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Solution-processed phosphorus-tungsten oxide film as hole injection layer for application in efficient organic light-emitting diode
Materials Science in Semiconductor Processing 85 (2018) 106–112
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Solution-processed phosphorus-tungsten oxide film as hole injection layer for application in efficient organic light-emitting diode Weiwei Denga,b, Shirong Wanga,b, Yin Xiaoa,b, Nonglin Zhoua,b, Xianggao Lia,b, a b
T
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School of Chemical Engineering and Technology, Tianjin University, Tianjin 300354, China Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Phosphotungstic acid Phosphorus-tungsten oxide Hole injection layer Spin-coating process Organic light-emitting diode
The research on new functional layer materials and optimization of fabrication in organic light-emitting diode (OLED) has been continuously concerned. In this work, phosphotungstic acid solution used as a precursor is spincoated onto ITO anode, and then a phosphorus-tungsten oxide (WO3-P2O5) film is obtained after annealing treatment. Various parameters which include film composition, surface morphology, work function and optical property are measured. It suggests that the film has the composition of 24 WO3·P2O5 annealed at 200 °C, which exhibits work function of 5.55 eV, surface root mean square (RMS) roughness value of 1.525 nm and optical transmittance of over 92%. The WO3-P2O5 film is used as hole injection layer (HIL), and the OLED with the structure of [ITO/ WO3-P2O5/ TPD/ Alq3/ LiF/ Al] is fabricated. The performance of device shows turn-on voltage of 2.6 V, maximum luminance of 13553 cd/m2, maximum current efficiency of 5.87 cd/A and maximum power efficiency of 3.44 lm/W, respectively. The results provide a new way to deposit HIL based on solutionprocessed metal oxide in efficient OLED.
1. Introduction Organic light-emitting diode (OLED) has gained more attention in panel displays and solid-state lighting for its distinguished advantage in efficiency [1], color rendering index [2] and flexibility [3]. In general, the structure of OLED basically contains [anode/ hole injection layer (HIL)/ hole transport layer (HTL)/ emitting layer (EML)/ electron transport layer (ETL)/ electron injection layer (EIL)/ cathode]. In detail, indium tin oxide (ITO) anode used in OLED has a work function of about 4.8 eV [4] and the highest occupied molecular orbital (HOMO) energy level of typical HTL material is about 5.4 – 6.1 eV [5,6]. The large energy barrier (between ITO anode and HTL) is adverse for hole injection from ITO to HTL. Hence, the interface modification of anode become one of the research hotspots in OLED, which can reduce the injection barrier to improve the device performance [7,8]. There are mainly two ways to modify the ITO interface which can adjust work function of the ITO or improve the contact between ITO and HTL. The plasma treatment is verified to be effective way because it can increase the work function and wettability of the ITO surface. It usually includes oxygen plasma treatment [9], polymerized fluorocarbon (CFx) plasma [10] and chlorine (Cl2) plasma [11,12]. However, the ITO treated by plasma should be used as soon as possible because it cannot be conserved for long. The other way is adopting HIL. HIL is
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usually prepared by the deposition of transition metal oxides (TMOs) or organic compounds. Poly(3,4-ethylenedioxy-thiophene): poly(styrenesulfonate) (PEDOT:PSS) is known as one of the most widely used HIMs for its excellent mechanical flexibility, appropriate HOMO energy level and high transmittance [13]. However, because of insulating PSS, solvent post treatment is usually used for increasing conductivity of PEDOT:PSS [14,15]. TMOs such as MoO3 [16,17]， NiO [18] and WO3 [19,20] have been investigated to be used as HIL materials, because they possess high work function, favorable semiconducting properties and good transparency [21]. Among TMO used in OLED, WO3 gained more attention due to its high work function and highly transparent in visible region [22]. The deposition of WO3 usually adopts thermally evaporate or magnetron sputtering process. Unlike the vacuum deposition methods, solution processes such as roll-to-roll, spin-coating and ink-jet print are low-cost, simple methods to form thin film. Therefore, solution processed WO3 used as HIL material get more popular. Yang's group reported a solution-processed WO3 HIL for polymer light-emitting diodes (PLEDs), which exhibited the maximum luminance of 27560 cd/m2 at 10 V. The device showed higher current density and luminance compared to the PLEDs fabricated with only ITO [23]. Colsmann's group demonstrated a solution processed blue OLED comprising WO3 HIL. The device current efficiency increases from 8 cd/A when using PEDOT:PSS to 14 cd/A upon
Corresponding author at: School of Chemical Engineering and Technology, Tianjin University, Tianjin 300354, China. E-mail address: [email protected] (X. Li).
https://doi.org/10.1016/j.mssp.2018.06.007 Received 4 April 2018; Received in revised form 7 May 2018; Accepted 7 June 2018 Available online 20 June 2018 1369-8001/ © 2018 Published by Elsevier Ltd.
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the incorporation of WO3 [24]. However, the employment of solution process to deposit WO3 film becomes difficult due to the harsh dissolution conditions and complex synthesis process [25]. Therefore, efforts have been made to develop a precursor for WO3 HIL. It has been reported that phosphoric acid (H3PO4) can increase the ITO work function [26]. Unfortunately, the work function of acid-treated ITO dramatically decreases after deposition of HTL. In this contribution, inorganic acid has the potential to be precursor for HIL. Herein, this work intends to prepare precursor using the excellent solubility of phosphotungstic acid (H3PW12O40·xH2O) which contains transition metal (tungsten), and to obtain the coating solution which is suitable for various solution processing. Then HIL film is obtained by annealing the phosphotungstic acid film. Afterward, the film composition, surface morphology and work function are studied. At last, highperformance OLED is fabricated due to the hole injection property.
Fig. 1. TGA curve of phosphotungstic acid powder.
2. Experiment heavily the performance of device especially solution process fabrication [27,28]. It should be possible that phosphotungstic acid solution can avoid the corrosion phenomenon because of its insoluble characteristics in chlorobenzene and toluene when it is used as a HIL precursor. Further film formation experiment suggests that it can form stable film when using methanol and acetonitrile as solvent. Considering methanol has the lower boiling point, the acetonitrile is a good candidate for dissolving phosphotungstic acid powders, and the concentration of phosphotungstic acid is 10 mg/mL.
The ITO glass substrate (sheet resistance of about 8 Ω/sq) was purchased from Advanced Election Tech which was used as anode in OLEDs. Methanol, 2-propanol, acetone, acetonitrile, tetrahydrofuran, toluene, chlorobenzene and dimethyl sulfoxide (DMSO) were purchased from Tianjin Concord Technology. Phosphotungstic acid (H3PW12O40·xH2O), tungsten trioxide (WO3), N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)-benzidine (TPD), Tris-(8-hydroxyquinoline) aluminum(Ⅲ) (Alq3), LiF and aluminum (Al) were purchased from Shanghai Energy Chemical. PEDOT:PSS (BAYTRON®P VP CH 8000) was purchased from Xi’an Polymer Light Technology Corp. OLED was designed with the structures of [ITO/ HIL/ TPD (25 nm)/ Alq3 (60 nm)/ LiF (1 nm)/ Al (100 nm)]. Firstly, ITO glass substrates were successively cleaned with DI water, 2-propanol, acetone and methanol in an ultrasonic bath for 20 min. Then they were boiled in 2propanol and dried with nitrogen gas flow, followed by the oxygen plasma treatment (3 min, 80 W and 1mbar). WO3-P2O5 and PEDOT:PSS, which were used as HIL materials, were spin-coated onto the ITO glass substrates in air at a spinning speed of 2000 rpm for 30 s. WO3 layer and other functional layers were prepared by the method of vacuum thermal evaporation under a pressure of 5 × 10−4 Pa. In detail, the thermal evaporation rates of WO3, TPD, Alq3, LiF and Al were 0.3, 1.7, 1.7, 0.3 and 10 Å/s with the thickness of 3 nm, 25 nm, 60 nm, 1 nm and 100 nm, respectively. The measurement conditions of HIL film and device performance were as follows: thermal property of phosphotungstic acid was measured by TGA-Q500. Optical property, surface morphology, chemical analysis and work function of WO3-P2O5 films were investigated by UV–Vis spectrometer, atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS) and photoelectron spectrometer (Riken Keiki AC-2). The current density–voltage–luminescence (J–V–L) characteristics of OLEDs were investigated using a chroma meter CS-2000 (Konica Minolta) and a source measure unit (Keithley 2400). All the devices characterizations were performed at room temperature without any encapsulation.
