Abstract

The presence of a solution-processed hybrid PEDOT:PSS-MoO3-based hole injection layer (HIL) promotes a good interfacial contact between the indium tin oxide anode and hole-transporting layer for efficient operation of organic light-emitting diodes (OLEDs). This work reveals that the use of the hybrid HIL benefits the performance of phosphorescent OLEDs in two ways: (1) to assist in efficient hole injection, thereby improving power efficiency of OLEDs, and (2) to improve electron-hole current balance and suppression of interfacial defects at the organic/anode interface. The combined effects result in the power efficiency of 89.2 lm/W and external quantum efficiency of 23.9% for phosphorescent green OLEDs. The solution-processed hybrid PEDOT:PSS-MoO3-based HIL is beneficial for application in solution-processed organic electronic devices.

© 2016 Optical Society of America

1. Introduction

Development of high performance organic light-emitting diodes (OLEDs) has attracted significant attention for application in next generation solid-state lighting and flat-panel displays, due to their advantages of high brightness, broad electroluminance spectrum, and energy-saving feature [1–3]. Efficient charge injection is one of the prerequisites for achieving high performance OLEDs [4–6]. Efficient carrier injection at the organic/electrode interface, often limited due to the mismatch in energy level between electrode and organic layer, is one of the crucial factors for improving the performance of solution-processed OLEDs. Poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) is a commonly used hole injection layer (HIL) with the advantages of high transparency, easy fabrication process, high conductivity, good surface morphology, thermally stability and excellent mechanical flexibility. The use of PEDOT:PSS HIL facilitates the charge injection at the interface between PEDOT:PSS and functional organic material with a low-lying highest occupied molecular orbital (HOMO) [7]. A suitable solution-porcessable HIL enabling efficient carrier injection is desired for attaining efficient carrier injection in OLEDs involving phosphorescent light-emitting materials with a high HOMO level.

In comparison to the solution-processed PEDOT:PSS HIL, different transitional metal oxide (TMO)-based HILs, prepared by sputtering, thermal evaporation and solution-process routes, for OLEDs have been reported. Molybdenum oxide (MoO3) is one of the TMO HILs adopted for assist in carrier injection in hole-transporting layer (HTL) with a low-lying HOMO level used in OLEDs. An external quantum efficiency (EQE) > 20% was reported for an OLED using a MoO3 HIL, achieved due to improved hole injection at the interface between the MoO3 HIL and a deep HOMO material of 4,4-Bis(N-carbazolyl)-1,1-biphenyl (CBP) [8]. Many studies have revealed that TMO-based HIL possess a high work function and a strong molecular electronegativity, allowing the extraction of electrons from the HOMO level of neighboring organic layer to its deep conduction band [9–12]. Much effort has been devoted in developing solution-processed TMO HIL. Different approaches, e.g. the sol-gel method [13–16], direct dissolving TMO powder in solvent [17, 18], and synthesis of TMO nanoparticles (NPs) solution [19, 20], have been reported to realize the solution-processed TMO HIL. TMO HIL prepared by the sol-gel method requires high temperature annealing treatment to convert the TMO precursors to the dry films by hydrolysis. However, a high sintering temperature is not suitable for devices made from flexible plastic substrates. For example, MoO3 sol-gel process requires a sintering temperature of 275°C [15]. The solubility of TMO powders in the solvent can be an issue, because most TMO powders are hard to dissolve in organic solvents for high quality thin films deposited. The synthesis of TMO NPs solution can be an adequate approach for solution-processed TMO HIL. However the films prepared by the TMO NPs have a large surface roughness and a high density of pin-holes, causing high leakage current [19, 20].

Solution-processed organic-inorganic hybrids consisting of PEDOT:PSS and different TMO nanoparticles e.g., MoO3 and vanadium oxide (V2O5), have been used as a HTL for application in organic solar cells (OSCs) [21–24]. Wang et al. reported a hybrid HTL derived from a mixture of PEDOT:PSS and MoO3 NPs for use in inverted OSCs [22]. The mixing PEDOT:PSS helps to suppress the aggregation of MoO3 nanoparticles, and also to passivate the hydrophilic PSS chains through preferentially connection with MoO3 NPs, leading to the stronger adhesion between the hybrid HTL and photoactive layer for application in inverted OSCs. A mixed PEDOT:PSS and MoO3-NPs interlayer layer was also used to improve the printing of a silver nanowire mesh for application in semitransparent OSCs [23]. It is shown that MoO3 exhibited intense photoluminescence in the wavelength range from 350 to 550 nm. The use of the MoO3-doped PEDOT:PSS helps to improve the carrier collection efficiency in c-Si/PEDOT:PSS heterojunction solar cells via absorption enhancement, caused by MoO3 induced intense photoluminescence over the wavelength range from 350 to 550 nm [24]. Recently, the hybrids of conductive polymer with different metal oxide nanoparticles serving as the solution-processable transparent electrode have also been developed for application in solution-processed ITO-free transparent OLEDs [25], and charge generation layer in white emitting tandem OLEDs [26].

In this work, we report our effort of a comprehensive and systemic analysis on the effect of different solution-processed HILs on (1) electron-hole current balance, and (2) suppression of interfacial defects at the organic/anode interface in OLEDs. The focus is to achieve enhanced power efficiency of phosphorescent OLEDs using a synergetic approach of photoelectron spectroscopy measurements, experimental optimization and process integration.

2. Experiment

2.1. Configuration of OLEDs with solution-processed HIL

Indium tin oxide (ITO) anodes modified with different solution-processed hole injection layers of solution-processed MoO3, PEDOT:PSS and hybrids compositing of PEDOT:PSS with different volume ratios of MoO3 nanoparticles were used for application in the OLEDs. A HTL of CBP and an electron transporting layer (ETL) of 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB) were used in the phosphorescent OLEDs. A set of structurally identical OLEDs of anode/HIL/HTL/ETL/cathode, but with different HILs was fabricated for comparison studies. A phosphorescent dopant Bis[2-(2-pyridinyl-N)phenyl-C](2,4-pentanedionato-O2,O4)iridium(III) (Ir(ppy)2acac), with an EL peak emission at 520 nm, was doped in the host material of CBP [8]. The device architecture and the schematic diagram of energy levels of the functional materials used in the OLEDs are illustrated in Fig. 1.

 figure: Fig. 1

Fig. 1 (a) The cross-sectional view of the phosphorescent OLED, and (b) the schematic diagram of energy levels of the functional materials used in the OLEDs.

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A bare ITO glass substrate, with a sheet resistance of 15 Ω/sq, was cleaned following the standard procedures [27]. The MoO3 NPs solution was synthesized according to the reported procedures [28]. The MoO3 NPs solution (0.1 mole/mL, dissolved in ethanol) was mixed with the PEDOT:PSS (CleviosTM P VP Al 4083, H. C. Starck, 1%) solution, with different volume ratios of PEDOT:PSS to MoO3 NPs of 1:1, 2:1, 3:1 and 4:1. The mixture of PEDOT:PSS and MoO3 NPs was spin-casted on the ITO glass substrate to form a 30 nm thick hybrid HIL. For comparison studied, pure MoO3 NPs solution was also spin-casted to form a 10 nm thick MoO3 HIL on ITO/glass. After annealing at 120°C for 10 min, the samples were loaded into a N2-purged glove box, which is connected to a 10-source evaporator system for device fabrication. The functional organic layers were deposited by thermal evaporation in a high vacuum system with a base pressure of less than 5.0 × 10−4 Pa. Phosphorescent OLEDs with an evaporated MoO3 (eMoO3) HIL and a solution-processed MoO3 (sMoO3) HIL have the same device configuration of ITO (80 nm)/MoO3 (10 nm)/CBP (50 nm)/CBP:Ir(ppy)2acac (7%, 15 nm)/TmPyPB (55 nm)/LiF (1 nm)/Al (100 nm). For OLEDs with a hybrid PEDOT:PSS-MoO3 HIL, the devices have a configuration of ITO (80 nm)/hybrid HIL with different volume ratios of PEDOT:PSS to MoO3 (1:1, 2:1, 3:1 and 4:1, 30nm)/CBP (30 nm)/CBP:Ir(ppy)2acac (7%, 15 nm)/TmPyPB (55 nm)/LiF (1 nm)/Al (100 nm). The thickness of the corresponding layers was optimized by the theoretical optical simulation [29].

2.2. Characterization of OLEDs with solution-processed HIL

The phosphorescent OLEDs were encapsulated in the glove box and then characterized in ambient condition. Current density–voltage–luminance (JVL) characteristics of the solution-processed OLEDs were measured by a Keithley source measurement unit (Keithley Instruments Inc., Model 236 SMU), which was calibrated using a silicon photodiode. The EL spectra were measured by spectra colorimeter (Photo Research Inc., Model 650) spectrophotometer. The surface morphology of different HILs was characterized by the atomic force microscope (AFM) (Multimode V) using tapping mode. Surface electronic properties of different HILs were analyzed using X-ray Photoelectron Spectroscopy (XPS) and Ultraviolet Photoelectron Spectroscopy (UPS) measurements were performed in the system equipped with an electron spectrometer (Sengyang SKL-12) and an electron energy analyzer (VG CLAM 4 MCD) operated at a base pressures of <2 × 10−9 mbar. The XPS spectra were measured by an achromatic Mg Kα excitation (1253.6 eV) at 10 kV, with an emission current of 15 mA. The UPS spectra of the samples were measured using He I line (21.22 eV) with a biased of −5.0 eV to observe the edge of the secondary cutoff.

3. Results and discussions

The performance of a set of structurally identical OLEDs with different HILs was investigated. Figure 2(a) shows the JV characteristics of OLEDs with different HILs inserted between ITO and CBP. Without the interlayer at anode/organic interface, the OLED obviously shows an extremely low current density without EL emission over an entire operation voltage range from 0 – 9 V. This indicates that the hole injection is not ideal due to a high injection barrier at the ITO/CBP interface [30]. An obvious increase in the injection current in OLED is obtained after the insertion of a MoO3 HIL between ITO and CBP, as shown in Fig. 2(a). It indicates that MoO3 HIL makes a good contact with the CBP [8]. OLEDs with an eMoO3 HIL or a sMoO3 HIL have a comparable current density and a low turn-on voltage of ~3.0 V. At a low driving voltage, the luminance of phosphorescent OLEDs with a sMoO3 HIL is slightly lower than that of the OLEDs with an eMoO3 HIL, as shown in Fig. 2(b), due to a high leakage current. Figure 2(a) depicts the JV characteristics of the devices in semi-log plot. OLEDs with a sMoO3 HIL have a high leakage current, e.g., ~2 orders of magnitude higher than that of the OLEDs with an eMoO3 HIL at the operating voltage <3.0 V.

 figure: Fig. 2

Fig. 2 (a) J–V and (b) L–V characteristics of a set of OLEDs fabricated using different HILs of pristine PEDOT:PSS (red dots), eMoO3 (green triangles), sMoO3 (inverted blue triangles) and hybrid PEDOT:PSS-MoO3 (sky blue diamonds).

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The introduction of the hybrid HIL exhibits a superior hole injection leading to high performing OLEDs. The mixture of PEDOT:PSS and MoO3 NPs solutions with an optimal volume ratio of 3:1 (PEDOT:PSS to MoO3) was used. In comparison to the performance of the OLEDs with an eMoO3 HIL, the OLEDs with a hybrid PEDOT:PSS-MoO3 HIL demonstrates a remarkable enhancement in injection current as shown in Fig. 2(a). The turn-on voltage of the OLEDs with a hybrid PEDOT:PSS-MoO3 HIL is slightly higher than that of the OLEDs with an eMoO3 HIL, as shown in Fig. 2(b). It can be seen that the luminance of the phosphorescent OLEDs with a hybrid HIL increases rapidly with the operating voltage, achieving efficient hole injection above 3.2 V.

In comparison to the performance of the OLEDs made different HILs of eMoO3, sMoO3 and hybrid PEDOT:PSS-MoO3, it is obvious that the performance of the structurally identical OLEDs with a pristine PEDOT:PSS interlayer apparently is less than satisfactory. In the following discussion, we concentrate on the high performing OLEDs to unraveling the fundamental mechanisms of hybrid PEDOT:PSS-MoO3-induced performance enhancement of OLEDs.

In addition, the efficiency of OLEDs made with different MoO3-based HILs was also studied. Figure 3 shows the power efficiency as a function of the luminance for a set of structurally identical OLEDs made with different HILs. The OLEDs having a hybrid PEDOT:PSS-MoO3 HIL has the highest power efficiency as compared to the efficiency of OLEDs with either an eMoO3 HIL or a sMoO3 HIL, e.g. 89.2 lm/W @ 100 cd/m2 and 73.5 lm/W @ 1000 cd/m2. The overall power efficiency of the hybrid HIL-contained OLEDs is almost 2 times higher than that of OLEDs made with a sMoO3 HIL. Figure 4 plots the EQE as a function of the luminance for a set of structurally identical OLEDs with different HILs. A high EQE for OLEDs with a hybrid HIL, e.g. >20%, was obtained over a wide luminance range from 10 to 10000 cd/m2. EQE of OLEDs with a hybrid PEDOT:PSS-MoO3 HIL is comparable to the best record of the OLEDs with an eMoO3 HIL [8]. It implies that the performance of phosphorescent OLEDs with a solution-processed hybrid PEDOT:PSS-MoO3 HIL is almost the same as that of the structurally identical devices having an eMoO3 HIL. Figure 4(b) shows the normalized EL spectra measured for OLEDs operated at 5.0 V. All OLEDs with different HILs have almost identical emission spectra with an EL peak position at 520 nm.

 figure: Fig. 3

Fig. 3 Comparison of power efficiency as a function of the luminance obtained for a set of structurally identical OLEDs made with different HILs of pristine PEDOT:PSS (red dots), eMoO3 (green triangles), sMoO3 (inverted blue triangles) and hybrid PEDOT:PSS-MoO3 (sky blue diamonds).

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 figure: Fig. 4

Fig. 4 (a) EQE as a function of the luminance for a set of structurally identical OLEDs having different HILs of pristine PEDOT:PSS (red dots), eMoO3 (green triangles), sMoO3 (inverted blue triangles) and hybrid PEDOT:PSS-MoO3 (sky blue diamonds), and (b) the normalized EL spectra of different OLEDs measured at 5.0 V.

