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Sökning: WFRF:(Tvingstedt Kristofer)

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31.
  • Vandewal, Koen, et al. (författare)
  • Polarization anisotropy of charge transfer absorption and emission of aligned polymer: fullerene blend films
  • 2012
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 86:3, s. 035212-
  • Tidskriftsartikel (refereegranskat)abstract
    • An improved understanding of the electronic structure of interfacial charge transfer (CT) states is of importance due to their crucial role in charge carrier generation and recombination in organic donor-acceptor (DA) solar cells. DA combinations with a small difference between the energy of the CT state (E-CT) and energy of the donor exciton (E-D*) are of special interest since energy losses due to electron transfer are minimized, resulting in an optimized open-circuit voltage. In that case, the CT state can be considered as a resonance mixture, containing character of a fully ionic state (D+ A(-)) and of the local polymer excited state (D* A). We show that the D* A contribution to the overall CT state wave function can be determined by measurements of the polarization anisotropy of CT absorption and emission of polymer: fullerene blends with aligned polymer chains. We study two donor polymers, P3HT and TQ1, blended with fullerene acceptors with different ionization potentials, allowing variation of the E-D* -E-CT difference. We find that, upon decreasing E-D* -E-CT, the local excitonic D* A character of the CT state increases, resulting in a decreased fraction of charge transferred and an increased transition dipole moment. For typical polymer: fullerene systems, this effect is expected to become detrimental for device performance if E-D* - E-CT andlt; 0.1 eV. This however, depends on the electronic coupling between D* A and D+ A(-), which we experimentally estimate to be similar to 6 meV for the TQ1: PCBM system.
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32.
  • Vandewal, Koen, et al. (författare)
  • Quantification of Quantum Efficiency and Energy Losses in Low Bandgap Polymer:Fullerene Solar Cells with High Open-Circuit Voltage
  • 2012
  • Ingår i: Advanced Functional Materials. - : Wiley-VCH Verlag Berlin. - 1616-301X .- 1616-3028. ; 22:16, s. 3480-3490
  • Tidskriftsartikel (refereegranskat)abstract
    • In organic solar cells based on polymer:fullerene blends, energy is lost due to electron transfer from polymer to fullerene. Minimizing the difference between the energy of the polymer exciton (ED*) and the energy of the charge transfer state (ECT) will optimize the open-circuit voltage (Voc). In this work, this energy loss ED*-ECT is measured directly via Fourier-transform photocurrent spectroscopy and electroluminescence measurements. Polymer:fullerene photovoltaic devices comprising two different isoindigo containing polymers: P3TI and PTI-1, are studied. Even though the chemical structures and the optical gaps of P3TI and PTI-1 are similar (1.4 eV1.5 eV), the optimized photovoltaic devices show large differences in Voc and internal quantum efficiency (IQE). For P3TI:PC71BM blends a ED*-ECT of similar to 0.1 eV, a Voc of 0.7 V and an IQE of 87% are found. For PTI-1:PC61BM blends an absence of sub-gap charge transfer absorption and emission bands is found, indicating almost no energy loss in the electron transfer step. Hence a higher Voc of 0.92 V, but low IQE of 45% is obtained. Morphological studies and field dependent photoluminescence quenching indicate that the lower IQE for the PTI-1 system is not due to a too coarse morphology, but is related to interfacial energetics. Losses between ECT and qVoc due to radiative and non-radiative recombination are quantified for both material systems, indicating that for the PTI-1:PC61BM material system, Voc can only be increased by decreasing the non-radiative recombination pathways. This work demonstrates the possibility of obtaining modestly high IQE values for material systems with a small energy offset (andlt;0.1 eV) and a high Voc.
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33.
  • Vandewal, Koen, et al. (författare)
  • Relating the open-circuit voltage to interface molecular properties of donor:acceptor bulk heterojunction solar cells
  • 2010
  • Ingår i: PHYSICAL REVIEW B. - : American Physical Society. - 1098-0121. ; 81:12, s. 125204-
  • Tidskriftsartikel (refereegranskat)abstract
    • The open-circuit voltage (V-oc) of polymer:fullerene bulk heterojunction solar cells is determined by the interfacial charge-transfer (CT) states between polymer and fullerene. Fourier-transform photocurrent spectroscopy and electroluminescence spectra of several polymer:fullerene blends are used to extract the relevant interfacial molecular parameters. An analytical expression linking these properties to V-oc is deduced and shown to be valid for photovoltaic devices comprising three commonly used conjugated polymers blended with the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). V-oc is proportional to the energy of the CT states E-CT. The energetic loss q Delta V between E-CT and qV(oc) vanishes when approaching 0 K. It depends linearly on T and logarithmically on illumination intensity. Furthermore q Delta V can be reduced by decreasing the electronic coupling between polymer and fullerene or by reducing the nonradiative recombination rate. For the investigated devices we find a loss q Delta V of similar to 0.6 eV at room temperature and under solar illumination conditions, of which similar to 0.25 eV is due to radiative recombination via the CT state and similar to 0.35 eV is due to nonradiative recombination.
