| 1. |
- Fan, Baobing, et al.
(författare)
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Correlation of Broad Absorption Band with Small Singlet-Triplet Energy Gap in Organic Photovoltaics
- 2023
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Ingår i: Angewandte Chemie International Edition. - : WILEY-V C H VERLAG GMBH. - 1433-7851 .- 1521-3773.
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Tidskriftsartikel (refereegranskat)abstract
- Organic photovoltaics (OPV) are one of the most effective ways to harvest renewable solar energy, with the power conversion efficiency (PCE) of the devices soaring above 19% when processed with halogenated solvents. The superior photocurrent of OPV over other emerging photovoltaics offers more opportunities to further improve the efficiency. Tailoring the absorption band of photoactive materials is an effective way to further enhance OPV photocurrent. However, the field has mostly been focusing on improving the near-infrared region photo-response, with the absorption shoulders in short-wavelength region (SWR) usually being neglected. Herein, by developing a series of non-fullerene acceptors (NFAs) with varied side-group conjugations, we observe an enhanced SWR absorption band with increased side-group conjugation length. The underpinning factors of how molecular structures and geometries improve SWR absorption are clearly elucidated through theoretical modelling and crystallography. Moreover, a clear relationship between the enhanced SWR absorption and reduced singlet-triplet energy gap is established, both of which are favorable for the OPV performance and can be tailored by rational structure design of NFAs. Finally, the rationally designed NFA, BO-TTBr, affords a decent PCE of 18.5% when processed with a non-halogenated green solvent.
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| 2. |
- Fan, Baobing, et al.
(författare)
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Correlation of Local Isomerization Induced Lateral and Terminal Torsions with Performance and Stability of Organic Photovoltaics
- 2023
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Ingår i: Journal of the American Chemical Society. - : AMER CHEMICAL SOC. - 0002-7863 .- 1520-5126. ; 145:10, s. 5909-5919
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Tidskriftsartikel (refereegranskat)abstract
- Organic photovoltaics (OPVs) have achieved great progress in recent years due to delicately designed non-fullerene acceptors (NFAs). Compared with tailoring of the aromatic heterocycles on the NFA backbone, the incorporation of conjugated side-groups is a cost-effective way to improve the photoelectrical properties of NFAs. However, the modifications of side-groups also need to consider their effects on device stability since the molecular planarity changes induced by side-groups are related to the NFA aggregation and the evolution of the blend morphology under stresses. Herein, a new class of NFAs with localisomerized conjugated side-groups are developed and the impact of local isomerization on their geometries and device performance/stability are systematically investigated. The device based on one of the isomers with balanced side- and terminal-group torsion angles can deliver an impressive power conversion efficiency (PCE) of 18.5%, with a low energy loss (0.528 V) and an excellent photo- and thermal stability. A similar approach can also be applied to another polymer donor to achieve an even higher PCE of 18.8%, which is among the highest efficiencies obtained for binary OPVs. This work demonstrates the effectiveness of applying local isomerization to fine-tune the side-group steric effect and non-covalent interactions between side-group and backbone, therefore improving both photovoltaic performance and stability of fused ring NFA-based OPVs.
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| 3. |
- Jiang, Kui, et al.
(författare)
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Suppressed recombination loss in organic photovoltaics adopting a planar-mixed heterojunction architecture
- 2022
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Ingår i: Nature Energy. - : NATURE PORTFOLIO. - 2058-7546. ; 7:11, s. 1076-1086
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Tidskriftsartikel (refereegranskat)abstract
- At present, high-performance organic photovoltaics mostly adopt a bulk-heterojunction architecture, in which exciton dissociation is facilitated by charge-transfer states formed at numerous donor-acceptor (D-A) heterojunctions. However, the spin character of charge-transfer states originated from recombination of photocarriers allows relaxation to the lowest-energy triplet exciton (T-1) at these heterojunctions, causing photocurrent loss. Here we find that this loss pathway can be alleviated in sequentially processed planar-mixed heterojunction (PMHJ) devices, employing donor and acceptor with intrinsically weaker exciton binding strengths. The reduced D-A intermixing in PMHJ alleviates non-geminate recombination at D-A contacts, limiting the chance of relaxation, thus suppressing T-1 formation without sacrificing exciton dissociation efficiency. This resulted in devices with high power conversion efficiencies of >19%. We elucidate the working mechanisms for PMHJs and discuss the implications for material design, device engineering and photophysics, thus providing a comprehensive grounding for future organic photovoltaics to reach their full promise. Organic solar cells with a bulk-heterojunction architecture suffer from photocurrent loss driven by triplet states. Now, Jiang et al. show that sequentially deposited donor-acceptor planar-mixed heterojunctions suppress triplet formation, enabling efficiencies over 19%.
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