Richard J. Curry

Efficient oxide phosphors for light upconversion; green emission from Yb3+ and Ho3+ co-doped Ln2BaZnO5 (Ln = Y, Gd)

The optical properties of Yb3+ and Ho3+ co-doped Y2BaZnO5, synthesized by solid-state reactions, are investigated in detail. Under 977 nm excitation (∼25 × 10−3 W mm−2), bright green upconversion emission is observed. Concentration dependence studies at room temperature show that relatively high infrared to visible upconversion efficiencies are obtained with values up to ∼2.6%. The results of power dependence studies and temperature-dependent lifetime measurements allow us to determine the dominant upconversion mechanisms in Yb3+:Ho3+ co-doped Y2BaZnO5oxides. The materials presented in this article constitute new and efficient upconversion phosphors which may find utility in a variety of applications.

Silicon-based organic light-emitting diode operating at a wavelength of 1.5 μm

1.5-μm light-emitting diodes which operate at room temperature have been fabricated on silicon substrates. The devices use an erbium-containing organic light-emitting diode (OLED) structure which utilizes p++ silicon as the hole injection contact. The OLEDs use N, N′-diphenyl-N,N′-bis(3-methyl)-1,1′-biphenyl-4,4′-diamine as the hole transporting layer and erbium tris(8-hydroxyquinoline) as the electron conducting and emitting layer.

Oxide phosphors for efficient light upconversion: Yb3+ and Er3+ co-doped Ln2BaZnO5 (Ln = Y, Gd)

The optical properties of Yb3+ and Er3+ co-doped Ln2BaZnO5 (Ln = Y or Gd), synthesized by solid-state reaction, are investigated in detail. Two main emission bands centered around 548 nm (green) and 673 nm (red) are observed under 977 nm laser excitation via an upconversion process. Studies of the behavior as a function of dopant concentration are described and relatively high infrared to visible upconversion efficiencies of ∼5% are obtained at room temperature. Under modulated 977 nm excitation and for fixed dopant concentrations, the upconverted emission chromaticity can be varied by changing the excitation duration. The results of power dependence studies and lifetime measurements are presented. This detailed study of the upconversion processes allows us to identify the dominant upconversion mechanisms in Yb3+,Er3+ co-doped Ln2BaZnO5 oxides.

Structural and optoelectronic properties of C60 rods obtained via a rapid synthesis route

High purity single crystal C60 rods with uniform dimensions are synthesized by a rapid and facile approach which can be completed over a timescale of typically a few minutes. The morphology of the fullerene product has been characterized in detail by scanning electron microscopy, scanning transmission electron microscopy, and atomic force microscopy, demonstrating that the resulting materials are solid, hexagonal cross-sectioned rods with novel faceted tips. High resolution transmission electron microscopy investigations reveal that the rods are face-centered cubic packed single crystals. Vibrational and electronic spectroscopy studies provide compelling evidence that the rods are a van der Waals solid since the electronic structure of the component C60 molecules is largely preserved. The structures obtained are found to possess novel optoelectronic properties exhibiting low energy absorption not reported in related structures and materials to date. Furthermore significant room temperature photoluminescence is obtained from the C60 rods accompanied by a small blue shift of the spectra which is also observed for the first ‘allowed’ absorption transitions. Given their rapid synthesis, excellent purity, optical and charge transport properties these fullerene structures are expected to be a promising materials for nanoelectronic devices including thin film organic solar cells and photodetectors.

A PbS nanocrystal-C60 photovoltaic device for infrared light harvesting

PbS nanocrystal (nc-PbS)-C60 photovoltaic devices are demonstrated, in which nc-PbS function as electron donors, showing infrared photosensitivity up to 1600nm. Annealing nc-PbS is proved to remove capping oleic acid ligands, studied using x-ray photoelectron spectroscopy, significantly improving the short circuit current, open circuit voltage, and fill factor. The device performance is rationalized by quantum confinement in nc-PbS and energy level alignment at the heterojunction based on direct measurements of nc-PbS ionization potential using ultraviolet photoelectron spectroscopy.

