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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.

Low-frequency noise spectroscopy has been used to monitor electronic properties of solar cells under temperature or radiation stress. For all the investigated polymer:fullerene, silicon and perovskite solar cells this technique evidences a clear correlation of the recombination and the transport processes with the device performances.

The wet anisotropic etching process is generally used in the field of micromachining (MEMS), particularly for commercial products such as accelerometers. Hard masks like oxide or nitride play a key role in the transfer of patterns to the substrate during the lithography process. This work reports on the use of polycrystalline graphenic carbon as an etch mask for wet chemical processing and outlines a simple method to create patterned structures on (100) silicon wafers. Graphenic carbon (GC) was deposited on the silicon substrate by chemical vapor deposition (CVD) using C2H4 as precursor. The desired pattern was written in the spin-coated negative photoresist using UV laser lithography. Different geometrical shapes were printed on the substrate with dimensions ranging from 10 to 50 micrometers. In the next stage, the O2 plasma etched away the carbon from the area not covered by the photoresist, acting as an additional mask for this and the subsequent processing steps. Finally, the sample was immersed in the KOH bath saturated with isopropanol and the etching rate was evaluated for each crystal plane. Compared to the use of a sacrificial oxide mask, this technique is simpler and produces more reliable results.

The hole mobility of a semiconducting film with a new type of small molecule has been determined by electrical characterization with two different device geometries. The small molecule, namely Zn(OC)2, consists of oxadiazole groups (O), that act as electron conductor and blue emitter, and carbazole groups (C), that are hole conductors, which are arranged around a central Zn-atom. This disordered organic material has in principal ambipolar conduction properties. In both the organic devices investigated, that is, a vertical diode and a bottom-gate organic field effect transistor, the transport is dominated by holes. In the diode structure, the charge carrier transport shows a dependence from the electric field with a space charge limited current characteristic. The hole mobility value is several orders of magnitude higher as compared to the one extracted from the characteristics of the organic transistor. This large difference in the charge carrier transport properties for the two different device configurations is due to the strong influence of the dielectric/semiconductor interface which degrades the hole transport in the organic field effect transistor.

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.

In this paper, we investigate the electrical hole transport properties of an organic/inorganic heterostructure consisting of a thin organic film, that combines hole and electron conducting molecules around a bridging Zn-atom, deposited on top of an n-type crystalline silicon substrate. Current-voltage characteristics and capacitance voltage measurements have been used for the determination of the organic layer dielectric and hole conduction parameters.

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.

Organic/inorganic heterojunctions have been fabricated by spin-coating of an oxadiazole based polyether on top of either n-type or p-type crystalline silicon substrates. The prepared heterodiodes showed good rectification. During current-voltage measurements sharp current switching effects have been observed in the reverse bias characteristics for both types of heterodiodes. In particular a very stable hysteresis in the reverse characteristics of the polyether/n-type crystalline silicon has been measured at applied voltages below 1V. The application of this heterodiode as electrical memory with an on/off-ratio of about 2 orders of magnitude has been demonstrated (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

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.

A detailed electric noise investigation of polymer:fullerene solar cells, at 300 K under dark conditions, is reported. The experimental noise results are interpreted in terms of a model taking into account the device capacitance and recombination resistance. Relevant parameters of the solar cells can be computed through fluctuation spectroscopy, and the results have been compared with those obtained by alternative techniques. After a thermal treatment at 340 K, a modification of the voltage-spectral traces has been observed and related to a strong cell resistance reduction.

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.

We report on the application of a self electro-optic effect device in waveguiding configuration for the generation of a wide variety of different nonlinear optical transfer functions. It makes use of the Wannier-Stark effect in an InGaAs/InP short period superlattice and operates at room temperature for TE-polarized light around 1.55 μm. In particular, optical bistability, optical signal-frequency multiplication and the operation as an optical power discriminator are demonstrated.

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.

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.

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. >

The authors report the room temperature operation of an InGaAs/InP based Wannier-Stark modulator in a waveguide configuration as a self-electro-optic effect device for the generation of various nonlinear optical transfer functions for TE polarised light at 1.55 mu m. Optical bistability has been achieved and the application of the device as an optical power discriminator is demonstrated. >

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.

Polarisation-independent operation of an unstrained InGaAsP/InGaAsP multiquantum well electroabsorption modulator is reported. Contrast ratios of 20 and 30 dB for 400 mu m long devices have been achieved at 2.0 and 5.5V reverse bias voltage, respectively, with on-state attenuation less that 2 dB and at 1.55 mu m wavelength. Polarisation sensitivity at these voltage values is found to vanish.

