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The origin of the hole injection improvements at indium tin oxide/molybdenum trioxide/N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl- 4,4′-diamine interfaces

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Hyunbok Lee at Kangwon National University
Hyunbok Lee
Sang Wan Cho at Yonsei University
Sang Wan Cho
Kyul Han
Kyul Han
Kyul Han
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Pyung Eun Jeon
Pyung Eun Jeon
Pyung Eun Jeon
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Abstract and Figures

We investigated the interfacial electronic structures of indium tin oxide (ITO)/molybdenum trioxide ( MoO <sub>3</sub>)/N,N<sup>′</sup> -bis(1-naphthyl)- N,N<sup>′</sup> -diphenyl- 1,1<sup>′</sup> -biphenyl- 4,4<sup>′</sup> -diamine (NPB) using in situ ultraviolet and x-ray photoemission spectroscopy to understand the origin of hole injection improvements in organic light-emitting devices (OLEDs). Inserting a MoO <sub>3</sub> layer between ITO and NPB, the hole injection barrier was remarkably reduced. Moreover, a gap state in the band gap of NPB was found which assisted the Ohmic hole injection at the interface. The hole injection barrier lowering and Ohmic injection explain why the OLED in combination with MoO <sub>3</sub> showed improved performance.
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The origin of the hole injection improvements at indium tin
oxide/molybdenum trioxide/N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-
biphenyl- 4,4′-diamine interfaces
Hyunbok Lee, Sang Wan Cho, Kyul Han, Pyung Eun Jeon, Chung-Nam Whang et al.
Citation: Appl. Phys. Lett. 93, 043308 (2008); doi: 10.1063/1.2965120
View online: http://dx.doi.org/10.1063/1.2965120
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The origin of the hole injection improvements at indium tin oxide/
molybdenum trioxide/N,N⬘-bis„1-naphthyl…-N,N⬘-diphenyl-1,1⬘-biphenyl-
4,4⬘-diamine interfaces
Hyunbok Lee,1Sang Wan Cho,1,a兲Kyul Han,1Pyung Eun Jeon,1Chung-Nam Whang,1
Kwangho Jeong,1Kwanghee Cho,2and Yeonjin Yi3,b兲
1Institute of Physics and Applied Physics, Yonsei University, 134 Sinchon-dong, Seodaemoon-Gu, Seoul
120-749, Republic of Korea
2Research and Development Division, Hynix Semiconductor Inc., San 136-1, Ami-Ri Bubal-Eub Icheon-Si,
GyeungGi-Do 467-701, Republic of Korea
3Division of Advanced Technology, Korea Research Institute of Standards and Science, 209 Gajeong-Ro,
Yuseong-Gu, Daejeon 305-340, Republic of Korea
共Received 9 June 2008; accepted 8 July 2008; published online 1 August 2008兲
We investigated the interfacial electronic structures of indium tin oxide 共ITO兲/molybdenum trioxide
共MoO3兲/N,N⬘-bis共1-naphthyl兲-N,N⬘-diphenyl-1,1⬘-biphenyl-4, 4⬘-diamine 共NPB兲using in situ
ultraviolet and x-ray photoemission spectroscopy to understand the origin of hole injection
improvements in organic light-emitting devices 共OLEDs兲. Inserting a MoO3layer between ITO and
NPB, the hole injection barrier was remarkably reduced. Moreover, a gap state in the band gap of
NPB was found which assisted the Ohmic hole injection at the interface. The hole injection barrier
lowering and Ohmic injection explain why the OLED in combination with MoO3showed improved
performance. © 2008 American Institute of Physics.关DOI: 10.1063/1.2965120兴
Since the development of organic light-emitting devices
共OLEDs兲with thin-multilayer structures,1many studies on
OLEDs have been conducted by academic and industrial re-
searchers. However, more efforts are needed to solve invet-
erate problems such as high operation voltage, luminance
efficiency, and lifetime. These problems could be solved by
implementing proper interfaces among the layers composing
the organic devices. In the case of the interface between an
anode and a hole transport layer 共HTL兲, various organic or
inorganic interlayers have been adopted such as copper
phthalocyanine 共CuPc兲,23,4,9,10 perylenetetracarboxylic
dianhydride,3SiO2,4SiOxNy,5TiO2共Ref. 6兲, and several
metal oxides.7In general, it has been considered that these
materials act as a buffer layer to balance the concentrations
of holes and electrons and/or as a hole injection layer 共HIL兲
to reduce the hole injection barrier between anode and HTL.
In addition, the interlayer makes good contact between an
anode and a HTL by forming a smooth surface on the anode.
These interlayers have shown markedly improved device
properties.
