Maximum Internal Quantum Efficiency Research Articles

Formation of triplet excimers and dimers in amorphous organic thin films and light emitting devices

Abstract We examine triplet excimers in neat films of the phosphorescent molecule: platinum (II) (2-(4′,6′-difluorophenyl)pyridinato-N,C2) acetyl acetate (FPt1). This molecule was chosen because of its square planar structure, allowing it to facially pack in a crystal with an intermolecular separation of only 3.4±0.1 A, thereby facilitating excimer formation between adjacent molecules. The transient and steady-state photoluminescent and electroluminescent properties of FPt1 are used to understand the formation of triplet excimers ( 3 E 0 * ) and dimers ( 3 D * ) in amorphous films grown by thermal evaporation. We find that 3 E 0 * and 3 D * have peak emission intensities at 2.07 and 1.83 eV, and have lifetimes of (1.6±0.3) μs and (800±80) ns at room temperature, respectively. When films are optically pumped, triplet excitons form on monomers and then diffuse via thermally activated hopping to nearby excimer sites with an activation energy of (37±4) meV. We find that the energy transfer efficiency from the monomer to the excimer state approaches 100% at room temperature, and that the photoluminescence efficiency of the excimer is three times less than that of the monomer. Further, excitation of triplet aggregate states, or metal–metal dimers via the association of cations and anions of electrically pumped FPt1 is achieved in an organic light-emitting device, which achieves a maximum internal quantum efficiency of ∼14%. Electrical pumping also reveals dimer states unresolved in photoluminescent studies. The high efficiency of the triplet excimers of FPt1 is significant since dimeric species, especially triplets, are often associated with concentration quenching and low quantum yields.

Calculations of light trapping, responsivity, and internal quantum efficiency of In-doped silicon (n) structure

This research intends to investigate a mathematical model for In-doped silicon (n) structure, and calculate the absorbance of the simulated cell, then study the effect of impurity photovoltaic effect on the responsivity and internal quantum efficiency using the Shockley–Read–Hall model. It is found that the internal flux inside the simulated Lambertian cell could be enhanced as much as 25 times as a result of light trapping. Maximum responsivity and internal quantum efficiency of the simulated cell was obtained at a wavelength around 1 μm and 10 17 cm −3 indium concentration. Near infrared response of the simulated cell is improved due to the enhancements of sub-band gap response by indium doping. To compare the mathematical model with the practical results, few samples of In-doped n-type silicon structures have been fabricated using the thermal vacuum resistive technique. Maximum responsivity and maximum internal quantum efficiency are obtained at wavelength 0.9–1 μm and 3.96×10 17 cm −3 indium concentration. The results of the simulated and practical cells agree.

Ten-Milliwatt Operation of an AlGaN-Based Light Emitting Diode Grown on GaN Substrate

Ten-milliwatt output at 352 nm wavelength is obtained in the room-temperature cw operation of an AlGaN-based ultraviolet light emitting diode. The maximum internal quantum efficiency is estimated to be more than 80%. We also demonstrate the excitation of fluorescence of three basal colors by this wavelength.

Ten-Milliwatt Operation of an AlGaN-Based Light Emitting Diode Grown on GaN Substrate

Ten-milliwatt output at 352 nm wavelength is obtained in the room-temperature cw operation of an AlGaN-based ultraviolet light emitting diode. The maximum internal quantum efficiency is estimated to be more than 80%. We also demonstrate the excitation of fluorescence of three basal colors by this wavelength.

Infrared response and quantum efficiency of In-doped silicon (n) structure

In this paper the indium will be examined to be used as an electrical and optical dopant with the n-type silicon wafer to fabricate the indium-doped silicon (n) structure. The subgap response of the resulted structure is particularly strong and extends up to wavelengths of 1100 nm and above, while the response of the structure is very poor at the visible region. The structure has a maximum external and internal quantum efficiency at awavelength equal to 1100 nm.

Bulk and surface recombination in InAs/AlAs0.16Sb0.84 3.45 μm light emitting diodes

A study of the light generation efficiency of a series of InAs/AlAs0.16Sb0.84 light emitting diodes with p-type InAs active layers is reported. The bulk low-injection radiative efficiency of the p-type material is shown to be as high as 24% at a hole concentration of 1.5×1017 cm−3 but to decrease with increasing hole concentration in a manner consistent with the dominant low-injection nonradiative processes being Auger processes with two holes in their initial states and a total rate constant of 2×10−28 cm6 s−1. The maximum internal low-drive quantum efficiency achieved in the light emitting diodes is shown to be limited by interface recombination at the InAs/AlAs0.16Sb0.84 heterojunction and reabsorption in the active layer in addition to the bulk nonradiative processes in the InAs. A maximum value of ∼9% is achieved.

