Publications

The primary application of UV Fluorescence (UVF) imaging to date has been to detect defects and bill of materials variations in aged silicon solar panels in the field.  We demonstrate here the application of UVF for imaging defects in solar panels after environmental chamber testing.  Commercial panels that underwent chamber exposure at NREL were tested with a UVF camera system to examine defects such as cracked cells.  New single cell encapsulated coupons with different encapsulant and backsheet materials were fabricated at NREL, and these were also imaged with the UVF camera system to look at changes before and after encapsulation.  We see complementary information from UVF as compared to EL testing that justifies its use after most environmental chamber tests for panel designs using fluorescing encapsulants.  For panel designs that don’t use fluorescing encapsulants, substituting a fluorescing encapsulant for some stages of product development may be advantageous for detecting some defects such sealing problems and hidden cracks under wires.  

Collecting data to support an insurance claim for hail damaged commercial rooftop PV systems can be challenging since the small or non-existent spaces between panels does not allow access to the MC4 connectors to perform electroluminescence (EL) testing or even to place a luminescence camera system close to many of the panels.  UV Fluorescence (UVF) is an emerging technique for imaging cracked solar cells and other defects that is non-contact in nature and which is well suited toward imaging these rooftop systems.  However, while EL is well known to the PV community and to some insurance claim boards, UVF is unfamiliar to most, and this is a barrier to its implementation in insurance claims.  This poster shows UVF images captured for a variety of rooftop systems in Minnesota, and shares details on how these images were used in successful insurance claims. 

50th IEEE PVSC, Jun 12 2023

The thermal stresses associated with the soldering of interconnect wires onto the busbars of solar cells is one of the leading causes of cracks in silicon solar cells. Cracks will often branch outward from the busbar region so that they are easily seen in an electroluminescence (EL) image. However, since the wires are often wider than the busbar metallization, cracks can be located underneath or close to the wires and be “hidden” within the EL image. If the cracks remain beneath the busbar metallization, they may cause no reliability problems. However, if they propagate along the side of busbars and remain hidden under or next to the wires, they can prevent continuity of the gridlines to the busbars. The cracks may cause minimal problems in a new solar panel, but over time they can open up with thermal cycling and cyclic loading in the field. We demonstrate how these hidden cracks may be detected with the technique of UV Fluorescence, and we show examples of their signature in EL images. It is our observation that many groups are not familiar with these EL signatures, and do not consider that hidden cracks may be the cause of many gridline interruptions. We also show how gridline corrosion is strongly linked to hidden cracks where moisture can penetrate through the cracks.

RE+ Tradeshow, Anaheim, CA, Sep 19, 2022

This poster presentation gives examples of how electroluminescence (EL) and UV Fluorescence (UVF) testing can be used to reduce risks and improve quality/performance at different stages throughout the life cycle of a PV system.

2022 NREL PV Reliability Workshop, Feb 22, 2022

This narrated presentation provides an overview of UV Fluorescence (UVF) imaging of defects in solar panels and presents data from the field using BrightSpot’s UV flash camera system in both pole mount and drone mount configurations.  A comparison is made between thermal IR, EL, and UVF testing.

48th IEEE PVSC, Jun 23 2021

UV Fluorescence (UVF) is a relatively new “non- contact” method of detecting cracked cells in solar panels with potential high throughput and low cost. We report here on application of a pole-mount UV-flash camera system to the detection of defects on residential rooftops in Boulder Colorado. The majority of tested system exhibited useful UVF images, with most showing cracked cells, and a smaller percentage showing hot spot cells. Junction box heating and sealing problems and possible finger corrosion or encapsulant delamination were also seen. This data suggests that pole-mount UVF system is highly applicable and informative over a wide range of residential panel designs and ages and provides information complementary to that achieved from Electroluminescence imaging.

DuraMAT Cell Crack Working Group Meeting, March 15 2021

This narrated presentation covers research performed together with partners FSEC and D2 Solar under a DOE funded project.  Topics include our patent pending RailPad solution to limit module deflection and cell cracking under load, compressive stress solutions, cold induced microcracking, and the questions that remain unanswered.

2021 NREL PV Reliability Workshop, Feb 1 2021

This narrated presentation explores how short cracks, visible only in high-resolution quality EL images, almost always propagate into long cracks upon front side mechanical loading, and how few cells without such starting defects develop new cracks with loading.

47th IEEE PVSC, June 15, 2020

We present data at both the single cell coupon level and at the module level that demonstrate that the shunting from cracks reduces performance at low irradiances significantly more than at 1-Sun conditions, and that the effect scales with the total length of the cracks. We show through simulations that energy delivery may degrade significantly more than Pmax with damage from cracked cells. We conclude that the degradation from accelerated testing and field exposure should be assessed not just at an irradiance of 1-Sun but also at lower values as well.

