Doping core–shell nanoparticles into a solution-processed electron transporting layer for polymer light-emitting diodes

Abstract

Core–shell gold nanoparticles have been doped into the solution-processed electron-transporting layer (ETL) of polymer light-emitting diodes (PLEDs). By using this doping strategy, metal-enhanced fluorescence was realized in the device. The doped device has obtained enhanced luminance, enhanced luminous efficiency and a reduced turn-on voltage compared with that using the non-doped ETL.

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Organic light-emitting diodes (OLEDs) have been attracting attention in both the industry and research communities, since they are very promising candidates for application in flat-panel displays and solid-state lighting.^1–5 Solution processing techniques are generally adopted in the fabrication of OLEDs to simplify the manufacture process and reduce the cost, including spin-coating, screen printing, and ink-jet printing. However, the devices using solution processing usually show poorer performance than their thermal evaporation counterpart.^6–10 As a result, the improvement of the performance of solution-processed OLEDs is one of the key challenges in the large-scale commercialization of OLED technology.

Polymer light-emitting diodes (PLEDs) have been widely studied in recent years due to their compatibility with solution processing techniques.^3,4 Various methods, from the points of molecular design or device structure, are employed to optimize the performance of PLEDs.^11–13 Doping metallic nanoparticles (NPs) into organic electroluminescence (EL) devices is a strategy of great interest for researchers to improve device performance.^14–19 Provided that the fluorophore and the metallic NPs are kept a suitable distance, and that the spectrum of fluorophore emission and localized surface plasmon resonance (LSPR) of the metallic NPs are well overlapped, the metal-enhanced fluorescence (MEF) can be obtained.^20,21 Thus the MEF effect can improve the EL performance of devices. In addition, as the metallic NPs can be synthesized and dispersed in some common solvent, the devices incorporating metallic NPs can achieve all-solution processing.

To achieve the optimized improvement by doping metallic NPs into PLEDs, a spacer layer outside the metallic NPs is needed. This spacer layer can avoid the exciton quenching and can provide suitable distance to ensure MEF effect.¹⁴ Also, it is equally important that the incorporation of the spacer layer will not have some negative effect on the electrical property of the device. Based on the above considerations, it can be a practical method to introduce the metallic NPs into the carrier-transporting layer of EL devices. However, there were only a few studies about the carrier-transporting layer doping strategy using solution processing.^15,16

It has been reported that the non-conjugated polymer, polyethylene glycol (PEG), could enhance the electron injection ability of devices based on the ETL composed of 2,7-bis(diphenylphosphoryl)-9,9′-spirobifluorene (SPPO13).²² Due to the strong covalent bonding between the thiol groups with the gold surfaces,²³ the thiolated polyethylene glycol (PEG-SH) can be facilely coated on the gold nanoparticles surfaces. Thus the PEG-SH can be suitable to serve as the spacer layer to achieve MEF effect and improve the performance of PLEDs. Besides, the alcohol-solubility of PEG can make the Au core–PEG shell NPs dispersed well in isopropyl alcohol (IPA), which is a common solvent used for fabricating solution-processed ETL. Thus, the introducing of MEF effect into PLED devices can be facilely accomplished by merely blending the solution of core–shell NPs with the solution of electron-transporting material.

In this study, we show that the doping of core–shell PEGylated gold (Au@PEG) nanoparticles into the solution-processed electron-transporting layer (ETL) can dramatically improve the device performance of PLEDs. The sufficient overlap between exciton energy with localized surface plasmon resonance of gold nanoparticles results in the metal-enhanced fluorescence in PLEDs. The PEG shell of the gold nanoparticles not only can avoid the exciton quenching on the metal surface, but can alleviate the negative effect of gold nanoparticles on the electron injection ability of device. By using this PEGylated gold nanoparticles doped ETL in the PLEDs, both the luminance and the luminous efficiency of device were improved, and the lower device turn-on voltage was also obtained. The maximum luminous efficiency of device was enhanced by nearly 1.8-fold compared with the non-doped device, from 6.94 cd A⁻¹ to 12.38 cd A⁻¹.

