Turn-on voltage

The Schottky barriers were excellent diodes for films annealed at 600 °C, with turn on voltages of 0.6-0.8V and minimal reverse bias leakage.48 However, many of the contacts on the as-deposited films gave large reverse bias currents and nearly ohmic responses. This behavior is indicative of degeneracy of the semiconductor because of a high carrier density resulting from native defects. The improvement in the diode behavior of the annealed films is attributed to enhanced crystallinity and reduction of defects. 

EL from tetraalkoxy-substituted PPV 20, synthesized by Gilch polymerization, was recently reported [84], A multilayer device ITO/PEDOT/20/PBD/LiF/Al with PBD as an ETL emits green-yellow light with a luminescent efficiency of 0.121m/W, a maximum brightness of 8200 cd/m2, and a turn-on voltage of 5 V. 

Generally, silyl substituents seem to retard the hole-transporting ability of PPV. As a result, devices fabricated from silyl-substituted PPVs suffer from a high turn-on voltage. To improve the EL efficiency of PLEDs fabricated from Si-PPVs, the introduction of additional hole injection layer or copolymerization with electron-rich comonomers is required. 

In push pull polymer 77, both the absorption and emission maxima are red-shifted relative to 1. The LED performance of these materials appeared to be rather low (the EL efficiency of 0.002cd/A and the maximum luminance of 100cd/m2 was achieved at 30 V), and the turn-on voltage for the push-pull polymer 77 (4 V) was lower than that in more electron-deficient polymers 75 and 76. 

Blends of yellowish-green-light emissive carbazole-containing PPV-based copolymer 105 (Apl = 490, 520 nm, AEL = 533 nm) with blue-emissive oxadiazole- poly-/ -phenylene (PPP) copolymer 106 (AEL = 426 nm) allowed to tune the emission of PLEDs (ITO/polymer blend/Al) from AEL = 533 nm to AEL = 451 nm, although the device turn-on voltage was essentially higher for the blends with increased content of 106 [149] (Chart 2.22). 

Neumann and coworkers [165] synthesized tetrafluorinated-PPV copolymer 133 and studied its light-emitting properties. However, this material was quite unsuccessful for LED applications increasing the amount of fluorinated comonomer resulted in a dramatic decrease of the PLQY and the turn-on voltage of the devices was above 30 V (which could only be realized in ac mode due to device shorting). The quenching was less pronounced for an analogous copolymer with MEH-PPV (134), which showed an EL efficiency of up to 0.08 cd/A (in ITO/PEDOT/134/Ca diode) [166] (Chart 2.26). 

Several groups introduced an oxadiazole moiety as a part of the PPV backbone (polymers 139a [169,170], 139b [171], 140 [172], 141 [169], and 142 [170]). Not unexpected, the oxadiazole moieties lowered the LUMO energy of these polymers (as demonstrated by CV measurements). The decreased electron injection barrier is manifested by lowered turn-on voltage (6 V for ITO/139b/Al) [171]. However, relatively low efficiency (0.15% for 139b [171]) was reported for these copolymers (Chart 2.28). 

At Kodak, researchers used a rigid adamantane moiety to separate the luminescent oligo(phenylene vinylene) blocks (copolymers 182, 183) [213], The EL color can be tuned from blue (AEL = 470 nm) to green (AEL = 516 nm) by replacing a phenylene unit in 182 for 2,7-naphthylene (183). A very low turn-on voltage of 5.5 V (as for this class of materials) was achieved in the device ITO/182/Mg Ag, but no EL efficiency was reported (Chart 2.40). 

Kim et al. have introduced silicon atoms in PPV block copolymers to confine the conjugation length and achieve blue EL materials. Copolymers 188-190 [215] and 191 [216] have been synthesized by Wittig-Horner and Knoevenagel condensation, respectively. The emission band in this series can be tuned between 410 and 520 nm, and ITO/polymer/Al PLEDs with turn-on voltages 7 V have been reported (Chart 2.41). 

Introduction of electron-accepting hi thieno[3,2-6 2, 3 -e]pyri dine units resulted in copolymer 308 with ca. 0.5 V lower reduction potential compared to the parent homopolymer PFO 195 [398]. Upon excitation at 420 nm (A ax =415 nm), copolymer 308 exhibited blue-green emission with two peaks at 481 and 536 nm. Preliminary EL studies of an ITO/PEDOT/308/A1 device showed two peaks positioned as in the PL spectra. The PLED exhibited low turn-on voltage ( 4 V) but at higher voltages of 6-9 V, a slight increase in the green component was observed (Chart 2.83). 

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