Changing inter-molecular spin-orbital coupling for generating magnetic field effects in phosphorescent organic semiconductors

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Changing inter-molecular spin-orbital coupling for generating magnetic field effects in phosphorescent organic semiconductors
  See discussions, stats, and author profiles for this publication at: Changing inter-molecular spin-orbital couplingfor generating magnetic field effects inphosphorescent organic semiconductors  Article   in  Applied Physics Letters · January 2012 DOI: 10.1063/1.3673561 CITATIONS 8 READS 24 6 authors , including: Some of the authors of this publication are also working on these related projects: Growth of semiconductor films using reactive sputtering   View projectFabrication of memresistive devices based on CoxS and CuxS for logic and memory storage applicationsView projectLiang YanUniversity of North Carolina at Chapel Hill 34   PUBLICATIONS   517   CITATIONS   SEE PROFILE Carlos F. O. Graeff São Paulo State University 196   PUBLICATIONS   1,693   CITATIONS   SEE PROFILE Ivo HummelgenUniversidade Federal do Paraná 164   PUBLICATIONS   1,840   CITATIONS   SEE PROFILE ma DonggeSouth China University of Technology 504   PUBLICATIONS   13,241   CITATIONS   SEE PROFILE All content following this page was uploaded by Carlos F. O. Graeff  on 23 December 2016. The user has requested enhancement of the downloaded file.  Changing inter-molecular spin-orbital coupling for generatingmagnetic field effects in phosphorescent organic semiconductors Liang Yan, Ming Shao, Carlos F. O. Graeff, Ivo Hummelgen, Dongge Ma et al.   Citation: Appl. Phys. Lett. 100 , 013301 (2012); doi: 10.1063/1.3673561   View online:   View Table of Contents:   Published by the American Institute of Physics.   Related Articles Changing inter-molecular spin-orbital coupling for generating magnetic field effects in phosphorescent organicsemiconductors   APL: Org. Electron. Photonics 5, 1 (2012)   Gate-controlled electron g-factor in lateral quantum dot molecules   J. Appl. Phys. 110, 124309 (2011)   Kinetic and relativistic effects on the surface alloy formation of submonolayer Au adsorbed on Si(111)-×-Pbsurface   Appl. Phys. Lett. 99, 211912 (2011)   Spin-polarized transport in zigzag graphene nanoribbons with Rashba spin–orbit interaction   J. Appl. Phys. 110, 103702 (2011)   Excitonic couplings and Stark effect in individual quantum dot molecules   J. Appl. Phys. 110, 083511 (2011)   Additional information on Appl. Phys. Lett. Journal Homepage:    Journal Information:   Top downloads:   Information for Authors:   Downloaded 03 Jan 2012 to Redistribution subject to AIP license or copyright; see  Changing inter-molecular spin-orbital coupling for generating magnetic fieldeffects in phosphorescent organic semiconductors Liang Yan, 1 Ming Shao, 1 Carlos F. O. Graeff, 2 Ivo Hummelgen, 3 Dongge Ma, 4 and Bin Hu 1,5,a) 1  Department of Materials Science and Engineering, University of Tennessee, Knoxville,Tennessee 37996, USA 2  Departamento de Fı´sica-FC, Universidade Estadual Paulista—UNESP, Bauru 17033-360, Brazil 3  Departamento de Fı´sica, Universidade Federal do Parana´, 81531-980 Curitiba, Parana´, Brazil 4 Changchun Institute of Applied Chemistry, Changchun 130022, China 5 Wu Han National Laboratory for Optoelectronics, Huazhong University of Science and Technology,Wu Han, China (Received 15 July 2011; accepted 30 November 2011; published online 3 January 2012)Phosphorescent organic semiconductors normally show negligible magnetic field effects in electronicand optic responses. These phenomena have been generally attributed to strong spin-orbital couplingwhich can dominate internal spin-dephasing process as compared with applied magnetic field. Thispaper reports both positive and negative magnetocurrents from phosphorescent organic semiconductorsthrough dissociation and charge-reaction channels when the intermolecular spin-orbital coupling ischanged based on materials mixing. Our experimental results indicate that inter-molecular spin-orbitalcoupling is essentially responsible for the generation of magnetic field effects in phosphorescentorganic semiconductors. V C  2012 American Institute of Physics . [doi:10.1063/1.3673561]Organic semiconductors can be divided into fluorescentand phosphorescent types based on singlet and triplet emis-sion. It has been experimentally found that fluorescent organicsemiconductors can show magnetic responses in electricalcurrent 1 – 3 to externally applied magnetic field ( < 100mT),leading to magnetocurrent (MC) with nonmagnetic electrodes.However, phosphorescent organic semiconductors alwaysexhibit negligible MC due to strong spin-orbital coupling(SOC). 