Near-ultraviolet electroluminescence from polysilanes

Hiroyuki Suzuki*, Satoshi Hoshino, Chien-Hua Yuan, Michiya Fujiki, Seiji Toyoda1,Nobuo Matsumoto

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

Abstract

We report the electroluminescent (EL) characteristics of a new class of polymeric material, polysilanes, which were employed in light-

emitting diodes (LEDs) as an emissive material. In contrast to the LEDs utilizing p-conjugated polymers and small molecules that have been

reported to date, LEDs made from polysilanes exhibit EL in the near-ultraviolet (NUV) or ultraviolet (UV) region due to their s-conjugation.

Three types of polysilanes, dialkyl, monoalkyl-aryl and diaryl polysilanes, have been used as the emissive material, together with an indium-

tin-oxide (ITO) and metal electrode for the injection of holes and electrons, respectively. The LED characteristics were observed to depend

strongly on the chemical, optical and electronic properties of the emissive polysilanes. The development of emissive polysilanes has led to

the successful fabrication of single-layer LEDs which emit NUV light at 407 nm (3.05 eV) with a quantum ef®ciency of 0.1% photons/

electron at room temperature. q 1998 Elsevier Science S.A. All rights reserved.

Keywords: Polymer light-emitting diodes; Electroluminescence; Polysilanes; Ultraviolet light source

1. Introduction

Polysilanes are linear silicon (Si)-backbone polymers

substituted with a variety of alkyl and/or aryl side groups

[1,2]. The optical and electronic properties of polysilanes

have been the subject of intensive studies both experimen-

tally and theoretically [2]. These studies have revealed that,

in contrast to p -conjugated polymers, polysilanes are quasi

one-dimensional (1D) materials with delocalized s -conju-

gated electrons along the polymer backbone chain, and their

optical and electronic properties are primarily ascribable to

the effect of quantum con®nement on these s-conjugated

electrons [3,4]. Owing to their 1D direct-gap nature, poly-

silanes exhibit a sharp photoluminescence (PL) with a high

quantum ef®ciency usually in the NUV or UV region in

solid state as well as in solution. However, previous

attempts to utilize polysilanes in technological applications

have been limited to microlithography [2,5] and xerography

[6,7], and no attempt has been reported to employ them as

an emissive material in active optical devices in spite of

their ef®cient PL in the UV or NUV region.

The recent successful fabrication of high ef®ciency light-

emitting diodes (LEDs) made from conjugated polymers [8]

and sublimed molecular ®lms [9] has stimulated rapid

developments in this ®eld targeted at their potential optoe-

lectronics applications. For instance, ef®cient whole visible

colors, including blue, have already been realized with an

acceptable device durability. The use of polysilanes in

LEDs is attractive since this can provide novel types of

NUV- or UV-light sources which are dif®cult to achieve

with other organic materials. In fact, recent studies demon-

strated that polysilanes emit NUV- or NU-EL very weakly

at low temperatures [10±15] and at room temperature [16].

In this article, we report recent progress made in our

laboratory towards utilizing polysilanes as an emissive

material for NUV- or UV-LEDs by describing the basic

EL characteristics in relation to the chemical, optical and

electronic properties of the emissive polysilanes and the

LED structure. In our work we used ®ve polysilanes,

which can be classi®ed into the following three types: (1)

three dialkyl polysilanes: poly(di-n-butylsilane) (PDBS),

poly(di-n-pentylsilane) (PDPS) and poly(di-n-hexylsilane)

(PDHS), (2) one monoalkyl-aryl polysilane: poly(methyl-

phenylsilane) (PMPS), and (3) one diaryl polysilane; poly[-

bis(p-butylphenyl)silane] (PBPS).

2. Experimental

The con®guration of the LEDs and the chemical struc-

H.

Thin Solid Films 331 (1998) 64±70

0040-6090/98/$ - see front matter q 1998 Elsevier Science S.A. All rights reserved.

PII S0040-6090(98)00947-X

* Corresponding author. Fax: 181 462 70 2353;

e-mail: hiroyuki@will.brl.ntt.co.jp.1 Present address: NTT Opto-electronisc Laboratories, Tokai, Ibaraki,

319-1193, Japan.

tures of the polysilanes used in this study are depicted in Fig.

