Time-resolved photoluminescence study of PPV derivatives with electron — and hole — transporting moieties.

ELSEVIER

Synthetic Metals

102 (1999)

961-962

Time-resolved photoluminescence study of PPV derivatives with electron - and hole - transporting moieties. Yong Hee Kim”, Sae Chae Jeoung’, Dongho Kim”‘, Sung - Jae Chungb and Jung - I1 Jinb ‘Spectroscopy

Lab., Korea Research Institute of Standards and Science, Taejon 305-600, Korea

bDepartment of chemistry and Advanced Material Chemistry Research Center, Korea University,

Seoul 136-70 1, Korea

Abstract The decay dynamics of photoexcited poly[(Z carbazol)-p-phenylene-vinylene](PCzPV) and poly[(2-(4-biphenylyl)-5-(4-tert-butylphenyl) - 1,3,4 - oxadiazole) -p-phenylene- vinylene](PPPDPV) were investigated. The cw photoluminescence spectrum of PCzPV is composed of well-resolved two emission bands and a weak but apparent emission band at much lower energy. On the other hand, PPPDPV exhibits a broad and featureless emission band. The photoluminescence decay PPPDPV is almost independent of excitation energy and temperature. However, when the side chain substituent is changed to carbazole, the overall decay dynamics strongly depend on the excitation wavelength as well as temperature. The decay time constant at 580 nm of photoexcited PCzPV is shortened with a change in the excitation energy from 400 nm to 300 nm at 13 K. These results in the PCzPV and PPPDPV are interpreted in terms of the electronic properties of substituent and excimer process between substituent and main chain Keywords:

Poly(phenylene

vinylene) derivatives, Time-resolved

fast spectroscopy,

1. Introduction

which acts as a PL quenching

Conjugated polymers have drawn much attention because of their feasibility in tuning electrical and optical properties. One of the most exciting applications is the electroluminescent devices based on PPV’. It is well known that the photoluminescence (PL) and electroluminescence (EL) properties of PPV and its derivatives provide new opportunities for the development of light emitting diodes based on the organic polymer films and the PL and EL in PPV are originated from the same excitons’. Therefore, Photoluminescence measurements help us to understand the photophysics of electroluminescent polymeric systems. In order to achieve high EL efftciency, it is important to maintain the balance between electron- and hole- injection rates from the opposite electrodes into the devices. In recent years, many investigators have employed the hole- or electrontransport layers’ between the anode or cathode and emitting polymer to obtain high EL efficiency and confirmed its high EL efficiency. Chung et al.4 have designed a new copolymer (PPPDPV and PCzPV) possessing a butyl PBD (electron transporting moiety) and carbazole (hole transporting moiety)units as a pendent group, not used as a layer. They reported that the EL efftciency of PPPDPV is higher than those To elucidate the reason for the high EL of PPV and PCzPV. efficiency in PPPDPV, it is necessary to investigate the exciton we have investigated the decay relaxation pathway. Thus, dynamics of photoexcitations in PCzPV and PPPDPV by using the time-resolved PL. It was found that PCzPV forms an excimer state through the interaction of carbazole with PPV main chain,

2. Experimental

0379-6779/99/$ - see front matter PII: SO379-6779(98)00995-3

0 1999

Elsevier

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state.

Section

O P--J@Q’ PCZPV

PCzPV and PPPDPV were synthesized through the Gilch and Wheelwright method which was previously reported3 .The steady state PL spectra were recorded with an excitation at 325 nm by using a cw He:Cd laser. Fluorescence lifetimes were measured by a time-correlated single photon counting (TCSPC) method The excitation source is a cavity-dumped picosecond dual-jet dye laser synchronously pumped by a mode-locked argon ion laser. To excite the sample, the dye laser pulse was frequencydoubled by a /3-BBO (bariumborate) crystal. As for the excitation at 400 nm, the second harmonic from femtosecond Ti:sapphire laser was employed. 3. Results and Discussion Fig. 1 displays the ground - state absorption and PL spectra of PCzPV and PPPDPV. The absorption edges of PCzPV(510 nm) and PPPDPV(505 nm) are slightly blue-shifted as compared reserved.

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Y. H. Kim et al. / Synthetic

I .oo

0.00

300

350

400

450

500

550

Wavelength

600

650

(nm)

Fig. 1. Absorption spectra of PCzPV (A) and PPPDPV (a), and PL spectra of PCzPV (B: 13 K, C: 295 K) and PPPDPV (b: 13 K, c: 295 K)

= 400

“Ill

3

.

probe 7,

-

rise

520 100

-0.5

0.0

F : _

7,

sl .c

.

