Principle and Application of EL (Electroluminescence) in Solar Testing

What is Electroluminescence?

Electroluminescence (EL) is the phenomenon where electrical energy is directly converted into light. It can be categorized into two types based on the applied electric field: low-field and high-field electroluminescence.

Low-field EL, also known as injection-type electroluminescence, is the working principle behind Light Emitting Diodes (Leds). When a forward voltage is applied across a PN junction, charge carriers are injected and cross the junction, leading to a carrier concentration that exceeds thermal equilibrium levels. These excess carriers then recombine, releasing energy either as heat (phonons) or light (photons). The photon emission process—where electrical energy is converted to light—is what defines injection-type EL.

In doped silicon crystals, various energy levels exist due to donors, acceptors, impurities, and defects. When non-equilibrium carriers recombine at these energy levels, they emit photons. Since these energy levels lie below the bandgap (Eg), the emitted photons have wavelengths longer than 1110 nm.

The emission spectrum of silicon under electrical injection shows a peak at approximately 1110 nm, corresponding to inter-band recombination. Due to the thermal distribution of carriers, not all electrons are at the conduction band minimum nor are holes at the valence band maximum. This causes the spectrum to broaden. In silicon, inter-band emission typically spans from 1000 nm to 1300 nm, while additional emission between 1300 nm and 1700 nm results from defect-level recombination.

The intensity of electroluminescence increases with the number of non-equilibrium minority carriers. It is also positively correlated with minority carrier diffusion length and current density.

Evolution of EL Testing Technology

The phenomenon of electroluminescence was first observed in 1907 by British scientist H.J. Round while working on silicon carbide diodes. However, it wasn't until 1936 that Georges Destriau of the University of Paris demonstrated the luminescent behavior of ZnS-phosphor-infused castor oil under an electric field. This discovery helped lay the theoretical foundation for EL technology.

While early research did not target the photovoltaic (PV) industry specifically, the underlying physics—electron transitions and photon emission under an electric field—parallels the photoelectric conversion process in Solar cells. This alignment eventually paved the way for the integration of EL in PV diagnostics.

EL testing in photovoltaics was first applied to detect defects in silicon wafers. By applying a forward bias, technicians observed the luminescence emitted by the wafer under electrical excitation to locate hidden cracks, uneven diffusion, and other anomalies. This non-contact, non-destructive inspection method greatly improved silicon wafer quality control.

When a forward bias is applied to a solar cell, the barrier of the PN junction is reduced, allowing charge carriers to cross the barrier and enter the diffusion region as non-equilibrium minority carriers. These carriers continuously recombine with majority carriers and emit light, which is captured by a CCD camera and converted into an image. The more recombination occurs, the brighter the corresponding area appears in the EL image; less recombination results in dimmer areas.

The brightness of the EL image is directly proportional to the minority carrier lifetime (or diffusion length) and current density. In areas with defects, the minority carrier diffusion length is shorter, leading to weaker light emission. Since defective regions in the solar cell emit little or no infrared light, they appear as “dark areas” or “black spots” in the EL image.

As the PV industry advanced, EL testing became indispensable across multiple stages: raw material inspection, product development, mass production quality control, final product validation, and post-installation maintenance. Today, it is a critical tool that supports the reliable and sustainable development of the solar industry.

Key Advantages of EL Testing Technology

• Efficient Quality Control: Detects micro-cracks, hidden fractures, and hot spots that could compromise solar cell and module performance or lifespan.
• Non-Destructive Inspection: EL testing does not harm cells or modules and can be performed multiple times throughout production.
• Improved Production Efficiency: Enables rapid and accurate defect detection, reducing reliance on manual inspection and lowering operational costs.
• Optimized Product Design: Helps manufacturers understand product performance better and make informed design improvements.

Industry Standards for EL Testing

There are well-established standards guiding EL testing in crystalline silicon photovoltaic products, including:

• T/CPIA0020-2020: Methods for EL Testing of Crystalline Silicon PV Cells
• T/CPIA0009-2019: Imaging Methods for Detecting Defects in PV Modules
• IEC TS 60904-13: Electroluminescence Testing for Photovoltaic Modules

These standards define procedures, conditions, and technical parameters for EL testing. Adhering to them ensures early detection of defects and helps maintain consistent performance and reliability in field applications.

Overview of the EL Testing System

Based on the electroluminescence principle in crystalline silicon, EL testing systems apply a forward bias to inject non-equilibrium carriers into the cell. High-resolution CCD cameras then capture near-infrared images of the module, enabling defect detection and diagnosis.

The system typically includes:

• Constant Current Source: Provides stable and controllable excitation current;
• EL Imaging Camera: Captures emitted IR/NIR light;
• Darkroom Environment: Ensures no ambient light interferes with imaging;
• Image Processing Software: Analyzes captured images to identify defects.

EL systems are divided into two major categories based on their application environments: indoor EL testing and outdoor EL testing.

Indoor EL Testing

Indoor EL testing is conducted in controlled laboratory environments with stable temperature, humidity, and lighting—ideal conditions for detecting internal defects such as broken grid lines, micro-cracks, and concentric rings.

Applications include:

• Product Development: Verifying new designs meet performance standards.
• In-Line Quality Control: Real-time inspection during production.
• Final Inspection: Ensuring every module meets quality benchmarks before shipping.
• Failure Analysis: Diagnosing issues in malfunctioning modules.

Its greatest strengths lie in data repeatability and high image quality, making it ideal for detailed and accurate analysis.

Outdoor EL Testing

Outdoor EL testing is subject to environmental variables such as weather, light conditions, and wind, which may affect image clarity and test accuracy. It includes two main subtypes: portable and drone-based EL testing.

Portable EL Testing

This method is suitable for diagnosing individual modules in the field. It effectively identifies major defects like broken grid lines, micro-cracks, and fragments. However, since the camera is fixed on a tripod and must be manually moved from one module to the next, efficiency is low—making it unsuitable for large-scale solar farms.

Drone-Based EL Testing

This emerging technique involves mounting EL imaging systems on drones, dramatically improving inspection speed for large installations. Drones can cover vast areas quickly, eliminating the need for constant manual repositioning of equipment.

However, imaging quality may be compromised by the drone’s flight stability and external conditions like sunlight, wind, and weather. Additionally, because industry standards for drone-based EL testing are still under development, operators must possess both PV knowledge and advanced drone piloting skills.

As such, while drone EL testing offers efficiency, its results may vary due to subjective and environmental factors. It is best used in combination with other inspection methods for a more reliable evaluation of module performance.