for next-generation NFAs to advance high-performance LBL-OSCs.

Efficient Planar Heterojunction Organic Solar Cell with Enhanced Crystallization and Diffusivity of Acceptor

In the field of organic solar cells (OSCs), planar heterojunctions (PHJs) have received less attention. This study demonstrates that enhancing the crystallization and diffusivity of nonfullerene small-molecule acceptors (NF-SMAs) through sequential deposition significantly optimizes the morphology of PHJ OSCs, driving notable performance enhancements. An additive 1,2,4,5-tetrabromobenzene (TBrB) is employed during sequential deposition, enabling such desirable morphological control in OSCs. In situ UV-vis absorption spectroscopy reveals that TBrB selectively induces rapid aggregation of NF-SMAs, L8-BO, within subseconds. Structural analysis confirms that TBrB promotes the formation of a 3D "honeycomb" structure of L8-BO. Simultaneously, TBrB enhances L8-BO diffusivity into the D18 layer, resulting in a widened and well-intermixed region. These morphological optimizations improve the charge transfer efficiency and reduce bimolecular recombination, achieving a peak power conversion efficiency (PCE) of 19.25%. This study underscores the critical role of fine-tuning solidification processes in sequential deposition to optimize the morphology and performance of OSCs.

Achieving 20% Toluene-Processed Binary Organic Solar Cells via Secondary Regulation of Donor Aggregation in Sequential Processing

Sequential processing (SqP) of the active layer offers independent optimization of the donor and acceptor with more targeted solvent design, which is considered the most promising strategy for achieving efficient organic solar cells (OSCs). In the SqP method, the favorable interpenetrating network seriously depends on the fine control of the bottom layer swelling. However, the choice of solvent(s) for both the donor and acceptor have been mostly based on a trial-and-error manner. A single solvent often cannot achieve sufficient yet not excessive swelling, which has long been a difficulty in the high efficient SqP OSCs. Herein, two new isomeric molecules are introduced to fine-tune the nucleation and crystallization dynamics that allows judicious control over the swelling of the bottom layer. The strong non-covalent interaction between the isomeric molecule and active materials provides an excellent driving force for optimize the swelling-process. Among them, the molecule with high dipole moment promotes earlier nucleation of the PM6 and provides extended time for crystallization during SqP, improving bulk morphology and vertical phase segregation. As a result, champion efficiencies of 17.38% and 20.00% (certified 19.70%) are achieved based on PM6/PYF-T-o (all-polymer) and PM6/BTP-eC9 devices casted by toluene solvent.

Isomerization-Controlled Aggregation in Photoactive Layer: An Additive Strategy for Organic Solar Cells with Over 19.5 % Efficiency

Morphology control of the photoactive layer is crucial for achieving high-performance organic solar cells (OSCs), yet it remains a significant challenge in this field. One effective approach is the additive strategy, which fine-tunes the morphology of the photoactive layer. However, the underlying mechanisms governing the impact of different types of additives from liquid, solid, to volatile solid, on the bulk heterojunction morphology and device performance are not fully understood. Herein, we present an aggregation regulation strategy for acceptor molecules by incorporating three novel isomeric additives: 4-bromo-1,2-dichlorobenzene (LCB), 1-bromo-2,4-dichlorobenzene (SCB), and 2-bromo-1,4-dichlorobenzene (VCB) into the blend active layer. This approach optimizes the bulk heterojunction morphology and enhances the photovoltaic performance of OSCs. Our results reveal that these additives induce stepwise regulation of acceptor molecule aggregation during film formation. The liquid additive LCB primarily extends solvent evaporation time, effectively preventing excessive aggregation, while the solid additive SCB significantly shortens the aggregation period during the film evolution, resulting in the most compact molecular π-π stacking. Furthermore, the volatile solid additive VCB fine-tunes the intermolecular interactions and crystallization within the active layer, promoting optimal molecular self-assembly and aggregation for ideal molecular stacking. Consequently, the power conversion efficiencies of 19.33 % and 19.51 % were achieved for the VCB-processed D18 : L8-BO- and PM6 : L8-BO-based OSCs, respectively, outperforming the LCB-processed and SCB-processed devices.

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.

Morphology Regulation Is Achieved by Volatile Solid Additives in Halogen-Free Solvents to Fabricate Efficient Polymer Solar Cells

The meticulous control of micromorphology in high power conversion efficiency (PCE) of polymer solar cells (PSCs) typically relies on halogenated solvents, which pose serious threats to both environmental sustainability and human health. In this work, a green and efficient method for fabricating high PCE PSCs with halogen-free solvents is developed. By introducing volatile solid additives 1-bromo-2,6-dichlorobenzene (DIB) and 1-bromo-2,3,5-trichlorobenzene (TIB) into toluene solvents, the aggregation behaviors of PM6:L8-BO were meticulously regulated, forming distinct fibrous morphology; in detail, the micromorphology of vertical direction exhibited a distinct pattern of acceptor enrichment at the top and donor enrichment at the bottom, which leads to enhanced exciton dissociation efficiency, improved charge transport performance, significantly reducing charge recombination, and finally improved PCEs, as the maximum PCEs were 18.56 and 17.67%, respectively, which are notably higher than those of devices without additives. Furthermore, since the solid additives can be completely removed from the active layer, the additive-treated devices exhibit superior morphology and photovoltaic stability. This work, therefore, unveils a straightforward and environmentally friendly method for preparing efficient PSCs, which is instrumental in facilitating the large-scale commercialization of PSC technology.

