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Spacial distribution of electroluminescence from oriented phenylenevinylene oligomer Langmuir-Blodgett film
ELSEVIER
Synthetic Metals 91 ( 1997) 83-35
Spatial distribution of electroluminescence from oriented phenylenevinylene oligomer Langmuir-Blodgett film Masanao
Era a.*, Jun-ichi Koganemaru a, Tetsuo Tsutsui a, Atsushi Watakabe b, Toyoki Kunitake b
Abstract An organic electroluminescent (EL) device with a well-defined molecular orientation was prepared by using aphenylenevinyleneoligomer (MOPV) Langmuir-Blodgett ( LB) film. The device consisted of an oxadiazole electron-transporting layer. a diamine hole-transporting layer and an emissive layer of the MOPV LB film where MOPV molecules oriented nearly normal to the film plane. Because of the normal orientation of MOPV molecules in the emissive layer, the device exhibited a quite different spatial distribution of EL intensity from the Lambertian distribution: EL intewity has a maximum at a viewing angle of about 40” to the normal direction to the device surface, 0 1997 Elsevier Science S.A. Ke.worc/s:
Phrnylenevinylcne
oligomers: Electrolulninescrncr: Langmuir-Blodgett
1. Introduction In organic electroluminescent (EL) devices. introducing molecular orientation into the emissive layer is one of the promisingapproachesfor control of their emissionproperties. For example. in-plane molecular orientation in the emissive layer provides polarized electroluminescence [ l-31. Furthermore, molecularorientation in the normal direction of the emissive layer is expected to cause drastic change in the spatial distribution of EL intensity [A]. In this work, we preparedan organic EL device with well-defined molecular orientation by usingthe Langmuir-Blodgett (LB) technique and studied the effect of molecular orientation on the spatial distribution of EL intensity.
2. Experiments Amphiphilic phenylenevinylene oligomer (MOPV) was employed asemissive material. The MOPV formed a stable monolayer at the air/water interface. and the monolayer was depositedon substratesas Z-type film at a surface pressure of 15 mN m-’ by the LB technique. The formation of noncentrosymmctrical Z-type tilm structure in the MOPV LB ” Corresponding
author.
0379-6779/97/$17.00 PIISO379-6779197)03981-7
0 1997 Eisevicr Science S.A. All riphts reserved
films
film hasbeenconfirmed by the second-harmonicgeneration measurement[ 51. Using the MOPV LB film, we prepared a double heterostructure device consisting of a diamine (TAD) hole-transporting layer, an oxadiazole (OXD7) electron-transporting layer, and an emissive layer of the MOPV LB film. First, a TAD layer (50 nm thick) wasvacuum-depositedon indiumtin oxide (ITO) substrate at about lo-’ Torr. Then, an MOPV LB film (bimolecular layer) was deposited on the substrate. Finally, an OXD7 layer (50 nm thick) and an MgAg electrode (200 nm thick) were successivelydeposited. Molecular structuresof materialsemployed in this study are shown in Fig. 1. In the spatial distribution measurement of EL intensity from the double heterostructuredevice. electroluminescence passing through the IT0 substrate was detectedin the range of viewing anglebetween - 90 and90” with a 50 cm monochromatoranda multichannelCCD detector (HamamatsuPhotonicsCo., PMA 100).
3. Molecular orientation in MOPV LB film Fig. 2 showsthe absorptionspectrumof anMOPV LB film when the probe light was incident normal to the film plane. The absorptionpeaksdue to the transition momentsalong the long and short axes of the MOPV molecule are observedat
M. Eiw
et ni./Swh3ic
Metals 91 (1997)
c3-&$-Jy-z TAD
OXD7 Fig. 1. Molecular swuctures of phenplenevinylenc diamine (TAD) and an oxadiazole (OXD7).
oligomer
(MOPV),
a
S3-85
The absorption spectrumof an MOPV bimolecular layer film sandwichedwith vacuum-depositedfilms of TAD and OXD7 was almostidentical with that of the MOPV LB film on glasssubstrate;the spectrumshoweda large blue shift of the absorptionband due to the transition moment in the long axis direction of the MOPV molecule. Furthermore, absorbance due to the transition moment in the short axis of the MOPV molecule was larger than that in the long axis direction. From the spectrum,MOPV wasdemonstratedto form a similar molecular orientation in the sandwichedfilm as that on glasssubstrate.The result proves that one can incorporate an emissivelayer with a well-defined molecular orientation into an EL device by usingthe MOPV LB film.
