Chapter 6

Polymer Blends from Optoelectronics toSpintronics

Liang Yan, Yue Wu, and Bin Hu*

Department of Materials Science and Engineering, University of Tennessee,Knoxville, Tennessee 37996, USA

*bhu@utk.edu

This chapter reports recent experimental studies on electro-optically active polymer blends in organic spintronics. Theexperimental results indicate that polymer blends offer aconvenient methodology to modify the critical parameter:spin-orbital coupling, in spintronics through inter-molecularinteraction. Furthermore, the energy transfer in polymer blendscan carry magnetic field effects from one component to anotherand consequently amplify the magnetic field effects in polymerblends. As a result, polymer blends are an important class ofmaterials in organic spintronics.

Introduction

Polymer blending presents a fundamental method to generate nanoscalemorphological structures. The nanoscale morphological structures can offereffective control on charge transport and excited processes in optoelectronicswhere electronic and optic processes can be mutually controlled. Recently,experimental studies have found that polymer blends can have tunableinter-molecular spin-orbital coupling which is a critical parameter in spintronicswhere magnetic, optic, and electronic processes can be mutually controlled.As a result, polymer blends have become an important class of materials fromoptoelectronics to spintronics. In optoelectronics organic light emitting diodes(OLEDs) have been widely investigated due to their high potential applicationsin flexible display and large-area solid-state lighting (1, 2). Polymer molecularcomposites (3) or polymer blends (4) are widely used to control the keyprocesses: balancing degree of bipolar electron and hole injection, electron-hole

© 2010 American Chemical Society

Dow

nloa

ded

by U

NIV

OF

DE

LA

WA

RE

MO

RR

IS L

IB o

n Ju

ne 2

4, 2

012

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e (W

eb):

Dec

embe

r 14

, 201

0 | d

oi: 1

0.10

21/b

k-20

10-1

061.

ch00

6

In Contemporary Science of Polymeric Materials; Korugic-Karasz, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

recombination, energy transfer, and light emission efficiency in polymer basedwhite OLEDs. Furthermore, it has recently been found from magnetic fieldeffect on electroluminescence (MFEEL) that the light intensity changes withmagnetic field in the OLEDs (5–7). This experimental finding indicates thatorganic semiconducting materials including polymers and polymer blends canbe used for organic spintronics. There are two possible mechanisms for MFEEL.One is that the magnetic field changes the formation rate of both the singlet andtriplet excited states and further leads to their emission changing (8). The otherpossible mechanism is that the external magnetic field changes singlet/tripletexcited states ratio by affecting intersystem crossing (6, 9, 10). In the intersystemcrossing mechanism, the competition between internal magnetic interaction,such as spin-orbital coupling, and the external magnetic field is very importantto determine singlet/triplet ratio change caused by the external magnetic field inorganic semiconductors. Polymer molecular composites or polymer blends canalso show MFEEL. In these mixed structures, energy transfer and spin-orbitalcoupling are two existing key factors in the determination of magnetic fieldeffects. In our recent work, we studied the MFEEL in polymer blend-baseddevices with energy transfer, in composites of poly(N-vinylcarbazole) (PVK) andtris[2-phenylpyridine] iridium (Ir(ppy)3). We observed by using energy transferfrom a strong spin-orbital-coupling material to a weak spin-orbital-couplingmaterial that the MFEEL in strong spin-orbital coupling materials could behighly amplified. This experimental finding indicates that the use of polymerblends presents a new opportunity to amplify magnetic field effects in organicspintronics.

Experimental

The polymer molecular blend was prepared by dispersing the heavy-metalcomplex Ir(ppy)3 into a poly(N-vinylcarbazole) (PVK) matrix at 0.1wt%. ThePVK and Ir(ppy)3 blend was dissolved in chloroform and spun cast on pre-cleanedindium tin oxide (ITO) coated glass substrates. An aluminum metal electrodewas then thermally evaporated on the PVK+Ir(ppy)3 blend film under a vacuumof 2×10−6 Torr to fabricate light emitting devices with the architecture of ITO/PVK+Ir(ppy)3/Al. The MFEEL was measured at constant current density 20 mA/

cm2 in liquid nitrogen. The MFEEL was defined as , where IEL(B)and IEL(0) are the electroluminescence intensity at constant current condition withand without magnetic field, respectively.