Non-fullerene acceptors with high crystallinity and photoluminescence quantum yield enable >20% efficiency organic solar cells

The rational design of non-fullerene acceptors (NFAs) with both high crystallinity and photoluminescence quantum yield (PLQY) is of crucial importance for achieving high-efficiency and low-energy-loss organic solar cells (OSCs). However, increasing the crystallinity of an NFA tends to decrease its PLQY, which results in a high non-radiative energy loss in OSCs. Here we demonstrate that the crystallinity and PLQY of NFAs can be fine-tuned by asymmetrically adapting the branching position of alkyl chains on the thiophene unit of the L8-BO acceptor. It was found that L8-BO-C4, with 2-butyloctyl on one side and 4-butyldecyl on the other side, can simultaneously achieve high crystallinity and PLQY. A high efficiency of 20.42% (certified as 20.1%) with an open-circuit voltage of 0.894 V and a fill factor of 81.6% is achieved for the single-junction OSC. This work reveals how important the strategy of shifting the alkyl chain branching position is in developing high-performance NFAs for efficient OSCs.

Thiophene Copolymer Donors Containing Ester-Substituted Thiazole for Organic Solar Cells

Organic solar cells have seen significant progress in the past 2 decades with power conversion efficiencies (PCEs) exceeding 20% but mostly based on high-cost photovoltaic materials. Polythiophenes (PTs) without a fused-ring structure are good candidates as low-cost donor materials, deserving more attention for studying. In this work, ester-substituted thiazole (E-Tz) was explored as the electron-withdrawing unit to design PTs, and further optimization on the fluorinated/nonfluorinated donor segment contents via copolymerization strategy was simultaneously performed, yielding polymer donors of PTETz-100F, PTETz-80F, and PTETz-0F. Suitable temperature-dependent aggregation for reasonable phase separation and compact molecular packing for improved charge transport were achieved in the PTETz-80F-based system, resulting in higher exciton dissociation probability and charge collection probability. Thereby, devices based on PTETz-80F:L8-BO exhibited the best photovoltaic performance with a PCE of 12.69%. In addition, the synthetic complexity of PTETz-XF polymers is 46.05%, which is significantly lower than those of other representative high-performance polymer donors. This work demonstrates the feasibility of designing PTs with an E-Tz unit and the effectiveness of the copolymerization strategy on material property and device performance optimization.

Donor-Interacting Arylated Carbazole Self-Assembled Monolayer Enables Highly Efficient and Stable Organic Photovoltaics

Carbazole-derived self-assembled monolayers (SAMs) are promising materials for hole-extraction layer (HEL) in conventional organic photovoltaics (OPVs). Here, a SAM Cbz-2Ph derived from 3,6-diphenylcarbazole is demonstrated. The large molecular dipole moment of Cbz-2Ph allows the modulation of electrode work function to facilitate hole extraction and maximize photovoltage, thus improving the OPV performance. Additionally, the flanking aryls of Cbz-2Ph help establish CH-π interactions for forming a dense and well-organized SAM HEL and exhibit stronger van der Waals interactions with the donor PM6 than acceptor BTP-eC9. The stronger SAM-donor interactions modulate the PM6 distribution in PM6:BTP-eC9 bulk-heterojunction film, leading to PM6 enrichment near HEL to facilitate efficient hole extraction to the ITO anode in conventional p-i-n OPVs. Consequently, binary PM6:BTP-eC9-based devices incorporating the Cbz-2Ph HEL demonstrate an impressive efficiency of 19.18%. These cells also showcase excellent operational stability, with a T80 lifetime of ≈1260 h at the maximum power point, over 10 times longer than those using the traditional PEDOT:PSS HEL (T80 ≈96 h). Furthermore, the universal applicability of Cbz-2Ph as a HEL is evident through its successful implementation in PM6:BTP-eC9:L8-BO-F-based ternary devices and PM6:BTP-eC9-based printed OPV devices, achieving a PCE of 19.30% and 16.96%, respectively.

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.

