Organic emissive materials and devices for photonic communication

POLYMERS FOR ADVANCED TECHNOLOGIES

Polym. Adv. Technol. 2004; 15: 75–80

Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/pat.443

Organic emissive materials and devices for photonic

communication

Hiroyuki Suzuki1*, Atsushi Yokoo2 and Masaya Notomi2

1NTT Photonics Laboratories, NTT Corporation, 3-1 Morinosato Wakamiya, Atsugi, Kanagawa, 243-0198, Japan2NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato Wakamiya, Atsugi, Kanagawa, 243-0198, Japan

Received 4 July 2003; Accepted 15 July 2003

This article describes our recent progress on the development of organic emissive materials and

devices for photonic communication applications. Luminescence properties in the infrared (IR)

region were examined for two types of organic emissive material, an organic rare earth complex

and an organic ionic dye, and their potential for use as emissive materials in photonic communica-

tion applications was discussed. The luminescence mechanism elucidated in this study suggested a

guideline for the future development of organic IR emissive materials. Novel organic emissive

devices were fabricated by combining organic emissive materials with various types of photonic

crystal structure utilizing nanotechnology. These devices utilize the advantages and unique proper-

ties of organic materials, and so signal the way towards the breakthroughs needed to realize emis-

sive devices for the photonic communication networks of the future. Copyright # 2004 John Wiley

& Sons, Ltd.

KEYWORDS: conjugated polymers; dyes/pigments; LEDs; luminescence; nanotechnology

INTRODUCTION

Organic electronics and nanotechnology are recognized as

the key areas from which future technological innovations

will arise to challenge the essential limitations of conven-

tional technologies. For instance, success in the development

of organic light-emitting diodes (LEDs) in the visible region

for display applications has demonstrated the potential of

organic materials for practical use. However, this means

that the development of organic emissive materials has so

far been focused on the visible region. Currently efforts are

being made to extend the emission wavelength of organic

materials to the infrared (IR) region in order to apply them

to photonic communication and information processing.1–7

This is because photonic communication networks are con-

structed using silica optical fibers that have two main win-

dows in the loss spectrum in the 1.3 and 1.55 mm

wavelength regions. Only a limited number of studies have

focused on organic IR emissive materials suitable for this

application. Most organic IR emissive materials reported to

date are organic complexes with trivalent rare earth ions

such as Er3þ, 2–4 Nd3þ 5,6 and Pr3þ.7 The electroluminescent

(EL) originates from their radiative 4f–4f transitions since

the EL wavelengths from these complexes are typical for

the rare earth ions they contain. However, since the f–f transi-

tion is in principle a parity-forbidden process, there remains

an essential challenge to improve EL quantum efficiency.

Therefore, it is necessary to try to find a novel class of organic

IR emissive materials, for instance, containing no rare earth

ions, in order to exploit the possibility of realizing organic

IR light-emitting devices. Ionic dyes are a class of materials

that can meet requirements as regards emission wavelength,

efficiency and lasing possibility because they are well known

as efficient laser dyes in the ultraviolet to IR region. The obser-

vation of near-IR EL at around 800 nm from an ionic laser dye,

LDS821 was recently reported.8

Since the successful development of organic LEDs,

scientific interest in the area of organic electronics has been

shifting towards organic solid-state lasers (SSLs).9,10 This is

because these two types of emissive device can share the

organic materials that have already been developed and

organic SSLs have a wider field of application. Efforts at

realizing organic SSLs are concentrating on the adoption of

various device structures. In this respect, photonic crystal

(PhC) structures, which are fabricated by using nanotechnol-

ogy, are particularly important since they can be introduced

directly into organic materials11 or hybridized with organic

materials.12,13 This relies on the advantages and unique

properties of organic materials in terms of processability and

softness.

In this article, recent progress on the development of

organic emissive materials and devices for photonic com-

munication applications are described. Two types of organic

material as possible emissive materials in the IR region are

investigated, an organic rare earth complex and an organic

ionic dye. In addition, novel light-emitting devices are

fabricated by combining two emerging technologies, organic

electronics and nanotechnology. This approach was realized

by incorporating PhC structures into organic emissive

materials, either by hybridization or nano-imprinting.

Copyright # 2004 John Wiley & Sons, Ltd.

