In this work, the process of mist deposition is explored as a method used to deposit organic semiconductors for applications in organic light emitting diodes (OLEDs). The deposition kinetics of a specially formulated hole transport agent is studied. The results indicate that the mist-deposited organic film thickness varies linearly with precursor concentration, deposition time and substrate potential. Depending upon process parameters, a deposition rate in the range of 50 nm min−1 is readily achievable. Evolution of surface roughness revealed distinct stages in the film formation process. The growth of secondary layers was observed before the formation of a complete initial film layer. A working OLED with the hole transport layer deposited by mist deposition was demonstrated. The luminance of the device was measured to be a maximum of 3000 cd m−2 and the efficiency was 6.7 cd A−1

Huang, J.S.

Jiang, H.

Li, W.

Price, S.C.

Ruzyllo, J.

Shanmugasundaram, K.

Wang, Q. K.

Yang, Y.

Carolina Digital Repository

Semiconductor Science and TechnologyStudies of mist deposition in the fabrication of blueorganic light emitting diodesTo cite this article: K Shanmugasundaram et al 2008 Semicond. Sci. Technol. 23 075036 View the article online for updates and enhancements.Related contentStudies of mist deposition for the formationof quantum dot CdSe filmsS C Price, K Shanmugasundaram, SRamani et al.-Nickel as an alternative semitransparentanode to indium tin oxide for polymerLEDapplicationsD Krautz, S Cheylan, D S Ghosh et al.-A deep look into the spray coating processin real-time—the crucial role of x-raysStephan V Roth-Recent citationsSurface potential measurement of n-typeorganic semiconductor thin films by mistdeposition via Kelvin probe microscopyAkihiro Odaka et al-The analysis of the current–voltagecharacteristics of the high barrierAu/Anthracene/n-Si MIS devices at lowtemperaturesH. Kaçu et al-Patterned mist deposition of tri-colourCdSe/ZnS quantum dot films toward RGBLED devicesS. Pickering et al-This content was downloaded from IP address 152.2.71.53 on 15/01/2020 at 17:59Studies of mist deposition in thefabrication of blue organic light emittingdiodesK Shanmugasundaram1, S C Price1, W Li2, H Jiang2, J Huang2,Q K Wang2, Y Yang3 and J Ruzyllo11 Department of Electrical Engineering, The Pennsylvania State University, 11 Electrical EngineeringWest, University Park, PA 16802, USA2 Agiltron Inc., 15 Cabot Rd, Woburn, MA 01801, USA3 Department of Materials Science and Engineering, University of California, Los Angeles, CA, USAE-mail: kxs423@psu.eduReceived 24 February 2008, in final form 2 April 2008Published 28 May 2008Online at stacks.iop.org/SST/23/075036AbstractIn this work, the process of mist deposition is explored as a method used to deposit organicsemiconductors for applications in organic light emitting diodes (OLEDs). The depositionkinetics of a specially formulated hole transport agent is studied. The results indicate that themist-deposited organic film thickness varies linearly with precursor concentration, depositiontime and substrate potential. Depending upon process parameters, a deposition rate in therange of 50 nm min−1 is readily achievable. Evolution of surface roughness revealed distinctstages in the film formation process. The growth of secondary layers was observed before theformation of a complete initial film layer. A working OLED with the hole transport layerdeposited by mist deposition was demonstrated. The luminance of the device was measured tobe a maximum of 3000 cd m−2 and the efficiency was 6.7 cd A−1.of applications including high-k dielectrics, photoresist,ferroelectric materials and nanocrystal quantum dots[10–12]. In this process, the liquid precursor is convertedinto a very fine mist which is then carried by nitrogen tothe deposition chamber where submicron droplets coalesceat room temperature into the substrate covering its surfacewith a uniform film of viscous liquid which is then solidifiedusing low-temperature thermal curing. Details of the processare discussed elsewhere (e.g. [11]). Mist deposition offersadvantages over other physical liquid deposition methodsin that it allows selective deposition [13] and patterning ofdeposited materials, eliminating the need for post-depositionlithography and etches. At the same time it does not differ fromother solution processing