Resilient 3D Cross-Linked Conductive Carbon Dot Networks for Efficient Flexible Organic Solar Cells With Enhanced Photoelectrical-Mechanical Stability

Abstract

To enable large-area applications of flexible organic solar cells (OSCs), a novel electron transport layer (ETL) material with excellent high conductivity, outstanding bending performance, and good stability has been developed in this study. These characteristics make the material highly significant in enhancing the performance of flexible OSCs. Through synergistic hydrogen bonding and covalent interactions between the ─COOH/─OH groups on carbon dots (CDs) and the –NH₂ in polyethyleneimine (PEI), a dense three-dimensional (3D) cross-linked network is established. This network creates continuous conductive pathways, imparting the material with superior electrical conductivity and excellent thickness tolerance. Furthermore, nano-sized CDs act as physical crosslinking points, effectively dissipating stress and significantly enhancing mechanical toughness and bend tolerance. As a result, flexible OSCs with this ETL achieve a PCE of 16.24% and retain 91.5% of their initial efficiency after 10,000 bending cycles at a radius of 5 mm, significantly outperforming PEI-based (51.6%) counterparts, while also demonstrating excellent stability under ambient air and UV irradiation. Moreover, the CDs:PEI-based flexible OSCs maintain good bending stability and structural integrity even under different bending radius. This solution-processable material synergistically optimizes optoelectronic and mechanical properties, offering a robust interfacial solution for Roll-to-Roll manufacturing of flexible OSCs.

Keywords

3D cross-linked network / carbon dots / interface layer / mechanical robustness / organic solar cells

References

[1] J. Yi, G. Zhang, H. Yu, and H. Yan, “Advantages, Challenges and Molecular Design of Different Material Types Used in Organic Solar Cells,” Nature Reviews Materials 9, no. 1 (2023): 46–62.

[2] P. Cai, C. Song, Y. Du, et al., “Recent Progress of Solution-Processed Thickness-Insensitive Cathode Interlayers for High-Performance Organic Solar Cells,” Advanced Functional Materials 35, no. 20 (2025): 2422023.

[3] H. Chen, Y. Huang, R. Zhang, et al., “Organic Solar Cells With 20.82% Efficiency and High Tolerance of Active Layer Thickness Through Crystallization Sequence Manipulation,” Nature Materials 24, no. 3 (2025): 444–453.

[4] C. Li, J. Song, H. Lai, et al., “Non-Fullerene Acceptors With High Crystallinity and Photoluminescence Quantum Yield Enable > 20% Efficiency Organic Solar Cells,” Nature Materials 24, no. 3 (2025): 433–443.

[5] S. Li, Z. Li, X. Wan, and Y. Chen, “Recent Progress in Flexible Organic Solar Cells,” eScience 3, no. 1 (2023): 100085.

[6] F. Yang, Y. Huang, Y. Li, and Y. Li, “Large–Area Flexible Organic Solar Cells,” NPJ Flexible Electronics 5, no. 1 (2021): 30.

[7] N. Ahmad, J. Yuan, and Y. Zou, “One More Step Towards Better Stability of Non–Fullerene Organic Solar Cells: Advances, Challenges, Future Perspectives, and the Era of Artificial Intelligence,” Energy & Environmental Science 18, no. 11 (2025): 5093–5158.

[8] M. Wu, B. Ma, S. Li, J. Han, and W. Zhao, “Powering the Future: A Critical Review of Research Progress in Enhancing Stability of High-Efficiency Organic Solar Cells,” Advanced Functional Materials 33, no. 50 (2023): 2305445.

[9] J. Wan, Y. Xia, J. Fang, et al., “Solution–Processed Transparent Conducting Electrodes for Flexible Organic Solar Cells With 16.61% Efficiency,” Nano-Micro Letters 13, no. 1 (2021): 44.

