Guoli Tu

Conjugated Zwitterionic Polyelectrolyte as the Charge Injection Layer for High-Performance Polymer Light-Emitting Diodes

A new zwitterionic conjugated polyelectrolyte without free counterions has been used as an electron injection material in polymer light-emitting diodes. Both the efficiency and maximum brightness were considerably improved in comparison with standard Ca cathode devices. The devices showed very fast response times, indicating that the improved performance is, in addition to hole blocking, due to dipoles at the cathode interface, which facilitate electron injection.

Synthesis and characterization of low bandgap conjugated donor-acceptor polymers for polymer: PCBM solar cells

We report on the synthesis, characterization and photovoltaic performance of three novel semiconducting polymers based on poly[bis-N,N′-(4-octylphenyl)-bis-N,N′-phenyl-1,4-phenylenediamine-alt-5,5′-4′,7′,-di-2-thienyl-2′,1′,3′-benzothiadiazole]. They differ only in the presence and position of hexyl side-chains on the thienyl groups. T8TBT-0 has no such side-chains, they face towards the benzothiadiazole in T8TBT-in and away in T8TBT-out. Based on electron-donating triarylamine and electron-accepting dithienyl-benzothiadiazole groups, the new polymers exhibit low bandgaps and enhanced absorption in the red part of the visible spectrum. Despite their identical backbone they differ in their synthesis and photophysics: T8TBT-0 and T8TBT-in can be synthesized by direct Suzuki coupling but a new synthesis procedure is necessary for T8TBT-out. In absorption and luminescence a blue shift is induced by the inward facing, and to a lesser extent by the outward-facing side-chains. From comparison of the photophysics in solutions and films, we conclude that the addition of side-chains reduces formation of aggregates in films and that this effect is stronger for inward-facing side-chains. By blending the three polymers with PCBM in a standard photovoltaic device architecture, T8TBT-0 performs best with a power conversion efficiency (PCE) of 1.0% (under AM1.5G illumination at 100 mW cm−2) compared to 0.17% and 0.27% for T8TBT-out and T8TBT-in, respectively.

Poly(9,9-dioctylfluorene)-based conjugated polyelectrolyte: extended pi-electron conjugation induced by complexation with a surfactant zwitterion.

Adv. Mater. 2010, 22, 2073–2077 2010 WILEY-VCH Verlag G Over the past two decades semiconducting polymers have been proved to be important materials for use in optoelectronic applications. Polymers can bring the added value of solution processability and flexible chemical tailoring. Among polymers, conjugated polyelectrolytes (CPEs) combine the semiconducting properties of the conjugated backbone with the physical chemistry, in terms of solubility, self-assembly and reactivity, of polyelectrolytes. In addition, new polymer film morphologies can be explored due to CPEs amphiphilicity and capability to electrostatically interact with oppositely charged chemical species, in particular surfactants. It has been recently shown that in donor-acceptor polymer, ionic dipoles can foster the formation of long lived, weakly emissive charge-transfer states via Coulombic interactions, making CPEs very interesting materials for photovoltaic applications. The potential application of CPEs as efficient charge-injection layer in polymer light-emitting diodes (PLED) has already been explored extensively. Our work is focused on the study of ionomeric derivatives of poly(9,9-dioctylfluorene) (F8). F8 has been thoroughly characterized in the past because of its efficient blue electroluminescence and good charge-transport properties. The ability to align the F8 polymer chains enabled the fabrication of PLED with highly polarized emission. Extensive studies have been devoted to the characterization of the F8 polymorphic phases, with particular attention to the crystalline a-phase and the mesomorphic b-phase. It has been suggested that in the b-phase, adjacent fluorene units twist to give a more planar configuration of the conjugated backbone. This metastable phase has been found to show higher luminescence efficiency and improved chargetransport properties, due to the improved electronic delocalization and favourable interchain interactions. Evidence of its presence in the polymer film is the appearance of a characteristic absorption peak at 437 nm, together with a characteristic vibronic structure in the photoluminescence (PL) spectra. The long-range structural order achieved by this mesomorphic phase, extends to the mesoscale with the formation of sheetlike structures. The importance of achieving better control over the b-phase formation is shown by the higher performances achieved in PLEDs prepared with an increasing content of this phase. This has also been considered responsible for the high efficiencies of the amplified stimulated emission (ASE) in optically pumped lasing. Different methods have been proposed to increase the fraction of b-phase in the F8 polymer film including, solvent vapour-assisted deposition, thermal treatment, casting from poor solvent and additive mixtures. We have worked with the F8-based alternating copolymer of dioctylfluorene and (dibutyl sodium sulphonate)-fluorene units shown in Figure 1 (F8M). The alkyl metal counterion