Recently, transition metal oxides, such as molybdenum
oxide 共MoO3兲, vanadium oxide 共V2O5兲, and tungsten oxide
共WO3兲have attracted much attention due to their hole injec-
tion barrier lowering properties8,9and charge generation abil-
ity in tandem OLEDs.10–12 You et al.13 reported significantly
improved OLEDs with a MoO3interlayer. The device with a
MoO3layer showed better performance than the device with
a CuPc layer which has been used commonly as a HIL to
enhance both luminance-voltage characteristics and life-
time. In addition, Matsushima et al.14 reported Ohmic hole
injection by inserting the MoO3layer between indium tin
oxide 共ITO兲and N,N⬘-bis共1-naphthyl兲-N,N⬘-diphenyl-1,1⬘-
biphenyl-4,4⬘-diamine 共NPB兲. However, the mechanisms for
the hole injection barrier lowering and the Ohmic injection
from ITO to NPB are not clear yet. The interfacial electronic
structures and the energy level alignments at ITO/MoO3/
NPB interfaces would play a crucial role in the improve-
ments in device performance. We performed in situ ultravio-
let photoemission spectroscopy 共UPS兲and x-ray photoemis-
sion spectroscopy 共XPS兲measurements to study the
ITO/MoO3/NPB interfaces hence clarifying the origin of the
improvement in device performance. UPS and XPS allowed
us to understand the electronic structures of the interfaces
from the information of the vacuum level, highest occupied
molecular orbital 共HOMO兲, and core levels.
We fabricated a multilayer film of ITO /MoO3/NPB
structure and performed in situ UPS and XPS measurements.
Silver paste was employed at the contact region between an
ITO-coated glass substrate and the sample holder in order to
secure the electrical contact between the ITO and analyzer. A
clean ITO surface was obtained with Ar+ion sputtering so
the surface carbon was adequately reduced. We exposed the
ITO surface to the Ar+ions for as short as possible time in
order to maintain its pristine stoichiometry. MoO3and NPB
were thermally evaporated onto the ITO substrate in a depo-
sition chamber maintained below 5⫻10−8 Torr. The deposi-
tion rates of both MoO3and NPB were controlled to be
0.01 nm/s with evaporation temperatures of 430 and 130 °C,
respectively. To investigate the interface formation,
MoO3共5.0 nm兲and NPB 共0.1, 0.3, 0.5, 1.0, and 2.0 nm兲
were deposited on the clean ITO in a stepwise manner. Be-
fore and after each deposition step, the sample was trans-
ferred to the analysis chamber and analyzed without break-
ing the vacuum. The analysis chamber was composed of a
hemispherical electron energy analyzer 共PHI 5700 spectrom-
eter兲, monochromatic x-ray 共Al K
␣
, 1486.6 eV兲, and ultra-
violet 共He I, 21.21 eV兲discharge source. The UPS spectra
a兲Present address: Department of Physics, Boston University, 590 Common-
wealth Avenue, Boston, Massachusetts 02215, USA.
b兲Author to whom correspondence should be addressed. Electronic mail:
yeonjin@kriss.re.kr.
APPLIED PHYSICS LETTERS 93, 043308 共2008兲
0003-6951/2008/93共4兲/043308/3/$23.00 © 2008 American Institute of Physics93, 043308-1
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were obtained with a sample bias of −15 V in normal emis-
sion geometry to measure the secondary electron cutoff. We
plotted all spectra with respect to the weak but well defined
Fermi level 共EF兲of ITO. We also prepared an ITO/NPB
multilayer film and carried out similar measurements as a
control sample.
Figure 1shows the XPS core level spectra of each ele-
ment during the step-by-step deposition. The In 3dpeak 共a兲
was measured from the sputter cleaned ITO surface and it
attenuated completely after deposition of the 5 nm MoO3
layer, implying that the ITO surface was fully covered with
MoO3. The Mo 3dpeak position 关232.9 eV; Fig. 1共b兲兴
showed good agreement with the MoO3phase compared to
literature.15,16 The intensities of the Mo 3dpeaks attenuated
gradually with the deposition of NPB. However, they did not
show significant shifts or shape changes during the NPB
deposition, which shows there was neither band bending 共Vb兲
nor chemical reactions in the MoO3layer during the NPB
deposition. Figure 1共c兲shows the series of C 1sspectra dur-
ing the NPB layer deposition followed by the MoO3deposi-
tion on ITO. There is no significant carbon contamination on
the sputter cleaned ITO and MoO3deposited. The C 1s
peaks appeared and intensified as the NPB layer thickened.