4 – 6 Based on electrical drifting theory, the MC can begenerated by magnetically changing charge mobility or density, forming mobility-based MC (Refs. 7 – 12) or  density-based MC. 1,13 – 15 In mobility-based MC, organicsemiconductors need to have spin-spin interaction betweencharge carriers during transport. Applying magnetic fieldcan disturb this spin-spin interaction through magnetic scat-tering and, consequently, modifies charge mobilities, gener-ating mobility-based MC. However, experimental studieshave found that or ganic semiconductors exhibit negligible mobility-based MC. 16 – 19 These experimental findings implythat bulk organic semiconductors lack appreciable inter-charge spin interaction during transport at normal operationconditions. As a result, magnetically changing chargedensity becomes a practical method to generate MC, leading todensity-based MC in fluorescent organic semiconductors. 1,13 – 15 More specifically, there are two channels, namely, dissocia-tion and charge-reaction, to generate density-based MC. Indissociation channel, an applied magnetic field can increasethe singlet r atio in polaron-pair states through intersystemcrossing. 20,21 This can cause a positive MC throughdissociation 1,13 – 15 based on the experimental argument thatsinglets have larger dissociation rates due to stronger ionicproperties in wavefunctions as compared to triplets. 20,22 Incharge-reaction channel, the excitons formed from injectedcharge carriers can react with excessive charges, generatingexciton-char ge reaction, when they are within close proximity. 23 – 25 The exciton-charge reaction can break exci-tons into free charges. 26 – 28 Although both singlet and tripletexcitons can be involved in charge reaction, triplet excitonscan dominate charge reaction due to the long lifetimes. 13 An applied magnetic field can perturb the spin interactionbetween a triplet exciton and a charge in triplet-charge reac-tion and, consequently, decreases the triplet-charge reac-tion-rate constant, 29 Therefore, charge reaction channel canlead to a negative MC. This paper reports the experimentalstudies on the effects of inter-molecular SOC, which comesfrom inter-molecular magnetic interaction between an elec-tron spin of molecule A and a magnetic field from orbitalcurrent of molecule B when molecules A and B are placedwithin close proximity, on MC based on phosphorescentmolecules.The iridium heavy-metal complex molecule: tris(2-phe-nylpyridine) iridium (Ir(ppy) 3 ), and poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT) were purchasedfrom America Dye Source, Inc., and H. C. Starck, respec-tively, for experimental studies. The poly (methyl methacry-late) (PMMA) and polystyrene (PS), purchased from Sigma-Aldrich Co., were used as inert polymer matrices with differ-ent dielectric constants to host heavy-metal complex Ir(ppy) 3 molecules for the modification of inter-molecular SOC. Theorganic light-emitting diodes were fabricated with indium tinoxide (ITO) with 30 nm PEDOT on top and aluminum (Al)electrodes. The MC was measured by recording the changein the device current as a function of magnetic field at con-stant voltage. The MC amplitude was given by the relativechange in electrical current caused by applied magnetic fieldbased on  I   B   I  0  I  0 , where  I   B  and  I  0  are the electrical injection cur-rents with and without an applied magnetic field, respec-tively. The electron paramagnetic resonance (EPR) wasmeasured by JES-FA200 Electron Spin Resonance a) Author to whom correspondence should be addressed. Electronic 0003-6951/2012/100(1)/013301/3/$30.00  V C  2012 American Institute of Physics 100 , 013301-1 APPLIED PHYSICS LETTERS  100 , 013301 (2012) Downloaded 03 Jan 2012 to Redistribution subject to AIP license or copyright; see  Spectrometer from JEOL, Inc., working at X-band (9GHz)to characterize inter-molecular SOC.It can be seen in Fig. 1 that the heavy-metal complexIr(ppy) 3  exhibits a negligible MC. This negligible MC canbe commonly attributed to the strong SOC with the energyof    100 l eV (Ref. 30), which can largely dominate internalspin-dephasing processes as compared to applied