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1.
  • Andersson, Mattias, et al. (författare)
  • Mixed C60/C70 based fullerene acceptors in polymer bulk-heterojunction solar cells
  • 2012
  • Ingår i: Organic Electronics: physics, materials, applications. - : Elsevier BV. - 1566-1199 .- 1878-5530. ; 13:12, s. 2856-2864
  • Tidskriftsartikel (refereegranskat)abstract
    • Different mixtures of identically substituted C60 and C70 based fullerens have been used as acceptors in three polymer: fullerene systems that strongly express various performance limiting aspects of bulk heterojunction solar cells. Results are correlated with, and discussed in terms of e.g. morphology, charge separation, and charge transport. In these systems, there appears to be no relevant differences in either mobility or energy level positions between the identically substituted C60 and C70 based fullerenes tested. Examples of how fullerene mixtures influence the nano-morphology of the active layer are given. An upper limit to the open circuit voltage that can be obtained with fullerenes is also suggested.
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2.
  • Camacho Dejay, Rafael, et al. (författare)
  • Polarization Imaging of Emissive Charge Transfer States in Polymer/Fullerene Blends
  • 2014
  • Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 0897-4756 .- 1520-5002. ; 26:23, s. 6695-6704
  • Tidskriftsartikel (refereegranskat)abstract
    • Photoexcitation of conjugated polymerfullerene blends results in population of a local charge transfer (CT) state at the interface between the two materials. The competition between recombination and dissociation of this interfacial state limits the generation of fully separated free charges. Therefore, a detailed understanding of the CT states is critical for building a comprehensive picture of the organic solar cells operation. We applied a new fluorescence microscopy method called two-dimensional polarization imaging to gain insight into the orientation of the transition dipole moments of the CT states, and the associated excitation energy transfer processes in TQ1:PCBM blend films. The polymer phase was oriented mechanically to relate the polymer dipole moment orientation to that of the CT states. CT state formation was observed to be much faster than energy transfer in the polymer phase. However, after being formed an emissive CT state does not exchange excitation energy with other CT states, suggesting that they are spatially and/or energetically isolated. We found that the quantum yield of the CT emission is smaller for CT states spatially located in the highly oriented polymer domains, which is interpreted as the result of enhanced CT state dissociation in highly ordered structures.
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3.
  • Gadisa, Abay, et al. (författare)
  • Bipolar Charge Transport in Fullerene Molecules in a Bilayer and Blend of Polyfluorene Copolymer and Fullerene
  • 2010
  • Ingår i: ADVANCED MATERIALS. - : Wiley. - 0935-9648. ; 22:9, s. 1008-
  • Tidskriftsartikel (refereegranskat)abstract
    • Efficient polymer solar cells typically contain the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), which promotes dissociation of excited states and enhances charge transport. The ability of PCBM to transport holes in solar cell bulk heterojunction films is monitored via the electroluminescence emission of a bulk heterojunction blend of PCBM and a polyfluorene copolymer. In polymer/fullerene bilayer diodes, fullerene emission is also observed.
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4.
  • Ma, Zaifei, et al. (författare)
  • Structure-Property Relationships of Oligothiophene-Isoindigo Polymers for Efficient Bulk-Heterojunction Solar Cells
  • 2014
  • Ingår i: energy and environmental science. - : Royal Society of Chemistry. - 1754-5692. ; 17:1, s. 361-369
  • Tidskriftsartikel (refereegranskat)abstract
    • A series of alternating oligothiophene (nT)-isoindigo (I) copolymers (PnTI) were synthesized to investigate the influence of the oligothiophene block length on the photovoltaic (PV) properties of PnTI:PCBM bulk-heterojunction blends. Our study indicates that the number of thiophene rings (n) in the repeating unit alters both, polymer crystallinity and polymer-fullerene interfacial energetics, which results in a decreasing open-circuit voltage (Voc) of the solar cells with increasing n. The short-circuit current density (Jsc) of P1TI:PCBM devices is limited by the absence of a significant driving force for electron transfer. Instead, blends based on P5TI and P6TI feature large polymer domains, which limit charge generation and thus Jsc. The best PV performance with a power conversion efficiency of up to 6.9% was achieved with devices based on P3TI, where a combination of favorable morphology and optimal interface energy level offset ensures efficient exciton separation and charge generation. The structure-property relationship demonstrated in this work is a valuable guideline for the design of high performance polymers with small energy losses during the charge generation process, allowing for the fabrication of efficient solar cells that combine a minimal loss in Voc with a high Jsc.
