| 1. |
- Aarnio, Harri, et al.
(author)
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Spontaneous Charge Transfer and Dipole Formation at the Interface Between P3HT and PCBM
- 2011
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In: Advanced Energy Materials. - : Wiley-VCH Verlag Berlin. - 1614-6832 .- 1614-6840. ; 1:5, s. 792-797
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Journal article (peer-reviewed)abstract
- In the pursuit of developing new materials for more efficient bulk-heterojunction solar cells, the blend poly (3-hexylthiophene):[ 6,6]-phenyl-C(61)-butyric acid methyl ester (P3HT:PCBM) serves as an important model system. The success of the P3HT: PCBM blend comes from efficient charge generation and transport with low recombination. There is not, however, a good microscopic picture of what causes these, hindering the development of new material systems. In this report UV photoelectron spectroscopy measurements on both regiorandom-(rra) and regioregular-(rr) P3HT are presented, and the results are interpreted using the Integer Charge Transfer model. The results suggest that spontaneous charge transfer from P3HT to PCBM occurs after heat treatment of P3HT: PCBM blends. The resulting formation of an interfacial dipole creates an extra barrier at the interface explaining the reduced (non-)geminate recombination with increased charge generation in heat treated rr-P3HT: PCBM blends. Extensive photoinduced absorption measurements using both above-and below-bandgap excitation light are presented, in good agreement with the suggested dipole formation.
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| 2. |
- Bao, Qinye, et al.
(author)
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Oxygen- and Water-Based Degradation in [6,6]-Phenyl-C-61-Butyric Acid Methyl Ester (PCBM) Films
- 2014
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In: Advanced Energy Materials. - : Wiley-VCH Verlag. - 1614-6832 .- 1614-6840. ; 4:6
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Journal article (peer-reviewed)abstract
- Effects of in situ oxygen/water exposure on the energetics of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) films are presented. For oxygen exposure, the work function is downshifted by ≈0.15 eV compared to the ideal integer charge transfer (ICT) curve for pristine PCBM, which is incompatible with significant introduction of electron trap states or p-doping. Water induces the highest occupied molecular orbital (HOMO) structure to undergo strong, irreversible modifications accompanied by a chemical interaction with PCBM.
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| 3. |
- Bao, Qinye, et al.
(author)
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Trap-Assisted Recombination via Integer Charge Transfer States in Organic Bulk Heterojunction Photovoltaics
- 2014
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In: Advanced Functional Materials. - : Wiley-VCH Verlag. - 1616-301X .- 1616-3028. ; 24:40, s. 6309-6316
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Journal article (peer-reviewed)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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| 4. |
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| 5. |
- Beneš, Vít, et al.
(author)
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The Czech EU policy : domestic political context and key priorities
- 2011
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In: Revista de Historia Actual. - 1697-3305. ; 9:9, s. 65-79
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Journal article (peer-reviewed)abstract
- The article summarizes the ideological points of departure, the domestic political context and the key priorities of the Czech Republic’s European policy since 2007. We have identified three most important (and most controversial) topics: 1) the Czech EU presidency, 2) the Lisbon Treaty and 3) the eurozone. The most significant event for the country during the period was the Council Presidency in 2009; which actually delivered some concrete results but will probably most be remembered for the accompanying domestic quarrels. The Lisbon Treaty ratification exemplified the divisions among the country’s political elite; with a powerful group of euro-sceptics who advocate the loosening of the European integration and who are increasingly opposed to the Czech Republic’s participation in EU’s (or eurozone’s) policies. The eurozone crisis hardened Czech opposition to euro. The majority of politicians and the general public intend to remain outside of the euro-zone, the country is moving in the direction of ‘splendid isolation’.
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| 6. |
- Braun, Mats
(author)
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EU Climate Norms in East-Central Europe
- 2014
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In: Journal of Common Market Studies. - : John Wiley & Sons. - 0021-9886 .- 1468-5965. ; 52:3, s. 445-460
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Journal article (peer-reviewed)
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| 7. |
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| 8. |
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| 9. |
- Braun, Mats
(author)
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The Europeanization of Czech Environmental Policy Post Conditionality : Lessons from the REACH regulation and the Climate-Energy Package
- 2013
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In: Politics in Central Europe. - 1801-3422. ; 9:1, s. 5-26
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Journal article (peer-reviewed)abstract
- This paper aims to contribute to the literature on social norms in IR. The intention is to further specify how social norms influence state behaviour. The empirical material consists of a case study of how the Czech Republic has participated in EU policy making in the field of environmental policy. The paper first discusses the different reasons for why member states adapt to EU norms: i.e. rational conditionality and constructivist social learning. The former of the two is viewed as typical for the pre-accession period whereas the latter is important for our understanding of the post-accession period. In order to outline the actual socialization process a detailed case study of how the Czech Republic has dealt with two different EU policy initiatives (REACH and the climate-energy package) is presented. The conclusions suggest that even if we can find indications of socialization the process is complicated by a strong presence of competing norms.
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| 10. |
- Braun, Slawomir, et al.
(author)
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Fermi level equilibrium at donor-acceptor interfaces in multi-layered thin film stack of TTF and TCNQ
- 2010
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In: Organic electronics. - Amsterdam, Netherlands : Elsevier BV. - 1566-1199 .- 1878-5530. ; 11:2, s. 212-217
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Journal article (peer-reviewed)abstract
- Organic hetero-junctions in multi-layered thin film stacks comprising alternate layers of the molecular donor-tetrathiafulvalene (TTF) and the acceptor - tetracyanoquinodimethane (TCNQ), have been studied by ultraviolet photoelectron spectroscopy ( UPS). We show that the energy level alignment at the organic-organic interfaces in the stacks depends only upon the relative energy structure of the donor and acceptor molecules, in particular, the molecular integer charge transfer (ICT) states. The observed interfacial dipoles, across the multi-layered organic stacks, correspond to the difference in energy between the positive and the negative charge transfer states of the molecules constituting the interface. Consequently, Fermi level across the multi-layer system is pinned to those states, since the energetic conditions for the charge transfer across the interface are fulfilled. Hence the energy level alignment at donor - acceptor interfaces studied can be rationalized on the basis of integer charge transfer model (ICT-model). Moreover, we present the photoelectron spectra where 0.85 eV shift of the highest occupied molecular orbital (HOMO) of TTF during formation of TCNQ over-layer is directly observed. These studies contribute to the understanding of the nature of the offset between the frontier electronic levels of the donor and acceptor components which is of high importance in the engineering of efficient organic solar cells.
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