Photoconductive Cathode Interlayer for Enhanced Electron Injection in

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Photoconductive Cathode Interlayer for Enhanced Electron Injection in Inverted Polymer Light-Emitting Diodes Yinqi Luo, Tiancheng Yu, Li Nian, Linlin Liu, Fei Huang, Zengqi Xie, and Yuguang Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01758 • Publication Date (Web): 14 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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ACS Applied Materials & Interfaces

Photoconductive Cathode Interlayer for Enhanced Electron Injection in Inverted Polymer LightEmitting Diodes Yinqi Luo, † Tiancheng Yu,† Li Nian, † Linlin Liu, Fei Huang, Zengqi Xie,* Yuguang Ma Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China. †

These authors contributed equally to this work.

KEYWORDS: photoconductivity, cathode interlayer, light emitting diode, electron injection, Zinc oxide, perylene bisimide, P-PPV

ABSTRACT: Cathode interlayer is of crucial importance for efficient electron injection in inverted polymer light-emitting diodes (PLEDs) to realize high electroluminescence efficiency. Here, a novel photoconductive cathode interlayer based on organic dye doped ZnO (ZnO:PBI-H) is applied as cathode buffer layer in PLEDs, and dramatically enhanced device performance is obtained. The photo doping of ZnO may greatly promote the electron injection ability under the device working conditions, which increases the electron-hole recombination efficiency when using P-PPV as the light emitting material. Thanks to the decreased energy barrier between the cathode interlayer and the light emitting layer, the turn-on voltage of the PLEDs is obviously

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reduced when using the photoconductive cathode interlayer. Our results indicate that photo doping of the cathode interlayer is a promising strategy to increase the interlayer performance in light emitting diodes.

INTRODUCTION In the past three decades vast of organic semiconducting small molecules and polymers have been developed and applied in organic electronic devices such as organic light-emitting diodes (OLEDs),1-3 organic field effect transistors (OFETs),4-6 and organic photovoltaics (OPVs).7-11 Ptype organic materials are widely used as the light-emitting materials, in which holes dominate the charge transportation giving electron-hole combination profile close to the anode in the reported OLEDs.12-14 Whilst balanced charge injection and transportation is essentially important to obtain high electron-hole combination efficiency, various kinds of cathode interlayer materials have been developed to enhance the electron injection efficiency to adapt the less electron transportation behavior of p-type light emitting materials.15 Initially, n-type metal oxides, such as ZnO, TiO2, ZrO2, were employed to modify ITO electrode as electron injection and transport layers (EIL/ETL) as well hole blocking layers (HBL) in inverted polymer light-emitting diodes (PLEDs),16-19 which have many merits including exceptional air-stability, low cost, non-toxicity, and high transparency. However, the efficiencies of PLEDs based on these n-type metal oxides were less than satisfactory, because there is still large electron injection barrier from their conduction band (4.0-4.2 eV) to the lowest unoccupied molecular orbital (LUMO) of the emitters. To further reduce effectively the work function (WF) of cathode, two fruitful strategies were demonstrated and, consequently, highly efficient PLEDs became a reality. One approach is to build an interface dipole moment oriented away from the


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cathode contact, pulling up the vacuum level of the cathode and lowering the electron injection barrier.20 So far, the self-assembled monolayers (SAMs) of dipolar organic molecules,21 conjugated polymer with polar pendants,22-24 nonconjugated polar polymers,25-27 conjugated polyelectrolytes28 and polar solvents29 have been utilized successfully as cathode interlayer deposited on either ITO or n-type metal oxides. In this case, however, the device performance is sensitive to the thickness of the organic/polymer interlayer, typically less than 10 nm, resulting from the low conductivity and imperfect solvent resistance, which severely limits the mass production of large area devices. Another way is to carry out n-type doping of n-type metal oxides, such as TiO2 nanocrystalline film doped with Cs as reported by Park et al and doped ZnO film by adding Cs2CO3 directly into the precusor solution as reported by Bolink et al, and highly efficient OLEDs were presented in both cases.30,31 As is well known n-type doping not only reduces the WF but also increases the conductivity of the matrix materials. However, there is only few n-type dopant except active alkali metals and alkali metal salts.32 Most recently, a ZnO based photoconductive interlayer material was developed and applied successfully in high performance OPVs, in which organic dye molecule were used as dopant to enhance the electron mobility and conductivity of the interlayer by photo induced electron transfer from organic molecules to the conduction band (CB) of ZnO.33-37 Such photoconductive interlayer also showed decreased WF under light irradiation, which is a key factor for the cathode interlayer to adapt the energy level of the electron acceptor materials in OPVs. In principle, it is reasonable to use this kind of photoconductive material as cathode interlayer in inverted OLEDs since the light from the light-emitting layer can activate the interlayer to show enhanced electron injection behavior. In this paper, we report the dramatically enhanced electroluminescence (EL) efficiency and brightness of a P-PPV based PLEDs. The maximum


