Inverted polymer light-emitting devices using a conjugated starburst macromolecule as an interlayer

Abstract

We present an investigation of inverted polymer light-emitting diodes (PLEDs) with an alcohol-soluble conjugated starburst macromolecule TrOH as an interlayer. The introduction of a TrOH interlayer between the light-emitting polymer layer and ZnO has been shown to significantly enhance the device efficiency due to efficient electron injection as well as efficient blocking of exciton quenching. It was found that the thickness of the TrOH interlayer is an important factor impacting the device performance. As a result, an efficient inverted PLED with a maximum luminous efficiency of 1.43 cd A⁻¹ was achieved using a blue polymer poly(9,9-dioctylfluorene) (PFO) as the emitting layer.

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1. Introduction

Polymer light-emitting devices (PLEDs) using polymer semiconductors as the emitting layer have been drawing much attention because of their great potential in cost-effective and large-area displays and lighting applications.^1–3 In particular, PLEDs can be fabricated by the large-area manufacturing technologies such as ink-jet printing, spray coating, screen-printing and roll-to-roll coating.⁴ The conventional structure of PLEDs is indium tin oxide (ITO)/PEDOT : PSS/light emitting polymer/cathode, where poly(3,4-ethylenedioxythiophene) : poly(styrenesulfonate) (PEDOT : PSS) is used as a hole injection layer. For the purpose of facilitating electron injection, vacuum-deposited low work-function metal cathodes such as Ca and Ba are generally used to reduce operational voltage and increase luminous efficiency.⁵ However, these low work-function metals are sensitive to oxygen and humidity, thus deteriorating device operational stability. Although some ultrathin alkali salt interlayers such as LiF, CsF and Cs₂CO₃ have been developed for efficient electron injection from high work function metals such as aluminium,⁶ those materials are mostly incompatible with low-cost, large-area solution-processing manufacturing technologies.⁴

To overcome these problems of conventional PLEDs, it is desirable to achieve efficient PLEDs with stable cathode materials and structures. Recently, great efforts have been made to develop inverted-structure PLEDs with a typical configuration of ITO/metal oxide/light emitting polymer/metal oxide/anode.^7–12 In such devices, air-stable n-type metal oxides such as ZnO and TiO₂, and high work-function metal oxides such as MoO₃ and WO₃ are commonly utilized as electron injection layers (EILs) and hole injection layers (HILs), respectively.⁷ The hole-injection is effective because of the formation of ohmic contact at the interface between the high work-function metal oxides and the active layer. However, the electron injection is poor due to an energy level mismatch between the conduction band of n-type metal oxide (4.0–4.4 eV) and the lowest unoccupied molecular orbital (LUMO) of the active layer (2.4–3.0 eV).^10,13 This large electron injection energy barrier (1–2 eV) impedes electron injection into the active layer and limits overall electron–hole balance. Furthermore, the exciton dissociation at n-type metal oxide/polymer interface and exciton quenching caused by defect sites on the metal oxide surface reduce device efficiency.^9,12,14

To address these issues, various solution-processable interfacial materials including self-assembled dipole molecules,^8,15,16 polarized polymers,^17,18 alkali metal compounds such as Cs₂CO₃,¹³ ion liquid¹⁹ and n-doped carbon nanotube²⁰ have been utilized to improve electron injection, in combination with high device efficiency and a low operating voltage. Employing these interlayers on the n-type metal oxides can induce an interfacial dipole moment and thereby reduce the electron-injection barrier.⁹ As we know, an ideal interlayer with good environmental stability should efficiently inject electrons and block excitons quenching. For the alkali metal salt interlayers, although decomposed alkali metal ions such as Cs⁺ can enhance electron injection by doping an active layer to n-type,²¹ diffused metal ions in the active layer can form exciton quenching sites,¹³ resulting in worse device performance. Polarized polymers such as polyethyleneimine (PEI),^9,10 poly(9,9′-bis(6′′-N,N,N-trimethylammoniumhexyl)fluorene-co-alt-phenylene) with bromide counterions (FPQ-Br),¹⁷ polyethylenimine ethoxylated (PEIE)^9,22 and tetrakis(imidazolyl)borate of poly(9,9′-bis(6′′-(N,N,N-trimethyl ammonium)hexyl) fluorene-alt-co-phenylene) (PFN-BIm4)¹⁸ have been employed as interlayers in the inverted PLEDs. Inserting a thin layer of polarized polymer between the active layer and n-type metal oxide can enhance electron injection and suppress exciton quenching.^9,10 Though these polarized polymers have excellent film-forming properties and device performance, they suffer from problems such as batch-to-batch variation in terms of molecular weight and polydispersity, difficulty in purification and poor device reproducibility. Recently, our group developed a water/alcohol soluble monodisperse conjugated starburst macromolecule TrOH using as an efficient electron injection layer for PLEDs.^23–25 When a thin TrOH layer is inserted between the EML and Al cathode, the performance of the multilayered device is even better than that of the device using Ca as cathode. Despite the excellent performance of TrOH achieved in conventional PLEDs, the application of TrOH in inverted devices has not been investigated.

