Hsin-Hung Lu

Polyfluorenes for Device Applications

This article mainly reviews the approaches that have been proposed to improve the device performance of polyfluorene (PF)-based polymer light-emitting diodes (PLEDs). Chemical modifications on main chains, side chains, and chain ends of PFs via the incorporation of charge-transport moieties can reduce the hole- and electron-injection barriers; while physical manipulation on main-chain structures of PFs, poly(9,9-di-n-octylfluorene) (PFO), after dipping in mixed solvent/non-solvent (tetrahydrofuran/methanol) can generate β-phase with extended conjugation length, leading to a promoted charge balance and stable pure blue emission. Hole- and electron-injection barriers can be effectively lowered by the insertion of hole-transport and electron-injection layers, respectively. The recent development of PFs with various emission colors, via physical blending or chemical modification, are presented for a comprehensive understanding of PFs for device applications. The deliberate choice of cathode material with work function matching the lowest unoccupied molecular orbital (LUMO) levels of PFs is another efficient method for increasing electron flux, and is also discussed in this review.

Effect of structure ordering on charge carrier mobilities in green-emitting poly(phenylene vinylene)s

Charge transport behaviors and performances of electroluminescent (EL) devices of the two greenemitting poly(phenylene vinylene)s, the homopolymer of 2-[3′-(3,7-dimethyloctyloxy)phenylene]p-phenylene-vinylene and the copolymer of 50% by mole 2-[4′-(3,7-dimethyloctyloxy)phenylene]p-phenylene-vinylene and 50% 2-[3′-(3,7-dimethyloctyloxy)phenylene]-p-phenylene-vinylene are investigated. The former is found to have more ordered chain alignment than the latter. Such ordered alignment leads to significant increases of the mobilities of charge carriers, especially for that of an electron, which is promoted to a level equivalent to that of a hole. The balance of electron and hole mobility could be the reason for its high brightness (100 000 cd/m2) and efficiency (12 cd/A) of the EL device.

Enhancing shielding of triplet energy transfer to poly(p-phenylene)s from phosphor dopant by addition of branched alcohol for highly efficient electrophosphorescence.

To obtain an efficient electrophosphorescent device, one needs to consider quenching of phosphor phosphorescence brought by the low triplet energy of the host because the exothermic energy transfer can effectively quench phosphor phosphorescence and markedly lower the device efficiency. Here, a facile approach of adding a branched alcohol (3-tert-butyl-2,2,4,4-tetramethylpentan-3-ol, ROH) into green emission phosphor-doped dialkoxyl-substituted poly(para-phenylene)s (PPPs) is demonstrated to effectively enhance shielding of triplet energy transfer to PPPs from the phosphor, resulting from a formation of self-assembly structure that block direct contact between phosphor and the main chains. The green electrophosphorescent device performance can be improved from 7.1 and 32.2 cd/A to 25.1 and 42 cd/A for PPP with dioctoxyl substituents (dC(8)OPPP) and with carbozole (Cz)-capped dialkoxyl-substituents (CzPPP), respectively. The latter result 42 cd/A is the highest record for green emission in polymer light emitting diode. This finding suggests that promotion of specific electro-optical properties for small molecule and polymer can be obtained through a self-assembling interaction in addition to chemical structure modification.

Polyethyleneoxide/sodium dodecyl sulfate as hole-blocking/electron-transporting layer for high-performance blue polymer light-emitting diode with oxygen- and moisture-stable aluminum cathode

We present the case of the blend of polyethyleneoxide (PEO) with sodium dodecyl sulfate (SDS) as a hole-blocking (HB)/electron-transporting (ET) layer to allow the use of oxygen- and moisture-stable aluminum (Al) as the cathode for achieving high-performance polymer light-emitting diode. With inserting the PEO-SDS layer (at the weight ratio 1:1.25), the blue-emitting device with poly(9,9-di-n-octylfluorene) exhibits the maximum brightness 12 300 cd/m2 and current efficiency 2.8 cd/A, much higher than the device without this layer (0.3 cd/m2 and 0.005 cd/A) and that using CsF/Al as the cathode (5835 cd/m2 and 1.06 cd/A). This HB-ET layer can also improve the performances of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene]-based device with Al as the cathode.