Publication database

Among the parameters that characterize a solar cell and define its power-conversion efficiency, the fill factor is the least well understood, making targeted improvements difficult. Here we quantify the competition between charge extraction and recombination by using a single parameter θ, and we demonstrate that this parameter is directly related to the fill factor of many different bulk-heterojunction solar cells. Our finding is supported by experimental measurements on 15 different donor:acceptor combinations, as well as by drift-diffusion simulations of organic solar cells in which charge-carrier mobilities, recombination rate, light intensity, energy levels and active-layer thickness are all varied over wide ranges to reproduce typical experimental conditions. The results unify the fill factors of several very different donor:acceptor combinations and give insight into why fill factors change so much with thickness, light intensity and materials properties. To achieve fill factors larger than 0.8 requires further improvements in charge transport while reducing recombination.

We have observed extreme ultraviolet (EUV) spectra from terbium (Tb) ions in optically thin and thick plasmas for a comparative study. The experimental spectra are recorded in optically thin, magnetically confined torus plasmas and dense laser-produced plasmas (LPPs). The main feature of the spectra is quasicontinuum emission with a peak around 6.5-6.6 nm, the bandwidth of which is narrower in the torus plasmas than in the LPPs. A comparison between the two types of spectra also suggests strong opacity effects in the LPPs. A comparison with the calculated line strength distributions gives a qualitative interpretation of the observed spectra.

In this paper, experiments are described in which cylindrical vacuum insulator samples and samples inclined at 45° relative to the cathode were stressed by microsecond timescale high-voltage pulses and illuminated by focused UV laser beam pulses. In these experiments, we were able to distinguish between flashover initiated by the laser producing only photo-electrons and when plasma is formed. It was shown that flashover is predominantly initiated near the cathode triple junction. Even dense plasma formed near the anode triple junction does not necessarily lead to vacuum surface flashover. The experimental results directly confirm our conjecture that insulator surface breakdown can be avoided by preventing its initiation [J. G. Leopold et al., Phys. Rev. ST Accel. Beams 10, 060401 (2007)] and complement our previous experimental results [J. Z. Gleizer et al., IEEE Trans. Dielectr. Electr. Insul. 21, 2394 (2014) and J. Z. Gleizer et al., J. Appl. Phys. 117, 073301 (2015)].

We report ultra-broadband supercontinuum generation in high-confinement Si3N4 integrated optical waveguides. The spectrum extends through the visible (from 470 nm) to the infrared spectral range (2130 nm) comprising a spectral bandwidth wider than 495 THz, which is the widest supercontinuum spectrum generated on a chip.

Organic photodetectors are interesting for low cost, large area optical sensing applications. Combining organic semiconductors with discrete absorption bands outside the visible wavelength range with transparent and conductive electrodes allows for the fabrication of visibly transparent photodetectors. Visibly transparent photodetectors can have far reaching impact in a number of areas including smart displays, window-integrated electronic circuits and sensors. Here, we demonstrate a near-infrared sensitive, visibly transparent organic photodetector with a very high average visible transmittance of 68.9%. The transmitted light of the photodetector under solar irradiation exhibits excellent transparency colour perception and rendering capabilities. At a wavelength of 850 nm and at −1 V bias, the photoconversion efficiency is 17% and the specific detectivity is 10¹² Jones. Large area photodetectors with an area of 1.6 cm² are demonstrated.

Interfaces between organic electron-donating (D) and electron-accepting (A) materials have the ability to generate charge carriers on illumination. Efficient organic solar cells require a high yield for this process, combined with a minimum of energy losses. Here, we investigate the role of the lowest energy emissive interfacial charge-transfer state (CT₁) in the charge generation process. We measure the quantum yield and the electric field dependence of charge generation on excitation of the charge-transfer (CT) state manifold via weakly allowed, low-energy optical transitions. For a wide range of photovoltaic devices based on polymer:fullerene, small-molecule:C₆₀ and polymer:polymer blends, our study reveals that the internal quantum efficiency (IQE) is essentially independent of whether or not D, A or CT states with an energy higher than that of CT₁ are excited. The best materials systems show an IQE higher than 90% without the need for excess electronic or vibrational energy.

We present a compact TW-class OPCPA system operating at 800 nm. Broadband seed pulses are generated and pre-amplified to 25 μJ in a white light continuum seeded femtosecond NOPA. Amplification of the seed pulses to 35 mJ at a repetition rate of 10 Hz and compression to 9 fs is demonstrated.

The influcence of the pulse duration on the emission characteristics of nearly debris-free laser-induced plasmas in the soft x-ray region (λ ≈1-5 nm) was investigated, using six different target gases from a pulsed jet. Compared to ns pulses of the same energy, a ps laser generates a smaller, more strongly ionized plasma, being about 10 times brighter than the ns laser plasma. Moreover, the spectra are considerably shifted towards shorter wavelengths. Electron temperatures and densities of the plasma are obtained by comparing the spectra with model calculations using a magneto-hydrodynamic code.

Luminescence upconversion nanocrystals capable of converting two low-energy photons into a single photon at a higher energy are sought-after for a variety of applications, including bioimaging and photovoltaic light harvesting. Currently available systems, based on rare-earth-doped dielectrics, are limited in both tunability and absorption cross-section. Here we present colloidal double quantum dots as an alternative nanocrystalline upconversion system, combining the stability of an inorganic crystalline structure with the spectral tunability afforded by quantum confinement. By tailoring its composition and morphology, we form a semiconducting nanostructure in which excited electrons are delocalized over the entire structure, but a double potential well is formed for holes. Upconversion occurs by excitation of an electron in the lower energy transition, followed by intraband absorption of the hole, allowing it to cross the barrier to a higher energy state. An overall conversion efficiency of 0.1% per double excitation event is achieved.

Nanosecond timescale underwater electrical wire explosions of ring-shaped Cu wires were investigated using a pulsed generator with a current amplitude up to 50 kA. It was shown that this type of wire explosion results in the generation of a toroidal shock wave (SW). Time- and space-resolved optical diagnostics were used to determine azimuthal uniformity of the shock wave front and its velocity. It was found that the shock wave preserves its circular front shape in the range of radii 50 μm<r<5 mm. At r≤15μm, azimuthal irregularities of the SW front were obtained indicating the appearance of azimuthal instability. A surprising finding is that the shock wave propagates with a constant velocity of vsw=1.2M, where M is the Mach number. The dynamics of the leading part of the shock wave, based on the oblique shock wave theory, is presented, explaining the constant velocity of the shock wave.