magneticfield. However, positive and negative MC can be observedwhen the Ir(ppy) 3  molecules are dispersed into inert PS andPMMA matrices, respectively. The observed MC clearlysuggests that dispersing Ir(ppy) 3  molecules into an inertpolymer matrix can lead to a reduction on the SOC of heavy-metal complex molecules. It should be noted that theIr(ppy) 3  molecules have a strong orbital magnetic field 31 generated by orbital current due to large atomic number of Ir. This strong orbital magnetic field can interact with  p  elec-tron spins not only within a single molecule but also betweenadjacent molecules, generating both intra-molecular andinter-molecular SOC, as schematically shown in Fig. 2. As aresult, the overall SOC for heavy-metal complex moleculesconsists of intra- and inter-molecular components (Eq. (1)), ^  H  so  ¼  ^  H  intra þ  ^  H  inter   ¼ð n intra ^  L intra þ n inter   ^  L inter  Þ  ^ S ;  (1)where  ^  L  and  ^ S  are the orbital field and electron spin, respec-tively.  n  is the SOC constant. We know that the intra-molecular  ^  H  intra  is solely dependent of intra-molecular magnetic interac-tion between orbital field and  p  electrons in a given molecule.Therefore, once molecular structure is determined, the  ^  H  intra becomes a fixed quantity. However, the inter-molecular   ^  H  inter  depends on not only orbital field but also inter-molecular dis-tance. As a consequence, changing inter-molecular distancecan largely tune inter-molecular SOC, leading to a modificationon overall SOC for phosphorescent organic semiconductors.On contrast, the inter-molecular SOC becomes negligible influorescent molecules where orbital field is weak. 32 The decrease in current with magnetic field in Fig. 1 indi-cates that the triplet-charge reaction is a dominant channel inthe generation of density-based MC in the Ir(ppy) 3 :PMMAsystem. On contrast, the increase in current with magneticfield implies that the dissociation is a main channel in the gen-eration of density-based MC in the Ir(ppy) 3 :PS system.Clearly, the PMMA and PS matrices lead to negative and pos-itive MCs, respectively, for the phosphorescent Ir(ppy) 3  mole-cules. It is known that the PMMA and PS have differentdielectric constants 33 ( e PMMA ¼ 3.6,  e PS ¼ 2.5). Dielectric ma-trix can function as a local electric polarization to affect boththe dissociation in polaron-pair states and the charge reactionin excitonic states in the generation of density-based MC.Therefore, both dissociation and charge reaction channels canbe affected by matrix dielectric constant. On one hand,increasing matrix dielectric constant can increase the dissocia-tion rates for both singlet and triplet polaron pairs throughOnsager process 34,35 by changing the Onsager radius(  R c  ¼  e 2 4 pe 0 e  KT  ). This minimizes the difference between singletand triplet dissociation yields in polaron pairs. Therefore,increasing the matrix dielectric constant can then decrease theeffects of intersystem crossing on the dissociation in polaron-pair states. As a consequence, the dissociation channel of gen-erating positive MC becomes less important upon increasingmatrix dielectric constant. On the other hand, the excitonicstates are essentially metal-to-ligand charge-transfer states inthe Ir(ppy) 3  molecules. 36,37 The charge-transfer states aremore sensitive to local electrical polarization due to their stronger ionic wavefunctions as compared to Frenkel excitons.Therefore, increasing the matrix dielectric constant canenhance the ionic properties of wavefunctions of charge-transfer states through electrical polarization. This canincrease the Coulomb interaction between an excitonic stateand a charge in the generation of triplet-charge reaction. As aresult, the triplet-charge reaction channel of generating nega-tive MC becomes more important upon increasing the matrixdielectric constant. It can be clearly seen in Fig. 1 that thePMMA and PS matrices with higher and lower dielectric con-stants correspond to negative and positive MCs, respectively,by enhancing triplet-charge reaction and dissociation in thePMMA:Ir(ppy) 3  and PS:Ir(ppy) 3  composites when the inter-molecular SOC is weakened through molecular dispersion.Now we further examine the inter-molecular SOC whenthe phosphorescent Ir(ppy) 3  molecules are dispersed in an 0100200300-1.5-1.0- Ir(ppy) 3  in PS    C  u  r  r  e  n   t  c   h  a  n  g  e   (   %   ) Magnetic field (mT) Ir(ppy) 3  in PMMA Pure Ir(ppy) 3 FIG. 1. (Color online) Magnetocurrents are shown for Ir(ppy) 3 :PMMA andIr(ppy) 3 :PS composites with the weight ratio of 1:2 as compared to pureIr(ppy) 3  molecules. The experimental error for magnetocurrent is about 0.01%.   