Near-infrared Acceptor Materials for Highly Efficient and Stable Organic Solar Cells
用於高效穩定有機太陽能電池的近紅外受體材料
Student thesis: Doctoral Thesis
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Related Research Unit(s)
Detail(s)
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| Award date | 13 Jul 2023 |
Link(s)
| Permanent Link | https://scholars.cityu.edu.hk/en/theses/theses(4833a0f9-670f-428f-b70e-ac96c37b2481).html |
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| Other link(s) | Links |
Abstract
Organic solar cells (OSCs) as an emerging renewable energy technology have obtained tremendous progress due to their advantages in low cost, scalable solution-processing, and diverse form factors. Due to the rapid development of organic photovoltaic materials, the power conversion efficiency (PCE) of OSCs has ascended over 19% by employing the ternary strategy to blend conjugated semiconducting donors and acceptor. Among various materials, non-fullerene accepters (NFAs) have attracted significant attention due to their tunable energy levels, strong light-harvesting ability in visible and near-infrared (NIR) regions, and facile synthesis. However, the absorption of most NFAs is limited in the range between 600 and 900 nm, which hinders the further improvement of the short-circuit current density (Jsc) and PCE of derived solar cells. Therefore, it is important to exploit new acceptor materials that possess strong photo-responses in the NIR region to enhance light harvesting.
Apart from improving the efficiency of the OSCs, the trade-off between high efficiency and long-term stability is still very challenging for the commercialization of OSCs due to the quite complicated crystallization dynamics of active layer and its interfaces with the charge-transporting layers. The morphological instability induced by the low glass-transition temperatures and high diffusion properties of NFAs is one of the main reasons causes the degradation of OSCs. The high diffusion coefficients and tendency for self-aggregation of NFAs often result in high nucleation density and fast growth of the NFA crystals, leading to deteriorated stability. Especially, the originally well phase-separated nanoscale morphology between donor and acceptor can be easily destroyed by forming excessive aggregation under continuous aging to result in the degraded device lifetime and efficiency. To resolve these challenges, it is essential to design new types of acceptors with low diffusivity to stabilize the morphology for achieving long-term stability of OSCs.
In this thesis, the main work is aimed at improving the efficiency and long-term stability of OSCs. NIR acceptor materials were designed and synthesized by exploiting new fused-ring central cores to broaden the absorption and improve the Jsc and PCE. In addition, side-chain engineering was also introduced in the molecular design to provide a comprehensive understanding of the correlation between molecular structure, charge-transporting properties, and solar cell performance. In addition, dimer acceptors with different lengths flexible linkers were also exploited to not only stabilize the morphology of the active layer, but also realize high-efficiency OSCs with enhanced stability.
Specifically, NIR NFAs were designed and synthesized by introducing a weaker electron-withdrawing central core to enhance the intramolecular charge transfer (ICT) between the core and the end group to further broaden the absorption. A fused-ring π-core BzS was designed by combining weakly electron-withdrawing benzotriazole (Bz) and strongly electron-donating selenophene together. Besides, the length of N-alkyl chain on the Bz moiety was engineered to tune the morphology, affording two NFAs mBzS-4F and EHBzS-4F. Both NFAs possess an absorption edge approaching 1000 nm due to the enhanced intramolecular charge transfer and efficient intra- and inter-molecular interactions. As a result, binary photovoltaic devices based on PM6:mBzS-4F showed a high power conversion efficiency of 17.02% with a very high Jsc (27.72 mA cm-2) and a low energy loss (0.446 eV). This work provides a useful strategy to design efficient NIR-responsive materials.
Side-chain engineering was also adopted to tune the active layer’s thin-film morphology, in addition, X-ray crystallography was studied to reveal the influence of molecular packing on charge-transporting properties. A series of Bz-based NFAs, PN6SBO-4F, AN6SBO-4F, and EHN6SEH-4F were designed via regiospecific N-alkyl modification based on the initial high-performance NFA mBzS-4F that was synthesized. After analyzing the single-crystal structure, the strong interactions of terminal indanone groups in mBzS-4F and the J-aggregate-like packing in EHN6SEH-4F lead to the formation of ordered 3D networks in single-crystals with channels for efficient charge transport. Consequently, OSCs based on mBzS-4F and EHN6SEH-4F show efficient photon-to-current conversions, achieving the highest power conversion efficiency of 17.48% with a Jsc of 28.83 mA cm-2.
The third work is related to the investigation of the effect of regio-specific selenium substitution on narrow-bandgap NFAs capable of generating efficient photocurrents by manipulating their reorganization energies and triplet character. The multiple selenium-substituted S9SBO-F possesses tight molecular packing and considerably red-shifted absorption, which enable the derived solar cells to achieve an extraordinarily high short-circuit current density (>29 mA cm-2) for realizing a high-power conversion efficiency (>19%). More interestingly, pronounced triplet formation can be spectroscopically confirmed in neat S9SBO-F films, which is distinctly different from that observed for most of Y6-derived NFAs. These findings provide the basis to further exploit the intrinsic triplet character of narrow-bandgap NFAs for enhancing current generation and efficiency of OSCs.
