Precise Side-Chain Engineering Optimizes Polymer Pre-Aggregation and Crystallinity for Efficient Organic Solar Cells With Minimized Non-Radiative Energy Loss

Abstract

Minimizing energy loss (E_loss) to achieve high open-circuit voltage (V_OC) is essential for improving the efficiency of organic solar cells (OSCs). In addition to non-fullerene acceptors, aggregation-caused quenching in linear polymer donors also contributes to E_loss. Although polymer donors with strong aggregation characteristics are beneficial for enhancing crystallinity and improving charge transport, such strong aggregation often leads to increased non-radiative recombination losses (ΔE₃). Therefore, precisely optimizing crystallinity and aggregation is essential for reducing E_loss while maintaining efficient charge mobility. Here, we designed and synthesized a series of wide-bandgap polymer donors (P1–P6) based on chlorinated benzodithiophene (BDT) donor unit and diester-functionalized thieno[3,2-b]thiophene acceptor moiety (TT-Th). By systematically optimizing the alkyl side chains on both the BDT and ester-thiophene units, we achieved precise control over pre-aggregation behavior. Our results demonstrate that extending the side chains on the TT-Th unit progressively reduces polymer pre-aggregation and ΔE₃, but simultaneously weakens crystallinity and increases π–π stacking distance, thereby compromising charge transport. Among P1–P5, P4 with 2-butyloctyl side chains exhibited the best balance between pre-aggregation and ΔE₃, yielding the highest efficiency. Further optimization by shortening the BDT side chain to 2-ethylhexyl in P6 moderately enhanced both pre-aggregation and crystallinity. Although this led to a slight V_OC reduction, the improved charge transport properties enabled a champion efficiency of 15.74% with a low ΔE₃ of 0.22 eV. Notably, the efficiency of 15.74% is one of the highest values reported for D-A alternating polymers based on ester-bithiophene units. This work present an effective strategy to optimize pre-aggregation and crystallinity, offering valuable insights into reducing E_loss and enhancing OSC performance.

Keywords

energy loss / molecular aggregation / organic solar cells / polymer donor / π–π stacking

References

[1] F. Cheng, Y. Cui, F. Ding, et al., “Terpolymerization and Regioisomerization Strategy to Construct Efficient Terpolymer Donors Enabling High-Performance Organic Solar Cells,” Advanced Materials 35 (2023): 2300820.

[2] R. Ma, Z. Luo, Y. Zhang, et al., “Organic Solar Cells: Beyond 20%,” Science China Materials 68 (2025): 1689-1701.

[3] Q. Wang, S. Zhao, H. Ding, et al., “Enhancing Stability and Performance of Pseudo-Planar Heterojunction Organic Solar Cells Using a Hindered Phenolic Antioxidant With Over 19% Efficiency,” Science China Materials 68 (2025): 838-849.

[4] Y. Cui, P. Zhu, H. Hu, et al., “Impact of Electrostatic Interaction on Non-radiative Recombination Energy Losses in Asymmetric Acceptors based Organic Solar Cells,” Angewandte Chemie International Edition 62 (2023): 04931.

[5] K. L. Jiang, F. Liu, G. Ran, et al., “20.6% Efficiency Organic Solar Cells Enabled by Incorporating a Lower Bandgap Guest Nonfullerene Acceptor Without Open-Circuit Voltage Loss,” Advanced Materials 37 (2025): 2500282.

[6] X. Li, X. Kong, G. Sun, and Y. Li, “Organic small molecule acceptor materials for organic solar cells,” Escience 3 (2023): 100153.

[7] Y. Bai, T. Chen, X. Ji, et al., “Multi-Selenophene Strategy Enables Dimeric Acceptors-Based Organic Solar Cells with over 18.5% Efficiency,” Advanced Energy Materials 14 (2024): 2400938.

[8] Y. Jiang, S. Sun, R. Xu, et al., “Non-Fullerene Acceptor With Asymmetric Structure and Phenyl-Substituted Alkyl Side Chain for 20.2% Efficiency Organic Solar Cells,” Nature Energy 9 (2024): 975-986.

[9] J. Yuan, Y. Zhang, L. Zhou, et al., “Single-Junction Organic Solar Cell With Over 15% Efficiency Using Fused-Ring Acceptor With Electron-Deficient Core,” Joule 3 (2019): 1140-1151.

[10] M. Deng, X. Xu, W. Qiu, et al., “Improving Miscibility of Polymer Donor and Polymer Acceptor by Reducing Chain Entanglement for Realizing 18.64% Efficiency All Polymer Solar Cells,” Angewandte Chemie International Edition 62 (2024): 11937.

[11] T. Duan, J. Wang, X. Zuo, et al., “The Anti-Correlation Effect of Alkyl Chain Size on the Photovoltaic Performance of Centrally Extended Non-Fullerene Acceptors,” Materials Horizons 11 (2024): 4413-4423.

[12] J. Gao, H. Bai, P. Li, et al., “Halogenated Dibenzo[f,h]Quinoxaline Units Constructed 2D-Conjugated Guest Acceptors for 19% Efficiency Organic Solar Cells,” Advanced Science 11 (2024): 2403334.