1. The weight-averaged molecular weight of these polysi-

lanes is about 105±106. This was determined by gel permea-

tion chromatography based on poly(styrene) standards. In

the LED fabrication, the polysilane layer was spin-coated

from a toluene solution onto thoroughly cleaned indium-tin-

oxide (ITO) coated quartz substrates (20 V/A). Electron

injection was undertaken with Al or Mg/Ag (10:1 at.%)

alloy electrodes, which were fabricated by vacuum deposi-

tion or radio-frequency (RF) magnetron sputtering deposi-

tion, respectively. The active area of these LEDs was about

0.1±0.2 cm2.

We used a liquid-nitrogen-cooled charge coupled device

(CCD) with a UV-coating attached to a 15-cm monochro-

mator to measure the EL spectra. The current±voltage±EL

intensity curves were measured with a combination of a

photomultiplier and a source measure unit. The LEDs

were placed in a He-¯ow type cryostat which enabled the

temperature to be changed between room temperature and

liquid He temperature.

3. Results and discussion

3.1. Poly(methylphenylsilane)

3.1.1. Single-layer LEDs

We start this section by describing the characteristics of

LEDs made from PMPS since this was the ®rst polysilane in

which we observed EL originating from Si-backbone chains

[9], and its EL characteristics have been the most exten-

sively studied [9±12,14].

PMPS is an amorphous polysilane, whose electronic

structure is perturbed by substituted phenyl groups through

the interaction of the p -orbitals of the phenyl groups with

the s -orbitals of the Si-backbones. Until very recently

PMPS has been utilized as a hole transporting material in

multilayer LEDs because it provides non-dispersive hole

transport with a comparatively large effective mobility at

room temperature (,1023 cm2/Vs) as well as being trans-

parent over the whole visible region [17±20]. Since the

discovery of NUV-EL originating from the Si-backbone

chains, the characteristics and mechanism of the EL have

been investigated in detail for single-layer PMPS-LEDs.

H. Suzuki et al. / Thin Solid Films 331 (1998) 64±70 65

Fig. 1. The con®guration of the single-layer LEDs fabricated in this study

and the chemical structures of the polysilanes used as the emissive materi-

als.

Fig. 2. Normalized PL spectra of (a) PMPS (X) and PBPS (W), and (b)

PDBS (X), PDPS (P) and PDHS (W) ®lms at room temperature.

Since the emissive species of the EL are generally the

same as those of the photoluminescence (PL), it is worth

mentioning brie¯y the PL characteristics of PMPS in a solid

state. Upon photoexcitation, PMPS shows a strong narrow

PL band (lmax � 353 nm) (see Fig. 2a), which exhibits a

typical mirror image with respect to the lowest energy

absorption band with a small Stokes shift at room tempera-

ture. The PL quantum ef®ciency is larger than 0.1 in a solid

state, indicating the potential for using the polymer as an

emissive material in LEDs. This emission can be attributed

to the relaxation of quasi 1D excited singlet excitons delo-

calized along the Si backbone chains [3,4]. The PL spectrum

of `normal' PMPS has two typical characteristics: one is

additional broad emission bands in the visible region, and

the other is the strong temperature dependence of its inten-

sity. When the temperature is decreased, the intensity of the

NUV emission changes only slightly, whereas the visible

emission intensity increases continuously, as shown in Fig.

3a. This observation indicates that the whole relaxation

process of the excited state of PMPS is composed of several

processes which compete with one another.

Fig. 4 shows the variation in the EL intensity as a function

of the applied electric-®eld strength for PMPS-LEDs. The

PMPS-LEDs exhibit typical diode behavior with a recti®ca-

tion ratio of greater than 103. The threshold electric-®eld

strength (Eth) for the EL emission, which is obtained

under the assumption that it is uniform throughout the

whole thickness, is 1:0 £ 106 V/cm at room temperature,

and increases to 1:7 £ 106 V/cm at 126 K. Since polysilanes

are strongly hole conductive by nature, the current is domi-

nated by holes, and the onset of the EL re¯ects the supply of

electrons into the polymer. Thus, the increase in Eth with the

decrease in the temperature indicates that electron injection

into polysilanes is a thermally activated process with a ®nite

barrier height. An analysis of the current±voltage curves for

the PMPS-LEDs revealed that they can be well-described by

the space-charge-limited current model. In addition, the

depth of the hole traps in the polymer was obtained to be

about 0.4 eV [11] which is in good agreement with the

activation energy for the effective hole mobility (0.37 eV)