Result and Discussion

It is known that the electroluminescence is generated by radiative emission ofintra-molecular electron-hole pairs, namely excitons. We know that the excitonshave both singlet (S) and triplet (T) states. In general, the singlet/triplet ratioin excitonic states is 1/3 when electron-hole pairs are formed. A magnetic field

86

Dow

nloa

ded

by U

NIV

OF

DE

LA

WA

RE

MO

RR

IS L

IB o

n Ju

ne 2

4, 2

012

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e (W

eb):

Dec

embe

r 14

, 201

0 | d

oi: 1

0.10

21/b

k-20

10-1

061.

ch00

6

In Contemporary Science of Polymeric Materials; Korugic-Karasz, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

can change electroluminescence by changing the singlet and triplet formationrate or by changing singlet-triplet intersystem crossing. For singlet and tripletformation, the experimental result that both of fluorescence and phosphorescenceemission in electroluminescence increase with applied magnetic field suggeststhat both singlet and triplet exciton formation rates increase under the influence ofa magnetic field (8). For singlet-triplet intersystem crossing, an external magneticfield needs to be strong enough, relative to the internal magnetic field generatedby spin orbital coupling, to generate MFEEL. When strong spin-orbital-couplingIr(ppy)3 molecules are dispersed in a weak spin-orbital-coupling PVK matrix,the penetration of delocalized π electrons from the PVK matrix into the magneticfield from an orbital current in the Ir(ppy)3 molecules can inevitably generateinter-molecular spin-orbital coupling. This inter-molecular spin-orbital couplingcan be largely tuned by adjusting the Ir(ppy)3 dispersion concentration. Inparticular, this tunable inter-molecular spin-orbital coupling forms a convenientand effective mechanism to modify the spin-orbital coupling strengths for eachcomponent in such polymer blends.

Figure 1a shows the MFEEL from individual PVK and pure Ir(ppy)3components. The characteristic electroluminescence peaks are around 405 nmand 505 nm for individual PVK and Ir(ppy)3 components. It should be notedthat the pure PVK exhibits a large MFEEL with an amplitude of about +10% butthe pure Ir(ppy)3 does not show an appreciable MFEEL. It is obvious that thePVK yields fluorescence from its singlet excitons while the Ir(ppy)3 generatesphosphorescence from its triplet excitons due to the strong spin-orbital couplingcaused by the heavy metal Ir complex structure. For weak spin-orbital-couplingmaterials such as PVK, a low magnetic field of around 10~100 mT is strongerthan the internal magnetic field from spin-orbital coupling. As a consequence,significant MFEEL can be expected. For the strong-spin-orbital-coupling materialssuch as Ir(ppy)3, a low magnetic field less than 1T is much weaker than theinternal magnetic field generated by spin-orbital coupling (11). As a result,negligible MFEEL can be observed by a first order approximation from the pureIr(ppy)3 electroluminescence.

After we lightly doped Ir(ppy)3 into the PVK matrix, the PVK + Ir(ppy)3blend spectrum (Figure 2a) shows both characteristic peaks at 405 nm fromthe PVK matrix and at 505 nm from the dispersed Ir(ppy)3 molecules inelectroluminescence. More importantly, both electroluminescence peaks showclear MFEEL. As shown in Figure 2b and 2c, the PVKmatrix is around 5%MFEELsmaller than the pure PVK device. But the dispersed Ir(ppy)3 surprisingly shows+3%MFEEL while the pure Ir(ppy)3 device gives negligible MFEEL. The decreaseof MFEEL for the PVK matrix compared to pure PVK is due to the increasein its spin-orbital coupling strength caused by the inter-molecular spin-orbitalcoupling (12). The positive MFEEL for the dispersed Ir(ppy)3 molecules cancome from two possibilities. First, the spin-orbital coupling strength of dispersedIr(ppy)3 molecules was weakened by the PVK matrix through inter-molecularspin-orbital coupling effects. Then a low magnetic field becomes strong enoughto compete with the weakened spin-orbital coupling strength in the dispersedIr(ppy)3. Thus the electroluminescence from the dispersed Ir(ppy)3 moleculescan exhibit significant MFEEL. Second, the energy transfer from the PVK matrix

87

Dow

nloa

ded

by U

NIV

OF

DE

LA

WA

RE

MO

RR

IS L

IB o

n Ju

ne 2

4, 2

012

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e (W

eb):

Dec

embe

r 14

, 201

0 | d

oi: 1

0.10

21/b

k-20

10-1

061.

ch00

6

In Contemporary Science of Polymeric Materials; Korugic-Karasz, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

to dispersed Ir(py)3 molecules can carry the MFEEL from the PVK matrix to thedispersed Ir(ppy)3 molecules, leading to a MFEEL in the dispersed Ir(ppy)3 in thePVK + Ir(ppy)3 blend. The observed positive MFEEL from the dispersed Ir(ppy)3molecules can rule out the first possibility a spin-orbital coupling mechanism,because the spin-orbital coupling mechanism should give negative MFEELin triplet electroluminescence through intersystem crossing (10). Therefore,the energy transfer mechanism is mainly accountable for the positive MFEELobserved from the PVK + Ir(ppy)3 blend.

