We report magnetoconductance measurements on polymer:fullerene photovoltaic devices in the regime of high fields and low temperatures, where spin polarization of injected carriers plays an important role. The current either decreases or increases with magnetic field, depending on whether the interfacial charge-transfer state lies above or below the intramolecular triplet state in energy. Rapid transitions from triplet charge-transfer states to lower-lying triplet excitons constitute an important loss mechanism that is responsible for the negative magnetoconductance observed

Chow, Philip C. Y.

Etherington, Marc

Greenham, Neil C.

Wang, Jianpu

Applied Physics Letters

English

Marc K. Etherington

Jianpu Wang

Philip C. Y. Chow

Neil C. Greenham

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Recombination pathways in polymer:fullerene photovoltaics observed through spin polarization measurements

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Northumbria Research LinkCitation:  Etherington,  Marc,  Wang,  Jianpu,  Chow,  Philip  C.  Y.  and  Greenham,  Neil  C.  (2014) Recombination  pathways  in  polymer:fullerene  photovoltaics  observed  through  spin  polarization measurements. Applied Physics Letters, 104 (6). 063304. ISSN 0003-6951 Published by: American Institute of PhysicsURL: http://dx.doi.org/10.1063/1.4865203 <http://dx.doi.org/10.1063/1.4865203>This version was downloaded from Northumbria Research Link: http://nrl.northumbria.ac.uk/41875/Northumbria University has developed Northumbria Research Link (NRL) to enable users to access the University’s research output. Copyright © and moral rights for items on NRL are retained by the individual author(s) and/or other copyright owners.  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Lett. 104, 063304 (2014); https://doi.org/10.1063/1.4865203 104, 063304© 2014 Author(s).Recombination pathways inpolymer:fullerene photovoltaics observedthrough spin polarization measurementsCite as: Appl. Phys. Lett. 104, 063304 (2014); https://doi.org/10.1063/1.4865203Submitted: 30 October 2013 . Accepted: 22 January 2014 . Published Online: 12 February 2014Marc K. Etherington, Jianpu Wang, Philip C. Y. Chow, and Neil C. GreenhamARTICLES YOU MAY BE INTERESTED INTwo-layer organic photovoltaic cellApplied Physics Letters 48, 183 (1986); https://doi.org/10.1063/1.96937Detailed Balance Limit of Efficiency of p-n Junction Solar CellsJournal of Applied Physics 32, 510 (1961); https://doi.org/10.1063/1.1736034Morphology control of perovskite light-emitting diodes by using amino acid self-assembledmonolayersApplied Physics Letters 108, 141102 (2016); https://doi.org/10.1063/1.4945330Recombination pathways in polymer:fullerene photovoltaics observedthrough spin polarization measurementsMarc K. Etherington, Jianpu Wang,a),b) Philip C. Y. Chow, and Neil C. Greenhama)Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE,United Kingdom(Received 30 October 2013; accepted 22 January 2014; published online 12 February 2014)We report magnetoconductance measurements on polymer:fullerene photovoltaic devices in theregime of high fields and low temperatures, where spin polarization of injected carriers plays animportant role. The current either decreases or increases with magnetic field, depending on whetherthe interfacial charge-transfer state lies above or below the intramolecular triplet state in energy.Rapid transitions from triplet charge-transfer states to lower-lying triplet excitons constitute animportant loss mechanism that is responsible for the negative magnetoconductance observed. VC2014 Author(s). All article content, except where otherwise noted, is licensed under a CreativeCommons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4865203]Organic photovoltaics (OPVs) offer the potential forcheap large-area processing but are held back by efficiencieswhich remain lower than conventional inorganic photovol-taics despite rapid recent improvements.1–4 Key challenges toreach the highest level of efficiency include eliminating gemi-nate recombination of charge pairs formed by charge transfer(CT) at the donor-acceptor heterojunction,5–9 reducing theenergy required to drive the charge transfer reaction,10,11 andreducing the rate constant for unnecessary bimolecular recom-bination so that the operating voltage approaches the maxi-mum possible