determined by the time-of-¯ight technique [21]. In contrast

to the PL spectrum, the room temperature EL spectrum is

composed only of broad emission bands in the visible region

because the defect levels responsible for these visible bands

can act as main radiative trapping centers. In addition, the

short durability of the LEDs limits NUV-EL detection with

a reasonable signal to noise ratio at room temperature. The

temperature dependence of the EL spectrum reveals that a

sharp UV-EL band becomes detectable at temperatures

lower than 270 K, in addition to the visible broad emission

bands (Fig. 3b). However, the variation in the EL with the

temperature change is very different from that in the PL in

terms of intensity and spectral shape. These observed EL

characteristics re¯ect the fact that the EL and PL emitting

zones are different. The PL of polysilanes originates from

the bulk of the polymer ®lms because, at the excitation

wavelength used for the PL measurements, the absorption

depth is comparable to the ®lm thickness. On the other hand,

the EL originates from the polysilane layer near the inter-

face with the electron injecting electrode (EIE) due to the

strong hole conductive nature of polysilanes, and so defect

levels existing at this interface play an essential role in the

EL process. The external quantum ef®ciency is in the 1023±

1024 and 1025±1026% (photons/electron) range for visible

(VIS-) and NUV-EL, respectively, at temperatures between

110 and 240 K. This low quantum ef®ciency of the NUV- or

UV-EL results from the inef®cient electron injection into

H. Suzuki et al. / Thin Solid Films 331 (1998) 64±7066

Fig. 3. The temperature dependence of (a) the PL and (b) the EL spectra of

PMPS. The inset in (a) shows the temperature dependence of the PL inten-

sity in the NUV and VIS regions. The ®gures indicated in (a) are the

measurement temperatures.

the polysilanes from the Al electrode via a large barrier, and

also from the fact that defect levels at the polymer-EIE

interface act as ef®cient radiative trapping centers and

energy acceptors of the quasi 1D singlet excitons responsi-

ble for the NUV-EL.

We noticed that the durability of the PMPS-LEDs is

determined by the deterioration in the EIE caused by the

excess Joule heat generated during operation. This excess

heat is generated mainly by the need to inject electrons into

the polymer via the large barrier between the Al electrode

and the polymer. The room-temperature durability of the

PMPS-LEDs is typically 5±10 min at a current density of

6±7 mA/cm2 at 30 V. Lowering the operating temperature

noticeably improves their durability, but it is still shorter

than hours.

3.1.2. Effects of device structure

As described above, the EL characteristics of single-layer

PMPS-LEDs with an electron injecting Al electrode must be

greatly improved, compared with those of the LEDs made

from p -conjugated polymers reported to date. Since the

device characteristics of LEDs made from polysilanes are

mainly determined by the supply of electrons into emissive

polysilanes, we investigated the effects of the EIE and the

electron injection layer on the device characteristics of

LEDs made from PMPS [12,13].

As an EIE with a lower work-function than Al, we

utilized an Mg/Ag alloy (10:1) electrode, which was fabri-

cated by RF magnetron sputtering deposition (hereafter,

abbreviated to Mg/Ag(sp)). We have already con®rmed

that the characteristics of the EL originating from the bulk

emissive layer are improved by fabricating the EIE with the

RF-sputtering technique without causing any noticeable

change in the EL spectrum [22,23]. With an Mg/Ag(sp)

electrode, PMPS-LEDs exhibit a lower Eth value

(6:7 £ 105 V/cm at room temperature), and a total EL inten-

sity (NUV- and VIS-EL) which is about two orders of

magnitude larger than with an Al electrode. The EL spec-

trum of the PMPS-LEDs with the Mg/Ag(sp) electrode is,

however, dominated by VIS-EL because defect levels,

which act as radiative trapping centers, are produced at

the PMPS-EIE interface during the EIE electrode fabrica-

tion. This is because the energy of the metal particles used is

higher with the sputtering technique (0.2±10 eV) than with

the conventional vacuum evaporation technique (0.1±0.2

eV). This marked change observed in the EL spectrum of

PMPS-LEDs with an Mg/Ag(sp) electrode is additional

evidence that the EL is emitted from the vicinity of the

PMPS±EIE interface. We have also noted that the tempera-

ture at which the NUV-EL is detectable decreases from 270

K for a vacuum-deposited Al electrode to 244 K for an Mg/

Ag(sp) electrode. This observation indicates that these

defect levels also act as energy acceptors for the quasi-1D

excitons responsible for the NUV-EL, resulting in the reduc-

tion of its quantum ef®ciency.