Naphthalimide side-chain polymers for organic light-emitting diodes: Band-offset engineering and role of polymer thickness

We report the fabrication of efficient green light-emitting diodes using a side-chain random polymer based on a high electron affinity (EA) naphthalimide moiety (PNI). The chromophore is attached to a polymethacrylate backbone through a spacer, and emits in the green with high efficiency (30% photoluminescence quantum yield). In single-layer light-emitting diodes (LEDs), we find that the electroluminescence quantum efficiency is not limited by Al cathodes as for poly(p-phenylene vinylene), PPV, and we attribute this to the increased EA. We also report maximum internal quantum efficiencies of about 1.7% for Ca and 0.9% for Al in double-layer devices where PPV serves as both hole injector and emitter. Compared to some oxadiazole based electron injection/transport layers, PNI gives higher efficiencies at high currents, and longer lifetimes. Tuning of emission in the red is possible by dye doping (at high concentration) the PNI and causing the emission to happen in this layer. We discuss the properties of the different device configurations from the viewpoint of the electronic structure of the materials and, in particular, the influence of the thickness of the individual layers on both quantum (photon/electron) and luminous (Lumen/W) efficiency and driving conditions. Unexpectedly, we find that not only does the dye doping of PNI redshift the emission spectrum, but also affects significantly the charge transport properties, and in particular reduces the driving field necessary for electroluminescence in both single and double-layer LEDs.

Naphthalimide polymers for organic light-emitting diodes

We used a side-chain polymer based on a high-electron affinity (EA) naphthalimide moiety (PNI), to fabricate single and double-layer light-emitting diodes (LEDs) with improved efficiency in the green spectral region. The chromophore is attached to a polymethacrylate backbone through a spacer and is characterised by a 30% photoluminescence quantum yield. In single-layer light-emitting diodes we find that the electroluminescence efficiency is not limited by A1 cathodes as for poly( p-phenylene vinylene), PPV, as expected from consideration of the EA. We also report maximum internal quantum efficiencies of about 1.7% for Ca and 0.9% for Al in double-layer devices where PPV serves as both hole-injector and emitter. Tuning of emission in the red is possible by dye-doping (at high concentration) the PNI and causing the emission to happen in this layer. Unexpectedly, not only does the dye-doping of PNI red-shifts the emission spectrum, but also affects significantly the charge transport properties and in particular reduces the driving field necessary for electroluminescence in both single- and double-layer LEDs and we propose this effect as one of the factors for the lifetime increase upon doping recently reported in the literature.

Ultraviolet-sensitive, visible-blind GaN photodiodes fabricated by molecular beam epitaxy

GaN p–i–n photovoltaic diode arrays were fabricated from epitaxial films deposited on sapphire by molecular beam epitaxy. Peak UV responsivity was 0.11 A/W at 360 nm, corresponding to 48% internal quantum efficiency. Visible rejection over 400–800 nm was 3–4 orders of magnitude. Typical pulsed time response was measured at 8.2 μs. Spectral response modeling was performed to analyze the photocurrent contributions from photogenerated carrier drift in the depletion region and from minority carrier diffusion in the p and n layers. With the model, a maximum internal quantum efficiency of 55% at 360 nm was calculated for the photovoltaic diode structure.

Properties of GaP Schottky barrier diodes at elevated temperatures

GaP Schottky barrier diodes were studied between 25 and 500°C. A Ni-Ge alloy provided good ohmic contacts over this entire temperature range whereas Cr or Ni were satisfactory metals for the Schottky barriers. The slope of 1n I vs. V plots was 1.1 at all temperatures. The barrier height for Cr was found to be 1.2 eV from either C-V or I s - T measurements. The barrier height did not change with the impurity concentration but was a weak function of temperature. The diodes had reverse breakdown voltages above 250 V at room temperature and showed significant rectifying characteristics up to 500°C. When these devices were used as photodiodes, they showed a maximum internal quantum efficiency of approximately 25 per cent at wavelengths around 4500 Å.