Solution Processed Aluminum Paper for Flexible Electronics

Aug 9, 2012 - Solution Processed Aluminum Paper for Flexible Electronics. Hye Moon Lee,*. ,†. Ha Beom Lee,. ‡. Dae Soo Jung,. §. Jung-Yeul Yun,. ...
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Solution Processed Aluminum Paper for Flexible Electronics Hye Moon Lee,*,† Ha Beom Lee,‡ Dae Soo Jung,§ Jung-Yeul Yun,† Seung Hwan Ko,*,‡ and Seung Bin Park§ †

Powder Technology Department, Korea Institute of Materials Science (KIMS), 797 Changwondaero, 642-831 Changwon, Republic of Korea ‡ Mechanical and Aerospace Engineering Department, Korea Advanced Institute of Science and Technology (KAIST),192 Daehak-ro, 305-701 Daejeon, Republic of Korea § Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), 192 Daehak-ro, 305-701 Daejeon, Republic of Korea S Supporting Information *

ABSTRACT: As an alternative to vacuum deposition, preparation of highly conductive papers with aluminum (Al) features is successfully achieved by the solution process consisting of Al precursor ink (AlH3{O(C4H9)2}) and low temperature stamping process performed at 110 °C without any serious hydroxylation and oxidation problems. Al features formed on several kinds of paper substrates (calendar, magazine, and inkjet printing paper substrates) are less than ∼60 nm thick, and their electrical conductivities were found to be as good as thermally evaporated Al film or even better (≤2 Ω/□). Strong adhesion of Al features to paper substrates and their excellent flexibility are also experimentally confirmed by TEM observation and mechanical tests, such as tape and bending tests. The solution processed Al features on paper substrates show different electrical and mechanical performance depending on the paper type, and inkjet printing paper is found to be the best substrate with high and stable electrical and mechanical properties. The Al conductive papers produced by the solution process may be applicable in disposal paper electronics.

1. INTRODUCTION Paper is one of the most important inventions, having made a great contribution to the advance of civilization. Paper has been mainly used as an important medium for packaging and for expression, documentation, and propagation of knowledge and information. However, beyond the packaging and storing of information, paper has expanded its applications toward the development of cutting-edge electronics.1−7 Research applying paper as a substrate to flexible electronics has been an active and vibrant research field due to paper’s several intriguing attributes, which can achieve goals to increase device performance and to lower manufacturing cost at the same time.1,2,5,8,9 Flexible electronic devices consist of organic and/or inorganic features on flexible substrates. In order to use paper as a substrate in flexible electronic devices, the development of a low temperature process (≤150 °C) is essential because paper is easily degraded or burns at high temperature. Therefore, organic materials have usually been used in flexible electronic devices because they can maintain their critical attributes after low temperature drying or firing treatment by both solution- and vacuum-based processes. However, inorganic features formed on a paper substrate by solution-based process can hardly show their intrinsic electrical or mechanical properties because the allowable processing temperature on a paper substrate (≤150 °C) is too low to © 2012 American Chemical Society

remove surfactants covering nanoparticles in inorganic colloidal ink.10,11 Thus, a vacuum process, such as thermal evaporation or sputtering, instead of a solution process, has been generally used for preparation of inorganic features on flexible substrates. The solution-based process is a useful tool for low-cost preparation of electronic devices with large-area thin films consisting of functional materials on both rigid and flexible substrates.10−19 Solution based electronics fabrication processes have been intensively developed for organic electronic materials. However, despite their much superior electrical properties, inorganic materials have hardly been applied for solution based processes and flexible electronics due to their high process temperature and difficulties in preparing stable ink. Recently, several trials to apply a solution-based process to preparations of inorganic conductive films on paper substrates have been performed. Whitesides and colleagues prepared metal (tin, zinc, or nickel) features on paper substrate using a spray deposition method to resolve disadvantages stemming from the use of vacuum processes, but the prepared features were worse than those produced by vacuum-based processes in terms of resolution and electrical conductivity.6 Cui and Received: June 20, 2012 Revised: August 7, 2012 Published: August 9, 2012 13127