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The effect of the volume ratio of PEDOT:PSS to MoO3 NPs in the mixed solution for making hybrid HIL on the performance of OLEDs was also investigated. The performance of a set of OLEDs made with a hybrid HIL, prepared using mixed solution having different volume ratios of PEDOT:PSS to MoO3 NPs solutions, e.g., 1:1, 2:1, 3:1 and 4:1, was examined. Figure 5(a) shows the JV characteristics of the set of OLEDs. It is obvious that the OLEDs with different hybrid HILs have a higher injection current than that of the OLEDs with an eMoO3 HIL. Apart from the hybrid HIL prepared with a mixed solution having a volume ratio of PEDOT:PSS to MoO3 NPs of 1:1, OLEDs with hybrid HILs having other ratios show similar injection currents. Figure 5(b) presents the EQE, as a function of luminance for a set of OLEDs with different hybrid HILs, having different volume ratios of PEDOT:PSS to MoO3 NPs in the mixed solution. The performance of OLEDs with an eMoO3 HIL is presented in Fig. 5(b) for comparison. Compared to the control OLED, there is a remarkable enhancement in EQE of OLEDs with a hybrid PEDOT:PSS-MoO3 HIL. When the volume ratio of PEDOT:PSS in the PEDOT:PSS-MoO3 mixed solution increased from 1:1 to 3:1, the EQE of the resulting hybrid HIL-contained OLEDs increased from 21.8% to 24.5%, measured at 1000 cd/m2. The optimal volume ratio of PEDOT:PSS to MoO3 is found to be 3:1. For PEDOT:PSS with a higher volume ratio of PEDOT:PSS in the mixed solution, e.g. 4:1, a slight reduction in EQE of the resulting OLEDs was observed.

 figure: Fig. 5

Fig. 5 (a) J–V characteristics as a function of luminance and (b) EQE as a function of luminance, measured for a set of structurally identical OLEDs with different hybrid PEDOT:PSS-MoO3 HILs, having different volume ratios of PEDOT:PSS to MoO3 in the mixed solutions, e.g. 1:1, 2:1, 3:1 and 4:1.

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The surface topography of HILs, e.g. the morphology and the surface roughness, were analyzed by AFM. AFM images measured for the surfaces of the bare ITO, eMoO3-modified ITO and sMoO3-modified ITO, are shown in Figs. 6(a) - 6(c), respectively. The bare ITO surface has a root mean square (rms) roughness of 2.3 nm. Figures 6(b) and 6(c) are the AFM images measured for the surface of an eMoO3-modified ITO and a sMoO3-modified ITO, respectively. Both images illustrate some large grains formed on the surface, showing the grain size in sub-nanometer range. In fact, it is quite common for metal oxides to aggregate forming large clusters with an average domain size ranging from few nanometer to sub-nanometer [31, 32]. However, there is a clear difference in the surface roughness of an eMoO3-modified ITO and a sMoO3-modified ITO. The eMoO3 film has a low rms roughness of 1.75 nm, while the sMoO3 film has a slightly higher rms roughness of 3.55 nm. It is possible that a rougher sMoO3 HIL is responsible for a high leakage current observed in the OLEDs, and thereby resulting in a deterioration in the device performance.

 figure: Fig. 6

Fig. 6 AFM images measured for the surfaces of (a) a bare ITO, (b) an eMoO3-modified ITO, and (c) a sMoO3-modified ITO. An area of 5.0 µm × 5.0 µm was characterized in the AFM measurements using tapping mode.

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The surface roughness and the morphology of hybrid PEDOT:PSS-MoO3 hole injection layers were also characterized. A pristine PEDOT:PSS film was fabricated as the reference. Figure 7(a) depicts the AFM image measured for PEDOT:PSS film deposited on the ITO. In comparison to the AFM images measured for the surfaces of an eMoO3-modified ITO and a sMoO3-modified ITO, as shown in Figs. 6(b) and 6(c), it is clear that PEDOT:PSS has a smoother surface with a rms roughness of 0.99 nm. The AFM images measured for the surfaces of the hybrid PEDOT:PSS-MoO3 layers, prepared using mixed solution with different volume ratios of PEDOT:PSS to MoO3 NPs, are shown in Figs. 7(b)-7(e). The hybrid films have similar surface morphology as compared to that of the pristine PEDOT:PSS film. The rms roughness of the hybrid HIL, prepared using mixed solution with 1:1 volume ratio of PEDOT:PSS to MoO3 NPs, is ~1.2 nm. For hybrid HIL made with mixed solution with other ratios, e.g. 2:1, 3:1 and 4:1, the film roughness is getting close to that of the surface of the pristine PEDOT:PSS film. In comparison to the sMoO3 thin film, the hybrid PEDOT:PSS-MoO3 layer on ITO possesses a smoother surface without showing large grains. It demonstrates that the presence of PEDOT:PSS in the hybrid HIL avoids the aggregation of MoO3 NPs, which otherwise would induce a rough surface. The use of the hybrid PEDOT:PSS-MoO3 HIL also favors forming a smoother contact resulting in improving device performance through suppression of interfacial defects at the organic/anode interface. This is consistent with the enhancement in performance of OLEDs with a solution-processed hybrid PEDOT:PSS-MoO3 HIL.

 figure: Fig. 7

Fig. 7 AFM images measured for (a) PEDOT:PSS HIL and, hybrid PEDOT:PSS-MoO3 HILs prepared using mixed solution with different volume ratios of PEDOT:PSS to MoO3 NPs, (b) 1:1, (c) 2:1, (d) 3:1, and (e) 4:1.

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In-depth investigation of the surface electronic properties of the sMoO3 was carried out by photoelectron spectroscopy. In some TMOs, e.g. MoO3, WO3, and V2O5 etc., the presence of the oxygen vacancy defects can assist in the hole injection at metal oxide/organic interface [33]. The oxygen deficiency can introduce the lower oxidation state of metal cations and influence of the work function in TMO [9]. Therefore, it is important to understand the injection properties by characterizing the work function and the electronic states in the sMoO3 as well as hybrid PEDOT:PSS-MoO3 HIL used in the OLEDs. The secondary cut-off and valence band edge of the UPS spectra measured for the sMoO3 thin film are shown in Figs. 8(a) and 8(b). The work function of sMoO3 HIL derived from the UPS measurement is ~5.62 eV. Figure 8(c) presents the Mo 3d XPS spectrum measured for the sMoO3 thin film. The Mo 3d XPS spectrum shows the characteristic of Mo 3d5/2 and 3d3/2 peaks, due to the Mo6+ contribution at 232.6 eV and 235.6 eV, respectively, showing the Mo6+ oxidation state in the pure sMoO3 thin film.

 figure: Fig. 8

Fig. 8 (a) The secondary-electron cut-off and (b) the valance band edge of the UPS spectra, and (c) Mo 3d XPS spectrum, measured for the sMoO3-coated ITO samples. The deconvoluted XPS double peaks, illustrating Mo 3d5/2 and 3d3/2 due to Mo6+, are also presented

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As shown in Fig. 8(c), Mo 3d XPS spectrum measured for the sMoO3 film can be fitted perfectly with the dual peak deconvoluted for Mo 3d5/2 and 3d3/2 double peaks, revealing the Mo6+ oxidization state with 3d5/2 and 3d3/2 at binding energies of 232.8 eV 30 and 235.9 eV respectively. However, the similar fitting approach cannot be perfectly realized for Mo 3d5/2 and 3d3/2 XPS spectrum measured for the hybrid PEDOT:PSS-MoO3. For example, the Mo 3d XPS dual peak measured for the hybrid HIL can only be fitted well with two pairs of the Mo 3d5/2 and 3d3/2 peaks located at 232.8 eV and 236 eV, which can be assigned to Mo6+ state, and that located at 231.7 eV and 234.6 eV, which can be assigned to Mo5+. It is apparent that the contribution due to Mo6+ in the Mo 3d XPS spectrum measured for the hybrid HIL is dominate. In a separate work (not shown here), the absorbance spectra of PEDOT:PSS layers in its pristine state, and doped with MoO3 were compared with the absorbance spectrum of a PEDOT:PSS layer. The absorption band (750 nm) is distinctive of radical cations suggesting a higher density of charge carriers compared with PEDOT:PSS, the results are associated with an increased concentration of charge carriers in the conductive polymer, and measured by the four-point probe along with along with XPS measurements, revealing the additional charge carriers in the polymer and an increase of its oxidation level, caused by deficiency of the MoO3 NPs. The presence of Mo5+ oxidation state in the hybrid HIL is related to the deficiency of the MoO3 in the hybrid PEDOT:PSS-MoO3.

The electronic properties of the hybrid PEDOT:PSS-MoO3 HIL were also analyzed using XPS and UPS. Figures 9(a) and 9(b) illustrate the UPS spectra measured for hybrid PEDOT:PSS-MoO3 (ratio of 3:1) HIL. For comparison, the UPS spectra of pristine PEDOT:PSS film was also plotted in the same figure. The work function of PEDOT:PSS film is ~5.22 eV, which agrees well with the reported results [4, 7]. There is an obvious shift in the the secondary-electron cut-off of the UPS spectrum, measured for the hybrid HIL, as compared to the one obtained for the pristine PEDOT:PSS film, as shown in Fig. 9(a). The hybrid HIL with a higher work function of 5.54 eV is observed. The increase in the work function of the hybrid HIL helps to improve the energy alignment between HIL and HTL, assisting in efficient hole injection, thereby enhancing power efficiency of OLEDs.

 figure: Fig. 9

Fig. 9 (a) The secondary-electron cut-off, and (b) the valance band edge of the USP spectra measured for the hybrid HIL, XPS spectra measured for (c) pristine PEDOT:PSS thin film, indicating the component of S 2s peak and (d) PEDOT:PSS-MoO3 layer, illustrating the presence of Mo6+ and Mo5+states in the hybrid HIL.

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The electronic properties of PEDOT:PSS and hybrid PEDOT:PSS-MoO3 HILs were also analyzed by XPS. For pristine PEDOT:PSS film, the S 2s XPS peaks, located at 231.7 eV and 228.3 eV as shown in Fig. 9(c), originate from PSS. They also presented in the XPS spectrum measured for the hybrid PEDOT:PSS-MoO3 film, along with the Mo 3d5/2 and 3d3/2 double peaks over the binding energy range from 225 to 240 eV, as shown in Fig. 9(d). The measured Mo 3d XPS spectrum can be fitted by two pairs of Mo 3d5/2 and 3d3/2 double peaks due to the existence of both Mo6+ and Mo5+ states. This implies that there are two possible Mo oxidation states in the hybrid HIL. The Mo 3d5/2 and 3d3/2 double peaks located at 235.6 eV (3d3/2) and 232.6 eV (3d5/2), due to Mo6+, are dominated in the hybrid HIL, while the contribution of Mo 3d5/2 and 3d3/2 double peaks located at 234.6 eV (3d3/2) and 231.7 eV (3d5/2) arises from Mo5+ state. The relative composition ratio of Mo5+ to Mo6+ is about 1:3.25. The presence of Mo5+ component is an indication of oxygen deficiency in MoO3 NPs in the hybrid HIL. The concentration of Mo5+ in the hybrid PEDOT:PSS-MoO3 HIL decreased after it was annealed in air.

The sMoO3 HIL has a high work function, however its surface roughness is not ideal as compared to that of an eMoO3 HIL. The high leakage current in the OLEDs, caused by the pin-hole in the sMoO3 HIL, is responsible for the poor performance of OLEDs. The use of a hybrid PEDOT:PSS-MoO3 HIL promotes a good interfacial contact between the ITO and low-lying HOMO HTL, e.g., CBP, favoring the efficient hole injection, and thus enhancing the performance of the OLEDs, as shown in Figs. 2−4. The superior device performance and the performance reproducibility are attributed to the combined effects of improved interfacial contact for efficient hole injection and an improved electron-hole current balance. This results in the power efficiency of 89.2 lm/W and external quantum efficiency of 23.9% for phosphorescent green OLEDs.

AFM measurements reveal a smooth and a pin-hole-free morphology in the hybrid HILs. The introduction of hybrid PEDOT:PSS-MoO3 helps to improve the surface smoothness as compared to the pure sMoO3 HIL. At an optimal volume ratio of PEDOT:PSS to MoO3 NPs (3:1), rms surface roughness of the hybrid HIL can be reduced to ~1.0 nm, which is comparable to that of the pristine PEDOT:PSS film. The smooth and pin-hole-free hybrid HIL is beneficial in forming the subsequent layer on its surface and therefore a good contact for hole injection.

The UPS studies have shown that the hybrid films possess a high work function. In the optimal condition, the hybrid HIL, made with a mixed solution having a volume ratio of PEDOT:PSS to MoO3 NPs of 3:1, has a work function of 5.54 eV, which is close to the HOMO level of CBP. In comparison to the PEDOT:PSS pristine film, an increase of 0.3 eV in the work function of the hybrid HIL is observed. The increase in the work function of HIL is desired for reducing the energy barrier at the HIL/HTL interface, and thus facilitating the hole injection. The XPS spectra have shown the presence of the oxygen vacancies in the hybrid films. Different studies have been demonstrated that the oxygen defects in transition metal oxides can modify the Fermi level and facilitate the charge transfer at the HIL/organic interface [9–12, 34, 35]. The presence of oxygen deficiency in TMO results its Fermi level located close to the conduction band, making TMO an n-type semiconductor. The low-lying conduction band of the hybrid HIL is capable to extract electrons from the HOMO level of the adjacent CBP layer. This charge transfer process would generate free hole carriers in the CBP layer. Therefore, this defect-assisted charge transfer favors the efficient hole injection and assists in achieving balanced electron-hole current balance, thereby improving power efficiency of OLEDs. The use of solution-processed hybrid HIL has a potential for application in large area OLEDs.

4. Conclusions

High performance OLEDs with a solution-processed hybrid PEDOT:PSS-MoO3 HIL, formulated using PEDOT:PSS and MoO3 NPs in aqueous solution, were demonstrated. It is clearly seen that the solution-processed hybrid HIL has profound impact on the performance of phosphorescent OLEDs having a low-lying HOMO level functional material. A remarkable improvement in the power efficiency of the phosphorescent OLEDs was realized. The use of hybrid PEDOT:PSS-MoO3 HIL not only significantly enhances the EL efficiency through improved electron-hole current balance, but also greatly increases the power efficiency at high luminance, achieved by elimination of the interfacial defects. The maximum power efficiency of 89.2 lm/W and EQE of 23.9% for phosphorescent green OLEDs were achieved. Our results reveal clearly that the use of a solution-processed PEDOT:PSS-MoO3 HIL promotes forming a good interfacial contact, facilitating the efficient hole injection, thereby enhancing the device efficiency and improving the performance reproducibility. The solution-processed PEDOT:PSS-MoO3 HIL is beneficial for application in full solution-processed organic electronic devices.

