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

Low-Volatility Fused-Ring Solid Additive Engineering for Synergistically Elongating Exciton Lifetime and Mitigating Trap Density Toward Organic Solar Cells of 20.5% Efficiency

Volatile solid additives (VSAs) with single or fused-ring structures have attracted much attention for enhancing power conversion efficiencies (PCEs) of organic solar cells (OSCs). While the working mechanisms of high-volatility single-ring additives have been well studied, the influence of low-volatility fused-ring VSAs on molecular aggregations and exciton/carrier dynamics remains still unclear. Herein, 3,6-dibromothieno[3,2-b]thiophene (3,6TTBr) is selected as a representative low-volatility fused-ring VSA to elucidate its working mechanism. Via the theoretical and experimental joint investigation, it is found that rigid and planar 3,6TTBr molecules adsorb onto the terminal units of L8-BO (acceptor), inducing loose space for adjacent molecules. The low-volatility 3,6TTBr thus favors the L8-BO center-terminal packing with a larger interfragment distance, which relieves the L8-BO over-aggregation and induces the ordered packing. Consequently, the 3,6TTBr treatment reduces aggregation-caused quenching, enhancing the photoluminescence quantum yield and exciton lifetime of L8-BO film. The combination of the above properties with the reduced trap density and improved carrier transport in the 3,6TTBr-treated devices contributed to PCE of 20.1%. To validate the broad applicability of the findings, 1,5-dibromonaphthalene (1,5-BN), another low-volatility fused-ring solid, is explored. The devices with 1,5-BN achieved an impressive PCE of 20.5%, verifying the validity of the low-volatility fused-ring VSA strategy for boosting OSC performances.

Synergistically Halogenated and Methoxylated Thiophene Additive Enables High-Performance Organic Solar Cells

Morphology control plays a key role for improving efficiency and stability of bulk heterojunctions (BHJ) organic solar cells (OSCs). Halogenation and methoxylation are two separate ways successfully adopted in additives for morphology optimization. In this work, these two strategies are combined together. A series of halogenated methoxylated thiophenes is designed and synthesized as volatile additives to control the evolution of the BHJ morphology. Specifically, the addition of 2,5-diiodo-3,4-dimethoxythiophene (MT-I) prominently improves the performance and photostability of OSCs. Computational simulations reveal the noncovalent interactions of MT-I with the active layer materials that corresponds to the inhibition of excessive aggregation behavior of PM6 and Y6 during the film-forming process, facilitating favorable phase separation and enhanced molecular stacking. Consequently, PM6:Y6-based binary OSCs with the treatment of MT-I achieves a high PCE of 17.93%. Furthermore, MT-I demonstrates broad feasibility across diverse high-efficiency BHJ OSCs, leading to superior photovoltaic performance (PCE over 18%). This study offers valuable guidance for the design and application of high-performance additives in future endeavors.

Asymmetrified Benzothiadiazole-Based Solid Additives Enable All-Polymer Solar Cells with Efficiency Over 19

Disordered polymer chain entanglements within all-polymer blends limit the formation of optimal donor-acceptor phase separation. Therefore, developing effective methods to regulate morphology evolution is crucial for achieving optimal morphological features in all-polymer organic solar cells (APSCs). In this study, two isomers, 4,5-difluorobenzo-c-1,2,5-thiadiazole (SF-1) and 5,6-difluorobenzo-c-1,2,5-thiadiazole (SF-2), were designed as solid additives based on the widely-used electron-deficient benzothiadiazole unit in nonfullerene acceptors. The incorporation of SF-1 or SF-2 into PM6 : PY-DT blend induces stronger molecular packing via molecular interaction, leading to the formation of continuous interpenetrated networks with suitable phase-separation and vertical distribution. Furthermore, after treatment with SF-1 and SF-2, the exciton diffusion lengths for PY-DT films are extended to over 40 nm, favoring exciton diffusion and charge transport. The asymmetrical SF-2, characterized by an enhanced dipole moment, increases the power conversion efficiency (PCE) of PM6 : PY-DT-based device to 18.83 % due to stronger electrostatic interactions. Moreover, a ternary device strategy boosts the PCE of SF-2-treated APSC to over 19 %. This work not only demonstrates one of the best performances of APSCs but also offers an effective approach to manipulate the morphology of all-polymer blends using rational-designed solid additives.

