Reactive & Functional Polymers 45 (2000) 183–188www.elsevier.com/ locate / react

Dispersive-transport-controlled luminescence in theheterostructure of ladder-type poly(p-phenylene)

*W.L. Wang , K.J. Liao, S.X. WangDepartment of Applied Physics, Chongqing University, Chongqing 400044, P.R. China

Received 4 January 2000; received in revised form 12 April 2000; accepted 13 April 2000

Abstract

The electroluminescence (EL) and photoluminescence (PL) spectra in the heterostructure of the ladder-type poly(p-phenylene) were investigated. The microcavity effect could be achieved by adjusting the organic layer between the aluminumand gold electrode. The temperature dependence of the EL and PL showed that there are different carrier recombinationprocesses in amorphous organic materials. The thermalized model was used to describe the temperature dependence ofphotoluminescence. The difference between the EL and PL spectra can be explained by dispersive-transport-controlledrecombination. 2000 Elsevier Science B.V. All rights reserved.

Keywords: Ladder-type poly(p-phenylene); Photoluminescence; Electroluminescence; Recombination

1. Introduction ence (EL) layer lost 50% of its initial ELintensity in 100 h when operated in dry Ar at a

2Recently, conjugated polymers have been drive current 1 of 5 mA/cm [6]. A later reportstudied extensively for their electrical conduc- indicated that EL devices based on vapor-sub-tivity, optical and electroluminescent properties limed Alq thin films were relatively stable, withdue to their importance for future applications a luminance half-life of about 4000 h for ain efficient light-emitting devices [1–5]. How- multilayer EL device with an undoped Alqever, there are still several problems concerning emitter [7]. Recently, a highly stable EL devicethe stability and reproducibility of polymeric has been demonstrated using N,N-di-light-emitting device fabrication. Early inves- methylquinacridone (DMQA)/Alq as the dopedtigations indicated that organic light emitting emitter. A luminance half-life of the order ofdevices (OLED) have a limited lifetime. An about 7500 h has been achieved [8], which hasOLED made using tris(8-hydroxyquinoline) already approached to the practical applicationaluminum (Alq ) as the active electroluminesc- level. But the differences between photo-3

luminescence (PL) and EL spectra, which havebeen observed frequently, are not fully under-

*Corresponding author. Tel.: 186-23-6510-3490; fax: 11-86-stood.23-6510-3490.

E-mail address: wanluw@cqu.edu.cn (W.L. Wang). In this paper, the PL and EL spectra in

1381-5148/00/$ – see front matter 2000 Elsevier Science B.V. All rights reserved.PI I : S1381-5148( 00 )00024-9

184 W.L. Wang et al. / Reactive & Functional Polymers 45 (2000) 183 –188

ladder-type poly(p-phenylene) (LPPP) were in-vestigated. The experimental studies showed thedifference between PL and EL can be explainedby dispersive-transport-controlled nongeminaterecombination in trap-rich materials.

2. Experimental

The microcavity light emitting devices werefabricated based on a LPPP and Alq. The Fig. 2. Structure of light emitting devices.

chemical structures of LPPP and Alq are shownin Fig. 1. The ladder structure of LPPP with theribbon-type macromolecules exhibits a higher sion, Alq was used as the electron transporter.3

thermal and chemical stability than their linear Gold and aluminum were the hole and electronanalog [9]. LPPP shows a broad PL and EL injecting contacts. Alq organic layers were3

25emission with a maximum at 560 nm, allowing prepared in a vacuum chamber (about 10tunability over the full visible region. Light- Torr) by vapor deposition using resistivelyemitting devices with emission in the blue, heated tantalum boats. The substrates were thegreen or red spectral region have been fabri- TiO –fused-silica plate. The LPPP thin films2

cated on LPPP [10]. were fabricated by spin coatings from a solutionThe structure of the light-emitting devices is of LPPP in toluene. The concentration of the

shown in Fig. 2. The emitting layer was LPPP solution was about 10–25 mg/ml, and spinningwith hole transportation. To enhance light emis- speed ranged from 1000 to 2500 rpm. The Al

and Au electrodes were deposited by the vaporevaporation. The thickness was 25 mm for thegold electrode and 56 nm for the aluminumelectrode. Devices of Au–LPPP–Alq –Al–3

TiO –fused-silica configuration were thus ob-22tained. The active areas were about 4 mm .

3. Results and discussion

Fig. 3 shows normalized electroluminescencespectra of three Au–LPPP–Alq –Al–TiO –3 2

fused silica with LPPP layer thicknesses of 40,200 and 100 nm; the thickness of Alq was3

about 20, 50 and 50 nm, respectively. Theemission maximal peaks of the devices locatedat 449, 583 and 640 nm, respectively. Theemission peaks have been achieved simply byadjusting the organic layer thickness. The spec-tra of the emission were redistributed by thefundamental microcavity mode.