Organic Solar Cells Based on Non-Fullerene Low Molecular Weight Organic Semiconductor Molecules

The development of narrow bandgap A-D-A- and ADA'DA-type non-fullerene small molecule acceptors (NFSMAs) along with small molecule donors (SMDs) have led to significant progress in all-small molecule organic solar cells. Remarkable power conversion efficiencies, nearing the range of 17-18 %, have been realized. These efficiency values are on par with those achieved in OSCs based on polymeric donors. The commercial application of organic photovoltaic technology requires the design of more efficient organic conjugated small molecule donors and acceptors. In recent years the precise tuning of optoelectronic properties in small molecule donors and acceptors has attracted considerable attention and has contributed greatly to the advancement of all-SM-OSCs. Several reviews have been published in this field, but the focus of this review concerns the advances in research on OSCs using SMDs and NFSMAs from 2018 to the present. The review covers the progress made in binary and ternary OSCs, the effects of solid additives on the performance of all-SM-OSCs, and the recently developed layer-by-layer deposition method for these OSCs. Finally, we present our perspectives and a concise outlook on further advances in all-SM-OSCs for their commercial application.

Dual Additive Strategy with Quasi-Planar Heterojunction Architecture Assisted in Morphology Optimization for High-Efficiency Organic Solar Cells

Achieving high-performance and stable organic solar cells (OSCs) remains a critical challenge, primarily due to the precise optimization required for active layer morphology. Herein, this work reports a dual additive strategy using 3,5-dichlorobromobenzene (DCBB) and 1,8-diiodooctane (DIO) to optimize the morphology of both bulk-heterojunction (BHJ) and quasi-planar heterojunction (Q-PHJ) based on donor D18 and acceptor BTP-eC9. The systematic results reveal that the dual additive strategy significantly promotes phase separation while inhibiting excessive aggregation, which, in turn, improves molecular order and crystallization. As a result, BHJ and Q-PHJ OSCs processed with dual additive DIO + DCBB achieve impressive power conversion efficiencies of 17.77% and 18.60%, respectively, the highest reported values for dual additive-processed OSCs. The superior performance is attributed to improved charge transport and reduced recombination losses, as evidenced by higher short-circuit current densities (JSC) and fill factors (FF). Importantly, Q-PHJ OSCs processed with either DCBB or DIO + DCBB, in comparison to BHJ OSCs, exhibit exceptional shelf-stability, maintaining 80% of their initial power conversion efficiency after 2660 and 2193 h, respectively. These findings underscore the potential of dual additive strategies to advance the development of stable, high-efficiency OSCs suitable for large-area fabrication, marking a significant step forward in renewable energy technology.

Solution Sequential Deposition Pseudo-Planar Heterojunction: An Efficient Strategy for State-of-Art Organic Solar Cells

Organic solar cells (OSCs) are considered as a promising new generation of clean energy. Bulk heterojunction (BHJ) structure has been widely employed in the active layer of efficient OSCs. However, precise regulation of morphology in BHJ is still challenging due to the competitive coupling between crystallization and phase separation. Recently, a novel pseudo-planar heterojunction (PPHJ) structure, prepared through solution sequential deposition, has attracted much attention. It is an easy-to-prepare structure in which the phase separation structures, interfaces, and molecular packing can be separately controlled. Employing PPHJ structure, the properties of OSCs, such as power conversion efficiency, stability, transparency, flexibility, and so on, are usually better than its BHJ counterpart. Hence, a comprehensive understanding of the film-forming process, morphology control, and device performance of PPHJ structure should be considered. In terms of the representative works about PPHJ, this review first introduces the fabrication process of active layers based on PPHJ structure. Second, the widely applied morphology control methods in PPHJ structure are summarized. Then, the influences of PPHJ structure on device performance and other property are reviewed, which largely expand its application. Finally, a brief prospect and development tendency of PPHJ devices are discussed with the consideration of their challenges.

Molecular engineering and structure-property relationship based on D-A chlorophyll derivative and the application in organic solar cells

The photoactive layer materials of organic solar cells (OSCs) play a critical role in achieving excellent performance. Chlorophyll derivatives are commonly used due to their environmental friendliness, low cost, and easy accessibility. However, the efficiency of OSCs based on chlorophyll is limited by their photoelectric properties. Here, we focused on the D-A structure of chlorophyll derivative ZnChl-1 and designed four molecules through rational molecular engineering. The photoelectric properties at the microscopic level were systematically studied using density functional theory (DFT). Our findings reveal that T-ZnChl-1 with triphenylamine donor unit has a smaller energy gap and ionization energy, as well as a larger spectral red shift and absorption range. This suggests that intramolecular charge transfer will be enhanced, leading to an improvement in short-circuit current. Furthermore, Y6 is used as the acceptor to construct the heterojunction interfaces. The results indicate that the T-ZnChl-1/Y6 interface exhibits more charge transfer states and higher exciton dissociation rate KCS, which will promote charge separation and lead to excellent photovoltaic performance. This work clarifies the structure-property relationship of chlorophyll derivatives and the photo-response mechanism of intermolecular charge transfer, providing a theoretical basis for developing valuable chlorophyll-based OSCs.

