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The morphological, rheological and electrical properties of nanocomposites based on multi-walled carbon nanotubes (MWNTs) and high density polyethylene (HDPE), prepared by melt mixing, have been investigated. The effects of temperature, MWNT concentration and nanotube surface treatment have been analyzed. A good dispersion of oxidized nanotubes in the HDPE matrix was found using electron microscopy investigations, as has also been found in the case of related nonfunctionalized MWNTs. The rheological oscillatory shear tests in the melt state have been carried out with strain amplitude values within the linear viscoelastic region. The rheological measurements on the nonfunctionalized MWNT/HDPEs revealed that the percolation threshold was temperature dependent with lower values at higher temperatures. At low frequencies, the oxidized MWNTs/HDPE composites showed storage modulus and complex viscosity values lower than the corresponding values measured for nanocomposites prepared with nonfunctionalized MWNTs. The conductivity decrease is more than 2 orders of magnitude for the sample with 2.5 wt% oxidized MWNTs, compared to the un-oxidized comparator materials. A constant voltage, long-term stability test allowed to estimate the average number of electrically active percolation paths and showed that stepwise conductivity loss is preceded by a strong increase in the noise amplitude related to the electrical current. POLYM. ENG. SCI., 57:665–673, 2017. © 2017 Society of Plastics Engineers

The concentrating photovoltaic system represents one of the most promising solar technologies because it allows a more efficient energy conversion. When a CPV system is designed, the main parameter which has to be considered is the concentration factor that affects both the system energy performances and its configuration. An experimental characterization of a CPV system previously realized at the University of Salerno, is presented in this paper considering several aspects related to the optical configuration, the concentration factor and the solar cell used. In particular, the parameters of an Indium Gallium Phosphide/Gallium Arsenide/Germanium triple-junction solar cell are investigated as function of the concentration factor determined by means of an experimental procedure that uses different optical configurations. The maximum concentration factor reached by the CPV system is 310 suns. The cell parameters dependence on the concentration is reported together with an electroluminescence analysis of the Indium Gallium Phosphide/Gallium Arsenide/Germanium cell. A monitoring of the electrical power provided by the system during its working is also presented corresponding to different direct irradiance values. A mean power of 2.95 W with an average efficiency of 32.8% is obtained referring to a mean irradiance of 930 W/m 2 ; lower values are obtained when the irradiance is highly fluctuating. The concentrating photovoltaic system electric energy output is estimated considering different concentration levels; the maximal obtained value is 23.5 W h on a sunny day at 310×. Finally, the temperature of the triple-junction solar cell is evaluated for different months in order to evaluate the potential annual thermal energy production of the concentrating photovoltaic system.

A new type of tissue, based on the combination of tobacco cells and multi-walled carbon nanotubes exhibited good mechanical stability even after more than 2 years of room temperature storage. The original intermediate value of electrical conductivity in coplanar configuration, decreased strongly after high electric field stress before the storage period. In the visible wavelength range optical transmission values between 0.1% and 1% have been measured for the more than 100 μm thick sample. The bio/nano-composite in the low-conductivity state revealed to be a sensitive photoreceiver in the visible wavelength range, when illuminated with white LED light. A notable decrease of the low-frequency current noise amplitude with LED illumination has been observed.

In this work the preparation of flexible polymeric films with controlled electrical conductivity, light transmission and surface wettability is reported. A drop casted graphene oxide thin film is photo-reduced at different levels by UV light or laser irradiation. Optical microscopy, IR spectroscopy, electrical characterization, Raman spectroscopy and static water contact angle measurements are used in order to characterize the effects of the various reduction methods. Correlations between the optical, electrical and structural properties are reported and compared to previous literature results. These correlations provide a useful tool for independently tuning the properties of these films for specific applications.

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.