4. Electroluminescence from double heterostructure device with MOPV LB film as an emitter
Wavelength(nm) Fig. 2. Absorption spectra of an MOPV LB lilm (solid line) and MOPV in dimethpl sulfoxide (dotted line). Inset shows the incident angle dependence of dichroic ratio A,/A, at300 nm of the MOPV LB tilm.
300 and 255 nm, respectively. The former absorptionpeak is more than 70 nm blue-shifted from that in the dimethyl sulfoxide solution of MOPV (375 nm). In addition, absorbance in the short axis of MOPV was larger than that in the long axis. The results suggestthat MOPV molecules form Haggregatesin the LB film, asa result of orientation of MOPV moleculesvertical to the film plane [ 61. The inset in Fig. 2 showsthe incident angledependenceof the dichroic ratio A,/ A, of an MOPV LB film, whereA,, andA, are absorbancesof the MOPV LB film at 300 nm measuredwith p- and s-polarized light, respectively. The dichroic ratio is increasedfrom 0.92 to 2.4 on increasingthe incident angleof polarized light from 0 to 45”. This result demonstratesqualitatively that MOPV moleculesnearly orient in the vertical direction to the film plane. From X-ray diffraction measurement,the structure of the MOPV LB film was evaluated. Three diffraction peaks assignedto the (OOn) planewere observedin the small-angle region of the X-ray diffraction profile of an MOPV LB film (20 layers). From the peaks,the layer spacingwasevaluated to be 2.1 m-n.Comparisonbetweenthe layer spacingand the molecular length of MOPV (2.4 nm) suggeststhat MOPV moleculesform a layer structure in the LB film where the long axis of the MOPV molecule tilts at an angle of about 30” to the film normal.
When the EL deviceswith an MOPV emissivelayer were driven at room temperature,their device performancerapidly degradedand the brightnessof the devices wasvery low. On the other hand. the EL devices exhibited stable and good device performanceat a low temperatureof about 80 K (luminancewasmorethan lOOcdm-‘at IOmAcm-‘).Therefore, all EL measurementson the device were carried out at low temperature. Fig. 3 showsEL spectraof the EL device with an MOPV emissive layer at 80 K. The spectrum was measuredat a viewing angle of 0”. The EL spectrumcorrespondswell to the photoluminescent(PL) spectrumof the MOPV LB film. No emissionfrom the TAD layer and the OXD7 layer was observed. This result suggeststhat the emissionregion is located only within the MOPV bimolecular layer owing to the confinementeffect of injected chargecarriersandexcitons createdby the injected carriersin the emissivelayer [ 71. Fig. 4 shows spatial distribution of non-polarized EL intensity at 520 nm in the device with an MOPV emissive layer, where 0 is the outer emissionangle. The spatial distribution is quite different from the Lambertian distribution. The EL intensity hasa maximum at a viewing angle of about 40” to the normal direction to the device surface.in addition,
300
400 Wavelegth
(nm)
6oo
7oo
Fig. 3. EL spectrum of the double heterostructure device with an MOPV LB film as an emissive layer (solid line). Dotted lines are PL spectra of an MOPV LB tilm, a vacuum-deposited TAD film, and a vacuum-deposited OXD7 film.
M. Em el cd. / Synrheric
I
1
0
EL intensity
(a.u.)