*Correspondence to: H. Suzuki, NTT Photonics Laboratories,NTT Corporation, 3-1 Morinosato Wakamiya, Atsugi,Kanagawa, 243-0198, Japan.E-mail: [email protected]

EXPERIMENTAL

Organic IR emissive material developmentAs organic IR emissive materials, an EL organic Er3þ com-

plex, erbium(III) tris(8-hydroxyquinoline) (ErQ) and an ionic

dye, IR1051 were chosen.14 The structure of ErQ is shown in

Fig. 1(a), whereas that of IR1051 has not yet been published,

and only its molecular formula is known (C41H33BCl2F4N2).

The photoluminescence (PL) properties of vacuum-depos-

ited thin-films of ErQ were measured by using either an

Arþ laser (six wavelengths between 457.9 and 488.0 nm) or

a blue laser diode (wavelength: 405 nm) as an excitation

source. Spin-coated thin-films of poly(N-vinylcarbazole)

(PVK) (Aldrich, secondary standard) doped with IR1051 (1

wt%) was used together with 2-(4-biphenylyl)-5-(4-t-butyl-

phenyl)-1,3,4-oxadiazole (PBD) (30 wt%, Dojindo Labora-

tories, scintillator grade) for both the PL and EL

measurements. The PL spectra of IR1051 were measured

with a 980 or 1064 nm laser diode as an excitation source.

To fabricate the EL devices, an indium-tin-oxide (ITO) coated

on a glass substrate (20�/&) and a vacuum-evaporated Al

electrode as the hole and electron injecting electrode, respec-

tively were used. The luminescence characteristics in the IR

region were measured by using an InGaAs charge-coupled-

device (CCD) and an IR photomultiplier. All the measured

emission spectra were corrected for the spectral response of

the detection system used.

Organic emissive device developmentAll the emissive devices described here contain an organic

emissive material and a PhC structure. The lattice constant

of the PhC structures was in the 210–440 nm range, which

meant that the emission wavelength of the organic emissive

materials must be in the red region. A fabrication process sui-

table for the combined PhC structure is also an important fac-

tor when selecting organic emissive materials. The organic

emissive materials used in the present study were 4-dicyan-

methylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran

(DCM, exciton laser dye) and 3-hexylpolythiophene (P3HT,

molecular weight: 87 000). The structures of the two materials

are shown in Fig. 1(b) and 1(c). These were chosen from the

stock of conventional materials since the knowledge already

accumulated on their basic luminescence characteristics facil-

itates an understanding of the emission properties of the

devices that are under study. The PhC structures that were

adopted here are two-dimensional (2D) hexagonal SiO2

PhCs, alumina nanohole arrays and those fabricated by the

nano-imprinting technique. The procedures for the fabrica-

tion of these PhC structures have been reported

elsewhere.15–17 A DCM doped thin-film of tris(8-hydroxy-

quinoline) aluminum (AlQ) was prepared by the co-evapora-

tion technique on either 2D hexagonal SiO2 PhCs or Si

substrates. The PhC structure was then fabricated directly

in the DCM/AlQ film deposited on Si substrates by the

nano-imprinting technique. By contrast, P3HT was posi-

tion-selectively injected from a solution into an alumina

nanohole array to fabricate point defects acting as active cen-

ters. Photoexcitation was carried out with an N2 laser

(337 nm) for DCM/AlQ and a YAG/SHG laser (532 nm) for

P3HT. The emissions were detected with a CCD.

RESULTS AND DISCUSSION

Development of organic IR emissive materialsAlthough ErQ is reported to be EL, only preliminary studies

have been undertaken with respect to its PL characteristics.2

As a first step in the development of organic IR emissive

materials utilizing organic rare earth complexes, the PL prop-

erties of vacuum deposited ErQ thin-films were examined.18

Figure 2 shows the PL spectrum of ErQ thin-film in the IR

region (1000–1700 nm) when it was excited at 457.9, 488.0

and 514.5 nm. In this wavelength region, two PL bands peak-

ing at around 1.2 and 1.55 mm were observed. The origin of

these two PL bands is assigned to the 4S3/2! 4I11/2 and4I13/2! 4I15/2 transitions for the 1.2 and 1.55 mm bands,

