Inkjet Printed Polymer Layer on Flexible Substrate for OLED Applications

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Inkjet Printed Polymer Layer on Flexible Substrate for OLED Applications Fulvia Villani,* Paolo Vacca, Giuseppe Nenna, Olga Valentino, Gianbattista Burrasca, Tommaso Fasolino, Carla Minarini, and Dario della Sala Enea Centro Ricerche Portici, piazzale Enrico Fermi 1, 80055 Portici (NA), Italy ReceiVed: October 29, 2008; ReVised Manuscript ReceiVed: June 19, 2009

In optoelectronics, inkjet printing (IJP) technology is being developed as an alternative to the traditional techniques for organic materials deposition. In this work, we report the fabrication of organic light-emitting diodes (OLEDs) on the flexible substrate by studying the effect of a surface chemical treatment on the inkjet printed polymer film morphology. The employed piranha treatment increases the substrate surface energy and improves the wettability, thus inducing a decrease in the IJ printed drop thickness. The IJ printed polymer (poly(9,9-dihexyl-9H-fluorene-2,7-diyl)) is the hole-transporting layer (HTL) of a hybrid structure in which the other layers are deposited by vacuum thermal evaporation. Furthermore, in order to determine the effect of the IJ deposition method on the manufactured OLED performances, we compare them to those of devices fabricated using standard technologies. With this aim, OLEDs with the same structure are fabricated by replacing the IJ printed polymer with a spin-coated film employing the same polymer solution. The electrical and optical properties of the electroluminescent devices are investigated and discussed. Despite the lack of thickness uniformity in IJ printed film, which is an intrinsic, technological limit, OLEDs with IJ printed HTL show electro-optical characteristics that are similar to the ones of OLEDs with spin-coated HTL. Introduction Organic light-emitting diodes (OLEDs) and polymer lightemitting diodes (PLEDs) have great potential for extensive applications such as multicolor electroluminescent displays, indicator lights, and logos.1,2 One of the major advantages of OLEDs and PLEDs is their color-tuning capability with various emission wavelengths which are easily obtained by changing the chemical structure of the organic compounds. Another advantage is the solution processability of the conjugated organic material. Traditionally, OLEDs are produced by the thermal evaporation of organic materials in an ultrahigh-vacuum environment, while the polymer solutions employed in PLED manufacturing are deposited by spin-coating, a common processing technique for polymers, which takes advantage of their solution processability properties.1,3-5 There are many disadvantages associated with these simple technologies, such as solution wastage and lack of lateral patterning capability, since they do not allow selective distribution of the materials over the substrate autonomously. All these issues limit their commercial application in organic electronics.6 In fact, a single material covers entirely the substrate, so that, generally, only devices of a single color can be fabricated. The straightforward integration of multiple organic layers (to fabricate red, green, and blue emitters for color displays) would require the patterning of the individual organic layers. The patterning of organic materials by conventional photoresist is difficult because of their solubility in and sensitivity to aqueous solutions and many solvents. To realize the aforementioned applications, lateral control is required during the deposition of the different polymers: this drawback of the conventional deposition technologies can be * Corresponding author. Fulvia Villani, CR ENEA Portici, piazzale E. Fermi 1, 80055 Portici (NA), Italy, tel. +39 081 7723285, fax +39 081 7723344. [email protected].

overcome by using the inkjet printing (IJP) technology.2 IJP has emerged as an attractive deposition technique for organic semiconductors in optoelectronic applications.7 This technique is able to carry out material deposition and patterning at the same time. To carry out patterning, IJP does not require any chemical processes that employ wet etching of photoresist or other related material that might induce defects in the functional organic layers. The major advantage of IJP is that small amounts of functional materials can be deposited by solution on defined surface areas, in the desired shape. In this way, this technique minimizes waste products and eliminates the need for expensive masks and conventional photolithography, both of which increase production costs. Other remarkable advantages of the IJP technology consist of the fact that the method is noncontact, so there is no sensitivity to substrate defects, it can be applied to almost any low-viscosity liquid, and very thin films can be created. Due to the aforementioned advantages of this technology, high-resolution polymer light-emitting diodes can be manufactured through the direct deposition of patterned polymer inks.6-9 Nevertheless, the IJP technology does have some intrinsic limits, such as the technical difficulty in keeping the nozzle clear, the printed film roughness generated by joined drops, and a reduction in thickness uniformity due to drying processes.10,11 The accuracy of organic layer thicknesses is an important OLED manufacturing issue, since it directly affects brightness and color uniformity. Moreover, it is crucial to fabricate devices with uniform thicknesses in each layer because nonuniformities may lead to localized, high electric currents, localized overheating, and gradual destruction of the device. Last but not least, because of its potential for printing on a range of substrate types, both nonflexible and flexible, IJP technology has assumed a key role in the field of the optoelectronic applications, where the manufacturing of flexible electronic devices is a dynamic, continually evolving industry which is changing the way the world interacts with electronics.

