| 1. |
- Bao, Qinye, et al.
(författare)
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Trap-Assisted Recombination via Integer Charge Transfer States in Organic Bulk Heterojunction Photovoltaics
- 2014
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Ingår i: Advanced Functional Materials. - : Wiley-VCH Verlag. - 1616-301X .- 1616-3028. ; 24:40, s. 6309-6316
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Tidskriftsartikel (refereegranskat)abstract
- Organic photovoltaics are under intense development and significant focus has been placed on tuning the donor ionization potential and acceptor electron affinity to optimize open circuit voltage. Here, it is shown that for a series of regioregular-poly(3-hexylthiophene): fullerene bulk heterojunction (BHJ) organic photovoltaic devices with pinned electrodes, integer charge transfer states present in the dark and created as a consequence of Fermi level equilibrium at BHJ have a profound effect on open circuit voltage. The integer charge transfer state formation causes vacuum level misalignment that yields a roughly constant effective donor ionization potential to acceptor electron affinity energy difference at the donor-acceptor interface, even though there is a large variation in electron affinity for the fullerene series. The large variation in open circuit voltage for the corresponding device series instead is found to be a consequence of trap-assisted recombination via integer charge transfer states. Based on the results, novel design rules for optimizing open circuit voltage and performance of organic bulk heterojunction solar cells are proposed.
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| 2. |
- Bubnova, Olga, et al.
(författare)
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Advantageous thermoelectric properties of a semimetallic polymer
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Thermoelectric generation potentially holds a solution for waste heat recovery issues provided that the availability of inexpensive, biodegradable and highly efficient thermoelectric materials is insured in the near future. Plastic thermoelectrics could successfully comply with the said requirements if the thermoelectric efficiency (ZT) of conducting polymers was higher. However, given the novelty of the subject, at present there are no clear guidelines for ZT optimization in this class of materials. The most important piece of information that is currently missing is the description of a specific electronic makeup that conducting polymers must possess in order to enable good thermoelectric performance. In the present study the thermoelectric properties of poly(3,4-ethylenedioxythiophene) derivatives with two types of counterions, i.e. poly(styrenesulfonate) (PSS) and tosylate (Tos) are evaluated. A striking variation in their thermoelectric performance is attributed to structural and morphological differences between two polymers that manifest itself in dissimilar charge transport mechanism. The superior properties of PEDOT-Tos presumably originate from a high degree of crystallinity and structural order that predetermines the tendency for bipolaron band formation. Unlike polaronic PEDOT-PSS with slowly varying density of localized states (DOS) near the Fermi level (EF), the DOS in PEDOT-Tos is characterized by higher asymmetry and higher charge carrier density at EF (similar to semimetals), which allows for higher thermopower and electrical conductivity. Therefore, we conclude that the polymers with semimetallic electronic makeup are expected to exhibit promising thermoelectric properties with bigger variation in thermopower upon doping.
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| 3. |
- Bubnova, Olga, et al.
(författare)
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Corrigendum: Semi-metallic polymers
- 2014
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Ingår i: Nature Materials. - : Springer Science and Business Media LLC. - 1476-1122 .- 1476-4660. ; 13, s. 662-662
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Tidskriftsartikel (refereegranskat)
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| 4. |
- Bubnova, Olga, et al.
(författare)
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Semi-metallic polymers
- 2014
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Ingår i: Nature Materials. - : Nature Publishing Group. - 1476-1122 .- 1476-4660. ; 13:2, s. 190-194
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Tidskriftsartikel (refereegranskat)abstract
- Polymers are lightweight, flexible, solution-processable materials that are promising for low-cost printed electronics as well as for mass-produced and large-area applications. Previous studies demonstrated that they can possess insulating, semiconducting or metallic properties; here we report that polymers can also be semi-metallic. Semi-metals, exemplified by bismuth, graphite and telluride alloys, have no energy bandgap and a very low density of states at the Fermi level. Furthermore, they typically have a higher Seebeck coefficient and lower thermal conductivities compared with metals, thus being suitable for thermoelectric applications. We measure the thermoelectric properties of various poly( 3,4-ethylenedioxythiophene) samples, and observe a marked increase in the Seebeck coefficient when the electrical conductivity is enhanced through molecular organization. This initiates the transition from a Fermi glass to a semi-metal. The high Seebeck value, the metallic conductivity at room temperature and the absence of unpaired electron spins makes polymer semi-metals attractive for thermoelectrics and spintronics.
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| 5. |
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| 6. |
- Dagnelund, Daniel, 1980-, et al.
(författare)
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Activation of defects in GaNP by post-growth hydrogen treatment
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Effect of post-growth hydrogen treatment on defects and their role in carrier recombination in molecular beam epitaxial GaNP alloys is examined by means of photoluminescence and optically detected magnetic resonance. We present direct experimental evidence for effective activation of several different defects in carrier recombination by the hydrogen treatment. Among them, two defect complexes are identified to contain a Ga interstitial (Gai). None of the activated Gai complexes was previously observed in GaNP. Possible mechanisms for the hydrogen-induced defect activation are discussed.
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| 7. |
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| 8. |
- Dagnelund, Daniel, et al.
(författare)
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Antiferromagnetic coupling in CdSe/ZnMnSe quantum dot structures
- 2012
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Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 101:5, s. 052405-1-052405-
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Tidskriftsartikel (refereegranskat)abstract
- Spin polarization of nonmagnetic CdSe quantum dots (QDs) coupled to adjacent ZnMnSe diluted magnetic semiconductor (DMS) is investigated by CW and time-resolved magneto-optical spectroscopy under tunable laser excitation. Efficient enhancement in the degree of σ− circular polarization of photoluminescence from the CdSe QDs is observed under optical excitation at the σ+-active exciton state of the DMS. The fact that the enhancement persists much longer than the exciton lifetime of the DMS rules out a role of the DMS excitons. A possible explanation is discussed in terms of antiferromagnetic coupling between the excitons in QDs and aligned Mn ions in DMS.
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| 9. |
- Dagnelund, Daniel, 1980-, et al.
(författare)
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Carrier and spin injection from ZnMnSe to CdSe quantum dots
- 2008
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Optical carrier/exction and spin injection processes from a ZnMnSe dilute magnetic semiconductor (DMS) to CdSe quantum dots (QD’s) are studied in detail by means of spinpolarized magneto- photoluminescence (PL) and PL excitation spectroscopies. Efficiency of carrier/exciton transfer is found to be practically independent of width (Lb) of a ZnSe barrier layer inserted between the DMS and QD’s. This is tentatively explained in terms of photonexchange energy transfer. In sharp contrast, spin injection efficiency is found to be largely suppressed in the structures with large Lb, pointing towards increasing spin loss.
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| 10. |
- Dagnelund, Daniel, et al.
(författare)
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Carrier and spin injection from ZnMnSe to CdSe quantum dots
- 2009
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Optical carrier/exction and spin injection processes from a ZnMnSe dilute magnetic semiconductor (DMS) to CdSe quantum dots (QD’s) are studied in detail by means of spinpolarized magneto- photoluminescence (PL) and PL excitation spectroscopies. Efficiency of carrier/exciton transfer is found to be practically independent of width (Lb) of a ZnSe barrier layer inserted between the DMS and QD’s. This is tentatively explained in terms of photonexchange energy transfer. In sharp contrast, spin injection efficiency is found to be largely suppressed in the structures with large Lb, pointing towards increasing spin loss.
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