In this work, we presented a study in efficient inverted PLEDs using a solution-processed conjugated starburst macromolecule TrOH as the interlayer. We found that the performance of the PLEDs was significantly improved by the insertion of a TrOH interlayer between EML and ZnO due to efficient electron injection as well as efficient blocking of exciton quenching. A luminous efficiency of 1.43 cd A⁻¹ was achieved using blue polymer poly(9,9-dioctylfluorene) (PFO) as EML, which was superior to that of the device using ZnO as EIL. Open-circuit voltage measurement demonstrated that electron injection barrier height of the device was reduced by inserting a TrOH interlayer between ZnO and EML.

2. Experimental

The inverted PLEDs were fabricated with a configuration of ITO/ZnO/TrOH/PFO/MoO₃/Al. PFO end-capped with phenyl was synthesized by Yamamoto coupling polycondensation in our laboratory.²⁶ PFO exhibits number-average molecular weight of 6.94 × 10⁴ with a polydispersity of 1.33 in this work by gel permeation chromatography (GPC) analysis with polystyrene standard. The conjugated starburst macromolecule TrOH was synthesized in our laboratory.²³ The number average molecular weight of precursor polymer estimated by GPC using polystyrene as the standard and THF as the eluent is 18 000 (with a polydispersity of 1.8). The chemical structure of TrOH is shown in Fig. 1(b).

Fig. 1 Inverted device structure and molecular structure of TrOH.

In the experiment, the ZnO films were prepared according to the literature.²⁷ Sol–gel-derived ZnO films were prepared using zinc acetate dehydrate [Zn(CH₃COO)₂·2H₂O, Aldrich, 99.9%] in 2-methoxyethanol as a precursor solution. The concentration of zinc acetate dehydrate was 75 mg mL⁻¹ and the molar ratio of ethanolamine to zinc acetate dihydrate was kept at 1 : 1. This solution was stirred 1 h to get a clear and homogeneous solution. The ZnO layers were spin-coated onto the ITO substrate after the prepared solution was aged at room temperature for 12 h in order to make it more glutinous. Approximately 25 nm ZnO thin films were obtained by annealing at 200 °C for 1 h. Afterward, TrOH was spin-coated on top of ZnO from ethanol solutions with different concentrations of 0.5 mg mL⁻¹, 1.0 mg mL⁻¹ and 2.0 mg mL⁻¹, respectively. Subsequently, these corresponding films were annealed at 80 °C for 10 min in ambient atmosphere. The thicknesses of the films were estimated by an absorbance–thickness curve that assumed a linear dependence of the absorbance on thickness and found to be 8.7 nm, 12.1 nm and 16.1 nm corresponding to solution concentrations of 0.5 mg mL⁻¹, 1.0 mg mL⁻¹ and 2.0 mg mL⁻¹, respectively.²⁸ Following that, the EMLs were sequentially deposited from chloroform solution. Finally, after annealing at 120 °C for 10 min, a 6 nm MoO₃ and a 100 nm Al were deposited by thermal evaporation under a pressure of 5 × 10⁻⁴ Pa. For the electron-only devices, the 1,3,5-tris[N-(phenyl) benzimidazole]-benzene (TPBi, 100 nm), LiF (1 nm), and Al (100 nm) were thermally deposited, respectively. The active area of the devices is 13.5 mm².

The thickness of PFO and ZnO films was measured using a spectroscopic ellipsometry (α-SE, J.A. Woollam Co. Inc.). The surface morphology measurement was carried out by an atomic force microscope (AFM). The current–voltage–luminescence characteristics were measured by a Keithley 2602 source meter with a calibrated silicon photodiode. The electroluminescence (EL) spectra of the devices were analyzed with a spectrometer (PR655). For photovoltaic measurements, the photocurrent–voltage characteristics were recorded using a computer-controlled Keithley 2400 source meter and a solar simulator (Newport 91160) under AM 1.5 G condition at 100 mW cm⁻². All the device testing was carried out in ambient atmosphere.