Fig. 4. Angular dependence of EL intensity a~ 520 nm in the double hererostructure device with an MOPV LB tilm as an emissive layer.
the shape of the EL spectrum of the device was independent of viewing angle. This result demonstrates that the drastic change in spatial distribution of EL intensity does not originate from the interference effect. as observed in EL devices with microcavity structure [ 81. but from the normal orientation of MOPV molecules in the emissive layer. In this work, we successfully prepared an organic EL device containing an emissive layer with a well-defined molecular orientation by using a phenylenevinylene oligomer LB film. Because of the molecular orientation normal to the layer plane in the emissive layer, the EL device exhibited a different spatial distribution of EL intensity from the Lam-
M~rnls
91 [ 1997)
83-85
SS
bertian distribution, From this result, control of molecular orientation in the normal direction to the emissive layer is expected to enhance the coupling efficiency of emitted light to a specific light propagating mode? in particular, a waveguiding mode. The approach in which LB films with a welldefined molecular orientation are used as an emissive layer, we believe, is useful in constructing advanced EL devices, such as waveguide-type devices with distributed feedback structure and EL devices with microcavity devices.
References [ 1 ] 0. Inganls, M. Berggren, M.R. Andersson, G. Guatafsaon. T. Hjertberg, 0. Wennerstrom, P. Dyreklev and M. Granstrom, Synth. Met., 69-7 1 (1995) 2121. [2] M. Era, T. Tsutsui and S. Saito, Appl. Phys. Lett., 67 ( 1995) 2436. [3] M. Hamaguchi and K. Yoshino. Jpn. J. Appl. Phys., 34 ( 1995) L714. [4] R.N. Marks, G. Biscarini, R. Zamboni and C. Taliani, Europhys. Lett.. 32 (1995) 523. [ 51 A. Watakabe. H. Okada and T. Kunitake. Langmuir, 10 ( 1994) 2722. 161 X. Xu, M. Era. T. Tsutsui and S. Saito, Chem. Len. ( 19S8) 773. [7] M. Era, C. Adachi, T. Tsutsui and S. Saito, Chem. Phys. Lett., 178 (1991) 488. [X] N. Takada, T. Tsutsui and S. Saito, Appl. Phys. Lett., 63 ( 1993) 2032.
Synthetic Metals 91 ( 1997) 83-35
Spatial distribution of electroluminescence from oriented phenylenevinylene oligomer Langmuir-Blodgett film Masanao
Era a.*, Jun-ichi Koganemaru a, Tetsuo Tsutsui a, Atsushi Watakabe b, Toyoki Kunitake b
Abstract An organic electroluminescent (EL) device with a well-defined molecular orientation was prepared by using aphenylenevinyleneoligomer (MOPV) Langmuir-Blodgett ( LB) film. The device consisted of an oxadiazole electron-transporting layer. a diamine hole-transporting layer and an emissive layer of the MOPV LB film where MOPV molecules oriented nearly normal to the film plane. Because of the normal orientation of MOPV molecules in the emissive layer, the device exhibited a quite different spatial distribution of EL intensity from the Lambertian distribution: EL intewity has a maximum at a viewing angle of about 40” to the normal direction to the device surface, 0 1997 Elsevier Science S.A. Ke.worc/s:
Phrnylenevinylcne
oligomers: Electrolulninescrncr: Langmuir-Blodgett
1. Introduction In organic electroluminescent (EL) devices. introducing molecular orientation into the emissive layer is one of the promisingapproachesfor control of their emissionproperties. For example. in-plane molecular orientation in the emissive layer provides polarized electroluminescence [ l-31. Furthermore, molecularorientation in the normal direction of the emissive layer is expected to cause drastic change in the spatial distribution of EL intensity [A]. In this work, we preparedan organic EL device with well-defined molecular orientation by usingthe Langmuir-Blodgett (LB) technique and studied the effect of molecular orientation on the spatial distribution of EL intensity.
2. Experiments Amphiphilic phenylenevinylene oligomer (MOPV) was employed asemissive material. The MOPV formed a stable monolayer at the air/water interface. and the monolayer was depositedon substratesas Z-type film at a surface pressure of 15 mN m-’ by the LB technique. The formation of noncentrosymmctrical Z-type tilm structure in the MOPV LB ” Corresponding
author.
