Towards fully spray coated organic light emitting devices

Towards fully spray coated organic light emitting devices

Koen Gilissen*a,c, Jeroen Stryckersa,c, Jean Mancab,c, Wim Defermea,c aInstitute for Materials Research (IMO-IMOMEC) – Engineering Materials and Applications, Hasselt University, Wetenschapspark 1, 3590 Diepenbeek, Belgium; b Institute for Materials

Research (IMO-IMOMEC) – Materials Physics, Hasselt University, Wetenschapspark 1, 3590 Diepenbeek, Belgium; cIMEC, IMOMEC, Universitaire Campus - Wetenschapspark 1, 3590

Diepenbeek, Belgium

ABSTRACT

Pi-conjugated polymer light emitting devices have the potential to be the next generation of solid state lighting. In order to achieve this goal, a low cost, efficient and large area production process is essential. Polymer based light emitting devices are generally deposited using techniques based on solution processing e.g.: spin coating, ink jet printing. These techniques are not well suited for cost-effective, high throughput, large area mass production of these organic devices. Ultrasonic spray deposition however, is a deposition technique that is fast, efficient and roll to roll compatible which can be easily scaled up for the production of large area polymer light emitting devices (PLEDs). This deposition technique has already successfully been employed to produce organic photovoltaic devices (OPV)1. Recently the electron blocking layer PEDOT:PSS2 and metal top contact3 have been successfully spray coated as part of the organic photovoltaic device stack. In this study, the effects of ultrasonic spray deposition of polymer light emitting devices are investigated. For the first time – to our knowledge -, spray coating of the active layer in PLED is demonstrated. Different solvents are tested to achieve the best possible spray-able dispersion. The active layer morphology is characterized and optimized to produce uniform films with optimal thickness. Furthermore these ultrasonic spray coated films are incorporated in the polymer light emitting device stack to investigate the device characteristics and efficiency. Our results show that after careful optimization of the active layer, ultrasonic spray coating is prime candidate as deposition technique for mass production of PLEDs. Keywords: Organic light emitting diodes, ultrasonic spray coating, solution processing.

1. INTRODUCTION Since the discovery of organic light-emitting diodes (OLEDs)4 this research field has expanded and progressed rapidly. OLEDs based on small molecules are currently already available in a number of commercial applications 5. The active layers of these small-molecule-based OLEDs, are generally deposited by thermal evaporation, which is a relative expensive vacuum technology. The discovery of electro-luminescent conjugated polymer made solution processable deposition techniques possible for the fabrication of polymer based light-emitting diode (PLEDs) 6. This solution processability is obtained by the property of the conjugated polymers to be easily dissolvable in common organic solvents. Solution processing allows a high throughput, a cost effective, easily industrial scalable production process and is roll-to-roll compatible. Commonly used printing techniques are spin coating, ink-jet printing and screen printing. Another plausible technique, ultrasonic spray coating, is a deposition technique used in a variety of sectors, e.g. medical, automotive, electronics, and food processing. In recent years, polymer based organic solar cells (OSCs) were successfully (ultrasonically) spray coated1,7,8. Also transparent electrode materials like high conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) 9, zinc oxide (ZnO) 10, single wall carbon nanotubes (SWCNs) 11 and nanoparticle based silver top contacts 12 were ultrasonically spray coated. In this work we investigate a process for the ultrasonic spray deposition of thin and uniform films of workhorse PLED material Super Yellow. Uniform thin films are achieved by systematic variation of single solvent and multi solvent solutions and spray coater parameter optimizations. The optimal layer thickness is obtained by systematically varying the flow rate with constant coating speed and the coating speed with constant flow rate. Finally we show an ultrasonically spray coated PLED with a uniform emission and the luminous power efficacy is compared to state-of-the-art, spin coated Super Yellow PLEDs.

Organic Light Emitting Materials and Devices XVIII, edited by Franky So, Proc. of SPIE Vol. 9183, 918311© 2014 SPIE · CCC code: 0277-786X/14/$18 · doi: 10.1117/12.2060994