[13] J. Huang, T. Chen, L. Mei, et al., “On the Role of Asymmetric Molecular Geometry in High-Performance Organic Solar Cells,” Nature Communications 15 (2024): 3287.

[14] J. Cheng, C. Guo, L. Wang, et al., “Device Engineering of Non-Fullerene Organic Photovoltaics With Extrapolated Operational T80 Lifetime Over 45,000 h in Air,” Joule 21 (2024): 2250-2264.

[15] D. Qiu, C. Tian, H. Zhang, J. Zhang, Z. Wei, and K. Lu, “Correlating Aggregation Ability of Polymer Donors With Film Formation Kinetics for Organic Solar Cells With Improved Efficiency and Processability,” Advanced Materials 36 (2024): 2313251.

[16] R. Zeng, S. Xu, J. Deng, et al., “Molecular Precision Engineering for Efficient Binary Organic Photovoltaics Through Energy Level and Fibrillar Structure Modulation,” Advanced Energy Materials 14 (2024): 2401561.

[17] Y. Cai, X. Zhang, X. Xue, D. Wei, L. Huo, and Y. Sun, “High-Performance Wide-Bandgap Copolymers Based on Indacenodithiophene and Indacenodithieno[3,2-b]Thiophene Units,” Journal of Materials Chemistry C 5 (2017): 7777-7783.

[18] F. Li, A. Tang, B. Zhang, and E. Zhou, “Indacenodithieno[3,2-b]Thiophene-Based Wide Bandgap D-π-A Copolymer for Nonfullerene Organic Solar Cells,” ACS Macro Letters 8 (2019): 1599-1604.

[19] Z. Liu, X. Chen, S. Huang, et al., “Novel Efficient Accptor1-Acceptor2 Type Copolymer Donors: Vinyl Induced Planar Geometry and High Performance Organic Solar Cells,” Chemical Engineering Journal 419 (2021): 129532.

[20] S. Pang, Z. Wang, X. Yuan, et al., “A Facile Synthesized Polymer Featuring B-N Covalent Bond and Small Singlet-Triplet Gap for High-Performance Organic Solar Cells,” Angewandte Chemie International Edition 60 (2021): 8813-8817.

[21] T. Liu, R. Ma, Z. Luo, et al., “Concurrent Improvement in JSC and VOC in High-Efficiency Ternary Organic Solar Cells Enabled by a Red-Absorbing Small-Molecule Acceptor With a High LUMO Level,” Energy & Environmental Science 13 (2020): 2115-2123.

[22] Q. Nie, A. Tang, Q. Guo, and E. Zhou, “Benzothiadiazole-Based Non-Fullerene Acceptors,” Nano Energy 87 (2021): 106174.

[23] C. Zhu, J. Yuan, F. Cai, et al., “Tuning the Electron-Deficient Core of a Non-Fullerene Acceptor to Achieve Over 17% Efficiency in a Single-Junction Organic Solar Cell,” Energy & Environmental Science 13 (2020): 2459-2466.

[24] F. Zhao, S. Dai, Y. Wu, et al., “Single-Junction Binary-Blend Nonfullerene Polymer Solar Cells With 12.1% Efficiency,” Advanced Materials 29 (2017): 1700144.

[25] J. Zhou, P. Cong, L. Chen, et al., “Gradually Modulating the Three Parts of D-π-A Type Polymers for High-Performance Organic Solar Cells,” Journal of Energy Chemistry 62 (2021): 532-537.

[26] J. Li, F. Li, B. Zhang, and E. Zhou, “Synthesis of 1-Formyl-3-Bromo-Thieno[3,4-c]Pyrrole-4,6-Dione and the Application in A₂-A₁-D-A₁-A₂ Type Non-Fullerene Acceptor,” Journal of Physical Chemistry C 124 (2020): 9795-9801.

[27] J. Li, C. Sun, A. Tang, et al., “Utilizing an Electron-Deficient Thieno[3,4-c]Pyrrole-4,6-Dione (TPD) Unit as a π-Bridge to Improve the Photovoltaic Performance of A-π-D-π-A Type Acceptors,” Journal of Materials Chemistry C 8 (2020): 15981-15984.

[28] Y. Liu, W. Zhao, Y. Wu, et al., “Enhancing the Power Conversion Efficiency of Polymer Solar Cells to 9.26% by a Synergistic Effect of Fluoro and Carboxylate Substitution,” Journal of Materials Chemistry A 4 (2016): 8097-8104.

[29] Y. Tang, H. Sun, Z. Wu, et al., “A New Wide Bandgap Donor Polymer for Efficient Nonfullerene Organic Solar Cells With a Large Open-Circuit Voltage,” Advanced Science 6 (2019): 1901773.

[30] G. E. Park, S. Choi, S. Y. Park, D. H. Lee, M. J. Cho, and D. H. Choi, “Eco-Friendly Solvent-Processed Fullerene-Free Polymer Solar Cells With Over 9.7% Efficiency and Long-Term Performance Stability,” Advanced Energy Materials 7 (2017): 1700566.

