Eric Schneller

Evolution of Leakage Current Paths in MC-Si PV Modules From Leading Manufacturers Undergoing High-Voltage Bias Testing

The evolution of leakage currents in photovoltaic modules undergoing outdoor high-voltage bias testing is studied using data from high-voltage bias testing of multicrystalline silicon modules from leading manufacturers. An analysis of the module leakage currents as a function of environmental conditions including temperature, relative humidity, rain, and wetness is carried out. The behavior of the modules was found to be dependent on the module construction and the materials used. The Arrhenius model was used to fit the experimental data and activation energies were computed for various relative humidity values. The effect of dew and rain (wetness) on the front glass was investigated. Changes in the leakage current during dry conditions were studied using the temperature dependence of resistivity of bulk soda-lime glass. Because of the approximately tenfold increase in leakage currents during the wet conditions, it is suggested that the accelerated tests should not be limited exclusively to noncondensing environments but should also be complemented with tests that include wet conditions.

Device for detailed analysis of leakage current paths in photovoltaic modules under high voltage bias

High voltages used in photovoltaic (PV) systems are known to induce long-term power loss in PV modules due to leakage current flowing through the module packaging materials. It has been difficult to identify the specific materials and interfaces responsible for degradation based on an analysis of only the total leakage current. A detailed investigation of the leakage current paths within the PV modules, under high voltage bias, is carried out by utilizing a device that measures the independent contributions of various paths in real-time. Knowledge about dominant leakage current paths can be used to quantify the physical and chemical changes occurring within the module packaging materials.

The reliability of bypass diodes in PV modules

The operating conditions of bypass diodes in PV modules deployed in the field are considerably harsher than the conditions at which the diode manufacturers test the diodes. This has a potential to significantly reduce the operating life of bypass diodes and has raised concerns about the safety and reliability of PV modules as a whole. The study of modes and mechanisms of the failures encountered in bypass diodes used in PV modules can provide important information which would be useful to predict the module lifetime. This paper presents the review of the failure modes and mechanisms observed in bypass diodes and current work related to reliability testing of bypass diodes. The International PV Module Quality Assurance Task Force has recommended following four potential areas of research to understand the reliability issues of bypass diodes: Electrostatic Discharge, reverse bias thermal runaway testing, forward bias overheating and transition testing of forward bias to reverse bias. As a joint collaborative effort between Florida Solar Energy Center and Solar and Environmental Test Laboratory at Jabil Inc., laboratory testing of bypass diodes on the guidelines provided by the International PV Module Quality Assurance Task Force has been initiated. Preliminary results from this work are presented in this paper.

Survey of potential-induced degradation in thin-film modules

Abstract. Two CdTe and two copper indium gallium (di)selenide (CIGS)-type modules were tested for potential-induced degradation (PID) with positive and negative 1000 V biases applied to the active cell circuit in an 85°C, 85% relative humidity environmental chamber. Various degradation mechanisms could be seen with signatures such as shunting, transparent conductive oxide (TCO) corrosion, charge carrier lifetime reduction, and dead active layer at edges along with resulting cell mismatch. All modules tested exhibited degradation by system voltage stress in chamber, but only one module type has degraded in parallel field tests. I−V curve data indicated that one CdTe-type module sequentially exhibited shunting followed by a recovery and then series resistance losses. This module type showed TCO delamination from the glass in the environmental chamber tests and also exhibited power degradation within 5 weeks in field tests. Relative rates of Coulomb transfer from the voltage-biased active cell circuit to ground are compared for the modules in chamber tests to those placed outdoors under system voltage stress to extrapolate the anticipated time to failure in the field. This analysis correctly indicated which module type failed in the field first.

Finite element analysis based model to study the electric field distribution and leakage current in PV modules under high voltage bias

The maximum system voltage for Photovoltaic systems is 1000 V in US. Some modules are designed to operate even at 1500 V, which is the limit for IEC low voltage systems. The high voltage bias between the cell circuit and frame of the module leads to a leakage current flowing through the insulation of the module to the ground. Over time, this leakage current causes migration of various species to and from the cell circuit, can result in slow degradation of the performance of PV module. It is important to understand the electric field distribution and leakage current pathways in the PV modules in order to study the system voltage induced degradation of PV modules. The leakage current from the PV modules deployed outdoor and under high voltage bias strongly varies with the environmental conditions. The lumped resistance models described in literature that attempt to explain the leakage current flow through the PV module do not provide adequate information about the distribution of leakage current through different layers of insulation present in the PV modules. In this paper, a Finite Element Analysis (FEA) based model for the insulation of PV module is described. It yields useful information about the distribution of electric field, potential and leakage current flowing through different layers of module. The model is also used to predict and analyze the changes in leakage current with changes in module packaging materials and grounding configurations.

