-
The ARGO-YBJ Collaboration,
B. Bartoli,
P. Bernardini,
X. J. Bi,
I. Bolognino,
P. Branchini,
A. Budano,
A. K. Calabrese Melcarne,
P. Camarri,
Z. Cao, [......],
P. Zhang,
X. Y. Zhang,
Y. Zhang,
J. Zhao,
Zhaxiciren,
Zhaxisangzhu,
X. X. Zhou,
F. R. Zhu,
Q. Q. Zhu,
G. Zizzi
[show abstract]
[hide abstract]
ABSTRACT: We report the observation of a very high energy \gamma-ray source, whose
position is coincident with HESS J1841-055. This source has been observed for
4.5 years by the ARGO-YBJ experiment from November 2007 to July 2012. Its
emission is detected with a statistical significance of 5.3 standard
deviations. Parameterizing the source shape with a two-dimensional Gaussian
function we estimate an extension \sigma=(0.40(+0.32,-0.22}) degree, consistent
with the HESS measurement. The observed energy spectrum is dN/dE =(9.0-+1.6) x
10^{-13}(E/5 TeV)^{-2.32-+0.23} photons cm^{-2} s^{-1} TeV^{-1}, in the energy
range 0.9-50 TeV. The integral \gamma-ray flux above 1 TeV is 1.3-+0.4 Crab
units, which is 3.2-+1.0 times the flux derived by HESS. The differences in the
flux determination between HESS and ARGO-YBJ, and possible counterparts at
other wavelengths are discussed.
03/2013;
-
The ARGO-YBJ Collaboration,
B. Bartoli,
P. Bernardini,
X. J. Bi,
C. Bleve,
I. Bolognino,
P. Branchini,
A. Budano,
A. K. Calabrese Melcarne,
P. Camarri, [......],
P. Zhang,
X. Y. Zhang,
Y. Zhang,
J. Zhao,
Zhaxiciren,
Zhaxisangzhu,
X. X. Zhou,
F. R. Zhu,
Q. Q. Zhu,
G. Zizzi
[show abstract]
[hide abstract]
ABSTRACT: As one of the brightest active blazars in both X-ray and very high energy
$\gamma$-ray bands, Mrk 501 is very useful for physics associated with jets
from AGNs. The ARGO-YBJ experiment is monitoring it for $\gamma$-rays above 0.3
TeV since November 2007. Starting from October 2011 the largest flare since
2005 is observed, which lasts to about April 2012. In this paper, a detailed
analysis is reported. During the brightest $\gamma$-rays flaring episodes from
October 17 to November 22, 2011, an excess of the event rate over 6 $\sigma$ is
detected by ARGO-YBJ in the direction of Mrk 501, corresponding to an increase
of the $\gamma$-ray flux above 1 TeV by a factor of 6.6$\pm$2.2 from its steady
emission. In particular, the $\gamma$-ray flux above 8 TeV is detected with a
significance better than 4 $\sigma$. Based on time-dependent synchrotron
self-Compton (SSC) processes, the broad-band energy spectrum is interpreted as
the emission from an electron energy distribution parameterized with a single
power-law function with an exponential cutoff at its high energy end. The
average spectral energy distribution for the steady emission is well described
by this simple one-zone SSC model. However, the detection of $\gamma$-rays
above 8 TeV during the flare challenges this model due to the hardness of the
spectra. Correlations between X-rays and $\gamma$-rays are also investigated.
09/2012;
-
B. Bartoli,
P. Bernardini,
X. J. Bi,
C. Bleve,
I. Bolognino,
P. Branchini,
A. Budano,
A. K. Calabrese Melcarne,
P. Camarri,
Z. Cao, [......],
L. Zhang,
P. Zhang,
X. Y. Zhang,
Y. Zhang,
Zhaxiciren,
Zhaxisangzhu,
X. X. Zhou,
F. R. Zhu,
Q. Q. Zhu,
G. Zizzi
[show abstract]
[hide abstract]
ABSTRACT: The ARGO-YBJ experiment detects extensive air showers in a wide energy range by means of a full-coverage detector which is in stable data taking in its full configuration since November 2007 at the YBJ International Cosmic Ray Observatory (4300 m a.s.l., Tibet, People’s Republic of China). In this paper the measurement of the light-component spectrum of primary cosmic rays in the energy region (5÷200) TeV is reported. The method exploited to analyze the experimental data is based on a Bayesian procedure. The measured intensities of the light component are consistent with the recent CREAM results and higher than that obtained adding the proton and helium spectra reported by the RUNJOB experiment.
