Precision in Detail, Quality for the Future: The Accurate Detection of HJT PV Modules

Amidst the wave of photovoltaic technology advancement, heterojunction (HJT) technology is rapidly emerging as a powerful contender for the next generation of mainstream technology, thanks to its high conversion efficiency, low temperature coefficient, high bifaciality factor, and excellent low-light performance. However, behind this exceptional performance lie extremely precise and complex manufacturing processes. For HJT modules, "perfected by precision, born of inspection" – a precise, comprehensive, and stringent inspection system is the lifeline that ensures their superior performance is realized and remains stable and reliable over the long term.

I. Why are Inspection Requirements for HJT Modules More Stringent?

The unique structure of HJT modules dictionaries and heightened requirements of their inspection compared to traditional PERC or other module types.

Firstly, its core is the ultra-pure amorphous silicon/crystalline silicon interface. The essence of a heterojunction is the deposition of ultra-thin amorphous silicon layers on both sides of a crystalline silicon substrate, creating excellent passivation. This interface exists on an atomic scale; any minute contamination, defect, or deposition non-uniformity can severely compromise passivation quality, leading to a sharp efficiency drop and potential degradation, much like a weak spot in a dam. Therefore, inspection must be capable of "seeing into" the quality of this interface.

Secondly, low-temperature processes and sensitive thin films. HJT production utilizes low-temperature processes throughout (typically <200°C). This avoids thermal damage to the silicon wafer but also means the transparent conductive oxide films and metal electrodes are more "delicate." The conductivity, transmittance, thickness uniformity, and adhesion between these TCO layers directly impact the module's current output and long-term reliability.

Consequently, inspection of HJT modules cannot stop at conventional power and IV testing. It demands a deeper, higher-precision, multi-dimensional inspection system that runs through materials, processes, and finished products.

II. Core Inspection Dimensions: From Material Interfaces to Long-Term Reliability

1. Accurate Assessment of Electrical Performance and Spectral Response

Initial power testing is just the "entrance exam." For HJT modules, fine-grained analysis of the Current-Voltage characteristic curve is more critical.

• Low-Irradiance Performance Evaluation: The excellent low-light response of HJT is a key selling point. Inspection needs to measure its power generation efficiency under various low-light conditions to verify its real-world energy yield gain during cloudy weather, dawn, and dusk.

• Precise Bifaciality Factor Calibration: Bifaciality is another major advantage. Inspection must accurately measure front-side and rear-side efficiency under standardized conditions to calculate the true bifaciality factor. This involves precise simulation and control of rear-side illumination conditions.

• Quantum Efficiency Testing: This is a crucial tool for insight into the module's "inherent potential." By measuring the response efficiency to photons of different wavelengths, it can accurately assess the absorption of short-wavelength light by the amorphous silicon layers, the impact of TCO layers on transmittance, and the utilization of long-wavelength light by the bulk silicon, thereby guiding the optimization of process parameters for each layer.

2. Non-Destructive Structural Defect Detection and Interface Analysis

"Seeing" the invisible defects is key to ensuring HJT module quality.

• Electroluminescence Imaging: EL imaging acts as an "internal diagnostician" for HJT inspection. Under current injection, micro-cracks, poor soldering, contamination, and severe interface defects manifest as dark areas, black patches, or abnormal bright lines in the EL image. High-resolution EL detection effectively screens out products with inherent defects.

• Photoluminescence Imaging: PL inspection is more "proactive." It can be performed at the cell stage, even after coating but before electrode printing. By using a laser to excite photoluminescence from the silicon wafer, PL can detect carrier recombination defects caused by poor passivation layer quality at an earlier stage, enabling upfront interception of quality issues and preventing waste in subsequent process steps.

3. Material and Process Characteristic Analysis

This involves laboratory-grade analysis of the physical and chemical properties of key thin films.

• TCO Film Performance Inspection: This is the "window" and "circulatory system" of the HJT cell. It requires precise measurement of its sheet resistance, visible light transmittance, infrared reflectance, and thickness uniformity. Haze is also an important parameter, affecting light scattering within the module and thus its light-trapping capability.

• Passivation Quality Assessment: Using minority carrier lifetime testers, one can directly measure the carrier lifetime of the silicon wafer after amorphous silicon passivation. A higher lifetime value indicates better interface passivation and greater potential for high open-circuit voltage and efficiency. This is the gold standard for monitoring the stability of the core passivation process.

4. Accelerated Aging and Long-Term Reliability Validation

Excellent initial performance must withstand the test of time. Reliability testing for HJT modules needs to simulate environments often harsher than standard tests.

• Damp Heat Testing: Although HJT itself has a lower power temperature coefficient, the stability of its multi-layer film structure under damp heat is crucial. The double 85 test is foundational, with even longer duration tests sometimes required to evaluate the resistance to hydrolysis and corrosion of the TCO layers and the interfaces between films and electrodes.

• Potential Induced Degradation Testing: PID remains a potential threat to all high-performance modules. For HJT, due to its unique band structure and material system, its resistance to PID under different bias voltages, temperatures, and humidity conditions must be carefully evaluated to ensure stability in complex grid environments and under high system voltages.

• Mechanical Load and Hail Impact Testing: HJT often uses ultra-thin silicon wafers, making their mechanical stress resistance a key industry focus. Static and dynamic mechanical load testing, alongside hail impact tests, are necessary to verify structural integrity under strong wind, heavy snow, and severe weather. EL inspection should be performed again after mechanical tests to identify any new micro-cracks induced by stress.

III. Challenges and Outlook: Towards a Smarter Inspection Future

HJT module inspection also faces new challenges. For instance, how to perform effective inline EL inspection on the rear side of bifacial modules? How can lab-grade film analysis techniques be simplified into fast, non-destructive online monitoring methods suitable for production lines? This drives continuous innovation in inspection technology.

In the future, intelligent inspection based on big data and artificial intelligence is an inevitable trend. By collecting full-process inspection data for every module on the production line and using AI algorithms for deep learning and correlation analysis, predictive models linking "process parameters" to "final performance and defects" can be built. This not only enables millisecond-level automatic defect classification but also allows for intelligent feedback to optimize process parameters, creating a closed loop of "inspection-analysis-optimization." This ultimately enables continuous improvement in the production yield and quality of HJT modules.

Conclusion

In the journey where heterojunction technology leads the photovoltaic industry to new heights, precise inspection is not merely the "goalkeeper" ensuring the performance and reliability of every module leaving the factory; it is also the "compass" driving continuous technological optimization and maturation. It spans from the microscopic atomic interface to the macroscopic power plant, building a quality assurance and trust system throughout the product's entire lifecycle. For the market, a publicly transparent, stringent inspection process that far exceeds industry standards is, in itself, the most powerful commitment to customers and the best embodiment of the high-end, reliable brand value represented by "Heterojunction." Investing in cutting-edge inspection technology is an investment in the future of the industry.