New luminescent polymers for LEDs

ELSEVIER

Synthetic

Met&

91 ( 1997) 279-252

Abstract Recent results are reported on the synthesis of statistical copolymers 5 and 9 derived from the highly luminescent silyl PPV derivative (DMOS-PPV) a,ith various alternative building blocks carrying side chains capable of supporting polyelectrolytes. These polymers are prepared by Gilch dehydrohalogenation polymerization. It is of interest to study the effect on charge transport in standard electroluminescent (EL) devices using these PPV copolymers. Moderately efficient conventional single-layer devices are obtained for copolymer 9. When fabricated in a light-emitting electrochemical cell (LEC) configuration (lithium triflate) without any extra supporting polymer electrolyte (e.g., poly( ethylene oxide), PEO), devices ~~UZYIon both 8 and 9 show loiver turn-on voltages and improved efficiency. 0 1997Elsevier Science S.A. K~~~rr,or~lr:Electroluminescentcopolymers;Liph-emitting

devices

1. Introduction Interest in the aynthesis of luminescent polymeric materials continues to grow as applications in light-emitting devices ( LEDs) become a realistic possibility. The major emphasis has been on materials with hi@ photoluminescence (PL) quantum yields. good processibility. improved charge-transporting properties and long operating lifetimes. The discovery of a new generation of light-emitting devices by Pei et al., with mobile ions incorporated into the active polymer layer. has resulted in the introduction of light-emitting electrochemical cells ( LECs) [ l-k]. These devices consist of two electrodes. and an active polymer layer blended with lithium triflate-doped poly( ethylene oxide) ( PEO) which facilitates ionic conductivity. The active polymer layer in the LEC configuration then emits light in the spectral range corresponding to its single-layer LED. The LECs have been reported to be highly efficient with low turn-on voltages. The use of a polymeric blend in these devices can result in phase separation between the emissive layer and the polyelectrolyte which, in turn, could be detrimental to device * Corresponding author. Tel.: t 14 1213 334 370: fax: + 11 1223 334 866; e-mail: abhl @cus.cnm.nc.uk ’ Prwent address: ETRI. Yusong PO Box 106. Txjon 305-600. South Korea. ’ Present addrss: LG Electronic5 Rexarch Center. 16, Woomyron-dong. Seocho-gu, Seoul 137-130, South Korea. 0379-6779/97/$17.00 0 1997 Elsevier PZZSO379-6779197iO1031-0

Science S.A. All rights reserved

performance. To overcome this problem, we considered the synthesis of emissive polymers which contain ion-transporting side chains [ 51. i.e., the homopolymers poly[ Z-methoxy5-( triethoxpmethoxy) -1,4-phenylene vinylene] (MTEMPPV) 1 and poly[ 2,5-bis( triethoxymethoxy) - 1.4-phenylene vinylene] (BTEM-PPV) 2 with polyether side chains attached to the aromatic ring of a PPV analogue. OpOjy3

f-6 RO

‘_‘

\‘i n

1;R=CH3 2; R = (CHzCH20)3CH3

i/ d &+$=J+ Me0 3;y=o 4; x, y # 0

We have recently reported the synthesis and the effects of silicon substitution on the luminescence properties of a new polymer, poly( 2-dimethyloctylsilyl1,4-phenylenevinylene) (DMOS-PPV) 3 [ 61. This polymer exhibited an external efficiency of 0.05% in a single-layer LED (IT0/3/Al)

280

layer LEDs and LECs fabricated with these copolymers are reported here.

OfOCH3 / KO’Bu, c(j=$“+

+

5; R = CH3 6; R = (CH2CH20)3CH3

3r&r

THF

6h

-

2. Results and discussion

0

All polymerizations were performed using KO%u in THF as solvent (Scheme 1) with the exception of the homopolymers 1 and 2, which were synthesized using NaH/DMF I.81 to maintain polymers in solution. A 1: 1 monomer feed ratio was used for the polymerization of the bischloromethyl oxygenated monomer with the bisbromomethyl silyl monomer. ‘H NMR spectroscopy showed that the copolymers 8 and 9 contained 45 and 37%, respectively, of the monomeric unit 7 (see Table 1). Films of the homopolymers 1 and 2 absorb at maximum wavelengths of 494 and 498 nm, respectively, in the UV-Vis and emit in the red (601 and 640 nm, respectively). They therefore resemble MEH-PPV, and exhibit even lower PL quantum efficiencies (0.6 and 8.8%, respectively). DMOSPPV 3 has an absorption maximum of 414 nm and emits at a maximum of 523 nm with an efficiency of 60% [ 61. The resulting copolymers S and 9 exhibit absorption and luminescence properties which are intermediate between those of the two homopolymer units, as shown in Table 2. The PL spectrum of thin films of S has two peaks in the orange-red region (644 and 598 nm), while the copolymer 9 has a max-

/J

&

/-0

RO

-/-

OCH3

ii-

Si.

a; R= CH3 9; R = (CH2CH20)3CH3

n

Scheme 1. Copolymcrization of the biachioromethpl monomers 5 and 6 with the biabromomethyl monomer 7 to yield the copolymers 8 and 9.