Phys. Rev. D. 05/2012; 85(9).
-
B. Bartoli,
P. Bernardini,
X. J. Bi,
C. Bleve,
I. Bolognino,
P. Branchini,
A. Budano,
A. K. Calabrese Melcarne,
P. Camarri,
Z. Cao, [......],
P. Zhang,
X. Y. Zhang,
Y. Zhang,
J. Zhao,
Zhaxiciren,
Zhaxisangzhu,
X. X. Zhou,
F. R. Zhu,
Q. Q. Zhu,
G. Zizzi
[show abstract]
[hide abstract]
ABSTRACT: We report the observation of TeV gamma-rays from the Cygnus region using the
ARGO-YBJ data collected from 2007 November to 2011 August. Several TeV sources
are located in this region including the two bright extended MGRO J2019+37 and
MGRO J2031+41. According to the Milagro data set, at 20 TeV MGRO J2019+37 is
the most significant source apart from the Crab Nebula. No signal from MGRO
J2019+37 is detected by the ARGO-YBJ experiment, and the derived flux upper
limits at 90% confidence level for all the events above 600 GeV with medium
energy of 3 TeV are lower than the Milagro flux, implying that the source might
be variable and hard to be identified as a pulsar wind nebula. The only
statistically significant (6.4 standard deviations) gamma-ray signal is found
from MGRO J2031+41, with a flux consistent with the measurement by Milagro.
01/2012;
-
G. Aielli,
C. Bacci,
B. Bartoli,
P. Bernardini,
X. J. Bi,
C. Bleve,
P. Branchini,
A. Budano,
S. Bussino,
A. K. Calabrese Melcarne, [......],
Jianli Zhang,
L. Zhang,
P. Zhang,
X. Y. Zhang,
Y. Zhang,
Zhaxisangzhu,
X. X. Zhou,
F. R. Zhu,
Q. Q. Zhu,
G. Zizzi
[show abstract]
[hide abstract]
ABSTRACT: The ARGO-YBJ experiment at YangBaJing in Tibet (4300 m a.s.l.) has been
taking data with its full layout since October 2007. Here we present a
few significant results obtained in gamma-ray astronomy and cosmic-ray
physics. Emphasis is placed on the analysis of gamma-ray emission from
point-like sources (Crab Nebula, MRK 421), on the preliminary limit on
the antiproton/proton flux ratio, on the large-scale cosmic-ray
anisotropy and on the proton-air cross-section. The performance of the
detector is also discussed, and the perspectives of the experiment are
outlined.
Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 12/2011; 661:50. · 1.21 Impact Factor
-
The ARGO-YBJ Collaboration,
B. Bartoli,
P. Bernardini,
X. J. Bi,
C. Bleve,
I. Bolognino,
P Branchini,
A. Budano,
A. K. Calabrese Melcarne,
P. Camarri, [......],
L Zhang,
P Zhang,
X Y Zhang,
Y Zhang,
Zhaxiciren,
Zhaxisangzhu,
X. X. Zhou,
F. R. Zhu,
Q. Q. Zhu,
G. Zizzi
[show abstract]
[hide abstract]
ABSTRACT: ARGO-YBJ is an air shower detector array with a fully covered layer of
resistive plate chambers. It is operated with a high duty cycle and a large
field of view. It continuously monitors the northern sky at energies above 0.3
TeV. In this paper, we report a long-term monitoring of Mrk 421 over the period
from 2007 November to 2010 February. This source was observed by the
satellite-borne experiments Rossi X-ray Timing Explorer and Swift in the X-ray
band. Mrk 421 was especially active in the first half of 2008. Many flares are
observed in both X-ray and gamma-ray bands simultaneously. The gamma-ray flux
observed by ARGO-YBJ has a clear correlation with the X-ray flux. No lag
between the X-ray and gamma-ray photons longer than 1 day is found. The
evolution of the spectral energy distribution is investigated by measuring
spectral indices at four different flux levels. Hardening of the spectra is
observed in both X-ray and gamma-ray bands. The gamma-ray flux increases
quadratically with the simultaneously measured X-ray flux. All these
observational results strongly favor the synchrotron self-Compton process as
the underlying radiative mechanism.