Proc. of SPIE Vol. 9183 918311-1

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2. EXPERIMENTAL The ultrasonically spray coated Super Yellow thin films were prepared, in ambient conditions, on a glass substrate containing a layer of approximately30 nm of Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS). The PEDOT:PSS (Clevios P VP.AL 4083) layer was spin coated in ambient conditions and annealed at 120°C for 10 minutes in inert atmosphere. Prior to spin coating PEDOT:PSS, the glass substrates were thoroughly cleaned in an ultrasonic bath by using soap, demineralized water, acetone and isopropanol, followed by a UV/O3-treatment for 15 min. All solutions containing Super Yellow, (PDY-132) obtained from Merck, were prepared in an inert atmosphere glove box system and stirred overnight at 50°C and were cooled down to room temperature before deposition. The droplet formation ability of the ultrasonic spray nozzle decreases with increasing viscosity of the solution, therefore a concentration of 1 mg/ml was used in the first experiment with the tetrahydrofuran (THF) based solution. To prevent fast evaporation of sprayed THF based solution the primary solvent was switched to a 1,2-Dichlorobenzene (oDCB) solution with a concentration of 1 mg/ml. All experiments using oDCB – mesitylene solutions were carried out using 2.5 mg/ml oDCB. The Super Yellow was first dissolved overnight before adding the desired amount of cosolvent (10 – 40 v/v% of mesitylene). After adding the co-solvent, in inert atmosphere, the solution was stirred vigorously for 10 min. For the experiments using the pure oDCB Super Yellow solution the concentration was varied from 1.0 mg/ml to 3.5 mg/ml. Optical microscopy measurements were carried out on an Axiovert 40 MAT microscope. For profilometric measurements a Bruker dektak XT with a 2 μm tip was used. AFM was performed on a Bruker multimode 8 connected to a nanoscope 5 controller in peak force tapping mode. The PLED device stack consist out of a glass substrate with patterned ITO with a thickness of 100 nm (Kintec, sheet resistivity 20 Ω/sq), a spin coated PEDOT:PSS layer of 30 nm, ultrasonically spray coated Super Yellow, thermal evaporated calcium (30 nm) and aluminum (80 nm). The Super Yellow layers were deposited by a Sono-Tek ExactaCoat system using an Accumist spray nozzle. The calcium aluminum electrode was thermally evaporated at a base pressure of 10E-6 Bar. The current, voltage and luminous flux characteristics were measured using a Keithley 2401 source meter and an absolute calibrated integrating sphere spectrometer from Avantes.

3. RESULTS AND DISCUSSION The formation of thin polymer films by the ultrasonic spray coating technique is obtained by the spreading and merging of micro-meter sized droplets and subsequently solvent evaporation. Ultrasonic nozzles employ standing waves to atomize the solution, these standing waves are a result of the mechanical vibrations produced by the (ceramic) piezoelectric transducers inside the ultrasonic nozzle. As the solution is fed to and emerges onto the atomizing surface, the kinetic energy is dissipated causing atomization of the solution. The formed droplets are hereafter directed towards the substrate by nitrogen as carrier gas (nitrogen shroud). The first choice of solvent was tetrahydrofuran (THF) due to its low boiling point 65 °C and its use for ink-jet printing of conjugated polymers 13–15. The deposited film, from a 1mg/ml THF-Super Yellow solution, as shown in Figure 1, has distinct ring patterns, also known as coffee rings 16, caused by drying of the individual droplets before merging into a liquid bulk. This film shows a strong similarity to those obtained by Girotto et al. 17.

Figure 1. Super Yellow thin film deposited from THF solution.

To improve the formation of the ultrasonically spray coated Super Yellow films, the high boiling point solvent 1,2-Dichlorobenzene (oDCB - 180.3 °C) was used to dissolve 1 mg/ml Super Yellow. The ultrasonic spray deposition of



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pure oDCB at 1mg/ml resulted in the formation of a uniform liquid bulk. However during evaporation of the solvent the liquid bulk contracted towards the center of the substrate as previously described as a terraced hill 1. To reduce the surface tension of the Super Yellow - oDCB solution, 1,3,5-Trimethylbenzene (mesitylene) was added in concentrations of 10 – 40 v/v% 18. After systematic optimization of the flow rate and nozzle speed, uniform films were obtained by adding 40 v/v% mesitylene as co-solvent to the Super Yellow - oDCB solution as depicted in Figure 2. These atomic force microscope (AFM) images show the Super Yellow films prepared by spin coating (left) and by ultrasonic spray deposition (right) on top of spin coated PEDOT:PSS films. The ultrasonically spray coated films show a high degree of similarity with the spin coated film and both films show a good featureless coverage and low surface roughness.

Figure 2. AFM topography of Super Yellow film from Super Yellow - oDCB with 40v/v% mesitylene solution. (left) Deposited by

spin coating Ra = 0.72 nm. (right) Deposited by ultrasonic spray coating Ra = 0.87 nm. The overall film thickness can vary by the concentration of the solution, the solution flow rate and the nozzle speed. To achieve the optimal Super Yellow layer thickness of 80 nm 19 the concentration of the solution was varied from 1 mg/ml to 3 mg/ml with increments of 0.5 mg/ml. In order to find a relation between final film thickness and solution concentration, pure oDCB was utilized without co-solvents. Using spray coater parameters from the previous experiments it was found that pure oDCB yielded equal uniform films as the Super Yellow – oDCB mesitylene co-solvent solution. Figure 3 shows the Super Yellow layer thickness as function of the solution concentration. All the Super Yellow films were ultrasonically spray coated from their specific Super Yellow – oDCB concentration with their individual spray coating parameters, e.g.: flow rate, nozzle speed, to yield full substrate coverage and uniform low roughness films.