The core–shell Au@PEG NPs were synthesized using a method according to previous reports.²³ Briefly, Au NPs were prepared via citrate reduction method, and then 5 mL of the resulting colloidal solution was added dropwise to a PEG-SH solution (10 mg mL⁻¹, 5 mL) under vigorous stirring. One hour later, the Au@PEG NPs were separated from the reaction solution via centrifugation, washed with IPA and finally re-dispersed in IPA for further experiments.

The device structure used in this study was ITO/PEDOT:PSS/P-PPV/ETL/Al. Poly(3,4-ethylene-dioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS, CLEVIOS PVP Al 4083) was used as the hole-injecting/-transporting layer, and was fabricated on patterned ITO-coated glass substrates with a sheet resistance of 15 Ω sq⁻¹. Prior to spin-coating the PEDOT:PSS layer, the ITO substrates were pre-cleaned routinely and were ultraviolet (UV)-ozone treated for 5 min. The obtained 40 nm PEDOT:PSS layer was then baked at 150 °C for 10 min in nitrogen environment. The fluorescent polymer emitting material, poly(2-(4-(3′,7′-dimethyloctyloxy)-phenyl)-1,4-phenylene-vinylene) (P-PPV, purchased from Canton OLEDKING Optoelectric Materials Co. Ltd.),¹⁶ serving as the 60 nm thick greenish emissive layer (EML), was spin-coated in a chlorobenzene solution (5 mg mL⁻¹) at 1250 rpm for 40 s on the PEDOT:PSS layer. The baking of the EML was performed at 100 °C for 20 min. SPPO13, which was doped with Au@PEG NPs, was employed as the ETL. We prepared the ETL solution by dispersing Au@PEG NPs into the IPA solution of SPPO13. The ETL were processed from alcohol solvent (IPA), in which most conjugated polymers are insoluble.²⁴ This could avoid the emissive layer to be dissolved when depositing the ETL. The final concentration of Au@PEG NPs in the ETL solution was 0.33 mg mL⁻¹, to obtain the optimized device performance. The ETL of the reference devices was neat SPPO13 film, and SPPO13 film doped with pure PEG or Au NPs without PEG encapsulated. All these ETL solution had the same SPPO13 concentration of 3 mg mL⁻¹ and were spin-coated onto the EML to form a 40 nm thick ETL, followed by the annealing process of 60 °C for 20 min. The Au@PEG NPs were dispersed well in the ETL solution and thus in the spin-coated ETL film. Therefore, the density of Au@PEG NPs in the final deposited ETL film could be calculated to be about 10 wt%. Al cathode was deposited by vacuum thermal evaporation at a pressure of 5 × 10⁻⁴ Pa. The active area of each device was 0.04 cm², defined by the overlap of ITO anode and Al cathode.

Fig. 1a shows the transmission electron microscope (TEM, JEOL, JEM-2010) image of the Au@PEG NPs we prepared. The Au cores were well-dispersed and spherical in shape, with an average diameter of ∼15 nm. The uniform PEG shell of the Au core was ∼4 nm thick. As mentioned above, since the PEG-SH can be facilely coated on the surface of the Au core via the strong covalent bonding between the thiol groups with the gold surfaces, it is a facile method to obtain the core–shell structured NPs. The PEG shell can make the composite Au@PEG NPs dispersed well in IPA, which is a commonly used solvent for alcohol-soluble ETL materials in solution-processed OLEDs. Thus it can be practical that using the solution-processed ETL doped with Au@PEG NPs to enhance the EL performance of the devices.

Fig. 1 (a) TEM image of the synthesized Au@PEG core–shell NPs. (b) Configuration of the device structure of the PLEDs studied in this experiment.

