Technical papers & presentations:

Compressive Stress Strategies for Reduction of Cracked Cell Related Degradation Rates in New Solar Panels and Power Recovery in Damaged Solar Panels ; WCPEC-4, 14Jun2018, Hawaii.
Presented at the 4th World Conference on Photovoltaic Energy Conversion, WCPEC-4, June 14 2018. This work describes compressive stress strategies and rear side pressure strategies that can be employed in new panel construction to prevent crack formation as front side mechanical loads from handling, wind, and snow are applied during shipping, installation and in the field. These strategies can also slow the opening of cracks and the related power loss for any cracks that do form. Furthermore, we present concepts for the retrofitting of older installed systems to close already open cracks and regain lost power, or to slow the future degradation of systems with panels that are sensitive to cracked cells.
Compressive Stress Solutions - BrightSpo[...]
Adobe Acrobat document [1.3 MB]
The Effect of Laminate Construction and Temperature Cycling on the Fracture Strength and Performance of Encapsulated Solar Cells; WCPEC-4, 15Jun2018, Hawaii.
Presented at the 4th World Conference on Photovoltaic Energy Conversion, WCPEC-4, June 15 2018. A critical aspect of silicon solar module reliability is the fracture characteristics of the solar cells under mechanical loads. Here, we use 3-point bend testing of coupons to investigate the effects of tabbing, encapsulant and thermal history on the fracture strength of silicon solar cells. We find that the fracture strength depends significantly on the encapsulant modulus and thickness. Doubling the encapsulant thickness can increase the load at fracture by 80%. In addition, short exposure to low temperatures (< -20C) can decrease the room temperature fracture strength by 80% or more. This low temperature effect on crack susceptibility is not currently captured in standard durability testi
Temp Cycling and Cell Cracking - D2Solar[...]
Adobe Acrobat document [623.5 KB]
Cyclic Mechanical Loading of Solar Panels – A Field Experiment; WCPEC-4, 15Jun2018, Hawaii.
Presented at the 4th World Conference on Photovoltaic Energy Conversion, WCPEC-4, June 15 2018. A panel mounted outdoors during Hurricane Irma underwent a large number of low amplitude cyclic mechanical load cycles, and these cycles caused some opening of closed cracks in the solar cells. Low amplitude cyclic loading on the LoadSpot tool was compared, and crack opening was also seen.
Cyclic loading field experiment - FSEC- [...]
Adobe Acrobat document [719.5 KB]
Accelerating Cyclic Loading; WCPEC-4, 13Jun2018, Hawaii.
Presented at the 4th World Conference on Photovoltaic Energy Conversion, WCPEC-4, June 13 2018. We performed four variations of cyclic load testing on four groups of modules using the LoadSpot tool. We found that in some cases, increasing the loading frequency actually reduces maximum power degradation with respect to the baseline, whereas increasing or decreasing the load amplitude respectively increases or decreases maximum power degradation with respect to the baseline.
Accelerating Cyclic Loading - FSEC and B[...]
Adobe Acrobat document [464.5 KB]
Electroluminescence Based Metrics to Assess the Impact of Cracks on Photovoltaic Module Performance; WCPEC-4, 12Jun2018, Hawaii.
Presented at the 4th World Conference on Photovoltaic Energy Conversion, WCPEC-4, June 12 2018. We present a methodology that utilizes EL images to predict power loss due to cell cracking. We explored pixel intensity histogram normalization methods to generalize this approach to a wide range of measurement conditions. The final optimized EL metric exhibits a strong correlation with power loss for a range of module technologies where a 1% increase in the dark area due to cracks results in a 3% loss in performance. This approach enables field EL mapping to translate to module Pmax mapping across a system.
EL quantification of power loss - FSEC a[...]
Adobe Acrobat document [2.8 MB]
Detailed Performance Loss Analysis of Silicon Solar Cells using High- Throughput Metrology Methods; WCPEC-4, 14Jun2018, Hawaii.
Presented at the 4th World Conference on Photovoltaic Energy Conversion, WCPEC-4, June 14 2018. In this work, novel, high-throughput metrology methods are used to perform a detailed performance loss analysis of ≈400 industrial crystalline silicon solar cells. The characterization sequence includes a non-destructive transfer length method (TLM) measurement technique featuring circular TLM structures hidden within the busbar region of the cells. It also includes a very fast external quantum efficiency and reflectance measurement technique. More traditional measurements, like illuminated current-voltage, Suns- VOC, and photoluminescence imaging are also used to carry out the loss analysis. The variance of the individual loss parameters and their impact on cell performance are investigated a
Cell loss analysis- FSEC and BrightSpot [...]