device. Its high turn-on voltage ( 15 V for observable light emission) may, however, be a disadvantage and hence copolymerization to combine 3 with poly [ 2-methoxy-5-( 2’ethylhexyloxy) - 1,4-phenylenevinylene] (MEH-PPV) has produced poly( DMOSPV-co-MEHPV) 4, which reduced the turn-on voltage 171. This was also accompanied by a decrease in the PL and electroluminescence (EL) (ITOI4I Al) efficiencies as the ratio of MEHPV increased. The observed change in the properties of the copolymers 4 inspired us to investigate the outcome of combining the highly luminescent properties of 3 with the ion-transporting ability of the homopolymers 1 and 2 to produce the copolymers 8 and 9 (Scheme 1). The results ofthe studies on singleTable 1 Characterization

of polymers

8 and 9

Polymer

Monomer ratio

feed

polymer composition h in:m)

S

5:7=

1:l

55145

9

6:7=

I:1

63:37

Actual

Polydispersity

Yield (%I

30

5.0

133

8.2

82 78

Ka i x 10”)

’ GPC in CHClj using polystyrene standards. b Composition determined from ‘H NMR spectroscopy. Table 2 Optical and electrical

properties

of copolymers

S and 9

Polymers

MTEM-PPV

UV L,, ’ (nm)

494

49x

454

PL emission maximum a (nm) PL efficiency (5%) Luminous efficiency (lm W - ’ ) h Turn-on voltages (V) Reduction onset potential ’ (V) Oxidation onset potential ’ (V)

601 0.6 very low

640 0.04

598 (644) 19 0.005

6

10.5

- 1.5 0.5

- 1.5 0.6

- 1.4 0.5

1

BTEM-PPV

2

8.8

’ Measurements on polymer thin films. h Efficiencies measured in the configuration ITO/polymer/Al. ’ Not measured: external efficiency 0.05% [9]. ’ Cyclic voltammetrp measurements werr performed on thin films of the polymers calibrated against ferrocene (FciFc’ 0.32 V), scan rate 20 mV s- ‘. The electrolyte (dried over CaH,).

MTEM-DMOS

PPV 8

BTEM-DMOS 4.58 598 (626) 14 0.2 4 - 1.5 0.6

PPV 9

DMOS-PPV

3

413 523 60 -c 15

- 1.6 1.1

on a platinum working electrode with Ag wire as the reference used was 0.1 M tetrabutplammonium perchlorate (Bu,NC104)

electrode in MeCN

1

1.5

2

2.5

3

3.5

Energy (eV) Fig. I. UV-Vis.

PL and EL hpcctra of BTEM-DMOS

PPV 9.

decrease in oxidation onset potentials indicates the expected stronger influence of the alkoxy unit versus the silyl unit. LEC devices in the configurations ITO/copolymer + LiOTf/Al were fabricated. using a blend of the copolymers and lithium triflate (LiOTf) (7-10s w/w to polymer) as the active layer. The blend was dissolved in chloroform/ cyclohexanone (5:l v/v) and spin-coated onto IT0 glass (thickness of 1500 A). The aluminium contact (about 200 nm in thickness) was deposited onto the polymer film by vacuum evaporation. Upon application of a bias voltage between the two electrodes, red-orange emission was observed for both 8 and 9. The turn-on voltages for the copolymers in LEC devices decreased compared with their respective single-1ayerLEDs. The turn-on voltages for the LEDs dropped to 2.5 V (MTEMDMOS PPV) and 2.8 V (BTEM-DMOS PPV) for the LECs (Figs. 2 and 3). The efficiency of a device made with copolymer 9 increased to 0.5 lm W- ’ with this LEC configuration? more than twice the efficiency of its corresponding LED (Table 2).

3. Conclusions 0

12

3

Voltage Fig. 2. I-C’-L PPV 9/A1.

characteristica

4

5

6

7

(V)

of a bingle-layer

Juice,

ITOIBTEM-DMOS

The combination of the MTEM-PPV 1 and BTEM-PPV 2 with the highly efficient DMOS-PPV 3 has yielded a group of statistical copolymers 8 and 9 with polyelectrolyte functionality. These now have higher PL and EL efficiencies compared with the homopolymers 1 and 2, thus showing promise as materials for applications in LEC devices.

Acknowledgements

OlO"y

Fig. 3. I-W’-L charxtcri~tics Al.

ofa LEC. ITOIBTEIM-DMOS

PPVY f LiOTfI

imum emission at 598 nm with a sharp shoulder at 626 nm (Fig. 1). The results show the influence of the silyl unit on the properties of the copolymers. Both S and 9 exhibit a marked improvement in PL and EL efficiencies, with betterperformnnce in their corresponding LED devices. The PL efficiencies of 19 and 13%. respectively, are comparable with that of MEH-PPV ( 15%) [ IO]. while the luminous efficiencies of the copolymers show a five-fold increase compared with their respective homopolymers (Table 2). The copolymers show similar electron affinity to the homopolymers. but the

We thank the Engineering and Physical Sciences Research Council (UK) (including the Swansea Mass Spectrometry Service), the Korea Science and Engineering Foundation, the British Council, the European Commission (ESPRITProject No. SO13 LEDFOS, BRITE-EURAM Contract BRE2-CT930592 PolyLED) and LG Electronics for supporting this work. The Cambridge Commonwealth Trust and the Committee of Vice-Chancellors and Principals are acknowledged for the award of an Overseas Research Studentship (B.S.C.).

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