value for the materials involved.12,13 Recently,increasing attention has focused on the role of spin in control-ling recombination in OPVs.14 As shown in Figure 1(a), thecharge-transfer state formed by dissociation of a photogener-ated singlet exciton will have singlet character (1CT) and isliable to recombine to the singlet ground state if it is not rap-idly dissociated into free charges. Intersystem crossing from1CT to the triplet (3CT) charge-transfer state may occur, andthis state is less likely to recombine to the ground state beforedissociation. However, where there is an energeticallylower-lying intramolecular triplet exciton state (T1), this statemay be rapidly formed from the 3CT state. Thesetightly-bound T1 states are typically hard to dissociate, so theirformation is a terminal loss mechanism. Similar effects arisein bimolecular recombination, where 1CT and 3CT states areformed in a 1:3 ratio, as in an organic light-emitting diode(LED). If the 3CT state is long-lived, it may redissociate intocharges, thus reducing the net bimolecular recombination rate.One way to avoid formation of T1 from3CT states is toarrange for the CT states to be lower in energy than T1.However, this requires a large amount of energy to be dissi-pated in the S1 to1CT transition and is therefore incompatiblewith a high open-circuit voltage. Recently, it has been shownthat even when T1 is the lowest-energy state, in some systemsthe transition from 3CT to T1 is sufficiently slow that3CTstates are efficiently dissociated.14Magnetic fields provide a useful way to control spinpopulations in devices, and a wide range of resulting mag-netic field effects (MFEs) have been reported in organicdevices.15–22 We have recently reported that at very highfields and low temperatures, carriers injected into organicLEDs and photovoltaics can become spin polarized, meaningthat bimolecular recombination leads only to the formationof triplet states.23 In P3HT:PCBM devices, a photovoltaicsystem where the CT states lie below the T1 state, spin polar-ization leads to the preferential formation of long-lived 3CTstates that can be redissociated, thus suppressing the net rateof bimolecular recombination and increasing the dark currentby up to 50%. These results are rather unexpected, hencemore work to substantiate the underlying model is desirable.Furthermore, the ability to directly control the spin of CTstates allows a sensitive test of models for spin-dependentrecombination in different photovoltaic systems.In the current work, we extend these measurements to arange of polymer:fullerene devices, where the energy separa-tion and ordering of the 3CT and T1 states can be varied. Insystems where T1 is the lowest state, the MFE becomes neg-ative rather than positive, clearly identifying bimolecularrecombination to T1 as a terminal loss mechanism in thesesystems. Energies of CT states are determined by monitoringthe weak luminescence from 1CT, and the magnetic field de-pendence of this emission provides further confirmation forthe spin polarization model.The chemical structures of the materials used are shown inFigure 1(b). The active layers in the photovoltaic device struc-tures were poly(3-hexylthiophene) (P3HT) blended with [6,6]-phenyl C61 butyric acid methyl ester (PC60BM), and poly[2-methoxy-5-(30,70-dimethyloctyloxy)-1,4-phenylenevinylene](MDMO-PPV), poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b0]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)](PCPDTBT), poly[N-90-heptadecanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)] (PCDTBT)and poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b0]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7) each blended with [6,6]-phenyl C71 butyric acid methyl ester (PC70BM). Deviceswere fabricated with the structure glass/indium tin oxidea)Authors to whom correspondence should be addressed. Electronicaddresses: iamjpwang@njut.edu.cn and ncg11@cam.ac.uk.b)Present address: Institute of Advanced Materials, Nanjing Tech University,Nanjing, China 211816.0003-6951/2014/104(6)/063304/4 VC Author(s) 2014104, 063304-1APPLIED PHYSICS LETTERS 104, 063304 (2014)(ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS)/active layer/aluminum. Hole-only deviceswere also made using the same structure but with palladiumin place of aluminum. The active layer was spin-coated