Uwe R. Kortshagen

Hybrid Solar Cells from P3HT and Silicon Nanocrystals

We are reporting new hybrid solar cells based on blends of silicon nanocrystals (Si NCs) and poly-3(hexylthiophene) (P3HT) polymer in which a percolating network of the nanocrystals acts as the electron-conducting phase. The properties of composite Si NCs/P3HT devices made by spin-coating Si NCs and P3HT from a common solvent were studied as a function of Si NC size and Si NC/P3HT ratio. The open-circuit voltage and short-circuit current are observed to depend on the Si NC size due to changes in the bandgap and surface-area-to-volume ratio. Under simulated one-sun A.M. 1.5 direct illumination (100 mW/cm2), devices made with 35 wt % Si NCs 3-5 nm in size showed 1.15% power conversion efficiency.

The 2012 Plasma Roadmap

Low-temperature plasma physics and technology are diverse and interdisciplinary fields. The plasma parameters can span many orders of magnitude and applications are found in quite different areas of daily life and industrial production. As a consequence, the trends in research, science and technology are difficult to follow and it is not easy to identify the major challenges of the field and their many sub-fields. Even for experts the road to the future is sometimes lost in the mist. Journal of Physics D: Applied Physics is addressing this need for clarity and thus providing guidance to the field by this special Review article, The 2012 Plasma Roadmap.

Silicon nanocrystals with ensemble quantum yields exceeding 60

Silicon nanocrystals with diameters of less than 5nm show efficient photoluminescence at room temperature. For ensembles of silicon quantum dots, previous reports of photoluminescence quantum yields have usually been in the few percent range, and generally less than 30%. Here we report the plasma synthesis of silicon quantum dots and their subsequent wet-chemical surface passivation with organic ligands under strict exclusion of oxygen. Photoluminescence quantum yields exceeding 60% have been achieved at peak wavelengths of about 789nm.

High-efficiency silicon nanocrystal light-emitting devices

We demonstrate highly efficient electroluminescence from silicon nanocrystals (SiNCs). In an optimized nanocrystal-organic light-emitting device, peak external quantum efficiencies of up to 8.6% can be realized with emission originating solely from the SiNCs. The high efficiencies reported here demonstrate for the first time that with an appropriate choice of device architecture it is possible to achieve highly efficient electroluminescence from nanocrystals of an indirect band gap semiconductor.

Nanoscale design to enable the revolution in renewable energy

The creation of a sustainable energy generation, storage, and distribution infrastructure represents a global grand challenge that requires massive transnational investments in the research and development of energy technologies that will provide the amount of energy needed on a sufficient scale and timeframe with minimal impact on the environment and have limited economic and societal disruption during implementation. In this opinion paper, we focus on an important set of solar, thermal, and electrochemical energy conversion, storage, and conservation technologies specifically related to recent and prospective advances in nanoscale science and technology that offer high potential in addressing the energy challenge. We approach this task from a two-fold perspective: analyzing the fundamental physicochemical principles and engineering aspects of these energy technologies and identifying unique opportunities enabled by nanoscale design of materials, processes, and systems in order to improve performance and reduce costs. Our principal goal is to establish a roadmap for research and development activities in nanoscale science and technology that would significantly advance and accelerate the implementation of renewable energy technologies. In all cases we make specific recommendations for research needs in the near-term (2–5 years), mid-term (5–10 years) and long-term (>10 years), as well as projecting a timeline for maturation of each technological solution. We also identify a number of priority themes in basic energy science that cut across the entire spectrum of energy conversion, storage, and conservation technologies. We anticipate that the conclusions and recommendations herein will be of use not only to the technical community, but also to policy makers and the broader public, occasionally with an admitted emphasis on the US perspective.