Acknowledgments

This work was supported by Research Grants Council of the Hong Kong Special Administrative Region, China, Project No. [T23-713/11].

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4. J. Lee, N. Chopra, S. H. Eom, Y. Zheng, J. G. Xue, F. So, and J. Shi, “Effects of triplet energies and transporting properties of carrier transporting materials on blue phosphorescent organic light emitting devices,” Appl. Phys. Lett. 93(12), 123306 (2008). [CrossRef]  

5. H. Wang, K. P. Klubek, and C. W. Tang, “Current efficiency in organic light-emitting diodes with a hole-injection layer,” Appl. Phys. Lett. 93(9), 093306 (2008). [CrossRef]  

6. Y. Q. Miao, Z. X. Gao, R. Tao, H. P. Shi, H. Wang, Y. H. Li, H. S. Jia, W. H. Choi, and F. R. Zhu, “Realization of ultra-high color stable hybrid white organic light-emitting diodes via sequential symmetrical doping in emissive layer,” Sci. Adv. Mater. 8(2), 401–407 (2016). [CrossRef]  

7. S. C. Tse, S. W. Tsang, and S. K. So, “PEDOT:PSS polymeric conducting anode for small organic transporting molecules in dark injection experiments,” J. Appl. Phys. 100(6), 063708 (2006). [CrossRef]  

8. Z. B. Wang, M. G. Helander, J. Qiu, D. P. Puzzo, M. T. Greiner, Z. W. Liu, and Z. H. Lu, “Highly simplified phosphorescent organic light emitting diode with >20% external quantum efficiency at >10,000cd/m2,” Appl. Phys. Lett. 98(7), 073310 (2011). [CrossRef]  

9. M. T. Greiner, L. Chai, M. G. Helander, W. M. Tang, and Z. H. Lu, “Transition metal oxide work functions: the influence of cation oxidation state and oxygen vacancies,” Adv. Funct. Mater. 22(21), 4557–4568 (2012). [CrossRef]  

10. J. Meyer, K. Zilberberg, T. Riedl, and A. Kahn, “Electronic structure of vanadium pentoxide: An efficient hole injector for organic electronic materials,” J. Appl. Phys. 110(3), 033710 (2011). [CrossRef]  

11. M. Krjöger, S. Hamwi, J. Meyer, T. Riedl, W. Kowalsky, and A. Kahn, “Role of the deep-lying electronic states of MoO3 in the enhancement of hole-injection in organic thin films,” Appl. Phys. Lett. 95(12), 123301 (2009). [CrossRef]  

12. J. Meyer, M. Krjöger, S. Hamwi, F. Gnam, T. Riedl, W. Kowalsky, and A. Kahn, “Charge generation layers comprising transition metal-oxide/organic interfaces: electronic structure and charge generation mechanism,” Appl. Phys. Lett. 96(19), 193302 (2010). [CrossRef]  

13. Z. Tan, L. Li, C. Cui, Y. Ding, Q. Xu, S. Li, D. Qian, and Y. Li, “Solution-processed tungsten oxide as an effective anode buffer layer for high-performance polymer solar cells,” J. Phys. Chem. C 116(35), 18626–18632 (2012). [CrossRef]  

14. H. Lee, Y. Kwon, and C. Lee, “Improved performances in organic and polymer light-emitting diodes using solution-processed vanadium pentoxide as a hole injection layer,” J. Soc. Inf. Disp. 20(12), 640–645 (2012). [CrossRef]  

15. C. Girotto, E. Voroshazi, D. Cheyns, P. Heremans, and B. P. Rand, “Solution-processed MoO₃ thin films as a hole-injection layer for organic solar cells,” ACS Appl. Mater. Interfaces 3(9), 3244–3247 (2011). [CrossRef]   [PubMed]  

16. T. Yang, M. Wang, Y. Cao, F. Huang, L. Huang, J. Peng, X. Gong, S. Z. D. Cheng, and Y. Cao, “Polymer solar cells with a low-temperature-annealed sol-gel-derived MoOx film as a hole extraction layer,” Adv. Energy Mater. 2(5), 523–527 (2012). [CrossRef]  

17. Y. W. Kwon, Y. N. Kim, H. K. Lee, C. H. Lee, and J. H. Kwak, “Composite film of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and MoO3 as an efficient hole injection layer for polymer light-emitting diodes,” Org. Electron. 15(6), 1083–1087 (2014). [CrossRef]  

18. M. F. Xu, L. S. Cui, X. Z. Zhu, C. H. Gao, X. B. Shi, Z. M. Jin, Z. K. Wang, and L. S. Liao, “Aqueous solution-processed MoO3 as an effective interfacial layer in polymer/fullerene based organic solar cells,” Org. Electron. 14(2), 657–664 (2013). [CrossRef]  

19. J. Meyer, R. Khalandovsky, P. Görrn, and A. Kahn, “MoO3 films spin-coated from a nanoparticle suspension for efficient hole-injection in organic electronics,” Adv. Mater. 23(1), 70–73 (2011). [CrossRef]   [PubMed]  

20. T. Stubhan, T. Ameri, M. Salinas, J. Krantz, F. Machui, M. Halik, and C. J. Brabec, “High shunt resistance in polymer solar cells comprising a MoO3 hole extraction layer processed from nanoparticle suspension,” Appl. Phys. Lett. 98(25), 253308 (2011). [CrossRef]  

21. S. J. Lee, H. P. Kim, A. R. bin Mohd Yusoff, and J. Jang, “Organic photovoltaic with PEDOT:PSS and V2O5 mixture as hole transport layer,” Sol. Energy Mater. Sol. Cells 120, 238–243 (2014). [CrossRef]  

22. Y. Wang, Q. Luo, N. Wu, Q. Wang, H. Zhu, L. Chen, Y. Q. Li, L. Luo, and C. Q. Ma, “Solution-processed MoO3:PEDOT:PSS hybrid hole transporting layer for inverted polymer solar cells,” ACS Appl. Mater. Interfaces 7(13), 7170–7179 (2015). [CrossRef]   [PubMed]  

23. H. Lu, J. Lin, N. Wu, S. H. Nie, Q. Luo, C. Q. Ma, and Z. Cui, “Inkjet printed silver nanowire network as top electrode for semi-transparent organic photovoltaic devices,” Appl. Phys. Lett. 106(9), 093302 (2015). [CrossRef]  

24. Q. Liu, I. Khatri, R. Ishikawa, K. Ueno, and H. Shirai, “Effects of molybdenum oxide molecular doping on the chemical structure of poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate) and on carrier collection efficiency of silicon/poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate) heterojunction solar cells,” Appl. Phys. Lett. 102(18), 183503 (2013). [CrossRef]  

25. M. Zhang, S. Höfle, J. Czolk, A. Mertens, and A. Colsmann, “All-solution processed transparent organic light emitting diodes,” Nanoscale 7(47), 20009–20014 (2015). [CrossRef]   [PubMed]  

26. S. Höfle, A. Schienle, C. Bernhard, M. Bruns, U. Lemmer, and A. Colsmann, “Solution processed, white emitting tandem organic light-emitting diodes with inverted device architecture,” Adv. Mater. 26(30), 5155–5159 (2014). [CrossRef]   [PubMed]  

27. W. H. Choi, H. L. Tam, D. Ma, and F. Zhu, “Emission behavior of dual-side emissive transparent white organic light-emitting diodes,” Opt. Express 23(11), A471–A479 (2015). [CrossRef]   [PubMed]  

28. Z. H. Wu, B. Wu, H. L. Tam, and F. R. Zhu, “An insight on oxide interlayer in organic solar cells: from light absorption and charge collection perspectives,” Org. Electron. 31, 266–272 (2016). [CrossRef]  

29. G. M. Ng, E. L. Kietzke, T. Kietzke, L. W. Tan, P. K. Liew, and F. R. Zhu, “Optical enhancement in semitransparent polymer photovoltaic cell,” Appl. Phys. Lett. 90(10), 103505 (2007). [CrossRef]  

30. Z. B. Wang, M. G. Helander, J. Qiu, Z. W. Liu, M. T. Greiner, and Z. H. Lu, “Direct hole injection in to 4,4′-N,N′-dicarbazole-biphenyl: A simple pathway to achieve efficient organic light emitting diodes,” J. Appl. Phys. 108(2), 024510 (2010). [CrossRef]  

31. A. W. Castleman and K. H. Bowen, “Clusters: Structure, energetics, and dynamics of intermediate states of matter,” J. Phys. Chem. 100(31), 12911–12944 (1996). [CrossRef]  

32. X. Tang, D. Bumueller, A. Lim, J. Schneider, U. Heiz, G. Gantefoer, D. H. Fairbrother, and K. H. Bowen, “Catalytic dehydration of 2-propanol by size-selected (WO3)n and (MoO3)n metal oxide clusters,” J. Phys. Chem. C 118(50), 29278–29286 (2014). [CrossRef]  

33. M. V. Ganduglia-Pirovano, A. Hofmann, and J. Sauer, “Oxygen vacancies in transition metal and rare earth oxides: Current state of understanding and remaining challenges,” Surf. Sci. Rep. 62(6), 219–270 (2007). [CrossRef]  

34. S. Deb and J. Chopoorian, “Optical properties and color‐center formation in thin films of molybdenum trioxide,” J. Appl. Phys. 37(13), 4818–4825 (1966). [CrossRef]  

35. T. S. Sian and G. Reddy, “Optical, structural and photoelectron spectroscopic studies on amorphous and crystalline molybdenum oxide thin films,” Sol. Energy Mater. Sol. Cells 82(3), 375–386 (2004). [CrossRef]  