The Vbof the NPB layer 共0.15 eV兲was estimated from the
shift of the C 1speaks during the step-by-step measure-
ments and it was cross-checked with the HOMO shift ob-
tained from the UPS spectra since the band bending should
occur throughout all the energy levels.
Figures 2共a兲–2共c兲show the secondary electron cutoff re-
gion, full range, and HOMO region of UPS spectra, respec-
tively. Figure 2共b兲shows spectral changes clearly with the
deposition of MoO3and NPB on ITO. The MoO3valence
band structures are clearly seen at 3–10 eV 共Ref. 17兲
with the MoO3共5兲spectrum and the spectral features of NPB
dominating the spectra as the NPB layer thickens. In Fig.
2共a兲, the cutoff position shifted toward lower energy by 2.35
eV immediately after the 5 nm MoO3layer was deposited.
This shows the formation of a huge interface dipole18 on ITO
since the MoO3has very large work function.17 As NPB was
deposited on the MoO3共5nm兲covered ITO, the position of
secondary cutoff gradually moved back toward higher bind-
ing energies and the shift was saturated with 2 nm NPB
deposition. The total secondary cutoff shift during NPB
deposition was estimated to be 1.90 eV. Figure 2共c兲shows
the HOMO region spectra with background removal corre-
sponding to each deposition step. The HOMO onset posi-
tions were determined by line fitting of the HOMO onset
after considering the analyzer broadening 0.1 eV for our
system.19 The Vbwas determined from the shift of the
HOMO onset position as NPB layer thickened. The deter-
mined Vbwas 0.15 eV which accords well with the core level
shift 共see C 1speaks in Fig. 1兲.
The most remarkable changes in the spectra are the for-
mation of gap state in the gap of NPB with the MoO3layer
insertion. The gap state at 0.15 eV is seen clearly at the first
共0.1 nm兲and second 共0.3 nm兲deposition steps of NPB on
MoO3. The gap state was not observed at the interface of
ITO/NPB 共not shown兲. As the NPB layer thickened over 0.5
nm, the gap state disappeared, implying that the gap state
exists at very short range from the interface between MoO3
and NPB. This gap state could be generated by weak inter-
action since the XPS spectra did not show any strong chemi-
cal reactions.20
Combining the information of HOMO and vacuum level
shifts in combination with the band bending, we drew the
energy level diagrams of ITO/NPB and ITO/MoO3/NPB in
Figs. 3共a兲and 3共b兲.共The detailed procedures are presented
elsewhere.19兲The energy level diagram of ITO/NPB was
evaluated with the similar UPS and XPS measurements. The
position of the lowest unoccupied molecular orbital was es-
timated from a previously reported energy gap of 3.10 eV for
NPB.21 The ionization energy of NPB 共Eion兲showed good
agreement for both cases compared to previous results within
the error margin of our measurement 共0.05– 0.1 eV兲.22 In the
case of the ITO/NPB interface 共a兲, the interface dipole 共eD兲
was negligible 共0.03 eV兲and the hole injection barrier 共⌽h兲
was determined to be 1.64 eV. On the other hand, an ex-
tremely large interface dipole was induced after the deposi-
tion of the MoO3layer. Although the vacuum level was low-
ered again with the subsequent NPB deposition, the amount
of the lowering did not exceed the initial vacuum level rising
FIG. 1. 共Color online兲Measured XPS core level spectra of 共a兲In 3d,共b兲
Mo 3d,and共c兲C1sin between the MoO3and NPB layer depositions.
Each spectrum was obtained from sputter cleaned ITO, ITO/MoO3共5nm兲
and ITO/MoO3共5nm兲/NPB 共0.1, 0.3, 0.5, 1.0, and 2.0 nm兲.
FIG. 2. 共Color online兲The measured UPS spectra of the 共a兲secondary
electron cutoff region normalized for easy comparison, 共b兲full range, and
共c兲HOMO region with background removal before and after the MoO3and
NPB layer deposition. Each spectra was obtained from sputter cleaned ITO,
ITO/MoO3共5nm兲and ITO/MoO3共5nm兲/NPB 共0.1, 0.3, 0.5, 1.0, and 2.0
nm兲. The fitting procedures for determining the HOMO onset is indicated.
043308-2 Lee et al. Appl. Phys. Lett. 93, 043308 共2008兲
Downloaded 01 Oct 2013 to 202.116.1.149. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions
with the MoO3layer. Therefore, the vacuum level was effec-
tively raised, resulting in the ⌽hof 0.89 eV, greatly reduced
from its previous value, with MoO3insertion 共b兲. From the
view point of the charge injection, Matsushima et al.14 re-
ported Ohmic hole injection with the MoO3interlayer be-
tween ITO and NPB. We directly observed a gap state in the
gap of NPB indicated with a dotted line. This state could be
attributed to the charge transfer.14 This state was very close
to the Fermi level so that the charge carriers could be in-
jected easily from ITO to NPB; i.e., Ohmic hole injection
could occur. This explains the origin of the Ohmic hole in-
jection at the interface.