3.2. The composition of phosphotungstic acid and thin film As phosphotungstic acid (H3PW12O40·xH2O) contains the combined water, the annealing condition of the film can be determined according to its thermal behavior. TGA curve of phosphotungstic acid is shown in Fig. 1. From the curve, phosphotungstic acid powder has rapid weightless when the temperature increases, and then it becomes very gentle near 200 °C. It indicates that the combined water of phosphotungstic acid can easily lose. Furthermore, the weightlessness rate is 7.2% at 200 °C. Hence, phosphotungstic acid can be calculated with composition of H3PW12O40·11H2O. To confirm the composition of thin film, the atomic ratio on the surface has been analyzed by XPS (Tab.S2). It can be concluded that W/ P atomic ratio of the film without annealing treatment is 11.4:1. Furthermore, with the increase of processing temperature, W/P ratio increases gradually. When the annealing temperature is 200 °C, W/P ratio is 12.3:1. It means that the film has composition of 24 WO3·P2O5, which accords with Keggin heteropolyacids structure of phosphotungstic acid (H3PW12O40) [29]. When the annealing temperature increased up to 300 °C, W/P ratio increases as the phosphorus oxide has a little weightlessness visibly. 3.3. Valence state of W and work function of thin film The valence states of metal elements such as Mo and W in TMO thin films can result in different work function [30]. Fig. 2 indicates the W 4 f signals of the XPS spectra in samples which contained WO3 and WO3-P2O5 (treated at different conditions). The W 4f7/2 and W 4f5/2 peaks are carried out through using the C 1 s peak position with binding energy (BE). The position and shape of photoemission peaks are helpful to confirm the valence state of W atom. It has been proved that the position and the shape of W 4f photoemission peak represent W atoms with a valence state of VI, when W 4f7/2 situates at BE of 36.0 ± 0.1 eV and W 4f5/2 centers at 38.2 ± 0.1 eV (with a peak ratio of 4:3). Besides, another lower BEs (BE of W 4f7/2 is 31.3 eV, and of W 4f5/2 is 33.5 eV with a peak ratio 4:3) are attributed to W atoms with zero valence [29,31]. For the WO3 thin film, it completely confirms to the character peak of W (VI) (BE of W 4f7/2 is 36.0 eV, and of W 4f5/2 is 38.0 eV). And for the WO3-P2O5 films (unannealed), the character peaks
3. Results and discussion 3.1. Solubility of phosphotungstic acid In order to choose appropriate solvent, phosphotungstic acid powders are put into eight kinds of solutions which are water, methanol, 2propanol, acetone, acetonitrile, tetrahydrofuran, toluene and chlorobenzene. The conditions of dissolving are shown in Tab. S1. As can be seen from the results, phosphotungstic acid dissolves easily into all of solvents but toluene and chlorobenzene. As known to all, most of the hole transport materials (HTM) have high solubility in chlorobenzene or toluene. From the structure of OLED, HIL is contact with HTL, therefore the two layers inevitably corrode one another to depress 107
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Fig. 2. W 4f signals of the XPS spectra in (a) WO3; (b) WO3-P2O5 (annealed by different temperature).
analysis. As the thin film annealed at 200 °C, the combined water in phosphotungstic acid film loses completely and the film (WO3-P2O5 film) becomes densification with the RMS value of 1.525 nm. It is interesting to note that the RMS value of film increases from 1.525 nm to 2.239 nm, when the temperature rises to 250 °C. It indicates that phosphorus and oxygen element in the film begin to decrease, so that the surface becomes rough. And RMS value becomes smaller (1.935 nm) when the film annealed at 300 °C. And the RMS values of PEDOT:PSS and WO3 films are also measured (Fig. S2), which are 4.263 nm and 3.471 nm, respectively. WO3-P2O5 film is flat and uniform. It indicates that WO3-P2O5 film can improve ITO surface, make the surface flat, and have the potential to improve the interface between HTL and ITO. 3.5. Transmittance of thin film Fig. 3. The Photoelectron Spectrometer (PES) spectra of WO3-P2O5 films.
For OLED, transmittance of anode is a very important index [32]. To verify whether WO3-P2O5 films have influence on the transmittance of ITO glass substrates or not, the optical property is measured in Fig. 5 which shows the optical absorbance (a) and transmittance (b). It can be seen from Fig. 5a that WO3-P2O5 films are measured at an absorption peak of ultraviolet range (318 nm) and has a weak absorption peak at a wavelength of 456 nm. According to Fig. 5b, the optical transmittance curves show similar shapes, and the samples show quite high optical transmittances of over 92% (Tab.S3) in the visible wavelengths between 390 and 780 nm. Hence, the transmittance of ITO does not significantly change with the deposition of WO3-P2O5.
of W atom (BE of W 4f7/2 is 36.4 eV, and of W 4f5/2 is 38.6 eV) have a large gap with that of W (VI), which may result from Keggin structure of phosphotungstic acid and O atom in H2O. When the annealed temperature is 200 °C, the peaks (BE of W 4f7/2 is 36.2 eV, and of W 4f5/2 is 38.2 eV) match with the character peak of W (VI), and it indicates that the film performs character of WO3 unit. When the temperature rises to 300 °C, the peaks (BE of W 4f7/2 is 36.2 eV, and of W 4f5/2 is 38.4 eV) also have different performance with that of W (VI). And this may be due to the loss of phosphorus and oxygen content in the WO3-P2O5 film. In addition, phosphorus oxides are confirmed with the BE of P 2p (135.0 eV) (Fig. S1). From the above figures, it can be seen that the best annealing temperature is 200 °C. Based on this result, the work function of the WO3-P2O5 film (200 °C) is 5.55 eV measured by photoelectron energy spectrum (AC-2) (Fig. 3).
3.6. Performance of OLEDs with phosphorus-tungsten oxide film The work function of ITO measured with photoelectron spectrometer (AC-2) is 4.82 eV. As existing energy barrier between ITO and TPD (HOMO energy level of TPD is 5.5 eV), hole injection is not efficient. Therefore, HIMs are used to improve OLED performance, such as PEDOT:PSS (HOMO energy level is 5.2 eV) and TMO. Because the thermally evaporated WO3 is a kind of n-type semiconductor with work function of 6.65 eV (measured by AC-2), and oxide energy alignment is governed by electron-chemical-potential equilibration to enable thermal excitation of electrons, therefore, charge can be effectively transmitted [23,30]. WO3-P2O5 film has the work function of 5.55 eV. As the valence state of W in WO3-P2O5 film appears W (VI) state, WO3P2O5 presents similar property with WO3. It suggests that WO3-P2O5 film annealed in air is inclined to be a kind of n-type semiconductor. Due to the existing P2O5, the work function of WO3 is close enough to HOMO energy level of TPD. The interface dipole is appeared in [ITO/ WO3-P2O5/ TPD] after depositing WO3-P2O5 [33]. Furthermore, hole injection from ITO to TPD can easily occur as WO3-P2O5 films exhibit smooth. To obtain the hole injection property from WO3-P2O5 to TPD, hole
3.4. Surface morphology of thin film A flat film with fewer pin-holes and spikes is conducive to improve charge transporting and increase the efficiency of OLED. The surface morphology of the ITO and WO3-P2O5 films are exhibited in Fig. 4. According to the picture, there are some differences on the surface among these samples, and the root mean square (RMS) roughness value was 4.030 nm, 1.585 nm, 2.901 nm, 1.525 nm, 2.239 nm and 1.935 nm, respectively. It dramatically improves film flatness when annealing at 100 °C, and the RMS value reduces from 4.030 nm (ITO) to 1.585 nm. It indicates that the phosphotungstic acid can smooth the ITO surface to improve the contact between ITO and HTL. However, the RMS value rises from 1.585 nm to 2.901 nm when the film is annealed at 150 °C. The possible reason is that the combined water in phosphotungstic acid film continued to lose, the combined water volatilized and caused coarse film. This result is consistent with above thermal properties 108
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Fig. 4. AFM image of WO3-P2O5 film annealed at different temperature (a) bare-ITO; (b) 100 °C; (c) 150 °C; (d) 200 °C; (e) 250 °C; (f) 300 °C.
