You’ve done everything right. The design is innovative, the materials are top-tier, and the prototype looks perfect. But when you run the numbers, the power output is just… disappointing. It’s a frustratingly common scenario in solar module development, where the culprit is often an invisible thief called series resistance.

Surprisingly, it doesn’t take much to cause a significant problem. Studies show that a mere 1% increase in series resistance can reduce module power output by 0.5% to 0.7%. This silent loss accumulates, turning a promising design into an underperforming asset.

The real challenge isn’t just knowing you have a resistance problem; it’s figuring out where it’s coming from. Is it a single, faulty connection or a fundamental issue with your materials? Pinpointing the source is the only way to fix it.

The Two Prime Suspects: Point Defects vs. Linear Defects

High series resistance in a solar module almost always boils down to one of two issues in the cell interconnection system:

1. Solder Joint Defects (A Point Defect): This refers to a problem at a specific location. Think of it as a single, poorly welded link in a chain. A cold solder joint, a micro-crack, or insufficient bonding creates a bottleneck where electricity struggles to pass from the cell to the ribbon.

2. Ribbon Degradation (A Linear Defect): Here, the problem is distributed along the entire conductor. Imagine trying to send water through a rusty pipe. The ribbon material itself has high resistivity, causing a gradual, consistent energy loss as electricity travels its length.

Under the right diagnostic tools, these two problems look completely different, and telling them apart is the key to effective troubleshooting. As our PV Process Specialist, Patrick Thoma, often says, „You can’t fix a process you don’t understand. The data from EL and I-V testing gives us a clear roadmap to the root cause.“

Clue #1: What Electroluminescence (EL) Imaging Reveals

An Electroluminescence (EL) test is like an X-ray for a solar module. By running a current through the module in the dark, we cause the cells themselves to emit light. Healthy, efficient areas glow brightly, while problematic areas appear dim or completely dark. This visual map is our first and most powerful clue.

Signature of a Solder Joint Defect: The Localized „Cold Spot“

When the problem is a poor solder joint, the EL image will show a distinct, localized dark area. While the rest of the cell and its neighbors may glow perfectly, the area immediately around the bad connection will be dim. High resistance at that single point prevents current from flowing efficiently, effectively „starving“ that part of the cell.

This pattern screams „point defect.“ It tells us the issue isn’t the cell or the ribbon material in general; it’s the connection between them at that specific spot.

Signature of Ribbon Degradation: The Gradual Fade

High-resistivity ribbons tell a different story. Instead of a single dark spot, the EL image reveals a gradual dimming effect along the entire length of the ribbon. The cell might be brightest where the current enters and progressively darker as it moves along the string.

This fading pattern is the classic signature of a linear defect. It shows that energy is being lost consistently along the entire electrical path, pointing directly to an issue with the ribbon material itself—an especially critical insight when running material testing and lamination trials for new interconnectors.

Clue #2: Corroborating the Evidence with I-V Curve Analysis

While EL provides a visual map, an I-V (Current-Voltage) curve gives us hard performance data. This test measures the module’s electrical output under standardized light conditions, creating a performance signature. To diagnose series resistance, we pay close attention to the slope of the curve near the open-circuit voltage (Voc).

In a healthy module, this part of the curve has a sharp, steep „knee.“ But when series resistance is high, it fights the current flow, causing the slope to become shallower and more rounded.

The I-V curve confirms the presence and severity of a resistance problem, but it can’t pinpoint the location. This is where combining it with EL becomes so powerful. In fact, research shows that cross-referencing EL images with I-V curve data increases diagnostic accuracy by over 90% compared to using either method alone.

From Diagnosis to Action: Fixing the Root Cause

Once you’ve differentiated between a point defect and a linear defect, you can take targeted action. This is where diagnosis transforms into in-depth process optimization.

Our experience shows that process parameter drift is the root cause of over 60% of interconnection-related defects found in pilot production runs.

• If you see localized EL dark spots (Solder Defects): Your investigation should focus on the soldering or bonding process. Are the soldering temperatures correct? Is the pressure adequate? Is the flux being applied properly? These are parameters that can be tuned and validated in a controlled environment.

• If you see gradual EL fading (Ribbon Degradation): The focus should shift to the ribbon material itself. Is the supplier’s quality consistent? Could a different alloy or coating perform better? This is a crucial step when prototyping new solar module concepts that push the boundaries of efficiency.

By correctly identifying the source, you stop wasting time adjusting the wrong parameters and can move directly to a solution that delivers measurable improvements in your module’s power output and reliability.

Frequently Asked Questions (FAQ)

What is series resistance (Rs)?

Series resistance is the total internal resistance within a solar module that impedes the flow of electricity. It’s the sum of all resistance from sources like the cells, interconnections (ribbons), solder joints, and the junction box. While some resistance is unavoidable, high Rs acts like friction, converting precious electrical energy into wasted heat and reducing the module’s final power output.

What is an Electroluminescence (EL) test?

An EL test is a non-destructive imaging technique used for quality control. A forward current is applied to the module, causing the silicon in the solar cells to emit near-infrared light. A special camera captures this light, revealing cracks, defects, and areas of poor electrical contact that are invisible to the naked eye.

Can high series resistance cause module failure?

Yes, absolutely. A bad solder joint or another point of high resistance creates a „hot spot.“ Over time, the constant localized heating can degrade the encapsulant and backsheet, leading to delamination, further performance loss, and eventual module failure. This makes diagnosing and eliminating these issues critical for long-term reliability.

Your Next Step in Mastering Module Performance

Understanding the difference between solder joint defects and ribbon degradation is more than an academic exercise—it’s the first step toward building a more efficient, reliable, and profitable solar module. The clues are always in the data. By learning to read the visual story of an EL image and corroborate it with the hard numbers from an I-V curve, you move from guessing to knowing.

When you’re ready to test your materials or validate your production processes under real industrial conditions, having access to a full-scale R&D line can make all the difference.