Vivek Gade

Development of sputtering systems for large-area deposition of CuIn1−xGaxSe1−ySy thin-film solar cells

CuIn1−xGaxSe1−ySy (CIGS) thin-film modules are expected to become cheaper than crystalline silicon modules within 5 yr. At present, commissioning and reaching full production of thin film modules is delayed because of nonavailability of turnkey manufacturing plants. Very few universities are conducting research on development of PV plants. CIGS thin-film solar cells are being prepared routinely at Florida Solar Energy Center (FSEC) on glass and metallic foil substrates for terrestrial and space applications. Earlier, the substrate size was limited to 3×3 cm2. This article presents results of development of large-area sputtering systems for preparation of large (15.2×15.2 cm2) CIGS thin-film solar cells. The facilities have the potential of serving as a nucleus of a pilot plant for fabrication of CIGS minimodules. Initial problems of bowing of the brass diaphragm, restriction of effective water flow and consequent heating of the target material were resolved by increasing the thickness of the backing plate and redesigning the structural members. Thickness uniformity was improved by modifying the magnetic field distribution in the middle 15 cm portion of the 10.2×30.5 cm2 magnetron sputtering sources by selectively removing nickel-coated soft-iron pieces at the rear. This resulted in Mo layer thickness uniformity of ±3% over 10.2×10.2 cm2. The magnetic field was boosted at extremities to avoid precipitous ∼15% drop beyond 10.2 cm. With this, thickness uniformities of ±2.5% for Mo and ±4.5% for ZnO over 12.7×10.2 cm2 have been achieved however with a continuing drop beyond 12.7 cm width. Modifying the magnetic field to achieve better distribution by preferentially removing soft irons pieces and also boosting of the magnetic field at the ends are two new concepts introduced and successfully utilized in this study. Scaling up of the large-area uniform deposition of metallic precursor layers was a challenging task. The efforts were directed towards obtaining similar thickness and uniformity that have provided very good photovoltaic efficiencies of 10.4% (Air mass AM 1.5) in small area CIGS thin film solar cells on stainless steel foils in earlier research. Preliminary results on large area CIGS solar cells are encouraging.CuIn1−xGaxSe1−ySy (CIGS) thin-film modules are expected to become cheaper than crystalline silicon modules within 5 yr. At present, commissioning and reaching full production of thin film modules is delayed because of nonavailability of turnkey manufacturing plants. Very few universities are conducting research on development of PV plants. CIGS thin-film solar cells are being prepared routinely at Florida Solar Energy Center (FSEC) on glass and metallic foil substrates for terrestrial and space applications. Earlier, the substrate size was limited to 3×3 cm2. This article presents results of development of large-area sputtering systems for preparation of large (15.2×15.2 cm2) CIGS thin-film solar cells. The facilities have the potential of serving as a nucleus of a pilot plant for fabrication of CIGS minimodules. Initial problems of bowing of the brass diaphragm, restriction of effective water flow and consequent heating of the target material were resolved by increasing the thickness of the backing plate...

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.

Predicting the long term power loss from cell cracks in PV modules

Long term power loss due to cell cracks can become a significant PV wear out mechanism. An experimental framework to assist in predicting the power loss from cell cracks during module service life is presented. The paper is primarily structured around cell crack origin in laminated modules, crack orientation, oriented crack reproduction and climatic testing of custom mini-modules. The results are experimentally verified by a series of accelerated tests involving mechanical load and humidity freeze tests. Periodic characterization using EL and flash I-V are used to study evolution of crack types/categories and performance loss due to cell cracks. Manufacturing modules cell crack data was collected from a population of a week of data post lamination. The data analyzed is for a certain batch of polycrystalline cell modules. Jabil has produced few GW of modules over the past years.