References

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  1. S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, and K. Leo, “White organic light-emitting diodes with fluorescent tube efficiency,” Nature 459(7244), 234–238 (2009).
    [Crossref] [PubMed]
  2. S. J. Su, E. Gonmori, H. Sasabe, and J. Kido, “Highly efficient organic blue-and white-light-emitting devices having a carrier- and exciton-confining structure for reduced efficiency roll-off,” Adv. Mater. 20(21), 4189–4194 (2008).
  3. W. H. Choi, H. L. Tam, F. R. Zhu, D. G. Ma, H. Sasabe, and J. Kido, “High performance semitransparent phosphorescent white organic light emitting diodes with bi-directional and symmetrical illumination,” Appl. Phys. Lett. 102(15), 153308 (2013).
    [Crossref]
  4. J. Lee, N. Chopra, S. H. Eom, Y. Zheng, J. G. Xue, F. So, and J. Shi, “Effects of triplet energies and transporting properties of carrier transporting materials on blue phosphorescent organic light emitting devices,” Appl. Phys. Lett. 93(12), 123306 (2008).
    [Crossref]
  5. H. Wang, K. P. Klubek, and C. W. Tang, “Current efficiency in organic light-emitting diodes with a hole-injection layer,” Appl. Phys. Lett. 93(9), 093306 (2008).
    [Crossref]
  6. Y. Q. Miao, Z. X. Gao, R. Tao, H. P. Shi, H. Wang, Y. H. Li, H. S. Jia, W. H. Choi, and F. R. Zhu, “Realization of ultra-high color stable hybrid white organic light-emitting diodes via sequential symmetrical doping in emissive layer,” Sci. Adv. Mater. 8(2), 401–407 (2016).
    [Crossref]
  7. S. C. Tse, S. W. Tsang, and S. K. So, “PEDOT:PSS polymeric conducting anode for small organic transporting molecules in dark injection experiments,” J. Appl. Phys. 100(6), 063708 (2006).
    [Crossref]
  8. Z. B. Wang, M. G. Helander, J. Qiu, D. P. Puzzo, M. T. Greiner, Z. W. Liu, and Z. H. Lu, “Highly simplified phosphorescent organic light emitting diode with >20% external quantum efficiency at >10,000cd/m2,” Appl. Phys. Lett. 98(7), 073310 (2011).
    [Crossref]
  9. M. T. Greiner, L. Chai, M. G. Helander, W. M. Tang, and Z. H. Lu, “Transition metal oxide work functions: the influence of cation oxidation state and oxygen vacancies,” Adv. Funct. Mater. 22(21), 4557–4568 (2012).
    [Crossref]
  10. J. Meyer, K. Zilberberg, T. Riedl, and A. Kahn, “Electronic structure of vanadium pentoxide: An efficient hole injector for organic electronic materials,” J. Appl. Phys. 110(3), 033710 (2011).
    [Crossref]
  11. M. Krjöger, S. Hamwi, J. Meyer, T. Riedl, W. Kowalsky, and A. Kahn, “Role of the deep-lying electronic states of MoO3 in the enhancement of hole-injection in organic thin films,” Appl. Phys. Lett. 95(12), 123301 (2009).
    [Crossref]
  12. J. Meyer, M. Krjöger, S. Hamwi, F. Gnam, T. Riedl, W. Kowalsky, and A. Kahn, “Charge generation layers comprising transition metal-oxide/organic interfaces: electronic structure and charge generation mechanism,” Appl. Phys. Lett. 96(19), 193302 (2010).
    [Crossref]
  13. Z. Tan, L. Li, C. Cui, Y. Ding, Q. Xu, S. Li, D. Qian, and Y. Li, “Solution-processed tungsten oxide as an effective anode buffer layer for high-performance polymer solar cells,” J. Phys. Chem. C 116(35), 18626–18632 (2012).
    [Crossref]
  14. H. Lee, Y. Kwon, and C. Lee, “Improved performances in organic and polymer light-emitting diodes using solution-processed vanadium pentoxide as a hole injection layer,” J. Soc. Inf. Disp. 20(12), 640–645 (2012).
    [Crossref]
  15. C. Girotto, E. Voroshazi, D. Cheyns, P. Heremans, and B. P. Rand, “Solution-processed MoO₃ thin films as a hole-injection layer for organic solar cells,” ACS Appl. Mater. Interfaces 3(9), 3244–3247 (2011).
    [Crossref] [PubMed]
  16. T. Yang, M. Wang, Y. Cao, F. Huang, L. Huang, J. Peng, X. Gong, S. Z. D. Cheng, and Y. Cao, “Polymer solar cells with a low-temperature-annealed sol-gel-derived MoOx film as a hole extraction layer,” Adv. Energy Mater. 2(5), 523–527 (2012).
    [Crossref]
  17. Y. W. Kwon, Y. N. Kim, H. K. Lee, C. H. Lee, and J. H. Kwak, “Composite film of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and MoO3 as an efficient hole injection layer for polymer light-emitting diodes,” Org. Electron. 15(6), 1083–1087 (2014).
    [Crossref]
  18. M. F. Xu, L. S. Cui, X. Z. Zhu, C. H. Gao, X. B. Shi, Z. M. Jin, Z. K. Wang, and L. S. Liao, “Aqueous solution-processed MoO3 as an effective interfacial layer in polymer/fullerene based organic solar cells,” Org. Electron. 14(2), 657–664 (2013).
    [Crossref]
  19. J. Meyer, R. Khalandovsky, P. Görrn, and A. Kahn, “MoO3 films spin-coated from a nanoparticle suspension for efficient hole-injection in organic electronics,” Adv. Mater. 23(1), 70–73 (2011).
    [Crossref] [PubMed]
  20. T. Stubhan, T. Ameri, M. Salinas, J. Krantz, F. Machui, M. Halik, and C. J. Brabec, “High shunt resistance in polymer solar cells comprising a MoO3 hole extraction layer processed from nanoparticle suspension,” Appl. Phys. Lett. 98(25), 253308 (2011).
    [Crossref]
  21. S. J. Lee, H. P. Kim, A. R. bin Mohd Yusoff, and J. Jang, “Organic photovoltaic with PEDOT:PSS and V2O5 mixture as hole transport layer,” Sol. Energy Mater. Sol. Cells 120, 238–243 (2014).
    [Crossref]
  22. Y. Wang, Q. Luo, N. Wu, Q. Wang, H. Zhu, L. Chen, Y. Q. Li, L. Luo, and C. Q. Ma, “Solution-processed MoO3:PEDOT:PSS hybrid hole transporting layer for inverted polymer solar cells,” ACS Appl. Mater. Interfaces 7(13), 7170–7179 (2015).
    [Crossref] [PubMed]
  23. H. Lu, J. Lin, N. Wu, S. H. Nie, Q. Luo, C. Q. Ma, and Z. Cui, “Inkjet printed silver nanowire network as top electrode for semi-transparent organic photovoltaic devices,” Appl. Phys. Lett. 106(9), 093302 (2015).
    [Crossref]
  24. Q. Liu, I. Khatri, R. Ishikawa, K. Ueno, and H. Shirai, “Effects of molybdenum oxide molecular doping on the chemical structure of poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate) and on carrier collection efficiency of silicon/poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate) heterojunction solar cells,” Appl. Phys. Lett. 102(18), 183503 (2013).
    [Crossref]
  25. M. Zhang, S. Höfle, J. Czolk, A. Mertens, and A. Colsmann, “All-solution processed transparent organic light emitting diodes,” Nanoscale 7(47), 20009–20014 (2015).
    [Crossref] [PubMed]
  26. S. Höfle, A. Schienle, C. Bernhard, M. Bruns, U. Lemmer, and A. Colsmann, “Solution processed, white emitting tandem organic light-emitting diodes with inverted device architecture,” Adv. Mater. 26(30), 5155–5159 (2014).
    [Crossref] [PubMed]
  27. W. H. Choi, H. L. Tam, D. Ma, and F. Zhu, “Emission behavior of dual-side emissive transparent white organic light-emitting diodes,” Opt. Express 23(11), A471–A479 (2015).
    [Crossref] [PubMed]
  28. Z. H. Wu, B. Wu, H. L. Tam, and F. R. Zhu, “An insight on oxide interlayer in organic solar cells: from light absorption and charge collection perspectives,” Org. Electron. 31, 266–272 (2016).
    [Crossref]
  29. G. M. Ng, E. L. Kietzke, T. Kietzke, L. W. Tan, P. K. Liew, and F. R. Zhu, “Optical enhancement in semitransparent polymer photovoltaic cell,” Appl. Phys. Lett. 90(10), 103505 (2007).
    [Crossref]
  30. Z. B. Wang, M. G. Helander, J. Qiu, Z. W. Liu, M. T. Greiner, and Z. H. Lu, “Direct hole injection in to 4,4′-N,N′-dicarbazole-biphenyl: A simple pathway to achieve efficient organic light emitting diodes,” J. Appl. Phys. 108(2), 024510 (2010).
    [Crossref]
  31. A. W. Castleman and K. H. Bowen, “Clusters: Structure, energetics, and dynamics of intermediate states of matter,” J. Phys. Chem. 100(31), 12911–12944 (1996).
    [Crossref]
  32. X. Tang, D. Bumueller, A. Lim, J. Schneider, U. Heiz, G. Gantefoer, D. H. Fairbrother, and K. H. Bowen, “Catalytic dehydration of 2-propanol by size-selected (WO3)n and (MoO3)n metal oxide clusters,” J. Phys. Chem. C 118(50), 29278–29286 (2014).
    [Crossref]
  33. M. V. Ganduglia-Pirovano, A. Hofmann, and J. Sauer, “Oxygen vacancies in transition metal and rare earth oxides: Current state of understanding and remaining challenges,” Surf. Sci. Rep. 62(6), 219–270 (2007).
    [Crossref]
  34. S. Deb and J. Chopoorian, “Optical properties and color‐center formation in thin films of molybdenum trioxide,” J. Appl. Phys. 37(13), 4818–4825 (1966).
    [Crossref]
  35. T. S. Sian and G. Reddy, “Optical, structural and photoelectron spectroscopic studies on amorphous and crystalline molybdenum oxide thin films,” Sol. Energy Mater. Sol. Cells 82(3), 375–386 (2004).
    [Crossref]

2016 (2)

Y. Q. Miao, Z. X. Gao, R. Tao, H. P. Shi, H. Wang, Y. H. Li, H. S. Jia, W. H. Choi, and F. R. Zhu, “Realization of ultra-high color stable hybrid white organic light-emitting diodes via sequential symmetrical doping in emissive layer,” Sci. Adv. Mater. 8(2), 401–407 (2016).
[Crossref]

Z. H. Wu, B. Wu, H. L. Tam, and F. R. Zhu, “An insight on oxide interlayer in organic solar cells: from light absorption and charge collection perspectives,” Org. Electron. 31, 266–272 (2016).
[Crossref]

2015 (4)

M. Zhang, S. Höfle, J. Czolk, A. Mertens, and A. Colsmann, “All-solution processed transparent organic light emitting diodes,” Nanoscale 7(47), 20009–20014 (2015).
[Crossref] [PubMed]

Y. Wang, Q. Luo, N. Wu, Q. Wang, H. Zhu, L. Chen, Y. Q. Li, L. Luo, and C. Q. Ma, “Solution-processed MoO3:PEDOT:PSS hybrid hole transporting layer for inverted polymer solar cells,” ACS Appl. Mater. Interfaces 7(13), 7170–7179 (2015).
[Crossref] [PubMed]

H. Lu, J. Lin, N. Wu, S. H. Nie, Q. Luo, C. Q. Ma, and Z. Cui, “Inkjet printed silver nanowire network as top electrode for semi-transparent organic photovoltaic devices,” Appl. Phys. Lett. 106(9), 093302 (2015).
[Crossref]

W. H. Choi, H. L. Tam, D. Ma, and F. Zhu, “Emission behavior of dual-side emissive transparent white organic light-emitting diodes,” Opt. Express 23(11), A471–A479 (2015).
[Crossref] [PubMed]

2014 (4)

X. Tang, D. Bumueller, A. Lim, J. Schneider, U. Heiz, G. Gantefoer, D. H. Fairbrother, and K. H. Bowen, “Catalytic dehydration of 2-propanol by size-selected (WO3)n and (MoO3)n metal oxide clusters,” J. Phys. Chem. C 118(50), 29278–29286 (2014).
[Crossref]

S. J. Lee, H. P. Kim, A. R. bin Mohd Yusoff, and J. Jang, “Organic photovoltaic with PEDOT:PSS and V2O5 mixture as hole transport layer,” Sol. Energy Mater. Sol. Cells 120, 238–243 (2014).
[Crossref]

S. Höfle, A. Schienle, C. Bernhard, M. Bruns, U. Lemmer, and A. Colsmann, “Solution processed, white emitting tandem organic light-emitting diodes with inverted device architecture,” Adv. Mater. 26(30), 5155–5159 (2014).
[Crossref] [PubMed]

Y. W. Kwon, Y. N. Kim, H. K. Lee, C. H. Lee, and J. H. Kwak, “Composite film of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and MoO3 as an efficient hole injection layer for polymer light-emitting diodes,” Org. Electron. 15(6), 1083–1087 (2014).
[Crossref]

2013 (3)

M. F. Xu, L. S. Cui, X. Z. Zhu, C. H. Gao, X. B. Shi, Z. M. Jin, Z. K. Wang, and L. S. Liao, “Aqueous solution-processed MoO3 as an effective interfacial layer in polymer/fullerene based organic solar cells,” Org. Electron. 14(2), 657–664 (2013).
[Crossref]

W. H. Choi, H. L. Tam, F. R. Zhu, D. G. Ma, H. Sasabe, and J. Kido, “High performance semitransparent phosphorescent white organic light emitting diodes with bi-directional and symmetrical illumination,” Appl. Phys. Lett. 102(15), 153308 (2013).
[Crossref]

Q. Liu, I. Khatri, R. Ishikawa, K. Ueno, and H. Shirai, “Effects of molybdenum oxide molecular doping on the chemical structure of poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate) and on carrier collection efficiency of silicon/poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate) heterojunction solar cells,” Appl. Phys. Lett. 102(18), 183503 (2013).
[Crossref]

2012 (4)

T. Yang, M. Wang, Y. Cao, F. Huang, L. Huang, J. Peng, X. Gong, S. Z. D. Cheng, and Y. Cao, “Polymer solar cells with a low-temperature-annealed sol-gel-derived MoOx film as a hole extraction layer,” Adv. Energy Mater. 2(5), 523–527 (2012).
[Crossref]

M. T. Greiner, L. Chai, M. G. Helander, W. M. Tang, and Z. H. Lu, “Transition metal oxide work functions: the influence of cation oxidation state and oxygen vacancies,” Adv. Funct. Mater. 22(21), 4557–4568 (2012).
[Crossref]

Z. Tan, L. Li, C. Cui, Y. Ding, Q. Xu, S. Li, D. Qian, and Y. Li, “Solution-processed tungsten oxide as an effective anode buffer layer for high-performance polymer solar cells,” J. Phys. Chem. C 116(35), 18626–18632 (2012).
[Crossref]

H. Lee, Y. Kwon, and C. Lee, “Improved performances in organic and polymer light-emitting diodes using solution-processed vanadium pentoxide as a hole injection layer,” J. Soc. Inf. Disp. 20(12), 640–645 (2012).
[Crossref]

2011 (5)

C. Girotto, E. Voroshazi, D. Cheyns, P. Heremans, and B. P. Rand, “Solution-processed MoO₃ thin films as a hole-injection layer for organic solar cells,” ACS Appl. Mater. Interfaces 3(9), 3244–3247 (2011).
[Crossref] [PubMed]

Z. B. Wang, M. G. Helander, J. Qiu, D. P. Puzzo, M. T. Greiner, Z. W. Liu, and Z. H. Lu, “Highly simplified phosphorescent organic light emitting diode with >20% external quantum efficiency at >10,000cd/m2,” Appl. Phys. Lett. 98(7), 073310 (2011).
[Crossref]

J. Meyer, R. Khalandovsky, P. Görrn, and A. Kahn, “MoO3 films spin-coated from a nanoparticle suspension for efficient hole-injection in organic electronics,” Adv. Mater. 23(1), 70–73 (2011).
[Crossref] [PubMed]

T. Stubhan, T. Ameri, M. Salinas, J. Krantz, F. Machui, M. Halik, and C. J. Brabec, “High shunt resistance in polymer solar cells comprising a MoO3 hole extraction layer processed from nanoparticle suspension,” Appl. Phys. Lett. 98(25), 253308 (2011).
[Crossref]

J. Meyer, K. Zilberberg, T. Riedl, and A. Kahn, “Electronic structure of vanadium pentoxide: An efficient hole injector for organic electronic materials,” J. Appl. Phys. 110(3), 033710 (2011).
[Crossref]

2010 (2)

J. Meyer, M. Krjöger, S. Hamwi, F. Gnam, T. Riedl, W. Kowalsky, and A. Kahn, “Charge generation layers comprising transition metal-oxide/organic interfaces: electronic structure and charge generation mechanism,” Appl. Phys. Lett. 96(19), 193302 (2010).
[Crossref]

Z. B. Wang, M. G. Helander, J. Qiu, Z. W. Liu, M. T. Greiner, and Z. H. Lu, “Direct hole injection in to 4,4′-N,N′-dicarbazole-biphenyl: A simple pathway to achieve efficient organic light emitting diodes,” J. Appl. Phys. 108(2), 024510 (2010).
[Crossref]

2009 (2)

M. Krjöger, S. Hamwi, J. Meyer, T. Riedl, W. Kowalsky, and A. Kahn, “Role of the deep-lying electronic states of MoO3 in the enhancement of hole-injection in organic thin films,” Appl. Phys. Lett. 95(12), 123301 (2009).
[Crossref]

S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, and K. Leo, “White organic light-emitting diodes with fluorescent tube efficiency,” Nature 459(7244), 234–238 (2009).
[Crossref] [PubMed]

2008 (3)

S. J. Su, E. Gonmori, H. Sasabe, and J. Kido, “Highly efficient organic blue-and white-light-emitting devices having a carrier- and exciton-confining structure for reduced efficiency roll-off,” Adv. Mater. 20(21), 4189–4194 (2008).