10.1021/jp8095538 CCC: $40.75  2009 American Chemical Society Published on Web 07/07/2009

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TABLE 1: Summary of ITO Parameters (Thickness, rms Roughness, Polar, θpol, and Dispersed, θdisp, Component of Contact Angle, Surface Energy SE, Polarity Xp, Sheet Resistance) before and after Treatment with Piranha Solution ITO treatment

thickness (nm)

rms roughness (nm)

θpol (°)

θdisp (°)

SE (mJ/m2)

Xp

sheet resistance (Ω/square)

untreated piranha

578 566

8.6 6.3

106.6 16.5

62.1 33.0

29.0 69.8

∼0 0.62

140 ((40) 120 ((20)

Consequently, investment in research into the development of flexible electronic technology in order to replace conventional glass substrates with a polymer substrate is becoming increasingly important. It is highly unlikely that in the foreseeable future flexible electronic devices will be completely organic, but they will most probably be a hybrid of organic and inorganic layers. Anyway, the choice of a substrate with suitable surface properties is essential because it is the basis of the electronic device and must be compatible with the successive device layer in order to be successfully integrated. Since the organic film is normally in direct contact with the anode, several studies have been carried out on the effects of surface treatments on the electrical, chemical, and morphological properties of this electrode and on OLED performance.12 In particular, the control of the wettability of the anode surface is a crucial point in IJP technology, because the printed solution can be spread in a controlled manner.13 In this work, we studied the effects of indium tin oxide (ITO) treatments on inkjet printed poly(9,9-dihexyl-9H-fluorene-2,7diyl) (PF6) drop morphology in order to manufacture OLED structures on flexible substrates (PET, polyethylene terephthalate). The polymer solution used as ink was made of PF6 in toluene, and the IJ printed material was inserted into a hybrid structure in which the other layers were deposited by thermal evaporation. OLEDs with structure ITO/PF6/Alq3 (tris-8-hydroxyquinoline aluminum salt)/Al were fabricated and their electrical and optical properties investigated. OLEDs with the same hybrid structure were fabricated by replacing the inkjet printed hole-transporting layer (HTL) with a spin-coated polymer employing the same PF6 solution to determine how the deposition technique affects the optoelectronic properties of the device. Experimental Section ITO-coated commercial PET was used as substrate. It is 600 nm thick with a sheet resistance of about 100 Ω/square. For LED fabrication, we carried out ITO patterning by conventional photolithography to define the OLED geometry. We used a chemical wet process as surface treatment. Piranha solution (H2SO4/H2O2 4:1 (v/v)) was prepared as oxidant treatment, and the substrates were dipped in the solution at room temperature for 5 min. After this treatment, the ITO substrates were rinsed in distilled water and finally dried with nitrogen. In order to evaluate any modification induced by the oxidant wet treatment, transmittance spectra of ITO-coated PET substrates were performed with a Perkin-Elmer λ 900 spectrophotometer. The effects of this treatment on sheet resistance were studied using a four-point probe. A surface profilometer (KLA Tencor P-10 surface profiler) was used to detect possible alterations in ITO morphology induced by piranha treatment. Results are reported in terms of ITO thickness and root-mean-square surface roughness. A Dataphysics OCA 20 equipment at 21 °C and 50% relative humidity was employed to carry out the contact angle measurements on substrates, before and after chemical treatment. Water (polar) and diiodomethane (nonpolar) solvents were used to evaluate the polar and dispersion components of the surface