0379-6779/97/$17.00 PIISO379-6779197)03981-7
0 1997 Eisevicr Science S.A. All riphts reserved
films
film hasbeenconfirmed by the second-harmonicgeneration measurement[ 51. Using the MOPV LB film, we prepared a double heterostructure device consisting of a diamine (TAD) hole-transporting layer, an oxadiazole (OXD7) electron-transporting layer, and an emissive layer of the MOPV LB film. First, a TAD layer (50 nm thick) wasvacuum-depositedon indiumtin oxide (ITO) substrate at about lo-’ Torr. Then, an MOPV LB film (bimolecular layer) was deposited on the substrate. Finally, an OXD7 layer (50 nm thick) and an MgAg electrode (200 nm thick) were successivelydeposited. Molecular structuresof materialsemployed in this study are shown in Fig. 1. In the spatial distribution measurement of EL intensity from the double heterostructuredevice. electroluminescence passing through the IT0 substrate was detectedin the range of viewing anglebetween - 90 and90” with a 50 cm monochromatoranda multichannelCCD detector (HamamatsuPhotonicsCo., PMA 100).
3. Molecular orientation in MOPV LB film Fig. 2 showsthe absorptionspectrumof anMOPV LB film when the probe light was incident normal to the film plane. The absorptionpeaksdue to the transition momentsalong the long and short axes of the MOPV molecule are observedat
M. Eiw
et ni./Swh3ic
Metals 91 (1997)
c3-&$-Jy-z TAD
OXD7 Fig. 1. Molecular swuctures of phenplenevinylenc diamine (TAD) and an oxadiazole (OXD7).
oligomer
(MOPV),
a
S3-85
The absorption spectrumof an MOPV bimolecular layer film sandwichedwith vacuum-depositedfilms of TAD and OXD7 was almostidentical with that of the MOPV LB film on glasssubstrate;the spectrumshoweda large blue shift of the absorptionband due to the transition moment in the long axis direction of the MOPV molecule. Furthermore, absorbance due to the transition moment in the short axis of the MOPV molecule was larger than that in the long axis direction. From the spectrum,MOPV wasdemonstratedto form a similar molecular orientation in the sandwichedfilm as that on glasssubstrate.The result proves that one can incorporate an emissivelayer with a well-defined molecular orientation into an EL device by usingthe MOPV LB film.
4. Electroluminescence from double heterostructure device with MOPV LB film as an emitter
Wavelength(nm) Fig. 2. Absorption spectra of an MOPV LB lilm (solid line) and MOPV in dimethpl sulfoxide (dotted line). Inset shows the incident angle dependence of dichroic ratio A,/A, at300 nm of the MOPV LB tilm.
300 and 255 nm, respectively. The former absorptionpeak is more than 70 nm blue-shifted from that in the dimethyl sulfoxide solution of MOPV (375 nm). In addition, absorbance in the short axis of MOPV was larger than that in the long axis. The results suggestthat MOPV molecules form Haggregatesin the LB film, asa result of orientation of MOPV moleculesvertical to the film plane [ 61. The inset in Fig. 2 showsthe incident angledependenceof the dichroic ratio A,/ A, of an MOPV LB film, whereA,, andA, are absorbancesof the MOPV LB film at 300 nm measuredwith p- and s-polarized light, respectively. The dichroic ratio is increasedfrom 0.92 to 2.4 on increasingthe incident angleof polarized light from 0 to 45”. This result demonstratesqualitatively that MOPV moleculesnearly orient in the vertical direction to the film plane. From X-ray diffraction measurement,the structure of the MOPV LB film was evaluated. Three diffraction peaks assignedto the (OOn) planewere observedin the small-angle region of the X-ray diffraction profile of an MOPV LB film (20 layers). From the peaks,the layer spacingwasevaluated to be 2.1 m-n.Comparisonbetweenthe layer spacingand the molecular length of MOPV (2.4 nm) suggeststhat MOPV moleculesform a layer structure in the LB film where the long axis of the MOPV molecule tilts at an angle of about 30” to the film normal.
