Quality improvement of organic thin films deposited on vibrating substrates

Thin Solid Films 520 (2011) 1416–1421

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Quality improvement of organic thin films deposited on vibrating substrates Y.A. Paredes, P.G. Caldas, R. Prioli, M. Cremona ⁎ Physics Department, Pontifícia Universidade Católica do Rio de Janeiro, Rua Marques de São Vicente, 225-Gávea, Rio de Janeiro, RJ 22453-970, Brazil

a r t i c l e

i n f o

Available online 20 October 2011 Keywords: OLEDs Organic thin film Atomic force microscopy Substrate vibration

a b s t r a c t Most of the Organic Light-Emitting Diodes (OLEDs) have a multilayered structure composed of functional organic layers sandwiched between two electrodes. Thin films of small molecules are generally deposited by thermal evaporation onto glass or other rigid or flexible substrates. The interface state between two organic layers in OLED device depends on the surface morphology of the layers and affects deeply the OLED performance. The morphology of organic thin films depends mostly on substrate temperature and deposition rate. Generally, the control of the substrate temperature allows improving the quality of the deposited films. For organic compounds substrate temperature cannot be increased too much due to their poor thermal stability. However, studies in inorganic thin films indicate that it is possible to modify the morphology of a film by using substrate vibration without increasing the substrate temperature. In this work, the effect of the resonance vibration of glass and silicon substrates during thermal deposition in high vacuum environment of tris (8-quinolinolate)aluminum(III) (Alq3) and N,N′-Bis(naphthalene-2-yl)-N,N′-bis(phenyl)-benzidine (β-NPB) organic thin films with different deposition rates was investigated. The vibration used was in the range of hundreds of Hz and the substrates were kept at room temperature during the process. The nucleation and subsequent growth of the organic films on the substrates have been studied by atomic force microscopy technique. For Alq3 and β-NPB films grown with 0.1 nm/s as deposition rate and using a frequency of 100 Hz with oscillation amplitude of some micrometers, the results indicate a reduction of cluster density and a roughness decreasing. Moreover, OLEDs fabricated with organic films deposited under these conditions improved their power efficiency, driven at 4 mA/cm2, passing from 0.11 lm/W to 0.24 lm/W with an increase in their luminance of about 352 cd/m2 corresponding to an increase of about 250% in the luminance with respect to the same OLEDs fabricated in the same way and with the same conditions without substrate vibration. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Nowadays, the Organic Light-Emitting Diode (OLED) technology offers excellent performances with high brightness, low power consumption, wide viewing angle, fast response time and potentially low cost [1]. The majority of OLEDs have a multilayered structure composed of different functional organic layers sandwiched between two electrodes, one being transparent. The multilayer architecture is used in order to overcome several drawbacks which limit the OLED efficiency. A typical limitation is the higher mobility of positive charge carriers when compared with the negative ones [2,3]. Moreover, there is an energy barrier created by the difference between the metal cathode work function and the lowest unoccupied molecular orbital (LUMO) of the organic semiconductor, inhibiting the negative carrier injection. These drawbacks result in the formation of the majority of electron–hole pairs near the cathode neighborhood, suppressing light emission [4]. Even though several methodologies have been developed to avoid these difficulties, the most common uses the

⁎ Corresponding author. Tel.: + 55-21-3527-1258; fax: + 55-21-3527-1270. E-mail address: cremona@fis.puc-rio.br (M. Cremona). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.10.040

multilayer structure consisting of separated layers containing, each one, just electron or hole transporting molecules [5]. However, the interface state between organic layers [6] or between organic layer and the substrate/electrode depends strongly by the surface film morphology which in turn can influence the carrier mobility [7] and the carrier injection [8] in the OLED affecting its electrical and optical overall performances [9–11]. The morphology of a thermally deposited organic thin film depends on the deposition rate [12], on the kind of substrate (glass, graphite, silicon or metal [13–17]), on its contamination and temperature [18]. Compared to conventional inorganic nucleation from molecular beam epitaxy, organic semiconductors are rarely deposited on single crystalline surfaces and, therefore, no epitaxial growth is generally observed [19]. In the case of inorganic thin films it is possible that the high substrate temperature assists epitaxial growth by lowering the supersaturation levels, stimulating desorption of impurities, enhancing surface diffusion of adatoms into equilibrium sites, and promoting island coalescence [20]. On the other hand, increasing the substrate temperature beyond this range appreciably decreases the sticking coefficient of the semiconductor molecule, and nucleation does not occur [19], possibly limiting operational conditions for thin-film devices [21]. In multilayered small molecule based OLEDs each organic thin film is formed by