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Figure 1. Schematic diagram of the solution process for preparing an electrically conductive Al features on paper substrates at 110 °C using an Al precursor ink of AlH3{O(C4H9)2}. (a) Coating of Al ink on cold glass donor substrate via a drop-casting process. (b) Direct contact of dried Al ink on a target paper substrate preheated at 110 °C for preparation of Al thin film. (c) Al thin film formed on paper substrate.

active reaction of Al with oxygen and moisture. Chemical preparation of Al nanoparticles is generally achieved by the decomposition of the Al precursor (aluminum hydride; AlH3) into Al and 1.5H2,11,20−23 and the oxidation or hydroxylation of Al is performed immediately after the preparation of Al nanoparticles. Consequently, very thin oxide layers (3−10 nm) are formed on the surfaces of Al nanoparticles and the oxidation problem precludes the possibility of preparing electrically conductive Al features from the Al nanoparticle solutions on the substrates.11 In order to develop a solution process to preparation of Al feature with excellent electrical properties, it is necessary to directly prepare Al features on the surfaces of substrates without the use of Al nanoparticles by directly decomposing Al precursor on the surface of substrates. In this study, therefore, we tried to apply the direct decomposition of Al precursor ink (AlH3{O(C4H9)2}) on the surface of substrates to produce Al coated conductive paper for flexible electronics; this approach is found to be very effective in

colleagues produced conductive paper for energy-storage devices by coating aqueous CNT ink on paper.1 However, the effective thickness of the film coated on paper should be thicker than 2 μm in order to show a sheet resistance lower than 10 Ω/□, and this will result in high material cost due the CNT material consumption increase. Accordingly, inorganic ink has been considered to have to be cheap and suitable for solution process, and should not require any additives, which usually degrade conductivity, to produce conductive papers with excellent electrical and mechanical properties. Aluminum (Al) is the most excellent metal for the value of high electrical conductivity at low cost.6 Moreover, among various metals, Al possesses a very low work function,6,11 which means that it is the best material for electrical conductive features as well as for electrodes for ohmic contact. Intense interest and many research attempts have been focused on the development of a cost-effective solution-based process of highly conductive and cheap Al. However, attempts at solution processable Al deposition were not successful due to the very 13128

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Figure 2. SEM pictures of the microstructures of various paper substrates (a, calendar paper; b, magazine paper; and c, inkjet printing paper substrates) and their main components analyzed by EDS. preparing the Al precursor ink, while LAH was used as both a precursor and a reduction agent. Dibutyl ether was used as the solvent for the reaction of both chemicals. Twenty mM of AlCl3 and 60 mM of LAH were added to 100 mL of dibutyl ether. The mixed solution was heated at ∼70 °C under magnetic stirring for ∼1 h. To remove any byproduct of LiCl and any not fully reacted precursors, the resulting gray slurry was then filtered and discarded. The filtered clear solution was used as an Al precursor ink of AlH3{O(C4H9)2}. All these processes were performed in a glovebox containing an argon atmosphere to prevent the generation of aluminum hydroxide

the preparation of highly conductive Al papers with excellent electrical and mechanical properties.

2. EXPERIMENTAL SECTION Preparation of Al Precursor ink (AlH3{O(C4H9)2}). Al precursor ink was prepared by an ethereal reaction of aluminum chloride (AlCl3) with lithium aluminum hydride (LiAlH4; LAH) in dibutyl ether (O(C4H9)2). AlCl3, LAH, and O(C4H9)2 were purchased from Aldrich Chemical and used as received. AlCl3 was used as a precursor for 13129