only devices (HODs) with the structure of [ITO/ WO3-P2O5/ TPD (75 nm)/ WO3 (5 nm)/ Al (100 nm)] is fabricated [34]. The current density-voltage (J-V) curves shown in Fig. 6 are measured. Devices with WO3-P2O5 film annealed at different temperature show different characteristics. The threshold voltage (defined as the driving voltage when the current density increases sharply in the device) and maximum current density of HODs are 3.5 V, 207 mA/cm2 (100 °C); 2.0 V, 615 mA/cm2 (150 °C); 3.0 V, 436 mA/cm2 (200 °C); 5.0 V, 77 mA/cm2 (300 °C), respectively. All the HODs based on WO3-P2O5 films perform lower threshold voltage and higher maximum current density than HOD 0 (5.5 V, 51 mA/cm2) which was fabricated without any HIM. It indicates that the insertion of WO3-P2O5 film in HODs enhance hole injection at low applied voltage which can lead to lower turn-on voltage of OLED. On basis of data, hole transfer ability in HOD 2 (150 °C) is strongest in the four HODs, while HOD 4 (300 °C) shows the weakest one. It is worth noting that the hole transfer ability in HOD 1 (100 °C) and HOD 3 (200 °C) are moderate. Furthermore, HOD 3 expresses better than HOD 1. The hole current density is determined by the hole injection barrier at the ITO and TPD interface. According to the work function of WO3-P2O5 film, the utilization of WO3-P2O5 film can reduce the energy barrier between ITO and TPD, leading to increase of hole current density [35]. In conclusion, it suggests that WO3-P2O5 films annealed from 100 °C to 200 °C can enhance hole transfer ability of
Fig. 6. The current density-voltage (J-V) curves of HODs.
device at different levels. Considering WO3-P2O5 film demonstrates characters of smooth surface, high transmission, and promoting hole transfer ability of device, it is applied as HIL in OLED with the structure of [ITO/ HIL/ TPD (25 nm)/ Alq3 (60 nm)/ LiF (1 nm)/ Al (100 nm)]. Particularly, four OLEDs (device B, C, D and E) are fabricated with HIL using WO3-P2O5
Fig. 5. Optical characteristic of ITO and WO3-P2O5 films through different annealed temperature treatment: (a) absorption; (b) transmittance. 109
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15,981 cd/m2, 3.83 cd/A, 2.97 lm/W, respectively. Moreover, device B demonstrates smaller efficiency roll-off. According to the result of HOD 1, hole transfer rate possible matches with the electron-transfer rate in device B, and then carrier transporting balance occurs in OLED. Hence, device gains higher luminance and smaller efficiency roll-off. However, film treated at 100 °C still contains water, which is not conducive to prolonging the device lifetime. Device performance improves continuously along with the increase of annealed temperature. Device D (200 °C) shows the turn-on voltage of 2.6 V, maximum luminance of 13553 cd/m2, maximum CE of 5.87 cd/A, maximum PE of 3.44 lm/W, respectively, which are better than the performance of device C (150 °C) (2.6 V, 12,542 cd/m2, 5.36 cd/A, 3.35 lm/W). This result may be that surface of HIL in device C (RMS value is 2.901 nm) is rougher than that in device D (RMS value is 1.525 nm), although the hole transfer ability in HOD 2 is high. Figs. 7b and 7c show current efficiency-current density and power efficiency- current density curves, respectively. Device A exhibits poor performance at low applied voltage because of the inefficient hole injection. The efficiency in device A increased with the increasing current density, nevertheless, it is still quite low. It should be pointed out that the efficiency of C and D get improved significantly when are compared with device B. Whereas, there is an obvious efficiency roll-off in device C and D. The most likely reason is that holes are transported too fast in device C and D, which leads to the imbalance of carrier transporting [36]. Comparing with former three devices, the performance of device E (300 °C) (2.9 V, 10,681 cd/m2, 2.64 cd/A, 1.16 lm/W) is routine, which may result from change of W valence state in WO3-P2O5 film. According to the data of HOD, HIL annealed at 300 °C has a little impact on improving hole transfer ability in OLED. Therefore, the carrier transporting balance in device E becomes worse than those of device B, C and D, which results in the lower current efficiency in device E. Nevertheless, device E has little efficiency roll-off. Above all, it is potential to apply WO3-P2O5 film annealed at 200 °C as HIL to the fabrication of high performance OLED. OLEDs based on WO3-P2O5 film may be more stable than device A due to the efficient hole injection.
Table 1 The optoelectronic results of OLEDs with WO3-P2O5 via different treatments used as HIL. Decices
A (Bare-ITO) B (100 °C) C (150 °C) D (200 °C) E (300 °C) a b c d e f
Lmaxa (cd/m2)
9664 15,981 12,542 13,553 10,681
Vonb (V)
CIEe
λELf (nm)
(cd/A)
PE maxd (lm/ W)
4.09 3.83 5.36 5.87 2.64
1.04 2.97 3.35 3.44 1.16
(0.32,0.56) (0.31,0.55) (0.31,0.55) (0.31,0.55) (0.31,0.55)
522 522 522 522 522
CE max
3.3 3.0 2.6 2.6 2.9
c
The maximum luminance. Applied voltage at 1 cd/m2. The maximum current efficiency. The maximum power efficiency. The CIE coordinates of electroluminescence spectrum. The wavelength of EL spectrum.
films, which are annealed at different temperature. In comparison, blank control device (device A) is fabricated with the structure of [ITO/ TPD (25 nm)/ Alq3 (60 nm)/ LiF (1 nm)/ Al (100 nm)]. The electroluminescence (EL) performances of OLEDs are shown in Fig. 7 and Table1. It can be seen from Fig. 7a, b and c that device A (blank control device) shows the turn-on voltage (defined as the operation voltage at luminance of 1 cd/m2) of 3.3 V, maximum luminance of 9664 cd/m2, maximum current efficiency (CE) of 4.09 cd/A, maximum power efficiency (PE) of 1.04 lm/W, respectively. And the EL performances of devices are improved when using disparate WO3-P2O5 films as HIL due to the fact that the interface dipole is formed to accelerate the hole injection, which will decrease the turn-on voltage and increase the efficiency [16]. The turn-on voltage and threshold voltage of device B (100 °C) are 3.0 and 6.0 V, respectively. Although device B displays the maximum threshold voltage, its maximum luminance, CE, PE can reach
Fig. 7. The EL performances of OLEDs with WO3-P2O5 used as HIL: (a) current density–voltage-luminance (J-V-L) curves; (b) current efficiency-current density (CE-J) curves; (c) power efficiency-current density (PE-J) curves; (d) normalized EL spectra. 110
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Furthermore, device B, C and D may possess better stability than device E according to the hole transfer ability of HODs. Fig. 7d illustrates normalized EL spectra. Alq3 is often used for a green color material with its good film-forming, high thermostability and good electron transport properties [37–39]. The spectra demonstrate only one peak wavelength of 522 nm, and it conforms to the green emission of Alq3 which is irrespective of the annealed temperature [40]. Because there are no obvious shifts according to the EL spectra, it indicates that the recombination zone locates in emitting layer in all devices. Compared with device A, the full wavelength at half maximum (FWHM) of device B, C, D and E is narrower, indicating high purity of devices based on WO3-P2O5 films. Generally, the EL spectra mainly depends on the carrier recombination zone and the micro-cavity effects of the device [41,42]. Device B, C, D and E have weaker micro-cavity effects in comparison with device A, as the optical transmittance curves of WO3-P2O5 films show slightly different with that of ITO in Fig. 5. Difference in emission spectra of the devices is attributed to the different micro-cavity effect. In order to verify the superiority of the phosphorus-tungsten oxide film when used as HIL, device with the structure [ITO/ HIL/ TPD (25 nm)/ Alq3 (60 nm)/ LiF (1 nm)/ Al (100 nm)] is constructed (using PEDOT:PSS, WO3-P2O5 and WO3 as HIL). The J-V-L and efficiency curves of devices are shown in Fig. 8a, b and c, and the device data is summarized in Table2. Compared with device A (3.3 V, 9664 cd/m2, 4.09 cd/A, 1.04 lm/ W), device H (PEDOT:PSS) exhibits poor performance (3.2 V, 11,109 cd/m2, 3.89 cd/A, 0.86 lm/W), which may be due to the insulating PSS. The performance of device greatly depends on the solvent post treatment, which is usually used for increasing conductivity of PEDOT:PSS. While using vacuum evaporated WO3 film as HIL, the performance of device G (WO3) (17,274 cd/m2, 4.93 cd/A, 1.63 lm/W) is significantly improved. According to the energy-level alignment in [ITO/ HIL/ HTL], effective charge transfer from TPD to WO3 occurred, and formed a powerful interface dipole [13]. As a result, holes are significantly injected when WO3 is contact with TPD, which results in improving OLED performance. In comparison, as there is no strong
Table 2 The optoelectronic results of OLEDs with different HIL materials. Devices
A (bare-ITO) F (WO3P2O5) G (WO3) H (PEDOT: PSS) a b c d e f
Lmaxa (cd/m2)
Vonb (V)
PE
CE max
c
max
CIEe
λELf (nm)
d
(cd/A)
(lm/ W)
9664 13553
3.3 2.6
4.09 5.87
1.04 3.44
(0.32,0.56) (0.31,0.55)
522 522
17,274 11,109
2.8 3.2
4.93 3.89
1.63 0.86
(0.33,0.52) (0.30,0.54)
522 522
The maximum luminance. Applied voltage at 1 cd/m2. The maximum current efficiency. The maximum power efficiency. The CIE coordinates of electroluminescence spectrum. The wavelength of EL spectrum.
