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Photogenerated Carrier Mobility Significantly Exceeds Injected Carrier Mobility in Organic Solar Cells
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
Center for Physical Sciences and Technology Savanoriu, Lithuania.
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
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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. Vol. 7, no 9, 1602143
Keyword [en]
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: urn:nbn:se:liu:diva-135770DOI: 10.1002/aenm.201602143ISI: 000401719900010OAI: oai:DiVA.org:liu-135770DiVA: diva2:1083394
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
In thesis
1. Non-Equilibrium Charge Motion in Organic Solar Cells
Open this publication in new window or tab >>Non-Equilibrium Charge Motion in Organic Solar Cells
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:nbn:se:liu:diva-136479 (URN)10.3384/diss.diva-136479 (DOI)9789176855638 (ISBN)
Public defence
2017-05-19, Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
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Supervisors
Available from: 2017-04-18 Created: 2017-04-13 Last updated: 2017-08-09Bibliographically approved

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The full text will be freely available from 2018-01-06 15:29
Available from 2018-01-06 15:29

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