06/2011;
-
M. Amenomori,
X. J. Bi,
D. Chen,
S. W. Cui,
Danzengluobu,
L. K. Ding,
X. H. Ding,
C. Fan,
C. F. Feng,
Zhaoyang Feng, [......],
T. Yuda, H. M. Zhang,
J. L. Zhang,
N. J. Zhang,
X. Y. Zhang,
Y. Zhang,
Yi Zhang,
Ying Zhang,
Zhaxisangzhu,
X. X. Zhou
[show abstract]
[hide abstract]
ABSTRACT: Using the Tibet-III air shower array, we search for steady TeV
γ-rays from 18 pulsars in the Fermi Large Area Telescope pulsar
catalog. Among them, we observe 8 sources including the Crab instead of
the expected 0.41 sources at a significance of 2 σ or more excess.
Under the assumption of Poisson distribution, the chance probability is
estimated to be 1.4×10-8. When the Crab is excluded, it
becomes 1.8×10-7. These low chance probabilities
clearly show that the Fermi pulsars have a statistically significant
correlations with TeV γ-ray excesses.
Astrophysics and Space Sciences Transactions 05/2011; 7:211-215.
-
G. Aielli,
C Bacci,
B. Bartoli,
P. Bernardini,
X. J. Bi,
C. Bleve,
P Branchini,
A. Budano,
S Bussino,
A. K. Calabrese Melcarne, [......],
L Zhang,
P Zhang,
X Y Zhang,
Y Zhang,
Zhaxisangzhu,
Zhaxiciren,
X. X. Zhou,
F. R. Zhu,
Q. Q. Zhu,
G. Zizzi
[show abstract]
[hide abstract]
ABSTRACT: The sun blocks cosmic ray particles from outside the solar system, forming a
detectable shadow in the sky map of cosmic rays detected by the ARGO-YBJ
experiment in Tibet. Because the cosmic ray particles are positive charged, the
magnetic field between the sun and the earth deflects them from straight
trajectories and results in a shift of the shadow from the true location of the
sun. Here we show that the shift measures the intensity of the field which is
transported by the solar wind from the sun to the earth.
01/2011;
-
M. Amenomori,
X. J. Bi,
D. Chen,
S. W. Cui,
Danzengluobu,
L. K. Ding,
X. H. Ding,
C. Fan,
C. F. Feng,
Zhaoyang Feng, [......],
T. Yuda, H. M. Zhang,
J. L. Zhang,
N. J. Zhang,
X. Y. Zhang,
Y. Zhang,
Yi Zhang,
Ying Zhang,
Zhaxisangzhu,
X. X. Zhou
[show abstract]
[hide abstract]
ABSTRACT: The energy spectrum of cosmic rays around the knee measured by Tibet
air-shower experiment is summarized and its characteristic features are
discussed under two possible scenarios. The result of Tibet experiment
measured over wide range of 1014-1017 eV with high
statistics provided details of the knee at the energy around
4×1015 eV. The study of the chemical composition based
on measurements of proton and helium spectra obtained from air-shower
core detection indicates the dominance of heavy nuclei around the knee.
Such feature can be explained either by contribution of nearby sources
with source composition dominated by heavy nuclei or by nonlinear effect
in diffusive shock acceleration mechanism.
Astrophysics and Space Sciences Transactions 12/2010; 7:15-20.
-
G. Aielli,
C. Bacci,
B. Bartoli,
P. Bernardini,
X. J. Bi,
C. Bleve,
P. Branchini,
A. Budano,
S. Bussino,
A. K. Calabrese Melcarne, [......],
Jianli Zhang,
L. Zhang,
P. Zhang,
X. Y. Zhang,
Y. Zhang,
Zhaxisangzhu,
X. X. Zhou,
F. R. Zhu,
Q. Q. Zhu,
G. Zizzi
[show abstract]
[hide abstract]
ABSTRACT: In 2008, the blazar Markarian 421 entered a very active phase and was one of the brightest sources in the sky at TeV energies, showing frequent flaring episodes. Using the data of ARGO-YBJ, a full coverage air shower detector located at Yangbajing (4300 m a.s.l., Tibet), we monitored the source at gamma-ray energies E>0.3 TeV during the whole year. The observed flux was variable, with the strongest flares in March and June, in correlation with X-ray enhanced activity. While during specific episodes the TeV flux could be several times larger than the Crab Nebula one, the average emission from day 41 to 180 was almost twice the Crab level, with an integral flux of (3.6 ± 0.6) × 10–11 photons cm–2 s–1 for energies E>1 TeV, and decreased afterward. This Letter concentrates on the flares that occurred in the first half of June. This period has been deeply studied from optical to 100 MeV gamma rays, and partially up to TeV energies, since the moonlight hampered the Cherenkov telescope observations during the most intense part of the emission. Our data complete these observations, with the detection of a signal with a statistical significance of 3.8 standard deviations on June 11-13, corresponding to a gamma-ray flux about 6 times larger than the Crab one above 1 TeV. The reconstructed differential spectrum, corrected for the intergalactic absorption, can be represented by a power law with an index α = –2.1+0.7 –0.5 extending up to several TeV. The spectrum slope is fully consistent with previous observations reporting a correlation between the flux and the spectral index, suggesting that this property is maintained in different epochs and characterizes the source emission processes.
