| 1. |
- Fang, Junfeng, et al.
(författare)
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Conjugated Zwitterionic Polyelectrolyte as the Charge Injection Layer for High-Performance Polymer Light-Emitting Diodes
- 2011
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society. - 0002-7863 .- 1520-5126. ; 133:4, s. 683-685
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Tidskriftsartikel (refereegranskat)abstract
- A new zwitterionic conjugated polyelectrolyte without free counterions has been used as an electron injection material in polymer light-emitting diodes. Both the efficiency and maximum brightness were considerably improved in comparison with standard Ca cathode devices. The devices showed very fast response times, indicating that the improved performance is, in addition to hole blocking, due to dipoles at the cathode interface, which facilitate electron injection.
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| 2. |
- He, Ximin, et al.
(författare)
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Formation of Nanopatterned Polymer Blends in Photovoltaic Devices
- 2010
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Ingår i: Nano letters (Print). - : American Chemical Society. - 1530-6984 .- 1530-6992. ; 10:4, s. 1302-1307
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we demonstrate a double nanoimprinting process that allows the formation of nanostructured polymer heterojunctions of composition and morphology that can be selected independently. We fabricated photovoltaic (PV) devices with extremely high densities (10(14)/mm(2)) of interpenetrating nanoscale columnar features in the active polymer blend layer. The smallest feature sizes are as small as 25 nm on a 50 nm pitch, which results in a spacing of hererojunctions at or below the exciton diffusion length. Photovoltaic devices based on double-imprinted poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2,2 diyl) (F8TBT)/poly(3-hexylthiophene) (P3HT) films are among the best polymer polymer blend devices reported to date with a power conversion efficiency (PCE, eta(e)) of 1.9%.
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| 3. |
- He, Ximin, et al.
(författare)
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Formation of Well-Ordered Heterojunctions in Polymer: PCBM Photovoltaic Devices
- 2011
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Ingår i: Advanced Functional Materials. - : Wiley-VCH Verlag. - 1616-301X .- 1616-3028. ; 21:1, s. 139-146
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Tidskriftsartikel (refereegranskat)abstract
- The nanoscale morphology in polymer:PCBM based photovoltaic devices is a major contributor to overall device performance. The disordered nature of the phase-separated structure, in combination with the small length scales involved and the inherent difficulty of reproducing the exact morphologies when spin-coating and annealing thin blend films, have greatly hampered the development of a detailed understanding of how morphology impacts photo voltaic device functioning. In this paper we demonstrate a double nanoimprinting process that allows the formation of nanostructured polymer: PCBM heterojunctions of composition and morphology that can be selected independently. We fabricated photovoltaic (PV) devices with extremely high densities (10(14) mm(-2)) of interpenetrating nanoscale columnar features (as small as 25 nm; at or below the exciton diffusion length) in the active layer. By comparing device results of different feature sizes and two different polymer: PCBM combinations, we demonstrate how double imprinting can be a powerful tool to systematically study different parameters in polymer photovoltaic devices.
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| 4. |
- Howard, Ian A., et al.
(författare)
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Charge Recombination and Exciton Annihilation Reactions in Conjugated Polymer Blends
- 2010
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Ingår i: Journal of the American Chemical Society. - 0002-7863. ; 132:1, s. 328-335
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Tidskriftsartikel (refereegranskat)abstract
- Bimolecular interactions between excitations in conjugated polymer thin films are important because they influence the efficiency of many optoelectronic devices that require high excitation densities. Using time-resolved optical spectroscopy, we measure the bimolecular interactions of charges, singlet excitons, and triplet excitons in intimately mixed polyfluorene blends with band-edge offsets optimized for photoinduced electron transfer. Bimolecular charge recombination and triplet−triplet annihilation are negligible, but exciton−charge interactions are efficient. The annihilation of singlet excitons by charges occurs on picosecond time-scales and reaches a rate equivalent to that of charge transfer. Triplet exciton annihilation by charges occurs on nanosecond time-scales. The surprising absence of nongeminate charge recombination is shown to be due to the limited mobility of charge carriers at the heterojunction. Therefore, extremely high densities of charge pairs can be maintained in the blend. The absence of triplet−triplet annihilation is a consequence of restricted triplet diffusion in the blend morphology. We suggest that the rate and nature of bimolecular interactions are determined by the stochastic excitation distribution in the polymer blend and the limited connectivity between the polymer domains. A model based on these assumptions quantitatively explains the effects. Our findings provide a comprehensive framework for understanding bimolecular recombination and annihilation processes in nanostructured materials.
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| 5. |
- Yim, Keng-Hoong, et al.
(författare)
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Phase-Separated Thin Film Structures for Efficient Polymer Blend Light-Emitting Diodes
- 2010
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Ingår i: NANO LETTERS. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 10:2, s. 385-392
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Tidskriftsartikel (refereegranskat)abstract
- We report laterally and vertically phase-separated thin film structures in conjugated polymer blends created by polymer molecular weight variation. We find that micrometer-scale lateral phase separation is critical in achieving high initial device efficiency of light-emitting diodes, whereas improved balance of charge carrier mobilities and film thickness uniformity are important in maintaining high efficiency at high voltages. The optoelectronic properties of these blend thin films and devices are strongly influenced by the polymer chain order/disorder and the interface state formed at polymer/polymer heterojunctions.
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