interface dipole between PEDOT:PSS and TPD, device H performs slight improvement. Besides, due to the insulating PSS, the turn-on voltage of device H merely decreases from 3.3 V to 3.2 V. And the performance of device F (WO3-P2O5) (5.87 cd/A, 3.44 lm/W) is better than that of device G (4.93 cd/A, 1.63 lm/W). As the work function of WO3-P2O5 film is 5.55 eV, which is closer to HOMO energy level of TPD than WO3 film, coating of WO3-P2O5 film forms a strong interface dipole. The energy gap significantly decreases and efficient hole-injection from ITO to TPD is enabled [23]. Particularly, the maximum luminance of device F is 13553 cd/m2 (that of device G is 17,274 cd/m2). Meanwhile, all the devices fabricated by solution process show efficiency roll-off, and this may be that vacuum evaporated WO3 is more compact to ensure efficient hole injection at high voltages. Even so, device F show higher efficiency than other devices due to the high efficient hole injection of WO3-P2O5 film. In addition, Fig. 8d illustrates normalized EL spectra with one peak wavelength of 522 nm. Compared with device A and G, the FWHM of
Fig. 8. The EL performances of OLEDs with different HIL materials: (a) current density–voltage-luminance (J-V-L) curves; (b) current efficiency-current density (CE-J) curves; (c) power efficiency-current density (PE-J) curves; (d) normalized EL spectra. 111
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device F and H is narrower, indicating high purity of the two devices. It is reported that OLEDs with PEDOT:PSS express a weaker micro-cavity effect when compared with OLEDs with ITO [41]. Thus, the difference in emission spectra results from the different micro-cavity effect in device F and H. In conclusion, these results suggest that phosphorustungsten oxide has the potential to replace PEDOT:PSS.
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4. Conclusions An effective and low-cost method to form HIL in OLEDs is demonstrated. Compared with WO3 film, the work function of the phosphorustungsten oxide film is modified owing to the P2O5 unit. WO3-P2O5 film possesses high transmission and smooth surface characters. Inspired by this, it is employed as HIL to fabricate the OLED which demonstrates the maximum CE of 5.87 cd/A and maximum PE of 3.44 lm/W. The device exhibits performance comparable to that of OLED using WO3 or PEDOT:PSS as HIL. Since phosphorus-tungsten oxide layer is much more convenient and low-cost, it may benefit to the research on HIL materials with metal oxide in OLED. Acknowledgments This work was supported by the National Key R&D Program of China (2016YFB0401303), the National Nature Science Foundation of China (21676188) and the Key Projects in Natural Science Foundation of Tianjin (16JCZDJC37100). Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.mssp.2018.06.007. References [1] J. Wang, J. Liu, S. Huang, X. Wu, X. Shi, C. Chen, et al., High efficiency green phosphorescent organic light-emitting diodes with a low roll-off at high brightness, Org. Electron. 14 (2013) 2854–2858. [2] J. Yu, Y. Yin, W. Liu, W. Zhang, L. Zhang, W. Xie, et al., Effect of the greenish-yellow emission on the color rendering index of white organic light-emitting devices, Org. Electron. 15 (2014) 2817–2821. [3] L. Mo, J. Ran, L. Yang, Y. Fang, Q. Zhai, L. Li, Flexible transparent conductive films combining flexographic printed silver grids with CNT coating, Nanotechnology 27 (2016) 065202. [4] K. Sugiyama, H. Ishii, Y. Ouchi, K. Seki, Dependence of indium–tin–oxide work function on surface cleaning method as studied by ultraviolet and x-ray photoemission spectroscopies, J. Appl. Phys. 87 (1999) 295–298. [5] S. Logan, J.E. Donaghey, W. Zhang, I. McCulloch, A.J. Campbell, Compatibility of amorphous triarylamine copolymers with solution-processed hole injecting metal oxide bottom contacts, J. Mater. Chem. C 3 (2015) 4530–4536. [6] Z.B. Wang, M.G. Helander, J. Qiu, Z.W. Liu, M.T. Greiner, 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 (2010) 024510. [7] Y.J. Choi, S.C. Gong, C.S. Park, H.S. Lee, J.G. Jang, H.J. Chang, et al., Improved performance of organic light-emitting diodes fabricated on Al-doped ZnO anodes incorporating a homogeneous Al-doped ZnO buffer layer grown by atomic layer deposition, Acs Appl. Mater. Interfaces 5 (2013) 3650–3655. [8] A.P. Kulkarni, C.J. Tonzola, A. Babel, S.A. Jenekhe, Electron transport materials for organic light-emitting diodes, Chem. Mater. 16 (2004) 4556–4573. [9] I. Irfan, S. Graber, F. So, Y. Gao, Interplay of cleaning and de-doping in oxygen plasma treated high work function indium tin oxide (ITO), Org. Electron. 13 (2012) 2028–2034. [10] J.X. Tang, Y.Q. Li, L.R. Zheng, L.S. Hung, Anode/organic interface modification by plasma polymerized fluorocarbon films, J. Appl. Phys. 95 (2004) 4397–4403. [11] K. He, X. Yang, H. Yan, J. Gong, S. Zhong, Q. Ou, et al., Surface properties of indium tin oxide treated by Cl2 inductively coupled plasma, Appl. Surf. Sci. 316 (2014) 214–221. [12] X.A. Cao, Y.Q. Zhang, Performance enhancement of organic light-emitting diodes by chlorine plasma treatment of indium tin oxide, Appl. Phys. Lett. 100 (2012) 183304. [13] Z. Hongmei, X. Jianjian, Z. Wenjin, H. Wei, Effect of PEDOT:PSS vs. MoO3 as the hole injection layer on performance of C545T-based green electroluminescent lightemitting diodes, Displays 35 (2014) 171–175. [14] T.-R. Chou, S.-H. Chen, Y.-T. Chiang, Y.-T. Lin, C.-Y. Chao, Highly conductive PEDOT:PSS films by post-treatment with dimethyl sulfoxide for ITO-free liquid
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Solution-processed phosphorus-tungsten oxide film as hole injection layer for application in efficient organic light-emitting diode Weiwei Denga,b, Shirong Wanga,b, Yin Xiaoa,b, Nonglin Zhoua,b, Xianggao Lia,b, a b
T
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School of Chemical Engineering and Technology, Tianjin University, Tianjin 300354, China Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Phosphotungstic acid Phosphorus-tungsten oxide Hole injection layer Spin-coating process Organic light-emitting diode
The research on new functional layer materials and optimization of fabrication in organic light-emitting diode (OLED) has been continuously concerned. In this work, phosphotungstic acid solution used as a precursor is spincoated onto ITO anode, and then a phosphorus-tungsten oxide (WO3-P2O5) film is obtained after annealing treatment. Various parameters which include film composition, surface morphology, work function and optical property are measured. It suggests that the film has the composition of 24 WO3·P2O5 annealed at 200 °C, which exhibits work function of 5.55 eV, surface root mean square (RMS) roughness value of 1.525 nm and optical transmittance of over 92%. The WO3-P2O5 film is used as hole injection layer (HIL), and the OLED with the structure of [ITO/ WO3-P2O5/ TPD/ Alq3/ LiF/ Al] is fabricated. The performance of device shows turn-on voltage of 2.6 V, maximum luminance of 13553 cd/m2, maximum current efficiency of 5.87 cd/A and maximum power efficiency of 3.44 lm/W, respectively. The results provide a new way to deposit HIL based on solutionprocessed metal oxide in efficient OLED.