J. Lee, N. Chopra, S. H. Eom, Y. Zheng, J. G. Xue, F. So, and J. Shi, “Effects of triplet energies and transporting properties of carrier transporting materials on blue phosphorescent organic light emitting devices,” Appl. Phys. Lett. 93(12), 123306 (2008).
[Crossref]

H. Wang, K. P. Klubek, and C. W. Tang, “Current efficiency in organic light-emitting diodes with a hole-injection layer,” Appl. Phys. Lett. 93(9), 093306 (2008).
[Crossref]

2007 (2)

M. V. Ganduglia-Pirovano, A. Hofmann, and J. Sauer, “Oxygen vacancies in transition metal and rare earth oxides: Current state of understanding and remaining challenges,” Surf. Sci. Rep. 62(6), 219–270 (2007).
[Crossref]

G. M. Ng, E. L. Kietzke, T. Kietzke, L. W. Tan, P. K. Liew, and F. R. Zhu, “Optical enhancement in semitransparent polymer photovoltaic cell,” Appl. Phys. Lett. 90(10), 103505 (2007).
[Crossref]

2006 (1)

S. C. Tse, S. W. Tsang, and S. K. So, “PEDOT:PSS polymeric conducting anode for small organic transporting molecules in dark injection experiments,” J. Appl. Phys. 100(6), 063708 (2006).
[Crossref]

2004 (1)

T. S. Sian and G. Reddy, “Optical, structural and photoelectron spectroscopic studies on amorphous and crystalline molybdenum oxide thin films,” Sol. Energy Mater. Sol. Cells 82(3), 375–386 (2004).
[Crossref]

1996 (1)

A. W. Castleman and K. H. Bowen, “Clusters: Structure, energetics, and dynamics of intermediate states of matter,” J. Phys. Chem. 100(31), 12911–12944 (1996).
[Crossref]

1966 (1)

S. Deb and J. Chopoorian, “Optical properties and color‐center formation in thin films of molybdenum trioxide,” J. Appl. Phys. 37(13), 4818–4825 (1966).
[Crossref]

Ameri, T.

T. Stubhan, T. Ameri, M. Salinas, J. Krantz, F. Machui, M. Halik, and C. J. Brabec, “High shunt resistance in polymer solar cells comprising a MoO3 hole extraction layer processed from nanoparticle suspension,” Appl. Phys. Lett. 98(25), 253308 (2011).
[Crossref]

Bernhard, C.

S. Höfle, A. Schienle, C. Bernhard, M. Bruns, U. Lemmer, and A. Colsmann, “Solution processed, white emitting tandem organic light-emitting diodes with inverted device architecture,” Adv. Mater. 26(30), 5155–5159 (2014).
[Crossref] [PubMed]

bin Mohd Yusoff, A. R.

S. J. Lee, H. P. Kim, A. R. bin Mohd Yusoff, and J. Jang, “Organic photovoltaic with PEDOT:PSS and V2O5 mixture as hole transport layer,” Sol. Energy Mater. Sol. Cells 120, 238–243 (2014).
[Crossref]

Bowen, K. H.

X. Tang, D. Bumueller, A. Lim, J. Schneider, U. Heiz, G. Gantefoer, D. H. Fairbrother, and K. H. Bowen, “Catalytic dehydration of 2-propanol by size-selected (WO3)n and (MoO3)n metal oxide clusters,” J. Phys. Chem. C 118(50), 29278–29286 (2014).
[Crossref]

A. W. Castleman and K. H. Bowen, “Clusters: Structure, energetics, and dynamics of intermediate states of matter,” J. Phys. Chem. 100(31), 12911–12944 (1996).
[Crossref]

Brabec, C. J.

T. Stubhan, T. Ameri, M. Salinas, J. Krantz, F. Machui, M. Halik, and C. J. Brabec, “High shunt resistance in polymer solar cells comprising a MoO3 hole extraction layer processed from nanoparticle suspension,” Appl. Phys. Lett. 98(25), 253308 (2011).
[Crossref]

Bruns, M.

S. Höfle, A. Schienle, C. Bernhard, M. Bruns, U. Lemmer, and A. Colsmann, “Solution processed, white emitting tandem organic light-emitting diodes with inverted device architecture,” Adv. Mater. 26(30), 5155–5159 (2014).
[Crossref] [PubMed]

Bumueller, D.

X. Tang, D. Bumueller, A. Lim, J. Schneider, U. Heiz, G. Gantefoer, D. H. Fairbrother, and K. H. Bowen, “Catalytic dehydration of 2-propanol by size-selected (WO3)n and (MoO3)n metal oxide clusters,” J. Phys. Chem. C 118(50), 29278–29286 (2014).
[Crossref]

Cao, Y.

T. Yang, M. Wang, Y. Cao, F. Huang, L. Huang, J. Peng, X. Gong, S. Z. D. Cheng, and Y. Cao, “Polymer solar cells with a low-temperature-annealed sol-gel-derived MoOx film as a hole extraction layer,” Adv. Energy Mater. 2(5), 523–527 (2012).
[Crossref]

T. Yang, M. Wang, Y. Cao, F. Huang, L. Huang, J. Peng, X. Gong, S. Z. D. Cheng, and Y. Cao, “Polymer solar cells with a low-temperature-annealed sol-gel-derived MoOx film as a hole extraction layer,” Adv. Energy Mater. 2(5), 523–527 (2012).
[Crossref]

Castleman, A. W.

A. W. Castleman and K. H. Bowen, “Clusters: Structure, energetics, and dynamics of intermediate states of matter,” J. Phys. Chem. 100(31), 12911–12944 (1996).
[Crossref]

Chai, L.

M. T. Greiner, L. Chai, M. G. Helander, W. M. Tang, and Z. H. Lu, “Transition metal oxide work functions: the influence of cation oxidation state and oxygen vacancies,” Adv. Funct. Mater. 22(21), 4557–4568 (2012).
[Crossref]

Chen, L.

Y. Wang, Q. Luo, N. Wu, Q. Wang, H. Zhu, L. Chen, Y. Q. Li, L. Luo, and C. Q. Ma, “Solution-processed MoO3:PEDOT:PSS hybrid hole transporting layer for inverted polymer solar cells,” ACS Appl. Mater. Interfaces 7(13), 7170–7179 (2015).
[Crossref] [PubMed]

Cheng, S. Z. D.

T. Yang, M. Wang, Y. Cao, F. Huang, L. Huang, J. Peng, X. Gong, S. Z. D. Cheng, and Y. Cao, “Polymer solar cells with a low-temperature-annealed sol-gel-derived MoOx film as a hole extraction layer,” Adv. Energy Mater. 2(5), 523–527 (2012).
[Crossref]

Cheyns, D.

C. Girotto, E. Voroshazi, D. Cheyns, P. Heremans, and B. P. Rand, “Solution-processed MoO₃ thin films as a hole-injection layer for organic solar cells,” ACS Appl. Mater. Interfaces 3(9), 3244–3247 (2011).
[Crossref] [PubMed]

Choi, W. H.

Y. Q. Miao, Z. X. Gao, R. Tao, H. P. Shi, H. Wang, Y. H. Li, H. S. Jia, W. H. Choi, and F. R. Zhu, “Realization of ultra-high color stable hybrid white organic light-emitting diodes via sequential symmetrical doping in emissive layer,” Sci. Adv. Mater. 8(2), 401–407 (2016).
[Crossref]

W. H. Choi, H. L. Tam, D. Ma, and F. Zhu, “Emission behavior of dual-side emissive transparent white organic light-emitting diodes,” Opt. Express 23(11), A471–A479 (2015).
[Crossref] [PubMed]

W. H. Choi, H. L. Tam, F. R. Zhu, D. G. Ma, H. Sasabe, and J. Kido, “High performance semitransparent phosphorescent white organic light emitting diodes with bi-directional and symmetrical illumination,” Appl. Phys. Lett. 102(15), 153308 (2013).
[Crossref]

Chopoorian, J.

S. Deb and J. Chopoorian, “Optical properties and color‐center formation in thin films of molybdenum trioxide,” J. Appl. Phys. 37(13), 4818–4825 (1966).
[Crossref]

Chopra, N.

J. Lee, N. Chopra, S. H. Eom, Y. Zheng, J. G. Xue, F. So, and J. Shi, “Effects of triplet energies and transporting properties of carrier transporting materials on blue phosphorescent organic light emitting devices,” Appl. Phys. Lett. 93(12), 123306 (2008).
[Crossref]

Colsmann, A.

M. Zhang, S. Höfle, J. Czolk, A. Mertens, and A. Colsmann, “All-solution processed transparent organic light emitting diodes,” Nanoscale 7(47), 20009–20014 (2015).
[Crossref] [PubMed]

S. Höfle, A. Schienle, C. Bernhard, M. Bruns, U. Lemmer, and A. Colsmann, “Solution processed, white emitting tandem organic light-emitting diodes with inverted device architecture,” Adv. Mater. 26(30), 5155–5159 (2014).
[Crossref] [PubMed]

Cui, C.

Z. Tan, L. Li, C. Cui, Y. Ding, Q. Xu, S. Li, D. Qian, and Y. Li, “Solution-processed tungsten oxide as an effective anode buffer layer for high-performance polymer solar cells,” J. Phys. Chem. C 116(35), 18626–18632 (2012).
[Crossref]

Cui, L. S.

M. F. Xu, L. S. Cui, X. Z. Zhu, C. H. Gao, X. B. Shi, Z. M. Jin, Z. K. Wang, and L. S. Liao, “Aqueous solution-processed MoO3 as an effective interfacial layer in polymer/fullerene based organic solar cells,” Org. Electron. 14(2), 657–664 (2013).
[Crossref]

Cui, Z.

H. Lu, J. Lin, N. Wu, S. H. Nie, Q. Luo, C. Q. Ma, and Z. Cui, “Inkjet printed silver nanowire network as top electrode for semi-transparent organic photovoltaic devices,” Appl. Phys. Lett. 106(9), 093302 (2015).
[Crossref]

Czolk, J.

M. Zhang, S. Höfle, J. Czolk, A. Mertens, and A. Colsmann, “All-solution processed transparent organic light emitting diodes,” Nanoscale 7(47), 20009–20014 (2015).
[Crossref] [PubMed]

Deb, S.

S. Deb and J. Chopoorian, “Optical properties and color‐center formation in thin films of molybdenum trioxide,” J. Appl. Phys. 37(13), 4818–4825 (1966).
[Crossref]

Ding, Y.

Z. Tan, L. Li, C. Cui, Y. Ding, Q. Xu, S. Li, D. Qian, and Y. Li, “Solution-processed tungsten oxide as an effective anode buffer layer for high-performance polymer solar cells,” J. Phys. Chem. C 116(35), 18626–18632 (2012).
[Crossref]

Eom, S. H.

J. Lee, N. Chopra, S. H. Eom, Y. Zheng, J. G. Xue, F. So, and J. Shi, “Effects of triplet energies and transporting properties of carrier transporting materials on blue phosphorescent organic light emitting devices,” Appl. Phys. Lett. 93(12), 123306 (2008).
[Crossref]

Fairbrother, D. H.

X. Tang, D. Bumueller, A. Lim, J. Schneider, U. Heiz, G. Gantefoer, D. H. Fairbrother, and K. H. Bowen, “Catalytic dehydration of 2-propanol by size-selected (WO3)n and (MoO3)n metal oxide clusters,” J. Phys. Chem. C 118(50), 29278–29286 (2014).
[Crossref]

Ganduglia-Pirovano, M. V.

M. V. Ganduglia-Pirovano, A. Hofmann, and J. Sauer, “Oxygen vacancies in transition metal and rare earth oxides: Current state of understanding and remaining challenges,” Surf. Sci. Rep. 62(6), 219–270 (2007).
[Crossref]

Gantefoer, G.

X. Tang, D. Bumueller, A. Lim, J. Schneider, U. Heiz, G. Gantefoer, D. H. Fairbrother, and K. H. Bowen, “Catalytic dehydration of 2-propanol by size-selected (WO3)n and (MoO3)n metal oxide clusters,” J. Phys. Chem. C 118(50), 29278–29286 (2014).
[Crossref]

Gao, C. H.

M. F. Xu, L. S. Cui, X. Z. Zhu, C. H. Gao, X. B. Shi, Z. M. Jin, Z. K. Wang, and L. S. Liao, “Aqueous solution-processed MoO3 as an effective interfacial layer in polymer/fullerene based organic solar cells,” Org. Electron. 14(2), 657–664 (2013).
[Crossref]

Gao, Z. X.

Y. Q. Miao, Z. X. Gao, R. Tao, H. P. Shi, H. Wang, Y. H. Li, H. S. Jia, W. H. Choi, and F. R. Zhu, “Realization of ultra-high color stable hybrid white organic light-emitting diodes via sequential symmetrical doping in emissive layer,” Sci. Adv. Mater. 8(2), 401–407 (2016).
[Crossref]

Girotto, C.

C. Girotto, E. Voroshazi, D. Cheyns, P. Heremans, and B. P. Rand, “Solution-processed MoO₃ thin films as a hole-injection layer for organic solar cells,” ACS Appl. Mater. Interfaces 3(9), 3244–3247 (2011).
[Crossref] [PubMed]

Gnam, F.

J. Meyer, M. Krjöger, S. Hamwi, F. Gnam, T. Riedl, W. Kowalsky, and A. Kahn, “Charge generation layers comprising transition metal-oxide/organic interfaces: electronic structure and charge generation mechanism,” Appl. Phys. Lett. 96(19), 193302 (2010).
[Crossref]

Gong, X.

T. Yang, M. Wang, Y. Cao, F. Huang, L. Huang, J. Peng, X. Gong, S. Z. D. Cheng, and Y. Cao, “Polymer solar cells with a low-temperature-annealed sol-gel-derived MoOx film as a hole extraction layer,” Adv. Energy Mater. 2(5), 523–527 (2012).
[Crossref]

Gonmori, E.

S. J. Su, E. Gonmori, H. Sasabe, and J. Kido, “Highly efficient organic blue-and white-light-emitting devices having a carrier- and exciton-confining structure for reduced efficiency roll-off,” Adv. Mater. 20(21), 4189–4194 (2008).

Görrn, P.

J. Meyer, R. Khalandovsky, P. Görrn, and A. Kahn, “MoO3 films spin-coated from a nanoparticle suspension for efficient hole-injection in organic electronics,” Adv. Mater. 23(1), 70–73 (2011).
[Crossref] [PubMed]

Greiner, M. T.

M. T. Greiner, L. Chai, M. G. Helander, W. M. Tang, and Z. H. Lu, “Transition metal oxide work functions: the influence of cation oxidation state and oxygen vacancies,” Adv. Funct. Mater. 22(21), 4557–4568 (2012).
[Crossref]

Z. B. Wang, M. G. Helander, J. Qiu, D. P. Puzzo, M. T. Greiner, Z. W. Liu, and Z. H. Lu, “Highly simplified phosphorescent organic light emitting diode with >20% external quantum efficiency at >10,000cd/m2,” Appl. Phys. Lett. 98(7), 073310 (2011).
[Crossref]

Z. B. Wang, M. G. Helander, J. Qiu, Z. W. Liu, M. T. Greiner, and Z. H. Lu, “Direct hole injection in to 4,4′-N,N′-dicarbazole-biphenyl: A simple pathway to achieve efficient organic light emitting diodes,” J. Appl. Phys. 108(2), 024510 (2010).
[Crossref]

Halik, M.

