Gerrit Klaerner

Efficient, blue light-emitting diodes using cross-linked layers of polymeric arylamine and fluorene

Abstract Single-, double- and triple-layer polymer light-emitting diodes (PLEDs) with indium-tin oxide (ITO) anodes and Ca cathodes were fabricated, based on solution processable and thermally cross-linkable hole transporting polyarylamine and blue luminescent polyfluorene; and an electron transporting oxadiazole trimer. Triple-layer PLEDs show the highest efficiency, brightness and saturation of blue emission. These results are interpreted in terms of charge injection balance and electron-hole recombination efficiency. The maximum external quantum efficiencies achieved (>1%) are comparable to the best polymer blue PLEDs.

Color tuning in polyfluorenes by copolymerization with low band gap comonomers

Abstract Poly((9.9-dialkylfluorene-2,7-diyl) derivatives containing low band gap comonomers, have been synthesized through Ni(0) mediated Yamamoto polymerization. High molecular weight and thermally stable polymers were obtained which could be processed in common organic solvents. The absorption and fluorescence spectra of the new polymers were recorded. The fluorescence maxima of the copolymers red-shift as the band gap of the comonomer decreases. Although substituted fluorene homopolymers showed excimer emission after annealing at elevated temperature, excimer formation upon annealing was suppressed by incorporating low band gap comonomers into the fluorene main chain. In this paper, color tuning, exciton migration and trapping and the formation of excimers in poly(fluorene) derivatives is discussed.

Improved efficiencies of light-emitting diodes through incorporation of charge transporting components in tri-block polymers

9,9-Di-n-hexylfluorene-co-anthracene (DHF-ANT) random copolymer and triphenylamine (TPA)- and oxadiazole (OXA)-containing tri-block copolymers along with a crosslinked TPA polymer are used to fabricate single- and double-layer light emitting diodes (LEDs). In both single-layer and double-layer devices, TPA hole transporting components improve hole injection and transport, leading to improved device performance when low work function calcium (Ca) is used as the cathode, whereas OXA electron transporting components improve electron injection when high work function aluminum (Al) is used. This suggests that DHF-ANT copolymer and DHF-ANT blocks in the two tri-block polymers are hole-limited in devices with Ca cathodes, and electron-limited with Al cathodes. Furthermore, double-layer devices including a separate hole transporting crosslinked TPA layer increase device efficiencies by at least one order in magnitude over the corresponding single-layer devices, due to the improved charge injection, charge confinement and charge recombination.

Novel polymers based on poly(fluorene)s for LED applications

The emission color stability of poly(fluorene) derivatives upon thermal annealing or passage of current in an electroluminescent device is affected by the polymer molecular wight, the structure of the main chain polymer unit and particularly by the polymer chain end substituents. Proper attention to these features leads to colorfast blue emission in both photo and electroluminescence. Furthermore, the spectral emission characteristics can be tuned by the incorporation of various comonomers. Preliminary single layer device studies validate the potential utility of poly(fluorene) homo and copolymers for OLED applications.

Femtosecond study of exciton dynamics in polyfluorene statistical copolymers in solutions and thin films

The formation and decay dynamics of photogenerated excitons in polyfluorene statistical co-polymers in solutions and in thin films have been studied using femtosecond transient absorption spectroscopy. In solution photoexcitation of the polymer generates primarily intrachain singlet excitons which are initially hot and then relax quickly (< 200 fs) towards the equilibrium position in the excited state. The exciton subsequently decays following a double exponential with time constants of 30 ps and 330 ps in toluene. The fast decay is attributable to vibrational relaxation, spectral diffusion, or internal conversion (recombination) of the exciton from the excited to the ground electronic state through tunneling or thermal-activated barrier crossing before thermalization. The slow decay is assigned to conversion of the thermalized exciton to the ground state through both radiative and non-radiative pathways. In films the exciton dynamics are found to depend strongly on excitation intensity. At low intensity, the dynamics are similar to that in solutions, with a double exponential decay with time constants of 15 ps and 300 ps. At high intensities, a fast decay component with a time constant of 0.8 ps appears, which becomes more dominant at higher intensities. This fast decay is attributed to exciton- exciton annihilation due to high density of excitons created. The signal in films at both low and high excitation intensities is attributable to intrachain singlet excitons, as in solution. There is no evidence for formation of interchain bound polaron pairs in films at low intensities. At high intensities, the possibility cannot be ruled out completely, especially in relation to the fast decay. If bound polaron pairs are formed as indicated by the fast decay, they must be generated as a result of interaction between excitons on different chains since they are absent at low power, an they must be created and then decay within about 1 ps.