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luminous efficiency (LEmax) was increased to 15.4 cd A-1 from 2.0 cd A-1 by using photoconductive cathode interlayer (ZnO:PBI-H) to replace the pristine ZnO cathode interlayer. The turn-on voltage of the ZnO:PBI-H based device was obviously decreased indicating clearly reduced electron injection barrier was obtained in the device, and thus the enhanced device performance was attributed to the more balanced charge injection from electrodes in device. RESULTS AND DISCUSSION In general, the band gaps of interlayer materials are wide and they should be nearly colorless in consideration of the transmission of light.38 In spite of the absorption of PBI-H, the transmittance of ITO/ZnO:PBI-H cathode is still as high as 80-90% in the entire visible region comparable to that of ITO/ZnO cathode because of the slight doping of 1wt%. In addition, the film of ZnO:PBI-H possesses nice solvent resistance that PBI-H would not be released even if immersed in organic solvent like xylene for a long time, attributing to the special chemical N-Zn bonding between PBI-H and ZnO formed during the annealing process, and this facilitates the sequential spin-coating of light emitting layer. Furthermore, the surface morphology of PBI-Hdoped ZnO film is quite smooth with fully coverage of the ITO electrode, which is favorable to fabricate stable multilayer PLEDs. All of these basic chemical and physical properties of photoconductive cathode interlayer ZnO:PBI-H have been investigated thoroughly in the previous report.33 The PLEDs were then fabricated based on the device configuration of ITO/cathode interlayers (30 nm)/P-PPV (80 nm)/MoO3 (10 nm)/Al (100 nm) with three different cathode interlayers of ZnO, ZnO/PFN and ZnO:PBI-H, and the corresponding devices are referred to as device A, B and C, respectively. Figure 1a shows the device structure and the corresponding energy level diagram, where P-PPV acted as a classical green light-emitting layer, ZnO:PBI-H


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were applied as electron injection and transport layers, while the MoO3 served as hole injection layer. The chemical structure of P-PPV and PBI-H are depicted in Figure 1b, respectively. Figure 2 depicts the current density-luminance-voltage and luminous efficiency-current density characteristics of the PLEDs with three different cathode interlayers, respectively. The detailed performance data for different devices, such as the turn-on voltage, maximum luminance (Lmax), and maximum luminous efficiency (LEmax), are listed in Table 1. Amazingly, the slight doping of PBI-H into ZnO dramaticlly enhances the device performance of PLEDs. Compared with device A, the LEmax of device C was markedly increased from 2.2 cd A-1 to 15.4 cd A-1, the turn-on voltage was reduced observably from 4.8 V to 3.1 V and the Lmax was enhanced from 4097 cd m2

to 21050 cd m-2, respectively. The performance of device C is even better than device B in

which the classical cathode interlayer of PFN was used to modify the surface of ZnO (turn-on voltage of 3.2 V, Lmax of 17154 cd m-2 and LEmax of 14.4 cd A-1). It is noted that the leakage current in low voltage region of device B and C is obviously suppressed while the current density beyond build-in potential is observably larger than device A, as shown in Figure 2a. The results indicate that the ZnO:PBI-H cathode interlayer, similar to ZnO/PFN interlayer, possesses not only excellent electron injection and transport capacities but also good hole blocking ability. The increased electron transportation of ZnO:PBI-H must comes from its photoconductive property, while the enhanced hole blocking ability could be understood that the electron transfer from the HOMO of PBI-H to the covalent band of ZnO to fulfill the possible holes in ZnO. In our previous reports, very low dark current was observed in inverted polymer solar cells when applied a negative bias to the device, which also clearly indicates the enhanced hole blocking ability of ZnO:PBI-H interlayer.33 Compared with device B, it is found that device C shows a higher luminance and a larger current density at higher voltage, which might be attributed to the