Figure 1. a) MFEEL for pure PVK and Ir(ppy)3, b) The singlet and triplet energylevels for both PVK and Ir(ppy)3. S and T refer to singlet and triplet states.

88

Dow

nloa

ded

by U

NIV

OF

DE

LA

WA

RE

MO

RR

IS L

IB o

n Ju

ne 2

4, 2

012

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e (W

eb):

Dec

embe

r 14

, 201

0 | d

oi: 1

0.10

21/b

k-20

10-1

061.

ch00

6

In Contemporary Science of Polymeric Materials; Korugic-Karasz, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

Figure 2. a) Normalized electroluminescence spectrum for pure PVK, pureIr(ppy)3 and PVK+0.1% Ir(ppy)3 blend, b) MFEEL at 405 nm for 0.1%

Ir(ppy)3+PVK blend and pure PVK, c) MFEEL at 505nm for 0.1% Ir(ppy)3+PVKblend and pure Ir(ppy)3.

89

Dow

nloa

ded

by U

NIV

OF

DE

LA

WA

RE

MO

RR

IS L

IB o

n Ju

ne 2

4, 2

012

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e (W

eb):

Dec

embe

r 14

, 201

0 | d

oi: 1

0.10

21/b

k-20

10-1

061.

ch00

6

In Contemporary Science of Polymeric Materials; Korugic-Karasz, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

Figure 3. a) Absorption of Ir(ppy)3 and photoluminescence of PVK, b)Photoluminescence and electroluminescence of PVK with 0.1% Ir(ppy)3 , c)

Energy transfer process in electroluminescence of composite of PVK doped with0.1wt% Ir(ppy)3. S and T refer to singlet and triplet states.

Figure 3a shows the absorption spectrum of Ir(ppy)3 and photoluminescencespectrum of PVK. We can see that the PVK emission and Ir(ppy)3 absorption havea large spectral overlap to activate the Förster energy transfer from the PVKmatrixto the dispersed Ir(ppr)3 molecules. Figure 3b shows both photoluminescenceand electroluminescence spectra for the PVK + 0.1% Ir(ppy)3 blend. It shouldparticularly be noted that the photoluminescence and electroluminescence spectraare quite similar. Both of them show two peaks: one from PVK at 405nm, onefrom Ir(ppy)3 at 505nm. The spectral similarity between photoluminescenceand electroluminescence implies that the Förster energy transfer occurs in thePVK+Ir(ppy)3 blend device. Furthermore, the significant electroluminescencequenching in PVK matrix caused by the low 0.1% Ir(ppy)3 doping indicates thatthe Förster energy transfer is very efficient in the PVK + Ir(ppy)3 blend device.

Figure 3c schematically illustrates how energy transfer can amplify theMFEEL for the phosphorescence of heavy-metal complex Ir(ppy)3 throughpolymer blend design. When PVK and Ir(ppy)3 were mixed, a low magneticfield would change the intersystem crossing in PVK matrix with the consequenceof increasing singlet ratio in the PVK matrix. Then due to the energy transfer,the increased singlets in the PVK matrix are transferred to the singlet states inthe dispersed Ir(ppy)3 molecules through the Förster process. Because of thestrong spin-orbital coupling in the dispersed Ir(ppy)3 molecules, the intersystemcrossing process can be very efficient and especially independent on magnetic

90

Dow

nloa

ded

by U

NIV

OF

DE

LA

WA

RE

MO

RR

IS L

IB o

n Ju

ne 2

4, 2

012

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e (W

eb):

Dec

embe

r 14

, 201

0 | d

oi: 1

0.10

21/b

k-20

10-1

061.

ch00

6

In Contemporary Science of Polymeric Materials; Korugic-Karasz, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

field in the Ir(ppy)3 component. As a result, almost all singlets are converted intotriplets in the dispersed Ir(ppy)3 molecules. This means that triplet concentrationin the dispersed Ir(ppy)3 molecules can essentially increase due to (i) efficientenergy transfer from the PVK matrix to the dispersed Ir(ppy)3 molecules and (ii)strong intersystem crossing within the dispersed Ir(ppy)3 molecules. Therefore,the MFEEL from the dispersed Ir(ppy)3 molecules can be considerably amplifiedbased on polymer blend design as compared to pure Ir(ppy)3 molecules.