3. Results and discussion

As shown in Fig. 2, the surface morphology (3 μm × 3 μm images) of ZnO and ZnO/TrOH (8.7, 12.1 and 16.1 nm) films were characterized by AFM to explore the influence of TrOH on the surface roughness of ZnO film. The ZnO film has root-mean-square roughness (R_rms) of 2.78 nm. R_rms values of the ZnO/TrOH films gradually decrease from 1.98 nm to 1.61 nm with increasing the TrOH thickness. According to the result, the surfaces of ZnO/TrOH films are much smoother than that of ZnO film. This indicates that TrOH interlayers can reduce the surface roughness of ZnO, which is beneficial to the electron injection.

Fig. 2 AFM images (3 μm × 3 μm) of (a) ZnO and (b)–(d) modified ZnO by TrOH with different thickness (8.7 nm, 12.1 nm and 16.1 nm).

In order to estimate the effect of the TrOH interlayer on the device performance, the inverted PLEDs with a configuration of ITO/ZnO/TrOH/PFO/MoO₃/Al were examined, where the thickness of TrOH interlayer was tuned from 8.7 to 16.1 nm by adjusting the concentration of TrOH solution. For comparison, the control device with a structure of ITO/ZnO/PFO/MoO₃/Al was also fabricated. Fig. 3 shows the current density–voltage, luminance–voltage, luminous efficiency–current density and EL spectra characteristics of the devices. The detailed device performance is summarized in Table 1. It is found that the device without the TrOH interlayer exhibits very poor performance with a turn on voltage (the voltage at 1 cd m⁻²) of 5.6 V, a maximum luminance of 427 cd m⁻² and a maximum luminous efficiency of 0.07 cd A⁻¹ due to the large electron injection barrier between ZnO and light-emitting layer. When a thin layer of TrOH is inserted between ZnO and EML, the performance of the devices is much better than that of the control device. As shown in Fig. 3(a), the devices with TrOH interlayers show much higher current density than that of the control device with ZnO as EIL, indicating that the TrOH interlayers can effectively reduce the electron injection barrier and facilitate electron injection from ZnO into EML. Comparing with the control device, the luminance and efficiency of the devices with TrOH interlayers are also improved. The device with 8.7 nm TrOH as interlayer shows a low turn-on voltage of 3.1 V, a maximum luminance of 5052 cd m⁻² at 9.0 V and a peak luminous efficiency of 0.94 cd A⁻¹. With the increase of the TrOH interlayer thickness, the efficiency of the device can be further improved to 1.43 cd A⁻¹. However, it is notable that the current densities of the devices decrease gradually with increasing the TrOH thickness. Additionally, the turn-on voltages of the devices also slightly increases from 3.1 V to 3.5 V with the increment of TrOH thickness. These trends may be attributed to the increase in the electron injection barrier as the TrOH interlayer thickness increases, which will be discussed later. To estimate the influence of TrOH interlayer on the EL spectra, the EL spectra of the devices were examined. As shown in Fig. 3(d), the spectral feature of EL implies that their exciton recombination zone are all located in the bulk of PFO, and that the TrOH plays a role as an electron injection layer.

Fig. 3 Device characteristics of the inverted PLEDs with different thickness of TrOH interlayer: (a) current density–voltage, (b) luminance–voltage, (c) luminous efficiency–current density, and (d) EL spectra.

Table 1 Electroluminescence characteristics of the inverted devices (V_on is turn-on voltage. L_max is the maximum luminance. LE_max is maximum luminous efficiency.)

Device with various TrOH thickness	Von [V]	Lmax [cd m^−2]	LEmax [cd A^−1]
0 nm	5.6	427	0.07
8.7 nm	3.1	5052	0.94
12.1 nm	3.3	6124	1.29
16.1 nm	3.5	4333	1.43

