特色研究計畫-以刮刀塗佈技術製備垂 直式有機電晶體用於低成本有機發光 顯示器 交通大學物理研究所 孟心飛 交通大學光電系 冉曉雯 有機半導體實驗室研究生: 趙寅初、王凱瑞、徐永軒、羅芳財、 黃建豪、呂季遠、林洪正、蔡武衛 Limits of organic field-effect transistor Long channel length ~ 5 micron High operation voltage ~ 10 V Low output current Organic semiconductor Vertical space-charge-limited transistor Organic solid-state vacuum tube Jc ~ 0.01 mA/cm2 App. Phys. Lett. 88, 223510(2006) Space-charge-limited current (SCLC) • No background carrier • Current by injected carriers • Organic semiconductor valence band in place of vacuum Grid base fabrication polystyrene nano-spheres as mask O2 plasma Advantages of SCLT Short vertical channel ~ 300 nm Low operation voltage ~ 2 V No lithography for base grid OLED driving High current density ~ 100 mA/cm2 High aperture ratio by stacking OLED driven by SCLT Light emission OLED efficiency 10 cd/A Jc = 50 mA/cm2 Luminance = 5000 cd/m2 Active matrix by blade coating Ag Al TBPi PO-01-TB CBP NPB PEDOT Al OLED SCLT Cathode Al P3HT Al LiF ETL EML HTL MoO3 Collector Al Base PVP MoO3 Al Glass Emitter Multi-layer blade coating of organic semiconductors 50 -100 micron moving gap Rapid evaporation Polymer film (wet) Polymer film (dry) ITO Substrate Hot plate Polymer solution Gap Emission • layer Multi-layer solution deposition for energy cathode Electron blocking Emissive Hole blocking anode Appl. Phys. Lett. 93, 153308 (2008) • • • • high efficiency Arbitrary solvent Large area uniformity Low material waste Compatible with roll-to-roll process Blade coating for OLED and vertical transistor Organic layer ITO Hot plate ITO Hot plate 4×4 cm2 OLED ITO Hot plate J. Appl. Phys. 110, 094501 (2011) Hot wind ITO Hot plate Other applications RFID Pressure sensor array App. Phys. Lett. 95, 253306 (2009) Short channel length Short carrier transit time High frequency Progress of SCLT performance enough for OLED driving 2008-2012 國科會 卓越領航計劃 2011 交大 特色計劃 Enhancing the base control Al (collector) P3HT base 350 nm AlOx AlOx Al Al Al • enlarge P3HT thickness PVP PVP PVP • enable large base potential (2.5 V) to provide better control on channel 200 nm • reach on/off ratio 300000 at 1.5 V ITO (emitter) Glass Appl. Phys. Lett., 97, 223307, 2010 10 2.5% wt 3.5% wt 4.5% wt 1E-4 1 10 -1 10 -3 10 -5 10 -7 2 JC (mA/cm ) 2 P3HT mobility (cm /V-s) • enlarge PVP thickness AlOx 1E-5 500 1000 1500 2000 2500 3000 3500 4000 VBE = -1.10V -0.50V 0.10V 0.70V 1.30V 1.90V 2.50V 0.0 0.5 1.0 -VCE (V) 1.5 Enhancing the base control Al (collector) P3HT base 100 nm AlOx AlOx AlOx AlOx Al Al Al Al PVP PVP PVP ITO (emitter) Glass PVP • reduce channel diameter (from 200 nm to 100 nm) by using PS spheres with 100-nm diameter • enlarge(b) channel aspect ratio and enhance base control • obtain superior switching swing as 96 mV/dec (close to c-Si MOSFET theoretical value, 60 mV/dec) with a on/off ratio as 20000. Appl. Phys. Lett. 98, 223303, 2011 (c) (d) 2011 Breakthrough toward a real technology Jump in collector current Organic semiconductor blade coating Self-assembled monolayer PS sphere by blade coating 2011 Breakthrough toward a real technology Jump in collector current Organic semiconductor blade coating Self-assembled monolayer PS sphere by blade coating Conduction in Organic Semiconductors Van der Walls forces hold molecules together Charge transport is dependent on -bonding orbitals and quantum mechanical wave-function overlap (by hopping). Effective mobility increases with increasing temperature and