T. Stubhan, T. Ameri, M. Salinas, J. Krantz, F. Machui, M. Halik, and C. J. Brabec, “High shunt resistance in polymer solar cells comprising a MoO3 hole extraction layer processed from nanoparticle suspension,” Appl. Phys. Lett. 98(25), 253308 (2011).
[Crossref]

Hamwi, S.

J. Meyer, M. Krjöger, S. Hamwi, F. Gnam, T. Riedl, W. Kowalsky, and A. Kahn, “Charge generation layers comprising transition metal-oxide/organic interfaces: electronic structure and charge generation mechanism,” Appl. Phys. Lett. 96(19), 193302 (2010).
[Crossref]

M. Krjöger, S. Hamwi, J. Meyer, T. Riedl, W. Kowalsky, and A. Kahn, “Role of the deep-lying electronic states of MoO3 in the enhancement of hole-injection in organic thin films,” Appl. Phys. Lett. 95(12), 123301 (2009).
[Crossref]

Heiz, U.

X. Tang, D. Bumueller, A. Lim, J. Schneider, U. Heiz, G. Gantefoer, D. H. Fairbrother, and K. H. Bowen, “Catalytic dehydration of 2-propanol by size-selected (WO3)n and (MoO3)n metal oxide clusters,” J. Phys. Chem. C 118(50), 29278–29286 (2014).
[Crossref]

Helander, M. G.

M. T. Greiner, L. Chai, M. G. Helander, W. M. Tang, and Z. H. Lu, “Transition metal oxide work functions: the influence of cation oxidation state and oxygen vacancies,” Adv. Funct. Mater. 22(21), 4557–4568 (2012).
[Crossref]

Z. B. Wang, M. G. Helander, J. Qiu, D. P. Puzzo, M. T. Greiner, Z. W. Liu, and Z. H. Lu, “Highly simplified phosphorescent organic light emitting diode with >20% external quantum efficiency at >10,000cd/m2,” Appl. Phys. Lett. 98(7), 073310 (2011).
[Crossref]

Z. B. Wang, M. G. Helander, J. Qiu, Z. W. Liu, M. T. Greiner, and Z. H. Lu, “Direct hole injection in to 4,4′-N,N′-dicarbazole-biphenyl: A simple pathway to achieve efficient organic light emitting diodes,” J. Appl. Phys. 108(2), 024510 (2010).
[Crossref]

Heremans, P.

C. Girotto, E. Voroshazi, D. Cheyns, P. Heremans, and B. P. Rand, “Solution-processed MoO₃ thin films as a hole-injection layer for organic solar cells,” ACS Appl. Mater. Interfaces 3(9), 3244–3247 (2011).
[Crossref] [PubMed]

Höfle, S.

M. Zhang, S. Höfle, J. Czolk, A. Mertens, and A. Colsmann, “All-solution processed transparent organic light emitting diodes,” Nanoscale 7(47), 20009–20014 (2015).
[Crossref] [PubMed]

S. Höfle, A. Schienle, C. Bernhard, M. Bruns, U. Lemmer, and A. Colsmann, “Solution processed, white emitting tandem organic light-emitting diodes with inverted device architecture,” Adv. Mater. 26(30), 5155–5159 (2014).
[Crossref] [PubMed]

Hofmann, A.

M. V. Ganduglia-Pirovano, A. Hofmann, and J. Sauer, “Oxygen vacancies in transition metal and rare earth oxides: Current state of understanding and remaining challenges,” Surf. Sci. Rep. 62(6), 219–270 (2007).
[Crossref]

Huang, F.

T. Yang, M. Wang, Y. Cao, F. Huang, L. Huang, J. Peng, X. Gong, S. Z. D. Cheng, and Y. Cao, “Polymer solar cells with a low-temperature-annealed sol-gel-derived MoOx film as a hole extraction layer,” Adv. Energy Mater. 2(5), 523–527 (2012).
[Crossref]

Huang, L.

T. Yang, M. Wang, Y. Cao, F. Huang, L. Huang, J. Peng, X. Gong, S. Z. D. Cheng, and Y. Cao, “Polymer solar cells with a low-temperature-annealed sol-gel-derived MoOx film as a hole extraction layer,” Adv. Energy Mater. 2(5), 523–527 (2012).
[Crossref]

Ishikawa, R.

Q. Liu, I. Khatri, R. Ishikawa, K. Ueno, and H. Shirai, “Effects of molybdenum oxide molecular doping on the chemical structure of poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate) and on carrier collection efficiency of silicon/poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate) heterojunction solar cells,” Appl. Phys. Lett. 102(18), 183503 (2013).
[Crossref]

Jang, J.

S. J. Lee, H. P. Kim, A. R. bin Mohd Yusoff, and J. Jang, “Organic photovoltaic with PEDOT:PSS and V2O5 mixture as hole transport layer,” Sol. Energy Mater. Sol. Cells 120, 238–243 (2014).
[Crossref]

Jia, H. S.

Y. Q. Miao, Z. X. Gao, R. Tao, H. P. Shi, H. Wang, Y. H. Li, H. S. Jia, W. H. Choi, and F. R. Zhu, “Realization of ultra-high color stable hybrid white organic light-emitting diodes via sequential symmetrical doping in emissive layer,” Sci. Adv. Mater. 8(2), 401–407 (2016).
[Crossref]

Jin, Z. M.

M. F. Xu, L. S. Cui, X. Z. Zhu, C. H. Gao, X. B. Shi, Z. M. Jin, Z. K. Wang, and L. S. Liao, “Aqueous solution-processed MoO3 as an effective interfacial layer in polymer/fullerene based organic solar cells,” Org. Electron. 14(2), 657–664 (2013).
[Crossref]

Kahn, A.

J. Meyer, R. Khalandovsky, P. Görrn, and A. Kahn, “MoO3 films spin-coated from a nanoparticle suspension for efficient hole-injection in organic electronics,” Adv. Mater. 23(1), 70–73 (2011).
[Crossref] [PubMed]

J. Meyer, K. Zilberberg, T. Riedl, and A. Kahn, “Electronic structure of vanadium pentoxide: An efficient hole injector for organic electronic materials,” J. Appl. Phys. 110(3), 033710 (2011).
[Crossref]

J. Meyer, M. Krjöger, S. Hamwi, F. Gnam, T. Riedl, W. Kowalsky, and A. Kahn, “Charge generation layers comprising transition metal-oxide/organic interfaces: electronic structure and charge generation mechanism,” Appl. Phys. Lett. 96(19), 193302 (2010).
[Crossref]

M. Krjöger, S. Hamwi, J. Meyer, T. Riedl, W. Kowalsky, and A. Kahn, “Role of the deep-lying electronic states of MoO3 in the enhancement of hole-injection in organic thin films,” Appl. Phys. Lett. 95(12), 123301 (2009).
[Crossref]

Khalandovsky, R.

J. Meyer, R. Khalandovsky, P. Görrn, and A. Kahn, “MoO3 films spin-coated from a nanoparticle suspension for efficient hole-injection in organic electronics,” Adv. Mater. 23(1), 70–73 (2011).
[Crossref] [PubMed]

Khatri, I.

Q. Liu, I. Khatri, R. Ishikawa, K. Ueno, and H. Shirai, “Effects of molybdenum oxide molecular doping on the chemical structure of poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate) and on carrier collection efficiency of silicon/poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate) heterojunction solar cells,” Appl. Phys. Lett. 102(18), 183503 (2013).
[Crossref]

Kido, J.

W. H. Choi, H. L. Tam, F. R. Zhu, D. G. Ma, H. Sasabe, and J. Kido, “High performance semitransparent phosphorescent white organic light emitting diodes with bi-directional and symmetrical illumination,” Appl. Phys. Lett. 102(15), 153308 (2013).
[Crossref]

S. J. Su, E. Gonmori, H. Sasabe, and J. Kido, “Highly efficient organic blue-and white-light-emitting devices having a carrier- and exciton-confining structure for reduced efficiency roll-off,” Adv. Mater. 20(21), 4189–4194 (2008).

Kietzke, E. L.

G. M. Ng, E. L. Kietzke, T. Kietzke, L. W. Tan, P. K. Liew, and F. R. Zhu, “Optical enhancement in semitransparent polymer photovoltaic cell,” Appl. Phys. Lett. 90(10), 103505 (2007).
[Crossref]

Kietzke, T.

G. M. Ng, E. L. Kietzke, T. Kietzke, L. W. Tan, P. K. Liew, and F. R. Zhu, “Optical enhancement in semitransparent polymer photovoltaic cell,” Appl. Phys. Lett. 90(10), 103505 (2007).
[Crossref]

Kim, H. P.

S. J. Lee, H. P. Kim, A. R. bin Mohd Yusoff, and J. Jang, “Organic photovoltaic with PEDOT:PSS and V2O5 mixture as hole transport layer,” Sol. Energy Mater. Sol. Cells 120, 238–243 (2014).
[Crossref]

Kim, Y. N.

Y. W. Kwon, Y. N. Kim, H. K. Lee, C. H. Lee, and J. H. Kwak, “Composite film of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and MoO3 as an efficient hole injection layer for polymer light-emitting diodes,” Org. Electron. 15(6), 1083–1087 (2014).
[Crossref]

Klubek, K. P.

H. Wang, K. P. Klubek, and C. W. Tang, “Current efficiency in organic light-emitting diodes with a hole-injection layer,” Appl. Phys. Lett. 93(9), 093306 (2008).
[Crossref]

Kowalsky, W.

J. Meyer, M. Krjöger, S. Hamwi, F. Gnam, T. Riedl, W. Kowalsky, and A. Kahn, “Charge generation layers comprising transition metal-oxide/organic interfaces: electronic structure and charge generation mechanism,” Appl. Phys. Lett. 96(19), 193302 (2010).
[Crossref]

M. Krjöger, S. Hamwi, J. Meyer, T. Riedl, W. Kowalsky, and A. Kahn, “Role of the deep-lying electronic states of MoO3 in the enhancement of hole-injection in organic thin films,” Appl. Phys. Lett. 95(12), 123301 (2009).
[Crossref]

Krantz, J.

T. Stubhan, T. Ameri, M. Salinas, J. Krantz, F. Machui, M. Halik, and C. J. Brabec, “High shunt resistance in polymer solar cells comprising a MoO3 hole extraction layer processed from nanoparticle suspension,” Appl. Phys. Lett. 98(25), 253308 (2011).
[Crossref]

Krjöger, M.

J. Meyer, M. Krjöger, S. Hamwi, F. Gnam, T. Riedl, W. Kowalsky, and A. Kahn, “Charge generation layers comprising transition metal-oxide/organic interfaces: electronic structure and charge generation mechanism,” Appl. Phys. Lett. 96(19), 193302 (2010).
[Crossref]

M. Krjöger, S. Hamwi, J. Meyer, T. Riedl, W. Kowalsky, and A. Kahn, “Role of the deep-lying electronic states of MoO3 in the enhancement of hole-injection in organic thin films,” Appl. Phys. Lett. 95(12), 123301 (2009).
[Crossref]

Kwak, J. H.

Y. W. Kwon, Y. N. Kim, H. K. Lee, C. H. Lee, and J. H. Kwak, “Composite film of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and MoO3 as an efficient hole injection layer for polymer light-emitting diodes,” Org. Electron. 15(6), 1083–1087 (2014).
[Crossref]

Kwon, Y.

H. Lee, Y. Kwon, and C. Lee, “Improved performances in organic and polymer light-emitting diodes using solution-processed vanadium pentoxide as a hole injection layer,” J. Soc. Inf. Disp. 20(12), 640–645 (2012).
[Crossref]

Kwon, Y. W.

Y. W. Kwon, Y. N. Kim, H. K. Lee, C. H. Lee, and J. H. Kwak, “Composite film of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and MoO3 as an efficient hole injection layer for polymer light-emitting diodes,” Org. Electron. 15(6), 1083–1087 (2014).
[Crossref]

Lee, C.

H. Lee, Y. Kwon, and C. Lee, “Improved performances in organic and polymer light-emitting diodes using solution-processed vanadium pentoxide as a hole injection layer,” J. Soc. Inf. Disp. 20(12), 640–645 (2012).
[Crossref]

Lee, C. H.

Y. W. Kwon, Y. N. Kim, H. K. Lee, C. H. Lee, and J. H. Kwak, “Composite film of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and MoO3 as an efficient hole injection layer for polymer light-emitting diodes,” Org. Electron. 15(6), 1083–1087 (2014).
[Crossref]

Lee, H.

H. Lee, Y. Kwon, and C. Lee, “Improved performances in organic and polymer light-emitting diodes using solution-processed vanadium pentoxide as a hole injection layer,” J. Soc. Inf. Disp. 20(12), 640–645 (2012).
[Crossref]

Lee, H. K.

Y. W. Kwon, Y. N. Kim, H. K. Lee, C. H. Lee, and J. H. Kwak, “Composite film of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and MoO3 as an efficient hole injection layer for polymer light-emitting diodes,” Org. Electron. 15(6), 1083–1087 (2014).
[Crossref]

Lee, J.

J. Lee, N. Chopra, S. H. Eom, Y. Zheng, J. G. Xue, F. So, and J. Shi, “Effects of triplet energies and transporting properties of carrier transporting materials on blue phosphorescent organic light emitting devices,” Appl. Phys. Lett. 93(12), 123306 (2008).
[Crossref]

Lee, S. J.

S. J. Lee, H. P. Kim, A. R. bin Mohd Yusoff, and J. Jang, “Organic photovoltaic with PEDOT:PSS and V2O5 mixture as hole transport layer,” Sol. Energy Mater. Sol. Cells 120, 238–243 (2014).
[Crossref]

Lemmer, U.

S. Höfle, A. Schienle, C. Bernhard, M. Bruns, U. Lemmer, and A. Colsmann, “Solution processed, white emitting tandem organic light-emitting diodes with inverted device architecture,” Adv. Mater. 26(30), 5155–5159 (2014).
[Crossref] [PubMed]

Leo, K.

S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, and K. Leo, “White organic light-emitting diodes with fluorescent tube efficiency,” Nature 459(7244), 234–238 (2009).
[Crossref] [PubMed]

Li, L.

