Heinz C. Neitzert

Epoxy/MWCNT Composite as Temperature Sensor and Electrical Heating Element

An epoxy/carbon nanotubes (CNTs) composite material with a low concentration of multiwalled CNTs (0.5 wt%) has been shown to be applicable in a wide temperature range (up to 160°C) as heating and temperature-sensing element. It can be prepared in any type of geometry allowing a simple application to all kinds of surfaces that have to be sensed and heated. The composite material itself and the electric contacts have demonstrated excellent stability even under extreme ambient conditions. The electrical resistivity of the composite has shown a temperature dependence consistent with the fluctuation-induced tunneling model. This model assumes that the electrical resistance of the nanotube network is dominated by the interconnections between the individual nanotubes rather than by the nanotube resistance itself.

Bio-Nano-Composite Materials Constructed With Single Cells and Carbon Nanotubes: Mechanical, Electrical, and Optical Properties

Here, we report a procedure to obtain novel artificial materials using either fungal or isolated tobacco cells in association with different percentages of carbon nanotubes. The electrical, mechanical, and optical properties of these materials have been determined. The produced bio-nano-composite materials have linear electrical characteristics, high temperature stability up to 180 °C, linear increase of the electrical conductivity with increasing temperature and, in one case, also optical transparency. Using tobacco cells, we obtained a material with low mass density and mechanical properties suitable for structural applications along with high electrical conductivity. We also present theoretical models both for their mechanical and electrical behavior. These findings report a procedure for the next generation bio-nano-composite materials.

Candida albicans/MWCNTs: A Stable Conductive Bio-Nanocomposite and Its Temperature-Sensing Properties

A Candida albicans/multiwalled carbon nanotube (Ca/MWCNTs) composite material has been produced. It can be used as a temperature-sensing element operative in a wide temperature range (up to 100 °C). The Ca/MWCNTs composite has excellent linear current-voltage characteristics when combined with coplanar gold electrodes. We used growing cells of C. albicans to structure the CNT-based composite. The fungus C. albicans combined with MWCNTs coprecipitated as an aggregate of cells and nanotubes that formed a viscous material. Microscopic analyses showed that Ca/MWCNTs formed a sort of artificial tissue. Slow temperature cycling was performed up to 12 days showing a stabilization of the temperature response of the material.

Effect of concentration on low-frequency noise of multiwall carbon nanotubes in high-density polyethylene matrix

Transport and noise measurements of multiwall carbon nanotubes in high-density polyethylene matrix are reported. In these composites current transport occurs through a random tunnel junctions network, formed by adjacent carbon nanotubes. Low-frequency noise investigations reveal a 1/f behavior induced by resistance fluctuations. An unusual temperature dependence in samples with different nanotube concentration is found. This can be explained by a transition from a fluctuation-induced tunneling mechanism to a thermally activated regime, occurring at increasing nanotube concentration and resulting in a decrease in the overall noise.

Correlation between Electronic Defect States Distribution and Device Performance of Perovskite Solar Cells

Abstract In the present study, random current fluctuations measured at different temperatures and for different illumination levels are used to understand the charge carrier kinetics in methylammonium lead iodide CH3NH3PbI3‐based perovskite solar cells. A model, combining trapping/detrapping, recombination mechanisms, and electron–phonon scattering, is formulated evidencing how the presence of shallow and deeper band tail states influences the solar cell recombination losses. At low temperatures, the observed cascade capture process indicates that the trapping of the charge carriers by shallow defects is phonon assisted directly followed by their recombination. By increasing the temperature, a phase modification of the CH3NH3PbI3 absorber layer occurs and for temperatures above the phase transition at about 160 K the capture of the charge carrier takes place in two steps. The electron is first captured by a shallow defect and then it can be either emitted or thermalize down to a deeper band tail state and recombines subsequently. This result reveals that in perovskite solar cells the recombination kinetics is strongly influenced by the electron–phonon interactions. A clear correlation between the morphological structure of the perovskite grains, the energy disorder of the defect states, and the device performance is demonstrated.