The last part of this thesis is focused on exploiting new type of NIR acceptors to enhance the thermo- and photo-stability of high-efficiency OSCs. A series of dimer acceptor, dT9TBO, adopting different flexible linkers were used as the third component in the ternary system to not only optimize intermolecular packing of small molecular acceptors, but also suppress their molecular diffusion to afford stabilized morphology. Consequently, the champion PM6:Y6:dT9TBO-based device displays a high PCE of 18.41% and shows excellent thermal/photo stability. Moreover, these OSCs also exhibit very good mechanical properties, showing their potential to be used for wearable electronics.
Apart from improving the efficiency of the OSCs, the trade-off between high efficiency and long-term stability is still very challenging for the commercialization of OSCs due to the quite complicated crystallization dynamics of active layer and its interfaces with the charge-transporting layers. The morphological instability induced by the low glass-transition temperatures and high diffusion properties of NFAs is one of the main reasons causes the degradation of OSCs. The high diffusion coefficients and tendency for self-aggregation of NFAs often result in high nucleation density and fast growth of the NFA crystals, leading to deteriorated stability. Especially, the originally well phase-separated nanoscale morphology between donor and acceptor can be easily destroyed by forming excessive aggregation under continuous aging to result in the degraded device lifetime and efficiency. To resolve these challenges, it is essential to design new types of acceptors with low diffusivity to stabilize the morphology for achieving long-term stability of OSCs.
In this thesis, the main work is aimed at improving the efficiency and long-term stability of OSCs. NIR acceptor materials were designed and synthesized by exploiting new fused-ring central cores to broaden the absorption and improve the Jsc and PCE. In addition, side-chain engineering was also introduced in the molecular design to provide a comprehensive understanding of the correlation between molecular structure, charge-transporting properties, and solar cell performance. In addition, dimer acceptors with different lengths flexible linkers were also exploited to not only stabilize the morphology of the active layer, but also realize high-efficiency OSCs with enhanced stability.
Specifically, NIR NFAs were designed and synthesized by introducing a weaker electron-withdrawing central core to enhance the intramolecular charge transfer (ICT) between the core and the end group to further broaden the absorption. A fused-ring π-core BzS was designed by combining weakly electron-withdrawing benzotriazole (Bz) and strongly electron-donating selenophene together. Besides, the length of N-alkyl chain on the Bz moiety was engineered to tune the morphology, affording two NFAs mBzS-4F and EHBzS-4F. Both NFAs possess an absorption edge approaching 1000 nm due to the enhanced intramolecular charge transfer and efficient intra- and inter-molecular interactions. As a result, binary photovoltaic devices based on PM6:mBzS-4F showed a high power conversion efficiency of 17.02% with a very high Jsc (27.72 mA cm-2) and a low energy loss (0.446 eV). This work provides a useful strategy to design efficient NIR-responsive materials.
Side-chain engineering was also adopted to tune the active layer’s thin-film morphology, in addition, X-ray crystallography was studied to reveal the influence of molecular packing on charge-transporting properties. A series of Bz-based NFAs, PN6SBO-4F, AN6SBO-4F, and EHN6SEH-4F were designed via regiospecific N-alkyl modification based on the initial high-performance NFA mBzS-4F that was synthesized. After analyzing the single-crystal structure, the strong interactions of terminal indanone groups in mBzS-4F and the J-aggregate-like packing in EHN6SEH-4F lead to the formation of ordered 3D networks in single-crystals with channels for efficient charge transport. Consequently, OSCs based on mBzS-4F and EHN6SEH-4F show efficient photon-to-current conversions, achieving the highest power conversion efficiency of 17.48% with a Jsc of 28.83 mA cm-2.
The third work is related to the investigation of the effect of regio-specific selenium substitution on narrow-bandgap NFAs capable of generating efficient photocurrents by manipulating their reorganization energies and triplet character. The multiple selenium-substituted S9SBO-F possesses tight molecular packing and considerably red-shifted absorption, which enable the derived solar cells to achieve an extraordinarily high short-circuit current density (>29 mA cm-2) for realizing a high-power conversion efficiency (>19%). More interestingly, pronounced triplet formation can be spectroscopically confirmed in neat S9SBO-F films, which is distinctly different from that observed for most of Y6-derived NFAs. These findings provide the basis to further exploit the intrinsic triplet character of narrow-bandgap NFAs for enhancing current generation and efficiency of OSCs.
The last part of this thesis is focused on exploiting new type of NIR acceptors to enhance the thermo- and photo-stability of high-efficiency OSCs. A series of dimer acceptor, dT9TBO, adopting different flexible linkers were used as the third component in the ternary system to not only optimize intermolecular packing of small molecular acceptors, but also suppress their molecular diffusion to afford stabilized morphology. Consequently, the champion PM6:Y6:dT9TBO-based device displays a high PCE of 18.41% and shows excellent thermal/photo stability. Moreover, these OSCs also exhibit very good mechanical properties, showing their potential to be used for wearable electronics.