Fig. 5 shows the effect of the incorporation of an electron

injection layer made from a vacuum-deposited thin-®lm of

2,5-bis(4-biphenylyl)-1,3,4-oxadiazole (BBD) molecules

into PMPS-LEDs. The device characteristics were notice-

ably improved by the BBD layer as indicated in the I±V±EL

intensity curve at room temperature; the Eth value (5:0 £ 105

V/cm) is half, and the total EL intensity double that for

single-layer PMPS-LEDs [12]. These changes are ascrib-

able to the combined effects of improvements in electron

injection and carrier con®nement, and the separation of the

emissive PMPS layer from the EIE which is known to act as

a quencher of singlet excitons of emissive materials. The

decrease in the temperature caused an increase in the total

EL intensity (by three orders of magnitude between room

temperature and 140 K) and in the EL turn-on electric-®eld

strength (5:0 £ 105 V/cm at room temperature and 8:1 £ 105

V/cm below 230 K). At room temperature, a PMPS-LED

H. Suzuki et al. / Thin Solid Films 331 (1998) 64±70 67

Fig. 4. The EL intensity as a function of applied electric ®eld strength for

single-layer LEDs made from PMPS (W), PDBS (V), PDPS (A), PDHS (O)

and PBPS (X). The measurement temperature is 100 K for PDBS and

PDHS, 200 K for PDPS, 126 K for PMPS and room temperature for PBPS.

Fig. 5. The current (X) and EL intensity (W) as a function of applied

electric ®eld strength for the two-layer PMPS-LED with a BBD layer,

together with the EL intensity for the single-layer PMPS-LED (B). The

molecular structure of BBD and the con®guration of the two-layer PMPS-

LED are also shown.

with a BBD layer has an EL spectrum composed only of a

broad emission in the visible region as is the case for single-

layer PMPS-LEDs. The NUV-EL from PMPS becomes

detectable at temperatures below 230 K, although the EL

spectrum of these LEDs is mainly composed of a broad VIS-

EL (see Fig. 6). We con®rmed that the observed EL origi-

nated from the PMPS layer by measuring the temperature

dependence of the PL spectra of a BBD ®lm. In addition, the

incorporation of a BBD layer into the PMPS-LEDs caused a

red-shift in the EL spectrum, and the magnitude of this

spectral shift coincided with the energy difference between

the conduction band and/or the energy level of quasi 1D

excitons of PMPS, and the conduction band of the BBD

layer. This spectral red-shift in the EL was commonly

observed for PMPS-LEDs with an electron injection layer

composed of oxadiazole molecules [13]. We ascribe the

observed red-shift in the EL spectrum to the decrease in

the probability of injected electrons having energy enough

to generate quasi 1D excitons in PMPS-LEDs with a BBD

layer because electron injection is an energy selective

process occurring via surface defect levels which exist

below the conduction band of the BBD layer in terms of

energy. Thus, the use of an electron injecting layer with a

conduction band whose energy is higher than or comparable

to that of the emissive polysilane is essential to enhance the

intensity of the NUV- or UV-EL for polysilane-based LEDs.

We also observed a decrease in intensity at wavelengths

longer than 550 nm in the EL spectrum of the PMPS-

LEDs with a BBD layer. This implies that the defect levels

responsible for the VIS-EL have two different origins, that

is, some are generated during the EIE fabrication and some

exist prior to the EIE fabrication. This is because the PMPS

layer cannot be damaged during EIE fabrication by the

incorporation of the BBD layer.

3.2. Dialkyl polysilanes

The second type of polysilanes we introduced as an emis-

sive material in the LED con®guration were dialkyl poly-

silanes [24,25]. It is essential to use dialkyl polysilane to

obtain the fundamental EL characteristics which originate

from s -conjugated 1D Si chains because alkyl side chain

groups cause only a small perturbation in the electronic

structure of Si-backbones. Of the dialkyl polysilanes

reported so far, we chose PDBS, PDPS and PDHS because

these polysilanes have a large dispersion in the band gap

(4.6±5.2 eV), the lowest exciton energy level (3.35±3.95

eV) and the ionization potential (5.7±5.9 eV) [26]. It is

thus possible to extract the relation between the EL char-

acteristics and the electronic structure of emissive polysi-

lanes, which is of a great importance for the further

development of emissive polysilanes for future research.

