Blue electroluminescence and dynamics of charge carrier recombinationin a vacuum-deposited poly(p-phenylene) thin ®lm

C.H. Leea,*, G.W. Kanga, J.W. Jeona, W.J. Songb, C. Seoulb

aDepartment of Physics, Inha University, 253 Yonghyun-dong, Nam-ku, Inchon 402-751, South KoreabDepartment of Textile Engineering, Inha University, 253 Yonghyun-dong, Nam-ku, Inchon 402-751, South Korea

Abstract

We have studied the electroluminescence (EL) and the dynamics of charge carrier recombination in the vacuum-deposited poly(p-

phenylene) (PPP) thin ®lm that shows a bright blue EL emission at about 450 nm. The current±voltage±luminescence (I±V±L) characteristics

are systematically studied in the temperature range between 12 and 325 K in the light-emitting devices of ITO/PPP/Al and the bilayer devices

with hole transporting layers of aromatic diamine (TPD) and poly(9-vinylcarbazole) (PVK). The EL quantum ef®ciency (QE) of ITO/PPP/Al

and ITO/PVK/PPP/Al is almost temperature-independent, indicating carrier injection via tunnelling mechanism. However, ITO/TPD/PPP/Al

shows the increasing QE with decreasing temperature and the power-law I±V±L dependence, characteristics of a space-charge-limited current

in a trap-®lled insulator. From the time delay between the onset of the voltage pulse and that of the EL we can estimate the charge carrier

mobility of m < 4 £ 1026 cm2/V s in ITO/PPP/Al and m < 5 £ 1025 cm2/V s in ITO/PVK/PPP/Al. q 2000 Elsevier Science S.A. All rights

reserved.

Keywords: Blue electroluminescence; Charge carrier recombination; Thin ®lm; I±V±L characteristics

1. Introduction

Organic electroluminescence (EL) devices become very

attractive for their potential applications in large-area, full-

color, ¯at-panel displays [1]. Since Tang and Van Slyke [2]

®rstly reported a highly ef®cient light-emitting diode (LED)

with vacuum-deposited organic thin ®lms, various light-

emitting organic or polymeric materials have been studied

[3±11]. As a blue-emitter, poly(p-phenylene) (PPP) has

gained signi®cant interest because of its high photolumines-

cence (PL) ef®ciency and good thermal stability [12].

Although PPP is a highly stable conjugated polymer, it is

infusible and insoluble so that it is not easy to fabricate a

thin ®lm [7]. Therefore, various synthetic approaches have

been directed towards soluble PPP precursors or derivatives

in order to use simple spin-coating method [7±9]. Another

approach is to use the vacuum deposition method. The

advantage of this technique is that contamination of PPP

by various impurities in the solvent can be minimized and

the ®lm thickness can be easily controlled. The structure,

morphology, and ¯uorescence of the vacuum-deposited PPP

®lm have been reported [13±15]. In this paper, we report a

successful fabrication of the blue EL devices using the

vacuum-deposited PPP ®lm. The current±voltage±lumines-

cence (I±V±L) characteristics are systematically studied in

the single-layer ITO/PPP/Al device and the double-layer

devices with hole transporting layers of N,N 0-diphenyl-

N,N 0-bis(3-methylphenyl)-[1,1 0±biphenyl]-4,4 0-diamine

(TPD) and poly(9-vinylcarbazole) (PVK). We also carried

out the transient EL experiment to study the dynamics of

charge carrier recombination and the carrier mobility.

2. Experimental

Poly(p-phenylene) used in this study was synthesized by

following the Yamamoto method [16] and the details of

synthesis and characterization of PPP was reported else-

where [17]. The PPP thin ®lm was deposited onto the ITO

substrate with a sheet resistance of about 10 V/A, supplied

by Samsung Corning Co., Ltd., by heating the PPP powder

to 5008C under a vacuum of about 2 £ 1026 Torr. The thick-

ness of the vacuum-deposited PPP ®lm was controlled

approximately 100 nm. From the IR analysis, we found

that the average phenylene chain lengths of the parent

PPP powder and the vacuum-deposited PPP ®lm were

about n � 27±28, i.e. a molecular weight of about 2000,

and n � 8±9, respectively [14,17]. After the deposition of

the PPP layer, Al electrodes was deposited with the active

Thin Solid Films 363 (2000) 306±309

0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved.

