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Dispersion-Dominated Photocurrent in Polymer:Fullerene Solar Cells
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology. (Center of Organic Electronics (COE))
Center for Physical Sciences and Technology, Lithuania.
Center for Physical Sciences and Technology, Lithuania.
Center for Physical Sciences and Technology, Lithuania.
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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. Vol. 24, no 28, 4507-4514 p.
Keyword [en]
solar cell; organic solar cell; dispersion; photocurrent; charge carrier relaxation; Monte Carlo simulations
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:liu:diva-108324DOI: 10.1002/adfm.201400404ISI: 000339713900015OAI: oai:DiVA.org:liu-108324DiVA: diva2:729850
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: 2017-12-05Bibliographically 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)
Opponent
Supervisors
Available from: 2017-04-18 Created: 2017-04-13 Last updated: 2017-08-09Bibliographically approved

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Melianas, ArmantasInganäs, OlleKemerink, Martijn

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