In summary, we investigated the role of a MoO3layer
between ITO and NPB using in situ photoemission measure-
ments. The hole injection barrier was highly reduced with the
insertion of MoO3layer compared to the interface without
MoO3. Furthermore, the formation of the gap state in the
NPB gap assisted the Ohmic hole injection from the ITO
electrode. The hole injection barrier lowering and Ohmic
hole injection explain why the OLED in combination with
MoO3showed excellent performance.
This work was supported by Institute of Physics and
Applied Physics 共IPAP兲, Yonsei University, and BK21
project of the Korea Research Foundation 共KRF兲.
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FIG. 3. 共Color online兲Energy level diagrams of 共a兲ITO/NPB and 共b兲
ITO/MoO3/NPB. ⌽eand ⌽hare the electron and hole injection barrier. Eion
and eD are the ionization energy of NPB and interface dipole. The band
bending 共Vb兲was evaluated from the shift of HOMO and C 1slevel during
the deposition. Comparing with the diagrams, the hole injection barrier of
ITO/MoO3/NPB is much lower than that of ITO/NPB. Noticeable gap state
was observed with the insertion of MoO3layer.
043308-3 Lee et al. Appl. Phys. Lett. 93, 043308 共2008兲
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... Thus, charge-injection layers that modify the work function have been employed. MoO 3 is a popular hole injection layer (HIL) for efficient anode systems because of its high work function [5]. However, the accurate charge injection and transport mechanisms of MoO 3 HIL are still under investigation. ...
... NPB was selected as the HTL material because of its widespread use in OLEDs, and a 20 nm thickness of MoO 3 layer was chosen to saturate the work-function increase. The measured HOMO level of the NPB HTL was compared with that of ITO/MoO 3 (5 nm)/NPB interfaces reported previously [5]. The change in the HOMO level of the NPB HTL provides useful information for designing device architectures. ...
... Figure 3 shows the energy-level diagrams of (a) ITO/MoO 3 (20 nm)/NPB and (b) ITO/MoO 3 (5 nm)/NPB. The energy-level alignment of ITO/MoO 3 (20 nm)/NPB was determined based on the results of this study, whereas that of ITO/MoO 3 (5 nm)/NPB was obtained from a previous report [5]. The interface dipole was calculated using the equation of eD = ΔSEC − V b , where eD, ΔSEC, and V b represent the interface dipole, SEC shift, and band bending, respectively [23]. ...
View
... Perovskites, often used as active materials for LEDs and solar cells, have been used as HILs due to their electronic properties. As we know, if the contact/adhesion between interfaces is poor, the effective migration of holes from the ITO anode to the adjacent organic layer will be hindered [46]. Thus, the better contacts of perovskite at the ITO/HIL and HIL/HTL interfaces are beneficial for hole injection and transmission. ...
... interfaces is poor, the effective migration of holes from the ITO anode to the adjacent organic layer will be hindered [46]. Thus, the better contacts of perovskite at the ITO/HIL and HIL/HTL interfaces are beneficial for hole injection and transmission. ...
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  • Oct 2022
  • NAT PHOTONICS
Organic light-emitting diodes (OLEDs) with thick carrier transport layers are desirable for high production yields of OLED-based displays and lighting; however, high operating voltages are inevitably introduced to thick OLEDs due to the low carrier mobilities of organics. The associated Joule heating will also induce structural defects and lower operational stabilities. Here we demonstrate highly efficient and stable OLEDs with thicknesses of over 1 μm and low operating voltages. The OLEDs use MoO3/SimCP2 as a hole-injection layer and a thick layer of 4,4′-(cyclohexane-1,1-diyl)bis(N,N-di-p-tolylaniline) (TAPC) as a hole-transporting layer. We find that Ohmic hole injection can only be formed for TAPC layers with thicknesses of over 900 nm. In this configuration, we achieve external quantum efficiencies of 23.09%, 22.19% and 7.39%, and operating voltages of 5.11 V, 3.55 V and 6.88 V at 1,000 cd cm–2 for red, green and blue OLEDs, respectively. We also incorporate a thin layer of HAT-CN between the TAPC and electron-blocking layers to suppress electron leakage. The red, green and blue OLEDs in this work maintained the above-mentioned performances while also featuring excellent extrapolated LT95 operational lifetimes of around 55,000 h, 18,000 h and 1,600 h, respectively, at an initial luminance of 1,000 cd cm–2. We believe that our work paves the way for large-area OLED-based displays and lighting with high production yields. One-micrometre-thick OLEDs with low operating voltages of 5.11 V, 3.55 V and 6.88 V at 1,000 cd cm–2 for red, green and blue devices, respectively, and long lifetimes (55,000 h, 18,000 h and 1,600 h, respectively) are realized.