T, = 160

2

_ :

2

.

.

250

ps

(17%);

(no %), lZ = 430

ps

(

1 .o

1 tb) i.n=

“Ill

%),

probe

= 520

= 140 probe

r2 =

nm

ps

0.5

CD 0 =

2

(83

ps

580

= 138

_

310

ps

= 25

probe z,

nm

ps =

1.5

2.0

nm

(49%), 530

ps

B%),

T2 = 330

ps

(51%)

T’? = 450

ps

(15

:

nm

(85%),

X)

.

-1 P

-0.5

0.0

0.5 Time

1 .o

1 .s

2.0

(ns)

Fig. 2. Photoluminescence decay profile of PCzPV at 13 K with an excitation at 400 (a) and 3 10 nm (b). with the PPV(535 nm) and the absorption maxima of pendent groups are slightly red-shifted as compared with the monomer absorption bands. It is likely that there is a weak interaction between PPV main chain and pendent group. The PL spectra of PPPDPV exhibit a broad, featureless feature and show little temperature dependence. However, in the case of the PCzPV, the overall PL intensity increases with a peak shift to lower energy and the emission band at 580 nm becomes distinct as the temperature decreases. Fig. 2(a) shows the photoluminescence

Metals

102 (1999)

961-962

decay profiles of PCzPV measured at 520 and 580 nm following the excitation at 400 nm at 13 K. The overall PL decay with an excitation at 400 nm is faster at higher energy side at both temperatures. Also the PL decay profiles at lower energy side at 13 K exhibit an apparent rise component (r = -30 ps) while absent at 295 K (data not shown). However, the luminescence decay with an excitation at 3 10 nm, which mainly excites the carbazole unit (Fig. 2(b)) is faster at lower energy side unlikely with 400 nm excitation and does not show any rise component. Unlike PCzPV, the decay profiles PPPDPV with 400 nm excitation not exhibit any rise component even at 13 K and show the similar decay curve without regard to excitation wavelength and temperature (data not shown). These results suggest that the decay dynamics of PPPDPV are different from PCzPV. To explain the above results, we should consider the molecular structure of PCzPV and PPPDPV. Since the steric hindrance between hydrogen atom on PPV main chain and pendent group is higher in PCzPV than in PPPDPV, the rotation of carbazole moiety is prohibited especially at 13 K and the energy transfer from the carbazole to main chain is hindered. As a result, the charge separation takes places at 13 K and consequently the inter-chain excimer is formed. It is suggested that 580 nm emission band originate from the excimer state attained directly by 310 nm excitation while through barrier by 400 nm excitation and this barrier is lowered at 295 K. Since the PBD group in PPPDPV is less bulky than the carbazole group, the steric hindrance is weak as compared with the carbazole group in PCzPV. Therefore, the PBD group in PPPDPV is likely to be located in plain with the main chain, which makes the energy transfer from PBD to PPV takes place more easily even at 13 K and the possibility of inter-chain excimer formation is very low. These results are consistent with the steady-state PL, in which we could not observe any additional emission band even at 13 K. Also, the luminescence decay times of PPPDPV are longer than that of PCzPV. It is because that the PBD group in PPPDPV acts as an electron donating group while carbazol is a hole transporting group, the electron injection from PBD group are achieved efficiently and thus the electron- and hole- injection rates becomes balanced. By the same reason the neutral singlet excitons are produced more efficiently in PPPDPV as compared with PCzPV. These results consistent with Chung et aL4 in which PPPDPV shows the higher electroluminescence efftciency compared with PCzPV and PPV. In conclusion, our results show that the excimer state is formed directly in PCzPV polymeric system with an excitation at 300 nm. These seem to be an energy barrier to form the excimer state with an excitation at 400 nm. The excimer state acts as a quenching state in PCzPV and this behavior is one of the reasons for the low EL efficiency of PCzPV as compared with PPPDPV. Acknowledgment. This work has been financially supported by Creative Research Initiatives (DK) and Star Project (SCJ) of the Ministry of Science and Technology of Korea and the Korea Science and Engineering Foundation (JIJ). Reference [I] [2] [3] [4]

P. L. Burn et al., Nature (London) 256, 47 (1992) J. H. Burroughes et al., Nuture(London) 347, 539 (1990) A.R. Brown et al., Appl. Phys. Lett., 61, 2793 (1992) S.-J. Chung et al., Adv. Muter., in press