methods, from neither the point ofview of thermal, budget required nor type of solvents used.In this experiment, mist deposition is for the first timeemployed in the formation of thin organic semiconductor filmsin the fabrication of blue light emitting diodes. The kineticsof the film deposition process is investigated and feasibility ofOLED formation using mist deposition of solution-processedorganic semiconductors is explored.1. IntroductionRecently, organic semiconductor thin film devices, including organic light emitting diodes (OLEDs) [1–3] and organic thin film transistors (OTFTs) [4, 5], have been extensively studied for potential applications in display technology. Several reports have introduced working OLEDs and OTFTs fabricated from solution-processed organic thin films (e.g.[6, 7]). The performance of these devices has improved over the years to be comparable to organic semiconductor devices fabricated by vacuum processing technologies.Several techniques including spin coating and dip casting have been reported for the deposition of organic semiconductor thin films from liquid precursors [8, 9]. However, these methods may not be able to provide adequate control of film thickness, surface morphology and patternability for efficient devices for the manufacture of large displays. These limitations warrant the continued exploration of alternate technologies in the deposition of thin films for applications in organic displays. Mist deposition is one such technique that has been demonstrated with good results in a numberField ScreenWaferShower HeadHigh Volta ge ConnectionN2N2Liquid SourceAtomizerMistFigure 1. Schematic of the mist deposition system used in thisstudy.2. Experimental detailsThe principle of mist deposition is to convert the liquid sourcematerial into a fine mist of submicrometer size droplets, whichis then carried in a pressurized stream of N2 gas into thedeposition chamber (figure 1) where the droplets are allowed tocoalesce into the substrate at room temperature at atmosphericpressure. This process forms a uniform film of liquid on thesubstrate, which is then thermally treated to burn off the solventand leave a thin film of solid on the surface.In order to control the deposition rate beyond gravitationalinteractions, which in the case of submicrometer-sized dropletsare very weak, an electric field is created between the groundedfield screen and a wafer (figure 1). After deposition, thefilm is thermally cured at the temperature of 100–300 ◦C inambient air or in the controlled ambient of either O2 or N2 atthe atmospheric pressure.The polymeric material used in this study is a holetransport agent with potential uses in organic light emittingdevices and solar cells. It is a proprietary hole transportagent (AG I) provided by Agiltron, Inc. Three differentconcentrations of AG I in a toluene solvent were studied:0.1% (w/v), 0.5% (w/v) and 1% (w/v). The deposited filmswere subjected to a hot-plate anneal in ambient air at 100 ◦C for2 min to drive off the excess solvent. The OLEDs processed inthis experiment were ITO/AG I/PFO/Al structures in whichan active layer comprising 1% wt AG I was mist depositedand a poly(9, 9-dioctylfluorene) (PFO) layer spin coated. Inthe working diodes, the thickness of AG I was estimated to be200 nm and the PFO thickness was estimated to be between80 and 100 nm. Al contacts were then vacuum evaporatedthrough a shadow mask to define a 12 mm2 electrode area.The films were deposited on two different substrates:Si and ITO- coated glass. The substrates were prepared asfollows prior to deposition. Si substrates were subjected toa standard RCA1 (6:1.5:1 DI H2O:H2O2:NH4OH) clean [14]for 10 min at 80 ◦C followed by etch off in a 1:100 HF:DIH2O solution unless otherwise noted. The ITO-coated glasssubstrates were subjected to a solvent clean in acetone followedby IPA rinse. The remaining organics were burned off by alamp clean at 300 ◦C for 1 min [15]. Any other treatmentsthat were performed will be described accordingly. Deposited0601201802403003604204800 0.6 1.2Concentration (%)Thickness (nm)Substrate – Si <1-0-0>Deposition Time – 600sDeposition Voltage – 4kVThickness (nm)Figure 2. Thickness versus precursor concentration of a polymericfilm (AG I) on Si substrates.040801201602000 2 4 6 8 10Deposition Potential (kV)Thickness (nm)Substrate – Si <1-0-0>AG I Concentration – 