Student Project Abstracts 2005 - Pluto - University of Washington

Quinoxaline-Containing Polyfluorenes: Enhanced BlueElectroluminescence by Addition of a Hole Blocking LayerKelli A. IrvinMontana State UniversityJessica M. Hancock and Samson A. JenekheDeptartments of Chemical and Chemical EngineeringUniversity of WashingtonINTRODUCTIONThe commercial market today is saturated with products usingflat panel displays from cell phones to computer screens andpersonal data organizers to billboards. Currently, liquid crystaldisplays (LCDs) dominate the market with high resolution andaffordability 1 . The only challenger to this domination may perhapsbe the organic light-emitting diode (OLED). OLEDs showpromise to consume less energy, lower costs due to ease of manufacturingof conjugated polymers, and be brighter than LCDs. Inaddition, conjugated polymers have properties such as ease ofprocessing using spin casting, low drive voltages due to the excellentcharge mobility, and novel properties such as flexibility. 2OLEDs don’t have the need for backlighting and the overall productwould have a slimmer profile due to the nanoscale thicknessof the layers used in processing. 3Conjugated polymers were first recorded to be electroluminescentin the 1990s by J.H. Burroughes, et al., using poly(pphenylenevinylene), or PPV. 4 Since their initial discovery, greatadvancements have been made included tunable color, stablecolor across a voltage range, and more efficient devices usingmultiple layers.Challenges that need to be overcome for OLEDs includedisplay lifetimes and stable true blue color. Poly(9,9’-dioctylfluorenes),or PFO, has been widely researched as an emissivelayer yielding stable blue color. 5 PFO is a p-type material thattransport holes and blocks electrons with high mobility throughoutthe delocalized conjugated chain. Although PFO is capableof efficiently transporting holes, it lacks the ability to efficientlytransport electrons and block holes. Copolymerization with ann-type material shows promise to enhance the electron transportproperties of the polymer while maintaining the blue emission.Therefore, more research is needed to find a material that exhibitsstable blue color and has the ability to be a bifunctional emissivelayer.BACKGROUNDPreviously, Kulkarni, et al. 6 , completed work using six molar ratiosof the copolymer 2,3-bis-(p-phenylene)quinoxaline-containingpoly(9,9’-dioctyl-fluorenes) (QXF) (Figure 1a) as the emissivelayer in OLEDs to obtain stable blue electroluminescence.The motivation for incorporating the quinoxaline moiety was toimprove the electron transport of commercially produced PFO.Copolymerizing a p-type and n-type material could eventuallylead to the realization of a material capable of efficient electronand hole transport. The configuration of the devices incorporatedpoly(N-carbazole) (PVK) to enhance the injection of holes intothe device. The work outlined in the proceeding sections introduceda hole blocking layer into the architecture of the device inan effort to enhance brightness and efficiency. Ultimately, thetrue ability of QXF as an emitter will be shown or the realizationof QXF as a bifunctional material will be demonstrated.Figure 1. Chemical structures of (left) QXF and (right) TPBI.Fabrication and Characterization of LEDsThree configurations of devices were fabricated using three selectedmolar ratios based on the quinoxaline content as the emissivelayer. The three molar ratios will be noted as QXF2, QXF5, andQXF15 (Figure 1a). All devices were fabricated with the same anodeand cathode using indium-tin oxide (ITO) and LiF/Al, respectively.Poly(ethylenedioxythiophene) doped with poly(styrenesulfonate)(PEDOT) was spin cast for improved injection of holes.The first device as shown in Figure 2a was a single layer LED withthe copolymer QXF as the emissive layer. PVK was incorporatedinto the architecture of the second configuration (Figure 2b). Thethird device (Figure 2c) included both the PVK layer and incorporated1,3,5-tris(N-phenylbenzimidizole-2-yl)benzene (TPBI) (seeFigure1b) as a hole blocking layer. The three architectures based onQXF as the emissive layer were ITO/PEDOT/QXF/LiF/Al (DiodeI), ITO/PEDOT/PVK/QXF/LiF/Al (Diode II), and ITO/PEDOT/PVK/QXF/TPBI/LiF/Al (Diode III).CMDITR Review of Undergraduate Research Vol. 2 No. 1 Summer 2005 61