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successive layer by layer deposition of an organic compound onto a substrate, a metal or a previously deposited organic layer. Accordingly, the deposition of a layer is strongly influenced by the surface morphology of the previous one consequently affecting the quality of the final device. However, it is possible to modify the morphology of a thin film without increasing temperature but using instead substrate vibration as reported in the literature for some semiconductor (cadmium chalcogenide) and metal thin films [22–26]. In the present work, the quality improvement of tris(8-quinolinolate)aluminum(III) (Alq3) and N, N′-Bis(naphthalene-2-yl)-N,N′-bis(phenyl)-benzidine (β-NPB) organic thin films deposited onto silicon and glass substrates for OLED applications were studied using the substrate resonance vibration during their thermal deposition in high vacuum environment and with different deposition rates. Our results indicate a reduction of the island density and a roughness decreasing. Moreover, the OLEDs fabricated with organic films deposited under these conditions showed an improved efficiency with an increase in their luminance of about 352 cd/m 2 and in their power efficiency of about 0.13 lm/W. 2. Experimental Alq3, β-NPB and CuPc (copper phthalocyanine) compounds were used as received, without additional purification of the LUMTEC Corp. (Fig. 1). It is worth to mention that these materials have different behavior during the evaporation which contributes to differentiate the morphology of respective films: Alq3 sublimates while β-NPB melts and then evaporates. Organic layers were deposited onto silicon or ITO substrates with a sheet resistance of 8.1 Ω/square in high vacuum environment (base pressure 5–10 − 4 Pa) by thermal evaporation. The deposition rates (from 0.05 nm/s to 0.3 nm/s) were precisely controlled by using the INFICON Cignus deposition crystal thickness controller. The organic layer thickness of 70 nm was also monitored

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through the same controller during the deposition and confirmed by a successive profilometer measurement. Before the deposition, the substrates were cleaned by detergent, rinsed with deionized water, sonicated in organic solvents (acetone and alcohol) and cleaned in a UV-ozone chamber. In order to obtain the in situ substrate oscillations a low impedance loudspeaker (R= 8 Ω) that oscillates in the range of hundreds of Hz was fixed at the substrate holder. The speaker was set to vibrate in a direction parallel to the substrate surface plane with amplitude of a tenth of microns. The amplitude and frequency of the vibrating substrate were controlled through a waveform generator (Agilent 33210A) connected to the loudspeaker. The morphology of thin films was performed by atomic force microscopy (AFM) using a Multimode microscope with a Nanoscope IIIa electronics AFM (Veeco). The microscope was operated in tapping mode with a 10 nm radius silicon tip. A set of ten images with 3 μm × 3 μm was taken at different surface locations. The reported roughness values correspond to the surface height variation from the average surface height. The reported error corresponds to the standard variation of the roughness value. For the OLED fabrication the following architecture was employed (thickness in nm): ITO/CuPc(12)/β-NPB (25)/Alq3 (50)/Al (120). Even if OLEDs with this configuration are known to have not too much efficiency, this structure was preferred due to its simplicity in order to test the substrate vibration technique. Efficiencies from 0.5 up to 2.7 lm/W and luminance from 50 up to 3000 cd/m 2 can be found for such devices in literature [27–30] using similar thickness for the organics and Al or Al/LiF as cathode. The organic layers were deposited sequentially without breaking vacuum between the different phases. ITO is used as positive electrode while Al film was used as negative one. The deposition rates used were of 0.1 nm/s and 0.3 nm/s for the organics and the aluminum cathode, respectively. The devices were fabricated with and without a 100 Hz substrate vibration. The device active area was about 0.5 mm 2. The current density

Fig. 1. Structure of organic molecules used in this work: (a) Alq3, (b) β-NPB, (c) CuPc.