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better electron injection ability of ITO/ZnO:PBI-H cathode due to the low WF of 3.8 eV measured by ultraviolet photoelectron spectroscopy (UPS).33 The PLEDs based on ZnO:PBI-H shows a low efficiency roll-off that the device still keeps a high Lmax of 15.4 cd A-1 at a high luminance of 1000 cd m-2, which is ascribed to the balanced hole and electron injection from MoO3/Al anode and ITO/ZnO:PBI-H cathode. In addition, the device operational stability was initially evaluated, and the experiment result showed that the ZnO:PBI-H based device exhibit quite similar degradation time as ZnO/PFN based device (See SI).

Figure 1. (a) Device configuration and the corresponding energy level diagram of inverted PLEDs device based on ZnO:PBI-H cathode interlayer. (b) Chemical structures of PBI-H and PPPV.


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Figure 2. (a) Current Density-luminance-voltage (J-L-V) and (b) Luminous efficiency-current density (LE-J) of inverted PLEDs devices based on ZnO, ZnO/PFN and ZnO:PBI-H cathode interlayer, respectively. Table 1. The performance of the inverted PLEDs with different cathode interlayers.

Device A B C

Cathode interlayer ZnO ZnO/PFN ZnO:PBI

Vona

Lmaxb

LEmaxc

PEmaxd

LEmax @ 1000 cd m−2

[V]

[cd m-2]

[cd A-1]

[lm W-1]

[cd A-1]

4.8 3.2 3.1

4097 17154 21050

2.2 14.4 15.4

0.8 6.7 7.2

2.0 11.7 15.4

CIE x

y

0.36 0.59 0.37 0.59 0.36 0.59

a

Turn-on voltage recorded at a brightness of 1 cd cm-2. bMaximum luminance. cMaximum luminous efficiency. dMaximum power efficiency. Figure 3 shows the EL spectra of PLEDs with different cathode interlayers at a current

density of 12.5 mA/cm2. As can be seen, all the three devices show a main emission band located


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at 522 nm along with a shoulder emission band at 554 nm. The relative intensity of the two emission bands gives the information of the length of the light pathway through the devices. For the case of ZnO/PFN cathode interlayer based device B, an relative enhancement of the shoulder peak at 554 nm, i.e. reduced relative intensity of the emission band at 522 nm, in the EL spectrum is observed when compared with that of device A, could be attributed to the reabsorption of the emission light. The slight difference of the EL spectrum reflects that the exciton recombination zone shift away from cathode in device B, which is attributed to the enhanced electron injection of PFN interlayer.39,40 In principle, based on the above analysis, the electron injection ability of ZnO:PBI-H cathode interlayer is even prior to that of PFN, and thus more balanced charge injection could be achieved in device C, that is the profile of EL spectrum of device C should be similar to that of device B. However, as shown in Figure 3, the EL spectrum of device C is almost identical to that of device A. We speculate that the absorption of the ZnO:PBI-H interlayer used in device C might reduce the intensity of the emission band at 554 nm, based on the spectral overlap between absorption spectrum of PBI-H and EL spectrum of P-PPV that will be discussed in next section. Indeed, when we used a blue light emitting material in the device, for which the spectral overlap is small, the obtained EL spectrum from the device based on ZnO:PBI-H was observed quite similar to that from the device based on ZnO/PFN (See SI), which certify the above speculation well.