Beyond Conventional Enhancements: Self-Organization of a Buffer Material on Tin Oxide as a Game-Changer for Improving the Performance of Inverted Organic Solar Cells

Inverted organic solar cells (OSCs) have garnered significant interest due to their remarkable stability. In this study, the efficiency and stability of inverted OSCs are enhanced via the in situ self-organization (SO) of an interfacial modification material Phen-NaDPO onto tin oxide (SnO2). During the device fabrication, Phen-NaDPO is spin-coated with the active materials all together on SnO2. Driven by the interactions with SnO2 and the thermodynamic forces due to its high surface energy and the convection flow, Phen-NaDPO spontaneously migrates to the SnO2 interface, resulting in the formation of an in situ modification layer on SnO2. This self-organization of Phen-NaDPO not only effectively reduces the work function of SnO2, but also enhances the ordered molecular stacking and manipulates the vertical morphology of the active layer, which suppress the surface trap-assisted recombination and minimize the charge extraction. As a result, the SO devices based on PM6:Y6 exhibit significantly improved photovoltaic performance with an enhanced power conversion efficiency of 17.62%. Moreover, the stability of the SO device is also improved. Furthermore, the SO ternary devices based on PM6:D18:L8-BO achieved an impressive PCE of 18.87%, standing as one of the highest values for single-junction inverted organic solar cells to date.

Hierarchical Solid-Additive Strategy for Achieving Layer-by-Layer Organic Solar Cells with Over 19 % Efficiency

Layer-by-layer (LbL) deposition of active layers in organic solar cells (OSCs) offers immense potential for optimizing performance through precise tailoring of each layer. However, achieving high-performance LbL OSCs with distinct solid additives in each layer remains challenging. In this study, we explore a novel approach that strategically incorporates different solid additives into specific layers of LbL devices. To this end, we introduce FeCl3 into the lower donor (D18) layer as a p-type dopant to enhance hole concentration and mobility. Concurrently, we incorporate the wide-band gap conjugated polymer poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) into the upper acceptor (L8-BO) layer to improve the morphology and prolong exciton lifetime. Unlike previous studies, our approach combines these two strategies to achieve higher and more balanced electron and hole mobility without affecting device open-circuit voltage, while also suppressing charge recombination. Consequently, the power conversion efficiency (PCE) of the D18+FeCl3/L8-BO device increases to 18.12 %, while the D18/L8-BO+PFO device attains a PCE of 18.79 %. These values represent substantial improvements over the control device's PCE of 17.59 %. Notably, when both FeCl3 and PFO are incorporated, the D18+FeCl3/L8-BO+PFO device achieves a remarkable PCE of 19.17 %. In summary, our research results demonstrate the effectiveness of the layered solid additive strategy in improving OSC performance.

An ultraflexible energy harvesting-storage system for wearable applications

The swift progress in wearable technology has accentuated the need for flexible power systems. Such systems are anticipated to exhibit high efficiency, robust durability, consistent power output, and the potential for effortless integration. Integrating ultraflexible energy harvesters and energy storage devices to form an autonomous, efficient, and mechanically compliant power system remains a significant challenge. In this work, we report a 90 µm-thick energy harvesting and storage system (FEHSS) consisting of high-performance organic photovoltaics and zinc-ion batteries within an ultraflexible configuration. With a power conversion efficiency surpassing 16%, power output exceeding 10 mW cm-2, and an energy density beyond 5.82 mWh cm-2, the FEHSS can be tailored to meet the power demands of wearable sensors and gadgets. Without cumbersome and rigid components, FEHSS shows immense potential as a versatile power source to advance wearable electronics and contribute toward a sustainable future.

Electrostatic Potential Design of Solid Additives for Enhanced Molecular Order of Polymer Donor in Efficient Organic Solar Cells

Polymeric semiconducting materials struggle to achieve fast charge mobility due to low structural order. In this work, five 1H-indene-1,3(2H)dione-benzene structured halogenated solid additives namely INB-5F, INB-3F, INB-1F, INB-1Cl, and INB-1Br with gradually varied electrostatic potential are designed and utilized to regulate the structural order of polymer donor PM6. Molecular dynamics simulations demonstrate that although the dione unit of these additives tends to adsorb on the backbone of PM6, the reduced electrostatic potential of the halogen-substituted benzene can shift the benzene interacting site from alkyl side chains to the conjugated backbone of PM6, not only leading to enhanced π-π stacking in out-of-plane but also arising new π-π stacking in in-plane together with the appearance of multiple backbone stacking in out-of-plane, consequent to the co-existence of face-on and edge-on molecular orientations. This molecular packing transformation further translates to enhanced charge transport and suppressed carrier recombination in their photovoltaics, with a maximum power conversion efficiency of 19.4% received in PM6/L8-BO layer-by-layer deposited organic solar cells.