The Astrophysical Journal Letters 04/2010; 714(2):L208. · 5.53 Impact Factor
-
M. Amenomori,
X. J. Bi,
D Chen,
S. W. Cui,
Danzengluobu,
L. K. Ding,
X. H. Ding,
C Fan,
C. F. Feng,
Zhaoyang Feng, [......],
T Yuda, H M Zhang,
J L Zhang,
N. J. Zhang,
X Y Zhang,
Y Zhang,
Yi Zhang,
Ying Zhang,
Zhaxisangzhu,
X. X. Zhou
[show abstract]
[hide abstract]
ABSTRACT: We analyze the large-scale two-dimensional sidereal anisotropy of multi-TeV cosmic rays by Tibet Air Shower Array, with the data taken from 1999 November to 2008 December. To explore temporal variations of the anisotropy, the data set is divided into nine intervals, each in a time span of about one year. The sidereal anisotropy of magnitude about 0.1% appears fairly stable from year to year over the entire observation period of nine years. This indicates that the anisotropy of TeV Galactic cosmic rays remains insensitive to solar activities since the observation period covers more than a half of the 23rd solar cycle. Comment: 18 pages, 2 figures, accepted by The Astrophysical Journal
01/2010;
-
M. Amenomori,
X. J. Bi,
D Chen,
S. W. Cui,
Danzengluobu,
L. K. Ding,
X. H. Ding,
C Fan,
C. F. Feng,
Zhaoyang Feng, [......],
T Yuda, H M Zhang,
J L Zhang,
N. J. Zhang,
X Y Zhang,
Y Zhang,
Yi Zhang,
Ying Zhang,
Zhaxisangzhu,
X. X. Zhou
[show abstract]
[hide abstract]
ABSTRACT: Using the Tibet-III air shower array, we search for TeV gamma-rays from 27 potential Galactic sources in the early list of bright sources obtained by the Fermi Large Area Telescope at energies above 100 MeV. Among them, we observe 7 sources instead of the expected 0.61 sources at a significance of 2 sigma or more excess. The chance probability from Poisson statistics would be estimated to be 3.8 x 10^-6. If the excess distribution observed by the Tibet-III array has a density gradient toward the Galactic plane, the expected number of sources may be enhanced in chance association. Then, the chance probability rises slightly, to 1.2 x 10^-5, based on a simple Monte Carlo simulation. These low chance probabilities clearly show that the Fermi bright Galactic sources have statistically significant correlations with TeV gamma-ray excesses. We also find that all 7 sources are associated with pulsars, and 6 of them are coincident with sources detected by the Milagro experiment at a significance of 3 sigma or more at the representative energy of 35 TeV. The significance maps observed by the Tibet-III air shower array around the Fermi sources, which are coincident with the Milagro >=3sigma sources, are consistent with the Milagro observations. This is the first result of the northern sky survey of the Fermi bright Galactic sources in the TeV region. Comment: 17 pages, 2 figures, 1 table, Accepted for publication in ApJ Letters
12/2009;
-
G. Aielli,
C. Bacci,
B. Bartoli,
P. Bernardini,
X. J. Bi,
C. Bleve,
P. Branchini,
A. Budano,
S. Bussino,
A. K. Calabrese Melcarne, [......],
JianLi Zhang,
L. Zhang,
P. Zhang,
X. Y. Zhang,
Y. Zhang,
Zhaxisangzhu,
X. X. Zhou,
F. R. Zhu,
Q. Q. Zhu,
G. Zizzi
[show abstract]
[hide abstract]
ABSTRACT: The proton-air cross section in the energy range 1–100 TeV has been measured by the ARGO-YBJ cosmic ray experiment. The analysis is based on the primary cosmic ray flux attenuation for different atmospheric depths (i.e. zenith angles) and exploits the detector capabilities of selecting the shower development stage by means of hit multiplicity, density and lateral profile measurements at ground. The effects of shower fluctuations, the contribution of heavier primaries and the uncertainties of the hadronic interaction models, have been taken into account. The results have been used to estimate the total proton-proton cross section at center-of-mass energies between 70 and 500 GeV, where no accelerator data are currently available.