fromsolution to produce films of approximately 100 nm thickness.The devices were placed in the sample space of a CryogenicLtd 10 T split-pair optical cryo-magnetic system with inte-gral variable temperature insert to perform dark currentmeasurements in a temperature-controlled helium atmos-phere. Electroluminescence, which is attributed to 1CT emis-sion, was measured using a fiber-coupled spectrograph withan InGaAs CCD camera, either at room temperature in anintegrating sphere, or in-situ in the magnet.Figure 2 shows the effect of magnetic field on the darkcurrent in each device at 2 K and at fixed biases chosen togive currents of 0.08mA/cm2 at zero magnetic field.P3HT:PC60BM shows a large positive effect as reportedpreviously,23 whereas MDMO-PPV:PC70BM shows asmaller positive effect and the other blends show negativeeffects. The effects seen are much stronger than any effectsseen in hole-only devices (see supplementary material24) andare therefore associated with changes in electron-holerecombination. We focus here on the high-field effects,which only appear at low temperatures (see supplementarymaterial24), consistent with a spin polarization effect. Asexplained above, in materials showing a positive effect, pref-erential recombination to the long-lived 3CT state allowsmore time for redissociation of the CT state, reducing the netrate of bimolecular recombination. Reduced recombinationleads to increased current, since the region of overlap ofelectron and hole densities becomes more extended, thusreducing space-charge limitations on the current. For deviceswhere both electron and hole currents are space-charge-lim-ited, these effects can be modeled quantitatively according tothe theory of Parmenter and Ruppel.25 For the devices wherea negative MFE is seen, we propose that this is due to the T1state being below 3CT, leading to rapid losses of 3CT statesto T1 before redissociation can occur. Increasing the forma-tion of 3CT states through spin polarization thus leads toincreased net recombination and reduced current.FIG. 1. (a) Excited state energies and ki-netic processes in a donor-acceptor pho-tovoltaic blend. (b) Chemical structuresof the materials used.FIG. 2. Magnetoconductance in the dark current of polymer:fullerene photo-voltaic devices at 2K, measured at constant biases chosen to give a currentdensity of 0.08mA/cm2 at zero field.TABLE I Energies of the singlet exciton (S1), charge-transfer state (CT),and triplet exciton (T1) in the polymers used.Material S1 (eV)3CT (eV) T1 (eV)P3HT 2.05a 1.3 1.40bMDMO-PPV 2.1c 1.35 1.40bPCPDTBT 1.76d 1.2 1.0ePCDTBT 1.86f 1.35 1.16PTB7 1.6g 1.27 0.9aReference 31.bReference 10.cReference 32.dReference 33.eReference 34.fReference 35.gReference 2.063304-2 Etherington et al. Appl. Phys. Lett. 104, 063304 (2014)To confirm this proposal, we compare our data with in-dependent measurements of the CT and triplet state energies,as shown in Table I. The 3CT and 1CT states are very closein energy, even at high fields,26 so we can determine the 3CTenergy from the peaks of the electroluminescence spectra(Figure 3) which are attributed to emission from 1CT statesformed by bimolecular recombination. The energies of T1states are more difficult to determine. In some of our materi-als, measurements of these energies by triplet sensitizationexperiments have been reported,10 and for others we use theempirical rule due to K€ohler that the triplet state lies 0.7 eVbelow the absorption onset,27 an approach that has provedsuccessful in detailed spectroscopic studies of CT and T1states in photovoltaic blends.28,29 From Table I, there is aclear connection between the sign of the observed MFE andthe relative ordering of the CT and T1 states.We note that we also observe distinct changes in darkcurrent at low fields (<100mT). These effects have beenwidely studied16–19 and are attributed to a reduction in inter-system crossing once the external field exceeds the hyperfinefield.20–22 We note that in the low-field regime intersystemcrossing takes place in charge pairs sufficiently separated tohave negligible exchange energy;30 these charge pairs coex-ist with more tightly bound CT states that have sufficientwavefunction overlap to recombine to the ground state or toT1. We find that in each case, the sign of the low-field effectmatches that seen at high fields. Negative low-magnetic-fieldeffects