1. Introduction Organic light-emitting diode (OLED) has gained more attention in panel displays and solid-state lighting for its distinguished advantage in efficiency [1], color rendering index [2] and flexibility [3]. In general, the structure of OLED basically contains [anode/ hole injection layer (HIL)/ hole transport layer (HTL)/ emitting layer (EML)/ electron transport layer (ETL)/ electron injection layer (EIL)/ cathode]. In detail, indium tin oxide (ITO) anode used in OLED has a work function of about 4.8 eV [4] and the highest occupied molecular orbital (HOMO) energy level of typical HTL material is about 5.4 – 6.1 eV [5,6]. The large energy barrier (between ITO anode and HTL) is adverse for hole injection from ITO to HTL. Hence, the interface modification of anode become one of the research hotspots in OLED, which can reduce the injection barrier to improve the device performance [7,8]. There are mainly two ways to modify the ITO interface which can adjust work function of the ITO or improve the contact between ITO and HTL. The plasma treatment is verified to be effective way because it can increase the work function and wettability of the ITO surface. It usually includes oxygen plasma treatment [9], polymerized fluorocarbon (CFx) plasma [10] and chlorine (Cl2) plasma [11,12]. However, the ITO treated by plasma should be used as soon as possible because it cannot be conserved for long. The other way is adopting HIL. HIL is
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usually prepared by the deposition of transition metal oxides (TMOs) or organic compounds. Poly(3,4-ethylenedioxy-thiophene): poly(styrenesulfonate) (PEDOT:PSS) is known as one of the most widely used HIMs for its excellent mechanical flexibility, appropriate HOMO energy level and high transmittance [13]. However, because of insulating PSS, solvent post treatment is usually used for increasing conductivity of PEDOT:PSS [14,15]. TMOs such as MoO3 [16,17]， NiO [18] and WO3 [19,20] have been investigated to be used as HIL materials, because they possess high work function, favorable semiconducting properties and good transparency [21]. Among TMO used in OLED, WO3 gained more attention due to its high work function and highly transparent in visible region [22]. The deposition of WO3 usually adopts thermally evaporate or magnetron sputtering process. Unlike the vacuum deposition methods, solution processes such as roll-to-roll, spin-coating and ink-jet print are low-cost, simple methods to form thin film. Therefore, solution processed WO3 used as HIL material get more popular. Yang's group reported a solution-processed WO3 HIL for polymer light-emitting diodes (PLEDs), which exhibited the maximum luminance of 27560 cd/m2 at 10 V. The device showed higher current density and luminance compared to the PLEDs fabricated with only ITO [23]. Colsmann's group demonstrated a solution processed blue OLED comprising WO3 HIL. The device current efficiency increases from 8 cd/A when using PEDOT:PSS to 14 cd/A upon
Corresponding author at: School of Chemical Engineering and Technology, Tianjin University, Tianjin 300354, China. E-mail address: [email protected] (X. Li).
https://doi.org/10.1016/j.mssp.2018.06.007 Received 4 April 2018; Received in revised form 7 May 2018; Accepted 7 June 2018 Available online 20 June 2018 1369-8001/ © 2018 Published by Elsevier Ltd.
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the incorporation of WO3 [24]. However, the employment of solution process to deposit WO3 film becomes difficult due to the harsh dissolution conditions and complex synthesis process [25]. Therefore, efforts have been made to develop a precursor for WO3 HIL. It has been reported that phosphoric acid (H3PO4) can increase the ITO work function [26]. Unfortunately, the work function of acid-treated ITO dramatically decreases after deposition of HTL. In this contribution, inorganic acid has the potential to be precursor for HIL. Herein, this work intends to prepare precursor using the excellent solubility of phosphotungstic acid (H3PW12O40·xH2O) which contains transition metal (tungsten), and to obtain the coating solution which is suitable for various solution processing. Then HIL film is obtained by annealing the phosphotungstic acid film. Afterward, the film composition, surface morphology and work function are studied. At last, highperformance OLED is fabricated due to the hole injection property.
Fig. 1. TGA curve of phosphotungstic acid powder.
2. Experiment heavily the performance of device especially solution process fabrication [27,28]. It should be possible that phosphotungstic acid solution can avoid the corrosion phenomenon because of its insoluble characteristics in chlorobenzene and toluene when it is used as a HIL precursor. Further film formation experiment suggests that it can form stable film when using methanol and acetonitrile as solvent. Considering methanol has the lower boiling point, the acetonitrile is a good candidate for dissolving phosphotungstic acid powders, and the concentration of phosphotungstic acid is 10 mg/mL.
The ITO glass substrate (sheet resistance of about 8 Ω/sq) was purchased from Advanced Election Tech which was used as anode in OLEDs. Methanol, 2-propanol, acetone, acetonitrile, tetrahydrofuran, toluene, chlorobenzene and dimethyl sulfoxide (DMSO) were purchased from Tianjin Concord Technology. Phosphotungstic acid (H3PW12O40·xH2O), tungsten trioxide (WO3), N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)-benzidine (TPD), Tris-(8-hydroxyquinoline) aluminum(Ⅲ) (Alq3), LiF and aluminum (Al) were purchased from Shanghai Energy Chemical. PEDOT:PSS (BAYTRON®P VP CH 8000) was purchased from Xi’an Polymer Light Technology Corp. OLED was designed with the structures of [ITO/ HIL/ TPD (25 nm)/ Alq3 (60 nm)/ LiF (1 nm)/ Al (100 nm)]. Firstly, ITO glass substrates were successively cleaned with DI water, 2-propanol, acetone and methanol in an ultrasonic bath for 20 min. Then they were boiled in 2propanol and dried with nitrogen gas flow, followed by the oxygen plasma treatment (3 min, 80 W and 1mbar). WO3-P2O5 and PEDOT:PSS, which were used as HIL materials, were spin-coated onto the ITO glass substrates in air at a spinning speed of 2000 rpm for 30 s. WO3 layer and other functional layers were prepared by the method of vacuum thermal evaporation under a pressure of 5 × 10−4 Pa. In detail, the thermal evaporation rates of WO3, TPD, Alq3, LiF and Al were 0.3, 1.7, 1.7, 0.3 and 10 Å/s with the thickness of 3 nm, 25 nm, 60 nm, 1 nm and 100 nm, respectively. The measurement conditions of HIL film and device performance were as follows: thermal property of phosphotungstic acid was measured by TGA-Q500. Optical property, surface morphology, chemical analysis and work function of WO3-P2O5 films were investigated by UV–Vis spectrometer, atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS) and photoelectron spectrometer (Riken Keiki AC-2). The current density–voltage–luminescence (J–V–L) characteristics of OLEDs were investigated using a chroma meter CS-2000 (Konica Minolta) and a source measure unit (Keithley 2400). All the devices characterizations were performed at room temperature without any encapsulation.