1% (w/v)Deposition Time – 180sFigure 3. Thickness versus deposition potential of a polymeric film(AG I) on Si substrates.films were characterized by AFM analysis and spectroscopicellipsometry after thermal curing as described earlier. TheOLEDs formed were measured using a computer-controlledsource meter equipped with a calibrated silicon photodetector.3. Results and discussionsThe deposition kinetics of the AG I films deposited on Sisubstrates was studied first. Figure 2 shows the dependenceof material concentration on the thickness. Films in this casewere deposited at 4 kV for 600 s starting with precursorsof three different concentrations. It is seen from figure 2that thickness increases linearly with concentration, which isexpected. The thickness for the 0.1% concentration precursoris 97.5 nm and that for the 1% concentration solution is 482 nm.The difference in the film thickness is explained by the changein viscosity of the solution with material concentration. Kimand Marshall showed that the droplet size of a liquid increaseswith the viscosity of the solution [16]:D ∼ (η/ρσ)0.45,where D is the mean droplet diameter, η is the viscosity of theliquid, ρ is the density of the liquid and σ is the surface tension.As the material concentration increases, the diameter of themist droplets also increases thereby depositing a larger volume0501001502002503003504004500 200 400 600 800Deposition Time (s)Thickness (nm)Substrate – Si <1-0-0>AG I Concentration – 1% (w/v)Deposition Voltage – 4kVFigure 4. Thickness versus deposition time of a polymeric film(AG I) on Si substrates.0510152025303540450 200 400 600 800RMS Roughness (nm) Substrate – Si <1-0-0>AG I Concentration – 1% (w/v)Deposition Voltage – 4kVDeposition Time (s)Figure 5. RMS roughness versus deposition time of a polymericfilm (AG I) on Si substrates.of a liquid film on the substrate which when thermally annealedleaves more material on the surface. This relationship was(a) (b)(c)Figure 6. AFM images of a polymeric film (AG I) on Si substrates for three deposition: (a) 30 s, (b) 240 s, and (c) 300 s.confirmed by Chang et al [17] in the case of mist depositionof HfSiO4 precursors.The thickness versus the substrate potential relationshipfor 3 min depositions is shown in figure 3. As seen, theincrease in thickness with the mist deposition time is linear.The flattening of the growth rate between the 4 kV and 6 kVsubstrates is possibly due to nonlinearity in the film formation.This might be due to the growth of secondary and tertiary layersbefore the first layers are completely formed. As the potentialincreases, there is no growth in thickness but the depositedmaterial is used to fill the voids created by the early growth ofthe secondary layers. This phenomenon is discussed furtherin the subsequent sections.Figure 4 shows the increase in film thickness with thedeposition time. All films were deposited with a precursorconcentration of 1% w/v in toluene, at a substrate potentialof 4 kV. It is seen from figure 4 that the thickness increasesalmost linearly with the deposition time. This is typical ofmost materials deposited by mist deposition. It is to be notedthat the initiation time for the film growth is very short in thiscase, and there is a complete film with a thickness of about26 nm in the first 30 s of deposition.The RMS roughness data in figure 5 indicate that as thedeposition time increases the roughness increases and thendecreases abruptly, again indicating the possible formationof secondary film layers before the formation of completeinitial layers. The roughness after a 30 s deposition is8.2 nm which then decreases to 4.5 nm after 60 s of deposition,thereby confirming the formation of an initial film layer.The roughness then increases to about 47.1 nm for a 240 sdeposition after which it decreases to 7.5 nm after 300 s. Thisis believed to mark the growth of a complete film layer. It isalso important to note that the difference in thickness betweenthe complete formation of the first layer and the second layer-4 -2 0 2 4 6 8 10 1210-310-210-1100101102103101102103104Current Density [A/m2 ]Bias [V] Luminance [cd/m2 ]2 ]Figure 7. Luminance and current density versus applied bias of amist-deposited OLED.is significantly larger compared to the thickness of the initiallayer. This shows that there is a layer-by-layer growth mode onthe Si substrate immediately prior to the