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... The Al bottom electrode as well as the following semiconducting layers and the Ag top electrode were thermally evaporated onto the nanopatterned imprint resist, resulting in 1D grating-patterned electrode-semiconductor interfaces for charge injection into the devices, as shown schematically in figure 1(a). A 10 nm thin layer of the hole injection/electron blocking material MoO 3 was deposited between the hole transport layer and both electrodes to prevent electron injection which would lead to charge recombination inside the device [40]. Planar HODs holding no grating pattern were fabricated as reference devices by evaporating the device layers directly onto the surface of polished glass substrates. ...
Injection-limited and space charge–limited currents in organic semiconductor devices with nanopatterned metal electrodes
Article
Full-text available
Charge injection at metal-organic interfaces often limits the electric current in organic light-emitting diodes without additional injection layers. Integrated nanopatterned electrodes may provide a way to overcome this current injection limit by local field enhancements leading to locally space charge–limited currents. We compare electrical characteristics of planar and nanopatterned hole-only devices based on the charge transport material NPB with different thicknesses in order to investigate the nanopattern’s effect on the current limitation mechanism. Integration of a periodic nanograting into the metal electrode yields a current increase of about 1.5 to 4 times, depending on thickness and operating voltage. To verify the experimental results, we implement a finite element simulation model that solves the coupled Poisson and driftdiffusion equations in a weak form. It includes space charges, drift and diffusion currents, non-linear mobility, and charge injection at the boundaries. We find in experiment and simulation that the planar devices exhibit injection-limited currents, whereas the currents in the nanopatterned devices are dominated by space charge effects, overcoming the planar injection limit. The simulations show space charge accumulations at the corners of the nanopattern, confirming the idea of locally space charge–limited currents.
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... As a result, it has been revealed that it is an example of TMOs that can increase performance as an intermediate layer in photovoltaic applications thanks to its high WF [16]. In their research on organic lightemitting devices (OLEDs), Lee et al. reported that the MoO 3 interlayer makes device performance more stable and improved hole injection can be achieved [20]. There is a need to examine the changes caused by the high WF MoO 3 interface in the electrical characterization in photodiode applications. ...
The effect of different frequencies and illuminations on the electrical behavior of MoO3/Si heterojunctions
Article
Full-text available
  • Oct 2021
  • J MATER SCI-MATER EL
In this study, the rectifier properties of the transition metal oxide group n-type semiconductor molybdenum trioxide (MoO 3) were investigated. The MoO 3 material is a suitable material for the heterojunction structures with AFM, SEM, XRD, and 3D optical profilometer such as structural and morphological characterization result showed. Current-voltage (I-V), capacitance-voltage (C-V), and conductance-voltage (G-V) measurements of Cr/MoO 3 /n-Si and Cr/ MoO 3 /p-Si heterojunction devices were made in dark and different illuminations at 300 K. The basic diode parameters were determined by using Ther-mionic emission (TE), and Cheung and Norde method from the I-V characteristics of the devices in dark conditions. The ideality factors of Cr/ MoO 3 /n-Si and Cr/MoO 3 /p-Si devices were calculated as 1.25 and 1.22, respectively, and barrier heights of 0.69 and 0.71 eV of the devices were calculated by TE method. These results showed that the MoO 3 /Si heterojunction has rectifier properties. The high values of ideality can be attributed to the inhomogeneities at the interface and the series resistance. In addition, the photoconductivity properties were examined of the devices at 50 and 100 mW/ cm 2 illuminations. From the experimental results obtained, it was concluded that the devices can be used as photodiodes as well as showing good rectifier properties.
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... Conventionally, WO x and MoO x TMO layers have been applied as electron blocking and hole transport layers (HTLs) in normal, inverted; and tandem polymer and perovskite solar cells (PSCs) towards avoiding degradation issues associated with conventionally used PEDOT:PSS [25][26][27][28][29][30]. Efficient hole extraction had been demonstrated in PSCs with MoO x and WO x , which, owing to their p-type nature, aid in selectively extracting holes while blocking electrons due to a shallow ionization potential and low electron affinity. ...