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(Al(OH)3), aluminum oxide (Al2O3), or aluminum nitride (AlN), which deteriorates electrical properties. Solution Process for Stable Thickness of Al Features on Paper Substrate. Figure 1 shows a schematic diagram of the solution process for fabricating conductive Al features on paper substrates using the Al precursor ink of AlH3{O(C4H9)2}. The Al precursor ink was coated on a soda lime glass (SLG) by drop-casting and then dried for ∼5 min at room temperature (hereafter, this glass is referred to as the donor substrate). Calendar, magazine, and inkjet printing papers were used as flexible substrates in this study, and their microstructures and components are shown in Figure 2. Calendar (Figure 2a) and magazine papers (Figure 2b) are usually produced by pressing and heating the mixture of short cellulose fibers, pigments (carbonates and oxides), and natural binder, such as starch, which is not reactive with Al precursor ink, so that a lot of small pores were observed on their surfaces. But the inkjet printing paper (Figure 2c) consists of long cellulose fibers and additives which are same as the used for calendar and magazine papers, so that relatively small amounts of large sized pores were observed. Since all the components of paper substrates are not electrical conductive, the paper substrates are nonconductive. Paper substrate was preheated at 110 °C (hereafter, this preheated substrate is designated the target substrate), and about 1 μL of catalyst (Ti(O-i-Pr)4) was dropped on the surface of the heated plate together with the target substrate. The donor substrate with the Al precursor film was in an intimate contact with the surface of the preheated target substrate, and then Al features formed on the target substrate after around 60 s. The contact of donor to target substrates initiates the generation of Al nuclei on the surface of target substrates by activating the decomposition of AlH3{O(C4H9)2} into Al{O(C4H9)2} and 1.5H2, and subsequent decomposition of Al{O(C4H9)2} into Al and O(C4H9)2.11,20−23 Finally, the nucleated Al becomes large enough to cover the whole surface of the target substrate. Because film thickness has a significant effect on the electrical resistance, it is very important to prepare Al features with identical thickness using the solution process for reasonable comparison of Al features on paper substrates in terms of electrical resistance. Temperature, time, and the volume of Al precursor ink drop-casted on the surface of the donor substrate have significant effects on determining the Al film thickness as formed on target substrates by the solution process. Therefore, we made an effort to produce Al features with identical thickness on all different substrates by fixing the stamping temperature at 110 °C, the stamping time at 60 s, and the thickness of the Al precursor film coated on the donor substrate at ∼48 μm. This process was also performed in a glovebox containing an argon atmosphere. Samples for TEM Observation and EDS Analysis. Samples for TEM observation and EDS analysis were prepared by focused ionized beam (FIB; Nova 200, FEI company) treatment of Al features on paper substrates. The FIB used for TEM and EDS samples works with a highly focused gallium ion beam at 30 kV. Electrical Resistance Measurements. Electrical sheet resistances of Al features on paper substrates were measured by a four-point probe method (CMT-SR 1000N, Advanced Instrument Technology), and probe stations method (model 5500, MS-Tech, Korea) was used to measure the linear electrical resistance. Cyclic Bending Test of Al Features on Paper Substrates. The experimental setup for cyclic bending test of Al features on paper substrates is shown in Figure 3. By fixing one end (right) and moving end (left) in a cyclic motion and modifying the sample carrier properly to simulate simple supported (hinged) end configuration, a uniform bending deformation (same radius of curvature throughout the entire flexible substrate) could be created. Bending deformation cycles with radius of ∼25 mm were run from zero to 30000 cycles. This method is similar to the setup explained in a research paper by Grigoropoulos and his colleagues.24

Figure 3. Schematic diagram and digital pictures of the cyclic bending test setup at minimum and maximum bending deformation.

respectively, by the solution process, as described in Figure 1. The pictures in Figure S1 (Supporting Information) show the top view SEM images of the microstructures of corresponding paper substrates. Like the Al thin films prepared on a rigid substrate in our previous study,11 the solution process was successfully used to form line patterned Al features on various kinds of paper substrates with some differences in surface morphologies on calendar, magazine, and inkjet paper substrates. During the Al pattern formation on a paper by solution-based process, the Al precursor ink behaves totally differently on a paper substrate compared with a rigid substrate because a rigid substrate is impermeable to the ink but a paper substrate can soak up the ink very well. As shown in the results of cross-sectional TEM (transmission electron microscopy) observations and EDS (energy-dispersive X-ray spectroscopy) analyses of Al features on calendar (Figure 4d) and inkjet printing paper substrates (Figure 4e), Al films (indicated by blue symbols) approximately 50 nm thick were uniformly formed along the surfaces of substrates. The Al films formed on paper substrates by the solution process were so thin that their surface morphologies directly reflect the surface microstructure of the paper substrates, and thus the different surface morphologies of Al features presumably stemmed from the different microstructures of the paper substrates. This result could be clearly confirmed by the Al film formed on the pyramid structured surface of the Si substrate (see Supporting Information Figure S2). Moreover, EDS analyses revealed that calcium (Ca) and carbon (C), which are the main chemicals composing the surfaces of calendar and inkjet printing paper substrates, respectively, coexisted with Al in the interface region between Al features and surfaces of paper substrates; this indicates that the production of Al features via the solution process was performed both beneath and above the surface of the paper substrates due to Al precursor ink wicking and penetration into the paper substrates with porous structures. As described in the results of the film adhesion test, production of Al features both beneath and above the surface of the paper substrate causes a strong adhesion of Al features on the paper substrate Production of Al features via the solution process is achieved by the decomposition of the Al precursor of AlH3{O(C4H9)2}