Polymer Fiber Rigid Network with High Glass Transition Temperature Reinforces Stability of Organic Photovoltaics

Organic photovoltaics (OPVs) need to overcome limitations such as insufficient thermal stability to be commercialized. The reported approaches to improve stability either rely on the development of new materials or on tailoring the donor/acceptor morphology, however, exhibiting limited applicability. Therefore, it is timely to develop an easy method to enhance thermal stability without having to develop new donor/acceptor materials or donor-acceptor compatibilizers, or by introducing another third component. Herein, a unique approach is presented, based on constructing a polymer fiber rigid network with a high glass transition temperature (Tg) to impede the movement of acceptor and donor molecules, to immobilize the active layer morphology, and thereby to improve thermal stability. A high-Tg one-dimensional aramid nanofiber (ANF) is utilized for network construction. Inverted OPVs with ANF network yield superior thermal stability compared to the ANF-free counterpart. The ANF network-incorporated active layer demonstrates significantly more stable morphology than the ANF-free counterpart, thereby leaving fundamental processes such as charge separation, transport, and collection, determining the device efficiency, largely unaltered. This strategy is also successfully applied to other photovoltaic systems. The strategy of incorporating a polymer fiber rigid network with high Tg offers a distinct perspective addressing the challenge of thermal instability with simplicity and universality.

High Open-Circuit Voltage Organic Solar Cells with 19.2% Efficiency Enabled by Synergistic Side-Chain Engineering

Restricted by the energy-gap law, state-of-the-art organic solar cells (OSCs) exhibit relatively low open-circuit voltage (VOC) because of large nonradiative energy losses (ΔEnonrad). Moreover, the trade-off between VOC and external quantum efficiency (EQE) of OSCs is more distinctive; the power conversion efficiencies (PCEs) of OSCs are still <15% with VOCs of >1.0 V. Herein, the electronic properties and aggregation behaviors of non-fullerene acceptors (NFAs) are carefully considered and then a new NFA (Z19) is delicately designed by simultaneously introducing alkoxy and phenyl-substituted alkyl chains to the conjugated backbone. Z19 exhibits a hypochromatic-shifted absorption spectrum, high-lying lowest unoccupied molecular orbital energy level and ordered 2D packing mode. The D18:Z19-based blend film exhibits favorable phase separation with face-on dominated molecular orientation, facilitating charge transport properties. Consequently, D18:Z19 binary devices afford an exciting PCE of 19.2% with a high VOC of 1.002 V, surpassing Y6-2O-based devices. The former is the highest PCE reported to date for OSCs with VOCs of >1.0 V. Moreover, the ΔEnonrad of Z19- (0.200 eV) and Y6-2O-based (0.155 eV) devices are lower than that of Y6-based (0.239 eV) devices. Indications are that the design of such NFA, considering the energy-gap law, could promote a new breakthrough in OSCs.

Designing a Novel Wide Bandgap Small Molecule Guest for Enhanced Stability and Morphology Mediation in Ternary Organic Solar Cells with over 19.3% Efficiency

In this study, a novel wide-bandgap small molecule guest material, ITOA, designed and synthesized for fabricating efficient ternary organic solar cells (OSCs) ITOA complements the absorbance of the PM6:Y6 binary system, exhibiting strong crystallinity and modest miscibility. ITOA optimizes the morphology by promoting intensive molecular packing, reducing domain size, and establishing a preferred vertical phase distribution. These features contribute to improved and well-balanced charge transport, suppressed carrier recombination, and efficient exciton dissociation. Consequently, a significantly enhanced efficiency of 18.62% for the ternary device is achieved, accompanied by increased short-circuit current density (JSC), fill factor (FF), and open-circuit voltage (VOC). Building on this success, replacing Y6 with BTP-eC9 leads to an outstanding PCE of 19.33% for the ternary OSCs. Notably, the introduction of ITOA expedites the formation of the optimized morphology, resulting in an impressive PCE of 18.04% for the ternary device without any postprocessing. Moreover, the ternary device exhibits enhanced operational stability under maximum power point (MPP) tracking. This comprehensive study demonstrates that a rationally designed guest molecule can optimize morphology, reduce energy loss, and streamline the fabrication process, essential for achieving high efficiency and stability in OSCs, paving the way for practical commercial applications.

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.

Chalcogen Atoms Regulate the Organic Solar Cell Performance of B-N-Based Polymer Donors

Donor polymers play a key role in the development of organic solar cells (OSCs). B-N-based polymer donors, as new types of materials, have attracted a lot of attention due to their special characteristics, such as high E(T1), small ΔEST, and easy synthesis, and they can be processed with real green solvents. However, the relationship between the chemical structure and device performance has not been systematically studied. Herein, chalcogen atoms that regulate the OSCs performance of B-N-based polymer donors were systematically studied. Fortunately, the substitution of a halogen atom did not affect the high E(T1) and small ΔEST character of the B-N-based polymer. The absorption and energy levels of the polymer were systematically regulated by O, S, and Se atom substitution. The PBNT-TAZ:Y6-BO-based OSCs device demonstrated a high power conversion efficiency of 15.36%. Moreover, the layer-by-layer method was applied to further optimize the device performance, and the PBNT-TAZ/Y6-BO-based OSCs device yielded a PCE of 16.34%. Consequently, we have systematically demonstrated how chalcogen atoms modulated the electronic properties of B-N-based polymers. Detailed and systematic structure-performance relationships are important for the development of next-generation B-N-based materials.

