LONGi Back Contact Modules Achieve Industry-Leading Reliability Under Extreme Stress Testing

LONGi’s back contact (BC) solar modules have successfully completed an extended independent reliability program conducted by TUV SUD, demonstrating exceptional thermo-mechanical stability and moisture resistance that far exceeds international photovoltaic standards.

The results represent a definitive response to long-standing market questions regarding the durability of back contact cell architecture under real-world operating stresses. With degradation capped at 0.83% after 400 thermal cycles — double the standard IEC requirement — and 1.08% following 2,000 hours in a damp heat chamber, the LONGi BC module has demonstrated a reliability profile that substantially outperforms the IEC 61215 threshold of 5%.

Key Findings at a Glance

1. Minimal Degradation Under Thermal Cycling and Damp heat

Degradation remains low under both thermal cycling and damp heat conditions, with values below 1% up to TC400 and around ~1.1% after extended damp heat exposure, significantly lower than IEC allowable limits.

• Stable and Non-Progressive Behavior

Degradation increase from TC200 to TC400 is minimal ~0.14% and no signs of accelerated wear or thermal fatigue. This Indicates robust interconnection design and strong thermo-mechanical reliability

Under damp heat conditions, degradation remains controlled and consistent across samples, confirming stable performance under combined temperature and humidity stress.

• Strong Margin vs Industry Standards

According to IEC standards the maximum allowable degradation: ≤ 5% but LONGi BC module performance 0.83% at TC400 and ~1.08% under damp heat. Demonstrating significant reliability margin, exceeding standard requirements by a wide margin.

Source: Thermal Cycling test by TUV SUD Source: Damp heat test by TUV SUD

Importance of Thermal Cycling and Damp Heat test

Thermal cycling (TC) and Damp heat are the most critical reliability tests for photovoltaic modules, simulating long-term exposure to temperature fluctuations and moisture ingression that impact interconnections, solder joints, and overall structural integrity.

For back contact (BC) technology, these tests are often discussed in the market due to its unique cell architecture, but Longi Back-contact has successfully proven reliability along with high-level performance through independent third-party testing.

Test Scope (TC): The module performance was evaluated under standardized IEC thermal cycling conditions. The procedure involves subjecting the module to controlled temperature cycles between –40°C and +85°C inside an environmental chamber. Each cooling and heating phase is called a cycle.  The test is performed for defined no. of cycles as follows:

TC200: 200 thermal cycles (IEC 61215 requirement)

TC400: 400 thermal cycles (accelerated test)

Test conditions:

1000 W/m² irradiance

25 ± 2°C module temperature

Test Results:

The module shows low and stable degradation under thermal cycling (0.69% at TC200 and 0.83% at TC400), with no progressive trend. It also passed wet leakage and insulation tests with no visual defects, confirming strong electrical and structural integrity.

Stage	Pmax (W)	Degradation vs Initial
Initial	635.9 W	—
TC200	631.5 W	0.69%
TC400	630.6 W	0.83%

Test Scope (DH):

Damp heat testing was performed to assess long-term performance under high temperature and humidity conditions. The module was exposed to 85°C and 85% relative humidity for up to 1000 to 2000 hours, simulating prolonged environmental stress in humid climates.

Test Results:

Stage	Pmax (W)	Degradation vs Initial
Initial	635.4 W	—
1000 h	630.2 W	0.82%
2000h	628.6 W	1.08%

Conclusion:

The evaluated modules demonstrate stable performance under both thermal cycling and damp heat conditions, with degradation remaining below 1% up to TC400 and around ~1.1% under extended damp heat exposure. The modules also passed wet leakage and insulation tests with no visual defects, confirming strong resistance to thermo-mechanical stress.

Overall, the results indicate consistent performance under temperature and humidity stress, with degradation well below IEC limits, supporting reliable long-term operation.