respectively.19 The PL spectrum is markedly dependent on

the excitation wavelength, as shown in Fig. 2. The absorption

spectrum of the ErQ thin-film reveals that the dominant

photoexcited species change from Er3þ to organic ligands

(8-hydroxyquinoline) in accordance with the excitation

wavelength shift to the blue region. This means that the

photoexcited species are generated much more effectively

by the shorter wavelength excitation since the absorption of

Er3þ is forbidden whereas that of the organic ligand is an

allowed transition. In fact, the PL intensity at 1.55 mm was

enhanced by more than three orders of magnitude by chan-

ging the excitation wavelength from 514.5 to 405 nm. The

observed marked enhancement in the PL intensity at

1.55 mm indicates that the excitation energy transfer from

the lowest energy triplet excited state of the organic ligand

to the Er3þ via the electron exchange interaction plays an

essential role in the PL process.20 This ligand-sensitized PL

mechanism is unique to organic rare earth complexes and

can be utilized for enhancing the luminescence intensity ori-

ginating from the 4f–4f forbidden transitions of rare earth

ions. When excited at 514.5 nm the ErQ film exhibits very

weak PL at 1.55 mm. The main reason for this reduction is

the depopulation of the PL emitting 4I13/2 state caused by

excited state absorption (ESA). This was experimentally con-

firmed by the observation of up-converted PL in the ultravio-

let region.

Figure 1. The structures of the organic emissive materials

used in this study: (a) ErQ, (b) DCM and (c) P3HT.

Copyright # 2004 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2004; 15: 75–80

76 H. Suzuki, A. Yokoo and M. Notomi

Another potential organic emissive material in the IR

region is the ionic dye, IR1051.21 The fact that ionic dyes

contain no rare earth ions can be advantageous in terms of

improving the luminescence efficiency. The absorption and

EL spectra of IR1051 in PVK are shown in Fig. 3. The dye

exhibits an EL with two distinct peaks in the 0.9–1.2 mm

range. Although the wavelength range of the EL from IR1051

is unsuitable for use with silica optical fibers for long-range

optical communication, it is compatible with silicon-based

photocircuits, a new type of low-loss polymer optical fibers

(POF)22 and/or polymer waveguides23 making it possible to

realize active and/or passive optical devices for optical

information processing, communication, and interconnec-

tion. In addition, it was suggested that lasers emitting at

1.2 mm are promising for high-speed singlemode fiber local

area networks (LANs) of several Gbit/s.24,5 Figure 4 shows

the hypothetical band structure of the LED. This band

structure is based on the ionization potential and the bandgap

energy determined by spectroscopic and ionization potential

measurements. Figure 4 indicates that the doped IR1051

acted as an effective carrier trapping and radiative recombi-

nation center for the EL. The LED also exhibits broad visible

EL peaking at 544 nm, which shares about 60% of the total EL

intensity. It was concluded that the observed visible EL has its

origin not in the doped emissive dye but in PVK. The increase

in IR EL efficiency is an essential subject for future research

involving the optimization of the LED structure and the

combination of emissive and host materials.

This section is finalized by summarizing the advantages

and limits of the organic IR emissive materials reported to

date. Luminescence (PL and EL) in the 1.5 mm region has been

observed only in organic rare-earth complexes. There

Figure 2. The PL spectrum of a vacuum-deposited ErQ thin-film in the IR region (1000–

1700nm). The excitation wavelengths were 457.9, 488.0 and 514.5 nm from an Arþ laser.

Figure 3. The absorption and the EL spectra of IR1051 in PVK.

Organic emissive materials 77


remains, however, a vital challenge for improving their

luminescence efficiency despite the fact that the ligand-

sensitization scheme can provide a way to enhance it.

However, the development of organic emissive materials

containing no rare-earth elements can offer an alternative

way to meet the requirement for the luminescence efficiency.

However, they exhibit the luminescence of the wavelength

shorter than 1.3 mm.