increasing carrier concentration Space-charge-limited current: J 9 8 r 0 V L 2 3 Anisotropic Transport Charge transport in most organic semiconductor including conjugated polymers is anisotropic. Field-effect hole mobility in P3HT is higher than 0.1 cm2/Vs along the polymer backbone and the - orbital stacking and is lower than 2×10-4 cm2/Vs along the insulating side chain E.g. P3HT: poly(3-hexylthiophene) Q: How to improve the output current in organic transistor ? A: Improving molecular packing in the right direction! For conventional FET: Current flows in lateral direction. Edge-on direction is required to obtain high mobility. Reported methods: Using high boiling point solvent to improve molecular packing Using solvent annealing to improve molecular packing Using SAM to control orientation !! SAM treatment in OFETs Nature 5, 222, 2006 SAMs (Self-Assemble Monolayers): HMDS and OTS on SiO2 for P3HT FET S SAM D P3HT Oxide Gate Our idea : SAM on vertical sidewalls Conditions: 1.SAM: (a) HMDS and (b) OTS 1.Top injection : a) using symmetric EC metal to reduce built-in potential barrier b) Using MoO3/Al to adjust emitter work function as 5.3 eV 2.P3HT @ CB by both spin coating and blade coating (without spinning) Results: improved pore filling After OTS treatment, pore surface becomes hydrophilic STD (no SAM), spin coating STD, blade coating OTS treated, spin coating Results: improved bulk mobility OTS treated time : 2 (hours) -2 2 Mobility(cm /Vs) 10 -3 10 -4 10 Spin-coating STD OTS-18 450nm 450nm Blade-coating STD OTS-18 OTS-18 450nm 450nm 350nm Results: material analysis a 1800 OTS-18 treated without SAM 1600 1200 P3HT 1000 800 600 Glass 400 P3HT 1200 1000 800 600 G lass 400 200 0 0 Out of plane -200 0 5 10 1.0 15 20 0 25 d 0.8 0.6 Glass 0.4 0.2 OTS-18 treated without SAM 500 550 600 650 Wavelength(nm) 700 5 10 15 20 Diffraction angle (2 Theta) P3HT 0.0 Out of plane -200 Diffraction angle (2 Theta) Photoluminescence(a.u.) OTS-18 treated without SAM 1400 200 c 1800 1600 Intensity(a.u.) Intensity(a.u.) 1400 b 750 25 Results: transistor performances a 2 2.4 (V) , 54.7 (mA/cm ) 2 10 0 10 -1 10 -2 10 10 -1 10 -2 10 -3 10 -4 JC -5 JB 10 on/off ratio 10486 at 1.68V 0.0 0.5 1.0 1.5 -VCE(V) 2.0 2.5 0V 0.6V 1.2V 0V 0.6V 1.2V 0 -3 -4 VCE= 10 10 10 2 10 1 2 1 2 VBE(V)= -0.90 -0.35 0.20 0.75 1.30 1.85 2.40 10 -JC(mA/cm ) c 3 10 -J(mA/cm ) b VCE=-2 V 3.0 10 -1.0 -0.5 0.0 0.5 1.0 VB(V) 1.5 2.0 2.5 2011 Breakthrough toward a real technology Jump in collector current Organic semiconductor blade coating Self-assembled monolayer PS sphere by blade coating compatible to roll-to-toll process Blade coating PS spheres (a) PS sphere blade coating (b) Conventional Dipping Method PS Sphere Distribution in 1cm2 1 cm 1 cm Active Region Transistor Performance (Blade Coating PS Spheres) 小面積(1mm2)元件輸出特性 大面積(1cm2)元件輸出特性 On/off ratio ~ 40000 On/off ratio ~ 5000 0 2 -VCE (V) V BE -1 -0.60V -0.40V -0.20V 0.00V 0.20V 0.40V 0.60V 10 2 10 -1.20V 0 -0.60V 10 0.00V -1 0.60V 10 1.20V -2 1.80V 10 2.40V -3 10 -4 10 -5 10 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Jc (mA/cm ) 1 Jc (mA/cm ) 10 VBE -2 10 -3 10 -4 10 -5 10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 -VCE (V) Al/MoO3 pattern Top Emitter Connected OLED/SCLT VE IE VB on IB VC IC -4.60E-04 4.50E+00 0.00E+00 4.33E-04 -9.00E-01 1.09E-05 -4.45E-04 On state 4.50E+00 0.00E+00 4.43E-04 -9.00E-01 1.16E-05 VE IE VB off IB VC IC -1.96E-04 4.50E+00 Off state 0.00E+00 