The room temperature PL spectra of thin-®lms of PDBS,

PDPS and PDHS are shown in Fig. 2b. The peak wave-

lengths of these PL spectra depend markedly on the poly-

silane due mainly to the variations in the conformation of

the polymer backbone. On the basis of the PL peak wave-

lengths, the backbone conformation of PDBS, PDPS and

PDHS is assigned to the disorder, a mixture of the 7/3

helix and the trans-planar and the trans-planar conforma-

tion, respectively. Only PDBS shows additional broad emis-

sion bands in the visible region. However, in contrast to

PMPS, the intensity of the visible emission of PDBS is

only slightly dependent on temperature. This is because

the recombination centers for this visible emission have

different origins, that is, branching points generated during

polymerization [10]. By contrast, PDPS and PDHS exhibit

only the NUV or UV emission, and no additional emissions

are observed in the PL spectrum even at low temperatures.

This result suggests that the radiative deactivation occurs

exclusively through the conjugated Si chains for PDPS and

PDHS.

The variation in the EL intensity as a function of the

H. Suzuki et al. / Thin Solid Films 331 (1998) 64±7068

Fig. 6. The temperature dependence of the EL spectrum of the two-layer

PMPS-LED with a BBD layer. The inset shows the EL spectra in the NUV

region.

Fig. 7. The EL spectrum of PDBS (X), PDPS (O) and PDHS (W). The

spectrum was measured at 100 K for PDBS and PDHS and at 200 K for

PDPS.

applied electric-®eld strength for the dialkyl polysilane-

based LEDs is shown in Fig.4. These LEDs also exhibit

typical diode behavior. The Eth for the EL emission is 1:8 £106 at 100 K, 2:7 £ 106 at 200 K and 3:0 £ 106 V/cm at 100

K for PDBS, PDPS and PDHS, respectively. The Eth value is

inconsistent with that expected from the hypothetical band

structure of these LEDs; the LEDs which exhibit VIS-EL

have a lower Eth value than those with no VIS-EL. This

inconsistency can be attributed to the fact that electron

injection into the polysilane layer is facilitated by the forma-

tion of positive space charges via the accumulation of

trapped holes at defect levels in the vicinity of the EIE.

Fig. 7 shows the EL spectra of PDBS, PDPS and PDHS at

low temperatures. The NUV- or UV-EL of the dialkyl poly-

silanes was only measurable at low temperatures. A pure

UV-EL was observed only for PDHS while PDBS and

PDPS exhibited additional visible broad emission in the

EL spectrum. For these LEDs, the external quantum ef®-

ciency for the NUV- or UV-EL is estimated to be larger than

that for the PMPS-LEDs by about one-order of magnitude at

temperatures between 100 and 200 K. The low quantum

ef®ciency of the NUV- or UV-EL of these LEDs results

from the inef®cient electron injection into the polysilanes

from the Al electrode via a large barrier, and/or from the fact

that defect levels existing at the polymer-EIE interface act

as ef®cient radiative trapping centers and the energy accep-

tors of the quasi 1D singlet excitons responsible for the

NUV- or UV-EL. These dialkyl polysilane-based LEDs

exhibit a durability range similar to that of PMPS-LEDs at

temperatures between 100 K and room temperature.

This study has revealed that the EL characteristics of

dialkyl polysilane-based LEDs cannot be related straight-

forwardly to their electronic structure. The presence of

defect levels, irrespective of whether they are intrinsic or

extrinsic, also has a noticeable in¯uence on the EL charac-

teristics by means of their effects on the electron injection

into the polysilanes and the deactivation process of quasi 1D

excited singlet excitons.

3.3. Poly[bis(p-butylphenyl)silane]

Our studies on the EL characteristics of PMPS and the

dialkyl polysilanes revealed that an increase in electron

injection and a decrease in the defect concentration at the

polymer-EIE interface are crucial in terms of greatly

improving the EL characteristics of LEDs made from poly-

silanes. These requirements mean that we must develop

emissive polysilanes which have both a narrower band

gap and a higher glass transition temperature (Tg) than the

polysilanes described above. Emissive polysilanes with a

high Tg offer robustness against temperature increases

during operation and EIE fabrication, and thus the defect

concentration at the polymer-EIE interface becomes smal-

ler.