To investigate the effect of Au@PEG NPs on the EL performance of the devices, we have fabricated a series of PLEDs employing doped or non-doped SPPO13 as the ETL. The device structure of the PLEDs was ITO/PEDOT:PSS (40 nm)/P-PPV (60 nm)/ETL (40 nm)/Al (100 nm), as depicted in Fig. 1b. All the devices were fabricated with this same structure, excepting the non-doped ETL (Device 1), ETL doped with Au@PEG NPs (Device 2), pure PEG (Device 3) and Au NPs without PEG encapsulated (Device 4). The concentration of PEG (Device 3) and Au NPs (Device 4) in the ETL solution was 0.11 mg mL⁻¹ and 0.22 mg mL⁻¹, respectively, each of which was in the equal value to the corresponding concentration in ETL of Device 2. All the organic layers of the PLEDs were deposited using solution processing, which was compatible with large-size manufacture of devices. On account of that the ETL of the device was 40 nm in thickness, the Au@PEG NPs with 15 nm core and 4 nm shell can be well-dispersed in the ETL.

In consideration of that the film-forming property of solution-processed organic layers is crucial for device performance, the surface morphologies of the ETL in Device 1 and 2 were investigated. The ETL films were fabricated by spin-coating under the same condition with the PLED device. Fig. 2 shows the atomic force microscopy (AFM, Bruker Multimode 8) images of non-doped ETL and ETL doped with Au@PEG NPs on ITO/PEDOT:PSS/EML substrate. Both of the ETL film exhibited relatively smooth and homogeneous surface morphology, with root-mean-square roughness (R_q) value of 0.291 and 0.842 nm for non-doped ETL and Au@PEG NPs doped ETL, respectively. This indicates that the moderate doping concentration of Au@PEG NPs in ETL did not change the amorphous morphology and the thickness of SPPO13-based ETL severely and that the NPs were nearly buried entirely in ETL. Therefore, the doping only played a less role in degrading the electron transporting properties and induced nearly no interface changes. The slightly higher R_q of doped ETL showed the Au@PEG NPs were indeed incorporated into the ETL, and the doped NPs might act as the hopping sites for electrons.²⁵

Fig. 2 (a) AFM images (5 μm × 5 μm scale) of the (a) non-doped ETL and (b) ETL doped with Au@PEG NPs on ITO/PEDOT:PSS/EML substrate.

The ultraviolet-visible (UV-vis) absorption spectra of the synthesized Au NPs and the Au@PEG NPs are exhibited in Fig. 3a. Compared to the bare Au NPs with the same diameter of about 15 nm, the Au@PEG NPs showed a little red-shifted absorption peak at 525 nm. This was mainly caused by that the LSPR peak can be influenced by the dielectric constant of the spacer layer on the surface of metallic NPs.²⁶ The normalized EL spectra of Device 1 and 2 at a driving voltage of 8 V are also shown in Fig. 3a. It showed nearly no difference in EL spectrum between device with and without using Au@PEG NPs doped ETL. The EL peaks of the devices were located at about 524 nm, which could obviously overlap well with the LSPR energy of the Au@PEG NPs. This well matched spectrum of the LSPR energy with the emission energy could lead to the optimized MEF effect and contribute to the improved performance of PLED devices. We have also measured the absorption data of ETL film with and without Au@PEG NPs doped. As shown in Fig. 3b, the absorption peak at about 540 nm in doped film was not present in non-doped film. This indicated that LSPR in ETL film was generated by the Au@PEG NPs. The red-shifted absorption peak at 540 nm might be also caused by the difference of dielectric surrounding environment²⁶ and the LSPR energy could overlap well with the emission peak as well.

Fig. 3 (a) Normalized UV-vis absorption spectra of the synthesized Au NPs and Au@PEG NPs. Normalized EL spectra of Device 1 (ITO/PEDOT:PSS/P-PPV/SPPO13/Al) and Device 2 (ITO/PEDOT:PSS/P-PPV/Au@PEG NPs doped SPPO13/Al) at a driving voltage of 8 V. (b) Normalized UV-vis absorption spectra of ETL film with and without Au@PEG NPs doped.

