Marco Barbato

Local Shunting in Multicrystalline Silicon Solar Cells: Distributed Electrical Simulations and Experiments

In this paper, we analyze the effect of local shunts in photovoltaic (PV) solar cells by experimental characterization and distributed electrical simulations. To this purpose, we developed a quasi-3-D distributed electrical network that is based on two-diode circuit elementary units. It allows accounting for resistive losses associated to the transport through the emitter, the fingers and the busbars, and to local defects in the semiconductor. The electrical parameters of the equivalent circuit units are calibrated according to experiments performed on multicrystalline (mc-Si) silicon solar cells, including samples that feature local shunts due to localized defects, which lead to nonuniform distribution of electrical and optical properties. The distributed electrical simulations account for the degradation of fill factor and power conversion efficiency in case of local shunting. Moreover, by combining the proposed tool with a RC thermal network it is possible to estimate the temperature distribution in a shunted solar cell. Our analysis shows how a shunted cell that operates under hot-spot conditions is subject to significant local overheating, which possibly lead to permanent PV cell damages.

Influence of Shunt Resistance on the Performance of an Illuminated String of Solar Cells: Theory, Simulation, and Experimental Analysis

This paper presents an extensive study of how a solar cell with low shunt resistance can affect the performance and reliability of a solar panel. The analysis is based on both simulations and experimental tests and provides the following results: 1) the cell with low shunt resistance significantly reduces the efficiency of a panel; 2) this is particularly pronounced if the shunted cell is partially shaded: in this case, the shunt resistance of the cell acts as a load for the entire panel; 3) in these conditions, the shunted cell can significantly degrade: in fact, the small-size shunt paths are crossed by a high current density, generated by the other cells in the panel, thus reaching high temperature levels. Stress tests have been also carried out to fully characterize the degradation process and its dynamics and to understand the physical origin of the failure of shunted cells.

Effect of shunt resistance on the performance of mc-Silicon solar cells: a combined electro-optical and thermal investigation

In this paper we discuss the effect of shunt resistance on the electro-optical characteristics of multicrystalline silicon (mc-Si) solar cells at different illumination levels. The analysis is based on combined electro-optical characterization and thermographic measurements of solar cells with similar efficiencies, but with different shunt resistance levels. In order to understand how the shunt resistance can affect the performance of mc-Si solar cells, a special setup for J-V characterization at several illumination levels was developed. Results indicate that (i) a low shunt resistance is strongly correlated to the presence of hot spots, which can be identified by means of infrared thermography; (ii) solar cells with different shunt resistance levels can show significantly different fill factors and efficiencies, particularly at low irradiation levels. This can strongly influence the reliability of modules at low illumination conditions; (iii) the electrical characteristics of mc-Si solar cells can be modeled with good results, by considering the equivalent two-diode electrical model and solving it by a circuit simulator like SPICE.

Evolution of electrical parameters of dielectric-less ohmic RF-MEMS switches during continuous actuation stress

The evolution of the main electrical parameters of dielectric-less ohmic RF-MEMS cantilever-based switches during continuous actuation stress was investigated in this work. Thanks to different designs, the main electrical parameters changes were attributed to a charging phenomena of the oxide over the substrate near the polysilicon actuator, leading to both narrowing and shifting of traditional hysteresis-like curves. Recovery procedures were also analyzed. Furthermore, the breakdown occurrence was also investigated, supported by both emission microscope and optical images.

Study of the actuation speed, bounces occurrences, and contact reliability of ohmic RF-MEMS switches

The influence of the bias signal waveform on the electromechanical dynamic response of ohmic RF-MEMS switches is here investigated by means of electromechanical characterizations and modelling procedures. The actuation transient of ohmic RF-MEMS switches was studied in this work developing a fast to compute, but comprehensive electromechanical model, using electromechanical parameters from experimental results. The developed model was then used to investigate how different bias waveforms influence the switch dynamic, in terms of actuation time, and bounces occurrences, and a practical solution to limit bounces, without compromising the actuation time was presented. Furthermore, it was demonstrated how it is possible to improve the reliability to cycling stress using ad hoc shaped bias signals.