Figure 3. Super Yellow layer thickness as function of Super Yellow - pure oDCB solution concentration.



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The solution concentration is not the most practical parameter to vary when optimizing the active layer thickness of a PLED. For each variation of the solution concentration a new solution batch has to be made using relative expensive conjugated polymers and organic solvents 20. However the ultrasonic spray deposition technique also allows deposition control by varying the solution flow rate and nozzle speed at constant solution concentration. By varying the solution flow rate, the amount of solution that emerges at the atomizing surface of the nozzle is changed per unit of time and thus the spray density is varied. Figure 4 shows the resulting Super Yellow film thickness as function of the flow rate at a constant nozzle speed of 15 mm/s, using a 2.5 mg/ml Super Yellow - oDCB solution. At flow rates lower than 0.4 ml/min non-uniform and partially covered films were observed. At flow rates higher than 2.0 ml/min atomization at the ultrasonic nozzle was hindered.

Figure 4. Super Yellow layer thickness as function of solution flow rate at constant nozzle speed and constant concentration.

By varying the nozzle speed at a constant flow rate the final film thickness can also be tuned as depicted in Figure 5. The solution, 2.5 mg/ml Super Yellow – oDCB, was supplied to the ultrasonic nozzle at a constant flow rate of 1.5 ml/min.

Figure 5. Super Yellow layer thickness as function of nozzle speed at constant solution flow rate and constant concentration



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These results show that for a single solution concentration a wide range of film thicknesses can be deposited. The thickness of the emissive layer is of crucial importance for the charge transport in the PLED 21,22. The ability of this deposition technique makes it an ideal candidate for solution processing of PLEDs on an industrial scale as well as on laboratory scale. The applicability of this deposition technique was investigated by analyzing the device performance of polymer light emitting devices. The devices were prepared by ultrasonic spray deposition of the active layer with a thickness of ~80 nm.

Figure 6. (a) Experimental current density - voltage characteristic and total luminous flux of the spray coated PLEDs in ambient

atmosphere and spin coated reference device in inert atmosphere. (b) Experimental luminous power efficacy of the spray coated and spin coated PLEDs. (c) Digital photograph of the operated device with ultrasonically spray coated active layer, each device has a

physical dimension of 5 mm by 5 mm.



Figure 6 shows for both a spin coated and spray coated sample the current density – voltage (J – V) and total luminous flux – voltage (Φ – V) characteristics as well as the luminous power efficacy in function of the applied voltage (b). The uniform emission of the ultrasonically spray coated PLEDs is verified in Figure 6 c. At the lower forward bias regime, before the turn-on voltage, the spin coated devices show a lower current-density response compared to the ultrasonically spray coated devices, indicating a higher shunt resistance of the spin coated layer 23,24. A more striking difference is observed from the measured total luminous flux of the spin coated devices which is up to 3 times higher than the total luminous flux of the spray coated device. This total luminous flux deficiency of the ultrasonically spray coated devices shows its impact on the luminous power efficacy. As explained in the experimental section the ultrasonically spray coated active layers were processed in ambient conditions. The influence of this spray coating in ambient conditions, and the influence of the ultrasonication at the spray head are also examined but falls out of the scope of this paper. Further work is necessary to find the cause of this total luminous flux deficiency and to improve the efficacy of ultrasonically spray coated PLEDs.

4. CONCLUSION In conclusion this work shows the applicability of the ultrasonic spray deposition technique for the fabrication of polymer based light-emitting diodes. It is shown that after solution and ultrasonic spray coating parameter optimization comparable, uniform thin films, compared to spin coated films, can be produced. Furthermore it is shown that the final deposited layer thickness can easily be tuned by varying the flow rate and nozzle speed making it a versatile and ideal candidate for the roll-to-roll production of polymer based light emitting diodes. Devices with ultrasonically spray coated active Super Yellow layers, processed in ambient conditions, show a bright uniform emission. Device efficacies reach up to 81 % of device efficacies prepared by spin coating in inert atmosphere conditions.

ACKNOWLEDGEMENTS The author would like to thank financial contribution from the CORNET project POLEOT (IWT-TETRA-120629), the

INTERREG projects ORGANEXT and SolarFlare and the SoPPoM-project.

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Towards fully spray coated organic light emitting devices

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