Rheological and electrical properties of nanocomposites based on multi-walled carbon nanotubes (MWNTs) and high density polyethylene (HDPE), prepared by melt mixing in a micro-twin screw extruder, have been investigated. The effect of MWNT concentration (0.5 and 2.5 wt %) and nanotube surface treatment (oxidative treatment in a tubular furnace at 500°C for 1 hr in a 95% nitrogen, 5% oxygen atmosphere) has been analyzed. It has been found that the sample conductivity with oxidation of the nanotubes decreases more than 2 orders of magnitude.Scanning electron microscopy showed good adhesion and dispersion of nanotubes in the matrix, independently of the surface treatment. Electrical and rheological measurements revealed that the oxidative treatment, causing some reduction of the MWNT quality, decreases the efficiency of the nanotube matrix interaction.

Carbon nanotubes addition to polymer and epoxy material allows to realize a large variety of new types of sensors and actuators. For the development of these devices, a deeper understanding of the basic charge carriers transport mechanisms is needed and low-frequency noise spectroscopy can effectively contribute to this task. The dc and electrical noise characteristics of different polymer/carbon nanotubes composites are analyzed at temperatures from 10 to 300 K. It has been found that a random tunnel junctions resistive networks model describes all the systems under test. A crossover from a two-level tunneling mechanism, at low temperatures, to resistance fluctuations induced by percolative paths, in the high-temperature region, has also been observed. This behavior of the 1/f noise is a general feature of the investigated highly conducting nanocomposites, independent of the specific polymer matrix and nanotube concentration.

Some commercially available solar panels with very high efficiencies for terrestrial photovoltaic applications are based on the amorphous silicon on crystalline silicon material system. This type of heterostructure has more than 40 years' old history. The early development of the technology and the results, obtained in the last years with this type of solar cell are reviewed. In particular it is demonstrated why the physical understanding of the interface properties and band-structure was important for the development of high efficiency solar cells.

The effect of the electrical field on the conductivity of linear low-density polyethylene/carbon nanotubes/ (LLDPE/CNT) composites during temperature cycling has been investigated. Under applied voltage a positive resistivity temperature coefficient was observed during heating already at low (2–3 wt%) CNT content, followed by a large negative temperature coefficient during cooling whose value depends on the applied voltage. The resistivity values after thermal cycling were markedly lower, while they slightly increased in the absence of an electrical field. The effects of thermal cycling on structural and physical properties of the composites have been evaluated by X-ray diffraction and differential scanning calorimetry.

Carbon nanotubes added to polymer and epoxy matrices are compounds of interest for applications in electronics and aerospace. The realization of high-performance devices based on these materials can profit from the investigation of their electric noise properties, as this gives a more detailed insight of the basic charge carriers transport mechanisms at work. The dc and electrical noise characteristics of different polymer/carbon nanotubes composites have been analyzed from 10 to 300 K. The results suggest that all these systems can be regarded as random resistive networks of tunnel junctions formed by adjacent carbon nanotubes. However, in the high-temperature regime, contributions deriving from other possible mechanisms cannot be separated using dc information alone. A transition from a fluctuation-induced tunneling process to a thermally activated regime is instead revealed by electric noise spectroscopy. In particular, a crossover is found from a two-level tunneling mechanism, operating at low temperatures, to resistance fluctuations of a percolative network, in the high-temperature region. The observed behavior of 1/f noise seems to be a general feature for highly conductive samples, independent on the type of polymer matrix and on the nanotube density.

A biodegradable low-cost energy storage material has been obtained by using a self-assembly nanocomposite gelatin and graphene flakes. The blends with various ratio of graphene and gelatin have been prepared and characterized by Raman spectroscopy, cyclevoltammetry measurements, chronopotentiometry measurements and impedance spectroscopy. The ability of the biopolymer to bind the water–glycerol molecules and the graphene flakes leads to an improvement of the dielectric properties with a value of the surface capacitance of about 3 orders higher (10 mF cm −2 ) than that measured for the device based on activated carbon. An electrical oxidation at the anode contact has been observed by applying to the device a higher bias voltage. The modified active material exhibits an enhancement of the cycle stability with a further increase of the surface capacitance (100 mF cm −2 ) and of the respective specific capacitance up to a value of about 380 F g −1 .