3.2. The composition of phosphotungstic acid and thin film As phosphotungstic acid (H3PW12O40·xH2O) contains the combined water, the annealing condition of the film can be determined according to its thermal behavior. TGA curve of phosphotungstic acid is shown in Fig. 1. From the curve, phosphotungstic acid powder has rapid weightless when the temperature increases, and then it becomes very gentle near 200 °C. It indicates that the combined water of phosphotungstic acid can easily lose. Furthermore, the weightlessness rate is 7.2% at 200 °C. Hence, phosphotungstic acid can be calculated with composition of H3PW12O40·11H2O. To confirm the composition of thin film, the atomic ratio on the surface has been analyzed by XPS (Tab.S2). It can be concluded that W/ P atomic ratio of the film without annealing treatment is 11.4:1. Furthermore, with the increase of processing temperature, W/P ratio increases gradually. When the annealing temperature is 200 °C, W/P ratio is 12.3:1. It means that the film has composition of 24 WO3·P2O5, which accords with Keggin heteropolyacids structure of phosphotungstic acid (H3PW12O40) [29]. When the annealing temperature increased up to 300 °C, W/P ratio increases as the phosphorus oxide has a little weightlessness visibly. 3.3. Valence state of W and work function of thin film The valence states of metal elements such as Mo and W in TMO thin films can result in different work function [30]. Fig. 2 indicates the W 4 f signals of the XPS spectra in samples which contained WO3 and WO3-P2O5 (treated at different conditions). The W 4f7/2 and W 4f5/2 peaks are carried out through using the C 1 s peak position with binding energy (BE). The position and shape of photoemission peaks are helpful to confirm the valence state of W atom. It has been proved that the position and the shape of W 4f photoemission peak represent W atoms with a valence state of VI, when W 4f7/2 situates at BE of 36.0 ± 0.1 eV and W 4f5/2 centers at 38.2 ± 0.1 eV (with a peak ratio of 4:3). Besides, another lower BEs (BE of W 4f7/2 is 31.3 eV, and of W 4f5/2 is 33.5 eV with a peak ratio 4:3) are attributed to W atoms with zero valence [29,31]. For the WO3 thin film, it completely confirms to the character peak of W (VI) (BE of W 4f7/2 is 36.0 eV, and of W 4f5/2 is 38.0 eV). And for the WO3-P2O5 films (unannealed), the character peaks
3. Results and discussion 3.1. Solubility of phosphotungstic acid In order to choose appropriate solvent, phosphotungstic acid powders are put into eight kinds of solutions which are water, methanol, 2propanol, acetone, acetonitrile, tetrahydrofuran, toluene and chlorobenzene. The conditions of dissolving are shown in Tab. S1. As can be seen from the results, phosphotungstic acid dissolves easily into all of solvents but toluene and chlorobenzene. As known to all, most of the hole transport materials (HTM) have high solubility in chlorobenzene or toluene. From the structure of OLED, HIL is contact with HTL, therefore the two layers inevitably corrode one another to depress 107
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Fig. 2. W 4f signals of the XPS spectra in (a) WO3; (b) WO3-P2O5 (annealed by different temperature).
analysis. As the thin film annealed at 200 °C, the combined water in phosphotungstic acid film loses completely and the film (WO3-P2O5 film) becomes densification with the RMS value of 1.525 nm. It is interesting to note that the RMS value of film increases from 1.525 nm to 2.239 nm, when the temperature rises to 250 °C. It indicates that phosphorus and oxygen element in the film begin to decrease, so that the surface becomes rough. And RMS value becomes smaller (1.935 nm) when the film annealed at 300 °C. And the RMS values of PEDOT:PSS and WO3 films are also measured (Fig. S2), which are 4.263 nm and 3.471 nm, respectively. WO3-P2O5 film is flat and uniform. It indicates that WO3-P2O5 film can improve ITO surface, make the surface flat, and have the potential to improve the interface between HTL and ITO. 3.5. Transmittance of thin film Fig. 3. The Photoelectron Spectrometer (PES) spectra of WO3-P2O5 films.
For OLED, transmittance of anode is a very important index [32]. To verify whether WO3-P2O5 films have influence on the transmittance of ITO glass substrates or not, the optical property is measured in Fig. 5 which shows the optical absorbance (a) and transmittance (b). It can be seen from Fig. 5a that WO3-P2O5 films are measured at an absorption peak of ultraviolet range (318 nm) and has a weak absorption peak at a wavelength of 456 nm. According to Fig. 5b, the optical transmittance curves show similar shapes, and the samples show quite high optical transmittances of over 92% (Tab.S3) in the visible wavelengths between 390 and 780 nm. Hence, the transmittance of ITO does not significantly change with the deposition of WO3-P2O5.
of W atom (BE of W 4f7/2 is 36.4 eV, and of W 4f5/2 is 38.6 eV) have a large gap with that of W (VI), which may result from Keggin structure of phosphotungstic acid and O atom in H2O. When the annealed temperature is 200 °C, the peaks (BE of W 4f7/2 is 36.2 eV, and of W 4f5/2 is 38.2 eV) match with the character peak of W (VI), and it indicates that the film performs character of WO3 unit. When the temperature rises to 300 °C, the peaks (BE of W 4f7/2 is 36.2 eV, and of W 4f5/2 is 38.4 eV) also have different performance with that of W (VI). And this may be due to the loss of phosphorus and oxygen content in the WO3-P2O5 film. In addition, phosphorus oxides are confirmed with the BE of P 2p (135.0 eV) (Fig. S1). From the above figures, it can be seen that the best annealing temperature is 200 °C. Based on this result, the work function of the WO3-P2O5 film (200 °C) is 5.55 eV measured by photoelectron energy spectrum (AC-2) (Fig. 3).
3.6. Performance of OLEDs with phosphorus-tungsten oxide film The work function of ITO measured with photoelectron spectrometer (AC-2) is 4.82 eV. As existing energy barrier between ITO and TPD (HOMO energy level of TPD is 5.5 eV), hole injection is not efficient. Therefore, HIMs are used to improve OLED performance, such as PEDOT:PSS (HOMO energy level is 5.2 eV) and TMO. Because the thermally evaporated WO3 is a kind of n-type semiconductor with work function of 6.65 eV (measured by AC-2), and oxide energy alignment is governed by electron-chemical-potential equilibration to enable thermal excitation of electrons, therefore, charge can be effectively transmitted [23,30]. WO3-P2O5 film has the work function of 5.55 eV. As the valence state of W in WO3-P2O5 film appears W (VI) state, WO3P2O5 presents similar property with WO3. It suggests that WO3-P2O5 film annealed in air is inclined to be a kind of n-type semiconductor. Due to the existing P2O5, the work function of WO3 is close enough to HOMO energy level of TPD. The interface dipole is appeared in [ITO/ WO3-P2O5/ TPD] after depositing WO3-P2O5 [33]. Furthermore, hole injection from ITO to TPD can easily occur as WO3-P2O5 films exhibit smooth. To obtain the hole injection property from WO3-P2O5 to TPD, hole
3.4. Surface morphology of thin film A flat film with fewer pin-holes and spikes is conducive to improve charge transporting and increase the efficiency of OLED. The surface morphology of the ITO and WO3-P2O5 films are exhibited in Fig. 4. According to the picture, there are some differences on the surface among these samples, and the root mean square (RMS) roughness value was 4.030 nm, 1.585 nm, 2.901 nm, 1.525 nm, 2.239 nm and 1.935 nm, respectively. It dramatically improves film flatness when annealing at 100 °C, and the RMS value reduces from 4.030 nm (ITO) to 1.585 nm. It indicates that the phosphotungstic acid can smooth the ITO surface to improve the contact between ITO and HTL. However, the RMS value rises from 1.585 nm to 2.901 nm when the film is annealed at 150 °C. The possible reason is that the combined water in phosphotungstic acid film continued to lose, the combined water volatilized and caused coarse film. This result is consistent with above thermal properties 108
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Fig. 4. AFM image of WO3-P2O5 film annealed at different temperature (a) bare-ITO; (b) 100 °C; (c) 150 °C; (d) 200 °C; (e) 250 °C; (f) 300 °C.