are expected when the zero-field intersystem crossingrate is larger than the other rates, the dissociation rate for CTstates is comparable with the rate of formation of T1 from3CT, and the recombination rate from 1CT is lower than theother rates.24 The effect becomes positive as the rate of T1formation is reduced, consistent with the observed change asT1 moves above3CT in energy.As further confirmation of the role of spin-dependentbimolecular recombination when spins are polarized, wehave investigated the magnetic field dependence of the elec-troluminescence in MDMO-PPV:PC70BM devices (Figure4). At low temperatures, there is a substantial decrease inelectroluminescence with field (despite the slight increase incurrent). This is consistent with preferential formation ofnon-emissive 3CT states as predicted. Whilst some of thesestates might be redissociated and have another chance torecombine, the rest will decay non-radiatively to the groundstate (consistent with the small positive effect seen in thecurrent for this system). Thus, the fraction of recombinationthat is radiative is reduced by spin polarization, as seen pre-viously in conventional LED systems.23Our measurements clearly demonstrate a change in signof the magnetic field effect on the dark current in photovoltaicblend devices depending on the energetic ordering of thecharge-transfer and triplet exciton states. Combined with mea-surement of emission from charge-transfer states, these resultsestablish a detailed picture of spin-dependent recombinationpathways in photovoltaic systems, identifying recombinationpathways through triplet states that can be an important lossmechanism in devices. These measurements establish a usefulmethod to determine the energetic position of triplet stateswhose energies are difficult to measure directly.This work was supported by the Engineering and PhysicalSciences Research Council [Grant Nos. EP/G060738/1 andEP/G031088/1]. We are grateful to Sam Bayliss for helpfuldiscussions.1R. F. Service, Science 332, 293 (2011).2Y. Liang, Z. Xu, J. Xia, S.-T. Tsai, Y. Wu, G. Li, C. Ray, and L. Yu, Adv.Mater. 22, E135 (2010).3H. Zhou, Y. Zhang, J. Seifter, S. D. Collins, C. Luo, G. C. Bazan, T.-Q.Nguyen, and A. J. Heeger, Adv. Mater. 25, 1646 (2013).4Z. He, C. Zhong, S. Su, M. Xu, H. Wu, and Y. Cao, Nat. Photonics 6, 591(2012).5C. R. McNeill, S. Westenhoff, C. Groves, R. H. Friend, and N. C.Greenham, J. Phys. Chem. C 111, 19153 (2007).6S. Westenhoff, I. A. Howard, J. M. Hodgkiss, K. R. Kirov, H. A.Bronstein, C. K. Williams, N. C. Greenham, and R. H. Friend, J. Am.Chem. Soc. 130, 13653 (2008).7T. Offermans, S. C. J. Meskers, and R. A. J. Janssen, J. Chem. Phys. 119,10924 (2003).8V. Mihailetchi, L. Koster, J. Hummelen, and P. Blom, Phys. Rev. Lett. 93,216601 (2004).9S. De, T. Pascher, M. Maiti, K. G. Jespersen, T. Kesti, F. Zhang, O. Ingan€as,A. Yartsev, and V. Sundstr€om, J. Am. Chem. Soc. 129, 8466 (2007).10D. Veldman, S. C. J. Meskers, and R. A. J. Janssen, Adv. Funct. Mater. 19,1939 (2009).11K. Vandewal, K. Tvingstedt, A. Gadisa, O. Ingan€as, and J. V Manca,Nature Mater. 8, 904 (2009).12A. Pivrikas, G. Juska, A. Mozer, M. Scharber, K. Arlauskas, N. Sariciftci,H. Stubb, and R. €Osterbacka, Phys. Rev. Lett. 94, 176806 (2005).FIG. 3. Electroluminescence spectra of polymer:fullerene blend devices atroom temperature (symbols) and low temperature (dashed line) under posi-tive bias.FIG. 4. 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© 2014 Author(s). We report magnetoconductance measurements on polymer:fullerene photovoltaic devices in the regime of high fields and low temperatures, where spin polarization of injected carriers plays an important role. The current either decreases or increases with magnetic field, depending on whether the interfacial charge-transfer state lies above or below the intramolecular triplet state in energy. Rapid transitions from triplet charge-transfer states to lower-lying triplet excitons constitute an important loss mechanism that is responsible for the negative magnetoconductance observed.link_to_subscribed_fulltex

Etherington, Marc K.

Chow, Philip C.Y.

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Recombination pathways in polymer:fullerene photovoltaics observed through spin polarization measurements

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