A printed aluminum cathode with low sintering temperature for organic light-emitting diodes

Abstract

A printed aluminum cathode with low sintering temperature has been achieved using an aluminum precursor ink, AlH₃·O(C₃H₇)₂, which in the presence of a TiCl₄ catalyst can be printed to give the required pattern and then sintered at 80 °C for 30 s to form an Al film. The Al cathode of 50 nm thickness has a sheet resistance of 2.09 Ω □⁻¹ and work function of 3.67 eV. The study demonstrates that the low sintering temperature and work function of the printed film, together with its high conductivity and stability, mean that it is well suited for use as an OLED cathode and that it paves the way for fully printed flexible devices.

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Introduction

Printed electronics offer a revolutionary approach to electronics, as they are large in area, thin, lightweight, flexible and low in cost. They pave the way for further developments, for example in paper batteries, radio frequency identification (RFID), organic light-emitting diodes (OLEDs) and organic photovoltaics (OPV).¹ In fully printed electronic devices, printable conductive electrodes are attractive due to their simple processing technology, and their low cost in terms of energy consumption and equipment usage. The printing of anodes using conductive materials such as indium-tin oxide (ITO),² ZnO-doped In₂O₃ (IZO),³ nano-silver,⁴ carbon nanotubes (CNTs)⁵ and conductive polymers^6,7 has been widely reported. However, the printed cathode is still a big challenge due to the absence of suitable ink material with low work function – indeed suitable work function of the cathode is considered to be one of the most important factors in organic electronic devices, since it determines the efficiency of the charge carrier injection from the cathode.^8–12

For cathode materials in organic devices, Al electrodes are almost exclusively used, due to the high conductivity, matched work function, and low cost of Al. Currently, the principal technology used for making Al electrodes is either thermal evaporation or sputtering with a shadow mask.

It has been reported that nanoparticle-based Al inks can be patterned by printing and thermally cured in air to form conductive Al electrodes for solar cells.¹³ However, the sintering temperature is very high, between 550 and 940 °C. Such temperatures are too high for either organic active layers or plastics substrates. In addition, due to the highly oxidative conditions encountered during the preparation of Al nanoparticles or inks in air, an Al₂O₃ shell between 3 and 10 nm thick is inevitably present on the surface of the Al particles, giving low electrical conductivity.¹⁴

Recently H. M. Lee^14,15 has proposed the use of an Al solution process for stamping Al cathodes using an ink based on AlH₃[O(C₄H₉)₂]. In the presence of a Ti(O-i-Pr)₄ catalyst and an inert atmosphere this Al compound is converted at 150 °C into an Al film. However, this sintering temperature is still too high for OLED and OPV functional layers or a flexible organic substrate such as polyethylene terephthalate (PET). There is therefore a requirement for a suitable Al-based ink of low sintering temperature for printing the Al cathode.

In the present study we have examined a series of organometallic Al compounds and suitable catalysts for printing an Al cathode at a lower sintering temperature, without adversely affecting its stability.

Experimental

The Al precursor ink was typically prepared by reaction of aluminum chloride (AlCl₃) with lithium aluminum hydride (LiAlH₄) in an anhydrous ether such as diethyl ether (O(C₂H₅)₂),¹⁶ isopropyl ether (O(C₃H₇)₂), or butyl ether (O(C₄H₉)₂),^14,15 as shown in eqn (1).^14–18

AlCl₃ + LiAlH₄ + O(C_nH_2n+1)₂ → AlH₃ x[O(C_nH_2n+1)₂] + LiCl (1)

AlCl₃ and the ethers were purchased from Aladdin Reagents, Shanghai, and LiAlH₄ from Sigma-Aldrich. The ethers were further dried by sodium before use. All syntheses and the Al cathode printing process were conducted in a glove box, with oxygen and moisture levels each below 0.1 ppm.

We found the ink based on isopropyl ether (O(C₃H₇)₂) to be particularly effective in achieving compact Al films at relatively low temperature, and this ether is safer in use than the more familiar diethyl ether (O(C₂H₅)₂), which is highly flammable even at −5 °C, as mentioned by Schmidt et al.¹⁶ In addition, Al films prepared using AlH₃[O(C₄H₉)₂] can be successfully formed at 110–150 °C.^14,15 On the other hand, this temperature is still too high for some flexible substrates and organic active layers, as mentioned above, and we therefore adopted the highly volatile but easily manipulated solvent O(C₃H₇)₂ for printing the Al cathode, giving a sintering temperature of 80 °C.