Adobe Acrobat document [727.9 KB]
Non-Destructive Contact Resistivity Measurements on Solar Cells Using the Circular Transmission Line Method; 44th IEEE Photovoltaic Specialist Conference, June 25 2017, Washington D.C.
We have developed a new tool to measure the contact resistance and sheet resistance of solar cells in a nondestructive and novel manner. With a simple change in the artwork for the metallization, small circular structures can be hidden within the busbars of the solar cells to enable the measurements with no impact on cell efficiency or aesthetics following the soldering of wires over the busbars.
Circular TLM - UCF and BrightSpot - IEEE[...]
Adobe Acrobat document [1.0 MB]
Evaluating Solar Cell Fracture as a Function of Module Mechanical Loading Conditions; 44th IEEE Photovoltaic Specialist Conference, June 28 2017, Washington D.C.
The LoadSpot mechanical load tester was used to study crack formation and crack opening as a function of load for different module and cell types.
LoadSpot testing for different modules -[...]
Adobe Acrobat document [1.0 MB]
Forecasting Environmental Degradation Power Loss in Solar Panels with a Predictive Crack Opening Test; 44th IEEE Photovoltaic Specialist Conference, June 28 2017, Washington D.C.
The LoadSpot mechanical load tester was used for static loading studies, followed by environmental chamber exposure, followed by cyclic loading tests. Measuring EL and IV under a low level of applied pressure was found to be a predictor of subsequent EL and IV degradation in some cases. Excessive new cracks were formed by light loading after chamber exposure.
LoadSpot testing post env chamber - UCF [...]
Adobe Acrobat document [639.0 KB]
Look while you load: Electroluminescence and IV testing of solar panels under mechanical load
Presentation from the 2017 NREL/SNL/BNL PV Module Reliability Workshop, Lakewood CO, February 28 2017
Load Testing - 2017 PVRW presentation - [...]
Adobe Acrobat document [7.1 MB]
TCAD MODELING OF TLM CONTACT RESISTANCE STRUCTURES; 32nd European Photovoltaic Solar Energy Conference and Exhibition, 21Jun2016, Munich, Germany
We have constructed TCAD and circuit simulator models for the measurement of contact resistivity on strips cut from solar cells. The models correctly predict trends in the measured contact resistivity as the sample width is varied. The models also show how the TLM is affected by the presence of unprobed contacts when measuring solar cells. The results are used to create correction factors that can be incorporated into the TLM measurement.
Contact Resistivity Modeling - UCF and B[...]
Adobe Acrobat document [1.4 MB]
Mechanical Load Testing of Solar Panels – Beyond Certification Testing; 43rd IEEE Photovoltaic Specialists Conference, 10Jun2016, Portland, OR
This paper presents an overview of mechanical load testing and cell cracking. We present the LoadSpot tool as a solution for improved load testing with the added benefit of a predictive crack opening test. We also present finite element modeling of different module structures and mounting methods.
Load Testing PVSC2016 paper - BrightSpot[...]
Adobe Acrobat document [1.1 MB]
Dependence of Solar Cell Contact Resistivity Measurements on Sample Preparation Methods; 43rd IEEE Photovoltaic Specialists Conference, 10Jun2016, Portland, OR
This paper describes TLM contact resistance measurements which explore the effects of test sample strip width and laser scribing parameters.
ContactSpot PVSC2016 paper - BrightSpot.[...]
Adobe Acrobat document [924.0 KB]
A Better Way to Bend: Vacuum and Air Pressure for Mechanical Load Testing of Solar Panels; NREL Photovoltaic Module Reliability Workshop 2016 (PVMRW)
This poster introduces our new product, the LoadSpot, which can perform static and dynamic load testing of solar panels as per IEC specifications. It has advantages over other methods of performing load testing in terms of pressure uniformity and the ability to characterize the panels by IV and EL testing while under load. Vacuum applied to the rear side temporarily opens up closed cracks and can serve as a predictive test of potential power loss were these cracks to open up in the field.
LoadSpot poster - PVMRW2016.pdf
Adobe Acrobat document [5.8 MB]
Cracked cell solutions; NREL Photovoltaic Module Reliability Workshop 2015 (PVMRW)
Provides background on the origins of microcrack and crack generation, and outlines several approaches that can be taken at the wafer, cell, module and system levels to both reduce the occurrence of cracked cells in the first place, and to reduce their impact when they do occur.
BrightSpot - Cracked cell solutions - NR[...]
Adobe Acrobat document [1.2 MB]
"Contact resistance measurement - observations on technique and test parameters" - IEEE PVSC conference June 2015
Background of contact resistance measurement techniques, and discusses the effects of test method and analysis algorithm on results
Contact resistance - BrightSpot - PVSC20[...]
Adobe Acrobat document [639.5 KB]
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