AFM, micro-PL, and PV analyses of CuIn/sub 1-x/Ga/sub x/S/sub 2/ thin films solar cells on stainless steel foil

A dual-chamber, large-area magnetron-sputtering unit was fabricated. CuIn/sub 1-x/Ga/sub x/S/sub 2/ (CIGS2) thin films were prepared by sulfurization of DC magnetron-sputtered CuGa/In precursor on stainless steel foil substrates in Ar:H/sub 2/S(4%) mixture at 475/spl deg/C. CIGS2 thin films grew with chalcopyrite CuIn/sub 0.7/Ga/sub 0.3/S/sub 2/ phase with a = 5.67 /spl Aring/ and c = 11.34 /spl Aring/ and preferred {112} orientation. Inhomogeneous broadening, and variation of peak position and amplitude of microPL signal were probably caused by rough morphology and variation in the defect chemistry. Best efficiencies of CIGS2 solar cell on SS flexible foil measured at NREL and NASA GRC were 10.4% (AM 1.5) and 8.84% AM 0 respectively.

Predicting service life of bypass diodes in photovoltaic modules

Service life model for bypass diodes is a critical step towards developing service life models for PV modules. A review of failure mechanisms in bypass diodes along with the accelerated tests that could replicate them is presented. Susceptibility of commercially available bypass diodes for thermal runaway was studied. Accelerated tests were performed to study the diode wear out by High Temperature Forward Bias (HTFB) and Thermal Cycling (TC) mechanisms.

Climate and mounting specific testing of bypass diodes / junction boxes for improving long term reliability and cost reduction

It is well known that PV modules experience significantly different stressors in field depending on the climate and mounting of deployment. Therefore, the climate specific rating and associated tests for modules are being developed. However, currently there are no climate-specific tests for bypass diodes. It is shown by modeling and experimental results that the differences in the stressors experienced by bypass diodes in PV modules according to climate and mounting are significant. The benefits of designing bypass diode / junction box systems according to end use environment in terms of improved reliability and cost reduction are explained with the illustrative study of test development for High Temperature Forward Bias (HTFB) operation. Climate / mounting specific HTFB tests are developed for bypass diodes for three target climates (Hot-dry: Phoenix, Hot-humid: Miami, Temperate: Denver) and two mounting configurations (Rack mount and Roof mount).

Improving manufacturability, quality and durability of PV modules through PV ribbon evaluation program at Jabil Solar and Environmental Test Center (JSEC)

An introduction to the PV Component Evaluation Program developed by Jabil Solar and Environmental Test Center (JSEC) is provided with an illustrative case study of ribbon evaluation. This program aims to reduce time to market of new PV module materials and components by providing a systematic procedure for comparative assessment of the performance and reliability of products. Reliability and performance indices of ribbons from 5 different manufacturers were determined by evaluation of several metrics such as tensile properties, solder adhesion characteristics and specific resistance. Through the comparative analysis of performance and reliability of ribbons, typical values parameters / metrics that lead to optimized overall performance were identified.

Research, test, and development activities performed by junction box bypass diode task force # 4

The paper provides latest update on the activities performed by the group #4-diodes, shading and reverse bias of the PV Module Quality Assurance Task Force (PVQAT) in the areas such as electrostatic discharge testing and standards, thermal runaway testing, diode junction temperature measurement techniques, thermal endurance tests and analysis of field failures. Philosophy, motivation and future direction for the group #4 is also discussed.

Effect of shading on the switching of bypass diodes in PV modules

Bypass diodes are installed in Photovoltaic (PV) modules in order to prevent the application of high reverse voltage across the shaded cells in the event of partial shading of the module. Crystalline silicon (c-Si) modules have one bypass diode per 18-20 cells while thin film modules have at most one bypass diode per module. Ideally, bypass diodes are expected to turn on as soon as a current mismatch is detected between various strings of cells inside the module, which typically occurs in the event of partial shading. However, limited information is available on the actual switching characteristics of bypass diodes in field. In this paper, effect of incremental shading of various cells on the flasher I-V curve of a commercial 60-cell c-Si module was studied. Cell combinations in various strings were shaded with operational bypass diodes in the module and effect on module performance parameters such as Voc, Isc, Pmax and Fill Factor was discussed. Consequently one bypass diode in the module was short circuited and open circuited respectively and again the effect of shading on the I-V curve was investigated. Techniques for identifying short circuited and open circuited bypass diodes from I-V curve are presented.