Solution processable transition metal oxide ultra-thin films as alternative electron transport and hole blocking layers in dye sensitized solar cells
Article
  • May 2021
  • J PHOTOCH PHOTOBIO A
An electron transporting and hole-blocking layer (ETL) between the fluorine-doped tin oxide (FTO) and mesoporous titanium dioxide interface of photoanode is essential in dye-sensitized solar cells (DSCs) for improving their photovoltaic performance. In the present work, we have examined the potential for further improving device performance by using alternative materials for ETLs. Accordingly, the effect of replacing TiO2 based ETLs with spin coated ultra-thin transition metal oxides (TMOs), molybdenum trioxide (MoO3) and tungsten trioxide (WO3) films has been studied. TMO films of 10 nm thickness have been found to be optimal for obtaining good transparency and charge transport properties, without compromising device performance. Further, devices fabricated with surfactant assisted (SA-) MoO3 ETLs have shown efficiency ∼4%, which is comparable with devices based on conventionally prepared TiO2 ETLs. In addition, a significant improvement in efficiency (15 %) has been observed for DSCs prepared using SA-WO3 films. The overall improvement in device efficiency is attributed to the increase in short-circuit current density without loss of open circuit potential and fill factor. Detailed analyses of results suggest that, in addition to surface morphology, transparency, charge transport properties, oxygen vacancies in sub-stoichiometric TMOs play an important role in determining the device performance. The findings reveal that TMOs have significant potential to replace TiO2 ETL in DSCs and other third generation solar cell, viz. perovskite and polymer solar cells.
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Scalable Bilayer MoS 2 ‐Based Vertically Inverted p–i–n Light‐Emitting Diodes
Article
  • Oct 2023
Herein, a vertically inverted p–i–n architecture of light‐emitting diodes (LEDs) is designed for manufacturing feasibility and demonstrated scalable bilayer MoS 2 ‐based LEDs. A 4 inch scale bilayer MoS 2 is prepared by a two‐step growth method allocating the pre‐deposition of a few‐nm thick metal film and post‐sulfurization. To apply bilayer MoS 2 for an active layer in LEDs, the film is transferred over ZnO nanoparticle layers, an electron transfer layer, and then the rest of the LED components are constructed by thermal deposition. This vertically inverted LED architecture allows individual organic or inorganic components to incorporate without degradation during the wet‐transfer process and transfer electron or hole carriers across separate layers, resulting in efficient radiative recombination in the MoS 2 emitting layer. MoS 2 ‐based LEDs exhibit electroluminescence of ≈5.41 cd m ⁻² throughout four active areas of 6.25 mm ² at a driving voltage of 7 V. Therefore, this achievement can overcome the drawbacks of existing transition metal dichalcogenides (TMDs)‐based optoelectrical applications and extend its potential in various fields, such as flexible, ultrathin, or transparent displays.
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Degradation analysis of doped organic p-n heterojunction charge generation layers by impedance and optical spectroscopy
Article
  • Sep 2021
Investigation of the long-term stability of charge generation layers (CGLs) provides a fundamental and an essential approach in achieving highly efficient tandem organic electronic devices. Thus, in this foremost study, the degradation mechanism of electrically aged organic p-n heterojunction CGLs has been investigated by impedance and optical spectroscopy. Rubidium carbonate (Rb2CO3)–doped 2,2,2-(1,3,5-benzinetriyl)-tris(1-phenyl-1H-benzimidazole) (TPBi) and molybdenum trioxide (MoO3)–doped 1,4-bis[N-(1-naphthyl)-N′-phenylamino]-4,4′-diamine (NPB) are used as the n-type and p-type organic semiconductors, respectively. A detailed analysis from capacitance-frequency (C–F) and capacitance-voltage (C–V) characteristics reveals reduced charge generation and 19.6% reduction in the geometric capacitance of the CGL after electrical aging. Reduced peak intensity from UV–Vis–NIR spectra of the aged CGL points to 21.4% charge transfer complex decomposition of the Rb2CO3-doped TPBi. We propose that the rate-limiting step of charge generation in the CGL is caused by the electron transport in the TPBi:Rb2CO3 layer and not the charge generation itself at the TPBi:Rb2CO3/NPB:MoO3 heterojunction. This simple, comprehensive, and non-destructive technique facilitates a crucial analysis that underpins the mechanism of device degradation and further provides a fundamental approach in developing highly stable CGLs for efficient organic electronic devices.