When the EL deviceswith an MOPV emissivelayer were driven at room temperature,their device performancerapidly degradedand the brightnessof the devices wasvery low. On the other hand. the EL devices exhibited stable and good device performanceat a low temperatureof about 80 K (luminancewasmorethan lOOcdm-‘at IOmAcm-‘).Therefore, all EL measurementson the device were carried out at low temperature. Fig. 3 showsEL spectraof the EL device with an MOPV emissive layer at 80 K. The spectrum was measuredat a viewing angle of 0”. The EL spectrumcorrespondswell to the photoluminescent(PL) spectrumof the MOPV LB film. No emissionfrom the TAD layer and the OXD7 layer was observed. This result suggeststhat the emissionregion is located only within the MOPV bimolecular layer owing to the confinementeffect of injected chargecarriersandexcitons createdby the injected carriersin the emissivelayer [ 71. Fig. 4 shows spatial distribution of non-polarized EL intensity at 520 nm in the device with an MOPV emissive layer, where 0 is the outer emissionangle. The spatial distribution is quite different from the Lambertian distribution. The EL intensity hasa maximum at a viewing angle of about 40” to the normal direction to the device surface.in addition,
300
400 Wavelegth
(nm)
6oo
7oo
Fig. 3. EL spectrum of the double heterostructure device with an MOPV LB film as an emissive layer (solid line). Dotted lines are PL spectra of an MOPV LB tilm, a vacuum-deposited TAD film, and a vacuum-deposited OXD7 film.
M. Em el cd. / Synrheric
I
1
0
EL intensity
(a.u.)
Fig. 4. Angular dependence of EL intensity a~ 520 nm in the double hererostructure device with an MOPV LB tilm as an emissive layer.
the shape of the EL spectrum of the device was independent of viewing angle. This result demonstrates that the drastic change in spatial distribution of EL intensity does not originate from the interference effect. as observed in EL devices with microcavity structure [ 81. but from the normal orientation of MOPV molecules in the emissive layer. In this work, we successfully prepared an organic EL device containing an emissive layer with a well-defined molecular orientation by using a phenylenevinylene oligomer LB film. Because of the molecular orientation normal to the layer plane in the emissive layer, the EL device exhibited a different spatial distribution of EL intensity from the Lam-
M~rnls
91 [ 1997)
83-85
SS
bertian distribution, From this result, control of molecular orientation in the normal direction to the emissive layer is expected to enhance the coupling efficiency of emitted light to a specific light propagating mode? in particular, a waveguiding mode. The approach in which LB films with a welldefined molecular orientation are used as an emissive layer, we believe, is useful in constructing advanced EL devices, such as waveguide-type devices with distributed feedback structure and EL devices with microcavity devices.
References [ 1 ] 0. Inganls, M. Berggren, M.R. Andersson, G. Guatafsaon. T. Hjertberg, 0. Wennerstrom, P. Dyreklev and M. Granstrom, Synth. Met., 69-7 1 (1995) 2121. [2] M. Era, T. Tsutsui and S. Saito, Appl. Phys. Lett., 67 ( 1995) 2436. [3] M. Hamaguchi and K. Yoshino. Jpn. J. Appl. Phys., 34 ( 1995) L714. [4] R.N. Marks, G. Biscarini, R. Zamboni and C. Taliani, Europhys. Lett.. 32 (1995) 523. [ 51 A. Watakabe. H. Okada and T. Kunitake. Langmuir, 10 ( 1994) 2722. 161 X. Xu, M. Era. T. Tsutsui and S. Saito, Chem. Len. ( 19S8) 773. [7] M. Era, C. Adachi, T. Tsutsui and S. Saito, Chem. Phys. Lett., 178 (1991) 488. [X] N. Takada, T. Tsutsui and S. Saito, Appl. Phys. Lett., 63 ( 1993) 2032.