Fig. 1. Molecular structures of LPPP (a) and Alq (b). Fig. 4 shows the electroluminescence and

W.L. Wang et al. / Reactive & Functional Polymers 45 (2000) 183 –188 185

Fig. 3. Normalized electroluminescence spectra of three Au–LPPP–Alq –Al with different organic layer thickness: (a) LPPP: 40 nm; Alq :3 3

20 nm; (b) LPPP: 200 nm; Alq : 50 nm; (c) LPPP: 100 nm; Alq : 50 nm.3 3

Fig. 4. EL and PL spectra with total organic layer thickness of 150 nm for an excitation wavelength of 420 nm; (a) PL, (b) EL.

photoluminescence spectra with total organic for PL and EL, respectively. There is a ratherthickness of 150 nm for an excitation wave- broad band for PL spectrum. These values are inlength of 420 nm at the same sample spot. The good agreement with the theoretical value of 70full width at half-maximum was 80 and 69 nm nm estimated by the following relation [11].

186 W.L. Wang et al. / Reactive & Functional Polymers 45 (2000) 183 –188

]]2 dependence. The test samples were the same as1 2 R Rl œ 1 2]]] ]]]]Dl 5 ? (1) that in Fig. 4 The emission peak was shifted to1 / 4

p(R R )2On d 1 2i i low energy with increasing temperature. Thepeak value was about 1.4 eV at 300 K.

where d is the effective cavity length, n is the Fig. 6 compares the luminescence peak ener-i i

real refractive index in the cavity, R and R are gies as a function of temperature between PL1 2

the reflectivity of the mirrors. and EL peak in the same samples. The EL peakFig. 4 shows that the emission peaks of EL energy temperature dependence is weaker in the

and PL are centered at 640 and 630 nm, temperature range of 150–300 K than that ofrespectively. There is a remarkable difference the PL.between the EL and PL emission peaks at the In amorphous organic materials, continuallysame sample and spot. The difference can be distributed localized states have a significantexplained by dispersive-transport-controlled re- effect on the carrier recombination and transportcombination [12]. process. The carrier transport in disordered

The photoexcited electron–hole pair in the materials can be described by a multiple-trap-PL process occurred at the same space sites, ping mode [13,14], in which the carriers arewhich the carrier recombination was indepen- frequently trapped in the defect and released bydent of the carrier transport. However, electrons thermal excitation. In the model, the averageand holes were injected to the emitter layers trap energy of the carriers is given by thefrom opposite electrodes in the EL process. demarcation level energy ED

Thus, carrier recombination energy was closely E 5 KT ln (n t) (2)D 0dependent on the carrier transport process.Fig. 5 shows the EL spectrum temperature where n is the attempt-to-escape frequency for0

Fig. 5. EL spectrum temperature dependence with organic layer thickness of 150 nm.

W.L. Wang et al. / Reactive & Functional Polymers 45 (2000) 183 –188 187

Fig. 6. Comparison of the luminescence peak energy as a function of temperature between PL and EL peak energy with organic layerthickness of 150 nm. (a) PL, (b) EL.

excitation out of the trapping site, K is port level. Thus, the lower EL peak energy andBoltzmann constant, and t is the carrier radiate its weaker temperature dependence can be ex-recombination lifetime. plained in terms of dispersive transport-con-

The PL peak energy shift with the tempera- trolled recombination [15].ture and time can be given by the thermalizationmodel [15]

4. ConclusionsE (T ) 5 E (o) 2 E (T ) (3)P P D

The electroluminescence and photoluminesc-ence based on the hole transporter LPPP andE (t) 5 E (o) 2 E (t) (4)p p Dlight emitter, and the electron transporter tris(8-

According to Eqs. (2)–(4), PL peak energy was hydroxyquinaline) was investigated. The mi-decreased with increasing temperature due to crocavity effects could be achieved by adjustingthe thermalization. However, in the case of EL, the organic layer between the electrodes. Thethe dependence of the EL peaks on temperature experimental results showed that the differencecannot follow the thermalization, whereas the between the electroluminescence and the photo-carrier transport played a role in the EL tem- luminescence spectra could be explained byperature dependence. The carrier transport phe- dispersive-transporto-controlled nongeminatenomena was in accordance with the multiple- recombination.trapping model at high temperature, and thehopping transport must be considered at lowertemperature. The carrier-hopping rate was much Acknowledgementsfaster than the recombination rate. Accordingly,the carriers first hop down to their transport The authors gratefully acknowledge the kindenergy level and circle many times through the donation of the LPPP samples used in theseorganic layer, then recombine from their trans- studies by Dr. Ulrich Scherf. Financial support

188 W.L. Wang et al. / Reactive & Functional Polymers 45 (2000) 183 –188

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