S    B  O  r   b Intramolecular SOCIntermolecular SOC   S   B  O  r  b   S  B  O r b  H  i nt e r  H  i nt e r    H   i  n  t  r a   M  1  M  2  (a)(b) FIG. 2. (Color online) (a) Intra-molecular SOC generated by interactionbetween a  p  electron spin  l s  and an orbital magnetic field B Orb  locatedwithin a single molecule. (b) Inter-molecular SOC generated by interactionbetween a  p  electron spin  l s  and an orbital magnetic field B Orb  located onadjacent molecules. M 1  and M 2  represent two adjacent Ir(ppy) 3  molecules. 013301-2 Yan  et al.  Appl. Phys. Lett.  100 , 013301 (2012) Downloaded 03 Jan 2012 to Redistribution subject to AIP license or copyright; see  inert polymer matrix. We know that dispersing the Ir(ppy) 3 molecules into an inert polymer matrix can, in general, formthree phases: separated Ir(ppy) 3  molecules, aggregatedIr(ppy) 3  molecules, and continuous polymer morphologies inpolymer:Ir(ppy) 3  composites. For separated Ir(ppy) 3  mole-cules, changing molecular concentration can directly modifythe inter-molecular SOC by varying the inter-molecular dis-tances. For the aggregated Ir(ppy) 3  molecules, changing mo-lecular concentration can change their domain sizes and,consequently, affects the sum of inter-molecular SOC compo-nents within a given domain. Based on theoretical estima-tion, 38 we can suggest that the summation of individual inter-molecular SOC components can gradually increase the entireinter-molecular SOC for a single domain before reaching satu-ration with the domain size of    5nm. Overall, it can beargued that changing molecular concentration can conven-iently tune the inter-molecular SOC through separated andaggregated molecules in the Ir(ppy) 3 :polymer composite.Here, we use EPR to characterize the inter-molecular SOCupon Ir(ppy) 3  dispersion in an inert polymer matrix. Fig. 3(a)shows that the EPR peak clearly shifts to a lower magneticfield after the Ir(ppy) 3  molecules are dispersed in an inertpolymer matrix. This EPR peak shift indicates that the g factor decreases with increasing Ir(ppy) 3  concentration after theIr(ppy) 3  molecules are dispersed into the PS matrix. Thedecrease of g factor of the EPR spectrum suggests that inter-molecular SOC is reduced with decreasing the Ir(ppy) 3  con-centration in the Ir(ppy) 3 :PS composite. Therefore, the EPRresult confirms that dispersing Ir(ppy) 3  molecules in an inertpolymer matrix can lead to a reduction in inter-molecular SOC for phosphorescent organic molecules. Furthermore,decreasing the inter-molecular SOC can increase the MC am-plitude in the PS:Ir(ppy) 3  composite (Fig. 3(b)). As a result,changing the inter-molecular SOC can essentially tune adensity-based MC in phosphorescent organic semiconductors.In summary, the strong SOC from heavy-metal complexstructures leads to negligible magnetic field effects in phos-phorescent organic semiconductors. We find that dispersingphosphorescent Ir(ppy) 3  molecules into an inert polymer ma-trix can generate charge density-based MC. This experimen-tal observation suggests that inter-molecular SOC is a keyparameter to generate magnetic field effects in phosphores-cent organic semiconductors. The EPR results confirm thatdispersing Ir(ppy) 3  molecules into an inert polymer matrixcan reduce the inter-molecular SOC. Clearly, our experimen-tal studies indicate that changing inter-molecular SOC formsan effective mechanism to tune magnetic field effects inheavy-metal complex molecules. Furthermore, changing ma-trix dielectric constant can switch the density-based MCbetween dissociation and charge reaction channels, tuningthe MC between positive and negative values in phosphores-cent organic semiconductors.The research has been supported by U.S. NSF (ECCS-046645; OISE-0929566) and China NSF (50928302). Theauthors (C. F. O. Graeff and I. 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The currentdensity for MC measurements was adjusted to be 100 mA/cm 2 . 013301-3 Yan  et al.  Appl. Phys. Lett.  100 , 013301 (2012) Downloaded 03 Jan 2012 to Redistribution subject to AIP license or copyright; see View publication statsView publication stats
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