formation of the initialpolymer layer. The growth of the secondary layer proceedswith island formation with the growth of multiple layers beforethe islands coalesce to form a complete layer.Atomic force microscope images of the deposited films at60, 240 and 300 s are shown in figures 6(a)–(c), respectively.These images clearly show the distinct stages in the growthof the AG I layer with the growth of secondary layers beforethe initial layer is complete. The spikes seen on these imagesare large enough to be considered as the growing film. It ispostulated that the additional material deposited during thelonger deposition times is used in filling the voids createdby the earlier layers and does not contribute to a significantincrease in film thickness.In the last part of this experiment, the ITO/AG I/PFO/AlOLEDs in which an active layer comprised mist deposited1% wt AG I were processed. The measured luminance andcurrent density versus voltage characteristics of the fabricatedOLEDs are shown in figure 7. It is seen from this figurethat the diode has a maximum luminance of 3000 cd m−2 at10 V, and the turn-on voltage is 2.3 V. Figure 8 shows theelectroluminescence efficiency versus current density. Theefficiency reaches a maximum of 6.7 cd A−1 at a current densityof 150 mA cm−2. A survey of the current literature on OLEDsreveals that although the luminance and current density of ourdevices are comparable to those reported in the literature, theluminescence efficiency is still low which is possibly due tothe low hole injection in the AG I layer [18, 19]. It is believedthat with further studies, the electroluminescence efficiencyof OLEDs formed by mist deposition can be significantlyimproved.4. SummaryAn attempt to deposit organic semiconductor materials by mistdeposition for OLED applications was made in this study.The results obtained indicate that mist deposition is capableof forming very thin, mechanically coherent films of organicsemiconductors based on which OLEDs are formed. Filmformation and morphology are dependent on the substrate, and0 100 200 300 400 500 60002468Luminous Efficiency (cd/A)Current Density (A/m 2)Figure 8. Luminance efficiency versus current density of amist-deposited OLED.surface preparation prior to deposition may also be needed tofurther improve film properties. Working OLEDs fabricatedby mist deposition were also demonstrated in this study forthe first time. Mist deposition shows very good promise inthe deposition of thin films for organic photonics. Furtherstudies to optimize the deposition process and hence improvethe performance of the LED devices are necessary.AcknowledgmentsThis study was supported by a SBIR program (contract no.DE-FG02-04ER83894) from The Department of Energy. PennState authors would like to thank Agiltron, Inc. for supportof this project. The deposition tool used in this study wasprovided by Primaxx, Inc.References[1] Brown J, Kwong R, Tung Y J, Adamovich V, Weaver M andHack M 2004 Recent progress in high-efficiencyphosphorescent OLED technology J. Soc. Inf. Disp.12 329–32[2] Hatwar T K, Brown C T, Cosimbescu L, Ricks M L,Spindler J P, Begley W and Vargas J R 2004 Developmentin OLED formulations with improved efficiency andstability Proc. SPIE 5519 1–11[3] Kowalsky W, Gorrn P, Meyer J, Kroger M, Johannes H H andRiedl T 2007 See-through OLED displays Light-EmittingDiodes: Research, Manufacturing, and Applications XI,2007 Proc. SPIE 6486 64860F[4] Nomoto K, Hirai N, Yoneya N, Kawashima N, Noda M,Wada M and Kasahara J A 2008 A high-performanceshort-channel bottom-contact OTFT and its application toAM-TN-LCD IEEE Trans. Electron Devices 52 1519–26[5] Mizukami M et al 2006 Flexible AM OLED panel driven bybottom-contact OTFTs IEEE Electron Devices Lett.27 249–51[6] Duggal A R, Heller C M, Shiang J J, Liu J and Lewis L N2007 Solution-processed organic light-emitting diodes forlighting J. Disp. Technol. 3 184–92[7] Minakata T and Natsume Y Highly crystalline thin films ofpentacene formed by simple solution process and their FETperformances Proc. 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Studies of mist deposition in the fabrication of blue organic light emitting diodes

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Studies of mist deposition in the fabrication of blue organic light emitting diodes

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