only devices (HODs) with the structure of [ITO/ WO3-P2O5/ TPD (75 nm)/ WO3 (5 nm)/ Al (100 nm)] is fabricated [34]. The current density-voltage (J-V) curves shown in Fig. 6 are measured. Devices with WO3-P2O5 film annealed at different temperature show different characteristics. The threshold voltage (defined as the driving voltage when the current density increases sharply in the device) and maximum current density of HODs are 3.5 V, 207 mA/cm2 (100 °C); 2.0 V, 615 mA/cm2 (150 °C); 3.0 V, 436 mA/cm2 (200 °C); 5.0 V, 77 mA/cm2 (300 °C), respectively. All the HODs based on WO3-P2O5 films perform lower threshold voltage and higher maximum current density than HOD 0 (5.5 V, 51 mA/cm2) which was fabricated without any HIM. It indicates that the insertion of WO3-P2O5 film in HODs enhance hole injection at low applied voltage which can lead to lower turn-on voltage of OLED. On basis of data, hole transfer ability in HOD 2 (150 °C) is strongest in the four HODs, while HOD 4 (300 °C) shows the weakest one. It is worth noting that the hole transfer ability in HOD 1 (100 °C) and HOD 3 (200 °C) are moderate. Furthermore, HOD 3 expresses better than HOD 1. The hole current density is determined by the hole injection barrier at the ITO and TPD interface. According to the work function of WO3-P2O5 film, the utilization of WO3-P2O5 film can reduce the energy barrier between ITO and TPD, leading to increase of hole current density [35]. In conclusion, it suggests that WO3-P2O5 films annealed from 100 °C to 200 °C can enhance hole transfer ability of
Fig. 6. The current density-voltage (J-V) curves of HODs.
device at different levels. Considering WO3-P2O5 film demonstrates characters of smooth surface, high transmission, and promoting hole transfer ability of device, it is applied as HIL in OLED with the structure of [ITO/ HIL/ TPD (25 nm)/ Alq3 (60 nm)/ LiF (1 nm)/ Al (100 nm)]. Particularly, four OLEDs (device B, C, D and E) are fabricated with HIL using WO3-P2O5
Fig. 5. Optical characteristic of ITO and WO3-P2O5 films through different annealed temperature treatment: (a) absorption; (b) transmittance. 109
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15,981 cd/m2, 3.83 cd/A, 2.97 lm/W, respectively. Moreover, device B demonstrates smaller efficiency roll-off. According to the result of HOD 1, hole transfer rate possible matches with the electron-transfer rate in device B, and then carrier transporting balance occurs in OLED. Hence, device gains higher luminance and smaller efficiency roll-off. However, film treated at 100 °C still contains water, which is not conducive to prolonging the device lifetime. Device performance improves continuously along with the increase of annealed temperature. Device D (200 °C) shows the turn-on voltage of 2.6 V, maximum luminance of 13553 cd/m2, maximum CE of 5.87 cd/A, maximum PE of 3.44 lm/W, respectively, which are better than the performance of device C (150 °C) (2.6 V, 12,542 cd/m2, 5.36 cd/A, 3.35 lm/W). This result may be that surface of HIL in device C (RMS value is 2.901 nm) is rougher than that in device D (RMS value is 1.525 nm), although the hole transfer ability in HOD 2 is high. Figs. 7b and 7c show current efficiency-current density and power efficiency- current density curves, respectively. Device A exhibits poor performance at low applied voltage because of the inefficient hole injection. The efficiency in device A increased with the increasing current density, nevertheless, it is still quite low. It should be pointed out that the efficiency of C and D get improved significantly when are compared with device B. Whereas, there is an obvious efficiency roll-off in device C and D. The most likely reason is that holes are transported too fast in device C and D, which leads to the imbalance of carrier transporting [36]. Comparing with former three devices, the performance of device E (300 °C) (2.9 V, 10,681 cd/m2, 2.64 cd/A, 1.16 lm/W) is routine, which may result from change of W valence state in WO3-P2O5 film. According to the data of HOD, HIL annealed at 300 °C has a little impact on improving hole transfer ability in OLED. Therefore, the carrier transporting balance in device E becomes worse than those of device B, C and D, which results in the lower current efficiency in device E. Nevertheless, device E has little efficiency roll-off. Above all, it is potential to apply WO3-P2O5 film annealed at 200 °C as HIL to the fabrication of high performance OLED. OLEDs based on WO3-P2O5 film may be more stable than device A due to the efficient hole injection.
Table 1 The optoelectronic results of OLEDs with WO3-P2O5 via different treatments used as HIL. Decices
A (Bare-ITO) B (100 °C) C (150 °C) D (200 °C) E (300 °C) a b c d e f
Lmaxa (cd/m2)
9664 15,981 12,542 13,553 10,681
Vonb (V)
CIEe
λELf (nm)
(cd/A)
PE maxd (lm/ W)
4.09 3.83 5.36 5.87 2.64
1.04 2.97 3.35 3.44 1.16
(0.32,0.56) (0.31,0.55) (0.31,0.55) (0.31,0.55) (0.31,0.55)
522 522 522 522 522
CE max
3.3 3.0 2.6 2.6 2.9
c
The maximum luminance. Applied voltage at 1 cd/m2. The maximum current efficiency. The maximum power efficiency. The CIE coordinates of electroluminescence spectrum. The wavelength of EL spectrum.
films, which are annealed at different temperature. In comparison, blank control device (device A) is fabricated with the structure of [ITO/ TPD (25 nm)/ Alq3 (60 nm)/ LiF (1 nm)/ Al (100 nm)]. The electroluminescence (EL) performances of OLEDs are shown in Fig. 7 and Table1. It can be seen from Fig. 7a, b and c that device A (blank control device) shows the turn-on voltage (defined as the operation voltage at luminance of 1 cd/m2) of 3.3 V, maximum luminance of 9664 cd/m2, maximum current efficiency (CE) of 4.09 cd/A, maximum power efficiency (PE) of 1.04 lm/W, respectively. And the EL performances of devices are improved when using disparate WO3-P2O5 films as HIL due to the fact that the interface dipole is formed to accelerate the hole injection, which will decrease the turn-on voltage and increase the efficiency [16]. The turn-on voltage and threshold voltage of device B (100 °C) are 3.0 and 6.0 V, respectively. Although device B displays the maximum threshold voltage, its maximum luminance, CE, PE can reach
Fig. 7. The EL performances of OLEDs with WO3-P2O5 used as HIL: (a) current density–voltage-luminance (J-V-L) curves; (b) current efficiency-current density (CE-J) curves; (c) power efficiency-current density (PE-J) curves; (d) normalized EL spectra. 110
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Furthermore, device B, C and D may possess better stability than device E according to the hole transfer ability of HODs. Fig. 7d illustrates normalized EL spectra. Alq3 is often used for a green color material with its good film-forming, high thermostability and good electron transport properties [37–39]. The spectra demonstrate only one peak wavelength of 522 nm, and it conforms to the green emission of Alq3 which is irrespective of the annealed temperature [40]. Because there are no obvious shifts according to the EL spectra, it indicates that the recombination zone locates in emitting layer in all devices. Compared with device A, the full wavelength at half maximum (FWHM) of device B, C, D and E is narrower, indicating high purity of devices based on WO3-P2O5 films. Generally, the EL spectra mainly depends on the carrier recombination zone and the micro-cavity effects of the device [41,42]. Device B, C, D and E have weaker micro-cavity effects in comparison with device A, as the optical transmittance curves of WO3-P2O5 films show slightly different with that of ITO in Fig. 5. Difference in emission spectra of the devices is attributed to the different micro-cavity effect. In order to verify the superiority of the phosphorus-tungsten oxide film when used as HIL, device with the structure [ITO/ HIL/ TPD (25 nm)/ Alq3 (60 nm)/ LiF (1 nm)/ Al (100 nm)] is constructed (using PEDOT:PSS, WO3-P2O5 and WO3 as HIL). The J-V-L and efficiency curves of devices are shown in Fig. 8a, b and c, and the device data is summarized in Table2. Compared with device A (3.3 V, 9664 cd/m2, 4.09 cd/A, 1.04 lm/ W), device H (PEDOT:PSS) exhibits poor performance (3.2 V, 11,109 cd/m2, 3.89 cd/A, 0.86 lm/W), which may be due to the insulating PSS. The performance of device greatly depends on the solvent post treatment, which is usually used for increasing conductivity of PEDOT:PSS. While using vacuum evaporated WO3 film as HIL, the performance of device G (WO3) (17,274 cd/m2, 4.93 cd/A, 1.63 lm/W) is significantly improved. According to the energy-level alignment in [ITO/ HIL/ HTL], effective charge transfer from TPD to WO3 occurred, and formed a powerful interface dipole [13]. As a result, holes are significantly injected when WO3 is contact with TPD, which results in improving OLED performance. In comparison, as there is no strong
Table 2 The optoelectronic results of OLEDs with different HIL materials. Devices
A (bare-ITO) F (WO3P2O5) G (WO3) H (PEDOT: PSS) a b c d e f
Lmaxa (cd/m2)
Vonb (V)
PE
CE max
c
max
CIEe
λELf (nm)
d
(cd/A)
(lm/ W)
9664 13553
3.3 2.6
4.09 5.87
1.04 3.44
(0.32,0.56) (0.31,0.55)
522 522
17,274 11,109
2.8 3.2
4.93 3.89
1.63 0.86
(0.33,0.52) (0.30,0.54)
522 522
The maximum luminance. Applied voltage at 1 cd/m2. The maximum current efficiency. The maximum power efficiency. The CIE coordinates of electroluminescence spectrum. The wavelength of EL spectrum.