Isomerism Effect of 3D Dimeric Acceptors for Non-Halogenated Solvent-Processed Organic Solar Cells with 20 % Efficiency

Organic photovoltaic materials that can be processed via non-halogenated solvents are crucial for the large-area manufacturing of organic solar cells (OSCs). However, the limited available of electron acceptors with adequate solubility and favorable molecular packing presents a challenge in achieving efficient non-halogenated solvent-processed OSCs. Herein, inspired by the three-dimensional dimeric acceptor CH8-4, we employed a molecular isomerization strategy to synthesize its isomers, CH8-4A and CH8-4B, by tuning the position of fluorine (F) atom in the central unit. The differing intramolecular fluorine-sulfur non-covalent interactions among these isomers led to differences in molecular pre-aggregation abilities (CH8-4B Collapse

Key Words

• Dimeric acceptor
• isomerism effect
• non-halogenated solvent
• organic solar cells

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Grants

• 52303237 National Natural Science Foundation of China
• 21935007 National Natural Science Foundation of China
• 52025033 National Natural Science Foundation of China
• 22361132530 National Natural Science Foundation of China
• 2022YFB4200400 Ministry of Science and Technology of the People's Republic of China
• 2019YFA070590 Ministry of Science and Technology of the People's Republic of China
• 2023YFE0210400 Ministry of Science and Technology of the People's Republic of China
• 023- 632343116 the Fundamental Research Funds for the Central Universities, Nankai University

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Affiliation(s)

Jia Wang School of Materials Science and Engineering, National Institute for Advanced Materials, Nankai University, Tianjin, 300350, China
Peiran Wang School of Materials Science and Engineering, National Institute for Advanced Materials, Nankai University, Tianjin, 300350, China
Tianqi Chen School of Materials Science and Engineering, National Institute for Advanced Materials, Nankai University, Tianjin, 300350, China
Wenkai Zhao School of Materials Science and Engineering, National Institute for Advanced Materials, Nankai University, Tianjin, 300350, China
Jiaying Wang School of Materials Science and Engineering, National Institute for Advanced Materials, Nankai University, Tianjin, 300350, China
Baofa Lan School of Materials Science and Engineering, National Institute for Advanced Materials, Nankai University, Tianjin, 300350, China
Wanying Feng State Key Laboratory and Institute of Elemento-Organic Chemistry, Frontiers Science Center for New Organic Matter, The Centre of Nanoscale Science and Technology and Key Laboratory of Functional Polymer Materials, Renewable Energy Conversion and Storage Center (RECAST), College of Chemistry, Nankai University, Tianjin, 300071, China
Hang Liu State Key Laboratory and Institute of Elemento-Organic Chemistry, Frontiers Science Center for New Organic Matter, The Centre of Nanoscale Science and Technology and Key Laboratory of Functional Polymer Materials, Renewable Energy Conversion and Storage Center (RECAST), College of Chemistry, Nankai University, Tianjin, 300071, China
Yongsheng Liu State Key Laboratory and Institute of Elemento-Organic Chemistry, Frontiers Science Center for New Organic Matter, The Centre of Nanoscale Science and Technology and Key Laboratory of Functional Polymer Materials, Renewable Energy Conversion and Storage Center (RECAST), College of Chemistry, Nankai University, Tianjin, 300071, China
Xiangjian Wan State Key Laboratory and Institute of Elemento-Organic Chemistry, Frontiers Science Center for New Organic Matter, The Centre of Nanoscale Science and Technology and Key Laboratory of Functional Polymer Materials, Renewable Energy Conversion and Storage Center (RECAST), College of Chemistry, Nankai University, Tianjin, 300071, China
Guankui Long School of Materials Science and Engineering, National Institute for Advanced Materials, Nankai University, Tianjin, 300350, China
Bin Kan School of Materials Science and Engineering, National Institute for Advanced Materials, Nankai University, Tianjin, 300350, China
Yongsheng Chen State Key Laboratory and Institute of Elemento-Organic Chemistry, Frontiers Science Center for New Organic Matter, The Centre of Nanoscale Science and Technology and Key Laboratory of Functional Polymer Materials, Renewable Energy Conversion and Storage Center (RECAST), College of Chemistry, Nankai University, Tianjin, 300071, China

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Morphology Optimization by Non-Halogenated and Twisted Volatile Solid Additive for High-Efficiency Organic Solar Cells

