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Limits of organic field-effect transistorLong channel length ~ 5 micronHigh operation voltage ~ 10 VLow output current

Organic semiconductorApp. Phys. Lett. 88, 223510(2006)

Jc ~ 0.01 mA/cm2Vertical space-charge-limited transistorOrganic solid-state vacuum tube

Space-charge-limited current (SCLC)

No background carrier Current by injected carriers Organic semiconductor valence band in place of vacuumGrid base fabrication polystyrene nano-spheres as mask

O2 plasma

Advantages of SCLT

Short vertical channel ~ 300 nmLow operation voltage ~ 2 VNo lithography for base gridOLED drivingHigh current density ~ 100 mA/cm2High aperture ratio by stackingOLED driven by SCLTOLED efficiency 10 cd/AJc = 50 mA/cm2Luminance = 5000 cd/m2Active matrix by blade coatingHTLEMLEmitterBaseETLCollectorLiFPO-01-TBCBPP3HT

PVPAlAlAlMoO3AlGlassNPBTBPiAlAgOLEDSCLTAlPEDOTMoO3CathodeLight emissionMulti-layer blade coating of organic semiconductors

Polymer solutionPolymer film (wet)Gap movingPolymer film (dry)ITO SubstrateHot plateRapid evaporation50 -100 microngapAppl. Phys. Lett. 93, 153308 (2008)

Emission layerElectronblockingEmissiveHole blockingenergycathodeanodeMulti-layer solution deposition for high efficiencyArbitrary solventLarge area uniformityLow material wasteCompatible with roll-to-roll process

Hot plateITOHot plateITOHot plate ITO Hot windHot plate ITOOrganic layer Blade coating for OLED and vertical transistorJ. Appl. Phys. 110, 094501 (2011)

44 cm2OLED

Other applications Short channel lengthShort carrier transit timeHigh frequencyApp. Phys. Lett. 95, 253306 (2009)Pressure sensor arrayRFID

Progress of SCLT performanceenough for OLED driving

2008-2012 2011 - Enhancing the base control

Appl. Phys. Lett., 97, 223307, 2010 Enhancing the base control

Appl. Phys. Lett. 98, 223303, 2011 2011 Breakthroughtoward a real technology Jump in collector current Organic semiconductor blade coatingSelf-assembled monolayer

PS sphere by blade coating2011 Breakthroughtoward a real technology Jump in collector current Organic semiconductor blade coatingSelf-assembled monolayer

PS sphere by blade coatingConduction in Organic SemiconductorsVan der Walls forces hold molecules togetherCharge 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:

Anisotropic TransportCharge 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 210-4 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 packingUsing solvent annealing to improve molecular packingUsing SAM to control orientation !!

SAM treatment in OFETsNature 5, 222, 2006SAMs (Self-Assemble Monolayers): HMDS and OTS on SiO2 for P3HT FET

GateOxideP3HTSAMSD

Our idea : SAM on vertical sidewalls

Conditions:SAM: (a) HMDS and (b) OTS

Top injection : using symmetric EC metal to reduce built-in potential barrierUsing MoO3/Al to adjust emitter work function as 5.3 eV 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 coatingResults: improved bulk mobility

Results: material analysis

abc

dResults: transistor performances

bcaVCE=-2 V2011 Breakthroughtoward a real technology Jump in collector current Organic semiconductor blade coatingSelf-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 MethodPS Sphere Distribution in 1cm2Active Region1 cm1 cm123

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3Transistor Performance(Blade Coating PS Spheres)

(1mm2)On/off ratio ~ 40000(1cm2)On/off ratio ~ 5000

Connected OLED/SCLT42-bOn stateOff stateAl/MoO3 patternTop EmitteronVEIEVBIBVCIC0.00E+004.43E-04-9.00E-011.16E-05-4.50E+00-4.60E-040.00E+004.33E-04-9.00E-011.09E-05-4.50E+00-4.45E-04offVEIEVBIBVCIC0.00E+001.44E-040.00E+005.21E-05-4.50E+00-1.96E-040.00E+001.43E-040.00E+005.07E-05-4.50E+00-1.94E-04

Total voltage : 6 V (across OLED and SCLT)ON state: Base voltage = -0.9 V; OFF state: Base voltage = 0 VOther solution-processed OTFTField-effect mobilityOperation voltageOn/off ratioChannel materialGate dielectric2005 [JACS]0.004< 2 V100P3HTPolymer dielectric CPVP-C6 (thickness 10-20 nm)2005 [ APL]0.005< 2V200P3HTSAM2006 [Nature Material]0.2-0.7> 20 V> 106PBTTTOTS-treated SiO22007 [OE]0.017< 2 V3600P3HTPlasma oxidation on Al gate2007 [APL]0.2 ~ 1.8> 10 V> 106TIPS HMDS-treated SiO22007 [JACS]0.1< 2 V105P3HTIon-gel gate dielectric (for top gate); gel solution mixed with stirring over 12 hrs2008 [Nature Materials, Adv. Mat.]1.8 1.6 0.8 < 3 V104~105P3HTPQT-12F8T2Ion-gel gate dielectric (for top gate); slow process: solvent evaporated for 24 hrs and the ion-gel dried in vacuum over 2 days2008 [Nature Material, JACS, APL]1.5> 20 V> 106diF-TESADTPFBT-treated gold electrodes with HMDS-treated SiO22009 [Adv. Mat.]0.15> 30 V36700PETV12TPMMA, 500 nm2009 [ Adv. Mat.]0.340.10.1> 20 VN/APBTTTP3HTPQT-12OTS-treated SiO22011 [ Adv. Mat.]7-11> 20 V> 106C8-BTBT or C10-DNTTCYTOP (Asahi Glass Co. for top-gate)2011 [AIP Advances]3.5> 20 V> 106C8-BTBTFTS-treated SiO2SummaryWith 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 WorkDevelop process for integrated solution-processed OLED/SCLT.Develop large-area array process for SCLT.Integrate large-area SCLT array with large-area OLED to realize low-cost e-book (based on blade coating process).Proposed process is introduced hereafter.Integrated OLED / SCLTTwo Approaches:Fabricating OLED on top of SCLTConnecting large-area OLED with SCLT array panel HTLEMLEmitterBaseETLCollectorLiFPO-01-TBCBPP3HT

PVPAlAlAlMoO3AlGlassNPBTBPiAlAgOLEDSCLTAlPEDOTMoO3CathodeLight emissionGlassGlassOLEDLight emissionProposed Large Area Process(,)100 um10 umTentative process to define the bottom metal electrode:Low-end lithography with lift-off process, blanket bar coating (similar to blade coating) or interference lithographyProposed Large Area Process(,)100 um10 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(,)100 um10 um Base metal deposition with PR lift-off process Removing PS spheres by roller tapingProposed Large Area Process(,)100 um10 um Plasma etch through nanometer holes SAM treatment on nanometer holes P3HT coatingProposed Large Area Process(,)100 um10 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) DemonstrationPS sphere blade coating process.Solution processed OLED switched on and off by SCLT within 1-V base voltage.