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Non-Equilibrium Charge Motion in Organic Solar Cells
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.ORCID iD: 0000-0002-3443-0987
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Organic photovoltaic (OPV) devices based on semiconducting polymers and small molecules allow for a low cost alternative to inorganic solar cells. Recent developments show power conversion efficiencies as high as 10-12%, highlighting the potential of this technology. Nevertheless, further improvements are necessary to achieve commercialization.

To a large extent the performance of these devices is dictated by their ability to extract the photo-generated charge, which is related to the charge carrier mobility. Various time-resolved and steady-state techniques are available to probe the charge carrier mobility in OPVs but often lead to different mobility values for one and the same system. Despite such conflicting observations it is generally assumed that charge transport in OPV devices can be described by well-defined charge carrier mobilities, typically obtained using a single steady-state technique. This thesis shows that the relevance of such well-defined mobilities for the charge separation and extraction processes is very limited.

Although different transient techniques probe different time scales after photogeneration, they are mutually consistent as they probe the same physical mechanism governing charge motion – gradual thermalization of the photo-generated carriers in the disorder broadened density of states (DOS). The photo-generated carriers gradually lose their excess energy during transport to the extracting electrodes, but not immediately. Typically not all excess energy is dissipated as the photo-generated carriers tend to be extracted from the OPV device before reaching quasi-equilibrium.

Carrier motion is governed by thermalization, leading to a time-dependent carrier mobility that is significantly higher than the steady-state mobility. This picture is confirmed by several transient techniques: Time-resolved Terahertz Spectroscopy (TRTS), Time-resolved Microwave Conductance (TRMC) combined with Transient Absorption (TA), electrical extraction of photo-induced charges (photo-CELIV). The connection between transient and steady-state mobility measurements (space-charge limited conductivity, SCLC) is described. Unification of transient opto-electric techniques to probe charge motion in OPVs is presented.

Using transient experiments the distribution of extraction times of photo-generated charges in an operating OPV device has been determined and found to be strongly dispersive, spanning several decades in time. In view of the strong dispersion in extraction times the relevance of even a well-defined time-dependent mean mobility is limited.

In OPVs a continuous ‘percolating’ donor network is often considered necessary for efficient hole extraction, whereas if the network is discontinuous, hole transport is thought to deteriorate significantly, limiting device performance. Here, it is shown that even highly diluted donor sites (5.7-10 %) in a buckminsterfullerene (C60) matrix enable reasonably efficient hole transport. Using transient measurements it is demonstrated that hole transport between isolated donor sites can occur by long-range hole tunneling (over distances of ~4 nm) through several C60 molecules – even a discontinuous donor network enables hole transport

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2017. , 83 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1836
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:liu:diva-136479DOI: 10.3384/diss.diva-136479ISBN: 9789176855638 (print)OAI: oai:DiVA.org:liu-136479DiVA: diva2:1088707
Public defence
2017-05-19, Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2017-04-18 Created: 2017-04-13 Last updated: 2017-08-09Bibliographically approved
List of papers
1. Unified Study of Recombination in Polymer:Fullerene Solar Cells Using Transient Absorption and Charge-Extraction Measurements
Open this publication in new window or tab >>Unified Study of Recombination in Polymer:Fullerene Solar Cells Using Transient Absorption and Charge-Extraction Measurements
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2013 (English)In: Journal of Physical Chemistry Letters, ISSN 1948-7185, Vol. 4, no 12, 2069-2072 p.Article in journal (Refereed) Published
Abstract [en]

Recombination in the well-performing bulk heterojunction solar cell blend between the conjugated polymer TQ-1 and the substituted fullerene PCBM has been investigated with pump-probe transient absorption and charge extraction of photo-generated carriers (photo-CELIV). Both methods are shown to generate identical and overlapping data under appropriate experimental conditions. The dominant type of recombination is bimolecular with a rate constant of 7 x 10(-12) cm(-3) s(-1). This recombination rate is shown to be fully consistent with solar cell performance. Deviations from an ideal bimolecular recombination process, in this material system only observable at high pump fluences, are explained with a time-dependent charge-carrier mobility, and the implications of such a behavior for device development are discussed.

Place, publisher, year, edition, pages
American Chemical Society, 2013
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-95964 (URN)10.1021/jz4009745 (DOI)000320979400014 ()
Note

Funding Agencies|Swedish Research Council||Swedish Energy Agency (STEM)||Knut and Alice Wallenberg Foundation||ERC (VISCHEM)|226136|Wallenberg Scholar grant from the Knut and Alice Wallenberg Foundation||

Available from: 2013-08-19 Created: 2013-08-12 Last updated: 2017-04-20
2. Dispersion-Dominated Photocurrent in Polymer:Fullerene Solar Cells
Open this publication in new window or tab >>Dispersion-Dominated Photocurrent in Polymer:Fullerene Solar Cells
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2014 (English)In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 24, no 28, 4507-4514 p.Article in journal (Refereed) Published
Abstract [en]

Organic bulk heterojunction solar cells are often regarded as near-equilibrium devices, whose kinetics are set by well-defined charge carrier mobilities, and relaxation in the density of states is commonly ignored or included purely phenomenologically. Here, the motion of photocreated charges is studied experimentally with picosecond time resolution by a combination of time-resolved optical probing of electric field and photocurrent measurements, and the data are used to define parameters for kinetic Monte Carlo modelling. The results show that charge carrier motion in a prototypical polymer:fullerene solar cell under operational conditions is orders of magnitude faster than would be expected on the basis of corresponding near-equilibrium mobilities, and is extremely dispersive. There is no unique mobility. The distribution of extraction times of photocreated charges in operating organic solar cells can be experimentally determined from the charge collection transients measured under pulsed excitation. Finally, a remarkable distribution of the photocurrent over energy is found, in which the most relaxed charge carriers in fact counteract the net photocurrent.