In this paper, we discuss the conditions under which we can correctly extract a variety of module parameters of single mode bidirectional optoelectronic duplexer and diplexer modules during relatively slow thermal cycling between -40/spl deg/C and +85/spl deg/C. The obtained values of parameters such as tracking error, optical crosstalk, and the laser diode characteristic temperature have been compared for bidirectional modules from different manufacturers and the impact on these parameters of different interconnect techniques, like microoptic (Mo), silicon optical bench, and planar lightwave circuit technologies is discussed. It is shown that there is a conflict between small threshold currents and good temperature stability for InGaAsP-based Fabry-Perot laser diodes emitting at 1300 and 1550 nm. Then the results of spatially low coherence reflectometry for the different bidirectional module types are presented.

A HDPE/CNT composite has been fabricated based on a commercial polyethylene loaded with a high concentration of CVD grown multiwalled carbon nanotubes with a high aspect ratio. Exploiting the resulting high conductivity of this composite material, we tested the possibility to use it as absorber of electromagnetic waves at microwave frequencies up to 25 GHz. For this purpose the device under test was placed as substrate of a microstrip transmission line. Measurements of the scattering parameters, done with a vectorial network analyzer, have been used in order to obtain the power absorption at different frequencies. Special attention was paid to the optimization of the microstrip geometry. A comparison of the microwave power absorption spectrum with those of commercial cavity absorbers showed encouraging results, regarding the microwave absorption capability of the HDPE/CNT composite material.

Temperature is one of the most specific external parameters that can accelerate the degradation rate in polymer:fullerene solar cells. To detect modifications of the active layer materials, electric noise spectroscopy is a sensitive experimental technique. A detailed characterization of the dc electric transport and voltage–noise properties shows how thermal ageing is detrimental for the investigated bulk heterojunction photovoltaic system. In particular, an increase of the energy barrier height at the interface between the metal contact and the blend, and a simultaneous decrease of the charge carrier zero-field mobility, evaluated through the analysis of the flicker noise component, are observed as a consequence of a thermal treatment. These effects can be related to morphological changes of the solar cell active layer and interface, and are revealed by monitoring the noise level.

The sensitivity of vertical cavity surface-emitting lasers to electrostatic discharge (ESD) pulses has been investigated under human body model test conditions. Very similar degradation behavior has been found for vertical-cavity surface-emitting lasers (VCSELs) from two different manufacturers, both with proton-implantation for lateral current confinement. For all investigated devices we observed during forward bias stress that the optical degradation precedes the electrical degradation and the forward bias damage threshold pulse amplitudes were only slightly higher than the reverse bias values. At the initial stage of the VCSEL degradation, damage of the upper p-DBR mirror region has been observed without modification of the active layer. During the ESD tests we monitored the electrical and the optical parameters of the VCSELs and measured during forward bias stress additionally the optical emission transients. The optical transients during ESD pulsing enable a fast evaluation of the damage threshold and give also an indication of the time scale of the junction heating during ESD pulses.

Human Body Model ESD tests on two commercially available different types of wavelength division multiplexing bidirectional optoelectronic modules, intended for data transmission over multimode optical fibers at 820 and 1320 nm wavelengths, have been performed. These bidirectional modules incorporated pin diode receivers and light emitting diode in a single package. In particular, the 820 nm light emitting diodes had rather high damage threshold pulse amplitudes of about +10,000 V under forward bias and −5000 V under reverse bias conditions, and no active layer degradation has been observed. In-situ measurements of the optical transients emitted during forward bias ESD pulses enabled a damage pulse threshold detection of a LED without the need of further optoelectronic characterization. The receiver photodiodes, however, were very sensitive to reverse bias ESD pulses. The InGaAs photodiodes degraded during ESD pulses with amplitudes as low as −150 V and in the case of the Silicon photodiodes as low as −175 V.

Here we present new materials obtained using, either fungal or isolated tobacco cells in association with different percentages of carbon nanotubes (CNTs). As a proof of concept, we used eitherCandida albicansor a non-green Tobacco BY-2 cell line combined with multi-walled CNTs. The electrical, mechanical, and conductivity vs temperature properties for some of these materials have been determined.C. albicans-based tissues have high conductivity and are stable at elevated temperatures. By lowering the CNTs content, we obtained a stable, electrically conductive optical transparent film, though with a relatively high sheet resistance. Further, we produced, using tobacco cells, a material that exhibits good electrical as well as mechanical properties.