[31] H. Yao, Y. Cui, D. Qian, et al., “14.7% Efficiency Organic Photovoltaic Cells Enabled by Active Materials With a Large Electrostatic Potential Difference,” Journal of the American Chemical Society 141 (2019): 7743-7750.

[32] A. L. Jones, Z. Zheng, P. Riley, et al., “Acceptor Gradient Polymer Donors for Non-Fullerene Organic Solar Cells,” Chemistry of Materials 31 (2019): 9729-9741.

[33] G. P. Kini, J. Y. Choi, S. J. Jeon, I. S. Suh, and D. K. Moon, “Effect of Mono Alkoxy-Carboxylate-Functionalized Benzothiadiazole-Based Donor Polymers for Non-Fullerene Solar Cells,” Dyes and Pigments 164 (2019): 62-71.

[34] X. Liao, Z. Yao, K. Gao, et al., “Mapping Nonfullerene Acceptors With a Novel Wide Bandgap Polymer for High Performance Polymer Solar Cells,” Advanced Energy Materials 8 (2018): 1801214.

[35] J. Chen, L. Wang, J. Yang, et al., “Backbone Conformation Tuning of Carboxylate-Functionalized Wide Band Gap Polymers for Efficient Non-Fullerene Organic Solar Cells,” Macromolecules 52 (2018): 341-353.

[36] J. Wang, L. Ma, Y. W. Lee, et al., “Design of Ultra-High Luminescent Polymers for Organic Photovoltaic Cells With Low Energy Loss,” Chemical Communications 57 (2021): 9132-9135.

[37] S. Li, L. Ye, W. Zhao, et al., “A Wide Band Gap Polymer With a Deep Highest Occupied Molecular Orbital Level Enables 14.2% Efficiency in Polymer Solar Cells,” Journal of the American Chemical Society 140 (2018): 7159-7167.

[38] J. S. Murray and P. Politzer, “The Electrostatic Potential: An Overview,” Wiley Interdisciplinary Reviews: Computational Molecular Science 1 (2011): 153-163.

[39] X. Liao, Q. He, G. Zhou, et al., “Regulating Favorable Morphology Evolution by a Simple Liquid-Crystalline Small Molecule Enables Organic Solar Cells With Over 17% Efficiency and a Remarkable Jsc of 26.56 mA/cm2,” Chemistry of Materials 33 (2020): 430-440.

[40] G. Xu, H. Rao, X. Liao, et al., “Reducing Energy Loss and Morphology Optimization Manipulated by Molecular Geometry Engineering for Hetero-Junction Organic Solar Cells,” Chinese Journal of Chemistry 38 (2020): 1553-1559.

[41] X. Chen, B. Liu, Y. Zou, et al., “A New Benzo[1,2-b:4,5-b′]Difuran-Based Copolymer for Efficient Polymer Solar Cells,” Journal of Materials Chemistry A 22 (2012): 17724.

[42] I. Riedel, J. Parisi, V. Dyakonov, L. Lutsen, D. Vanderzande, and J. C. Hummelen, “Effect of Temperature and Illumination on the Electrical Characteristics of Polymer-Fullerene Bulk-Heterojunction Solar Cells,” Advanced Functional Materials 14 (2004): 38-44.

[43] Y. Wang, D. Qian, Y. Cui, et al., “Optical Gaps of Organic Solar Cells as a Reference for Comparing Voltage Losses,” Advanced Energy Materials 8 (2018): 1801352.

[44] D. He, F. Zhao, C. Wang, and Y. Lin, “Non-Radiative Recombination Energy Losses in Non-Fullerene Organic Solar Cells,” Advanced Functional Materials 32 (2022): 2111855.

[45] L. Ye, H. Hu, M. Ghasemi, et al., “Quantitative Relations Between Interaction Parameter, Miscibility and Function in Organic Solar Cells,” Nature Materials 17 (2018): 253-260.

[46] S. Nilsson, A. Bernasik, A. Budkowski, and E. Moons, “Morphology and Phase Segregation of Spin-Casted Films of Polyfluorene/PCBM Blends,” Macromolecules 40 (2007): 8291-8301.

[47] B. P. Lyons, N. Clarke, and C. Groves, “The Relative Importance of Domain Size, Domain Purity and Domain Interfaces to the Performance of Bulk-Heterojunction Organic Photovoltaics,” Energy & Environmental Science 5 (2012): 7657.

[48] Y. Zhang, K. Liu, J. Huang, et al., “Graded Bulk-Heterojunction Enables 17% Binary Organic Solar Cells via Nonhalogenated Open Air Coating,” Nature Communications 12 (2021): 4815.

[49] Y. Tang, J. Yu, H. Sun, et al., “Two Compatible Polymer Donors Enabling Ternary Organic Solar Cells With a Small Nonradiative Energy Loss and Broad Composition Tolerance,” Solar RRL 4 (2020): 2000396.

[50] H. Zhao, H. B. Naveed, B. Lin, et al., “Hot Hydrocarbon-Solvent Slot-Die Coating Enables High-Efficiency Organic Solar Cells With Temperature-Dependent Aggregation Behavior,” Advanced Materials 32 (2020): 2002302.

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