Charles D. Merritt

Photoluminescence quantum yield of pure and molecularly doped organic solid films

We present measurements of the absolute photoluminescence (PL) quantum yield, φPL, for a wide variety of organic compounds in solid films, pure and molecularly doped with strongly fluorescent materials. The procedure, which uses an integrating sphere, does not entail comparison to other standards, and provides accurate measure of the photoluminescence efficiency for submicron thick films, prepared by high vacuum vapor deposition. Host materials include N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4-4′-diamine (TPD), a common hole transport material for light emitting diodes, tris (8-hydroxyquinolinolato) aluminum (III) (Alq3) and its methyl derivative, Almq3, two aluminum chelates used as electron transport and/or green emitting materials. Dopants include tetraphenylnapthacene (rubrene) and N,N′-diethyl quinacridone (DEQ). Doping results in a substantial increase (∼a factor 2–4) of φPL in comparison with that of the pure host. For instance, measured φPL increases from 0.25 and 0.42 for pure Alq3 a...

Rebalancing of internally generated carriers for mid-infrared interband cascade lasers with very low power consumption

The interband cascade laser differs from any other class of semiconductor laser, conventional or cascaded, in that most of the carriers producing population inversion are generated internally, at semimetallic interfaces within each stage of the active region. Here we present simulations demonstrating that all previous interband cascade laser performance has suffered from a significant imbalance of electron and hole densities in the active wells. We further confirm experimentally that correcting this imbalance with relatively heavy n-type doping in the electron injectors substantially reduces the threshold current and power densities relative to all earlier devices. At room temperature, the redesigned devices require nearly two orders of magnitude less input power to operate in continuous-wave mode than the quantum cascade laser. The interband cascade laser is consequently the most attractive option for gas sensing and other spectroscopic applications requiring low output power and minimum heat dissipation at wavelengths extending from 3 μm to beyond 6 μm.

Interband Cascade Lasers With Low Threshold Powers and High Output Powers

The midwave infrared interband cascade laser (ICL) can operate at threshold power densities 30 times lower than those of the quantum cascade laser. This is ultimately attributable to the much longer interband carrier lifetime, rather than to specifics of the cavity dimensions and mirror reflectivities. The ICL is therefore an attractive candidate for insertion into the portable, battery-powered chemical sensors now being developed for this spectral region. We review the characteristics of ICLs operating at wavelengths from 2.9 to 5.5 μm, and show that their Auger coefficients vary by less than a factor of 3 throughout this range. Consequently, the ICL performance degrades only modestly with increasing wavelength. We report that an epitaxial-side-down-mounted ICL ridge of width 30 μm and λ = 3.7 μm emits more than 300 mW of continuous wave (CW) output power at room temperature with M2 ≤ 3.1. A distributed-feedback ICL with a fourth-order grating etched into its corrugated sidewalls produces 55 mW of CW power in a single spectral mode at T = 25 °C.

Significant improvement of device durability in organic light-emitting diodes by doping both hole transport and emitter layers with rubrene molecules

We have developed highly durable organic light-emitting diodes. The basic structure of the diodes is anode/hole injection layer/hole transport layer+dopant/emitter layer+dopant/cathode. Both the hole transport and the emitter layers were doped with the highly fluorescent rubrene molecules. With the doping of both layers, 85% of the initial luminance was successively maintained even after 1000 h of continuous operation under constant current driving. Doping of only one of these layers, either the hole transport layer or emitter layer, on the other hand, resulted in shorter lifetime. We mention the possible mechanisms of the doping that enhance the device duration.

Mid-IR Type-II Interband Cascade Lasers

The interband cascade laser (ICL) concept provides robust and efficient emission in the midwave infrared spectral band. While the geometry is somewhat analogous to that of a quantum cascade laser employing intersubband transitions, the ICL implementation exploits the type-II band alignment of the GaSb-based material system. A semimetallic band overlap at the boundary between the electron and hole injector regions automatically generates carriers with densities tunable by quantum confinement. Electrical injection then replenishes the carriers already present rather than creating the population inversion. In this paper, we describe and analyze the physical principles governing ICL operation, and discuss specific modifications to the active region, electron injector, hole injector, and waveguide designs that demonstrably improve the performance. The pulsed I- V and L-I characteristics of devices processed from over 50 wafers provide a statistically meaningful confirmation of the established trends.

