PTs as red light emitters

Polythiophene LEDs were first reported in 1991 by Ohmori et al. [30], who described poly(3-alkylthiophenes) 9a-c (prepared by oxidation of 3-alkylthiophenes with FeCb in chloroform [19]) as red-orange-emitting material (peak emission at 640 nm for 9a) in single-layer rrO/PT/Mg In devices. Shortly afterwards, Heeger s group reported EL in poly(3-octylthiophene) 9e, which showed red-orange luminescence with of 0.025% in an ITO/9e/Ca configuration [31]. It was shown that the 

Bolognesi and co-workers prepared [3-((o-methoxy)alkylthiophene)s lOa-c by Ni-initiated polymerization of 2,5-diiodothiophenes [34, 35, 36]. A small red shift in EL of polymer 10c, compared with polymer 9e (from 1.8 to 1.95 eV), was presented as an indication of a lower bandgap in the former [31, 34], although it could be the result of asymmetry of the wide emission band (comparison with poly(3-decylthiophene) (P3DT) 9d revealed a smaller blue shift of 0.05 eV [37]. Polymers 10a,b showed high (for polythiophenes) PL quantum yields in solution (38-45 % in THE) that decreased, however, in the films [36], A general 

Another reason for generally low performance of PT homopolymers in LED applications is poor electron injection/mobility brought about by electron-rich character of these materials. This can potentially be improved by introducing electron-withdrawing substituents or moieties into the polymer structure. 

Pomerantz et al. were the first to study the effect of carboxylic groups on the PL properties of photoluminescence. Interestingly, the synthetic method [Ullmann polymerization on Cu powder or Yamamoto polymerization with Ni(0) catalyst] affects the fluorescence properties of the resulting polymer (lla,b) [38]. Both polymers revealed close molecular weights (Afn 3000), although the Cu-prepared polymers showed less defects and lower polydispersity. Consequently, PL emission maxima for the Cu-prepared polymers lla,b were red shifted compared with the Ni-prepared polymers [by 13-15 nm ( 0.05-0.06 eV) in solution and 25-30 nm ( 0.08-0.10 eV) in films. Table 19.1]. This emphasizes that the properties of the polymer depend on the preparation method and, consequently, conclusions drawn from small shifts of 0.05-0.1 eV in PL/EL energies of the materials prepared by different methods may not always be valid. 

Zotti and co-workers used electrochemical polymerization of substituted terthiophene and quinquethio-phene to produce alkoxycarbonyl-substituted polymers 12 [39] and 13 [40], respectively. Polymer 13 showed red EL (CIE 0.54, 0.45) with about one order of magnitude better performance compared with 12 and allowed a reasonable EL efficiency of 0.13 % using a high work function Al anode, which can be explained by the improved electron injection due to electron-withdrawing substituents. 

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