Place, publisher, year, edition, pages
Weinheim, Germany: Wiley-VCH Verlagsgesellschaft, 2014
Keyword
solar cell; organic solar cell; dispersion; photocurrent; charge carrier relaxation; Monte Carlo simulations
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-108324 (URN)10.1002/adfm.201400404 (DOI)000339713900015 ()
Note

Funding agencies|Swedish Science Council (VR); Swedish Energy Agency; Knut and Alice Wallenberg foundation; European Social Fund under Global Grant measure

Available from: 2014-06-26 Created: 2014-06-26 Last updated: 2017-04-20Bibliographically approved
3. Photo-generated carriers lose energy during extraction from polymer-fullerene solar cells
Open this publication in new window or tab >>Photo-generated carriers lose energy during extraction from polymer-fullerene solar cells
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2015 (English)In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 6, no 8778Article in journal (Refereed) Published
Abstract [en]

In photovoltaic devices, the photo-generated charge carriers are typically assumed to be in thermal equilibrium with the lattice. In conventional materials, this assumption is experimentally justified as carrier thermalization completes before any significant carrier transport has occurred. Here, we demonstrate by unifying time-resolved optical and electrical experiments and Monte Carlo simulations over an exceptionally wide dynamic range that in the case of organic photovoltaic devices, this assumption is invalid. As the photo-generated carriers are transported to the electrodes, a substantial amount of their energy is lost by continuous thermalization in the disorder broadened density of states. Since thermalization occurs downward in energy, carrier motion is boosted by this process, leading to a time-dependent carrier mobility as confirmed by direct experiments. We identify the time and distance scales relevant for carrier extraction and show that the photo-generated carriers are extracted from the operating device before reaching thermal equilibrium.

Place, publisher, year, edition, pages
Nature Publishing Group, 2015
National Category
Biological Sciences Physical Sciences
Identifiers
urn:nbn:se:liu:diva-123824 (URN)10.1038/ncomms9778 (DOI)000366294700004 ()26537357 (PubMedID)
Note

Funding Agencies|Swedish Science Council and Energimyndigheten; Knut and Alice Wallenberg foundation; Deutsche Forschungsgemeinschaft [SPP1355]

Available from: 2016-01-11 Created: 2016-01-11 Last updated: 2017-04-20
4. Photogenerated Carrier Mobility Significantly Exceeds Injected Carrier Mobility in Organic Solar Cells
Open this publication in new window or tab >>Photogenerated Carrier Mobility Significantly Exceeds Injected Carrier Mobility in Organic Solar Cells
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2017 (English)In: Advanced Energy Materials, ISSN 1614-6840, Vol. 7, no 9, 1602143Article in journal (Refereed) Published
Abstract [en]

Charge transport in organic photovoltaic (OPV) devices is often characterized by space-charge limited currents (SCLC). However, this technique only probes the transport of charges residing at quasi-equilibrium energies in the disorder-broadened density of states (DOS). In contrast, in an operating OPV device the photogenerated carriers are typically created at higher energies in the DOS, followed by slow thermalization. Here, by ultrafast time-resolved experiments and simulations it is shown that in disordered polymer/fullerene and polymer/polymer OPVs, the mobility of photogenerated carriers significantly exceeds that of injected carriers probed by SCLC. Time-resolved charge transport in a polymer/polymer OPV device is measured with exceptionally high (picosecond) time resolution. The essential physics that SCLC fails to capture is that of photo­generated carrier thermalization, which boosts carrier mobility. It is predicted that only for materials with a sufficiently low energetic disorder, thermalization effects on carrier transport can be neglected. For a typical device thickness of 100 nm, the limiting energetic disorder is σ ≈71 (56) meV for maximum-power point (short-circuit) conditions, depending on the error one is willing to accept. As in typical OPV materials the disorder is usually larger, the results question the validity of the SCLC method to describe operating OPVs.

Place, publisher, year, edition, pages
John Wiley & Sons, 2017
Keyword
charge carrier relaxation, charge carrier transport, organic photovoltaics, space-charge limited currents (SCLC), time-dependent mobility
National Category
Condensed Matter Physics Chemical Process Engineering Atom and Molecular Physics and Optics Physical Chemistry
Identifiers
urn:nbn:se:liu:diva-135770 (URN)10.1002/aenm.201602143 (DOI)000401719900010 ()
Note

Funding agencies: Research Council of Lithuania [MIP-085/2015]; Science Council; Knut and Alice Wallenberg foundation through a Wallenberg Scholar grant

Available from: 2017-03-21 Created: 2017-03-21 Last updated: 2017-06-13Bibliographically approved
5. Charge Transport in Pure and Mixed Phases in Organic Solar Cells
Open this publication in new window or tab >>Charge Transport in Pure and Mixed Phases in Organic Solar Cells
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2017 (English)In: Advanced Energy Materials, ISSN 1614-6840Article in journal (Refereed) Epub ahead of print
Abstract [en]

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.

Place, publisher, year, edition, pages
John Wiley & Sons, 2017
Keyword
charge carrier transport, fullerene domains, low donor concentration, organic photovoltaics, tunneling
National Category
Physical Chemistry Condensed Matter Physics Biophysics Medical Biotechnology (with a focus on Cell Biology (including Stem Cell Biology), Molecular Biology, Microbiology, Biochemistry or Biopharmacy)
Identifiers
urn:nbn:se:liu:diva-139690 (URN)10.1002/aenm.201700888 (DOI)
Available from: 2017-08-09 Created: 2017-08-09 Last updated: 2017-08-29Bibliographically approved

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