The current density–luminance–voltage (J–L–V) and luminous efficiency–current density (η–J) characteristics of Device 1–4 are shown in Fig. 4, and the EL performance of these devices are summarized in Table 1. In Fig. 4a, the current density of Device 3 was higher than that of other three devices, indicating that the doping of PEG had an active effect on the electron injection property of device. The lowest current density in Device 4 with the increasing voltage could be caused by the negative effect of Au NPs on the carrier injection.²⁷ It can be found that Device 2 showed moderate current density when driving voltage was increased, indicating the PEG shell could alleviate the negative effect of Au NPs on the electron injection ability of device. Among these devices, the Device 2 and 3 showed the lowest turn-on voltage of 3.5 V. Such a turn-on voltage was lower than that of Device 1 (4.0 V) and Device 4 (6.4 V). What's more, the maximum luminance of Device 1–4 was 28 782, 34 981, 30 065, and 2351 cd m⁻², respectively.

Fig. 4 (a) J–L–V and (b) luminous efficiency–current density (η–J) characteristics of Device 1–4.

Table 1 EL performance of Device 1–4

Device	Von^a (V)	Lmax^b (cd m^−2)	ηmax^c (cd A^−1)
a Turn-on voltage which is defined as the voltage at a luminance of 1 cd m^−2.b Maximum luminance.c Maximum luminance.
1	4.0	28 782	6.94
2	3.5	34 981	12.38
3	3.5	30 065	6.40
4	6.4	2351	0.58

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In Fig. 4b, the maximum luminous efficiency of Device 1–4 was 6.94, 12.38, 6.40, and 0.58 cd A⁻¹, respectively. (As a reference, the EL performance of Device 1 was comparable with that of the device based on metal fluoride cathode.²⁸) This proved the doping of Au@PEG NPs in Device 2 could not only help to the electrical property, but could contribute to the improvement of device luminance and efficiency. Considering that the P-PPV is a classic p-type semiconducting polymer with highest occupied molecular orbital (HOMO) of 5.4 eV,¹⁶ and that SPPO13 has been widely used as a hole-blocking material with HOMO of ∼6.5 eV,²⁹ the hole injected from the anode might stack at the interface between EML and ETL. Thus the exciton recombination zone could be located at the vicinity of this interface. So the generated exciton could interact well with the energy of LSPR induced by Au@PEG NPs. As a result, the improvement in luminance and efficiency can be explained by the achievement of MEF effect in device. The MEF effect could increase the radiative decay rate of excitons and thus improve the luminance and luminous efficiency of device.¹⁴ However, the exciton would suffer from some quenching effect when the Au NPs were without the PEG spacer layer. This quenching effect, combined with the negative effect on electron injection of inducing Au NPs, led to the poor performance of Device 4.

In conclusion, core–shell Au@PEG NPs have been doped into the solution-processed electron-transporting material to act as the ETL of PLEDs. This doping strategy can significantly improve the device performance. Enhanced luminance, enhanced luminous efficiency and reduced turn-on voltage was obtained in device by using the doped ETL. The sufficient overlap between exciton energy with LSPR of Au NPs results in the MEF effect in PLEDs. Meanwhile the PEG spacer layer provides suitable distance to avoid quenching effect of exciton, and alleviates the negative effect on electrical property of inducing Au NPs. The maximum luminous efficiency of device was enhanced by nearly 1.8-fold compared with the non-doped device, from 6.94 cd A⁻¹ to 12.38 cd A⁻¹. This results gives an alternative strategy to obtain high performance solution-processed OLEDs.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (51403018, 51273020) and Research Fund for the Doctoral Program of Higher Education of China (20130006110007).

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Footnote

† These authors contributed equally to this work.

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