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From the above results, the superior performance of the devices with TrOH interlayer can be attributed to the improved electron injection from ZnO to EML. For further confirmation, the photovoltaic characteristics of the devices with and without TrOH interlayer were evaluated. The open-circuit voltage of a device reflects the built-in potential across the device, which can be used to evaluate the change in the carrier-charge injection barrier of the device. Fig. 4 shows the photovoltaic characteristics of the devices with and without TrOH interlayer. As deduced from the current density–voltage under illumination from a solar simulator, the open-circuit voltage of the device with ZnO as EIL is 0.81 V, while the open-circuit voltage of the device with 8.7 nm TrOH as interlayer increases to 1.59 V. These results indicate that electron injection barrier height of the device is reduced by inserting a TrOH interlayer between ZnO and EML, which leads to a more balanced injection of electrons and holes and better device performance. Additionally, it is noted that the open-circuit voltages slightly decrease from 1.59 V to 1.33 V when the thickness of the TrOH interlayers increases from 8.7 nm to 16.1 nm. This means that electron injection barriers of the devices are increased with increasing TrOH thickness. Therefore, as TrOH thickness increases, turn-on voltage tends to gradually increase, while current density tends to gradually decrease [Fig. 1(a) and (b)]. In order to further compare the electron injection ability of the interlayers with different thickness, electron-only devices with the structures of ITO/ZnO (25 nm)/TPBi (100 nm)/LiF (1 nm)/Al (100 nm) and ITO/ZnO (25 nm)/TrOH (8.7, 12.1, 16.1 nm)/TPBi (100 nm)/LiF (1 nm)/Al (100 nm) were fabricated. The current density–voltage characteristics of the devices are shown in Fig. 5. The devices with TrOH interlayers show much higher current densities than that of the ZnO device, which are consistent with the superior device performance. In addition, the current densities of TrOH devices decrease with increasing the TrOH thickness. These results indicate that electron injection ability decreases gradually with increasing the TrOH thickness.

Fig. 4 Photovoltaic characteristics of the inverted PLEDs.

Fig. 5 Current density–voltage characteristics of the electron-only devices.

Additionally, it should be noted that the efficiencies of the devices increase with increasing TrOH thickness although electron injection ability decreases [Fig. 3(c)]. In order to get insight into the dependence of device efficiency on the TrOH thickness, steady state PL spectra of quartz/ZnO/TrOH (0 nm, 8.7 nm, 12.1 nm and 16.1 nm)/PFO (25 nm) were measured. As shown in Fig. 6, the quartz/ZnO/PFO sample shows the lowest PL intensity, indicating the strong luminescence quenching nature of the ZnO layer. This quenching effect can be effectively suppressed with the insertion of a thin TrOH interlayer, as evidenced by the enhanced PL intensity compared with the quartz/ZnO/PFO sample. With increasing the TrOH thickness, the intensities of PL spectra tend to gradually increase, resulting from the increasing distance between generated excitons and the quenching ZnO interface. It should be noted that the exciton recombination zone is located close to the ZnO/PFO interface in the inverted device structure due to ohmic contact in PFO/MoO₃/AL and higher hole mobility of PFO than electron mobility.²⁹ Thus, the significant exciton quenching occurs at the ZnO/PFO interface due to the exciton dissociation induced the metallic nature of ZnO.⁹ This quenching effect can be effectively suppressed by inserting a TrOH interlayer to passivate the surface defects of ZnO. Based on these results, the dependence of device efficiency on the TrOH thickness can be easy to understand. When a thin layer of TrOH (8.7 nm) was used as the EIL, the resulting device exhibits a low efficiency due to exciton quenching by ZnO. With the increase of the TrOH interlayer thickness, exciton quenching by ZnO tend to gradually decrease although electron injection ability decreases slightly. Therefore, the efficiencies of the devices increase with increasing the TrOH thickness.

Fig. 6 PL intensities of PFO (25 nm) on quartz/ZnO/TrOH (0 nm, 8.7 nm, 12.1 nm and 16.1 nm).

4. Conclusions

In summary, we have demonstrated efficient and air-stable inverted PLEDs by employing alcohol-soluble TrOH as an interlayer between ZnO and EML. The introduction of TrOH as interlayer effectively improves the device performance. The device exhibits a maximum luminous efficiency of 1.43 cd A⁻¹, which is better than that of the control device using ZnO as an electron injection layer. Improvement of device efficiency can be attributed to the enhanced electron injection and increased blocking of exciton quenching by TrOH interlayer. Our study indicates that the alcohol-soluble conjugated starburst macromolecule is a promising interlayer material for applications in fully solution-processed large-area inverted devices.

Acknowledgements

This work was supported by the National Key Basic Research Program of China (2014CB648300 and 2012CB933301), the NSFC (61204048, 21674050 and 21422402), the Ministry of Education of China (NCET-13-0872), Specialized Research Fund for the Doctoral Program of Higher Education (20133223110008), Program for Jiangsu Specially-Appointed Professor (RK030STP15001), the NSF of Jiangsu Province (BK20161519, BK20140865, BK20130037 and BK20140060), the Scientific Research Foundation of NUPT (NY214178, NY214093 and NY215076), the Synergetic Innovation Center for Organic Electronics and Information Displays, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Footnote

† These authors contributed equally to this work.

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