Student Project Abstracts 2005 - Pluto - University of Washington

QUINOXALINE-CONTAINING POLYFLUORENES: ENHANCED BLUE ELECTROLUMINESCENCE BY ADDITION OF A HOLE BLOCKING LAYERFigure 5. Electronic Structures of (a) Diode I, (b) Diode II, and (c) Diode III.The EL spectra of Diode II and Diode II of each molar ratioare shown in Figure 6. No significant shifts in the peaks or lineshape were found for any diodes or molar ratios over the appliedvoltage range. All diodes were blue emitting with an average CIEof ~(0.16, 0.04), which is similar to the true blue color emittedfrom the PFO homopolymer. On average, a blue shift of ~15 nmwas shown for the EL emission relative to the thin film PL emission.The current density-voltage and luminance-voltage graphsof Diodes II and III are shown for each molar ratio in Figures7 and 8, respectively. The turn-on voltages for the QXF2 andQXF5 were ~5 V for all device configurations, while the QXF15had consistently higher turn-on voltages of ~6 V. The quinoxalinemoiety improves electron injection, but the higher turn-onvoltages for QXF15 suggest that the improved electron injectionlead to a charge imbalance.Maximum luminances for Diode I were 230, 160, and 24cd/m 2 with luminous efficiencies (LE) of 0.05, 0.03, and 0.01 cd/A for QXF2, QXF5, and QXF15, respectively. The EQE valueswere very low at 0.19, 0.14, and 0.032% for QXF2, QXF5,and QXF15. The low brightnesses and efficiencies for the devicewere expected due to the large barrier to hole injection of 0.6 Vbetween the PEDOT and QXF layers. Highest brightness for DiodeII was exhibited by QXF5 with a luminance of 1180 cd/m 2 ,LE of 0.25 cd/A, and EQE of 0.33%. For Diode III, the QXF2copolymer exhibited the highest brightness, relative to QXF5and QXF15, of 1623 cd/m 2 with a LE of 0.91 cd/A and EQE of1.52%.Figure 6. EL Spectra of QXF2, QXF5, and QXF15for (a) Diode II and (b) Diode III.Figure 7. (left column) Luminance-Voltage Characteristicsof (top) QXF2, (middle) QXF5, and (bottom) QXF15. Redis Diode III, Blue is Diode II unless otherwise noted.Figure 8. (right column) Current Density-Voltage Characteristics of (top) QXF2,(middle) QXF5, and (bottom) QXF15. Red is Diode II, Blue is Diode III.The addition of the TPBI hole blocking layer in Diode III forall molar ratios improved luminance and efficiency. For QXF2,Diode II had a maximum luminance of 310 cd/m 2 , LE of 0.20 cd/A, and EQE of 0.81%. The addition of the TPBI layer in DiodeIII for QXF2 showed a brightness of 1620 cd/m 2 , LE of 0.91 cd/A, and EQE of 1.52%. The brightness improved by a factor of 5and the EQE showed an increase of 0.7% in Diode III relative toDiode II for QXF2. Similar improvements between Diode II andDiode III were shown for QXF5 and QXF15. These improvementsin luminance, LE, and EQE between Diode II and DiodeIII for all molar ratios are due to the low lying IP of TPBI whichaides in the containment of charge recombination to the emissivelayer.All molar ratios exhibited an increase in efficiency and brightnesswhen TPBI was added to the device architecture. However,64 CMDITR Review of Undergraduate Research Vol. 2 No. 1 Summer 2005