Development of organic emissive devicesThe approach to realizing novel light-emitting devices is to

introduce PhC structures into organic emissive materials,

either by hybridization or nano-imprinting. The first example

is an organic PhC laser composed of a DCM/AlQ thin-film

vacuum-deposited on a silicon-based 2D PhC (lattice con-

stant: 210–440 nm),15 as shown in Fig. 5. In this laser, excited

states of DCM are generated by the photoexcited energy

transfer from the host AlQ when the DCM/AlQ film was

excited with an N2 laser. Distinct red lasing oscillations (las-

ing wavelength: 600–680 nm) from the doped DCM (see

Fig. 6) were detected, whose origin was assigned to the four

lowest photonic bandgaps (M1, K1, M2 and �1). A theoretical

consideration of the possible feedback mechanism at these

photonic bandgaps, together with the direct observation of

the in-plane lasing beam propagation, revealed that these las-

ing oscillations reflected the 2D nature of the organic PhC

Figure 4. The hypothetical band structure of the ITO/

IR1051:PBD:PVK/Al device.

Figure 5. The structure of an organic PhC laser composed

of a DCM/AlQ thin-film vacuum-deposited on a silicon-based

2D PhC.

Figure 6. The lasing spectra for hexagonal hole patterns

with lattice constants of (a) 380 nm and (b) 420 nm.

Figure 7. The fabrication procedure of an organic PhC laser

by imprinting periodic nanostructures directly into a DCM/AlQ

thin-film. The lattice constant is 400 nm.



laser. In particular, the lasing mode originating from the K1

point was due to a purely 2D feedback mechanism, unlike

that observed in conventional semiconductor distributed

feedback (DFB) lasers. The second example is another type

of organic PhC laser fabricated by imprinting periodic nanos-

tructures (lattice constant: 400 nm) directly into a DCM/AlQ

thin-film with the same composition.17 Figure 7 summarizes

the fabrication procedure. Red lasing oscillations from the

doped DCM at around 600 nm were observed. An interesting

feature of this organic PhC laser is that the threshold input

optical energy for the lasing exhibits a marked dependence

on the depth of the periodic structures and thus the magni-

tude of the pressure during imprinting. The nano-imprinting

technique relies on the uniqueness of organic materials in

softness, and so can be a way of best utilizing the advantages

of these materials. Our final example involved position-selec-

tively injecting a solution-processable p-conjugated emissive

polymer, P3HT, into an alumina nanohole array (lattice

Figure 8. The structure of an alumina nanohole array position-selectively injected

with P3HT to form point defects: (a) top view and (b) side view. The lattice constant is

300 nm.

Figure 9. The PL spectrum of P3HT position-selectively injected into an alumina nanohole

array, together with that of a spin-coated P3HT film.

Organic emissive materials 79


constant: 300 nm) to fabricate point defects acting as active

centers,16 as shown in Fig. 8. This technique also relies on

an advantage of organic materials; namely that they are solu-

ble in conventional solvents making it possible to use various

wet processes for the fabrication. Figure 9 shows the PL spec-

trum of P3HT position-selectively injected into an alumina

nanohole array, together with that of a spin-coated P3HT

film. A sharp emission (FWHM: 3 nm) was observed from

the P3HT/alumina nanohole array hybrid sample. This

sharp emission was confirmed to correspond to the defect

levels in a 2D PhC composed of the alumina nanohole array

on the basis of theoretical calculations. Although the organic

emissive materials that have so far been combined with PhC

structures exhibit the luminescence in the visible region, the

results described earlier are useful to extend the research in

the IR region. The suitable combination between an organic

IR emissive material and a PhC structure can be selected on

the basis of its processability and luminescence wavelength,

as carried out for organic visible emissive materials.

CONCLUSION

Recent efforts to develop novel types of organic emissive

materials and devices for photonic communication applica-

tions are described. The research on organic IR emissive

materials development is in its preliminary stage, and efforts

by a number of scientists will be needed to obtain materials

suitable for practical use. Note that the use of both the unique

properties of organic materials and nanostructures is a pro-

mising way to accelerate this research. The target of the cur-

rent efforts is straightforward, namely the realization of

organic IR lasers by combining organic IR emissive materials

with PhC structures. Further development both of organic

emissive materials and PhC structures will lead to the realiza-

tion of new types of laser and the clarification of the funda-

mental characteristics of PhCs.

AcknowledgmentsWe thank T. Tamamura, M. Nakao, H. Masuda, S. Hoshino

and Y. Nishida, K. Yuzawa, Y. Hattori, N. Matsumoto, Y.

Shimizu and M. Kumeda for their cooperation and helpful

discussions.

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Organic emissive materials and devices for photonic communication

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