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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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