Charge‐Carrier Balance and Color Purity in Polyfluorene Polymer Blends for Blue Light‐Emitting Diodes

Abstract

A study of an efficient blue light-emitting diode based on a fluorescent aryl polyfluorene (aryl-F8) homopolymer in an inverted device architecture is presented, with ZnO and MoO 3 as electron-and hole-injecting electrodes, respectively. Charge-carrier balance and color purity in these structures are achieved by incorporating poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)-diphenylamine (TFB) into aryl-F8. TFB is known to be a hole-transporting material but it is found to act as a hole trap on mixing with aryl-F8. Luminance efficiency of ≈6 cd A −1 and external quantum efficiency (EQE) of 3.1% are obtained by adding a small amount (0.5% by weight) of TFB into aryl-F8. Study of charge injection and transport in the single-carrier devices shows that the addition of a small fraction of hole traps is necessary for chargecarrier balance. Optical studies using UV-vis and fluorescence spectroscopic measurements, photoluminescence quantum yield, and fluorescence decay time measurements indicate that TFB does not affect the optical properties of the aryl-F8, which is the emitting material in these devices. Luminance efficiency of up to ≈11 cd A −1 and EQE values of 5.7% are achieved in these structures with the aid of improved out-coupling using index-matched hemispheres.

Highly efficient inverted polymer light-emitting diodes using surface modifications of ZnO layer

“…However, the charge injection and transport are unbalanced in iPLEDs that use ITO or fluorine-doped tin oxide (FTO) as the cathode; n-type metal oxide, such as zinc oxide (ZnO), hafnium oxide (HfO 2 ) or zirconium oxide (ZrO 2 ), as the electron-injection/transport layer; poly(9,9 0 -dioctylfluorene-cobenzo-thiadiazole) (F8BT) or poly(phenylvinylene): super yellow as the emissive layer; molybdenum oxide (MoO 3 ) or nickel oxide (NiO) as hole injection/transport layer and gold (Au) as the anode. In fact, the hole injection in this type of device indicates an ohmic contact from the MoO 3 /Au to the highest occupied molecular orbital level of the emissive layer 25,26 , whereas the electroninjection rates are fairly low because of the considerable energy barrier difference between the conduction band (CB) of the n-type metal oxides and the lowest unoccupied molecular orbital (LUMO) of the emissive layer [9][10][11][12][13][14][15][16][17][27][28][29][30] . Recently, various strategies have been applied to promote electron injection and transport by controlling the interface between the CB of the n-type metal oxide and the LUMO of the emissive layer by using an interlayer, such as ionic liquid molecules (ILMs) 27 , conjugated polyelectrolyte 28,29 , self-assembled dipole monolayer 15 Here we show highly efficient iPLEDs by introducing a spontaneously formed ripple-shaped nanostructure of ZnO (ZnO-R) and applying an amine-based polar solvent treatment using 2-methoxyethanol (2-ME) and ethanolamine (EA) cosolvents (2-ME þ EA) to the ZnO-R.…”

“…However, there are certain critical obstacles impeding the realization of highly efficient iPLEDs. In particular, the majority of the light generated in the polymeric emissive layer is fully reflected and trapped by in-plane waveguide (WG) optical modes because of the use of high-reflective-index layers such as metaloxide (n MO B1.8-2.4) as the electron-injection layer, which results in low Z e values for iPLEDs [14][15][16][17] .…”

“…One method of enhancing the out-coupling efficiency is to use a high-refractiveindex glass substrate and a hemisphere microlens thereon [16][17][18] . Moreover, a Bragg diffraction grating (BDG), a low-index grid and an internal scattering structure of the organic layer have been introduced in these devices for the light extraction of the indium tin oxide (ITO)/organic WG mode [19][20][21][22][23][24] .…”

Influence of the Molecular Weight and Size Dispersion of the Electroluminescent Polymer on the Performance of Air-Stable Hybrid Light-Emitting Diodes

“…18 The charge transport at voltages higher than V bi is thus bulk limited and is best described as pseudo-hole only space charge limited current, since the predominant carriers are holes. [24][25][26] The J-V curve can be modeled using the Mott-Gurney equation combined with field dependent mobility (equation 1):…”

Hole-enhanced electron injection from ZnO in inverted polymer light-emitting diodes

“…In Refs. [26,27] inverted PLEDs were made with ''super yellow'' and an aryl-polyfluorene homopolymer as active layers, respectively. The LUMOs of these two polymers are comparable to that of MEH-PPV.…”