ELSEVIER Thin Solid Films 281-282 (1996) 542-544

Direct observation of the crystallization in EL organic thin films by totalreflection X-ray diffractometer

Kenji Orita, Kouichi Hayashi, Toshihisa Horiuchi, Kazumi MatsushigeDepartment of Electronic Science andEllgineering. Faculty ofEngineering, Kyoto University, Yoshida-honmachl, Sakyo-ku, Kyoto 606-01, Japan

Abstract

The degradation of amorphous organic electroluminescent (EL) devices is generally assumed due to the crystallization enhanced by theJoule heat. Here, in order to reveal directly the structural changes inorganic ELfilms during an annealing process, we performed first totalreflection X-ray diffraction measurements for N,N'-diphehy!-N,N'·bis(3-methylphenyl)-[1,1'-biphenyl] -4,4'-diamine (TPD) films vapor­deposited on Si02 glass substrates. Atomic force microscopy (AFM) was also used to observe the morphological modifications throughannealing. TheX-ray data revealed that the TPD film remained amorphous below 100°C, followed bya gradual change into crystalline duringannealing. Moreover, combining this result with the AFM data, we found that there existed some relationship between morphological changeand crystallization in the film.

Keywords: Luminescence; Crystallization; X-ray diffraction: Atomic force microscopy

1. Introduction

Organic electroluminescent (EL) devices have beenexpected tobefuture large-area light-emitting displays sincethe thin-film EL cell with a multilayer structure could bedriven by low d.c. voltage (I]. The typical configuration oforganic EL cells is as follows; indium tin oxide (ITO)substrate [an anode] IN,N'-diphenyl-N,N'-bia(Svmethy­phenyl)-] 1,I'-biphenyl]-4,4'-diamine (TPD) [a hole trans­port layer] hris(8-hydroxyquinoline) aluminum (Alq3) [anelectron transport and emitting layer] Ia mixture of magne­sium and silver [a cathode]. Moreover, high luminescence,high efficiency and wide selection of emission colors wererecently realized [2--4].

Th... ;tractical use of organic EL diodes, however, has notyetbeen achieved because of their instability, butno decisiveelucidation has sofarbeen accomplished on the degradationmechanism. One of the reasons isusually supposed tobe thecrystallization in their organic amorphous layers, which isenhanced by the Joule heat generated in running organic ELdevices. Han et aI. [5] reported that they observed the mor­phology inTPD films, which were kept under ambient atmos­phere at room temperature, by scanning probe microscopy.They judged that significant morphological modificationswere induced by the crystallization in the films. Butit shouldbenoted that the morphological change asthey observed doesnotnecessarily prove such a crystallization process.

0040-6090/96/$15.00 e 1996 Elsevier Science SA All rights reservedP// S0040-6090 (96) 08723-8

X-ray diffraction methods are the most suitable for struc­tural evaluations of materials. But the conventional X-raytechniques are, ingeneral, not applicable toorganic thin films,because it is very difficult to observe diffracted Xrays fromsuch films, due tostrong diffractions andscatterings from thesubstrates. To overcome these difficulties we developed theenergy dispersive total reflection X-ray diffraction (TRXD)systems and have obtained valuable information on structuresinorganic thin films [6-9].

In this study, we utilized the total reflection X-ray diffrac­tometer toobserve directly the dynamic structural changes ina TPD film during annealing. In addition, morphologicaltransformations were examined by an atomic force micros­copy (AFM) in meier to investigate the relation betweenstructural and morphological changes.

2. Experimental

We fabricated the film sample by vacuum vapor depositionofTPD powder onanoptically flat Si02 glass substrate. Thevapor deposition was conducted under the base pressure ofabout 1X10- 4 Pa and at a substrate temperature of worntemperature. The deposition rate and thickness of the filmwere controlled so as to be 0.5-0.7 nm s-) and 300 nrn,respectively, bymonitoring with an oscillating quartz thick­ness monitor.

Sofar aswe know, there isnoreport on the crystal structureofTPD. Therefore, we measured the crystalline TPD powder

K. Orita er al./Thill Solid Films 281-282(1996) 542-544

(c)

(b)

" "

" . .

