The emission characteristics and external quantum efficiencies of ambipolar polymer light-emitting field-effect transistors are investigated as a function of applied voltage, current density, and ratio of hole to electron mobility. Green-emitting poly(9,9-di- n -octylfluorene-alt-benzothiadiazole) (F8BT) with balanced electron and hole mobilities and red-emitting poly((9,9- dioctylfluorene)-2,7- diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3- benzothiadiazole]- 2′, 2″ -diyl) (F8TBT) with strongly unbalanced hole and electron mobilities as semiconducting and emissive polymers are compared. The current-voltage and light output characteristics of the two types of light-emitting transistors were found to be fundamentally alike independent of mobility ratio. Device modeling allowing for a single (Langevin-type) charge recombination mechanism was able to reproduce the device characteristics for both cases but could not replicate the experimentally observed dependence of external quantum efficiency on current density. The increase of quantum efficiency with current density up to a saturation value could be indicative of a trap-assisted nonradiative decay mechanism at the semiconductor-dielectric interface. Optical output modeling confirmed that the maximum external quantum efficiency of F8BT light-emitting transistors of 0.8% is consistent with complete recombination of all charges and a singlet exciton fraction of 25%.
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The authors thank Cambridge Display Technologies Ltd. for providing F8BT and F8TBT. J.Z. thanks the Gates Cambridge Trust for financial support. The work at Los Alamos National Laboratory was supported by DOE Office of Basic Energy Sciences Work Proposal No. 08SCPE973. FIG. 1. (a) Schematic device structure of a bottom contact∕top gate transistor with gold electrodes, PMMA as the gate dielectric, and F8BT or F8TBT as the semiconducting layer on a glass substrate. (b) Chemical structures and photoluminescence spectra of F8BT (left) and F8TBT (right) thin films after annealing. FIG. 2. Current-voltage characteristics of an ambipolar F8TBT transistor with L = 20 μ m , W ∕ L = 500 , and C i = 7.1 nF cm − 2 . Transfer characteristics at different (a) negative and (b) positive source-drain voltages. Output characteristics for gate voltages (c) from 0 to − 80 V and (d) from 20 to 100 V in steps of 10 V . The hole and electron mobilities at saturation for this device are 5 × 10 − 4 and 3 × 10 − 5 cm 2 V − 1 s − 1 , respectively. FIG. 3. Optical images of light emission from (a) F8BT and (b) F8TBT light-emitting FETs with interdigitated source-drain electrodes (dark areas) with L = 20 μ m . The emission zone of the F8BT LFET reflects the polycrystallinity of the polymer film, while the emission from the amorphous F8TBT is featureless. Position of emission zone with respect to source (grounded) electrode and source-drain current vs gate voltage during a transfer scan of an ambipolar (c) F8BT and (d) F8TBT transistor with L = 20 μ m . FIG. 4. (a) Source-drain current, (b) light output, and (c) external quantum efficiency vs gate voltage (forward and reverse) of a light-emitting F8BT (green) transistor with L = 20 μ m , W ∕ L = 500 , C i = 5.9 nF cm − 2 , μ e = 5 × 10 − 4 cm 2 V − 1 s − 1 , μ h = 6 × 10 − 4 cm 2 V − 1 s − 1 . (d) Source-drain current, (e) light output, and (f) external quantum efficiency vs gate voltage (forward and reverse) of a light-emitting F8TBT (red) transistor with L = 20 μ m , W ∕ L = 1000 , C i = 6.8 nF cm − 2 , μ e = 3 × 10 − 5 cm 2 V − 1 s − 1 , μ h = 6 × 10 − 4 cm 2 V − 1 s − 1 . FIG. 5. Calculated source-drain current and position of recombination zone vs V g at constant V ds for ambipolar transistors with (a) F8BT ( μ h ∕ μ e = 1.2 , V ds = 100 V ) and (b) F8TBT ( μ h ∕ μ e = 20 , V ds = − 120 V ) as in Fig. 3 . FIG. 6. (a) Calculated source-drain current, (b) light output, and (c) quantum efficiency vs gate voltage of a light-emitting F8BT (green) transistor with balanced hole and electron mobilities ( μ h ∕ μ e = 1.2 ). (d) Calculated source-drain current, (e) light output, and (f) quantum efficiency vs gate voltage of a light-emitting F8TBT (red) transistor with unbalanced hole and electron mobilities ( μ h ∕ μ e = 20 ) . FIG. 7. Constant current measurements of LFETs in Fig. 4 : (a) Source-drain voltage and (b) light output vs gate voltage for various fixed negative source-drain currents of a F8BT-LFET. (c) Source-drain voltage and (d) light output vs gate voltage for various fixed positive source-drain currents of a F8TBT-LFET. The voltage range for which a plateau of the light output is observed corresponds to the emission zone moving away from the source electrode through the channel before reaching the opposite drain electrode. FIG. 8. Light output (squares) and external quantum efficiency (triangles) vs channel-width-normalized source-drain current of (a) F8BT ( W = 1 cm ) and (b) F8TBT ( W = 2 cm ) transistors. Data points were extracted from plateau part of light output vs V g of constant current measurements, as shown in Fig. 7 . FIG. 9. (a) Experimental (open circles) and modeled (lines) transmission spectra of a multilayer stack of glass∕F8BT ( 60 nm ) ∕PMMA ( 420 nm ) ∕gold with different modeled thicknesses of gold ( 13 nm , dashed line; 15 nm , solid line; and 17 nm , dash-dot line). Calculated external quantum efficiencies based on the multilayer stack in (a) for different thicknesses of (b) F8BT, (c) PMMA, and (d) gold assuming 55% PL efficiency, 25% singlet exciton fraction, and an emitting dipole orientation of k z ∕ k x = 0.25 .