In the present study we investigated a combination of quantum dots with multi-walled carbon nanotubes as a possible future additive to the active layer of polymer solar cells. In this case the quantum dots should serve to enhance the long wavelength response of the solar cell, while the nanotubes enhance the charge carrier collection efficiency by favoring charge carrier separation and enhancement of the lateral conduction of the films. In order to clarify the interplay of the nanoparticles only, we deposited them into a non-conducting and transparent polymethyl-methalacrylate (PMMA) matrix. InP/ZnS quantum dots with an emission peak wavelength of 660 nm have been chosen in this study, because their addition can enhance the long wavelength response of conventional poly(3-hexylthiophene) (P3HT): phenyl-C61-butyric acid methyl ester (PCBM) bulk heterostructure polymer solar cells. In our study we kept the quantum dot concentration constant and varied the concentration of the carbon nanotubes (CNTs) in the deposited films. The characterization of the film morphology by scanning electron microscopy (SEM) imaging and of the optical properties by photoluminescence and transmittance revealed a rather complex interplay between nanotubes and quantum dots. In particular we found a strong quenching of the photoluminescence and an inhomogeneous CNT distribution for carbon nanotube concentrations exceeding 1%. The decrease in optical transmittance of the films with increasing CNT concentration is less pronounced, when quantum dots (QDs) are added. The optical transmittance in a wavelength range between 380 nm and 800 nm of the composites could be expressed empirically as a simple second order polynomial function. © 2013 Elsevier B.V. All rights reserved.

An overview over typical defects and reliability aspects regarding the operation of InGaAs InP avalanche photodiodes for the use in fiber-optic communication systems is given. Measurements regarding the performance and stability of different types of top illuminated InGaAs InP Separate Absorption Grading Multiplication avalanche photodiodes have been compared with special emphasis on the homogeneity of the photoresponse. High resolution scanning optical microscopy measurements at different wavelengths allowed to identify specific technological problems. Guard ring misalignment resulting in increased responsivity of the junction perimeter have been found on devices from one manufacturer. The responsivity images of the active area of some of the investigated photodiodes showed sharp peaks due to microplasma formation. The impact of these microplasmas on dark current and photocurrent of the devices has been investigated in detail. A step-stress test performed at successively increasing temperatures resulted in the catastrophic failure of the tested devices from one family. An analysis of the dark current-voltage characteristics before and after degradation leads to the conclusion that a modification of the semiconductor-insulator interface properties caused a dramatic increase in the surface leakage currents.

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 degrees 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.

We report on electrical transport and noise processes of Epoxy/multi-wall carbon nanotube composites. The dc measurements suggest that the investigated system can be regarded as a random resistor network in which the resistors are located at the junctions between carbon nanotubes and the nodes are the conducting pathways connecting different junctions, namely the nanotubes themselves. The noise-spectral density measurements reveal the existence of a threshold voltage, above which a markedly nonlinear behavior is found and is tentatively related to the specific properties of the Epoxy-mediated contact barrier among the carbon nanotubes.

A new type of Epoxy/CNT composite has been fabricated based on bisphenol A-epoxy resin, loaded with 0.5wt% of commercial multiwall carbon nanotubes. Exploiting the resulting high conductivity of this composite material, we tested the possibility to use it as absorber of electromagnetic waves at microwave frequencies up to 25 GHz. For this purpose the sample, to be tested, was placed as substrate of a microstrip transmission line. Measurements of the scattering parameters, done with a vectorial network analyzer, have been used in order to obtain the power absorption (PA) spectra. Special attention was paid to the optimization of the microstrip geometry. A comparison of the PA spectrum with the absorption spectra of commercial cavity absorbers showed encouraging results, regarding the microwave absorption capability of the new nanocomposite material.

