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Increased upconversion performance for thin film solar cells: a trimolecular composition

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      Abstract

      A dual-emitter upconvertor is applied to thin-film solar cells for the first time, generating record figures of merit.

      Abstract

      Photochemical upconversion based on triplet–triplet annihilation (TTA-UC) is employed to enhance the short-circuit currents generated by two varieties of thin-film solar cells, a hydrogenated amorphous silicon (a-Si:H) solar cell and a dye-sensitized solar cell (DSC). TTA-UC is exploited to harvest transmitted sub-bandgap photons, combine their energies and re-radiate upconverted photons back towards the solar cells. In the present study we employ a dual-emitter TTA-UC system which allows for significantly improved UC quantum yields as compared to the previously used single-emitter TTA systems. In doing so we achieve record photo-current enhancement values for both the a-Si:H device and the DSC, surpassing 10 –3 mA cm –2 sun –2 for the first time for a TTA-UC system and marking a record for upconversion-enhanced solar cells in general. We discuss pertinent challenges of the TTA-UC technology which need to be addressed in order to achieve its viable device application.

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      Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals.

      Lanthanide ions exhibit unique luminescent properties, including the ability to convert near infrared long-wavelength excitation radiation into shorter visible wavelengths through a process known as photon upconversion. In recent years lanthanide-doped upconversion nanocrystals have been developed as a new class of luminescent optical labels that have become promising alternatives to organic fluorophores and quantum dots for applications in biological assays and medical imaging. These techniques offer low autofluorescence background, large anti-Stokes shifts, sharp emission bandwidths, high resistance to photobleaching, and high penetration depth and temporal resolution. Such techniques also show potential for improving the selectivity and sensitivity of conventional methods. They also pave the way for high throughput screening and miniaturization. This tutorial review focuses on the recent development of various synthetic approaches and possibilities for chemical tuning of upconversion properties, as well as giving an overview of biological applications of these luminescent nanocrystals.
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        Surface plasmonic effects of metallic nanoparticles on the performance of polymer bulk heterojunction solar cells.

        We have systematically explored how plasmonic effects influence the characteristics of polymer photovoltaic devices (OPVs) incorporating a blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM). We blended gold nanoparticles (Au NPs) into the anodic buffer layer to trigger localized surface plasmon resonance (LSPR), which enhanced the performance of the OPVs without dramatically sacrificing their electrical properties. Steady state photoluminescence (PL) measurements revealed a significant increase in fluorescence intensity, which we attribute to the increased light absorption in P3HT induced by the LSPR. As a result, the rate of generation of excitons was enhanced significantly. Furthermore, dynamic PL measurements revealed that the LSPR notably reduced the lifetime of photogenerated excitons in the active blend, suggesting that interplay between the surface plasmons and excitons facilitated the charge transfer process. This phenomenon reduced the recombination level of geminate excitons and, thereby, increased the probability of exciton dissociation. Accordingly, both the photocurrents and fill factors of the OPV devices were enhanced significantly. The primary origin of this improved performance was local enhancement of the electromagnetic field surrounding the Au NPs. The power conversion efficiency of the OPV device incorporating the Au NPs improved to 4.24% from a value of 3.57% for the device fabricated without Au NPs.
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          On the efficiency limit of triplet-triplet annihilation for photochemical upconversion.

          Photochemical upconversion is performed, whereby emitter triplet states are produced through triplet energy transfer from sensitizer molecules excited with low energy photons. The triplet emitter molecules undergo triplet-triplet annihilation to yield excited singlet states which emit upconverted fluorescence. Experiments comparing the 560 nm prompt fluorescence when rubrene emitter molecules are excited directly, using 525 nm laser pulses, to the delayed, upconverted fluorescence when the porphyrin sensitizer molecules are excited with 670 nm laser pulses reveal annihilation efficiencies to produce excited singlet emitters in excess of 20%. Conservative measurements reveal a 25% annihilation efficiency, while a direct comparison between the prompt and delayed fluorescence yield suggests a value as high as 33%. Due to fluorescence quenching, the photon upconversion efficiencies are lower, at 16%.
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            Author and article information

            Affiliations
            [a ] School of Chemistry , UNSW , Sydney , NSW 2052 , Australia . Email: timothy.schmidt@ 123456unsw.edu.au ; Tel: +61 439 386 109
            [b ] ARC Centre of Excellence for Electromaterials Science (ACES) , Intelligent Polymer Research Institute (IPRI) , The University of Wollongong , North Wollongong , NSW 2522 , Australia
            [c ] Institute for Silicon Photovoltaics , Helmholtz-Zentrum Berlin , D-12489 , Germany
            [d ] School of Chemistry , The University of Sydney , NSW 2006 , Australia
            Author notes

            †Contributed equally to this work.

            Journal
            Chem Sci
            Chem Sci
            Chemical Science
            Royal Society of Chemistry
            2041-6520
            2041-6539
            1 January 2016
            9 October 2015
            : 7
            : 1
            : 559-568
            5519954 c5sc03215f 10.1039/c5sc03215f
            This journal is © The Royal Society of Chemistry 2015

            This is an Open Access article distributed under the terms of the Creative Commons Attribution 3.0 Unported License ( http://creativecommons.org/licenses/by/3.0/) which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

            Categories
            Chemistry

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