The correlation between the photoluminescence intensity of a dye-sensitizer (methyl pheophorbide-a, MPP-a) and the efficiency of the singlet oxygen ( 1 𝑂2) generation is studied. The excitation spectrum of 1 𝑂2 emission at 1270 nm has one strong band at 403 nm that coincides with the Soret band of MPP-a and only a weak peak at 667 nm, which practically coincides with the strongest band of the MPP-a excitation spectrum. Therefore, the choice of an excitation wavelength for the efficient 1 𝑂2 generation should be based on direct measurements of the excitation spectra of the sensitized 1 𝑂2 emission. K e y w o r d s: singlet oxygen, photodynamic therapy, photosensitization, dye sensitizer, methyl pheophorbide-a.
A gas discharge is observed in light-emitting diodes based on an InGaN/GaN structure for currents significantly greater than the operating currents, under conditions of considerable self-heating with formation of blisters in the polymer case of the LED, localized above the emitting surface of the heterostructure. In this case, we detected threshold appearance of intense emission in the red and infrared ranges of the spectrum. We show that the major mechanism for this emission is formation of a gas-discharge plasma as a result of non-oxidative thermal degradation of the polymer, surrounding the heterostructure and current carrying contacts.Introduction. Special attention has always been focused on catastrophic degradation processes and establishing the reasons and mechanisms for failure of light-emitting diodes (LEDs) and LED-based devices [1,2]. In particular, the authors of [3], in studying the correlation between acoustic emission and reversible/irreversible changes in the electrophysical characteristics of light-emitting heterostructures based on InGaN and GaAsP during their degradation at critically high dc current densities I, for some samples detected the appearance of luminescence in the red and IR regions, comparable in intensity or even more intense than electroluminescence (EL) in the high-energy blue band. However, the mechanisms for this phenomenon were not studied in [3].The aim of this work was to study the mechanism for the appearance of low-energy luminescence in LEDs based on an InGaN/GaN structure in a polymer case made from epoxy resin or polymethylmethacrylate (PMMA) during partial degradation of the heterojunction and its formal failure when the threshold dc current I th is exceeded.Experiment. We studied commercial LEDs emitting blue, green, and white light that were based on In x Ga 1-x N/GaN heterostructures with stoichiometric indium composition x = 0.2 and 0.35 from different manufacturers and with different dimensions of the structures (320-340) × (320-340) μm, in which the contacts to the structure are made on top (Fig. 1a). The cases of the studied LEDs, of diameter 5 and 3 mm, were made from epoxy polymer [2] and PMMA.The electroluminescence spectra in the nanometer region (600-1050 nm) were recorded using a monochromator and a photoelectron multiplier. The spectra in the micrometer region (1-15 μm) were measured by a spectrometer. Due to the fall-off in sensitivity of the photoelectron multiplier, the diffraction limit of the grating of the monochromator (for λ > 1 μm) and the prism of the spectrometer (for λ < 1.8 μm), the measurements of the electroluminescence spectra in the 1.0-1.8 μm region were made with high uncertainty.The dc voltage-current characteristics were measured. For this purpose, we used a stabilized dc current source, combined GDM-8245 (Instek), DT-890F (Haoyue), and also independently a Keithley 2400 S in sweep mode.When the metal base of the LED reached a temperature of T c ~ 470 K as a result of self-heating of the active region of the heterostructure with a cu...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.