Temperature dependent dark current-voltage measurements performed on four different types of Separate Absorption Grading Multiplication avalanche photodiodes showed that the increase of the dark current can be described for all devices by a single activation energy and that the breakdown voltage increases strictly linearly with temperature, both in a wide temperature range between 25/spl deg/C and 150/spl deg/C. Three types of avalanche photodiodes showed no performance degradation during 5000 h of a temperature step-stress test at temperatures up to 150/spl deg/C, while for another device type with active layer inhomogeneities already at low temperatures a continuous lowering of the breakdown voltage, followed by a rapid increase of the dark current has been observed. During electrostatic discharge tests the avalanche diodes failed at pulse amplitudes between 700 V and 1400 V.

An electrostatic discharge sensitivity study of four different types of InGaAsAnP avalanche photodiodes revealed that destruction occurred generally during negative pulse application at pulse amplitudes between 700V and 1400V. Photoluminescence and differential phase contrast imaging has been used for failure localization. A correlation between the location of the device breakdown and the active area inhomogeneity has been found for one type of avalanche photodiode.

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.

The plasma-induced deposition of hydrogenated amorphous silicon on crystalline silicon substrates has been investigated in situ by contactless transient photoconductivity measurements. In the initial stage of the deposition process the signals reflect charge-carrier kinetics in the substrate with the surface recombination rate modified by the deposition process. In the final stage the signal is dominated by mobile electrons in the film deposited. Between these extrema an additional contribution of excess charge carriers optically induced in the amorphous silicon film and transported to the substrate is observed. It is shown that this transport is inhibited by a high defect density in the film deposited and by a highly defective interface

The signal of the Transient Microwave Detected Photoconductivity after laser pulse illumination is followed during the PECVD growth of intrinsic amorphous silicon on top of crystalline silicon (c-Si) substrates. Depending on the thickness of the amorphous layer the observed phototransients can be assigned to the excess charge carriers in different regions of the so grown heterojunction. For thin a-Si:H layers the signal amplitude decreases exponentially with deposition time. In the case of thick amorphous silicon the signal reflects only the properties of the amorphous silicon and the signal amplitude is constant. In an intermediate range a large contribution of charge carriers absorbed in the a-Si:H and subsequently transported to the crystalline part of the heterostructure to the measured signal can be observed. Due to their much higher mobility in the crystalline layer these excess charge carriers become visible only after reaching the crystalline silicon. This leads to the observed delay of the TRMC signal amplitude relative to the exciting laser pulse. The experiments show, however, that this charge carrier transfer takes place only in the case of a low defect a-Si:H layer to a measurable extent and is suppressed by the low carrier mobility in highly defective a-Si:H deposited at lower substrate temperatures.

The relaxation of pulse‐induced mobile charge carriers in a‐Si:H films is followed by transient photoconductivity measurements in the microwave frequency range (time resolved microwave conductivity measurements). It is shown that a‐Si:H films deposited at substrate temperatures lower than 150 °C are characterized by a lower electron drift mobility and a higher deep trap concentration for electrons than a‐Si:H films deposited at 250 °C, where films deposited at 150 °C have only a lower drift mobility. Annealing of a film deposited at 30 °C leads to a higher drift mobility and a lower deep trap concentration with increasing annealing temperature. However, the (opto)electronic properties of the annealed films are still inferior to those of films deposited at the annealing temperature.

The growth of n-i-p and p-i-n amorphous hydrogenated silicon structures, as used in solar cells, has been followed by measuring the transient photoconductivity with contactless microwave reflection measurements. The signals in p-layers are much lower than in intrinsic or n-type material due to the increased electron decay rate. Influences of a thin n- or p-layer on the transient photoconductivity signals during growth of a subsequently deposited intrinsic layer are discussed. The optical interference fringes can be used for in-situ determination of the deposition rate during growth.

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-D(2) 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. (c) 2006 The Electrochemical Society.

Transient photoconductivity measurements have been performed in situ during plasma-enhanced chemical vapor deposition of amorphous hydrogenated silicon by a contactless method that uses the change of the microwave reflection after laser pulse illumination. Through the use of the interference pattern of the amplitude of the transients of microwave reflection during the layer growth, the actual thickness of the amorphous film can be determined. In the case of crystalline silicon substrates, the change in the light absorption in the substrate modified by the growth of the amorphous layer is measured directly. An example of the optimization of antireflective layers on crystalline silicon substrates is shown. A good agreement is found between the experimental data and calculations of optical reflection and transmission on the multilayer structures.

During the film growth of a-Si:H the transient photoconductivity was monitored with a contactless microwave technique. Effects of defects and of space charge regions due to i-n-i interfaces were observed and briefly discussed.

It is shown that the deposition of a-Si:H films on c-Si wafers yields a reasonably passivated surface of the wafer. The passivation of the c-Si surface is better, if the a-Si:H film has better optoelectronic properties. Cleaning of the wafer surface deposition is required as it is shown for HF cleaning.