We used a diphenyl-substituted polysilanes, poly[bis(p-

butylphenyl)silane] (PBPS), as an emissive material, as an

example of a polysilane with a higher Tg and a narrower

band gap [24]. The Tg of PBPS appears much higher than

room temperature although the polymer decomposes before

exhibiting any Tg. Note, for instance, that the Tg is 233 and

403 K for PDBS [25] and PMPS, respectively. The electro-

nic structure of PBPS is perturbed by substituted phenyl

groups, and appears to be affected additionally by a steric

effect. The information on the conformation of PBPS is

rather limited at present, but the polymer is found to be

mesomorphic at room temperature on the basis of X-ray

and optical microscopy measurements. The conformation

of the polymer seems to be extended because of its distinctly

long-wavelength absorption compared with other members

of the polysilane family, together with its narrow spectral

bandwidth. PBPS exhibits only an NUV emission in the PL

spectrum (see Figs. 2 and 8), and no additional emissions are

observed in the PL spectrum even at low temperatures.

For PBPS-LEDs the EL is detectable at an Eth value of

6:8 £ 105 V/cm at room temperature (see Fig. 4). This low

Eth value is a result of its smaller energy gap relative to the

other polysilanes, leading to a lower barrier height for the

electron injection.

PBPS-LEDs exhibit only NUV-EL peaking at 407 nm

even at room temperature, as shown in Fig. 8. This is the

®rst observation of room-temperature `pure' NUV-EL from

polysilane-based LEDs without accompanying any VIS-EL

[27,28]. Note that previous room-temperature NUV-EL

from evaporated ®lms of a polysilane was detected together

with a major contribution of VIS emissions in the EL spec-

trum [16]. The EL bandwidth for the PBPS-LEDs is less

than 15 nm (0.11 eV), indicating that the spectral `purity' of

this EL is high. The external quantum ef®ciency of the

NUV-EL for PBPS-LEDs reaches the 0.01±0.1%

(photons/electron) range when external voltages between

30 and 35 V are applied. This large increase was achieved

at room temperature with an electron injecting Al electrode.

H. Suzuki et al. / Thin Solid Films 331 (1998) 64±70 69

Fig. 8. The PL (W) and EL (X) spectrum of the PBPS-LED at room

temperature.

The PL quantum ef®ciency of thin ®lms of PBPS is not

signi®cantly different from that of the other polysilanes.

Therefore, the essence of this large improvement in the

external quantum ef®ciency is the use of an appropriate

polysilane which possesses no radiative defect levels at

the interface near the EIE and a smaller band gap to

decrease the barrier height for the electron injection from

the Al electrode. The improvement in the device durability

is also signi®cant for PBPS-LEDs. The PBPS-LEDs can

emit NUV-EL continuously over a period of 12 h; the EL

intensity decreases to a level of approximately 80% of the

initial value and remains steady without any signi®cant

additional decrease. The PBPS layer can persist under

these operating conditions because the EL spectrum is the

same before and after the operation. The fact that the PBPS-

LEDs are more durable than the other polysilane-based

LEDs is attributable to the fact that PBPS has a glass transi-

tion temperature (Tg) well above room temperature, which

minimizes the problems of thermal instability, and there is

less excess Joule heat generation because the barrier height

for the electron injection into PBPS from the Al electrode is

lower than with the other polysilanes.

4. Conclusion

In this article, we have described our recent achievements

in developing LEDs made from polysilanes. These LEDs

are unique in emission wavelengths without losing any of

advantages provided by polymers such as robustness, low

cost, processability. In addition, we can adopt a simple LED

con®guration, such as a single-layer or a two-layer structure

with an electron transporting layer, because of the strong

hole conductive nature of polysilanes. The strategies

successfully utilized for improving the device characteris-

tics of LEDs made from p -conjugated polymers and small

molecules have also been found to be generally effective for

polysilane-based LEDs. However, the unique feature of

polysilane-based LEDs is the essential role played by the

defect levels at the polymer±EIE interface in the EL process

because their EL originates primarily from the vicinity of

the polymer±EIE interface.

The success in fabricating ef®cient room temperature

NUV-LEDs made from PBPS with an Al electrode has

provided solid evidence that our strategies for developing

the emissive polysilanes are appropriate for LED fabrica-

tion. Our preliminary studies on the effects of device struc-

ture on device characteristics suggest the realization of

further improvements in the device characteristics of

NUV- or UV-LEDs made from polysilanes through, for

instance, the combination of PBPS-LEDs with a lower

work-function EIE and/or an electron injection layer.

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