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What Controls the Rate of Ultrafast Charge Transfer and Charge Separation Efficiency in Organic Photovoltaic Blends Andreas C. Jakowetz, Marcus L. Böhm, Jiangbin Zhang, Aditya Sadhanala, Sven Huettner, Artem A. Bakulin, Akshay Rao, and Richard H. Friend J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b05131 • Publication Date (Web): 19 Aug 2016 Downloaded from http://pubs.acs.org on August 19, 2016
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What Controls the Rate of Ultrafast Charge Transfer and Charge Separation Efficiency in Organic Photovoltaic Blends 12
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Andreas C. Jakowetz1, Marcus L. Böhm1, Jiangbin Zhang1, Aditya Sadhanala1, Sven Huettner2, 15 16
Artem A. Bakulin1,3, Akshay Rao*1 and Richard H. Friend*1 17 18 19 1
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Cavendish Laboratory, Department of Physics, University of Cambridge, J J Thomson Avenue,
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Fakultät für Biologie, Chemie und Geowissenschaften, University Bayreuth, Universitätsstrasse
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30, 95440 Bayreuth, Germany 29 31
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Faculty of Natural Sciences, Department of Chemistry, JRF Suite, Royal College of Science,
South Kensington Campus, London, SW7 2AZ, United Kingdom 34 35 36 37 38 40
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KEYWORDS: Charge Generation, Driving Energy, Ultrafast Spectroscopy, Transient 42
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Absorption, Pump-Probe spectroscopy, SAXS, WAXS, PDS, Polymer, Fullerene, Organic 43 4
Photovoltaics 46
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Abstract: 5
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In solar energy harvesting devices based on molecular semiconductors, such as Organic 6 7
Photovoltaics (OPVs) and artificial photosynthetic systems, Frenkel excitons must be dissociated 10
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via charge transfer at heterojunctions to yield free charges. What controls the rate and efficiency 12
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of charge transfer and charge separation is an important question, as it determines the overall 13 14
power conversion efficiency (PCE) of these systems. In bulk heterojunctions between polymer 17
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donor and fullerene acceptors, which provide a model system to understand the fundamental 19
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dynamics of electron transfer in molecular systems, it has been established that the first step of 20 2
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photoinduced electron transfer can be fast, of order 100 fs. But here we report the first study 24
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which correlates differences in the electron transfer rate with electronic structure and 25 26
morphology, achieved with sub-20 fs time resolution pump-probe spectroscopy. We vary both 27 29
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the fullerene substitution and donor:fullerene ratio which allow us to control both aggregate size 31
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and the energetic driving force for charge transfer. We observe a range of electron transfer times 32 3
from polymer to fullerene, from 240 fs to as short as 37 fs. Using ultrafast electro-optical pump36
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push-photocurrent spectroscopy, we find the yield of free versus bound charges to be weakly 38
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dependent on the energetic driving force, but to be very strongly dependent on fullerene 39 41
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aggregate size and packing. Our results point towards the importance of state accessibility and 43
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charge delocalisation and suggest that energetic offsets between donor and acceptor levels are 45
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not an important criterion for efficient charge generation. This provides design rules for next46 48
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generation materials to minimise losses related to driving energy and boost PCE. 49 50 51 52 53 54 5 56 57 58 60
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Introduction: 5
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Like biological light-harvesting complexes (LHCs), in organic photovoltaic cells (OPVs) photon 6 7
absorption leads to the formation of Frenkel exciton states. In order to dissociate these excitons, 10
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OPVs use a heterojunction between p- and n-type organic semiconductors (OSCs), where 12
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energetic offsets drive charge transfer (CT).1–9 This energetic offset is often referred to as the 13 15
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driving energy (E). For OPV systems it is defined as the difference between the ionisation 17
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potential of the Donor (IPD), the electron affinity of the acceptor (EAA), and the energy of the 18 19
generated exciton (Eexciton). Figure 1 shows a scheme for the energy levels while the exciton 2
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energy is included in this one electron diagram for visualisation purposes only. ΔE can be 24
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described by the difference between the LUMO levels of donor and acceptor. 25 26 27 29
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Historically, electron transfer in OSCs and devices based on them have been described within a 31
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modified Marcus framework, which considers the tunnelling of point like charges.10 This 32 34
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description has also been extended to hybrid systems such as interfaces between molecular 36
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systems and metal oxides, which underpin dye-sensitized solar cells (DSSCs), and artificial 37 38
photosynthetic systems. Moving beyond CT, another crucial question is what process leads to 39 41
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long-range charge separation. Electron transfer dissociates the exciton, giving an electron on the 43
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acceptor and hole on the donor. The hole and electron are still 0.5-1 nm apart, at which 4 45
separation they should have significant Coulomb binding energy and form a charge-transfer state 48
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(CTS).10 A certain fraction of these CTS will dissociate into free charges and a certain fraction 50
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will remain trapped and recombine to the ground state. The dissociation of these CTSs has often 51 52
been described within a modified Onsager-Braun framework, within which thermal activation 5
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leads to hopping of charges within a disordered broadened density of states (DOS). 56 57 58 60
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However, it has also been recognised that there are many shortcomings in applying these 5
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modified versions of Marcus theory to solid-state systems, for instance the availability of many 6 7
states to couple to, rather than a single transition as in the Marcus framework. Indeed, many 10
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fundamental aspects of the CT process in the systems mentioned above, such as their ultrafast 12
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time scale (