Phys. Rev. D. 11/2009; 80(9).
-
M. Amenomori,
X. J. Bi,
D Chen,
S. W. Cui,
Danzengluobu,
L. K. Ding,
X. H. Ding,
C Fan,
C. F. Feng,
Zhaoyang Feng, [......], H M Zhang,
J L Zhang,
N. J. Zhang,
X Y Zhang,
Y Zhang,
Yi Zhang,
Ying Zhang,
Zhaxisangzhu,
X. X. Zhou,
J. Kota
[show abstract]
[hide abstract]
ABSTRACT: We develop a model anisotropy best-fitting to the two-dimensional sky-map of multi-TeV galactic cosmic ray (GCR) intensity observed with the Tibet III air shower (AS) array. By incorporating a pair of intensity excesses in the hydrogen deflection plane (HDP) suggested by Gurnett et al., together with the uni-directional and bi-directional flows for reproducing the observed global feature, this model successfully reproduces the observed sky-map including the "skewed" feature of the excess intensity from the heliotail direction, whose physical origin has long remained unknown. These additional excesses are modeled by a pair of the northern and southern Gaussian distributions, each placed ~50 degree away from the heliotail direction. The amplitude of the southern excess is as large as ~0.2 %, more than twice the amplitude of the northern excess. This implies that the Tibet AS experiment discovered for the first time a clear evidence of the significant modulation of GCR intensity in the heliotail and the asymmetric heliosphere. Comment: 4 pages, 1 figure, 31st International Cosmic Ray Conference (Lodz, Poland), 2009
09/2009;
-
M. Amenomori,
S. Ayabe,
S. W. Cui,
Danzengluobu,
L. K. Ding,
X. H. Ding,
C. F. Feng,
Z. Y. Feng,
X. Y. Gao,
Q. X. Geng, [......],
A. F. Yuan,
T. Yuda, H. M. Zhang,
J. L. Zhang,
N. J. Zhang,
X. Y. Zhang,
Y. Zhang,
Zhaxisangzhu,
X. X. Zhou,
and (The Tibet ASγ Collaboration
[show abstract]
[hide abstract]
ABSTRACT: Several strong TeV γ-ray flares were detected from Mrk 421 in the years 2000 and 2001 by the Tibet III air shower array at a level of statistical significance of 5.1 σ. Mrk 421 was unprecedentedly active at X-ray and TeV γ-ray energies during this period, and a positive correlation was found between the change of the all-sky monitor Rossi X-Ray Timing Explorer X-ray flux and the Tibet TeV γ-ray flux. When a power-law energy spectrum for γ-rays from this source is assumed, the spectral index is calculated to be -3.24 ± 0.69 at the most active phase in 2001. The spectral index observed by the Tibet air shower array is consistent with those obtained via imaging air Cerenkov telescopes.
The Astrophysical Journal 12/2008; 598(1):242. · 6.02 Impact Factor
-
M. Amenomori,
S. Ayabe,
D. Chen,
S. W. Cui,
Danzengluobu,
L. K. Ding,
X. H. Ding,
C. F. Feng,
Z. Y. Feng,
X. Y. Gao, [......],
A. F. Yuan,
T. Yuda, H. M. Zhang,
J. L. Zhang,
N. J. Zhang,
X. Y. Zhang,
Yi Zhang,
Y. Zhang,
Zhaxisangzhu,
X. X. Zhou
[show abstract]
[hide abstract]
ABSTRACT: Results of a steady TeV γ-ray point-source search using data taken from the Tibet HD (1997 February-1999 September) and Tibet III (1999 November-2001 October) arrays are presented. From 0° to 60° in declination, significant excesses from the well-known steady source Crab Nebula and the high state of the flare-type source Markarian 421 are observed. Because the levels of significance from other positions are not sufficiently high, 90% confidence level upper limits on the flux are set assuming different power-law spectra. To allow cross-checking, two independently developed analyses are used in this work.
