EL Inspection

 

 

EL inspection is a diagnostic method used to detect defects in solar cells, applicable to every level from individual cells to large-scale module arrays. Currently, 100% full inspection via EL has become standard practice during solar module manufacturing, and its application for on-site module array inspection in power plants is also gaining popularity.

But what exactly is EL inspection, and what kinds of defects can it detect? 

How EL Inspection Works

EL stands for Electroluminescence, commonly referred to as "EL" within the industry.

During EL inspection, a forward electrical current is injected into the module (or individual cell). As current flows through the solar cells, they emit near-infrared (NIR) light—similar to how an LED chip lights up when energized. However, because solar cells emit non-visible light, specialized cameras are required to capture EL images.

It is important to note that EL inspection must typically be conducted in a darkroom environment. For field inspections at project sites, daytime sunlight contains intense ambient NIR interference, making daytime EL imaging impossible; inspections must instead be conducted after sunset under dark conditions. While specialized daylight EL systems have been developed, their higher equipment and operational costs make them suitable primarily for niche applications.

The image below shows an example of an EL scan. The solid black, non-emitting areas represent region where current cannot reach because they are completely isolated by cracks—meaning any power generated in these zones cannot be collected. Thus, black zones in an EL image indicate completely inactive areas.

In contrast, gray or dimmed areas represent "partially inactive zones" that retain slight electrical contact with surrounding regions. A small amount of current can still pass through, indicating these areas can still generate power under sunlight, albeit with internal resistive losses.

Finally, fine black line patterns indicate cracks within the cell. Because the crack edges remain in physical contact with one another, current can still flow across them. These cracks are invisible to the naked eye and are therefore called "micro-cracks." Over time, micro-cracks tend to propagate and expand under mechanical stress and thermal cycling, causing crack gaps to widen and leading to progressive power loss.

 

Defects Detectable by EL Inspection

Beyond micro-cracks and "dead zones," EL imaging can reveal various other critical defects. Below are common examples:

 1. PID (Potential Induced Degradation)

In PV systems, modules are connected in series to reach voltages between 1000V and 1500V. Modules positioned near the high negative voltage end of a string are particularly susceptible to Potential Induced Degradation (PID), which can cause rapid power attenuation exceeding 50% in severe cases.

However, a key characteristic of PID is its reversibility: specialized recovery equipment can restore PID-degraded modules back to over 90% of their original capacity. Through EL inspection, PID issues can be diagnosed at a very early stage—even before noticeable power drop occurs. If project owners wait until system yield drops significantly before identifying the issue, substantial generation losses will have already accumulated. Early detection and timely remediation of PID represent one of the most cost-effective applications of EL inspection.

 

2. Shorted Bypass Diode

Bypass diode failures typically manifest as short circuits, which appear in EL images as an entire substring completely dark (as shown in the image below). Typically, a single shorted bypass diode results in the loss of one-third of the module's power output; however, once modules are connected in series/parallel strings, the overall system loss can be further magnified.

Because a shorted diode induces only a minor voltage drop, monitoring systems often misclassify this issue as a measurement error, making it difficult to detect through remote performance monitoring alone. In contrast, EL imaging enables immediate detection and pinpoints the exact defective module, facilitating rapid field maintenance and replacement.

 

3. LID (Light Induced Degradation)

After module installation, initial power degradation occurs upon exposure to sunlight. This stabilization process typically reaches equilibrium after about one week of light exposure, after which no further LID-related degradation occurs; this initial power loss is known as Light Induced Degradation (LID). LID issues primarily occur in P-type cells. In contrast, modern N-type cells are virtually immune to significant LID losses due to their inherently different substrate physics.

The rate of degradation in P-type modules varies depending on cell technology, wafer quality, and manufacturing processes, with severe cases experiencing power losses exceeding 10%. Fortunately, most modern cell manufacturers have implemented anti-LID process treatments, keeping the degradation rate below 3% in most cases. However, improperly processed batches occasionally still enter the market, leading to severe overall system yield losses (as shown in the image below). In EL images, affected cells appear noticeably darker than surrounding ones because their power output has degraded significantly more than adjacent cells.

 

4. Inherent Module Defects

Modules may leave the factory with inherent manufacturing defects resulting from process or material flaws, which manifest as various irregular patterns in EL images. A key example is poor soldering quality, as shown in the image below. This type of soldering defect is directly related to improper manufacturing processes. Furthermore, affected dead zones tend to expand over time due to thermal expansion and contraction, leading to continuous power degradation.

