Kang-Yung Peng

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.

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.

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.