Recently, volatile solid additives have attracted tremendous interest in the field of organic solar cells (OSCs), which can effectively improve device efficiency without sacrificing the reproducibility and stability of the device. However, the structure of reported solid additives is onefold and its working mechanism needs to be further investigated. Herein, a novel non-halogenated and twisted solid additive 1,4-diphenoxybenzene (DPB) is employed to optimize the morphology of the active layer in OSCs. The properties of additive DPB, morphology of active layer, and carrier dynamics behaviors have been systematically investigated through theoretical calculations, in situ and ex situ spectroscopy, grazing-incidence wide-angle X-ray scattering (GIWAXS), and grazing-incidence small-angle X-ray scattering (GISAXS) measurement, as well as ultrafast spectroscopy technology. The results reveal that the twisted additive DPB selectively interacts with acceptor Y6, and thus forms optimized morphology of active layer with increased molecular crystallinity, tight molecular packing, and favorable phase separation. As a result, the optimized devices deliver a remarkable power conversion efficiency (PCE) of 19.04%, which is the highest value for the D18-Cl:N3 system to date. These results demonstrate that non-halogenated and twisted solid additive DPB has broad prospects in the preparation of highly efficient OSCs, providing theoretical and experimental guidance for the development of high-performance solid additives.

Gradual Optimization of Molecular Aggregation and Stacking Enables Over 19% Efficiency in Binary Organic Solar Cells

Volatile solid additive is an effective and simple strategy for morphology control in organic solar cells (OSCs). The development of environmentally friendly new additives which can also be easily removed without high-temperature thermal annealing treatment is currently a trend, and the working mechanism needs to be further studied. Herein, a highly volatile and non-halogenated solid additive 1-benzothiophene (BBT) is reported to regulate molecular aggregation and stacking of active layer components. According to the film-forming kinetics process, a momentary intermediate phase is formed during spin-coating, which slows down the film-forming process and leads to more ordered molecular stacking in the solid film after introducing solid additive BBT. Subsequently, after solvent vapor annealing (SVA) further treatment, the resultant blend films exhibit a tighter and more ordered molecular stacking. Consequently, the synergistic effect of solid additive BBT and SVA treatment can effectively control morphology of active layer and improve carrier transport characteristics, thereby enhancing the performance of OSCs. Finally, in D18-Cl:N3 system, an impressive power conversion efficiency of 19.53% is achieved. The work demonstrates that the combination of highly volatile solid additives and SVA treatment is an effective morphology control strategy, guiding the development of efficient OSCs.

The anti-correlation effect of alkyl chain size on the photovoltaic performance of centrally extended non-fullerene acceptors

Contrary to previous results, a unique anti-correlation effect of the alkyl chain size on the photovoltaic performance of acceptors was observed. For a centrally-extended acceptor, replacing linear alkyl chains (n-undecyl for CH-BBQ) on the thienothiophene unit with branched ones (2-butyloctyl for CH-BO) leads to a plunge in the power conversion efficiency of organic solar cells (18.12% vs. 11.34% for binary devices), while the largely shortened ones (n-heptyl for CH-HP) bring a surge in performance (18.74%/19.44% for binary/ternary devices). Compared with CH-BO, the more compact intermolecular packing of CH-HP facilitates carrier transport. The characterization of organic field effect transistors and carrier dynamics also echoes the above results. Molecular dynamics simulations indicate that the encounter of the branched alkyl chains and the extended central core hinders the effective interfacial interaction of polymer donors and acceptors, thus deteriorating the device performance. This work suggests that the conventional strategy for alkyl chain engineering of Y-series acceptors might need to be reconsidered in other molecular systems.

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

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

Isomerization Engineering of Solid Additives Enables Highly Efficient Organic Solar Cells via Manipulating Molecular Stacking and Aggregation of Active Layer

Morphology control is crucial in achieving high-performance organic solar cells (OSCs) and remains a major challenge in the field of OSC. Solid additive is an effective strategy to fine-tune morphology, however, the mechanism underlying isomeric solid additives on blend morphology and OSC performance is still vague and urgently requires further investigation. Herein, two solid additives based on pyridazine or pyrimidine as core units, M1 and M2, are designed and synthesized to explore working mechanism of the isomeric solid additives in OSCs. The smaller steric hindrance and larger dipole moment facilitate better π-π stacking and aggregation in M1-based active layer. The M1-treated all-small-molecule OSCs (ASM OSCs) obtain an impressive efficiency of 17.57%, ranking among the highest values for binary ASM OSCs, with 16.70% for M2-treated counterparts. Moreover, it is imperative to investigate whether the isomerization engineering of solid additives works in state-of-the-art polymer OSCs. M1-treated D18-Cl:PM6:L8-BO-based devices achieve an exceptional efficiency of 19.70% (certified as 19.34%), among the highest values for OSCs. The work provides deep insights into the design of solid additives and clarifies the potential working mechanism for optimizing the morphology and device performance through isomerization engineering of solid additives.

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

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