High-quality Al conductive films were thus achieved by printing, using a sintering temperature of about 60–80 °C, using the AlH₃[O(C₃H₇)₂]-based ink and a catalyst of TiCl₄ vapor. The electrical and mechanical properties of the printed Al film were investigated, and a printing process for the Al cathode for an OLED developed. An Al precursor ink was first prepared using AlCl₃, whereas LiAlH₄ was used both as precursor and reducing agent in O(C₃H₇)₂. Typically, 0.50 g AlCl₃ and 0.43 g LiAlH₄ were mixed at room temperature in a flask with 50 mL O(C₃H₇)₂ as solvent. The mixed solution was magnetically stirred for 24 h and then filtered through a 0.045 μm filter. The clear solution was concentrated to 10 wt%, and following adjustment of its viscosity it could be used as an Al precursor ink for printing.

The preparation of conductive Al films was as follows. Firstly, the target substrate was treated in a covered glass bowl containing one drop (100 μL) of TiCl₄ catalyst at 80 °C for 30 s. The Al precursor ink was then printed on the substrate by drop casting, stamping or screen printing. Finally, the target substrate patterned with the Al ink was sintered at 80 °C for 1 min, resulting in a high-quality Al film.

The stamping process is complicated, but ensures Al films of high quality. Two substrates are used, the target and the source (glass). The target substrate is treated with TiCl₄ vapor, as described above. The Al precursor ink is coated on the glass source substrate by drop casting and dried for 5–10 min at room temperature. After heating the source and the target substrates at 80 °C for 1 min, the catalyst within the target substrate activates the decomposition of the Al precursor ink to form the Al film.

The decomposition follows the reactions in eqn (2) and (3). Firstly, AlH₃[O(C₃H₇)₂] decomposes to H₂ and Al[O(C₃H₇)₂], and the latter is in turn decomposed into Al and O(C₃H₇)₂.^14,19 In both reactions the TiCl₄ catalyst and sintering conditions play a key role. Once the decomposition of the Al precursor ink has been initiated, the process becomes self-sustaining and a continuous layer is formed over the whole surface of the target substrate.

AlH₃[O(C₃H₇)₂] → Al[O(C₃H₇)₂] + 1.5H₂ (2)

Al[O(C₃H₇)₂] → Al + O(C₃H₇)₂ (3)

The structure of the organometallic compounds synthesized was confirmed by their 400 MHz ¹HNMR spectra (Varian MR400 spectrometer) using THF-D₈ as solvent. The crystal structure of the Al films was determined using a Bruker AXS D8 Advance X-ray diffractometer. The sheet resistance for a 1 × 1 cm² portion was measured on a Keithley 4200 parameter analyser using a 6 mm four-probe set-up 2 mm apart. The surface potential was measured using a Kelvin probe force microscope (Vecco Dimension 3100).

Results and discussion

¹HNMR spectra were produced as above, with THF-D₈ as the solvent. Fig. 1 shows the NMR spectrum of the Al precursor ink, and it is seen that the H atoms attached to the Al of the AlH₃ group appeared at 3.17 ppm. The signals of O(C₂H₅)₂ were present at around 1.1 and 3.3 ppm, and the peaks at 1.72 and 3.57 ppm were due to the solvent, THF-D₈. The signal at around 4.53 ppm could be assigned to hydrogen, generated by reaction of the Al precursor exposed to the atmosphere. Apart from these signals, there were no other significant peaks. This confirmed both the purity of the Al precursor, and the chemical structure of the Al precursor ink as AlH₃·x[O(C₂H₅)₂].

Fig. 1 NMR spectrum of the Al precursor, AlH₃·x[O(C₂H₅)₂], dissolved in THF-D₈.

Crystal structure analysis was also carried out, using an X-ray diffractometer (XRD; Bruker AXS D8 Advance), and the diffraction patterns of the samples only showed patterns due to Al metal (Fig. 2). The diffraction peaks attributed to pure Al with a face-centered cubic structure (JCPDS no. 01-089-2769) were clearly detected in all samples, with different preparation processes, with only a slight deviation from the standard value. We believe this small deviation to be tolerable, bearing in mind that the nanostructures had somewhat different properties to bulk materials, a common phenomenon in nanostructures. Although the intensity of the printed Al film was slightly weak, its electrical conductivity was as expected. The measured sheet resistance of printed Al films sintered at 80 °C was below 2.09 Ω □⁻¹.

Fig. 2 XRD patterns of Al films synthesized using various processes and different temperatures.

The stability of the printed Al film was also tested, and as indicated in our previous report on AlH₃[O(C₂H₅)₂]²⁰ we again obtained good results, shown in Fig. 3. After exposure to ambient air for 5 days, the electrical sheet resistance of printed Al films was still 3.61 Ω □⁻¹, confirming the relative stability of the Al film. In the initial stages of exposure to air, the change in sheet resistance was, however, obvious, caused by the oxidation of Al nanoparticles, as mentioned earlier. A 3–10 nm Al₂O₃ shell was formed on the surface of the Al particles, causing a reduction in electrical conductivity.¹⁴

Fig. 3 Sheet resistance stability of Al film prepared by the stamping process in air.