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(J)–voltage (V) and luminance (L) characteristic curves were measured simultaneously using a Ketihley 2400 source meter and a Newport 1830-C as optical power meter. The electroluminescence (EL) spectra were measured using a Photon Technology International (PTI) steady state spectrofluorometer MQ1 model. 3. Results and discussion Changes in the morphology and roughness of organic films deposited onto steady and vibrated silicon substrates have been analyzed by AFM as a function of vibration frequency and deposition rate. Fig. 2 shows AFM images of a 70 nm thick Alq3 film deposited onto silicon using five different substrate vibration frequencies. It is possible to observe that the number of clusters produced during the thermal deposition process [31,32] decreases with the increase of the substrate vibration frequency from 0 (Fig. 2a) to 800 Hz (Fig. 2e). Differences in the morphology of the Alq3 films are indicated in Table 1 through the root-mean-square (RMS) roughness values. The

RMS roughness shows a progressive decrease with the increase of the frequency of substrate. As the mechanical vibration of the substrate enhances the mobility of adatoms and clusters [33] a higher coalescence rate is expected for films which are subjected to sound vibration during deposition in comparison with the film deposited in the same conditions but onto a steady substrate. Therefore, the vibration during deposition leads to an increased coalescence, resulting in a film with a better surface coverage and a small amount of clusters improving the film quality. This is what Fig. 2 and Table 1 clearly show. Indeed, from Fig. 2a to Fig. 2c a continuous decrease in the number and size of the morphology features (clusters) present in the organic thin film as a function of the substrate vibration frequency is observed. In particular, with frequencies of 300 Hz (Fig. 2d) and 800 Hz (Fig. 2e) there are no more evident clusters in the deposited Alq3 film. This changing in the film surface is still more evident observing a 3D representation of Fig. 2a and Fig. 2d reported in Fig. 2f and Fig. 2g, respectively. In this way, as the vibration frequency increases the number of large clusters decrease, thereby obtaining a

Fig. 2. 3 μm × 3 μm AFM images of thermally deposited 70 nm Alq3 thick film onto: (a) steady unvibrated silicon substrate and silicon substrate vibrated at (b) 50 Hz, (c) 100 Hz, (d) 300 Hz and (e) 800 Hz. (f) and (g) are 3D AFM images of figure (a) (arrow indicated area) and figure (d).

Y.A. Paredes et al. / Thin Solid Films 520 (2011) 1416–1421 Table 1 Root-mean-square (RMS) roughness values of Alq3 thin film under different silicon substrate vibration frequencies. Frequency (Hz)

RMS (nm)

0 50 100 300 800

1.30 ± 0.08 0.61 ± 0.07 0.51 ± 0.08 0.50 ± 0.01 0.36 ± 0.01

more homogeneous organic thin film. The same analysis was also performed for the β-NPB that is an organic compound widely used as Hole Transporting Layer (HTL) in the OLED manufacture. Also in this case, during the β-NPB deposition the silicon substrate was allowed to vibrate from zero to 300 Hz and the results were very similar to those obtained for the Alq3 films. Fig. 3 shows a comparison between Alq3 and β-NPB thin films deposited onto silicon substrates at room temperature, with the same deposition rate (0.1 nm/s) with steady and vibrating substrates. In this case, the substrate vibration frequency was set at 100 Hz. The data reported in this work show that it is possible to reduce the roughness of thermally deposited organic thin films by using a suitable substrate mechanical vibration during the deposition process. This result can be considered independent from the material deposition behavior (sublimation or melting and sublimation). Table 2 summarizes the roughness RMS values obtained for Alq3 and β-NPB thin films growth with a fixed deposition rate (0.1 nm/s) with and without substrate vibration (100 Hz). For comparison, Table 3 reports the roughness RMS values of both materials for a fixed substrate vibration (100 Hz) as a function of different deposition rates. Differently from the examples presented in the literature [22–26], the tables show that the lowest roughness values for organic

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Table 2 RMS roughness values (nm) for Alq3 and β-NPB thin films with 0,1 nm/s deposition rate as a function of the substrate vibration.

Alq3 β-NPB

Unvibrated substrate

Vibrated substrate (100 Hz)

1.30 ± 0.08 0.50 ± 0.01

0.51 ± 0.08 0.34 ± 0.01

thin films can be obtained with a deposition rate of 0.1 nm/s and a substrate vibration frequency of 100 Hz. Finally, the effect of the substrate vibration in the OLED response was also investigated in a specific device structure. The ITO/CuPc/βNPB/Alq3/Al device was fabricated under different condition with and without substrate vibration. The CuPc was deposited without vibration onto ITO in order to reduce the effective barrier between the ITO and the hole-transporting layer. It is worth to mention that the results presented here are in fact an average on three devices fabricated for each experimental condition. For the manufactured OLEDs the obtained curves present a standard deviation of 0.5 and 3% for Hz vibrated and unvibrated substrate, respectively. The luminance–voltage curves showed in Fig. 4a refer to a device fabricated with the substrate submitted at different vibration frequencies only during the Alq3 evaporation. There is an evident improvement when the substrate is allowed to vibrate and the frequency correspondent to the best OLED performance is 100 Hz. This behavior is confirmed and enhanced for the case in which both Alq3 and βNPB layers are deposited with vibrating substrate, as showed in Fig. 4b. Comparing the luminance values of OLEDs fabricated without substrate vibration with the luminance values of devices fabricated with 100 Hz substrate vibration during the Alq3 evaporation and during the Alq3 and β-NPB evaporation we obtain an increase of 173 cd/m 2 and 352 cd/m 2, respectively. The luminance value of