PII: S0040-6090(99)01023-8

www.elsevier.com/locate/tsf

* Corresponding author.

area (the overlap of ITO and Al) of about 4 mm2. For ITO/

TPD/PPP/Al we successively deposited TPD, PPP, and Al

under vacuum. For ITO/PVK/PPP/Al, we spin-coated the

PVK layer before the vacuum-deposition of PPP.

The EL devices were mounted in the vacuum cryostat

(Janis CCR-150), and the I±V±L characteristics were

measured in the temperature range between 12 and 300 K

using a Keithley 2400 SourceMeter and a Keithley 2000

multimeter equipped with a calibrated Si photodiode. The

temporal response of the EL upon the application of the

rectangular voltage pulses using a pulse generator (HP

214B) was detected by a PMT through an ARC 275 mono-

chromator. Both the applied voltage pulse and the transient

EL signal were simultaneously digitized with a Tektronix

TDS 644B digital storage oscilloscope. The EL and PL

spectra were measured with a Shimadzu Spectro¯uorophot-

ometer (RF-540). The absorption spectra of PPP thin ®lms

deposited onto quartz substrates were measured with a

Scinco S-2140 UV±vis Spectrophotometer.

3. Results and discussions

Fig. 1 compares the PL spectrum of the parent PPP

powder with the PL and optical absorption spectra of the

vacuum-deposited PPP ®lm. The chemical structure of PPP

is also shown. The optical absorption spectrum shows a

peak at about 314 nm, corresponding to the p±p * interband

transition of PPP. The PL spectrum of the vacuum-deposited

PPP shows narrower full width at half maximum (FWHM)

compared with that of the PPP powder. In addition, the PL

spectrum of the vacuum-deposited PPP exhibits stronger PL

intensity at shorter wavelengths. The results are attributed to

a narrow distribution of shorter conjugation lengths in the

vacuum-deposited PPP, compared with the PPP powder,

consistent with the IR analysis [17].

Fig. 2 shows the EL spectra of ITO/PPP/Al, ITO/TPD/

PPP/Al and ITO/PVK/PPP/Al devices under the bias of 12,

25 and 21 V at room temperature, respectively. All the

spectra are normalized at the peak wavelength. Both ITO/

PPP/Al and ITO/TPD/PPP/Al show an EL emission peak at

446 nm with well-resolved vibronic structures. Since both

EL spectra are very similar with the PL spectrum of PPP, the

EL emission in ITO/TPD/PPP/Al originates from the PPP

layer. However, the EL spectrum of ITO/PVK/PPP/Al

shows a broad shoulder around 550 nm as well as the

peaks at the shorter-wavelength region originating from

PPP. This longer-wavelength shoulder in ITO/PVK/PPP/

Al suggests the exciplex formation at the interface between

PVK and PPP. We noticed that the shoulder around 550 nm

was more pronounced at the low bias voltage and the EL

peaks originating from PPP become stronger with increas-

ing bias voltages. This behavior is consistent with the exci-

plex formation at the interface between PVK and PPP

because the increase in the bias voltage leads to an increase

in the number of injected holes and electrons in the bulk of

the PPP layer. Therefore, the carrier recombination in the

bulk of the PPP layer becomes larger at the high bias

voltages compared with that at the interface. Similar exci-

plex formation was reported at the interface between tris(8-

quinolinolato)aluminium (Alq3) and the hole transporting

layer [18].

The I±V±L characteristics of ITO/PPP/Al, ITO/TPD/PPP/

Al and ITO/PVK/PPP/Al devices at room temperature are

compared in Fig. 3. The I±V characteristics show the recti-

fying behavior with the forward bias de®ned as positive

voltage applied to the ITO electrode. The inset shows the

luminescence±current (L±I) characteristics of the same data.

