Yujian You

STUDY OF LASING ACTION BASED ON FORSTER ENERGY TRANSFER IN OPTICALLY PUMPED ORGANIC SEMICONDUCTOR THIN FILMS

We present a study of optically pumped waveguide and microcavity lasers based on vacuum-deposited thin films of small molecular weight organic semiconductors. Lasing action in waveguide lasers is characterized by high output peak power (50 W), high differential quantum efficiency (70%), low lasing threshold (1 μJ/cm2), and long operational lifetime (>106 laser pulses at 100 times the threshold pump power). Microcavity laser characteristics include 3 W peak output power, 300 μJ/cm2 lasing threshold, and lifetimes of >106 pump laser pulses (operating at 6 times the threshold power). We demonstrate wavelength variability from 460 to 700 nm by changing the composition of the organic films. The confinement of excitations on the dopant molecules leads to quantum dot-like behavior such as high temperature stability of the lasing threshold, output power, and emission wavelength in the temperature range from 0 to 140 °C. The linewidth of laser emission from microcavity structures is found to be 0.2±0.1 A and is tr...

Electroluminescence color tuning by dye doping in organic light-emitting diodes

Doping a small amount of a fluorescent dye into an organic light-emitting diodes (OLEDs) can lead to significant changes in the color of luminescence and an improvement in the device properties (e.g,, quantum efficiency, lifetime, etc.). The process of energy transfer from the OLED material to the dye in these devices may involve several different processes, including carrier trapping as well as Forster and Dexter energy transfer reactions. The important parameters for each of these processes are discussed. The color purity and chromaticities of a wide range of different dye-doped OLEDs are reviewed.

Optically pumped blue organic semiconductor lasers

Lasing at 460, 485, and 510 nm is demonstrated in optically pumped, vacuum-deposited amorphous thin films of a carbazole derivative doped with Coumarin 47, perylene, and Coumarin 30, respectively. Efficient, nonradiative Forster energy transfer between host and dopant organic molecules results in low lasing thresholds (5 μJ/cm2), high differential quantum efficiencies (15%), high peak output powers (20 W), and long operational lifetimes (>105 pulses at 100 times the threshold power).

Orange and red organic light-emitting devices using aluminum tris(5-hydroxyquinoxaline)

Abstract Energy transfer in dye-doped organic light-emitting diodes (OLEDs) often occurs by Forster energy transfer processes. In many cases this energy transfer process can be very efficient, leading to fairly large Forster radii. We have recently shown that saturated red emission can be achieved by doping tetraphenylporphine (TPP) into aluminum tris(8-hydroxyquinolate) (Alq 3 ) in an ITO/TPD/Alq 3 /Mg-Ag device (TPD = N,N ′-diphenyl- N,N ′-bis(3-methylphenyl)-1,1′biphenyl-4,4′ diamine; ITO = indium-tin oxide). Predominant red emission is observed even at very low doping levels, as expected for the large Forster radius for TPP in Alq 3 (33 A). In order to increase the degree of overlap between the dopant absorption for red fluorescent dyes and the host emission we prepared a strongly red-shifted analog of Alq 3 , i.e. aluminum tris(8-hydroxyquinoxalate) (Alx 3 ). The photoluminescent efficiency for Alx 3 is significantly lower than that of Alq 3 , leading to a significant decrease in the electroluminescent quantum yield for Alx 3 -based OLEDs relative to Alq 3 -based devices. We have examined the doping of several dyes into Alx 3 , giving red and orange OLEDs.

GROWTH AND CHARACTERIZATION OF POTASSIUM-DOPED SUPERFULLERIDE THIN FILMS

Growth conditions for the formation of thin films (100–300 A) of potassium-doped superfullerides (KxC60, x>6) are examined. Thin films of these compounds are formed by depositing C60 onto a potassium precovered single crystal quartz substrate maintained at 200 K or lower, in a proportion of K:C60>12:1, followed by annealing the surface to the K-sublimation temperature (300 K). In situ measurements of electrical and optical properties are used to identify the compounds. The formation of superfullerides is confirmed by C60 doping of these phases to check for the formation of insulating K6C60 with a characteristic absorption spectrum. The absorption spectrum of the superfullerides shows distinct features corresponding to the filling of the t1g band. The presence of two superfulleride phases is suggested, a near-metallic superfulleride KxC60 (x≈11.2) and a more insulating KxC60 (x≈8–9).