energy. Polarity (Xp) is reported as the ratio of the polar component to the total surface energy (SE). The standard contact angle measurement error is (0.2°. Before measuring, samples were cleaned in isopropyl alcohol and deionized water, dried with nitrogen, kept in a vacuum oven at 50 °C for 1 h, and finally cooled at 21 °C. The treated ITO substrates were used to fabricate double layer devices. PF6, a blue-emitting polymer, was used as a holetransporting material; it was dissolved in a toluene solution (15 mg/mL) and inkjet printed over the ITO. The inkjet equipment had been especially designed by Aurel SpA for printing functional materials on flexible substrates as reel or single sheet and nonflexible substrates. This printer uses piezoelectric drop on demand (DoD) technology to jet ink droplets through a Microdrop printhead with a 50-µm-opening nozzle. Sequences of overlapping droplets to create lines were printed at 1 Hz drop emission frequency and 200 µm/s printhead speed. After deposition, the printed material was baked in a vacuum at 60 °C for 3 h. Alq3 small molecules were used as electrontransporting and emitting material. They were deposited by thermal evaporation in a high-vacuum chamber at 10-7 mbar base pressure; the film thickness was 70 nm. The 200-nm-thick aluminum cathode was evaporated as a final layer. The active device area was 0.56 mm2. OLEDs with the same structure were fabricated by spincoating the HTL from the same PF6 polymer solution. The spincoating process was carried out using a Laurell model WS400B-6NPP spin coater. The spun films were baked in identical conditions to the inkjet-printed material. The film thickness was measured with the surface profilometer. Morphology was analyzed by an atomic force microscopy (AFM) with a Veeco Multimode Nanoscope V system. Currentvoltage (I-V) characteristics were measured with a Keithley 2400 Power Supply SourceMeter in voltage mode with constant increment steps and delay time of 1 s before each measurement point. Electroluminescence (EL) analysis was performed by using an integrating sphere and a photodiode (Newport 810UV) connected to a Keithley 6517A electrometer. All the characterization was performed in air at room temperature. Results and Discussion ITO-coated PET substrates were analyzed before and after the developed wet process in order to evaluate whether any modification in surface energy, morphology, transmittance, and sheet resistance had been produced. Results from morphological and wettability analyses, for treated and untreated ITO-coated PET substrates, are listed in Table 1. The morphological study indicates that the oxidant wet process does not affect ITO geometry; indeed, its thickness is still around 600 nm, even after treatment. However, piranha solution was found to slightly smooth the ITO surface, resulting in a lower surface roughness value (Table 1). These results confirm the strong oxidant action of piranha solution,14 which can react violently to contact with the organics of the ITO layer, resulting in a soft ITO etching effect and removal of residual organics. It is suspected that these residuals are the major source of contamination15 and, together with ITO spikes, can compro-

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Figure 1. Transmittance spectra for ITO-coated PET substrates.

mise the integrity and the performance of the final devices through the formation of dark spots. Moreover, results achieved by contact angle tests (Table 1) show that the oxidant wet process produces a marked increase in surface energy (SE) and polarity (Xp). We registered an evident decrease in contact angles after chemical treatment, for both polar (θpol) and dispersed (θdisp) components of contact angle (Table 1). This may confirm that the removal of organic contaminants is obtained with a subsequent increase in ITO surface energy. It is known that a substrate with a higher surface energy than the surface tension of the deposited solution induces improved ITO-polymer adhesion. Typical values of surface tension reported in literature for polymeric solutions, commonly deposited by spin-coating, range from 30 to 35 mJ/m2.12 In light of this statement, the surface energy measured for the ITO/PET substrate treated with piranha solution should improve wettability for functional materials deposition. The four-point probe measurement reveals more uniform ITO sheet resistance after the oxidant process. The standard deviation of sheet resistance measurements developed on different points of the substrate strongly decreases, and the sheet resistance mean value moves to a lower value (Table 1). This suggests, therefore, that a lower distribution of ITO spikes induces greater uniformity in layer electrical properties. Moreover, ITO-coated PET substrates were analyzed by UV-vis-NIR spectrometry; the recorded measurements are reported in Figure 1 and show that average transmittance remains unmodified. The detected fringe shift is correlated to thickness nonuniformity. After substrate characterization, we carried out prints of single and overlapping drop sequences on untreated ITO/PET substrates as shown in Figure 2. The thickness of the dots ranges from 95 to 100 nm. These thickness values are not suitable for the realization of HTL for functional OLED devices. In order to obtain more suitable devices, we printed drops on piranha-treated ITO/PET substrates and observed a decrease in drop thickness to around 50-60 nm. In this case, the drop widths ranged from 300 to 320 µm, as shown in Figure 3. As expected,16 these values are greater than the corresponding values for untreated substrates (250-260 µm). The printed product results on untreated ITO substrates are a useful reference tool in understanding the chemical treatment effects. The piranha treatment employed modifies substrate wettability, and recorded drop dimensions confirm the results of surface energy analysis carried out through contact angle measurements. The estimated values for PF6 solutions are very similar to the surface energy of the untreated ITO/PET layer.12 Piranha treatment produces an increase in surface energy, assuring improved wettability, and reduced contact angle. On

Villani et al.

Figure 2. Micrographs of single (a) and overlapping (b) PF6 IJ printed drop sequences on untreated ITO/PET substrate.