[10] J. Wu, Y. Li, F. Tang, et al., “Porphyrin-Modified Sno₂ Electron Transport Layer for Efficient and Stable Inverted Organic Solar Cells,” Advanced Functional Materials 35, no. 36 (2025): 2504623.

[11] B. Liu, O. J. Sandberg, J. Qin, et al., “Inverted Organic Solar Cells With An In Situ–Derived Sioxny Passivation Layer and Power Conversion Efficiency Exceeding 18%,” Nature Photonics 19, no. 2 (2025): 195–203.

[12] B. Zhang, N. Xu, J. Wan, et al., “Induced Oriented Attachment of a ZnO Electron Transport Layer Enables over 20% Efficiency in Solution–Processed Conventional Organic Solar Cells,” Energy & Environmental Science 18, no. 21 (2025): 9550–9560.

[13] X. Zhang, Y. Wang, X. Wang, et al., “An Alcohol–Soluble Cathode Interface Material With Low–Temperature Processing and Thickness Insensitivity for Efficient and Highly Stable Flexible Organic Solar Cells,” ACS Sustainable Chemistry & Engineering 13, no. 8 (2025): 3384–3392.

[14] C. Liu, C. Xiao, C. Xie, and W. Li, “Flexible Organic Solar Cells: Materials, Large–Area Fabrication Techniques and Potential Applications,” Nano Energy 89 (2021): 106399.

[15] X. Liu, Z. Zheng, J. Wang, et al., “Fluidic Manipulating of Printable Zinc Oxide for Flexible Organic Solar Cells,” Advanced Materials 34, no. 3 (2021): 2106453.

[16] X. Liu, Y. Ji, Z. Xia, et al., “In-Doped ZnO Electron Transport Layer for High-Efficiency Ultrathin Flexible Organic Solar Cells,” Advanced Science 11, no. 37 (2024): 2402158.

[17] G. Wang, J. Zhang, C. Yang, et al., “Synergistic Optimization Enables Large-Area Flexible Organic Solar Cells to Maintain Over 98% Pce of the Small-Area Rigid Devices,” Advanced Materials 32, no. 49 (2020): 2005153.

[18] J. Jang, J. H. Kim, S. Lee, et al., “Efficient Flexible Organic Solar Cells With a Low–Temperature–Processed Al–Doped Zinc Oxide Electron Transport Layer,” ACS Applied Energy Materials 6, no. 18 (2023): 9778–9787.

[19] L. Hu, L. Han, J. Quan, et al., “Doping of ZnO Electron Transport Layer With Organic Dye Molecules to Enhance Efficiency and Photo-Stability of the Non-Fullerene Organic Solar Cells,” Small 20, no. 21 (2023): 2310125.

[20] Z. Xia, X. Lei, Y. Hu, et al., “Ultrathin Flexible Indium Tin Oxide-Free Organic Solar Cells With Gradient Bilayer Electron Transport Materials,” Solar RRL 8, no. 12 (2024): 2400230.

[21] K. D. Kadam, H. Kim, S. Rehman, et al., “Optimization of ZnO:PEIE as An Electron Transport Layer for Flexible Organic Solar Cells,” Energy & Fuels 35, no. 15 (2021): 12416–12424.

[22] H. Nie, M. R. Busireddy, H. M. Shih, et al., “High–Performance Inverted Organic Solar Cells via the Incorporation of Thickness–Insensitive and Low–Temperature–Annealed Nonconjugated Polymers as Electron Transport Materials,” ACS Applied Materials & Interfaces 15, no. 1 (2023): 1718–1725.

[23] Y. Hu, S. Wan, Y. Li, H. Fu, and Q. Zheng, “Sn²⁺–Coordinated Polyethylenimine as An Electron Transport Layer for High–Efficiency and Stable Inverted Organic Solar Cells,” Journal of Materials Chemistry C 13, no. 14 (2025): 7300–7306.

[24] N. Fattahi, L. Gorgannezhad, S. F. Masoule, et al., “PEI-Based Functional Materials: Fabrication Techniques, Properties, and Biomedical Applications,” Advances in Colloid and Interface Science 325 (2024): 103119.