Fig. 4 shows the performance of the Al films, and it is seen that all the films had a shiny metallic appearance on substrates such as photo-paper, glass, PET, or the P3HT layer (the active layer for OPV). Some corrosion was observed on P3HT film, however, caused by the ethers in the Al precursor ink and the acid formed by decomposition of TiCl₄. It is thus suggested that direct printing of Al ink on top of the P3HT layer should be avoided. Atomic layer deposition of a 5 nm thickness Al₂O₃ film on the P3HT surface, followed by printing Al ink, effectively resolves the corrosion issue, although the insertion of an ultra-thin Al₂O₃ layer may decrease the electron injection.²¹ Attempts will be made in future research to find methods to avoid corrosion, for example by selecting new protective layers or chemically cross-linked organic layers.

Fig. 4 Al film prepared on various substrates by a stamping process at a sintering temperature of 80 °C.

Table 1 lists the electric resistance, roughness and work function of the Al films. All the samples showed conductivity similar to the Al films prepared by thermal evaporation. The surface morphology suggested that the grain size of the Al surface due to the stamping process was about 220 nm, compared to about 80 nm by thermal evaporation. It was also found that the surface of the Al film on glass substrates became smoother if the sintering temperature was decreased from 150 to 80 °C. It is believed that the decomposition of Al ink into H₂ and Al becomes more violent at higher temperatures, forming microscopic porous holes in the Al film. Furthermore, the Al films sintered at 80 °C showed the lowest work function, at 3.67 eV. This is explained by the Li elements contained in the LiAlH₄ precursor being present in the Al film, leading to a decrease in work function. It has been reported that the work function of Al substrates with a native oxide layer is 3.3 eV, indicating that Al films prepared at 80 °C by stamping may be used as the cathodes for OLEDs and OPVs.

Table 1 Mechanical and electrical properties of Al films produced by various methods

Al fabrication method	Surface morphology	R (Ω cm)	Ra (nm)	WF (eV)
Thermal evaporation		2.79	2.63	4.15
Stamping (80 °C) (glass substrate)		3.10	4.56	4.05
Stamping (80 °C) (PET substrate)		2.09	24.5	3.67
Stamping (100 °C) (glass substrate)		2.49	10.5	4.07
Stamping (150 °C) (glass substrate)		2.20	6.03	4.28

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We accordingly applied the stamped Al films to the cathode of an OLED, the device structure being shown in Fig. 5. The architecture of the device consisted of an ITO layer, a N,N′-di(naphthalene-l-yl)-N,N′-diphenylbenzidine (NPB) layer (40 nm), a tris(8-hydroxyquinoline)aluminum (Alq₃) layer (60 nm), a lithium fluoride (LiF) layer (1 nm), and an aluminum oxide (Al₂O₃) layer (5 nm), with a cathode of Al film (50 nm) prepared by the solution-stamping process. It is well known that the insertion of Al₂O₃ for interfacial modification of cathodes improves the electron injection, and the thickness was ca. 1 nm,²¹ although this thickness is not sufficient to protect organic layers against corrosion by solvent or acidic gases during the solution-stamping process for preparing the Al cathode. In the process of fabricating the OLED device a 5 nm Al₂O₃ layer was deposited by ALD at 80 °C, with trimethylaluminum as precursor and water vapor as oxidizing gas. It was clearly observed that green light was emitted from the prepared OLED, indicating that the Al film prepared by the stamping process was suitable for use in preparing cathodes for OLED and OPV devices.

Fig. 5 (a) Schematic structure of OLEDs with a cathode of thin Al film prepared by the stamping process, and (b) a photograph of its light emission.

Conclusions

We have successfully synthesized an Al precursor ink, AlH₃·x[O(C₃H₇)₂], which can be patterned by printing and decomposed into high quality Al films by catalysis with TiCl₄ at a low sintering temperature of 80 °C. Compared with traditional thermal evaporation, printed Al films demonstrate low resistance (2.09 Ω □⁻¹) and low work function (3.67 eV), and can be used as the cathode at relatively low cost in flexible OLED and OPV devices.

Acknowledgements

This work was supported by the project of the major research plan of the National Natural Science Foundation of China (Grant no. 91123034), Strategic Priority Research Program of the Chinese Academy of Sciences (Grant no. XDA09020201), knowledge innovation program of the Chinese Academy of Sciences (Grant no. KJCX2-EW-M02), and project supported by National Science and Technology Ministry (Grant no. 2012BAF13B05-402). The authors also thank the Natural Science Foundation of Jiangsu Province (BK2012631), and the Open Fund of the State Key Laboratory of Luminescent Materials and Devices (South China University of Technology, 2012-skllmd-05) for financial support.

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