Ultrahigh Performance C60 Nanorod Large Area Flexible Photoconductor Devices via Ultralow Organic and Inorganic Photodoping

One dimensional single-crystal nanorods of C60 possess unique optoelectronic properties including high electron mobility, high photosensitivity and an excellent electron accepting nature. In addition, their rapid large scale synthesis at room temperature makes these organic semiconducting nanorods highly attractive for advanced optoelectronic device applications. Here, we report low-cost large-area flexible photoconductor devices fabricated using C60 nanorods. We demonstrate that the photosensitivity of the C60 nanorods can be enhanced ~400-fold via an ultralow photodoping mechanism. The photodoped devices offer broadband UV-vis-NIR spectral tuneability, exhibit a detectivitiy >109 Jones, an external quantum efficiency of ~100%, a linear dynamic range of 80 dB, a rise time 60 µs and the ability to measure ac signals up to ~250 kHz. These figures of merit combined are among the highest reported for one dimensional organic and inorganic large-area planar photoconductors and are competitive with commercially available inorganic photoconductors and photoconductive cells. With the additional processing benefits providing compatibility with large-area flexible platforms, these devices represent significant advances and make C60 nanorods a promising candidate for advanced photodetector technologies.

Near infrared up-conversion in organic photovoltaic devices using an efficient Yb3+:Ho3+ Co-doped Ln2BaZnO5 (Ln = Y, Gd) phosphor

The first detailed study that combines the use of a new generation of high-efficiency Yb3+:Ho3+ co-doped Y2BaZnO5 near-infrared up-converting phosphors with organic photovoltaic devices is reported. We show that it is possible to obtain a Jsc of 16 μA cm−2 under 986 nm illumination (∼390 mW cm−2 corresponding to ∼37 suns) leading to an up-conversion external quantum efficiency (ηEQEUC) of 0.0052%. Through modification of the organic photovoltaic devices to incorporate transparent electrodes we show that ηEQEUC could be increased to 0.031 %, matching that achieved in amorphous-Si:H PV cells. Accounting for the full spectral range that may be absorbed by the phosphor (∼870–1030 nm) yields an up-conversion power conversion efficiency (ηPCEUC) of 0.073% which again could be improved to 0.45% using transparent electrodes. This technique for utilizing the near-infrared spectral region may therefore offer a potential route to improving the performance of organic photovoltaic devices as research into discovering ...

Temperature dependent optical properties of PbS nanocrystals

A comprehensive study of the optical properties of PbS nanocrystals (NCs) is reported that includes the temperature dependent absorption, photoluminescence (PL) and PL lifetime in the range of 3-300 K. The absorption and PL are found to display different temperature dependent behaviour though both redshift as temperature is reduced. This results in a temperature dependent Stokes shift which increases from ∼75 meV at 300 K with reducing temperature until saturating at ∼130 meV below ∼150 K prior to a small reduction to 125 meV upon cooling from 25 to 3 K. The PL lifetime is found to be single exponential at 3 K with a lifetime of τ(1) = 6.5 μs. Above 3 K biexponential behaviour is observed with the lifetime for each process displaying a different temperature dependence. The Stokes shift is modelled using a three-level rate equation model incorporating temperature dependent parameter values obtained via fitting phenomenological relationships to the observed absorption and PL behaviour. This results in a predicted energy difference between the two emitting states of ∼6 meV which is close to the excitonic exchange energy splitting predicted theoretically for these systems.

Infrared emitting PbSe nanocrystals for telecommunications window applications

We demonstrate the colloidal synthesis of PbSe nanocrystal quantum dots, via an organometallic-precursor route, developed from recently reported techniques. This synthesis typically yields a particle size distribution of approximately 5–10%, as may be inferred from the sharp spectral features seen in absorption and from our effective-mass model correlating spectral features to nanocrystal size. An accurate quantitative analysis, using an infrared reference dye, shows these nanocrystals to exhibit infrared photoluminescence from intrinsic quantum-confined states, with high quantum efficiencies of up to 60% in solution. The wavelength of the photoluminescence may also be conveniently size tuned in order to access the 1.3–1.5 µm ‘telecommunications window’. We discuss the significance of this work in the context of future optoelectronic applications.

Radiative recombination mechanisms in aluminum tris(8-hydroxyquinoline): Evidence for triplet exciton recombination

The photoluminescence of aluminum tris(8-hydroxyquinoline) (AlQ) has been studied as a function of temperature and excitation wavelength. It was found that as the temperature and excitation energy is reduced the peak of the photoluminescence moves to longer wavelengths and broadens significantly. The photoluminescence spectra obtained at all temperatures and excitation energies can be deconvolved into three distinct peaks originating from three levels within the molecule. A rate-equation approach has been used to model the observed behavior and to obtain the relative lifetimes of the three processes responsible for the photoluminescence. From this we infer that at low temperatures and excitation energies the radiative recombination of triplet excitons is responsible for a significant amount of the photoluminescence of AlQ. It is this process which is responsible for the low energy tail seen in the photoluminescence of AlQ but which is not present in the electroluminescence.