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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.
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.
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 (J–V–L) 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 J–V 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 J–V 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.
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).
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.
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 (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.
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 J–V 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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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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