Z. Tan, L. Li, C. Cui, Y. Ding, Q. Xu, S. Li, D. Qian, and Y. Li, “Solution-processed tungsten oxide as an effective anode buffer layer for high-performance polymer solar cells,” J. Phys. Chem. C 116(35), 18626–18632 (2012).
[Crossref]

Li, S.

Z. Tan, L. Li, C. Cui, Y. Ding, Q. Xu, S. Li, D. Qian, and Y. Li, “Solution-processed tungsten oxide as an effective anode buffer layer for high-performance polymer solar cells,” J. Phys. Chem. C 116(35), 18626–18632 (2012).
[Crossref]

Li, Y.

Z. Tan, L. Li, C. Cui, Y. Ding, Q. Xu, S. Li, D. Qian, and Y. Li, “Solution-processed tungsten oxide as an effective anode buffer layer for high-performance polymer solar cells,” J. Phys. Chem. C 116(35), 18626–18632 (2012).
[Crossref]

Li, Y. H.

Y. Q. Miao, Z. X. Gao, R. Tao, H. P. Shi, H. Wang, Y. H. Li, H. S. Jia, W. H. Choi, and F. R. Zhu, “Realization of ultra-high color stable hybrid white organic light-emitting diodes via sequential symmetrical doping in emissive layer,” Sci. Adv. Mater. 8(2), 401–407 (2016).
[Crossref]

Li, Y. Q.

Y. Wang, Q. Luo, N. Wu, Q. Wang, H. Zhu, L. Chen, Y. Q. Li, L. Luo, and C. Q. Ma, “Solution-processed MoO3:PEDOT:PSS hybrid hole transporting layer for inverted polymer solar cells,” ACS Appl. Mater. Interfaces 7(13), 7170–7179 (2015).
[Crossref] [PubMed]

Liao, L. S.

M. F. Xu, L. S. Cui, X. Z. Zhu, C. H. Gao, X. B. Shi, Z. M. Jin, Z. K. Wang, and L. S. Liao, “Aqueous solution-processed MoO3 as an effective interfacial layer in polymer/fullerene based organic solar cells,” Org. Electron. 14(2), 657–664 (2013).
[Crossref]

Liew, P. K.

G. M. Ng, E. L. Kietzke, T. Kietzke, L. W. Tan, P. K. Liew, and F. R. Zhu, “Optical enhancement in semitransparent polymer photovoltaic cell,” Appl. Phys. Lett. 90(10), 103505 (2007).
[Crossref]

Lim, A.

X. Tang, D. Bumueller, A. Lim, J. Schneider, U. Heiz, G. Gantefoer, D. H. Fairbrother, and K. H. Bowen, “Catalytic dehydration of 2-propanol by size-selected (WO3)n and (MoO3)n metal oxide clusters,” J. Phys. Chem. C 118(50), 29278–29286 (2014).
[Crossref]

Lin, J.

H. Lu, J. Lin, N. Wu, S. H. Nie, Q. Luo, C. Q. Ma, and Z. Cui, “Inkjet printed silver nanowire network as top electrode for semi-transparent organic photovoltaic devices,” Appl. Phys. Lett. 106(9), 093302 (2015).
[Crossref]

Lindner, F.

S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, and K. Leo, “White organic light-emitting diodes with fluorescent tube efficiency,” Nature 459(7244), 234–238 (2009).
[Crossref] [PubMed]

Liu, Q.

Q. Liu, I. Khatri, R. Ishikawa, K. Ueno, and H. Shirai, “Effects of molybdenum oxide molecular doping on the chemical structure of poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate) and on carrier collection efficiency of silicon/poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate) heterojunction solar cells,” Appl. Phys. Lett. 102(18), 183503 (2013).
[Crossref]

Liu, Z. W.

Z. B. Wang, M. G. Helander, J. Qiu, D. P. Puzzo, M. T. Greiner, Z. W. Liu, and Z. H. Lu, “Highly simplified phosphorescent organic light emitting diode with >20% external quantum efficiency at >10,000cd/m2,” Appl. Phys. Lett. 98(7), 073310 (2011).
[Crossref]

Z. B. Wang, M. G. Helander, J. Qiu, Z. W. Liu, M. T. Greiner, and Z. H. Lu, “Direct hole injection in to 4,4′-N,N′-dicarbazole-biphenyl: A simple pathway to achieve efficient organic light emitting diodes,” J. Appl. Phys. 108(2), 024510 (2010).
[Crossref]

Lu, H.

H. Lu, J. Lin, N. Wu, S. H. Nie, Q. Luo, C. Q. Ma, and Z. Cui, “Inkjet printed silver nanowire network as top electrode for semi-transparent organic photovoltaic devices,” Appl. Phys. Lett. 106(9), 093302 (2015).
[Crossref]

Lu, Z. H.

M. T. Greiner, L. Chai, M. G. Helander, W. M. Tang, and Z. H. Lu, “Transition metal oxide work functions: the influence of cation oxidation state and oxygen vacancies,” Adv. Funct. Mater. 22(21), 4557–4568 (2012).
[Crossref]

Z. B. Wang, M. G. Helander, J. Qiu, D. P. Puzzo, M. T. Greiner, Z. W. Liu, and Z. H. Lu, “Highly simplified phosphorescent organic light emitting diode with >20% external quantum efficiency at >10,000cd/m2,” Appl. Phys. Lett. 98(7), 073310 (2011).
[Crossref]

Z. B. Wang, M. G. Helander, J. Qiu, Z. W. Liu, M. T. Greiner, and Z. H. Lu, “Direct hole injection in to 4,4′-N,N′-dicarbazole-biphenyl: A simple pathway to achieve efficient organic light emitting diodes,” J. Appl. Phys. 108(2), 024510 (2010).
[Crossref]

Luo, L.

Y. Wang, Q. Luo, N. Wu, Q. Wang, H. Zhu, L. Chen, Y. Q. Li, L. Luo, and C. Q. Ma, “Solution-processed MoO3:PEDOT:PSS hybrid hole transporting layer for inverted polymer solar cells,” ACS Appl. Mater. Interfaces 7(13), 7170–7179 (2015).
[Crossref] [PubMed]

Luo, Q.

Y. Wang, Q. Luo, N. Wu, Q. Wang, H. Zhu, L. Chen, Y. Q. Li, L. Luo, and C. Q. Ma, “Solution-processed MoO3:PEDOT:PSS hybrid hole transporting layer for inverted polymer solar cells,” ACS Appl. Mater. Interfaces 7(13), 7170–7179 (2015).
[Crossref] [PubMed]

H. Lu, J. Lin, N. Wu, S. H. Nie, Q. Luo, C. Q. Ma, and Z. Cui, “Inkjet printed silver nanowire network as top electrode for semi-transparent organic photovoltaic devices,” Appl. Phys. Lett. 106(9), 093302 (2015).
[Crossref]

Lüssem, B.

S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, and K. Leo, “White organic light-emitting diodes with fluorescent tube efficiency,” Nature 459(7244), 234–238 (2009).
[Crossref] [PubMed]

Ma, C. Q.

H. Lu, J. Lin, N. Wu, S. H. Nie, Q. Luo, C. Q. Ma, and Z. Cui, “Inkjet printed silver nanowire network as top electrode for semi-transparent organic photovoltaic devices,” Appl. Phys. Lett. 106(9), 093302 (2015).
[Crossref]

Y. Wang, Q. Luo, N. Wu, Q. Wang, H. Zhu, L. Chen, Y. Q. Li, L. Luo, and C. Q. Ma, “Solution-processed MoO3:PEDOT:PSS hybrid hole transporting layer for inverted polymer solar cells,” ACS Appl. Mater. Interfaces 7(13), 7170–7179 (2015).
[Crossref] [PubMed]

Ma, D.

Ma, D. G.

W. H. Choi, H. L. Tam, F. R. Zhu, D. G. Ma, H. Sasabe, and J. Kido, “High performance semitransparent phosphorescent white organic light emitting diodes with bi-directional and symmetrical illumination,” Appl. Phys. Lett. 102(15), 153308 (2013).
[Crossref]

Machui, F.

T. Stubhan, T. Ameri, M. Salinas, J. Krantz, F. Machui, M. Halik, and C. J. Brabec, “High shunt resistance in polymer solar cells comprising a MoO3 hole extraction layer processed from nanoparticle suspension,” Appl. Phys. Lett. 98(25), 253308 (2011).
[Crossref]

Mertens, A.

M. Zhang, S. Höfle, J. Czolk, A. Mertens, and A. Colsmann, “All-solution processed transparent organic light emitting diodes,” Nanoscale 7(47), 20009–20014 (2015).
[Crossref] [PubMed]

Meyer, J.

J. Meyer, R. Khalandovsky, P. Görrn, and A. Kahn, “MoO3 films spin-coated from a nanoparticle suspension for efficient hole-injection in organic electronics,” Adv. Mater. 23(1), 70–73 (2011).
[Crossref] [PubMed]

J. Meyer, K. Zilberberg, T. Riedl, and A. Kahn, “Electronic structure of vanadium pentoxide: An efficient hole injector for organic electronic materials,” J. Appl. Phys. 110(3), 033710 (2011).
[Crossref]

J. Meyer, M. Krjöger, S. Hamwi, F. Gnam, T. Riedl, W. Kowalsky, and A. Kahn, “Charge generation layers comprising transition metal-oxide/organic interfaces: electronic structure and charge generation mechanism,” Appl. Phys. Lett. 96(19), 193302 (2010).
[Crossref]

M. Krjöger, S. Hamwi, J. Meyer, T. Riedl, W. Kowalsky, and A. Kahn, “Role of the deep-lying electronic states of MoO3 in the enhancement of hole-injection in organic thin films,” Appl. Phys. Lett. 95(12), 123301 (2009).
[Crossref]

Miao, Y. Q.

Y. Q. Miao, Z. X. Gao, R. Tao, H. P. Shi, H. Wang, Y. H. Li, H. S. Jia, W. H. Choi, and F. R. Zhu, “Realization of ultra-high color stable hybrid white organic light-emitting diodes via sequential symmetrical doping in emissive layer,” Sci. Adv. Mater. 8(2), 401–407 (2016).
[Crossref]

Ng, G. M.

G. M. Ng, E. L. Kietzke, T. Kietzke, L. W. Tan, P. K. Liew, and F. R. Zhu, “Optical enhancement in semitransparent polymer photovoltaic cell,” Appl. Phys. Lett. 90(10), 103505 (2007).
[Crossref]

Nie, S. H.

H. Lu, J. Lin, N. Wu, S. H. Nie, Q. Luo, C. Q. Ma, and Z. Cui, “Inkjet printed silver nanowire network as top electrode for semi-transparent organic photovoltaic devices,” Appl. Phys. Lett. 106(9), 093302 (2015).
[Crossref]

Peng, J.

T. Yang, M. Wang, Y. Cao, F. Huang, L. Huang, J. Peng, X. Gong, S. Z. D. Cheng, and Y. Cao, “Polymer solar cells with a low-temperature-annealed sol-gel-derived MoOx film as a hole extraction layer,” Adv. Energy Mater. 2(5), 523–527 (2012).
[Crossref]

Puzzo, D. P.

Z. B. Wang, M. G. Helander, J. Qiu, D. P. Puzzo, M. T. Greiner, Z. W. Liu, and Z. H. Lu, “Highly simplified phosphorescent organic light emitting diode with >20% external quantum efficiency at >10,000cd/m2,” Appl. Phys. Lett. 98(7), 073310 (2011).
[Crossref]

Qian, D.

Z. Tan, L. Li, C. Cui, Y. Ding, Q. Xu, S. Li, D. Qian, and Y. Li, “Solution-processed tungsten oxide as an effective anode buffer layer for high-performance polymer solar cells,” J. Phys. Chem. C 116(35), 18626–18632 (2012).
[Crossref]

Qiu, J.

Z. B. Wang, M. G. Helander, J. Qiu, D. P. Puzzo, M. T. Greiner, Z. W. Liu, and Z. H. Lu, “Highly simplified phosphorescent organic light emitting diode with >20% external quantum efficiency at >10,000cd/m2,” Appl. Phys. Lett. 98(7), 073310 (2011).
[Crossref]

Z. B. Wang, M. G. Helander, J. Qiu, Z. W. Liu, M. T. Greiner, and Z. H. Lu, “Direct hole injection in to 4,4′-N,N′-dicarbazole-biphenyl: A simple pathway to achieve efficient organic light emitting diodes,” J. Appl. Phys. 108(2), 024510 (2010).
[Crossref]

Rand, B. P.

C. Girotto, E. Voroshazi, D. Cheyns, P. Heremans, and B. P. Rand, “Solution-processed MoO₃ thin films as a hole-injection layer for organic solar cells,” ACS Appl. Mater. Interfaces 3(9), 3244–3247 (2011).
[Crossref] [PubMed]

Reddy, G.

T. S. Sian and G. Reddy, “Optical, structural and photoelectron spectroscopic studies on amorphous and crystalline molybdenum oxide thin films,” Sol. Energy Mater. Sol. Cells 82(3), 375–386 (2004).
[Crossref]

Reineke, S.

S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, and K. Leo, “White organic light-emitting diodes with fluorescent tube efficiency,” Nature 459(7244), 234–238 (2009).
[Crossref] [PubMed]

Riedl, T.

J. Meyer, K. Zilberberg, T. Riedl, and A. Kahn, “Electronic structure of vanadium pentoxide: An efficient hole injector for organic electronic materials,” J. Appl. Phys. 110(3), 033710 (2011).
[Crossref]

J. Meyer, M. Krjöger, S. Hamwi, F. Gnam, T. Riedl, W. Kowalsky, and A. Kahn, “Charge generation layers comprising transition metal-oxide/organic interfaces: electronic structure and charge generation mechanism,” Appl. Phys. Lett. 96(19), 193302 (2010).
[Crossref]

M. Krjöger, S. Hamwi, J. Meyer, T. Riedl, W. Kowalsky, and A. Kahn, “Role of the deep-lying electronic states of MoO3 in the enhancement of hole-injection in organic thin films,” Appl. Phys. Lett. 95(12), 123301 (2009).
[Crossref]

Salinas, M.

T. Stubhan, T. Ameri, M. Salinas, J. Krantz, F. Machui, M. Halik, and C. J. Brabec, “High shunt resistance in polymer solar cells comprising a MoO3 hole extraction layer processed from nanoparticle suspension,” Appl. Phys. Lett. 98(25), 253308 (2011).
[Crossref]

Sasabe, H.