 

5. Rear-Side Damage

Improper handling and installation procedures can cause physical scratches on the back of a module. When the underlying solar cell is damaged, it appears as a distinct black scratch mark in EL images (as shown in the image below).

More critically, a punctured or compromised backsheet allows moisture ingress, accelerating module degradation and creating significant electrical leakage risks. Electrical leakage can trigger inverter trips and shut down generation output, resulting in severe revenue loss. Because front-side visual inspections cannot detect this type of damage, identifying it via EL imaging is one of the most vital diagnostic capabilities of EL inspection.

 

Differences Between EL and IR Inspection

While both Electroluminescence (EL) and Infrared (IR) thermography are essential diagnostic tools for solar modules, they serve distinctly different functions. The following module test illustrates the differences between the two methods. The image below shows the same module captured via both EL and IR, with the two images overlaid for direct comparison. Key insights include:

(1) EL imaging can pinpoint every individual micro-crack and inactive area, whereas IR thermography only detects broader surface temperature variations.

(2) The cell with the highest temperature exhibits the most severe fracturing. However, a severely fractured cell does not necessarily run at a high temperature. Because cells within a string are connected in series, the most severely damaged cell restricts the current flowing through the entire string—much like a pinched hose reducing total water flow—thereby preventing other defective cells from reaching high temperatures.

(3) Hotter cells do not necessarily contain micro-cracks. In this specific case, three cells in the bottom-right corner operate at higher temperatures due to lower intrinsic cell efficiency compared to neighboring cells, despite showing no micro-cracks or physical defects in EL scans.

(4) In summary, EL imaging precisely identifies the exact fault mechanism within each individual cell, whereas IR thermography only detects localized high temperatures, making it difficult to determine the specific failure mode or assess defect severity. Observing a single hot cell on a module does not mean only one cell is damaged; EL or other complementary diagnostic tools remain necessary to evaluate the true extent of the damage.

 

Timing and Application of EL Inspection

Because EL imaging can precisely identify specific defect types and their exact locations, it can be deployed at every stage of a solar project's lifecycle. However, because field EL testing must be conducted at night and has a lower throughput compared to airborne thermography, its operational cost is relatively higher. Consequently, EL is typically performed on a sampling basis rather than as a 100% full-site inspection (which is standard for aerial IR thermography). Alternatively, EL serves as a secondary diagnostic tool—when IR detects thermal anomalies but cannot determine the underlying failure mechanism, EL is deployed to pinpoint the exact defect type and guide corrective actions.

 

The primary deployment timing and use cases for EL inspection are summarized below:

·       Module Delivery / On-Site Arrival Inspection (Inspected upon arrival at project site, prior to mounting)

o   Inherent manufacturing defects

o   Transportation and transit damage

·       System Commissioning & Acceptance Inspection

o   Material handling damage during construction

o   Improper installation practices (e.g., cell/module damage from worker trampling)

·       Periodic O&M Inspection

o  Damage from improper maintenance practices (e.g., foot-traffic damage during module cleaning)

o   Environmentally induced module failures (e.g., severe mechanical stress from typhoons)

·       Post-Incident Loss Assessment

o   Damage evaluation after accidental impacts (e.g., collapsed module stacks or dropped panels)

o   Insurance claim quantification (e.g., determining whether storm-dislodged modules remain usable)

·       System Underperformance Investigation

o   Root-cause diagnosis for arrays failing to meet expected energy yields

o   Performance investigation for rapid or abnormal power attenuation

 

About the Author

Jay Lin, warmly known as Dr. Jay among industry peers:

·    2004: Engaged in solar module R&D at the Industrial Technology Research Institute (ITRI).

·    2005: Partnered with TÜV Rheinland to establish Asia’s first solar testing and certification laboratory in Taiwan.

·    2007: Served as CTO at a2pak Power, a German-backed solar venture.

·    2011: Founded PV Guider, a consulting firm providing professional advisory services and solar power plant quality control solutions.

 

Current Roles:

·    Chief Consultant, PV Guider

·    Task Force Leader / Chairman, SEMI Standard Committee

·    Member, Technical Committee of CNS National Standards

·    Technical Expert, IEA PVPS Task 13 (Photovoltaic Power Systems Programme - Solar Reliability Workgroup)

·    Technical Committee Member, European Photovoltaic Solar Energy Conference and Exhibition (EU PVSEC)