interface dipole between PEDOT:PSS and TPD, device H performs slight improvement. Besides, due to the insulating PSS, the turn-on voltage of device H merely decreases from 3.3 V to 3.2 V. And the performance of device F (WO3-P2O5) (5.87 cd/A, 3.44 lm/W) is better than that of device G (4.93 cd/A, 1.63 lm/W). As the work function of WO3-P2O5 film is 5.55 eV, which is closer to HOMO energy level of TPD than WO3 film, coating of WO3-P2O5 film forms a strong interface dipole. The energy gap significantly decreases and efficient hole-injection from ITO to TPD is enabled [23]. Particularly, the maximum luminance of device F is 13553 cd/m2 (that of device G is 17,274 cd/m2). Meanwhile, all the devices fabricated by solution process show efficiency roll-off, and this may be that vacuum evaporated WO3 is more compact to ensure efficient hole injection at high voltages. Even so, device F show higher efficiency than other devices due to the high efficient hole injection of WO3-P2O5 film. In addition, Fig. 8d illustrates normalized EL spectra with one peak wavelength of 522 nm. Compared with device A and G, the FWHM of
Fig. 8. The EL performances of OLEDs with different HIL materials: (a) current density–voltage-luminance (J-V-L) curves; (b) current efficiency-current density (CE-J) curves; (c) power efficiency-current density (PE-J) curves; (d) normalized EL spectra. 111
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device F and H is narrower, indicating high purity of the two devices. It is reported that OLEDs with PEDOT:PSS express a weaker micro-cavity effect when compared with OLEDs with ITO [41]. Thus, the difference in emission spectra results from the different micro-cavity effect in device F and H. In conclusion, these results suggest that phosphorustungsten oxide has the potential to replace PEDOT:PSS.
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4. Conclusions An effective and low-cost method to form HIL in OLEDs is demonstrated. Compared with WO3 film, the work function of the phosphorustungsten oxide film is modified owing to the P2O5 unit. WO3-P2O5 film possesses high transmission and smooth surface characters. Inspired by this, it is employed as HIL to fabricate the OLED which demonstrates the maximum CE of 5.87 cd/A and maximum PE of 3.44 lm/W. The device exhibits performance comparable to that of OLED using WO3 or PEDOT:PSS as HIL. Since phosphorus-tungsten oxide layer is much more convenient and low-cost, it may benefit to the research on HIL materials with metal oxide in OLED. Acknowledgments This work was supported by the National Key R&D Program of China (2016YFB0401303), the National Nature Science Foundation of China (21676188) and the Key Projects in Natural Science Foundation of Tianjin (16JCZDJC37100). Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.mssp.2018.06.007. References [1] J. Wang, J. Liu, S. Huang, X. Wu, X. Shi, C. Chen, et al., High efficiency green phosphorescent organic light-emitting diodes with a low roll-off at high brightness, Org. Electron. 14 (2013) 2854–2858. [2] J. Yu, Y. Yin, W. Liu, W. Zhang, L. Zhang, W. Xie, et al., Effect of the greenish-yellow emission on the color rendering index of white organic light-emitting devices, Org. Electron. 15 (2014) 2817–2821. [3] L. Mo, J. Ran, L. Yang, Y. Fang, Q. Zhai, L. Li, Flexible transparent conductive films combining flexographic printed silver grids with CNT coating, Nanotechnology 27 (2016) 065202. [4] K. Sugiyama, H. Ishii, Y. Ouchi, K. Seki, Dependence of indium–tin–oxide work function on surface cleaning method as studied by ultraviolet and x-ray photoemission spectroscopies, J. Appl. Phys. 87 (1999) 295–298. [5] S. Logan, J.E. Donaghey, W. Zhang, I. McCulloch, A.J. Campbell, Compatibility of amorphous triarylamine copolymers with solution-processed hole injecting metal oxide bottom contacts, J. Mater. Chem. C 3 (2015) 4530–4536. [6] Z.B. Wang, M.G. Helander, J. Qiu, Z.W. Liu, M.T. Greiner, 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 (2010) 024510. [7] Y.J. Choi, S.C. Gong, C.S. Park, H.S. Lee, J.G. Jang, H.J. Chang, et al., Improved performance of organic light-emitting diodes fabricated on Al-doped ZnO anodes incorporating a homogeneous Al-doped ZnO buffer layer grown by atomic layer deposition, Acs Appl. Mater. Interfaces 5 (2013) 3650–3655. [8] A.P. Kulkarni, C.J. Tonzola, A. Babel, S.A. Jenekhe, Electron transport materials for organic light-emitting diodes, Chem. Mater. 16 (2004) 4556–4573. [9] I. Irfan, S. Graber, F. So, Y. Gao, Interplay of cleaning and de-doping in oxygen plasma treated high work function indium tin oxide (ITO), Org. Electron. 13 (2012) 2028–2034. [10] J.X. Tang, Y.Q. Li, L.R. Zheng, L.S. Hung, Anode/organic interface modification by plasma polymerized fluorocarbon films, J. Appl. Phys. 95 (2004) 4397–4403. [11] K. He, X. Yang, H. Yan, J. Gong, S. Zhong, Q. Ou, et al., Surface properties of indium tin oxide treated by Cl2 inductively coupled plasma, Appl. Surf. Sci. 316 (2014) 214–221. [12] X.A. Cao, Y.Q. Zhang, Performance enhancement of organic light-emitting diodes by chlorine plasma treatment of indium tin oxide, Appl. Phys. Lett. 100 (2012) 183304. [13] Z. Hongmei, X. Jianjian, Z. Wenjin, H. Wei, Effect of PEDOT:PSS vs. MoO3 as the hole injection layer on performance of C545T-based green electroluminescent lightemitting diodes, Displays 35 (2014) 171–175. [14] T.-R. Chou, S.-H. Chen, Y.-T. Chiang, Y.-T. Lin, C.-Y. Chao, Highly conductive PEDOT:PSS films by post-treatment with dimethyl sulfoxide for ITO-free liquid
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