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5.
  • Melianas, Armantas, 1988-, et al. (författare)
  • Charge Transport in Pure and Mixed Phases in Organic Solar Cells
  • 2017
  • Ingår i: Advanced Energy Materials. - : John Wiley & Sons. - 1614-6832 .- 1614-6840. ; 7:20
  • Tidskriftsartikel (refereegranskat)abstract
    • In organic solar cells continuous donor and acceptor networks are considered necessary for charge extraction, whereas discontinuous neat phases and molecularly mixed donor–acceptor phases are generally regarded as detrimental. However, the impact of different levels of domain continuity, purity, and donor–acceptor mixing on charge transport remains only semiquantitatively described. Here, cosublimed donor–acceptor mixtures, where the distance between the donor sites is varied in a controlled manner from homogeneously diluted donor sites to a continuous donor network are studied. Using transient measurements, spanning from sub-picoseconds to microseconds photogenerated charge motion is measured in complete photovoltaic devices, to show that even highly diluted donor sites (5.7%–10% molar) in a buckminsterfullerene matrix enable hole transport. Hopping between isolated donor sites can occur by long-range hole tunneling through several buckminsterfullerene molecules, over distances of up to ≈4 nm. Hence, these results question the relevance of “pristine” phases and whether a continuous interpenetrating donor–acceptor network is the ideal morphology for charge transport.
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6.
  • Müller, Christian, et al. (författare)
  • Phase behaviour of liquid-crystalline polymer/fullerene organic photovoltaic blends : thermal stability and miscibility
  • 2011
  • Ingår i: Journal of Materials Chemistry. - : RSC Publishing. - 0959-9428 .- 1364-5501. ; 21, s. 10676-10684
  • Tidskriftsartikel (refereegranskat)abstract
    • The thermal behaviour of an organic photovoltaic (OPV) binary system comprised of a liquidcrystalline fluorene-based polymer and a fullerene derivative is investigated. We employ variabletemperature ellipsometry complemented by photo- and electroluminescence spectroscopy as well as optical microscopy and scanning force nanoscopy to explore phase transitions of blend thin films. The high glass transition temperature correlates with the good thermal stability of solar cells based on these materials. Furthermore, we observe partial miscibility of the donor and acceptor together with the tendency of excess fullerene derivative to segregate into exceedingly large domains. Thus, for charge generation less adequate bulk-heterojunction nanostructures are poised to develop if this mixture is exposed to more elevated temperatures. Gratifyingly, the solubility of the fullerene derivative in the polymer phase is found to decrease if a higher molecular-weight polymer fraction is employed, which offers routes towards improving the photovoltaic performance of non-crystalline OPV blends.
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7.
  • Tang, Zheng, et al. (författare)
  • Interlayer for Modified Cathode in Highly Efficient Inverted ITO-Free Organic Solar Cells
  • 2012
  • Ingår i: Advanced Materials. - : Wiley. - 0935-9648 .- 1521-4095. ; 24:4, s. 554-
  • Tidskriftsartikel (refereegranskat)abstract
    • Inverted polymer solar cells with a bottom metal cathode modified by a conjugated polymer interlayer show considerable improvement of photocurrent and fill factor, which is due to hole blocking at the interlayer, and a modified surface energy which affects the nanostructure in the TQ1/[70]PCBM blend.