Figure 3. Micrographs of single (a) and overlapping (b) PF6 IJ printed drop sequences on piranha-treated ITO/PET substrate.

the other hand, the contact angle should decrease as residue quantity decreases and surface energy increases. As the contact angle decreases, the presence of a thinner liquid layer near the droplet contact line induces irregular, fast solvent evaporation throughout the liquid/gas interface. Fast diffusion of the polymer maximizes material transfer toward the edges via capillarity and increases roughness. This effect has been confirmed by AFM measurements. We investigated the average surface roughness of the droplets IJ printed on treated and untreated ITO/PET substrates. The AFM images and roughness values are shown in Figure 4. The results obtained by AFM analysis demonstrate that the drops printed on piranha-treated substrate exhibit a greater degree of roughness. However, we chose the drops IJ printed from PF6 solution on treated ITO/PET substrates as HTL of OLED device, because their thickness and morphology are a suitable compromise for manufacturing devices with good electrooptical performance. In order to determine how the deposition technique affects the device optoelectronic properties, OLEDs with the same structure were fabricated by replacing the inkjet-printed HTL with the same material (PF6) deposited by means of the spincoating technique. With this aim, we performed IJ prints of single drops on treated substrates and analyzed the surface profile in detail via AFM. Figure 5 shows a micrograph of IJ printed drop on piranha-treated ITO/PET substrate and its surface profile detected scanning the little ringed area. AFM investigation pinpointed a central dot area characterized by thickness values reduced to around 15 nm. Upon analyzing the surface profile in different areas inside the drop (excluding drop edges), the greatest drop thickness detected was around 60 nm. In light of this result, two PF6 samples were prepared by spincoating onto piranha-treated ITO/PET substrates, which were

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Figure 6. Current density-voltage characteristics for ITO/PF6/Alq3/ Al OLEDs. Figure 4. AFM images of PF6 IJ printed drop on piranha-treated (a) and untreated (b) ITO/PET substrate. Rq is the obtained average roughness value.

Figure 7. Electroluminescence vs voltage of OLED devices.

Figure 5. Micrograph of IJ printed drop on piranha-treated ITO/PET substrate and its AFM surface profile in the ringed area.

then employed as HTL in the OLED structure. These samples were obtained at different spin conditions, 4000 rpm/s for 30 s and 1000 rpm/s for 30 s, with thicknesses of 15 and 60 nm, respectively. In order to compare the performances of OLEDs fabricated with the two previously mentioned technologies, 70 nm Alq3 layer was grown on all the samples in the same evaporation run to ensure the same deposition conditions. All the devices were completed by simultaneously evaporating an aluminum cathode (200 nm). Finally, we realized three sets of samples, all with the structure PET/ITO/PF6/Alq3/Al, the first

with IJ printed PF6, the second with thin (15 nm) spin-coated PF6, and the third with thick (60 nm) spin-coated PF6. Each set included five devices. The electrooptical characteristics for manufactured OLEDs were evaluated and analyzed. Current density-voltage characteristics of OLED devices are reported in Figure 6. The electrical measurements showed good reproducibility and stability only over a short operation time, since the devices were not encapsulated. Furthermore, the electrical behaviors of the fabricated devices had almost the same shape. This result points out that the device’s electrical properties do not strictly depend on the deposition technique. The I-V curve of the IJ printed device is quite similar to that of the device with thinner spincoated HTL probably because the non-uniform thickness of the IJ drop defines preferential paths for the current flow across thinner area sections where the resistance is lower. In the electroluminescence-voltage curve reported in Figure 7, the optical threshold values for each device correspond to the electrical threshold values (5.3 V for device with thin spin-coated PF6, 8.1 V for OLED with IJ printed HTL, and 25.6 V for device with thick spin-coated PF6). Moreover, the IJ printed device luminance slope is slightly different from those of the spincoated devices. This difference can be justified in terms of printed film thickness variability. Indeed, the OLED optical turnon is the resultant of the contributions coming from the different sections of the device.

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Conclusions

References and Notes

The successful application of IJP technology in the manufacture OLEDs on flexible substrate treated with a chemical piranha processing technique is reported in this study. From the results obtained, we deduce that the increased surface energy obtained through this treatment modifies the ITO surface wettability. In this way, inkjet printed polymer film properties can be improved, thus making it possible to manufacture devices with good electro-optical performances. Furthermore, this treatment ensures good electronic contact between the holetransporter polymer and anodic layers with improved chargecarrier injection through the interface. The device realized with IJ printed HTL shows an optical and electrical turn-on voltage at around 8 V, and its current performance is comparable to devices realized in the same structure with spin-coated HTL. From our data, it has become evident that a comparison of the two technologies is not easy, and that IJ printed drop thickness variability must be taken into account.

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Acknowledgment. This work was supported by the TRIPODE project financed by the Ministero dell’Universita` e della Ricerca (MUR). The authors gratefully acknowledge Dr. Fabrizio Porro for his valuable support in AFM analysis.

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