Ching-Yang Liu

High brightness stable white and yellow light-emitting diodes from ambipolar polyspirofluorenes with high charge carrier mobility

We have used two polyspirofluorenes, spirodiethylhexyloxylpolyfluorene (spiro-diEHPF) and triphenylamine-grafted spirodiethylhexyloxylpolyfluorene (spiro-TPA50-diEHPF), which exhibit rather high charge mobilities in the order of 10−3–10−4cm2∕Vs. The light-emitting devices with rubrene-doped spiro-diEHPF show maximal luminances of 36 000 and 70000cd∕m2 with maximal efficiencies 3.5 and 9cd∕A for the white and yellow emissions, respectively. For rubrene-doped triphenylamine-grafted polymer, spiro-TPA50-diEHPF, the maximal luminances and efficiencies are 56000cd∕m2 and 9cd∕A for white-emitting devices and 72000cd∕m2 and 14cd∕A for yellow emission. Furthermore, the electroluminescence profiles of the investigated devices show nearly independent of the applied voltages.

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.

Hole mobility on isolated chains of poly(3-hexylthiophene) by microwave conductivity measurement

We demonstrate a facile method to investigate intrinsic charge mobility on isolated chains of conjugated polymers by use of microwave absorption method. Hole carriers are generated on conjugated polymer chains in dilute solution by doping with the p-type dopants NOSbF(6), instead of using excitation sources of pulsed laser and electron beam as reported in the literature. The number of hole carriers can be easily estimated by doping level. Measurements on poly(3-hexylthiophene) in benzene with various doping levels from 0.1% to 3% indicate that the hole mobility can be divided into two ranges. In the doping level 0.1%-0.3%, the hole mobility maintains at the constant level 0.03 cm(2)/V s, which can be regarded as that on an isolated chain since the average number of hole carriers per chain is only around one. As the doping level is higher than 0.3%, a presence of multiple hole carriers on a chain occurs, which results in a repulsion of hole carriers and leads to a reduced hole mobility.

Spectral response design of hydrogenated amorphous silicon p-i-n diodes for ambient light sensing

For the applications involving ambient light sensing, the spectral response of photodetectors is required to mimic that of human eyes and the cost must be low. This paper discusses the layer structure design of human-eye-like hydrogenated amorphous silicon (a-Si:H) p-i-n photodiodes. The resultant sample devices are insensitive to infrared light and have a normal spectral response in the light band ranging between 400 nm wavelength and 740 nm wavelength. The spectrum peak locates around 560 nm wavelength, similar to the case of human eyes. The devices are suitable for low-cost ambient light sensing applications.