A Combined Mechanical and Electrical Characterization Procedure for Investigating the Dynamic Behavior of RF-MEMS Switches

This paper shows the potentialities of two characterization procedures on the electrical and mechanical characterizations of ohmic RF microelectromechanical systems (MEMS) switches. The first is a “fast electrical” procedure that uses an electrical stimulus and monitors the RF signals at the input and output ports of the switch; the second is a “fast hybrid” procedure, i.e., electrical and mechanical, with an electrical input adopted to actuate the device and a mechanical measurement conducted with an optical profilometer, which monitors the displacement and the velocity of the moving membrane when the input electrical signal is applied. Both systems are validated on cantilever and clamped-clamped resistive RF-MEMS switches. We developed these measurement procedures to speed up the measurement process and, consequently, to limit charge trapping during the characterization process. In future analyses, the procedure will be systematically applied to investigate reliability issues when the switch is subjected to multiple impacts and long-term actuation. The use of such procedures will permit separating electrical and mechanical failure mechanisms.

A new measurement set-up to investigate the charge trapping phenomena in RF MEMS packaged switches

In this work we present a new measurement set up able to predict the lifetime of packaged ohmic RF MEMS submitted to long actuation periods. Experimental results were carried out for a relatively long time period in order to verify the degradation law relates to charge trapping and stiction on cantilever and clamped-clamped switches. Thanks to the use of a microcontroller we have been able to reach a complete control of the timing during the stress phase. Furthermore, the characterization phase has been remarkably reduced in order to minimally influence the charge trapping during the characterization of stressed device. Results are carried out on two different MEMS designs (clamped-clamped and cantilever configurations).

A Novel Technique to Alleviate the Stiction Phenomenon in Radio Frequency Microelectromechanical Switches

Radio frequency (RF) microelectromechanical system (MEMS) switches subject to long term actuation suffer from narrowing of the actuation and release voltages. This can lead to the failure of the device when the device remains actuated without external biasing due to stiction effects. The stiction phenomenon is one of the most challenging problems in RF MEMS switches, especially in applications where these devices have to remain actuated for an extended period of time (months or even years). In this letter, we show a novel recovery technique to alleviate the stiction phenomenon significantly increasing the device lifetime. In particular, we show how the flowing of a small current through the suspended membrane can be used to fully restore the device properties to its fresh conditions in just a few seconds.

Thermal and electrical investigation of the reverse bias degradation of silicon solar cells

This work presents a detailed analysis of the degradation of Si-based solar cells submitted to reverse-bias stress; the study is based on electrical, electro-optical and thermal measurements, carried out at the different stages of the stress tests. The results show that exposure to reverse bias may induce severe modifications of the cell electro-optical performance: the most relevant failure mechanism is the increase in localized shunt resistance components. The changes in the leakage paths have been investigated both through infrared thermal imaging and SEM measurements.

Demonstration of Field- and Power-Dependent ESD Failure in AlGaN/GaN RF HEMTs

This paper reports an extensive analysis of the electrostatic discharge (ESD) robustness and of the corresponding failure mechanisms of AlGaN/gallium nitride high-electron mobility transistors. Based on transmission line pulse testing, we demonstrate the following results: 1) when ESD pulses are applied to the drain under pinchoff conditions, failure occurs through a field-dependent mechanism; the failure voltage is independent on the gate bias, and approximately equal to 300 V; 2) when ESD pulses are applied to the drain under ON-state conditions, a power-dependent mechanism prevails, lowering the ESD robustness; 3) vertical (drain substrate) breakdown does not significantly affect the ESD stability of the devices; the failure threshold for vertical failure is higher than 450 V; and 4) the length of the field plate can significantly impact the ESD behavior of the devices, by influencing the failure threshold and by favoring soft-degradation processes. Scanning electron microscopy was extensively used to achieve a complete description of the failure processes.