The EL intensity increases linearly with the current in all

C.H. Lee et al. / Thin Solid Films 363 (2000) 306±309 307

Fig. 1. The PL spectra of the parent PPP powder (dotted) and the PL (thick

solid line) and optical absorption spectra (thin solid line) of the vacuum-

deposited PPP thin ®lm. The chemical structure of PPP is also shown.

Fig. 2. The EL spectra of ITO/PPP/Al (solid) and ITO/TPD/PPP/Al

(dotted) and ITO/PVK/PPP (dot-dashed line) devices under the bias of

12, 25, and 21 V at room temperature.

devices and the slope of the L±I curve is proportional to the

external quantum ef®ciency (QE) of the EL. The single-

layer ITO/PPP/Al device turned on at approximately 6 V

with an external QE of about 0.02%. The insertion of the

hole-transporting layer in double-layer devices increases the

QE, but it also increases the EL onset voltages. The QE

values of ITO/TPD/PPP/Al and ITO/PVK/PPP/Al devices

are about 0.07% and 0.15% at room temperature, respec-

tively.

Fig. 4 shows the temperature dependence of the QE in

ITO/PPP/Al, ITO/TPD/PPP/Al and ITO/PVK/PPP/Al. The

QE is almost temperature independent in both ITO/PPP/Al

and ITO/PVK/PPP/Al devices below room temperature,

implying that the tunnelling mechanism plays a dominant

role in the carrier injection. We found that the Fowler±

Nordheim tunnelling formula was found to ®t well the I±V

data of both devices [19]. However, the QE of the ITO/TPD/

PPP/Al device increases with the decreasing temperature.

We also observed that the I±V±L dependence of the ITO/

TPD/PPP/Al device followed the power law dependence,

I / Vm11, m � 1±2 at low voltage and m � 4±6 at high

voltage [19]. It is the characteristics of a space-charge-

limited (SCL) current in a trap-®lled insulator [20].

Finally, we carried out the transient EL experiment in

ITO/PPP/Al and ITO/PVK/PPP/Al devices in order to esti-

mate the charge carrier mobility. Fig. 5 displays the EL time

delay t between the onset of the EL and the voltage pulse as

a function of 1/V in ITO/PPP/Al and ITO/PVK/PPP/Al

devices. The EL time delay is larger in ITO/PPP/Al than

that in ITO/PVK/PPP/Al. This time delay results from the

charge carrier transport towards the recombination zone.

Since the carrier mobility is m � d2=tV where d is the

®lm thickness, m is smaller in PPP compared with PVK.

From the slope in Fig. 5, the effective carrier mobility was

calculated to be m < 4 £ 1026 cm2/Vs in ITO/PPP/Al and

m < 5 £ 1025 cm2/Vs in ITO/PVK/PPP/Al. The mobility

estimated in ITO/PVK/PPP/Al is similar to the hole mobi-

lity measured from the time-of-¯ight photoconductivity

[21]. Thus, the observed time delay in ITO/PVK/PPP/Al

is the transit time of the injected holes across the PVK

layer reaching the PVK/PPP interface where the EL emis-

C.H. Lee et al. / Thin Solid Films 363 (2000) 306±309308

Fig. 4. The temperature dependence of the external EL quantum ef®cien-

cies in ITO/PPP/Al (triangle), ITO/TPD/PPP/Al (circle), and ITO/PVK/

PPP/Al (square).

Fig. 5. The EL time delay measured in the ITO/PPP/Al (open circle) and

ITO/PVK/PPP/Al (solid circle) as a function of 1/V (circle). The solid lines

are the slopes of the straight-line approximation to the data.

Fig. 3. The I±V (solid) and L±V (dotted) characteristics of ITO/PPP/Al

(left), ITO/TPD/PPP/Al (middle), and ITO/PVK/PPP/Al (right data set)

devices at room temperature. The inset shows the L±I characteristics of

the ITO/PPP/Al (bottom), ITO/TPD/PPP/Al (middle), and ITO/PVK/PPP/

Al (top) devices.

sion begins, as evidenced from the EL emission due to the

exciplex formation at the interface shown in Fig. 2.

