You’ve checked the basics: the modules are clean, the sky is clear, and the inverters are online. Yet the system’s output is lagging, and you can’t pinpoint why. This is where standard troubleshooting ends and professional diagnostics begin. The real culprits of power loss—rising series resistance, potential-induced degradation (PID), or subtle cell mismatch—don’t show up in a simple visual inspection. They hide in the module’s electrical characteristics.
To find them, you need to go beyond surface-level checks and learn to interpret the language of module performance through I-V curves and Electroluminescence (EL) imaging. This guide offers a systematic framework for translating complex test data into actionable insights, helping you pinpoint the root cause of electrical losses and make informed decisions to protect your assets.
Diagnosing High Series Resistance (Rs)
Series resistance is the cumulative opposition to current flow through a solar module—think of it as electrical friction. While a small amount is unavoidable, any increase over time directly reduces power output by dissipating energy as heat.
The Parameter: Series resistance (Rs)
Series resistance (Rs) originates from multiple sources: the metal contacts on the cell, the resistance of the silicon itself, the interconnecting ribbons, and the junction box connections. A significant increase often points to corrosion, solder bond degradation, or junction box failure. As a leading cause of panel degradation, accounting for 35% of failures, microcracks are a primary suspect for increasing series resistance by disrupting electrical pathways.
PVTestLab’s Measurement Approach
On an I-V curve, the signature of high series resistance is unmistakable. A healthy module shows a sharp “knee” as it approaches its maximum power point (MPP), but a module with high Rs will display a much gentler, flatter slope near the open-circuit voltage (Voc). This change in slope directly measures the increased resistance choking the module’s output.
Data Correlation
A flattened I-V curve isn’t just a data anomaly; it’s a clear indicator of a physical problem. Our process engineers correlate this signature with specific manufacturing or degradation issues:
- Gradual Slope Change: Suggests uniform degradation, such as aging of cell metallization or interconnect ribbons.
- Sudden Drop-off: Points to a more acute failure, like a corroded or failed solder joint within the junction box.
By combining I-V curve tracing with thermal imaging, we can often see the high-resistance point glowing as a hot spot, providing visual confirmation of the electrical fault.
Mitigation and Validation
Once identified, the mitigation strategy depends on the cause. A junction box failure requires replacement, while degradation in the laminate shifts the focus to process improvement. At PVTestLab, we help manufacturers validate solutions by testing alternative ribbon materials, soldering profiles, or junction box designs to engineer more durable connections from the start. This is a core part of our Material Testing & Lamination Trials service, ensuring that modules are built to resist these failure modes.
Pinpointing Shunt Resistance (Rsh) and PID
If series resistance is like friction, low shunt resistance is like a leak. It creates an alternative path for current to bypass the external circuit, effectively short-circuiting parts of the cell and draining power. One of the most aggressive causes of this decreasing shunt resistance is Potential-Induced Degradation (PID).
The Parameter: Shunt resistance (Rsh)
Shunt resistance (Rsh) measures the opposition to this leakage current. In a perfect module, Rsh would be infinite. PID occurs when a high voltage potential between the cells and the grounded module frame creates leakage paths, causing a catastrophic drop in shunt resistance. The data is stark: PID can cause a decrease in shunt resistance of up to 93%, leading to a fill factor drop from a healthy 74% down to 63% or lower.
PVTestLab’s Measurement Approach
A drop in shunt resistance changes the I-V curve’s slope near the short-circuit current (Isc). A healthy module’s curve is nearly flat in this region, while a PID-affected module will show a distinct downward tilt. This slope is inversely proportional to the shunt resistance—the steeper the tilt, the worse the leak.
Data Correlation
The I-V curve reveals what is happening, but Electroluminescence (EL) imaging shows us where. EL testing involves running a current through the module in the dark, causing it to emit near-infrared light. Healthy cells glow uniformly, while shunted or inactive cells appear dark. For PID, the signature is unmistakable: cells near the module frame, particularly at the corners, will appear completely black, perfectly correlating with the power loss measured on the I-V curve.