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.

Non-Fully Conjugated Dimerized Giant Acceptors with Different Alkyl-Linked Sites for Stable and 19.13 % Efficiency Organic Solar Cells

Achieving both high power conversion efficiency (PCE) and device stability is a major challenge for the practical development of organic solar cells (OSCs). Herein, three non-fully conjugated dimerized giant acceptors (named 2Y-sites, including wing-site-linked 2Y-wing, core-site-linked 2Y-core, and end-site-linked 2Y-end) are developed. They share the similar monomer precursors but have different alkyl-linked sites, offering the fine-tuned molecular absorption, packing, glass transition temperature, and carrier mobility. Among their binary active layers, D18/2Y-wing has better miscibility, leading to optimized morphology and more efficient charge transfer compared to D18/2Y-core and D18/2Y-end. Therefore, the D18/2Y-wing-based OSCs achieve a superior PCE of 17.73 %, attributed to enhanced photocurrent and fill factor. Furthermore, the D18/2Y-wing-based OSCs exhibit a balance of high PCE and improved stability, distinguishing them within the 2Y-sites. Building on the success of 2Y-wing in binary systems, we extend its application to ternary OSCs by pairing it with the near-infrared absorbing D18/BS3TSe-4F host. Thanks to the complementary absorption within 300-970 nm and further optimized morphology, ternary OSCs obtain a higher PCE of 19.13 %, setting a new efficiency benchmark for the dimer-derived OSCs. This approach of alkyl-linked site engineering for constructing dimerized giant acceptors presents a promising pathway to improve both PCE and stability of OSCs.

Asymmetric Π-Bridge Engineering Enables High-Permittivity Benzo[1,2-B:4,5-b']Difuran-Conjugated Polymer for Efficient Organic Solar Cells

Organic solar cells (OSCs) exhibit complex charge dynamics, which are closely correlated with the dielectric constant (ɛr ) of photovoltaic materials. In this work, a series of novel conjugated copolymers based on benzo[1,2-b:4,5-b']difuran (BDF) and benzotriazole (BTz) is designed and synthesized, which differ by the nature of π-bridge from one another. The PBDF-TF-BTz with asymmetric furan and thiophene π-bridge demonstrates a larger ɛr of 4.22 than PBDF-dT-BTz with symmetric thiophene π-bridge (3.15) and PBDF-dF-BTz with symmetric furan π-bridge (3.90). The PBDF-TF-BTz also offers more favorable molecular packing and appropriate miscibility with non-fullerene acceptor Y6 than its counterparts. The corresponding PBDF-TF-BTz:Y6 OSCs display efficient exciton dissociation, fast charge transport and collection, and reduced charge recombination, eventually leading to a power conversion efficiency of 17.01%. When introducing a fullerene derivative (PCBO-12) as a third component, the PBDF-TF-BTz:Y6:PCBO-12 OSCs yield a remarkable FF of 80.11% with a high efficiency of 18.10%, the highest value among all reported BDF-polymer-based OSCs. This work provides an effective approach to developing high-permittivity photovoltaic materials, showcasing PBDF-TF-BTz as a promising polymer donor for constructing high-performance OSCs.

Binary All-polymer Solar Cells with a Perhalogenated-Thiophene-Based Solid Additive Surpass 18 % Efficiency

Morphological control of all-polymer blends is quintessential yet challenging in fabricating high-performance organic solar cells. Recently, solid additives (SAs) have been approved to be capable in tuning the morphology of polymer: small-molecule blends improving the performance and stability of devices. Herein, three perhalogenated thiophenes, which are 3,4-dibromo-2,5-diiodothiophene (SA-T1), 2,5-dibromo-3,4-diiodothiophene (SA-T2), and 2,3-dibromo-4,5-diiodothiophene (SA-T3), were adopted as SAs to optimize the performance of all-polymer organic solar cells (APSCs). For the blend of PM6 and PY-IT, benefitting from the intermolecular interactions between perhalogenated thiophenes and polymers, the molecular packing properties could be finely regulated after introducing these SAs. In situ UV/Vis measurement revealed that these SAs could assist morphological character evolution in the all-polymer blend, leading to their optimal morphologies. Compared to the as-cast device of PM6 : PY-IT, all SA-treated binary devices displayed enhanced power conversion efficiencies of 17.4-18.3 % with obviously elevated short-circuit current densities and fill factors. To our knowledge, the PCE of 18.3 % for SA-T1-treated binary ranks the highest among all binary APSCs to date. Meanwhile, the universality of SA-T1 in other all-polymer blends is demonstrated with unanimously improved device performance. This work provide a new pathway in realizing high-performance APSCs.