Conclusion

We have shown that polymer blend design can lead to a substantial tuningof spin-orbital coupling through inter-molecular interaction when a weak spin-orbital-coupling polymer is mixed with a strong spin-orbital-coupling molecules.The tuning of spin-orbital coupling comes from the penetration of delocalizedπ electrons from a weak spin-orbital-coupling polymer matrix into the magneticfield of orbital current from strong spin-orbital-coupling molecules. On the otherhand, the Förster energy transfer can occur from the polymer matrix to dispersedmolecules in polymer molecular blends. Especially, Förster energy transfer cancarry the MFEEL from weak-spin-orbital-coupling polymer matrix to strong spin-orbital-coupling molecules, leading to amplification on the MFEEL in the strongspin-orbital-coupling molecules. Therefore, polymer blends design presents a newmechanism to amplify magnetic field effects in organic spintronics based on (i)modification of spin-orbital coupling and (ii) Förster energy transfer.

References

1. Kanno, H.; Sun, Y.; Forrest, S. R. High-efficiency top-emissive white-light-emitting organic electrophosphorescent devices. Appl. Phys. Lett. 2005, 86,263502–263503.

2. Tokito, S.; Iijima, T.; Tsuzuki, T.; Sato, F. High-efficiency whitephosphorescent organic light-emitting devices with greenish-blue andred-emitting layers. Appl. Phys. Lett. 2003, 83, 2459–2461.

3. Al Attar, H. A.; Monkman, A. P.; Tavasli, M.; Bettington, S.; Bryce, M.R. White polymeric light-emitting diode based on a fluorene polymer/Ircomplex blend system. Appl. Phys. Lett. 2005, 86.

4. Hu, B.; Karasz, F. E. Blue, green, red, and white electroluminescence frommultichromophore polymer blends. J. Appl. Phys. 2003, 93, 1995–2001.

5. Kalinowski, J.; Cocchi, M.; Virgili, D.; Macro, P. D.; Fattori, V. Magneticfield effects on emission and current in Alq3-based electroluminescentdiodes. Chem. Phys. Lett. 2003, 380, 710.

6. Kalinowski, J.; Cocchi, M.; Virgili, D.; Fattori, V.; Di Marco, P. Magneticfield effects on organic electrophosphorescence. Phys. Rev. B 2004, 70 (7),205303.

7. Davis, A. H.; Bussmann, K. Large magnetic field effects in organiclight emitting diodesbased on tris(8-hydroxyquinoline aluminum)

91

Dow

nloa

ded

by U

NIV

OF

DE

LA

WA

RE

MO

RR

IS L

IB o

n Ju

ne 2

4, 2

012

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e (W

eb):

Dec

embe

r 14

, 201

0 | d

oi: 1

0.10

21/b

k-20

10-1

061.

ch00

6

In Contemporary Science of Polymeric Materials; Korugic-Karasz, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

(Alq3)/N,N′-Di(naphthalene-1-yl)-N, N′ diphenyl-benzidine (NPB) bilayers.J. Vac. Sci. Technol., A 2004, 22, 1885.

8. Reufer, M.; et al. Spin-conserving carrier recombination in conjugatedpolymers. Nat. Mater. 2005, 4, 340–346.

9. Hu, B.; Wu, Y. Tuning magnetoresistance between positive and negativevalues in organic semiconductors. Nat. Mater. 2007, 6, 985–991.

10. Hu, B.; Yan, L.; Shao, M. Magnetic-field effects in organic semiconductingmaterials and devices. Adv. Mater. 2009, 21, 1500.

11. Birks, J. B. Organic Molecular Photophysics; Wiley: London, 1975.12. Wu, Y.; Xu, Z.; Hu, B.; Howe, J. Tuning magnetoresistance and magnetic-

field-dependent electroluminescence through mixing a strong-spin-orbital-coupling molecule and a weak-spin-orbital-coupling polymer. Phys. Rev. B2007, 75, 035214–035216.

92

Dow

nloa

ded

by U

NIV

OF

DE

LA

WA

RE

MO

RR

IS L

IB o

n Ju

ne 2

4, 2

012

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e (W

eb):

Dec

embe

r 14

, 201

0 | d

oi: 1

0.10

21/b

k-20

10-1

061.

ch00

6

In Contemporary Science of Polymeric Materials; Korugic-Karasz, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.