"

•• •

. " "

28'C

•

•

· " .•

lOJ 4.35[.......,-.. I , i ' i ' , , i I. j-- 430' •::J..... / •H:::. . .'J)-II) ;

~ ~ 4.25

c·~ 420_"0 •

6.01F-t-+-t-+-+-+--l-+-f-+-I--+-+-t-4

.... 5.8ca--: ~ 5.6tU~-5 E 5.4'0::1.~.§ ~,2

.=: ~ 50If s . j4.8,L......l-O-J.-.-.i-J-.J-.o...J....-.J-, __L.

20 30 40 50 60 70 so 91.1 100

Temperaturej'C]

20=11.0'

4.15 •

11 12 13 14 15 16 17 18Energy[keV]

Fig. 2. Temperature changes in TRXD profiles of theTPD film observed'luring an annealing process. The curve drawn on theobserved pattern isbase ona Gaussian fiuing, Thearrow indicates thecenter oftile halo pattern.

5.11A 4.80A 4.52A MOK~5.74A \ I I 4.36At f,' /'

•

chose the fitting energy range from 11.0 eV to 16.4 eV,because theeffect of the Mo KG' peak was considered to benegligible in this range.

Fig. 3 shows fitting parameters, which were calculated fortheprofiles at different temperatures from room temperatureto 100°C. The center of the obtained curve was assumed hereto correspond to theintermolecular distance. The integratedintensity was theproduct of thefull width at halfmaximum(FWHM) and peak intensity. With increasing temperature,theintermolecular distance increased, while both of the inte­grated intensity andFWHM, which represents thedegree oforder in the film, decreased correspondingly. These factssuggest that some structural changes occurred prior to thecrystallization in theamorphous phase.

Fig. 3.Char-ges intheparameters calculated from theGaussian fitting tothehalo patterns observed at different temperature. during annealing; (a) theintermolecular distance, (b) the integrated intensity, and (c) thefull widthat halfmaximum.

3015 2~ 2528[deg.]

10

(a)

(b)

6.91;' 4.80A 4.52A

~ 7:l8A \ 5.11A \ ./4.36A

~ \ 5.74A\'ta 9.l3A~ \ 4.03A

III

~

by both of the TRXD and conventional (}""28 scan methodsand decided the lattice spacings of this crystal.

First, we observed TRXD profiles concerning stackingstructures in thefilm during annealing. The annealing processwas performed in ambient atmosphere. The incident whiteX-rays from a molybdenum (Mo) target were operated at40kV and 30 rnA. The glancing angle was about D.Ogo, justbelow the critical angle for total reflections at the substratesurface (X-ray energy, 10 keY).Thediffracted X-rays weredetected with a solid-state detector (SSD) at a fixed Braggangle, (J.

AFM was ut.ilized to observe the surface morphology offilms as-deposited and after the annealing process, The sam­ple preparation was similar to that in theX-ray measurement.The lateral images were measured with a Si3N4 cantilever inconstant height mode; the vertical position of the tip wascontrolled to be constantly distant from the sample surfaceand the cantilever deflection was detected by a laser beamdeflection technique. The instrument used in this study wasSEIKO SPA-300 unit controlled with SPI-3700 controller.

3. Results and discussion

3./. Structural evaluation

Fig. 1 shows the X-ray diffraction profiles of crystallineTPD powder byconventional (}""20scanning (Fig. 1(a)) andTRXD (Fig. 1(b») methods. The lattice spacings ofthecrys­tal were determined accurately through the (}-2(J X-ray dif­fraction patterns. Here, wecorresponded the diffraction peaksin Fig, 1(b) to thelattice spacings.

Fig. 2 shows the temperature variation in TRXD spectraobserved during annealing up to 160°C at the diffractioncondition of 2(]= 11°, revealing the structural change fromamorphous to crystalline phases. Below about 100°C weobserved thehalo pattern, which ischaracteristic ofanamor­phous structure, andwas fitted toa Gaussian peak shape. We

'?r ........-,---.--.--,...,,--.,o-='l,.--,,-..,.--.--r-~...-,.,~....§

~.0'(ij

§ ~--'-~--L...,,~~--J l-...,..,...'O-"~~-J.-..,l-,-J= 10 12 14 16- Energy[keV]

Fig. 1. X-ray diffraction profiles of the crystalline TPD powder by (a)6-20 scan and (b) TRXD methods.