Influence of Radiation on the Properties and the Stability of Hybrid Perovskites

Organic-inorganic perovskites are well suited for optoelectronic applications. In particular, perovskite single and perovskite tandem solar cells with silicon are close to their market entry. Despite their swift rise in efficiency to more than 21%, solar cell lifetimes are way below the needed 25 years. In fact, comparison of the time when the device performance has degraded to 80% of its initial value (T80 lifetime) of numerous solar cells throughout the literature reveals a strongly reduced stability under illumination. Herein, the various detrimental effects are discussed. Most notably, moisture- and heat-related degradation can be mitigated easily by now. Recently, however, several photoinduced degradation mechanisms have been observed. Under illumination, mixed perovskites tend to phase segregate, while, further, oxygen catalyzes deprotonation of the organic cations. Additionally, during illumination photogenerated charge can be trapped in the Nuf8ffH antibonding orbitals causing dissociation of the organic cation. On the other hand, organic-inorganic perovskites exhibit a high radiation hardness that is superior to crystalline silicon. Here, the proposed degradation mechanisms reported in the literature are thoroughly reviewed and the microscopic mechanisms and their implications for solar cells are discussed.

Optical in situ characterization of isotactic polypropylene crystallization using an LED array in avalanche-photoreceiver mode

An experiment that is useful in investigating crystallinity evolution during fast cooling, comparable with cooling rates attained in industrial processes, is extremely attractive. In this paper, a setup able to quench thin polymer films while recording the sample thermal history and light intensity of a laser beam transmitted by the sample is described. A particular feature of the optical-measurement setup is the use of the light-emitting diode (LED) array as a receiver, enabling the monitoring of changes in the polarization properties as changes in light scattering of the polymer during crystallization. Furthermore, it could be demonstrated that the LED array can be used as a linear optical detector with photocurrent gain values exceeding ten when polarized slightly below reverse-bias breakdown.

Gelatin/graphene systems for low cost energy storage

In this work, we introduce the possibility to use a low cost, biodegradable material for temporary energy storage devices. Here, we report the use of biologically derived organic electrodes composed of gelatin ad graphene. The graphene was obtained by mild sonication in a mixture of volatile solvents of natural graphite flakes and subsequent centrifugation. The presence of exfoliated graphene sheets was detected by atomic force microscopy (AFM) and Raman spectroscopy. The homogeneous dispersion in gelatin demonstrates a good compatibility between the gelatin molecules and the graphene particles. The electrical characterization of the resulting nanocomposites suggests the possible applications as materials for transient, low cost energy storage device.

Proton Damage in Amorphous Silicon/Crystalline Silicon Heterojunction Solar Cells Measurement and Simulation

The applicability of amorphous silicon/crystalline silicon (a-Si:H/c-Si) heterojunction solar cells for their use in space environment was tested. The cells were subject to proton irradiation at energies between 0.8 and 4 MeV. The optical and electrical parameters were measured. The diffusion lengths, L D , were deduced from the internal quantum efficiency. A plot of 1/L 2 D vs the applied doses yielded the damage constant. This procedure was repeated for the energy range of 0.8-4 MeV. The damage constants were lower or comparable to those of diffused solar cells. As in the case of conventional solar cells a maximum was found for an energy of 1.7 MeV. The spectral response calculated by means of the AFORSHET simulation program was fitted to the experimental data, using an inhomogeneous defect distribution within the crystalline silicon substrate after irradiation. The defect density used as a fit parameter was proportional to the implanted proton dose.

Admittance measurements on a-Si/c-Si heterojunction solar cells

Hydrogenerated amorphous silicon/crystalline silicon (a-Si:H/c-Si) solar cells with areas of 1 X 1 cm are produced by deposition of a-Si:H and indium-tin-oxide (ITO) on 3-in. wafers. Three types of samples have been prepared for admittance measurements, differing in the way how the effective area is defined. The measurement geometry is either defined by cutting, by etching the ITO layer outside the 1 cm 2 active area, or by etching the ITO and the a-Si:H outside the active area. Admittance measurements reveal that the lateral conductivity of the ITO is high enough up to a frequency of 1 MHz to ensure a lateral equipotential surface. A simple equivalent network consisting of a parallel resistor-capacitor branch in series to a second resistor controls the cut sample. For the sample with just ITO layer etching, the effects of a lateral channel due to the a-Si:H layer have to be included. The finite dimensions of the sample modify the low-pass character of the channel. The sample with ITO and a-Si:H layer etching delivers the best measurement conditions. In all three cases the dispersion allows the surface doping level of the substrate to be extracted from the CV characteristics measured at 1 MHz.