3. RESULTS AND DISCUSSION Figures 4a, b, and c show the line patterned Al features formed on calendar, magazine, and inkjet printing paper substrates, 13130

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Figure 4. Line patterned Al features formed on various paper substrates by the solution process and their microstructures. Al features on (a) a calendar paper, (b) a magazine paper, and (c) an inkjet printing paper substrates. Inset pictures of (a−c) show the top view SEM images of the microstructures of corresponding paper substrates. Cross-sectional TEM images and EDS analysis of the Al features formed on (d) a calendar paper and (e) an inkjet printing paper substrates. Notice the presence of the Al components inside the papers by Al precursor wicking effect during the process.

into Al{O(C4H9)2} and 1.5H2, and the subsequent decomposition of Al{O(C4H9)2} into Al and O(C4H9)2.11,21 It is necessary for Al features on paper substrates to contain sound electrical properties for their applications to electrodes of electronics. However, the reaction of AlH3{O(C4H9)2} with moisture or oxygen is so active that moisture or oxygen adsorbed in paper substrates presumably causes the generation of a nonconductive Al compound, such as Al(OH)3 or aluminum oxide Al2O3, in the solution process. Thus, we measured the electrical sheet resistance of the prepared Al features on paper substrates; the results are shown in Figure 5a. Although the Al features prepared on paper substrates by the solution process were less than ∼60 nm in thickness, their electrical properties were found to be excellent, with electrical sheet resistance lower than 2 Ω/□. Furthermore, no significant differences in the electrical sheet resistance values of Al features were observed on paper substrates prepared by solution-based (black symbol in Figure 5a) and evaporation deposition (blue symbol in Figure 5a) processes with the same thickness. Electrical performance of the Al features was also experimentally confirmed, as can be seen in the data in Figure 5b, which shows the patterned Al circuits connected to lightemitting diodes (LED) on the calendar, magazine, and inkjet paper substrates. Clear Al pathways (∼ 2 mm wide, ∼ 50 nm thick) were produced on all kinds of paper substrates by the

solution process. All the patterned Al features on paper substrates showed electrical sheet resistances below 2 Ω/□, and thus the LED lamps mounted on the Al features flashed on when a DC current was supplied. Reaction of Al with oxygen is so active that a thin Al oxide layer (3−10 nm) always exists on the surface of Al features. Once the Al oxide layers are formed on the surface of Al features, the oxidations of Al features do not show further serious progress. This is because the thin native Al oxide layers are so dense that oxygen hardly penetrates into the oxide layers to induce further oxidation even at atmospheric surroundings. Thus, the solution processed Al features show long-term electrical stability even in the atmospheric surroundings. Figure S3 shows the long-term electrical functionality measurement after its initial fabrication and 10 month storage in the air environment. Accordingly, the solution process can be a good alternative to the vacuum-deposition process for highly conductive Al features on paper substrates without any serious hydroxylation or oxidation problems, which deteriorate the electrical properties. Furthermore, conductive paper with Al features could completely burn out in a few seconds, as shown in Figure 5b. We expect that conductive papers produced by the solution process can be readily applicable in single-use disposal electrical devices with few problems of waste disposal. 13131