4. Conclusions

We have presented a comprehensive study of the electri-

cal and optical characterization of the blue LEDs fabricated

with the vacuum-deposited PPP thin ®lm. The EL spectra of

both ITO/PPP/Al and ITO/TPD/PPP/Al devices are very

similar to the PL spectra with the blue EL emission peak

at 446 nm and well-resolved vibronic structures. However,

the EL spectrum of ITO/PVK/PPP/Al shows a broad

shoulder around 550 nm as well as the EL peaks originating

from PPP, suggesting the exciplex formation at the interface

between PVK and PPP. The ITO/PPP/Al LED turned on at

approximately 6 V with an external QE of about 0.02%. The

insertion of the hole transporting layer of TPD and PVK

enhances the QE up to about 0.07 and 0.15% at room

temperature, respectively. The QE of both ITO/PPP/Al

and ITO/PVK/PPP/Al is almost temperature-independent,

indicating carrier injection via the tunnelling mechanism.

However, ITO/TPD/PPP/Al shows the increasing QE with

decreasing temperature and the power-law I±V dependence,

characteristics of a space-charge-limited current in a trap-

®lled insulator. The time delay between the onset of the EL

and the voltage pulse is attributed to the transit time of

holes. The hole mobility is estimated to be m < 4 £ 1026

cm2/V s in ITO/PPP/Al and m , 5 £ 1025 cm2/V s in ITO/

PVK/PPP/Al.

Acknowledgements

This work was supported by the Korea Science and Engi-

neering Foundation (KOSEF) and the 1998 Inha University

Research Fund.

References

[1] J.R. Sheats, H. Antoniadis, M. Hueschen, et al., Science 273 (1996)

884.

[2] C.W. Tang, S.A. Van Slyke, Appl. Phys. Lett. 51 (1987) 913.

[3] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, et al., Nature 347

(1990) 539.

[4] D. Braun, A.J. Heeger, Appl. Phys. Lett. 58 (1991) 1982.

[5] C. Adachi, T. Tsutsui, S. Saito, Appl. Phys. Lett. 57 (1990) 531.

[6] Y. Ohmori, M. Uchida, K. Muro, K. Yoshino, Jpn. J. Appl. Phys. 30

(1991) L1941.

[7] G. Grem, G. Leditzky, B. Ullrich, G. Leising, Adv. Mater. 4 (1992)

36.

[8] Y. Yang, Q. Pei, A.J. Heeger, J. Appl. Phys. 79 (1996) 934.

[9] G. Leising, S. Tasch, F. Meghdadi, L. Athouel, G. Froyer, U. Scherf,

Synth. Met. 81 (1996) 185.

[10] A.W. Grice, D.D.C. Bradley, M.T. Bernius, M. Inbasekaran, W.W.

Wu, E.P. Woo, Appl. Phys. Lett. 73 (1998) 629.

[11] Z. Gao, C.S. Lee, I. Bello, S.T. Lee, R.-M. Chen, T.-Y Luh, J. Shi,

C.W. Tang, Appl. Phys. Lett. 74 (1999) 865.

[12] J. Stamp¯, S. Tasch, G. Leising, U. Scherf, Synth. Met. 71 (1995)

2125.

[13] M. Komakine, T. Namikawa, Y. Yamazaki, Makromol. Chem. Rapid

Commun. 7 (1986) 139.

[14] K. Miyashita, M. Kaneko, Synth. Met. 68 (1995) 161.

[15] S. Kobayashi, Y. Haga, Synth. Met. 87 (1997) 31.

[16] T. Yamamoto, Y. Hayashi, A. Yamamoto, Bul. Chem. Soc. Jpn. 51

(1978) 2091.

[17] C. Seoul W. J. Song, (1999) in press.

[18] K. Itano, H. Ogawa, Y. Shirota, Appl. Phys. Lett. 72 (1998) 636.

[19] C.H. Lee, G.W. Kang, J.W. Jeon, W.J. Song, C. Seoul, (1999) in

press.

[20] P.E. Burrows, S.R. Forrest, Appl. Phys. Lett. 64 (1994) 2285.

[21] D.M. Pai, J. Chem. Phys. 52 (1970) 2285.

C.H. Lee et al. / Thin Solid Films 363 (2000) 306±309 309