W. H. Choi, H. L. Tam, F. R. Zhu, D. G. Ma, H. Sasabe, and J. Kido, “High performance semitransparent phosphorescent white organic light emitting diodes with bi-directional and symmetrical illumination,” Appl. Phys. Lett. 102(15), 153308 (2013).
[Crossref]

S. J. Su, E. Gonmori, H. Sasabe, and J. Kido, “Highly efficient organic blue-and white-light-emitting devices having a carrier- and exciton-confining structure for reduced efficiency roll-off,” Adv. Mater. 20(21), 4189–4194 (2008).

Sauer, J.

M. V. Ganduglia-Pirovano, A. Hofmann, and J. Sauer, “Oxygen vacancies in transition metal and rare earth oxides: Current state of understanding and remaining challenges,” Surf. Sci. Rep. 62(6), 219–270 (2007).
[Crossref]

Schienle, A.

S. Höfle, A. Schienle, C. Bernhard, M. Bruns, U. Lemmer, and A. Colsmann, “Solution processed, white emitting tandem organic light-emitting diodes with inverted device architecture,” Adv. Mater. 26(30), 5155–5159 (2014).
[Crossref] [PubMed]

Schneider, J.

X. Tang, D. Bumueller, A. Lim, J. Schneider, U. Heiz, G. Gantefoer, D. H. Fairbrother, and K. H. Bowen, “Catalytic dehydration of 2-propanol by size-selected (WO3)n and (MoO3)n metal oxide clusters,” J. Phys. Chem. C 118(50), 29278–29286 (2014).
[Crossref]

Schwartz, G.

S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, and K. Leo, “White organic light-emitting diodes with fluorescent tube efficiency,” Nature 459(7244), 234–238 (2009).
[Crossref] [PubMed]

Seidler, N.

S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, and K. Leo, “White organic light-emitting diodes with fluorescent tube efficiency,” Nature 459(7244), 234–238 (2009).
[Crossref] [PubMed]

Shi, H. P.

Y. Q. Miao, Z. X. Gao, R. Tao, H. P. Shi, H. Wang, Y. H. Li, H. S. Jia, W. H. Choi, and F. R. Zhu, “Realization of ultra-high color stable hybrid white organic light-emitting diodes via sequential symmetrical doping in emissive layer,” Sci. Adv. Mater. 8(2), 401–407 (2016).
[Crossref]

Shi, J.

J. Lee, N. Chopra, S. H. Eom, Y. Zheng, J. G. Xue, F. So, and J. Shi, “Effects of triplet energies and transporting properties of carrier transporting materials on blue phosphorescent organic light emitting devices,” Appl. Phys. Lett. 93(12), 123306 (2008).
[Crossref]

Shi, X. B.

M. F. Xu, L. S. Cui, X. Z. Zhu, C. H. Gao, X. B. Shi, Z. M. Jin, Z. K. Wang, and L. S. Liao, “Aqueous solution-processed MoO3 as an effective interfacial layer in polymer/fullerene based organic solar cells,” Org. Electron. 14(2), 657–664 (2013).
[Crossref]

Shirai, H.

Q. Liu, I. Khatri, R. Ishikawa, K. Ueno, and H. Shirai, “Effects of molybdenum oxide molecular doping on the chemical structure of poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate) and on carrier collection efficiency of silicon/poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate) heterojunction solar cells,” Appl. Phys. Lett. 102(18), 183503 (2013).
[Crossref]

Sian, T. S.

T. S. Sian and G. Reddy, “Optical, structural and photoelectron spectroscopic studies on amorphous and crystalline molybdenum oxide thin films,” Sol. Energy Mater. Sol. Cells 82(3), 375–386 (2004).
[Crossref]

So, F.

J. Lee, N. Chopra, S. H. Eom, Y. Zheng, J. G. Xue, F. So, and J. Shi, “Effects of triplet energies and transporting properties of carrier transporting materials on blue phosphorescent organic light emitting devices,” Appl. Phys. Lett. 93(12), 123306 (2008).
[Crossref]

So, S. K.

S. C. Tse, S. W. Tsang, and S. K. So, “PEDOT:PSS polymeric conducting anode for small organic transporting molecules in dark injection experiments,” J. Appl. Phys. 100(6), 063708 (2006).
[Crossref]

Stubhan, T.

T. Stubhan, T. Ameri, M. Salinas, J. Krantz, F. Machui, M. Halik, and C. J. Brabec, “High shunt resistance in polymer solar cells comprising a MoO3 hole extraction layer processed from nanoparticle suspension,” Appl. Phys. Lett. 98(25), 253308 (2011).
[Crossref]

Su, S. J.

S. J. Su, E. Gonmori, H. Sasabe, and J. Kido, “Highly efficient organic blue-and white-light-emitting devices having a carrier- and exciton-confining structure for reduced efficiency roll-off,” Adv. Mater. 20(21), 4189–4194 (2008).

Tam, H. L.

Z. H. Wu, B. Wu, H. L. Tam, and F. R. Zhu, “An insight on oxide interlayer in organic solar cells: from light absorption and charge collection perspectives,” Org. Electron. 31, 266–272 (2016).
[Crossref]

W. H. Choi, H. L. Tam, D. Ma, and F. Zhu, “Emission behavior of dual-side emissive transparent white organic light-emitting diodes,” Opt. Express 23(11), A471–A479 (2015).
[Crossref] [PubMed]

W. H. Choi, H. L. Tam, F. R. Zhu, D. G. Ma, H. Sasabe, and J. Kido, “High performance semitransparent phosphorescent white organic light emitting diodes with bi-directional and symmetrical illumination,” Appl. Phys. Lett. 102(15), 153308 (2013).
[Crossref]

Tan, L. W.

G. M. Ng, E. L. Kietzke, T. Kietzke, L. W. Tan, P. K. Liew, and F. R. Zhu, “Optical enhancement in semitransparent polymer photovoltaic cell,” Appl. Phys. Lett. 90(10), 103505 (2007).
[Crossref]

Tan, Z.

Z. Tan, L. Li, C. Cui, Y. Ding, Q. Xu, S. Li, D. Qian, and Y. Li, “Solution-processed tungsten oxide as an effective anode buffer layer for high-performance polymer solar cells,” J. Phys. Chem. C 116(35), 18626–18632 (2012).
[Crossref]

Tang, C. W.

H. Wang, K. P. Klubek, and C. W. Tang, “Current efficiency in organic light-emitting diodes with a hole-injection layer,” Appl. Phys. Lett. 93(9), 093306 (2008).
[Crossref]

Tang, W. M.

M. T. Greiner, L. Chai, M. G. Helander, W. M. Tang, and Z. H. Lu, “Transition metal oxide work functions: the influence of cation oxidation state and oxygen vacancies,” Adv. Funct. Mater. 22(21), 4557–4568 (2012).
[Crossref]

Tang, X.

X. Tang, D. Bumueller, A. Lim, J. Schneider, U. Heiz, G. Gantefoer, D. H. Fairbrother, and K. H. Bowen, “Catalytic dehydration of 2-propanol by size-selected (WO3)n and (MoO3)n metal oxide clusters,” J. Phys. Chem. C 118(50), 29278–29286 (2014).
[Crossref]

Tao, R.

Y. Q. Miao, Z. X. Gao, R. Tao, H. P. Shi, H. Wang, Y. H. Li, H. S. Jia, W. H. Choi, and F. R. Zhu, “Realization of ultra-high color stable hybrid white organic light-emitting diodes via sequential symmetrical doping in emissive layer,” Sci. Adv. Mater. 8(2), 401–407 (2016).
[Crossref]

Tsang, S. W.

S. C. Tse, S. W. Tsang, and S. K. So, “PEDOT:PSS polymeric conducting anode for small organic transporting molecules in dark injection experiments,” J. Appl. Phys. 100(6), 063708 (2006).
[Crossref]

Tse, S. C.

S. C. Tse, S. W. Tsang, and S. K. So, “PEDOT:PSS polymeric conducting anode for small organic transporting molecules in dark injection experiments,” J. Appl. Phys. 100(6), 063708 (2006).
[Crossref]

Ueno, K.

Q. Liu, I. Khatri, R. Ishikawa, K. Ueno, and H. Shirai, “Effects of molybdenum oxide molecular doping on the chemical structure of poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate) and on carrier collection efficiency of silicon/poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate) heterojunction solar cells,” Appl. Phys. Lett. 102(18), 183503 (2013).
[Crossref]

Voroshazi, E.

C. Girotto, E. Voroshazi, D. Cheyns, P. Heremans, and B. P. Rand, “Solution-processed MoO₃ thin films as a hole-injection layer for organic solar cells,” ACS Appl. Mater. Interfaces 3(9), 3244–3247 (2011).
[Crossref] [PubMed]

Walzer, K.

S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, and K. Leo, “White organic light-emitting diodes with fluorescent tube efficiency,” Nature 459(7244), 234–238 (2009).
[Crossref] [PubMed]

Wang, H.

Y. Q. Miao, Z. X. Gao, R. Tao, H. P. Shi, H. Wang, Y. H. Li, H. S. Jia, W. H. Choi, and F. R. Zhu, “Realization of ultra-high color stable hybrid white organic light-emitting diodes via sequential symmetrical doping in emissive layer,” Sci. Adv. Mater. 8(2), 401–407 (2016).
[Crossref]

H. Wang, K. P. Klubek, and C. W. Tang, “Current efficiency in organic light-emitting diodes with a hole-injection layer,” Appl. Phys. Lett. 93(9), 093306 (2008).
[Crossref]

Wang, M.

T. Yang, M. Wang, Y. Cao, F. Huang, L. Huang, J. Peng, X. Gong, S. Z. D. Cheng, and Y. Cao, “Polymer solar cells with a low-temperature-annealed sol-gel-derived MoOx film as a hole extraction layer,” Adv. Energy Mater. 2(5), 523–527 (2012).
[Crossref]

Wang, Q.

Y. Wang, Q. Luo, N. Wu, Q. Wang, H. Zhu, L. Chen, Y. Q. Li, L. Luo, and C. Q. Ma, “Solution-processed MoO3:PEDOT:PSS hybrid hole transporting layer for inverted polymer solar cells,” ACS Appl. Mater. Interfaces 7(13), 7170–7179 (2015).
[Crossref] [PubMed]

Wang, Y.

Y. Wang, Q. Luo, N. Wu, Q. Wang, H. Zhu, L. Chen, Y. Q. Li, L. Luo, and C. Q. Ma, “Solution-processed MoO3:PEDOT:PSS hybrid hole transporting layer for inverted polymer solar cells,” ACS Appl. Mater. Interfaces 7(13), 7170–7179 (2015).
[Crossref] [PubMed]

Wang, Z. B.

Z. B. Wang, M. G. Helander, J. Qiu, D. P. Puzzo, M. T. Greiner, Z. W. Liu, and Z. H. Lu, “Highly simplified phosphorescent organic light emitting diode with >20% external quantum efficiency at >10,000cd/m2,” Appl. Phys. Lett. 98(7), 073310 (2011).
[Crossref]

Z. B. Wang, M. G. Helander, J. Qiu, Z. W. Liu, M. T. Greiner, and Z. H. Lu, “Direct hole injection in to 4,4′-N,N′-dicarbazole-biphenyl: A simple pathway to achieve efficient organic light emitting diodes,” J. Appl. Phys. 108(2), 024510 (2010).
[Crossref]

Wang, Z. K.

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M. F. Xu, L. S. Cui, X. Z. Zhu, C. H. Gao, X. B. Shi, Z. M. Jin, Z. K. Wang, and L. S. Liao, “Aqueous solution-processed MoO3 as an effective interfacial layer in polymer/fullerene based organic solar cells,” Org. Electron. 14(2), 657–664 (2013).
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Figures (9)

Fig. 1
Fig. 1 (a) The cross-sectional view of the phosphorescent OLED, and (b) the schematic diagram of energy levels of the functional materials used in the OLEDs.
Fig. 2
Fig. 2 (a) J–V and (b) L–V characteristics of a set of OLEDs fabricated using different HILs of pristine PEDOT:PSS (red dots), eMoO3 (green triangles), sMoO3 (inverted blue triangles) and hybrid PEDOT:PSS-MoO3 (sky blue diamonds).
Fig. 3
Fig. 3 Comparison of power efficiency as a function of the luminance obtained for a set of structurally identical OLEDs made with different HILs of pristine PEDOT:PSS (red dots), eMoO3 (green triangles), sMoO3 (inverted blue triangles) and hybrid PEDOT:PSS-MoO3 (sky blue diamonds).
Fig. 4
Fig. 4 (a) EQE as a function of the luminance for a set of structurally identical OLEDs having different HILs of pristine PEDOT:PSS (red dots), eMoO3 (green triangles), sMoO3 (inverted blue triangles) and hybrid PEDOT:PSS-MoO3 (sky blue diamonds), and (b) the normalized EL spectra of different OLEDs measured at 5.0 V.
Fig. 5
Fig. 5 (a) J–V characteristics as a function of luminance and (b) EQE as a function of luminance, measured for a set of structurally identical OLEDs with different hybrid PEDOT:PSS-MoO3 HILs, having different volume ratios of PEDOT:PSS to MoO3 in the mixed solutions, e.g. 1:1, 2:1, 3:1 and 4:1.
Fig. 6
Fig. 6 AFM images measured for the surfaces of (a) a bare ITO, (b) an eMoO3-modified ITO, and (c) a sMoO3-modified ITO. An area of 5.0 µm × 5.0 µm was characterized in the AFM measurements using tapping mode.
Fig. 7
Fig. 7 AFM images measured for (a) PEDOT:PSS HIL and, hybrid PEDOT:PSS-MoO3 HILs prepared using mixed solution with different volume ratios of PEDOT:PSS to MoO3 NPs, (b) 1:1, (c) 2:1, (d) 3:1, and (e) 4:1.
Fig. 8
Fig. 8 (a) The secondary-electron cut-off and (b) the valance band edge of the UPS spectra, and (c) Mo 3d XPS spectrum, measured for the sMoO3-coated ITO samples. The deconvoluted XPS double peaks, illustrating Mo 3d5/2 and 3d3/2 due to Mo6+, are also presented
Fig. 9
Fig. 9 (a) The secondary-electron cut-off, and (b) the valance band edge of the USP spectra measured for the hybrid HIL, XPS spectra measured for (c) pristine PEDOT:PSS thin film, indicating the component of S 2s peak and (d) PEDOT:PSS-MoO3 layer, illustrating the presence of Mo6+ and Mo5+states in the hybrid HIL.

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