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34.
  • Wang, Xiangjun, et al. (författare)
  • Single and bilayer submicron arrays of fluorescent polymer on conducting polymer surface with surface energy controlled dewetting
  • 2005
  • Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 16, s. 437-443
  • Tidskriftsartikel (refereegranskat)abstract
    • Construction of luminescent single- and bilayer polymer arrays in micron and submicron scales through dewetting on a heterogeneous conducting polymer surface is demonstrated. We study the influence of the pattern geometry and film thickness of polymer dewetting upon annealing, and the morphology of created polymer arrays on the heterogeneous surface. The materials used for patterning are an insulating poly(methyl methacrylate) (PMMA) or a conjugated fluorescent polymer, poly(dioctylphenylthiophene) (PDOPT). The substrate used is the conducting polymer poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT–PSS), with modified heterogeneous surface energy obtained by application of a bare polydimethylsiloxane (PDMS) stamp.
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35.
  • Zhang, Fengling, et al. (författare)
  • High photovoltage achieved in low band gap polymer solar cells by adjusting energy levels of a polymer with the LUMOs of fullerene derivatives
  • 2008
  • Ingår i: Journal of Materials Chemistry. - : Royal Society of Chemistry (RSC). - 0959-9428 .- 1364-5501. ; 18:45, s. 5468-5474
  • Tidskriftsartikel (refereegranskat)abstract
    • Solar cells based on organic molecules or conjugated polymers attract great attention due to their unique advantages, such as low cost, and their use in flexible devices, but are still limited by their low power conversion efficiency (PCE). To improve the PCEs of polymer solar cells, more efforts have been made to increase short-circuit current (J(sc)) or open-circuit voltage (V-oc). However, the trade-off between J(sc) and V-oc in bulk heterojunctions solar cells makes it tricky to find a polymer with a low band gap to efficiently absorb photons in the visible and near infrared region of the solar spectrum, while maintaining a high V-oc in solar cells. Therefore, it is crucial to design and synthesize polymers with energy levels aligning with the LUMO (lowest unoccupied molecular orbital) of an electron acceptor to minimize the LUMO level difference between donor and acceptor to keep enough driving force for charge generation, thereby maximizing photovoltage in solar cells. Here a novel copolymer APFO-Green 9 was synthesized. Polymer solar cells based on APFO-Green 9 blended with a derivative of fullerene demonstrate high photovoltage by fine tuning the HOMO and LUMO level of APFO-Green 9. Solar cells based on APFO-Green 9 and [6,6]-phenyl-C71-butyric acid methyl ester ([70]PCBM) present a photoresponse extended to 900 nm with J(sc) of 6.5 mA cm(-2), V-oc of 0.81 V and PCE of 2.3% under illumination of AM1.5 with light intensity of 100 mW cm(-2). As a low band gap polymer with a V-oc bigger than 0.8 V, APFO-Green 9 is a promising candidate for efficient tandem solar cells.
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38.
  • Zhou, Yi, et al. (författare)
  • Observation of a Charge Transfer State in Low-Bandgap Polymer/Fullerene Blend Systems by Photoluminescence and Electroluminescence Studies
  • 2009
  • Ingår i: ADVANCED FUNCTIONAL MATERIALS. - : Wiley. - 1616-301X .- 1616-3028. ; 19:20, s. 3293-3299
  • Tidskriftsartikel (refereegranskat)abstract
    • The presence of charge transfer states generated by the interaction between the fullerene acceptor PCBM and two alternating copolymers of fluorene with donor-acceptor-donor comonomers are reported; the generation leads to modifications in the polymer bandgap and electronic structure. In one of polymer/fullerene blends, the driving; force for photocurrent generation, i.e., the gap between the lowest unoccupied molecular orbitals of the donor and acceptor, is only 0.1 eV, but photocurrent is generated. It is shown that the presence of a charge transfer state is more important than the driving force. The charge transfer states are visible through new emission peaks in the photoluminescence spectra and through electroluminescence at a forward bias. The photoluminescence can be quenched under reverse bias, and can be directly correlated to the mechanism of photocurrent generation. The excited charge transfer state is easily dissociated into free charge carriers, and is an important intermediate state through which free charge carriers are generated.
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