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into the paper and the formation of Al features occurred both on and inside the paper substrates, as evidenced in the crosssectional TEM and EDS pictures (Figure 4d and e). Figure 6c and d shows the electrical resistance measurements and the OM (optical microscopy) images after bending tests, respectively, when the substrates with Al features were bent to 10−40 mm radius. For the Al features formed on calendar (blue symbols) and magazine papers (green symbols), electrical resistance increased as the bending radius decreased. On the other hand, the electrical resistance values of Al features formed on inkjet printing paper (black symbols) and PET substrates (red symbols) were very stable regardless of bending radius. The deterioration of electrical conductivity under bending could be caused by damages to conductive features stemming from weak adhesion of the features to substrates or to the destruction of substrate microstructures. Since it has already been confirmed that Al features produced by the solution process possess strong adhesion to substrates, we could surmise that ruptures of paper substrates caused the deterioration of electrical conductivity of Al features on calendar and magazine paper substrates. Thus, we observed the surfaces of Al features on substrates after bending tests using optical microscopy and confirmed that cracks were not present in Al features on inkjet printing paper (Figure 6d, bottom left) and PET (Figure 6d, bottom right) substrates, while many cracks (indicated by red arrows) were clearly observed in Al features on calendar (Figure 6d, top left) and magazine (Figure 6d, top right) paper substrates. This is attributed to the intrinsic structure difference in each paper type. As described in the Experimental Section, inkjet paper has fibrous structures that can accommodate larger deformation while the calendar and magazine papers consist of flake type powders that can be easily fractured under mechanical deformation. Furthermore, these cracks were also observed even on the surface of calendar and magazine paper substrates without any Al features (see Supporting Information Figure S5). These observations indicate that the deterioration of electrical conductivity was mainly caused by cracks generated in the Al feature for the case of calendar and magazine paper substrates during the bending test and they also signify that the occurrence of cracks stemmed from the rupture of the substrate microstructure, not from the delamination of Al feature itself from the substrates. Though some cracks were observed in the Al features on the calendar and magazine paper substrates after bending tests, the Al features on paper substrate still showed moderately small electrical sheet resistances (≤3 Ω/□). However, it is not reasonable to assert whether the conductive papers with Al features have good electrical and mechanical properties or not from a single set of bending test results. For this reason, electrical characterization of Al features on paper and PET substrates was performed under a cyclic bending deformation. Electrical resistances of Al features formed on paper and PET substrates by the solution process were measured for every bending cycle; the results are shown in Figure 7. While Al features on inkjet printing paper (black symbols) and PET (red symbols) substrates show stable conductivity regardless of bending radius, which gradually and linearly increased with the cycle of bending deformation in electrical resistance, those of calendar (blue symbols) and magazine (green symbols) paper substrates showed serious increases in the electrical resistance with decreasing bending radius and convergence to a specific resistance value after sharp increases of electrical resistance at the beginning of the cyclic bending deformation. Electrical

Figure 5. Electrical properties of Al features formed on various papers and PET substrates by the solution process. (a) Electrical sheet resistance measurement of Al features formed on calendar, magazine, and inkjet printing paper substrates and PET substrate. (b) Electronic circuit demonstration consisting of Al features and LED lamps. The paper Al circuit can burn with little waste.

To apply the Al features formed on paper substrates to flexible electronic devices, the devices should maintain good electrical conductivity and mechanical properties under various mechanical deformations. Figure 6 shows the mechanical and electrical property measurement comparison of Al features on paper substrates and on PET substrate by performing Scotch tape (Figure 6a,b) and bending tests(Figure 6c,d). The solution processed Al feature on a PET substrate has shown the excellent electrical and mechanical properties against external forces11 because a PET substrate has extremely dense and flat surface structures (see Supporting Information Figure S4). 3M Scotch tape tests (Figure 6a and b) were performed to investigate the Al feature adhesion to the substrate and the durability against the damage, such as scratching and peelingoff. As shown in Figure 6a, Scotch tape tests were carried out by strongly attaching 3M Scotch tape on the paper substrate with the solution processed Al features and subsequently peel off; none of the Al features were delaminated from any of the types of paper substrates. Furthermore, the electrical resistances, measured after the tape test for all kinds of substrates, rarely changed from the initial value. These results signify that adhesion of Al features to the paper substrates was so strong that Al features prepared by our approach did not suffer from delamination, which causes a deterioration of electrical conductivity. The strong binding of Al features to the paper substrate was presumably because Al precursor ink penetrated 13132