The Astrophysical Journal 12/2008; 633(2):1005. · 6.02 Impact Factor
-
M. Amenomori,
S. Ayabe,
D. Chen,
S. W. Cui,
Danzengluobu,
L. K. Ding,
X. H. Ding,
C. F. Feng,
Z. Y. Feng,
X. Y. Gao, [......],
A. F. Yuan,
T. Yuda, H. M. Zhang,
J. L. Zhang,
N. J. Zhang,
X. Y. Zhang,
Y. Zhang,
Yi Zhang,
Zhaxisangzhu,
X. X. Zhou
[show abstract]
[hide abstract]
ABSTRACT: We searched for steady PeV gamma-ray emission from the Monogem ring region with the Tibet air shower array from 1997 February to 2004 October. No evidence for statistically significant gamma-ray signals was found in a region 111° ≤ R.A. < 114°, 125 ≤ decl. < 155 in the Monogem ring where the MAKET-ANI experiment recently claimed a positive detection of PeV high-energy cosmic radiation, although our flux sensitivity is approximately 10 times better than MAKET-ANI's. We set the most stringent integral flux upper limit at a 99% confidence level of 4.0 × 10-12 cm-2 s-1 sr-1 above 1 PeV on diffuse gamma rays extended in the 3° × 3° region.
The Astrophysical Journal 12/2008; 635(1):L53. · 6.02 Impact Factor
-
M Amenomori,
X J Bi,
D Chen,
S W Cui,
Danzengluobu,
L K Ding,
X H Ding,
C Fan,
C F Feng,
Z Feng, [......],
G C Yu,
A F Yuan,
T Yuda, H M Zhang,
J L Zhang,
N J Zhang,
X Y Zhang,
Y Zhang,
Zhaxisangzhu,
X X Zhou
[show abstract]
[hide abstract]
ABSTRACT: The Tibet air shower array, which has an effective area of 36,900 m2, has been in operation at Yangbajing in Tibet, China at an altitude of 4,300 m above sea level. In this paper, we will briefly introduce the recent gamma-ray observation with the present Tibet air shower array and our future plan which is called the Tibet muon detector (MD) project.
Journal of Physics Conference Series 07/2008; 120(6):062024.
-
Y Wang,
X. J. Bi,
S. W. Cui,
L. K. Ding,
Danzengluobu,
X. H. Ding,
C Fan,
C. F. Feng,
Zhaoyang Feng,
Z. Y. Feng, [......],
A. F. Yuan, H M Zhang,
J L Zhang,
N. J. Zhang,
X Y Zhang,
Y Zhang,
Yi Zhang,
Zhaxisangzhu,
X. X. Zhou,
Q Yuan
[show abstract]
[hide abstract]
ABSTRACT: Using the data taken from Tibet II High Density (HD) Array (1997 February-1999 September) and Tibet-III array (1999 November-2005 November), our previous northern sky survey for TeV $\gamma-$ray point sources has now been updated by a factor of 2.8 improved statistics. From $0.0^{\circ}$ to $60.0^{\circ}$ in declination (Dec) range, no new TeV $\gamma-$ray point sources with sufficiently high significance were identified while the well-known Crab Nebula and Mrk421 remain to be the brightest TeV $\gamma-$ray sources within the field of view of the Tibet air shower array. Based on the currently available data and at the 90% confidence level (C.L.), the flux upper limits for different power law index assumption are re-derived, which are approximately improved by 1.7 times as compared with our previous reported limits.
05/2008;
-
[show abstract]
[hide abstract]
ABSTRACT: A high efficiency organic light-emitting device (OLED) has been fabricated by using a vertical stack of two OLED units connected by Al/WO3/Ag. In the stacked OLEDs, each unit can be independently biased. It is found that the introduction of WO3 between Al and Ag enhances the electroluminescent (EL) efficiency by 3.7 cd A−1 for the top emissive unit. Meanwhile, the EL efficiency of the stacked OLED with the Al/WO3/Ag interconnecting electrode is improved by 6 cd A−1 with respect to the case of using Al/Ag as the connecting electrode. Our results indicate that Al/WO3/Ag is an excellent kind of connecting structure for the fabrication of independently controlled high efficiency stacked OLEDs.
Journal of Physics D Applied Physics 04/2008; 41(10):105108. · 2.54 Impact Factor