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8.
  • Tang, Zheng, et al. (författare)
  • Semi-Transparent Tandem Organic Solar Cells with 90% Internal Quantum Efficiency
  • 2012
  • Ingår i: Advanced Energy Materials. - : Wiley. - 1614-6840 .- 1614-6832. ; 2:12, s. 1467-1476
  • Tidskriftsartikel (refereegranskat)abstract
    • Semi-transparent (ST) organic solar cells with potential application as power generating windows are studied. The main challenge is to find proper transparent electrodes with desired electrical and optical properties. In this work, this is addressed by employing an amphiphilic conjugated polymer PFPA-1 modified ITO coated glass substrate as the ohmic electron-collecting cathode and PEDOT:PSS PH1000 as the hole-collecting anode. For active layers based on different donor polymers, considerably lower reflection and parasitic absorption are found in the ST solar cells as compared to solar cells in the standard geometry with an ITO/PEDOT:PSS anode and a LiF/Al cathode. The ST solar cells have remarkably high internal quantum efficiency at short circuit condition (similar to 90%) and high transmittance (similar to 50%). Hence, efficient ST tandem solar cells with enhanced power conversion efficiency (PCE) compared to a single ST solar cell can be constructed by connecting the stacked two ST sub-cells in parallel. The total loss of photons by reflection, parasitic absorption and transmission in the ST tandem solar cell can be smaller than the loss in a standard solar cell based on the same active materials. We demonstrate this by stacking five separately prepared ST cells on top of each other, to obtain a higher photocurrent than in an optimized standard solar cell.
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9.
  • Tvingstedt, Kristofer, et al. (författare)
  • Electroluminescence from Charge Transfer States in Polymer Solar Cells
  • 2009
  • Ingår i: JOURNAL OF THE AMERICAN CHEMICAL SOCIETY. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 131:33, s. 11819-11824
  • Tidskriftsartikel (refereegranskat)abstract
    • In this article we report the weak but omnipresent electroluminescence (EL) from several types of organic polymer:fullerene bulk heterojunction solar cells biased in the forward direction. The light emitted from blends of Some commonly used polymers and the fullerene molecule is significantly different from that of any of the pure materials comprising the blend. The lower energy of the blend EL is found to correlate with both the voltage onset of emission and the open-circuit voltage of the photovoltaic cell under solar illumination. WE., accordingly interpret the emission to originate from interfacial charge transfer state recombination and emphasize EL as a very valuable tool to characterize the charge transfer state present in donor/acceptor organic photovoltaic (OPV) cells.
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10.
  • Tvingstedt, Kristofer, et al. (författare)
  • On the Dissociation Efficiency of Charge Transfer Excitons and Frenkel Excitons in Organic Solar Cells: A Luminescence Quenching Study
  • 2010
  • Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society. - 1932-7447 .- 1932-7455. ; 114:49, s. 21824-21832
  • Tidskriftsartikel (refereegranskat)abstract
    • The field dependence of photocurrent found in many organic solar cells is a significant and detrimental setback for internal quantum efficiency. In this work we study the important contribution to this field dependence due to the dissociation efficiency of the weakly bound interfacial charge transfer (CT) state, crucial for organic bulk heterojunction solar cells. Three different donor polymers and two different acceptors are examined, and their respective dissociation characteristics are evaluated by photoluminescence (PL) quenching, both for Frenkel excitons and for the intermolecular charge transfer excitons. We observe that while the field-dependent photocurrent for pure polymers does correlate well with quenching efficiency, the CT exciton quenching from the blend generally displays a less pronounced correlation with extracted photocurrent. We further note that while the electroluminescence and photoluminescence of the pure polymer are identical, we observe a red shift for the blend electroluminescence. This indicates that lower energetic states, not visible in PL, are available in the blend. The emissive state of the blends probed by PL is therefore proposed to originate from sites that are involved in photocurrent generation to a lesser extent.
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