Organic semiconductor interfaces: Discrimination between charging and band bending related shifts in frontier orbital line-up measurements with photoemission spectroscopy

Gaq3 is a promising luminescent organic semiconductor for applications in organic light emitting diodes. The frontier orbital alignment at the tris (8-hydroxyquinolinato) gallium (Gaq3)/Pt organic Schottky contact was determined by combined x-ray and ultraviolet photoemission spectroscopy (XPS, UPS) measurements. A Gaq3 thin film was deposited in several steps on a previously Ar+ sputtered pure Pt foil. After each growth step, the sample was characterized by XPS and UPS. The combination of XPS and UPS measurements allows the precise evaluation of the interface dipole independent from the simultaneously occurring band bending at the interface and charging artifacts. The measurements show that the Pt/Gaq3 interface has a strong dipole of 0.71 eV indicating the transfer of negative charge from Gaq3 to Pt. Due to the large work function difference between Pt and Gaq3, strong band bending occurred. At Gaq3 coverages higher than 128 A strong charging shifts occurred in the overlayer related emission lines which...

Molecular organic light-emitting diodes with temperature-independent quantum efficiency and improved thermal durability

We demonstrate efficient and thermally stable molecular light-emitting diodes based on a multilayered structure that consists of two hole transport layers with high glass-transition temperatures, and an emitting layer doped with a highly fluorescent dye. Stable device operation was achieved up to 90° C. By using an emitting layer doped with a highly fluorescent dye, the electroluminescence quantum efficiency reaches 2.2% and does not decrease with increasing temperature in contrast to a device with an undoped emitting layer. The luminous efficiency at 100 cd/m2 increases from 4.6 lm/W at room temperature to 7.5 lm/W at 90 °C. The half-decay time of the initial luminance (∼800 cd/m2) of the devices is 3200 h (room temperature) and 200 h (80 °C) at constant dc current (10 mA/cm2).

Continuous-wave interband cascade lasers operating above room temperature at λ = 4.7-5.6 μm

We have substantially improved the performance of interband cascade lasers emitting at λ = 4.7 and 5.6 μm, by applying the recently-pioneered approach of heavily doping the injector regions to rebalance the electron and hole concentrations in the active quantum wells. Ridges of ≈10 μm width, 4 mm length, and high-reflectivity back facets achieve maximum continuous wave operating temperatures of 60°C and 48°C, respectively. The threshold power density of ≈1 kW/cm2 at T = 25°C is over an order of magnitude lower than for state-of-the-art quantum cascade lasers emitting in this spectral range.

High-power room-temperature continuous-wave mid-infrared interband cascade lasers

We demonstrate cw output powers >290 mW into a nearly diffraction-limited (M² ≈2.2) output beam from an interband cascade laser operating at λ = 3.6-3.7 μm at room temperature. The interband cascade laser was designed for nearly equal electron and hole populations in the active region with heavy electron-injector doping, and was processed into narrow ridges mounted epitaxial side down on a copper heat sink. A 15.7-μm-wide, 4-mm-long ridge with the back facet coated for high reflection (HR) and an anti-reflection-coated front facet produced 253 mW of cw output power at T = 25°C into a beam with M² ≈2.7. Furthermore, corrugating the sidewalls of the ridge leads to a 20% improvement in the brightness. A 15.7-μm-wide, 0.5-mm-long ridge with an HR-coated back facet and an uncoated front facet exhibited a maximum cw wall-plug efficiency of nearly 15% at room temperature.

Pulsed and CW performance of 7-stage interband cascade lasers.

We report a narrow-ridge interband cascade laser emitting at λ ≈3.5 μm that produces up to 592 mW of cw power with a wallplug efficiency of 10.1% and beam quality factor of M(2) = 3.7 at T = 25 °C. A pulsed cavity length study of broad-area lasers from the same wafer confirms that the 7-stage structure with thicker separate confinement layers has a reduced internal loss of ≈3 cm(-1). More generally, devices from a large number of wafers with similar 7-stage designs and wavelengths spanning 2.95-4.7 μm exhibit consistently higher pulsed external differential quantum efficiencies than earlier state-of-the-art ICLs.