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Figure 3. EL spectra of PLEDs with device configuration of ITO/Cathode interlayer(30 nm)/PPPV(80 nm)/MoO3(10 nm)/Al(120 nm), where cathode interlayer indicates ZnO, ZnO/PFN or ZnO:PBI-H. Furthermore, we prepared the electron-only devices of ITO/cathode interlayers/PPPV/PFN/Al with three kinds of cathode interlayers simultaneously and tested the electron current injected from ITO in the dark (Figure 4a) and under sunlight irradiation (Figure 4b), respectively. As is shown in Figure 4a, the deposition of PFN interlayer on top of ZnO increases markedly the electron current, this is due to a strong interface dipole oriented away from ZnO, pulling up the vacuum level of ITO/ZnO cathode and enhancing the electron injection.41 Whereas the doping of PBI-H into ZnO just increases the current a little in the dark. However, as can be seen in Figure 2a, the current density in device C is almost the same as or even larger than that in device B. In view of the same anode contact, the larger current density in PLEDs is ascribed to the larger electron current. To explain the contradiction of current density trends in PLEDs and electron-only devices based on ZnO/PFN and ZnO:PBI-H, the J-V characteristics of electron-only devices was tested under sunlight irradiation. As shown in Figure 4b, the electron current based on ZnO:PBI-H under AM 1.5G 100 mW/cm2 illumination is dramatically increased


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by 80-fold at 4.5 V compared to that of device in the dark, and is nearly 27 times larger than that of device based on ZnO/PFN which shows little change under sunlight irradiation. The distinct changes of electron current through the doping of PBI-H in two conditions fully illustrate that electron injection ability of ZnO:PBI-H interlayer owns a strong light dependence.

Figure 4. Current density-voltage (J-V) characteristics of electron-only devices of ITO/cathode interlayers/P-PPV/PFN/Al in dark (a) and under the irradiation of sunlight (b), respectively. The cathode interlayer indicates ZnO, ZnO/PFN or ZnO:PBI-H. However, the PLEDs were tested in a black box, in which the only light source was PLEDs itself. In other words, the electron current of electron-only devices based on ZnO:PBI-H is far less than that of ZnO/PFN-based electron-only devices in the dark while the current density of device C is higher than that of device B, this peculiar increase of electron current in bipolar


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device is caused by the light from the electroluminescence of P-PPV. As can be seen in Figure 5, there exist a big overlap between the EL spectrum of P-PPV and absorption spectrum of PBI-H, and PBI-H is a kind of organic color pigment with large molar extinction coefficient. Thus, We speculate that the small amount of PBI-H dispersed in the ZnO matrix is excited as soon as the PLEDs emits a faint green light at low bias voltage, and then the electron is transfer from the high-lying LUMO of excited PBI-H to the low-lying CB of ZnO, leading to an n-type doping of ZnO and a subsequent enhancement of device performance in PLEDs. It should be mentioned that the turn-on voltage of ZnO:PBI-H based device was not changed even applying a reverse bias, to exclude the possible preactivity by room light of the interlayer, before the measurement of the device (See SI). The study on the deep physics and the working mechanism of such photoconductive interlayer are ongoing in our lab now.

Figure 5. EL spectrum of inverted PLEDs device of ITO/ZnO(30 nm)/P-PPV(80 nm)/MoO3(10 nm)/Al(120 nm) and absorption spectrum of PBI-H in solution. CONCLUSIONS In conclusion, we uniquely employed a novel photoconductive cathode interlayer ZnO:PBIH in P-PPV based PLEDs, and obtained a high performance with a maximum LE of 15.4 cd A-1,


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a maximun luminance of 21050 cd m-2 and a low turn-on voltage of 3.1 V, exceeding the performance of ZnO/PFN based PLEDs. The photoconductive interlayer could be activated by the light from PLED itself in consideration of the overlap between the absorption spectrum of PBI-H and the EL spectrum of the emitter, and thus the ZnO could be online n-type doped in operation of the device. This online n-type photo doping method avoids effectively the use of active alkali metal and other n-type dopants which are vulnerable to oxygen, and provide a new tactic to achieve highly efficient OLEDs based on p-i-n configuration. Supporting Information. Detailed experimental procedures, Stability test, Room light effect experiment, bule light device. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We acknowledge the financial support from the National Natural Science Foundation of China (51761135101, 21733005, 51573055, 51473052), National Basic Research Program of China (973 Program) (2014CB643504), Fundamental Research Funds for the Central Universities, and Key Program of Guangzhou Scientific Research Special Project (201707020024).


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Photoconductive Cathode Interlayer for Enhanced Electron Injection in

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