Electrical current transport and low-frequency noise spectra of multiwall carbon nanotubes in high-density polyethylene matrix have been measured in a temperature range between 10 and 300K. The dc electrical investigations suggest that these composites can be regarded as a random resistor network, where the resistors are formed by tunnel junctions between carbon nanotubes. A crossover of the conduction from a low-field to a high-field regime is found in current-voltage characteristics. In particular, the high-field regime has a strong dependence on carbon nanotube concentration. Noise measurements reveal a standard 1/f behavior due to resistance fluctuations. However, in samples with different concentration of nanotubes, an unusual temperature dependence of the noise is observed. The samples with higher percentage of nanotubes seem to be the most promising ones for devices application, since their noise level is lower in the whole investigated temperature range.

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.

An ultra-low cost temperature sensor has been developed that uses as active sensing material an epoxy layer, mixed with a small concentration of multi-walled carbon nanotubes. A stable NTC characteristics has been measured during temperature cycling. Under Joule heating conditions, a linear current-temperature characteristics has been obtained.

The optoelectronic properties of a-Si:H films grown at different substrate temperatures between 30°C and 250°C were evaluated in-situ by contactless Time Resolved Microwave Conductivity (TRMC) measurements. The transient photoconductivity induced by 532nm laser pulses during the growth of multilayer systems composed of a-Si:H layers deposited at different temperatures made a direct comparison of these films possible. The maximum height of the TRMC signal was observed to decrease strongly with decreasing deposition temperature. Additionally, the annealing of layers deposited at low temperatures was studied by TRMC, stationary photoconductivity, IR and ESR measurements. Whereas a considerable increase of the photoconductivity and the TRMC signal and a decrease in the spin density was observed during annealing at 250°C, the IR transmission spectra remained unchanged.

At the moment there are efforts to replace the classical transparent conducting coating, Indium Tin Oxide, (or other TCO's) by a new class of materials, which are easier to handle and cheaper in mass production. Possible choices are transparent conductive coatings, which consist of polymers. In this investigation PEDOT:PSS is used. Carbon nanotubes are added in order to render the film electrically conductive. A prerequisite for its use is long-term stability. Therefore the degradation of PEDOT: PSS/ carbon nanotubes as a function of time is determined.

The electrical properties of various lead-free electrically conductive adhesives are investigated. They are intended to replace the solder, which is normally used to connect the interconnector tapes to the busbar. Compared to solder joints conductive adhesives offer the advantage of lower contact formation temperature with reasonable low contact resistances.

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.

The effect of the electrical field on the conductivity of linear low-density polyethylene/carbon nanotubes/ (LLDPE/CNT) composites during temperature cycling has been investigated. Under applied voltage a positive resistivity temperature coefficient was observed during heating already at low (2–3 wt%) CNT content, followed by a large negative temperature coefficient during cooling whose value depends on the applied voltage. The resistivity values after thermal cycling were markedly lower, while they slightly increased in the absence of an electrical field. The effects of thermal cycling on structural and physical properties of the composites have been evaluated by X-ray diffraction and differential scanning calorimetry.

We realized an InGaAs/InP pin-diode photodiode in monomode waveguide configuration. Measuring the photocurrent spectra as a function of the applied reverse bias voltage under illumination of the pin-diode with TE polarized light (1510 - 1580 nm), we observe one dominant peak, whose position shifts linearly with increasing wavelength towards higher reverse bias voltages. By comparing the peak position with the energies, calculated for the different interband transitions in the InGaAs quantum well and taking into account the polarization dependence, we assign this peak to the optical transition between the heavy-hole energy level and the second electron energy level in the quantum well. By operating the pin-diode in an resistor biased Self-Electrooptic Effect Device configuration we clearly observe bistability at a wavelength of 1550nm. The possible operation of this device as smart waveguide photoreceiver with flip-flop functionality is demonstrated.

An electroabsorption waveguide modulator with an extinction ratio for TE-polarized light at 1550 nm of more than 20 dB for a voltage swing of less than 3 V, and low insertion losses has been realized, which exploits the Wannier-Stark effect in an InGaAs-InP short period superlattice, grown by chemical beam epitaxy. Even for a fixed voltage swing of 2.6 V, a good transmission contrast has been obtained in a wide wavelength range. >

We performed step-stress human body model electrostatic discharge tests both under fonvard and under reverse bias conditions with the ESD pulse amplitude increasing in 50 V steps. In the case of the VCSEL forward bias stress also the resulting optical emission transients during the ESD tests have been measured using a fast photodiode.