NREL PV Reliability Workshop, 25Feb2020, Lakewood CO

Pre-cracked modules mounted with RailPads showed greatly reduced module deflection and virtually no new internal damage and power degradation from a mild hurricane near the Florida coast. RailPads show promise for reducing module degradation rates, whether as a retrofit or during new installs.

Mounting Rail Spacers for Improved Solar Panel Durability

46th IEEE Photovoltaic Specialist Conference, June 17 2019, Chicago

We explore a novel solar panel mounting scheme where RailPad spacer elements are placed between the rails of the support structure and the rear side of the panel. These spacers significantly reduce the panel deflection under load, and we have demonstrated a dramatic reduction in cell cracking at high load levels and in crack opening after cyclic loading. Such spacers can be applied to either the rear of the module or to the rails on the mounting structure and could be introduced for both new installations or as protective retrofits to existing systems.

Electroluminescence-Testing Induced Crack Closure in PV Modules

46th IEEE Photovoltaic Specialist Conference, June 18 2019, Chicago

Electroluminescence (EL) measurements of PV modules with cracked cells have showed some open cracks to close arbitrarily from day to day. We have found that variations in current and temperature generated from resistive heating during EL measurements strongly influence crack closure. Because crack closure can lead to some gain in maximum power, we consider the ramifications for IEC standards, namely that performing EL measurements before I-V measurements may lead to inflated results.

The Impact of Cold Temperature Exposure in Mechanical Durability Testing of PV Modules

46th IEEE Photovoltaic Specialist Conference, June 18 2019, Chicago

Existing mechanical durability testing sequences typically perform mechanical loading prior to environmental exposures such as thermal cycling or humidity freeze. Recent work has shown that the fracture strength of silicon solar cells can reduce after exposure to temperatures below -20°C. In an effort to better evaluate modules with respect to cell crack durability, we explore the use of a single thermal cycle prior to mechanical loading. The results show a significant increase in the number of cell cracks that are generated at a given load after a single cold exposure. We explore how this can be used to further optimize the qualification test sequence for mechanical durability.

A New Approach to Indoor Characterization of PV Module Energy Yield Parameters

46th IEEE Photovoltaic Specialist Conference, June 17 2019, Chicago

We use the new TempCoSpot tool to determine temperature coefficients as well as to assess power rating of multi c-Si, mono c-Si, CdTe, and CIGS-based PV modules in controlled indoor conditions per IEC 60891 and IEC 61853-1. We use the tool to explore how nonuniformities in module temperature affect the accuracy of the temperature coefficients.

Cracked Up: how should we classify and respond to various electroluminescence defects in silicon PV modules?

Presented at the 2019 PV Reliability Workshop, PVRW, Feb 27, 2019

In EL testing of modules, there is little in the literature to guide one on when to test these modules, how to enhance & interpret the EL images, how to classify and quantify various types of defects, and in particular on how to respond to these defects in different situations. We suggest solutions to these issues for different scenarios. We show various cell processing defects that can be confused for cracks and suggest ways to better differentiate between cell defects and cracks. We suggest ways of quantifying the crack statistics and ways to respond when those statistics fall within different ranges. We also discuss the possibility of various in-field repairs or panel enhancements to reduce degradation rates.

Re-Evaluating Solar Panel Mechanical Durability Testing: Case Study of Commercial Modules

Presented at the 2019 PV Reliability Workshop, PVRW, Feb 27, 2019

We show experimental testing data that displays a large variation in crack durability across commercially available modules. Mechanical loading after thermal cycling causes a significant number of new cracks for modules with solder interconnects. A modified testing sequence is proposed to evaluate module design with respect to crack durability.

Compressive Stress Strategies for Reduction of Cracked Cell Related Degradation Rates in New Solar Panels and Power Recovery in Damaged Solar Panels

WCPEC-4, 14 Jun 2018, 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.

The Effect of Laminate Construction and Temperature Cycling on the Fracture Strength and Performance of Encapsulated Solar Cells

WCPEC-4, 15 Jun 2018, 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 testing.

Cyclic Mechanical Loading of Solar Panels – A Field Experiment

WCPEC-4, 15 Jun 2018, 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.

Accelerating Cyclic Loading

WCPEC-4, 13 Jun 2018, 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.

Electroluminescence Based Metrics to Assess the Impact of Cracks on Photovoltaic Module Performance

WCPEC-4, 12 Jun 2018, 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.

Detailed Performance Loss Analysis of Silicon Solar Cells using High-Throughput Metrology Methods

WCPEC-4, 14 Jun 2018, 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.

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.

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.

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.

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

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.

Mechanical Load Testing of Solar Panels – Beyond Certification Testing

43rd IEEE Photovoltaic Specialists Conference, 10 Jun 2016, 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.

Dependence of Solar Cell Contact Resistivity Measurements on Sample Preparation Methods

43rd IEEE Photovoltaic Specialists Conference, 10 Jun 2016, Portland, OR

This paper describes TLM contact resistance measurements which explore the effects of test sample strip width and laser scribing parameters.

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.

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.

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.