High-voltage bias testing of PV modules in the hot and humid climate without inducing irreversible instantaneous degradation

High-voltage bias testing of PV modules is known to be useful for revealing design, material and process flaws in PV modules. It is accepted that the hot and humid climate under high-voltage bias imposes a severely harsh environment on the PV modules and enhances the possibility of degradation. Test location is, therefore, important. A test methodology is being presented here that can provide useful information for testing PV modules at high voltage in natures own laboratory in a relatively short time frame.

Overview of laser processing in solar cell fabrication

This paper will provide an overview of various laser processing techniques used in the fabrication of solar cells. There are numerous applications of lasers including laser doping, annealing, patterning, drilling and welding that vary based on material system (e.g. silicon wafer, polycrystalline thin-film) and the cell architecture. Laser annealing has been identified as a potential route to high quality thin-films for polycrystalline semiconductors such as CdTe and Cu(In,Ga)(Se,S)2. Also for thin-film solar cell technologies, including amorphous-silicon, laser patterning has been widely adopted within the industry for creating monolithic interconnections and performing edge isolation. For silicon wafer based technologies there are a number of promising laser processing techniques, however most of these techniques are still in the development stages and are not yet incorporated into industrial production lines. These techniques that represent the next generation of high-efficiency crystalline silicon devices, include laser-fired contacts, laser doping of selective emitters, laser drilling for “wrap-through” device structures, and laser grooved buried contacts. This paper will present a review of each technique, the specific processing applications and the current state of development.

Baseline testing procedures for PV modules beyond the qualification testing

The qualification tests described in IEC 61215 for the c-Si PV modules are essentially pass/fail tests that assist in avoiding infant mortality. This paper reports on the baseline test procedure carried out on PV modules at Florida Solar Energy Center that go beyond the pass/fail criteria of the qualification tests and obtain information about the degradation modes and mechanisms. The importance and limitations of the various characterization techniques are described. Electroluminescence imaging has been used to detect and categorize the faults at the cell level. Indoor infrared imaging has been used to study the quality of electrical interconnects in the module. The infrared imaging carried out on the modules while they are undergoing outdoor exposure has provided information about the presence and distribution of hot spots in these modules. Conventionally, the insulation resistance tester has been used mostly for the dry and wet leakage test. In this study, the importance of the polarization index test and voltage excursion test are described. The use of these tests is essential to provide insight into the modes and mechanisms of degradation, during reliability and durability studies of PV modules. A predictive model for the service life of a PV module may be developed through the results obtained from these characterization techniques in conjunction with long-term exposure and accelerated lifetime tests.

Silicon Heterojunction System Field Performance

A silicon heterostructure photovoltaic system fielded for 10 years has been investigated in detail. The system has shown degradation, but at a rate similar to an average Si system, and still within the module warranty level. The power decline is dominated by a nonlinear <italic>V</italic>_oc loss rather than more typical changes in <italic>I</italic>_sc or Fill Factor. Modules have been evaluated using multiple techniques including: dark and light <italic>I</italic>–<italic>V</italic> measurement, Suns-<italic>V</italic>_oc, thermal imaging, and quantitative electroluminescence. All techniques indicate that recombination and series resistance in the cells have increased along with a decrease of factor 2 in minority carrier lifetime. Performance changes are fairly uniform across the module, indicating changes occur primarily within the cells.

Impact of ozone-based cleaning on surface recombination with different passivation materials

In this work, the impact of different ozone-based cleaning processes on the level of surface passivation achieved is determined and compared against the RCA cleaning processes. Two different passivation materials are used in this study, including hydrogenated amorphous silicon and silicon nitride plasma enhanced chemical vapor deposition (PECVD). Photoconductance measurements and calibrated photoluminescence imaging are used to evaluate the level of passivation achieved and spatial uniformity for the different cleaning processes.