Mitigation and Validation
The best defense against PID is using PID-resistant materials, especially encapsulants and glass. At PVTestLab, we conduct extensive Material Testing & Lamination Trials to validate the performance of new materials under real industrial lamination conditions. By subjecting prototype modules to high-voltage stress tests, we provide manufacturers with the data they need to select materials that prevent PID before a single module ever reaches the field.
Identifying Cell and Module Mismatch
Mismatch loss occurs when cells or modules with different electrical characteristics are wired together in series. The string’s performance is limited by its weakest link, causing stronger cells to be underutilized while they dissipate their excess energy as heat.
The Parameter: Mismatch
Mismatch can stem from manufacturing variations, uneven degradation from factors like microcracks, or partial shading. Even a single underperforming cell can compromise an entire string’s output, activating a bypass diode and creating a distinct signature on the module’s performance curve.
PVTestLab’s Measurement Approach
Mismatch is easily identified by the presence of „steps“ or „notches“ in the I-V curve. Each step corresponds to a cell string’s bypass diode activating to route current around an underperforming or shaded section. The number of steps tells you how many strings are affected, and the shape of the step provides clues about the severity of the mismatch.
Data Correlation
By analyzing the I-V curve, our engineers can diagnose the extent of the mismatch:
- A single, sharp step: Indicates one entire string is being bypassed, often due to localized damage or shading.
- Multiple, shallower steps: Suggests several cells with varying degrees of degradation across different strings.
- A rounded, distorted curve: Can point to widespread mismatch from manufacturing variability or uneven aging across the module.
This I-V data, combined with EL imaging that visualizes the underperforming cells, provides a complete picture of the module’s internal health.
Mitigation and Validation
For manufacturers, the key is minimizing variance from the start. Our Prototyping & Module Development service allows developers to build and test new module designs in a controlled, industrial environment. We help fine-tune cell sorting strategies, optimize interconnection processes, and validate designs that are more resilient to mismatch losses, ensuring higher and more reliable energy yield over the module’s lifetime.
Frequently Asked Questions
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Can’t I diagnose these issues with a simple multimeter?
A multimeter provides single-point measurements, like Voc or Isc. While these are useful for basic continuity checks, they can’t reveal a module’s full performance characteristics. An I-V curve tracer measures the output across a range of voltages, providing hundreds of data points that create a unique signature. It’s the difference between taking a single snapshot and watching a full diagnostic video of your module’s health. -
How do you differentiate between shading and a permanent fault like PID on an I-V curve?
While both can cause distortions, shading often creates very sharp, distinct steps in the curve corresponding to hard shadows. PID-induced shunting typically results in a change to the curve’s slope rather than a sharp step. The best way to be certain is to use corroborating evidence: repeat the test when shading is not present or use EL imaging. Shading won’t appear on an EL image, while PID and other cell defects will be clearly visible. -
Is this level of advanced testing worth the investment?
For asset owners and manufacturers, undiagnosed, fleet-wide degradation represents a far greater financial risk than the cost of testing. Identifying a systemic issue like PID or a recurring manufacturing defect early can prevent millions in lost revenue and warranty claims. This level of diagnostics transforms maintenance from a reactive cost center into a proactive, data-driven strategy for asset optimization. -
How is PVTestLab different from a standard certification lab?
Certification labs test a finished product against a fixed standard and provide a pass/fail result. PVTestLab operates as an applied research partner. We don’t just identify a problem; we use our full-scale production line and German engineering expertise from J.v.G. Technology to help you diagnose the root cause in your materials or processes. Our goal is to bridge the gap between a test result and an implemented manufacturing solution.
Move from Guesswork to Certainty
Understanding the electrical signatures of your solar modules is key to protecting their performance and longevity. By moving beyond simple visual checks and embracing data-driven diagnostics, you can identify hidden losses, make smarter maintenance decisions, and drive continuous improvement in your technology.
Ready to turn your test data into a clear path for optimization? Explore how our Prototyping & Module Development services can validate your designs, or contact our process engineers today to discuss your specific diagnostic challenges.