Charge photogeneration dynamics in non-fullerene polymer solar cells with fluorinated and non-fluorinated acceptors

In this work, charge photogeneration and recombination processes of PM6:IDIC-4F and PM6:IDIC blend films were investigated by the steady-state and time-resolved spectroscopies, as well as the time-dependent density functional theory calculations. The peaks in absorption and photoluminescence (PL) spectra of IDIC and IDIC-4F solutions were assigned by combining the experiment and the simulation of UV-vis absorption and PL spectra. For neat acceptor films, the exciton diffusion length of neat IDIC and IDIC-4F films was estimated as ∼28.9 and ∼19.9 nm, respectively. For PM6-based blend films, we find that the fluorine substitution engineering on the IDIC acceptor material can increase the phase separate size of acceptor material in blend films, resulting in the reduction of dissociation efficiencies of acceptor excitons. In addition, we find that the charge recombination in PM6:IDIC-4F is dominated by bimolecular recombination, in comparison to geminate type carrier recombination in PM6:IDIC blend films. In addition, we find that thermal annealing treatment has a weak influence on carrier recombination but slightly reduces the exciton dissociation efficiency of acceptor in PM6:IDIC blend films, leading to a slightly reduced power conversion efficiency of PM6:IDIC solar cells. These results may shed light on the design of high-performance semiconductor molecules for application in solar cells.

Enhancing the Performance of Wide-Bandgap Polymer-Based Organic Solar Cells through Silver Nanorod Integration

Light trapping induced by the introduction of metallic nanoparticles has been shown to improve photo absorption in organic solar cells (OSCs). Researchers in the fields of plasmonics and organic photovoltaics work together to boost sunlight absorption and photon-electron interactions in order to improve device performance. In this contribution, an inverted OSC was fabricated by using indacenodithieno[3,2-b]thiophene-alt-2,2'-bithiazole (PIDTT-BTz) as a wide-band gap donor copolymer and (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM) as an acceptor. Silver nanorods (Ag-NRs), synthesized by precipitation method, were embedded in the active layer of the solar cell. The device fabricated with 1 wt % Ag-NRs in the active layer showed a 26% improvement in power conversion efficiency (PCE) when exposed to 100 mW/cm2 simulated solar illumination. The role of Ag-NRs in the performance improvement of the OSCs was analyzed systematically using morphological, electrical, and optical characterization methods. The light trapping and exciton generation were improved due to the localized surface plasmon resonance (LSPR) activated in Ag-NRs in the form of longitudinal and transverse modes. The photoactive layers (PIDTT-BTz:PC71BM) with the incorporation of 0.5 and 1 wt % Ag-NR showed increased absorption, while the absorption with 1.5 wt % Ag-NRs appeared to be reduced in the wavelength range from 400 to 580 nm. Ag-NRs play a favorable role in exciton photogeneration and dissociation due to the two LSPR modes generated by the Ag-NRs. In the optimized device, the short-circuit current density (JSC) increased from 11.92 to 14.25 mA/cm2, resulting in an increase in the PCE from 3.94 to 4.93%, which is attributed to the improved light-trapping by LSPR using Ag-NRs.

Thorough Optimization for Intrinsically Stretchable Organic Photovoltaics

The development of intrinsically stretchable organic photovoltaics (is-OPVs) with a high efficiency is of significance for practical application. However, their efficiencies lag far behind those of rigid or even flexible counterparts. To address this issue, an advanced top-illuminated OPV is designed and fabricated, which is intrinsically stretchable and has a high performance, through systematic optimizations from material to device. First, the stretchability of the active layer is largely increased by adding a low-elastic-modulus elastomer of styrene-ethylene-propylene-styrene tri-block copolymer (SEPS). Second, the stretchability and conductivity of the opaque electrode are enhanced by a conductive polymer/metal (denoted as M-PH1000@Ag) composite electrode strategy. Third, the optical and electrical properties of a sliver nanowire transparent electrode are improved by a solvent vapor annealing strategy. High-performance is-OPVs are successfully fabricated with a top-illuminated structure, which provides a record-high efficiency of 16.23%. Additionally, by incorporating 5-10% elastomer, a balance between the efficiency and stretchability of the is-OPVs is achieved. This study provides valuable insights into material and device optimizations for high-efficiency is-OPVs, with a low-cost production and excellent stretchability, which indicates a high potential for future applications of OPVs.