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Improved band alignment for hole injection by an interfacial layer in organic light emitting devices
Article
  • Aug 2000
  • APPL PHYS LETT
We demonstrate that a thin organic interfacial layer of 3,4,9,10 perylenetetracarboxylic dianhydride (PTCDA) can be utilized to improve the band alignment of N,N'-di-(3-methylphenyl)N,N'diphenyl-4,4'diaminobiphenyl (TPD) films on [indium-tin-oxide (ITO)] (InSnO) substrates in, e.g., organic electroluminescent devices. A photoemission study of the highest occupied molecular orbital (HOMO) and vacuum level position as a function of the organic overlayer thickness reveals that due to chemisorptive bonding a thin PTCDA interlayer results in a reduced barrier between the Fermi level of ITO and the HOMO of TPD. Furthermore we detect a new molecular state 0.6 eV below the Fermi level at the PTCDA/ITO interface. Both effects are expected to improve the hole injection from the ITO anode into the TPD hole transport layer, e.g., in organic light emitting devices.
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Interface Analysis of Naththyl-substituted Benzidine derivative and tris-8-(hydroxyquinoline) aluminum Using Ultraviolet and X-Ray Photoemission Spectroscopy
Article
  • Nov 1999
Naththyl-substituted Benzidine derivative (NPB) and tris-8-(hydroxyquinoline) aluminum (Alq) organic materials have been used to demonstrated highly efficient bi-layer light emitting devices (LEDs). In such a device, the holes from the NPB layer are injected into the electron transporting Alq and recombine with emission characteristics of Alq. Thus, the interface between the Alq and NPB layers are crucial to the device performance. We will report the interface studies between Alq and NPB using ultraviolet and x-ray photoemission spectroscopy (UPS) and (XPS), respectively. The UPS results have shown a vacuum level shift of more than 0.2 eV when thin layers Alq are deposited onto NPB. The XPS results also show shift as a function Alq thickness. Finally, we will discuss the local band alignments close to organic layer interface and possible impact on device performance. This work is supported by DARPA DAAL01-96-K-0086 and NSF DMR-9612370.
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Organic light-emitting diodes with a nanostructured TiO 2 layer at the interface between ITO and NPB layers
Article
  • Dec 2003
  • DISPLAYS
Organic light-emitting diodes (OLEDs) with a nanostructured TiO2 layer at the interface between indium tin oxide and a-naphtylphenyliphenyl diamine layers were fabricated using a vacuum evaporation method. The nanostructured TiO2 layer was achieved by the Sol–Gel method. Compared to the different thickness of the buffer layer, the OLEDs with the 6 nm buffer layer showed the highest efficiency. The enhancements in efficiency result from an improved balance of hole and electron injections and a more homogeneous adhesion of the buffer layer inserted.
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Determination of interface dipole and band bending at the Ag/tris (8-hydroxyquinolinato) gallium organic Schottky contact by ultraviolet photoemission spectroscopy
Article
  • Apr 2000
  • SURF SCI
A tris (8-hydroxyquinolinato) gallium (Gaq3) thin film was grown in several steps on a previously in situ evaporated Ag thin film. Ultraviolet photoemission spectroscopy (UPS) measurements carried out prior to growth and after each growth step allowed the determination of the alignment of the highest occupied molecular orbital (HOMO) relative to the Fermi level of the Ag substrate. The deposition of ultra-thin (submonolayer) initial Gaq3 films allowed us to distinguish between band bending and interface dipole related high binding energy cutoff (secondary cutoff) shifts, which is necessary to determine the interface dipole with high precision. In order to determine the band bending with high accuracy it was necessary to identify the HOMO position of the submonolayer Gaq3 films. This was accomplished by removing the Ag related emission background in the low coverage spectra using the Fermi edge intensity as a measure for the Ag related emission. Our results demonstrate that the interface dipole builds up during the growth of the first one or two monolayers during which the HOMO position remains constant. The offset between the HOMO cutoff (low binding energy cutoff of the UP spectra) and the Ag Fermi edge was determined to be 1.67eV, while the interface dipole amounted to 0.69eV. In order to find an estimate for the alignment of the lowest unoccupied molecular orbital (LUMO) relative to the Ag Fermi edge, the HOMO/LUMO gap (2.70eV) of Gaq3 was determined by optical absorption measurements. The LUMO offset was estimated to be −1.04eV.
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Evidence of gap state formed by the charge transfer in Alq 3/NaCI/AI interface studied by ultraviolet and x-ray photoelectron spectroscopy
Article
  • Mar 2005
  • APPL PHYS LETT
Electronic structures of Alq3/NaCl/Al and Alq3/Al were studied by UV and x-ray photoelectron spectroscopy (XPS). The initial energy level of the highest occupied molecular orbital (HOMO) of Alq3/Al was shifted when the ultrathin NaCl layer was inserted between them, although the vacuum level was not changed. The measured interface dipole was 1.1 eV, identical for both Alq3/NaCl/Al and Alq3/Al. Our experiment shows that the dipole is formed in very short range (less than 0.1 nm) from the interface. The onset of the HOMO level of Alq3 was shifted 0.2 eV toward high binding energy for the additional NaCl layer, which lowered the barrier height and improved injection characteristics of the device. Moreover, a gap state was observed at 1.1 eV below the Fermi level when the NaCl was inserted between Alq3 and Al. The XPS core-level spectra revealed that the interaction on Alq3 and NaCl was very weak, which generated an unusual gap state without breaking or forming chemical bonds. We suggest that the weak interaction would originate from the charge transfer from Alq3 to NaCl.
