• Energy Research

The admittance of metal oxide semiconductor (MOS) diode after switching from inversion to deep depletion is calculated. Special attention is given to the final stationary state in inversion conditions. For this state the relaxation equations must be modified because the usually assumed large signal carrier deficits are not valid. The results of these dispersion measurements are compared with the conventional Zerbst technique. Good agreement is obtained. The dispersion technique is a valuable tool for determining very short lifetimes

Abstract 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.

Abstract 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.

Abstract Current–voltage under illumination and quantum yield characteristics of an amorphous silicon/crystalline silicon hetero solar cell have been measured before and after exposure to high-energy (1.7 MeV) protons. A comparison of the measured wavelength-dependent quantum yield with calculated values enabled to determine the effective electron diffusion length of the crystalline silicon, that dropped from a value of 434 μm before to a value of 4 μm after irradiation with 5×10 12 cm −2 protons. Good agreement has been obtained between measured and simulated data using DIFFIN, 1 a finite-element simulation program for a-Si:H/c-Si heterojunction solar cells, enabling us to extract the depth profile of the recombination rate and the density of states distribution in the semiconductor layers before and after irradiation.

Noise spectroscopy is essentially focused on the investigation of electric fluctuations produced by physical mechanisms intrinsic to conductor materials. Very complex electrical transport phenomena can be interpreted through the study of the fluctuation properties, which provide interesting information both from the point of view of basic research and of applications. In this respect, low-frequency electric noise analysis was proposed more than twenty years ago to determine the quality of solar cells and photovoltaic modules, and, more recently, for the reliability estimation of heterojunction solar cells. This spectroscopic tool is able to unravel specific aspects related to radiation damage. Moreover, it can be used for a detailed temperature-dependent electrical characterization of the charge carrier capture/emission and recombination kinetics. This gives the possibility to directly evaluate the system health state. Real-time monitoring of the intrinsic noise response is also very important for the identification of the microscopic sources of fluctuations and their dynamic processes. This allows for identifying possible strategies to improve efficiency and performance, especially for emerging photovoltaic devices. In this work are the reported results of detailed electrical transport and noise characterizations referring to three different types of solar cells (silicon-based, organic, and perovskite-based) and they are interpreted in terms of specific physical models.

Abstract A concentrating photovoltaic (CPV) plant is a complex system that integrates different technologies as single or multi-junction photovoltaic cells and optical devices. The CPV system performance analysis should take into account the malfunctions that can occur during the working, especially when the system operates with high values of sunlight concentration. A critical analysis of the solar cells is necessary to define the CPV system potential. In this paper a specific configuration of a CPV system is considered, and the experimental analysis of a system with a degraded triple-junction InGaP/GaAs/Ge solar cell is investigated. In particular, the triple-junction solar cell is stressed in an accelerated aging process of about 500 operating hours with a concentration of 310 suns without using a cooling system. After this process, for another 100 working hours the system has been monitored in order to compare, corresponding to different light concentration factors, the solar cell electric characteristics, the energy production and the power conversion efficiency in the pristine and degraded states. The results show the effect of the overheating of the triple-junction solar cell caused by the excessive increase of the light intensity. Under the same irradiance of 930 W/m2 the short circuit current, open circuit voltage and fill factor values of the aged solar cell, compared to the pristine device, result to be strongly reduced, while the extracted value of the series resistance increases and the values of the shunt resistances decrease. The increased value of the diode ideality factor m after the thermal stress indicates a non-negligible contribution of non-radiative recombination within the solar cell. Similar findings are deduced comparing the electroluminescence spectra of the pristine and degraded solar cell. The thermal stress induces a marked drop of electroluminescence signal intensity in the whole investigated wavelength range. It should be note that high electroluminescence efficiency is a good indicator in solar cells for high power conversion efficiencies. In particular, the power conversion efficiency is reduced by 50% referring to a CPV system with a degraded cell, while the electric output power is decreased by 30%. Hence, it is clear that the triple-junction cell inefficiencies, principally caused by a strong thermal stress, lead to a drastic drop of the CPV system performances. Finally, an active cooling system is absolutely necessary when high values of light concentration are reached.

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. © 2014 AIP Publishing LLC.