Fig. 3. AFM images of thermally deposited 70 nm (a) Alq3 and (b) β-NPB thick films onto steady unvibrated and 100 Hz vibrated silicon substrate.

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Table 3 RMS roughness values (nm) for Alq3 and β-NPB thin films with 100 Hz substrate vibration as a function of deposition rate.

a 10

0

Alq3 β-NPB

0.05 nm/s

0.1 nm/s

0.2 nm/s

0.52 ± 0.02 0.74 ± 0.12

0.51 ± 0.08 0.34 ± 0.01

1.48 ± 0.39 0.95 ± 0.36

173 cd/m 2 agrees with values found for devices fabricated with the same materials, in similar conditions (current densities) and with similar architectures [27–30]. Definitely, changes in the metal cathode by using, for example, Ag/Mg or the addition of a LiF layer, improve considerably the performance of the device. However, considering that all our devices were fabricated with the same architecture and in the same environmental conditions our result maintains his significance and corresponds to a luminance improvement of about 250% with respect to an OLED fabricated with no vibrating substrate. This improvement can be also observed in Fig. 5 where the power efficiency curves are presented for the different devices. Also in this case, Fig. 5a reports the data for a device with just Alq3 deposited with vibrating substrate while in Fig. 5b both

Power efficiency (lm/W)

Deposition rate

-2

10

-4

10

10

-2

10

0

10

2

2

Current density (mA/cm )

b

0

10

a Power efficiency (lm/W)

Luminance (cd/m2)

101

-2

10

-2

Substrate frequency (Hz)

10

unvibrated 100 300 800 -4

10

10

0

10

2

2

Current density (mA/cm ) 0

3

6

9

12

15

Voltage (V)

b

Alq3 and β-NPB are deposited in this condition. The power efficiency of the OLED fabricated with the two organic layers evaporated onto a 100 Hz vibrating substrate is about 40% higher than those obtained for the others devices with no vibrating substrate.

10 4

Luminance (cd/m2)

Fig. 5. Power efficiency curves of the ITO/CuPc/β-NPB/Alq3/Al fabricated OLEDs for two different experimental conditions: (a) Alq3 film deposited with 100 Hz vibrating substrate; (b) both Alq3 and β-NPB films deposited with 100 Hz vibrating substrate.

4. Conclusions

10 1

Substrate frequency (Hz) unvibrated 100 300 800

10-2

0

3

6

9

12

15

Voltage (V) Fig. 4. Luminance–voltage characteristics of the ITO/CuPc/β-NPB/Alq3/Al fabricated OLEDs for two different experimental conditions: (a) Alq3 film deposited with 100 Hz vibrating substrate; (b) both Alq3 and β-NPB films deposited with 100 Hz vibrating substrate.

In this work, the effect of the resonance vibration of glass and silicon substrates during thermal deposition in high vacuum environment of tris(8-quinolinolate)aluminum(III) (Alq3) and N,N′-Bis (naphthalene-2-yl)-N,N′-bis(phenyl)-benzidine (β-NPB) thin films with different deposition rates was investigated. Differently from the case of metals and inorganic materials the vibrations used were in the range of hundreds of Hz and the substrates were always kept at room temperature during the process. By using AFM morphology and roughness measurements we were able to show that this technique can be successfully used to improve the quality of organic thin films without increasing the substrate temperature. Moreover, due to the direct consequence that film morphology has on the device performances, OLEDs fabricated using substrate vibration during Alq3 and β-NPB depositions increased their luminance from 173 cd/m 2 to 352 cd/m 2 with respect to the same OLEDs fabricated in the

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conventional way in our laboratory. This corresponds to an increase of about 40% in power efficiency with a current density of 4 mA/cm 2. Acknowledgments The authors are grateful to the Brazilian agencies CNPq and FAPERJ for financial support. One of the authors (Y.A. Paredes) would like to thank CLAF for support and fellowship.

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