[25] Y. Dong, R. Yu, B. Zhao, et al., “Revival of Insulating Polyethylenimine by Creatively Carbonizing With Perylene into Highly Crystallized Carbon Dots as the Cathode Interlayer for High–Performance Organic Solar Cells,” ACS Applied Materials & Interfaces 14, no. 1 (2022): 1280–1289.

[26] Z. Li, X. Zhang, C. Liu, et al., “Toward Efficient Carbon–Dots–Based Electron–Extraction Layer Through Surface Charge Engineering,” ACS Applied Materials & Interfaces 10, no. 46 (2018): 40255–40264.

[27] C. Xie, X. Wen, H. Chen, et al., “Over 20% Efficient Water-Based Layer-By-Layer Organic Solar Cells With High Thickness Tolerance Enabled by Surfactant Promoted Electrostatic Interaction,” Advanced Materials 37, no. 41 (2025): e08783.

[28] S. Yang, Z. Chen, J. Zhu, et al., “Guest Acceptors With Lower Electrostatic Potential in Ternary Organic Solar Cells for Minimizing Voltage Losses,” Advanced Materials 36, no. 26 (2024): 2401789.

[29] S. Lai, Y. Cui, Z. Chen, et al., “Impact of Electrostatic Interaction on Vertical Morphology and Energy Loss in Efficient Pseudo-Planar Heterojunction Organic Solar Cells,” Advanced Materials 36, no. 18 (2024): 2313105.

[30] 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, no. 30 (2023): 2300820.

[31] H. Li, B. Yu, and H. Yu, “An Efficient and Stable Inverted Structure Organic Solar Cell Using ZnO Modified by 2D ZrSe₂ as a Composite Electron Transport Layer,” Advanced Functional Materials 34, no. 37 (2024): 2402128.

[32] J. Peng, W. Gao, B. K. Gupta, et al., “Graphene Quantum Dots Derived From Carbon Fibers,” Nano Letters 12, no. 2 (2012): 844–849.

[33] Y. X. Li, J. Y. Lee, H. Lee, C. C. Hu, and T. C. Chiu, “Highly Fluorescent Nitrogen–Doped Carbon Dots for Selective and Sensitive Detection of Hg²⁺ and ClO^- Ions and Fluorescent Ink,” Journal of Photochemistry and Photobiology, A: Chemistry 405 (2021): 112931.

[34] H. Li, W. H. Shih, W. Y. Shih, L. Chen, S. J. Tseng, and S. C. Tang, “Transfection of Aqueous Cds Quantum Dots Using Polyethylenimine,” Nanotechnology 19, no. 47 (2008): 475101.

[35] P. Sukpoonprom, W. D. J. Tremlett, Z. Qiao, et al., “Complex Formation of Ferrocene Derivatives With Electron Transport Layers Enables Improved Performance and Photostability in Organic Solar Cells,” Joule 9, no. 10 (2025): 102107.

[36] B. Zhang, Z. Pan, W. Li, et al., “Natural Dextran as An Efficient Interfacial Passivator for Zno-Based Electron-Transport Layers in Inverted Organic Solar Cells,” Advanced Energy Materials 1 (2025): 2404297.

[37] Z. Huang, Y. Xu, L. Liu, et al., “A Multifunctional Strategy to Improve the Efficiency and Stability of Organic Solar Cells via a 2Pacz/Ma Composite Hole Transport Layer,” Journal of Materials Chemistry A 13, no. 21 (2025): 15574–15584.

[38] W. Zhang, X. Ding, J. Lv, et al., “Advancing Organic Photovoltaics: The Role of Dipole Distance and Acidity in Perylene-Diimide Electron Transport Layers,” Advanced Functional Materials 35, no. 26 (2025): 2420588.