An optically multistable device in waveguide configuration has been fabricated, which is based on the Wannier–Stark effect in an InGaAs/InP superlattice. It implements a waveguided version of the self‐electro‐optic effect device and it operates at room temperature at a wavelength of 1.55 μm.

Capacitance-voltage measurements are performed in order to investigate dynamical properties of the undoped layer of an amorphous silicon pin solar cell. A special MOS diode is prepared which allows the determination of the resistivity, the net carrier concentration, and the doping density in the i-layer. The C-V measurements indicate two clearly distinguishable dispersion behaviors in accumulation and inversion of the (slightly n-type doped) i-layer of the diode and inversion built-up. A numerical simulation computes the potential variation inside the sample, taking into account the trap density in the a-Si:H. A complete set of CV curves under quasistatic conditions is simulated and fitting to the experimental data yields the actual trap density. This information granted, one can calculate the generation lifetime of carriers in the depleted region of this diode biased to inversion.

The optical emission at 950 nm due to charge carrier recombination within the confining InP layers has been measured for buried heterostructure and ridge waveguide 1300 nm InGaAsP Multi Quantum Well Fabry-Perot laser diodes as a function of the temperature and of the injected current. Above laser threshold, this emission does only weakly depend on laser current for the ridge waveguide type while it increases exponentially with injection current in the case of Double Channel Planar Buried Heterostructure and Facet Selective Buried Heterostructure laser diodes. At high current densities, the 950 nm emission is mainly due to lateral leakage currents. For buried heterostructure laser diodes, a direct correlation has been found between the sublinearity of the optical emission vs. laser current characteristics and the integral of the InP emission. Second harmonic generation with a conversion efficiency below 10/sup -9/ has been observed.

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.

The hydrogen content is important to the performance of glow discharge amorphous silicon solar cells. In fact, a-Si:H was one of the first materials studied using the 15N NRA method [1]. However, there are some outstanding issues which must be addressed in order to understand the stability of this material in a stress environment (i. e. thermal annealing, radiation). The low temperature (20 - 400°C) diffusion of hydrogen in this material is also still of interest. Further, an investigation into the effects of doping on the concentration and spatial distribution of hydrogen in a-Si:H solar cell material is required.

The transient photoconductivity decay rate is monitored on crystalline silicon samples in-situ during the heating of the samples up to 250°C and subsequent cooling by measuring the changes in the microwave reflection of the silicon sample following laser pulse illumination. An irreversible change of the excess charge carrier lifetime is found, showing a much slower decay after the first heating to 250°C. Subsequent thermal cycles show only a weak temperature dependence of the excess charge carrier decay rate at different temperatures. The strong dependence of this effect on the wavelength of the exciting laser indicates that the effect is caused by a modification of a near surface region of the crystalline silicon. The dependence of the signal amplitude on the measurement temperature is used to derive the temperature dependence of the sum of the electron and hole mobilities in the crystalline silicon. Wahrend des Heizens von kristallinem Silizium auf 250°C und anschliesendem Abkuhlen auf Raumtemperatur wird in-situ die uberschusladungstragerkinetik mittels Messung der transienten Mikrowellenreflektion nach Laserpulsanregung verfolgt. Mit ansteigender Substrattemperatur wird wahrend des ersten Heizens ein kontinuierliches Anwachsen der Ladungstragerlebensdauer gefunden, wohingegen beim anschliesenden Abkuhlen nur eine geringe Temperaturabhangigkeit der Ladungstragerlebensdauer beobachtet wird. Die Abhangigkeit dieses irreversiblen Prozesses von der Wellenlange des anregenden Lichtes deutet auf eine Modifikation der Oberflache des kristallinen Siliziums hin. Des weiteren wird mittels der Messung der Amplitude des transienten Mikrowellenreflektionssignals wahrend des Abkuhlens die Temperaturabhangigkeit der Summe von Elektronen- und Locherbeweglichkeit bestimmt.