1.43E-04 0.00E+00 5.07E-05 -1.94E-04 4.50E+00 0.00E+00 1.44E-04 0.00E+00 5.21E-05 Total voltage : 6 V (across OLED and SCLT) ON state: Base voltage = -0.9 V; OFF state: Base voltage = 0 V 42-b Other solution-processed OTFT Field-effect mobility Operation voltage On/off ratio Channel material Gate dielectric 2005 [JACS] 0.004 <2V 100 P3HT Polymer dielectric CPVP-C6 (thickness 10-20 nm) 2005 [ APL] 0.005 < 2V 200 P3HT SAM 2006 [Nature Material] 0.2-0.7 > 20 V > 106 PBTTT OTS-treated SiO2 2007 [OE] 0.017 <2V 3600 P3HT Plasma oxidation on Al gate 2007 [APL] 0.2 ~ 1.8 > 10 V > 106 TIPS HMDS-treated SiO2 2007 [JACS] 0.1 <2V 105 P3HT Ion-gel gate dielectric (for top gate); gel solution mixed with stirring over 12 hrs 2008 [Nature Materials, Adv. Mat.] 1.8 1.6 0.8 <3V 104~10 5 P3HT PQT-12 F8T2 Ion-gel gate dielectric (for top gate); slow process: solvent evaporated for 24 hrs and the ion-gel dried in vacuum over 2 days 2008 [Nature Material, JACS, APL] 1.5 > 20 V > 106 diF-TESADT PFBT-treated gold electrodes with HMDS-treated SiO2 2009 [Adv. Mat.] 0.15 > 30 V 36700 PETV12T PMMA, 500 nm 2009 [ Adv. Mat.] 0.34 0.1 0.1 > 20 V N/A PBTTT P3HT PQT-12 OTS-treated SiO2 2011 [ Adv. Mat.] 7-11 > 20 V > 106 C8-BTBT or C10-DNTT CYTOP (Asahi Glass Co. for top-gate) 2011 [AIP Advances] 3.5 > 20 V > 106 C8-BTBT FTS-treated SiO2 Summary With high output current , low operation voltage and high on/off current ratio, vertical transistor SCLT is one of the best solution processed transistor in the world. Blade coating is successfully demonstrated on OLED and SCLT. Particularly, blade coating PS spheres facilitates the roll-to-roll large area nanostructure colloidal lithography . Solution processed OLED can be switched on and off by SCLT within 1-V base voltage. Future Work Develop process for integrated solutionprocessed OLED/SCLT. Develop large-area array process for SCLT. Integrate large-area SCLT array with large-area OLED to realize low-cost ebook (based on blade coating process). Proposed process is introduced hereafter. Integrated OLED / SCLT Light emission Two Approaches: Fabricating OLED on top of SCLT Connecting large-area OLED with SCLT array panel Light emission Glass OLED Glass Ag Al TBPi PO-01-TB CBP NPB PEDOT Al OLED Al SCLT Cathode P3HT Al LiF ETL EML HTL MoO3 Collector Al Base PVP MoO3 Al Glass Emitter Proposed Large Area Process (示意圖,基板可放大) 100 um 10 um Tentative process to define the bottom metal electrode: Low-end lithography with lift-off process, blanket bar coating (similar to blade coating) or interference lithography Proposed Large Area Process (示意圖,基板可放大) 100 um 10 um PVP coating and cross-linking 10-um wide PR line formation (low-end photolithography, blanket bar coating or interference lithography) PS spheres coating on PVP Proposed Large Area Process (示意圖,基板可放大) 100 um 10 um Base metal deposition with PR lift-off process Removing PS spheres by roller taping Proposed Large Area Process (示意圖,基板可放大) 100 um 10 um Plasma etch through nanometer holes SAM treatment on nanometer holes P3HT coating Proposed Large Area Process (示意圖,基板可放大) 100 um 10 um Passivation layer formation Pixel contact via formation and metal coating (patterned by liftoff process) Then: fabricating large-area OLED on top of array panel or connecting array panel with OLED (on the other substrate) Demonstration PS sphere blade coating process. Solution processed OLED switched on and off by SCLT within 1-V base voltage.