[39] S. Li, W. Chen, C. Shi, et al., “Designed Polar Cosolvent-Prepared Zinc Oxide Film for Efficient and Stable Inverted Organic Solar Cells,” Small 20, no. 49 (2024): 2405743.

[40] Y. Tong, H. Liu, W. Xiao, et al., “Conjugated Polyelectrolyte With Long-Range Ordering and Compact Molecular Packing as Hole–Transporting Material for Efficient and Flexible Organic Solar Cells,” Advanced Materials 37, no. 40 (2025): e09995.

[41] K. Zhong, J. Deng, R. Zeng, et al., “Brominated Perylene Diimides as Cathode Interface Materials for High Efficiency Organic Solar Cells,” Advanced Functional Materials 35, no. 6 (2024): 2414822.

[42] C. Liu, D. Zhang, Z. Li, et al., “Boosted Electron Transport and Enlarged Built–In Potential by Eliminating the Interface Barrier in Organic Solar Cells,” ACS Applied Materials & Interfaces 9, no. 10 (2017): 8830–8837.

[43] L. Liu, F. Yu, D. Hu, et al., “Breaking the Symmetry of Interfacial Molecules With Push–Pull Substituents Enables 19.67% Efficiency Organic Solar Cells Featuring Enhanced Charge Extraction,” Energy & Environmental Science 18, no. 4 (2025): 1722–1731.

[44] B. Fan, H. Gao, Y. Li, et al., “Integration of Polyoxometalate Clusters With Self–Assembled Monolayer for Efficient and Robust Organic Solar Cells,” Joule 8, no. 5 (2024): 1443–1456.

[45] F. Sun, X. Zheng, T. Hu, et al., “1,5–Diiodocycloctane: A Cyclane Solvent Additive That Can Extend the Exciton Diffusion Length in Thick Film Organic Solar Cells,” Energy & Environmental Science 17, no. 5 (2024): 1916–1930.

[46] H. Xia, Y. Zhang, K. Liu, et al., “Oligomeric Semiconductors Enable High Efficiency Open Air Processed Organic Solar Cells by Modulating Pre–Aggregation and Crystallization Kinetics,” Energy & Environmental Science 16, no. 12 (2023): 6078–6093.

[47] B. Zhao, L. Zhu, S. Xiong, et al., “Boosting Binary Organic Solar Cells over 20% Efficiency via Synchronous Modulation of Charge Transport and Phase Morphology,” Advanced Energy Materials 15 (2025): e04947.

[48] Y. Ji, L. Liang, T. Chen, et al., “High-Performance Ultrathin Flexible Organic Solar Cells Through the Modification of Self-Assembled Monolayers,” Advanced Energy Materials 15, no. 32 (2025): 2501698.

[49] X. Zeng, T. Xu, H. Chen, et al., “A Donor:Hole–Transport Layer Alloy for High–Efficiency and Stable Binary Organic Solar Cells With Promoted Hole Collection and Suppressed Recombination,” Energy & Environmental Science 17, no. 23 (2024): 9383–9393.

[50] C. Zhang, X. Duan, C. Liu, et al., “Star–Branched Polymer Donors Enabling High–Performance Organic Solar Cells With Superior Flexibility and Intrinsic Stretchability,” Nature Communications 16, no. 1 (2025): 10141.

[51] S. Zeng, H. Li, S. Liu, et al., “Interface Design Based on Strain Isolation Theory With An Optimized Neutral Mechanical Plane Enables Highly Ductile and Flexible Organic Photovoltaics,” Energy & Environmental Science 18, no. 5 (2025): 2318–2329.

[52] J. Zhang, H. Mao, K. Zhou, et al., “Polymer-Entangled Spontaneous Pseudo-Planar Heterojunction for Constructing Efficient Flexible Organic Solar Cells,” Advanced Materials 36, no. 7 (2023): 2309379.

RIGHTS & PERMISSIONS

2026 The Author(s). Carbon Neutralization published by Wenzhou University and John Wiley & Sons Australia, Ltd.