544 K. Orita etal,/TlJin SolidFilms 281-282 (1996) 542-54·!

Above 100°C, relatively sharp diffractions were observed,the lattice spacings of which were almost same as those ofthe crystalline powder, revealing thatthe TPD film started tocrystallize inpart. Thus, it was disclosed forthe first time thatthe organic EL film underwent thestructural changes in theamorphous phase and then crystallized upon heating.

3.2. Morphological evaluation

Fig. 4 shows the AFIA images of the films as deposited,after annealing upto80"C and160 "C, Theas-deposited filmexhibited a very smooth surface, asshown in Fig. 4( a). Themorphology shown in Fig. 4(b) was similar to that of thefilm kept in air fora long time [5]; deep valleys formed andthesubstrate surface was exposed. Han et a1. claimed that theobserved change was attributed to the crystallization. Thechange observed here, however, was assigned tosome struc­tural changes in the amorphous phase, by considering theresult of the above X-ray analysis that the halo patternremained below 100 "C. This suggests that a morphologicalchange isnotnecessar j a proofof crystallization.

From the TRXD data, it was revealed that this type ofmorphology shown asFig. 4( c) corresponded topartial crys­tallization. In the image observed were the characteristicgeometry of crystallized regions and the undulatory surfacesof simply-condensed ones. The very flat area in the imageswas a bare substrate surface. In this way AFM could observethe morphological changes in accordance with the TRX!)analysis.

4. Conclusions

Weobserved directly, for thefirst time, the dynamic behav­iors oforganic ELfilms during anannealing process byusingtheenergy dispersive TRXD system. It was revealed that theTPD film remained amorphous below 100 "C and changedpartly from amorphous into crystallineabove 100 "C. Inaddi­tion, AFM observed that morphological transformationsoccurred corresponding to the structural ones detected herebyTRXD method.

Recently, thestructural and morphological changes inAlq,single-layer and AIQ3/TPD double-layer film samples havebeen analyzed similarly by theTRXD system and AFM, thedetails of which will be reported elsewhere soon. Since thisstructur 11 evaluation canbeundertaken even under auelectricfield ar. .Jsimultaneously with ELspectrum analysis, thedirectrelationship bel. .veen the degradation of organic EL diodesand thestructural changes in theorganic layers will be elu­cidated more clearly.

References

[1] C.W. Tang andSA Var.Slyke••\ppl.l'hys. :.ett.• 51 (1.987) 91l[2] C.Adachi, T.Tsutsui and S.Saito, Jim. J.Appl. PIWf., 27 (1988)L269.[31 C.W. Tang, SA VanSlyke and C.H. Ch:.m. J. Apr;. 'Phys., 65 (1989)

3610.

Fig. 4. AFM i,m::lges of the morphological change in the TPD film duringannealing; (a) the as-deposited film, (b) thefilm annealed upto 80 DC and(c) thefilm annealed up10160 DC.

[4] C.Hosokawa, H.Tokallin, H. Higashi and T.Kusumoto, Acta Polytech.Scan. Appl. Phys. Ser., 170 (1990) 219.

[5] E. Han, L.Do, Y. Niidome and M. Fujihara, Chern. u«, (1994) 969.[6] T.Horiuchi, K. Fukao and K.Matsushige,Jpn. J.Appl. Phys.; 26 ( 1987)

L1839.[7] K.Fukao, T.Horiuchi and K.Matsushige, Thin SolidFilms. 171(1989)

359.[8] K. Hayashi. K. Ishida. T. Horiuchi and K. Matsushige, Ipn. J. Ap;.?J.

Phys.• 31 (199~;) 4081.[9] K. Ishida. K. Hayashi, Y. Y:,~;.ida, T. Horiuchi andK. Matsushige, J.

Appl. Phys.• 73 (1993) 733~.