Step-by-Step Modulation of Crystalline Features and Exciton Kinetics for 19.2% Efficiency Ortho-Xylene Processed Organic Solar Cells

With plenty of popular and effective ternary organic solar cells (OSCs) construction strategies proposed and applied, its power conversion efficiencies (PCEs) have come to a new level of over 19% in single-junction devices. However, previous studies are heavily based in chloroform (CF) leaving behind substantial knowledge deficiencies in understanding the influence of solvent choice when introducing a third component. Herein, we present a case where a newly designed asymmetric small molecular acceptor using fluoro-methoxylated end-group modification strategy, named BTP-BO-3FO with enlarged bandgap, brings different morphological evolution and performance improvement effect on host system PM6:BTP-eC9, processed by CF and ortho-xylene (o-XY). With detailed analyses supported by a series of experiments, the best PCE of 19.24% for green solvent-processed OSCs is found to be a fruit of finely tuned crystalline ordering and general aggregation motif, which furthermore nourishes a favorable charge generation and recombination behavior. Likewise, over 19% PCE can be achieved by replacing spin-coating with blade coating for active layer deposition. This work focuses on understanding the commonly met yet frequently ignored issues when building ternary blends to demonstrate cutting-edge device performance, hence, will be instructive to other ternary OSC works in the future.

Green-Solvent Processed Blade-Coating Organic Solar Cells with an Efficiency Approaching 19% Enabled by Alkyl-Tailored Acceptors

Power-conversion-efficiencies (PCEs) of organic solar cells (OSCs) in laboratory, normally processed by spin-coating technology with toxic halogenated solvents, have reached over 19%. However, there is usually a marked PCE drop when the blade-coating and/or green-solvents toward large-scale printing are used instead, which hampers the practical development of OSCs. Here, a new series of N-alkyl-tailored small molecule acceptors named YR-SeNF with a same molecular main backbone are developed by combining selenium-fused central-core and naphthalene-fused end-group. Thanks to the N-alkyl engineering, NIR-absorbing YR-SeNF series show different crystallinity, packing patterns, and miscibility with polymeric donor. The studies exhibit that the molecular packing, crystallinity, and vertical distribution of active layer morphologies are well optimized by introducing newly designed guest acceptor associated with tailored N-alkyl chains, providing the improved charge transfer dynamics and stability for the PM6:L8-BO:YR-SeNF-based OSCs. As a result, a record-high PCE approaching 19% is achieved in the blade-coating OSCs fabricated from a green-solvent o-xylene with high-boiling point. Notably, ternary OSCs offer robust operating stability under maximum-power-point tracking and well-keep > 80% of the initial PCEs for even over 400 h. Our alkyl-tailored guest acceptor strategy provides a unique approach to develop green-solvent and blade-coating processed high-efficiency and operating stable OSCs, which paves a way for industrial development.

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.

Manipulating the Alkyl Chains of Naphthodithiophene Imide-Based Polymers to Concurrently Boost the Efficiency and Stability of Organic Solar Cells

Morphology instability holds the major responsibility for efficiency degradation of organic solar cells (OSCs). However, how to develop polymer donors simultaneously with high efficiency and excellent morphology stability remains challenging. Herein, we reported naphtho[2,1-b:3,4-b']dithiophene-5,6-imide (NDTI)-based new polymers PNDT1 and PNDT2. The alkyl chain engineering leads to high crystallinity, high hole mobility (>10-3 cm2 V-1 S-1), and nanofibrous film morphology, which enable PNDT2 to exhibit an efficiency of 18.13% and a remarkable FF value of 0.80. Moreover, the NDTIs have short π-π stacking and abundant short interactions, and their polymers exhibit superior morphological stability. Therefore, the PNDT2-based OSCs exhibit much better device stability than that of PNDT1, PAB-α, and benchmark polymers PM6 and D18. This work suggests the great importance of the large conjugated backbone of the monomer and alkyl chain engineering to develop high-performance and morphology-stable polymers for OSCs.

A Wide Bandgap Acceptor with Large Dielectric Constant and High Electrostatic Potential Values for Efficient Organic Photovoltaic Cells

Low-bandgap materials have achieved rapid development and promoted the enhancement of power conversion efficiencies (PCEs) of organic photovoltaic (OPV) cells. However, the design of wide-bandgap non-fullerene acceptors (WBG-NFAs), required by indoor applications and tandem cells, has been lagging far behind the development of OPV technologies. Here, we designed and synthesized two NFAs named ITCC-Cl and TIDC-Cl by finely optimizing ITCC. In contrast with ITCC and ITCC-Cl, TIDC-Cl can maintain a wider bandgap and a higher electrostatic potential simultaneously. When blending with the donor PB2, the highest dielectric constant is also obtained in TIDC-Cl-based films, enabling efficient charge generation. Therefore, the PB2:TIDC-Cl-based cell possessed a high PCE of 13.8% with an excellent fill factor (FF) of 78.2% under the air mass 1.5G (AM 1.5G) condition. Furthermore, an exciting PCE of 27.1% can be accomplished in the PB2:TIDC-Cl system under the illumination of 500 lux (2700 K light-emitting diode). Combined with the theoretical simulation, the tandem OPV cell based on TIDC-Cl was fabricated and exhibited an excellent PCE of 20.0%.