Hao-En Tseng

Molecular oxygen and moisture as traps in poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene]: locations and detrapping by chain relaxation

We investigate locations of molecular oxygen and moisture (O2/H2O) serving as traps in poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) by measurements of field-induced thermally stimulated current, photoexcitation thermally stimulated current (PE-TSC), and dynamic mechanical analysis. Two broad peaks of PE-TSC are found at 212 and 325 K in the samples with O2/H2O-rich MEH-PPV, which are in reasonable agreement with those of the side chain and main chain relaxation, respectively. These traps can be attributed to the presence of O2/H2O (most likely molecular oxygen) in the side chain region and the amorphous main chain region, and the detrapping process is induced by chain motions under elevated temperatures.

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.

Measurements of charge mobility and diffusion coefficient of conjugated electroluminescent polymers by time-of-flight method

Charge mobilities (μ) and diffusion coefficients (D) of hole (μh, Dh) and electron (μe, De) of the conjugated electroluminescent polymers, poly(phenylene vinylene)s and polyfluorenes, have been measured by fitting of a theoretical photocurrent transient equation to time-of-flight photocurrent transients. The μ so obtained are in agreement with those from inflection points of photocurrent transients. The D value lumps all factors together that cause the dispersion of carriers, and the parameter Dq/μkT can be used as an indicator of the degree of dispersion. This fitting method allows extracting μ and D from highly dispersive photocurrent transients, even for the case in which no inflection point appears.

Determination of trap polarity in conjugated electroluminescent polymer by photoexcitation thermally stimulated current method

We succeed in using a photoexcitation thermally stimulated current (TSC) technique for a determination of trap states for poly[2-methoxy-5-(2’-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV). By use of this method, the trap polarity can be easily and clearly assigned. We provide direct evidence that molecular oxygen can form electron traps in MEH-PPV, while moisture cannot result in detectable hole traps. An unusual negative field-dependent TSC is observed for detrapping electron current, indicating a lowering of electron mobility under increased electric field. The state of observed hole trap is not affected by an exposure to ambient air and by a change of morphology, but can be attributed to extrinsic impurities or structure defects.

Determination of aggregates as charge trapping and recombination centers in poly[2 -methoxy -5-(2' -ethylhexyloxy)1,4 -phenylene vinylene] by time-resolved electroluminescence spectroscopy

A presence of aggregates can give critical influences on photophysical properties of conjugated polymer. For poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene], the authors found that the aggregates can serve as charge traps and recombination centers by using time-resolved electroluminescence spectroscopy, and that the emissions from isolated chains and aggregates show different dynamics. The authors also found that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of aggregates lie within the band gap of isolated chains. Thus, the aggregates act as shallow traps for both electron and hole so that a reexcitation to HOMO and LUMO of isolated chains is possible.

GaN Schottky diodes with single-crystal aluminum barriers grown by plasma-assisted molecular beam epitaxy

GaN-based Schottky barrier diodes (SBDs) with single-crystal Al barriers grown by plasma-assisted molecular beam epitaxy are fabricated. Examined using in-situ reflection high-energy electron diffractions, ex-situ high-resolution x-ray diffractions, and high-resolution transmission electron microscopy, it is determined that epitaxial Al grows with its [111] axis coincident with the [0001] axis of the GaN substrate without rotation. In fabricated SBDs, a 0.2 V barrier height enhancement and 2 orders of magnitude reduction in leakage current are observed in single crystal Al/GaN SBDs compared to conventional thermal deposited Al/GaN SBDs. The strain induced piezoelectric field is determined to be the major source of the observed device performance enhancements.