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Figure 6. Electrical and mechanical performance test of Al features on paper substrates under various mechanical bending conditions. (a) Scotch tape adhesion test for an Al feature on magazine paper substrate. (b) Electrical resistance change of Al features on paper substrates before (red bar) and after (blue bar) Scotch tape adhesion test. (c) Electrical resistance change of the Al features on various papers and PET substrate under various bending radius. (d) Magnified optical microscopic images of Al features on various papers and PET substrates after intensive bending tests. (i) Al feature on calendar paper, (ii) Al feature on magazine paper, (iii) Al feature on inkjet paper, and (iv) Al feature on PET substrate. Red arrows indicate the microcracks developed during the bending test, and they contribute to the resistance increase for calendar and magazine papers.

conductivity through the intensive bending tests, and this signifies that the Al features possess excellent electrical and mechanical properties that can readily applicable to high performance flexible electronics. Figure 8 shows the performance of the bent Al features formed on paper substrates by the solution process in a simple flexible device to demonstrate the applicability of the current approach for high performance real flexible devices. Figure 8a and b shows the letters “KIMS” and “KAIST”, consisting of many tiny LED lamps mounted on line patterned Al features

Figure 7. Normalized electrical resistance evolution of the Al features on paper and PET substrates during the 30000 times cyclic bending test.

resistances of Al features formed on paper substrates by the solution-stamping process increased by a factor of 1.2−1.6 after 30000 cycles of bending deformation. Considering their initial very low resistances (≤2 Ω/□), Al features produced on paper substrates by the solution process maintained good electrical

Figure 8. Digital pictures of the LED arrays on Al circuits on (a) a magazine and (b) an inkjet printing paper substrate. Note the LED arrays worked properly under various mechanical bending conditions. 13133

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on the magazine and inkjet printing paper substrates, which showed the largest and the smallest increase of electrical resistances after cyclic bending deformation, respectively. The LED lamps connected to the Al features on the paper substrates flashed on even under large substrate deformation. Furthermore, this excellent performance of Al features on paper substrates used as simple flexible electronics device can be also confirmed in images provided in the Supporting Information S6. Consequently, we expect that the Al features formed on paper substrates by the solution process will be easily applicable to low-cost and eco-friendly flexible electronics.

ASSOCIATED CONTENT

S Supporting Information *

SEM images of the Al features coated on the flexible substrates and Si wafers with pyramid structures, OM images showing cracks on the surface of paper substrates without Al features, and LED circuit demonstation of Al features on paper substrates used as simple flexible electronics device. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

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4. CONCLUSIONS We have tried to apply the solution process consisting of Al precursor ink of AlH3{O(C4H9)2} and low temperature stamping process performed at 110 °C to the production of highly conductive paper for low-cost flexible paper electronics. As an alternative to vacuum deposition, the solution process was very useful in the fabrication of Al features with excellent electrical conductivity and flexibility under various mechanical deformations on paper substrates at normal pressure and low temperature surroundings. Depending on the paper type, solution processed Al showed different electrical and mechanical performance due to the difference in the paper structure. Among the three paper substrates, inkjet printing paper was found to be the best paper substrate, with high and stable electrical conductivity under repeated bending, due to its unique fibrous structures. Thus, we expect that the solutionbased process using Al precursor ink will be easily applicable to the production of Al features with excellent electrical and mechanical properties on paper substrates for low-cost flexible electronics such as electrical circuit boards and radio frequency identification (RFID) tags for single use disposable products and electrodes for energy conversion and storage devices.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.M.L.); [email protected] (S.H.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by a grant from Korea Institute of Materials Science and a grant (M2009010025) from the Fundamental R&D program for Core Technology of Materials funded by the Ministry of Knowledge Economy (MKE) of Republic of Korea; it was also supported by a grant to KAIST from the National Research Foundation of Korea (NRF) funded by the Korea Government (MEST) (2011-0028662). 13134

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