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Highly efficient white organic electroluminescent devices based on tandem architecture
Article
  • Dec 2005
  • APPL PHYS LETT
Two types of tandem organic light-emitting diodes (OLEDs) with white-light emission have been developed by using Mg:Alq3/WO3 as the interconnecting layer. While the Commission Internationale d’Eclairage (CIE) coordinates of the tandem device with individual blue- and yellow-emitting OLEDs was sensitive to the viewing angle and the operating time, the tandem device connecting two white-emitting OLEDs was considerably less. At an optimal WO3 thickness of 5 nm, the tandem two-unit device produced three higher luminance efficiency than that expected of a single-unit device. A maximum efficiency of 22 cd/A was achieved by the tandem device comprised of two white-fluorescent OLEDs, and the projected half-life under the initial luminance of 100 cd/m2 was over 80 000 h.
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Improved performance and stability of organic light-emitting devices with silicon oxy-nitride buffer layer
Article
  • Aug 2003
  • APPL PHYS LETT
The use of silicon oxy-nitride (SiOxNy) as an anode buffer layer in organic light-emitting devices (OLEDs) with a configuration of indium tin oxide (ITO)/SiOxNy/α-naphtylphenyliphenyl diamine (NPB)/8-hydroxyquinoline aluminum/Mg:Ag has been studied. With a SiOxNy buffer layer several angstroms thick, the device efficiency increased from 3.0 to 3.8 cd/A. The buffer layer also protected the ITO surface from contamination due to air exposure. Upon exposing the cleaned ITO substrate to air for one day before device fabrication, the device current efficiency and turn-on voltage degraded to 2.1 cd/A and 4.3 V, respectively, from 3 cd/A and 3.3 V for the device fabricated on an as-cleaned ITO surface. In contrast, devices prepared on air-exposed SiOxNy/ITO surface had almost the same current efficiency (3.85 cd/A) and turn on voltage (3.7 V) comparing to devices (3.8 cd/A and 3.7 V) fabricated on freshly prepared SiOxNy/ITO surface. The results suggested that SiOxNy is a promising anode buffer layer for OLEDs, for both efficiency and stability enhancements. © 2003 American Institute of Physics.
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29.2: Highly Efficient and Stable Inverted Bottom-Emission Organic Light Emitting Devices
Article
  • Aug 2006
  • APPL PHYS LETT
The authors report the development of highly efficient and stable C545T doped green fluorescent Alq3 inverted bottom-emission organic light emitting device (OLED), with a device configuration of ITO/Mg/Cs2O:Bphen/Alq3/C545T:Alq3/NPB/WO3/Al, that achieved a maximum current efficiency of 23.7 cd/A and a power efficiency of 12.4 lm/W which are two times better than those of the conventional OLED. At a brightness level of 100 cd/m2, the device required driving current density only as low as 0.5 mA/cm2 at a driving voltage of only 5.0 V and its half-lifetime T1/2 in excess of 104 000 h.
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Organic Electroluminescent Devices with Improved Stability
Article
  • Oct 1996
Highly stable organic electroluminescent devices based on vapor‐deposited Alq thin films have been achieved. The improvement in stability is derived from several factors including: (1) a multilayer thin‐film structure with a CuPc stabilized hole‐injection contact, (2) a hole‐transport diamine layer using a naphthyl‐substituted benzidine derivative, and (3) an ac drive wave form. These emissive devices have shown an operational half‐lifetime of about 4000 h from an initial luminance of 510 cd/m2. © 1996 American Institute of Physics.
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Enhanced brightness and efficiency in organic electroluminescent devices using SiO2 buffer layers
Article
  • Apr 1999
  • APPL PHYS LETT
Organic electroluminescent devices using SiO2 as a hole-injecting buffer have been fabricated. With the presence of the buffer, the luminance of the device reaches 1820 cd/m2 at the current density of 20 mA/cm2, which corresponds to an efficiency of 9.1 cd/A. The enhancements in brightness and efficiency are attributed to an improved balance of hole and electron injections due to blocking of the injected holes by the buffer layer and a more homogeneous adhesion of the hole transporting layer to the anode. © 1999 American Institute of Physics.
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