ITO/PEDOT

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. 

The same group recently reported that the TBB defects can be brought below the nuclear magnetic resonance (NMR) detection limit by employing similar polymerization conditions (i-BuOK in THF at room temperature) in the synthesis of naphthyl-substituted PPVs 51-53 [112]. Although the absorption and PL spectra of all three polymers are similar, the EL can be finely tuned between 486 nm (for 52) and 542 nm (for 53). The external QE (studied for ITO/PEDOT/polymer/Ba/Al device) is also sensitive to the substituents pattern in the naphthyl pendant group 0.08% for 51, 0.02% for 52, and 0.54% for 53. 

SCHEME 2.15 Dependence of the electroluminescence of alkoxyphenyl-PPV copolymers on the comonomers ratio (PLED configuration ITO/PEDOT or PANI/polymer/Ca or Yb/Ag). 

Likewise, copolymers 90 have higher EL efficiency than homopolymers DHF-PPV 57 or MEH-PPV 13, due to more balanced charge injection and transport properties. Copolymer 90 with 7.5 wt% loading of dialkoxyphenylene comonomer gave the highest QE. The device with an ITO/PEDOT/90/Ca configuration showed remarkably higher efficiency (2.4cd/A) than devices fabricated with other copolymers in the series (0.65-1.0cd/A) [141],... 

The diphenylamino-substituted PPV107, with solubilizing alkoxy groups, was synthesized by Shi and Zheng [150] via Wittig-Horner reaction (Chart 2.23). Its PL (555 nm) is very similar to that of diamino-PPV 64 and dialkoxy-PPV 14 homopolymers. The PLQY is rather high in solution (80%), but it drops to only 8% in films. Consequently, only a moderate EL efficiency of 0.6cd/A was obtained with this material (device ITO/PEDOT/107/Mg/Al) [150]. Almost simultaneously, Kido and coworkers [151] reported a similar diphenylamino-substituted PPV copolymer 108, which affords very efficient PLEDs. The device ITO/PEDOT/108/Ca/Al... 

Ma and coworkers [154] synthesized a bipolar luminescent PPV-based polymer 111, which contained both donor triarylamine and acceptor oxadiazole moieties in the backbone. A device fabricated with this polymer (ITO/PEDOT/111/CsF/Al) showed a maximum brightness of 3600 cd/m2 and a maximum luminescent efficiency of 0.65 cd/A (< el = 0.3%), about 15 times brighter and more efficient than the device of the same configuration with a nonoxadiazole polymer 112. 

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). 

The ultraviolet and x-ray photoelectron spectroscopy (UPS and XPS) measurements are used to calculate /P of PFO at — 5.6 +0.05 eV, and the band gap at 3.1 +0.1 eV, which is also much closer to the optical band gap than to the value deduced from the electrochemistry in films [254]. Thus, the HOMO LUMO levels of PF can be reasonably well-matched by work functions of ITO/PEDOT (—5.1eV) and Ca electrode (ca. — 2.9 eV), respectively. However,... 

FIGURE 2.18 The efficiency-luminance plot for ITO/PEDOT/228/Ca device. The numbers along the curve are the CIE coordinates at the corresponding brightness levels. The insert shows the EL emission spectrum of a diode driven at 3.6 V. (From Miteva, T., Meisel, A., Knoll, W., Nothofer, H.G., Scherf, U., Muller, D.C., Meerholz, K., Yasuda, A., and Neher, D., Adv. Mater., 13, 565, 2001. With permission.)... 

Star-like PFs 236 with a silsesquioxane core have been prepared by Ni-mediated copolymerization of 2,7-dibromo-9,9-dioctylfluorene with octa(2-(4-bromophenyl)ethyl)octasilses-quioxane [333]. The polymer is thermally stable up to 424°C (TGA). In both chloroform solution and films, its absorption and PL spectra are very close to that for PFO 196, although a somewhat higher PL efficiency is observed in films (64 and 55%, respectively). The polymer 236, however, demonstrates a better PL color stability during thermal annealing. An ITO/PEDOT/236/Ca/Ag device can be turned on at 6.0 V, and shows a brightness of 5430 cd/m2 (at 8.8 V) with F] =0.44%, almost twice as high as that for the corresponding PFO device (Chart 2.60). 

Since the ionization potential of 238 matches closely the work function of PEDOT (5.1-5.3 eV) [335], the hole injection is dramatically improved. Accordingly, the device ITO/PEDOT/237 238(7 3)/Al has a significantly improved EL efficiency, tjel= 1.5cd/A, two orders of magnitude higher than that of single-layer PLED with 237, six times higher than that of bilayer PLED with triarylamine polymer HTL, and almost twice as high as that of PF blends with low molecular triphenylamine HT materials (in device with Ca electrode) [321]. 

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