Nonthermal plasma synthesis of semiconductor nanocrystals

Semiconductor nanocrystals have attracted considerable interest for a wide range of applications including light-emitting devices and displays, photovoltaic cells, nanoelectronic circuit elements, thermoelectric energy generation and luminescent markers in biomedicine. A particular advantage of semiconductor nanocrystals compared with bulk materials rests in their size-tunable optical, mechanical and thermal properties. While nanocrystals of ionically bonded semiconductors can conveniently be synthesized with liquid phase chemistry, covalently bonded semiconductors require higher synthesis temperatures. Over the past decade, nonthermal plasmas have emerged as capable synthetic approaches for the covalently bonded semiconductor nanocrystals. Among the main advantages of nanocrystal synthesis in plasmas is the unipolar electrical charging of nanocrystals that helps avoid or reduce particle agglomeration and the selective heating of nanoparticles immersed in low-pressure plasmas. This paper discusses the important fundamental mechanisms of nanocrystal formation in plasmas, reviews the range of synthesis approaches reported in the literature and discusses some of the potential applications of plasma-synthesized semiconductor nanocrystals.

Doping efficiency, dopant location, and oxidation of Si nanocrystals

Gas-phase plasma-synthesized silicon nanocrystals (Si-NCs) are doped with boron (B) or phosphorous (P) during synthesis. The doping efficiency of B is smaller than that of P, consistent with the theoretical prediction of impurity formation energies. Despite vastly different synthesis conditions, the effect of doping on the photoluminescence (PL) of gas-phase-synthesized Si-NCs is qualitatively similar to that of Si-NCs doped during solid phase nucleation. Studies of oxidation-induced changes in PL and etching-induced changes in dopant concentration show that P resides at or near the Si-NC surface, while B is in the Si-NC cores. The oxidation of Si-NCs follows the Cabrera–Mott mechanism [N. Cabrera and N. F. Mott, Rep. Prog. Phys. 12, 163 (1948)].

On the E - H mode transition in RF inductive discharges

In inductively coupled radio frequency (RF) plasmas a mode transition between a low-power mode with dominant capacitive coupling (E-mode) and a high-power mode with dominant inductive coupling (H-mode) has been frequently reported in the literature. We investigate this transition, which results in an increase of the light emission by up to two orders of magnitude. Furthermore we observe a hysteresis of this mode transition. Two major aspects of the transition are addressed in this paper. First we propose an explanation for the discontinuous character of the mode transition. The analysis is based on a simple, electrodynamic model of the inductive discharge. This analysis shows that a certain minimum maintenance RF coil current is required for the operation of an inductive discharge and that the skin effect of the RF field is essential for stable discharge operation. This point is supported by experimental observations. In the second part of the paper we present a simple, self-consistent analytic description of the discharge, which enables us to calculate the minimum maintenance RF coil current. The theoretical results are found to be in reasonable agreement with the measured minimum maintenance coil currents.

Air-stable full-visible-spectrum emission from silicon nanocrystals synthesized by an all-gas-phase plasma approach

A novel dual-plasma system has been developed to combine the synthesis of silicon nanocrystals (Si-NCs), the etching to controllably tailor the Si-NC size, and the surface functionalization of Si-NCs into one simple all-gas-phase process. Si-NCs are synthesized in SiH(4)-based plasma; they then travel through CF(4)-based plasma, where Si-NCs are etched and passivated by C and F. The resulting Si-NCs exhibit air-stable emission across the full visible spectrum. Structural and optical characterization indicates that the emission in the red-to-green range is based on the recombination of quantum-confined excitons in Si-NCs, while the blue emission originates from defect states. The quantum yields of stabilized photoluminescence from Si-NCs range from 16% at the red end to 1% at the blue end.

Radial structure of a low-frequency atmospheric-pressure glow discharge in helium

The spatial structure of a low-frequency atmospheric-pressure glow discharge was studied experimentally. The radial current distribution and discharge light emission were simultaneously measured at different phases during the ac voltage cycle. The glow discharge is formed by a radially propagating ionization wave. We also observed discharge regimes with several current pulses per half cycle corresponding to the successive, spatially separated breakdowns.