Failure Mechanism of HJT Solar Panel in Damp Heat

In the solar industry, Damp Heat (DH) testing is used to evaluate how sensitive solar modules are to long-term exposure to humid conditions. This test involves exposing the modules to an environment of 85°C and 85% relative humidity for 1000–2000 hours or even longer.

Soda-lime glass, which is low-cost and widely available, is the main type of glass used in module packaging. Many previous studies have pointed out that soda-lime glass can be a source of performance degradation in modules, especially due to the sodium ions it contains. In solar modules, a strong electric field between the solar cells and the metal frame can cause sodium ions to move out of the glass and migrate toward the solar cells, leading to a problem called PID (Potential-Induced Degradation). Even without electrical voltage, soda-lime glass can still release sodium ions when it comes into contact with moisture. Once released, these sodium ions can spread through the encapsulation material and move toward the solar cells.

At the cell level, some studies have looked at how sodium ions affect silicon heterojunction (SHJ) solar cells. In these studies, researchers manually added sodium by applying drops or sprays of sodium-containing solutions onto the cell surface. For example, Gnocchi and colleagues studied SHJ modules with different packaging designs and found that the glass itself is the key factor in DH-related degradation (as shown in Figure 1). They applied NaOH solution droplets to different solar cell surfaces and found that, after just 4 hours of DH exposure, the silicon heterojunction cells showed a significant drop in photoluminescence (PL) signal intensity in the droplet areas. After 24 hours, the PL signal dropped even in areas not touched by the droplets. In contrast, PERC solar cells showed much less degradation under the same conditions (see Figure 2).

Figure 1: DH Degradation Behavior of Silicon Heterojunction Solar Cells and Modules with Different Encapsulation Structures

Figure 2: Effect of NaOH Solution Droplets on DH Degradation of Silicon Heterojunction and PERC Solar Cells

Adachi and colleagues applied a 0.05 wt% NaHCO₃ (sodium bicarbonate) solution to the surface of silicon heterojunction solar cells, both with and without an added silicon oxide protective layer, as shown in Figure 3. After damp heat (DH) treatment, a noticeable decrease in photoluminescence (PL) intensity was observed in the droplet areas on the cells without the silicon oxide layer. This indicates that the passivation effect was compromised. Based on additional data, they concluded that sodium ions do not significantly affect the electrical performance of the ITO layer but can diffuse through it and degrade the performance of the amorphous silicon layer or the passivation interface. This leads to losses in both open-circuit voltage and short-circuit current in the silicon heterojunction solar cells.

Figure 3: Effect of NaHCO₃ Solution on PL in DH Cell Testing

In studies on the damp heat (DH) degradation behavior of silicon solar cell modules, the critical roles of moisture and sodium ions have been repeatedly confirmed. For example, Adachi and colleagues compared glass-backsheet modules using soda-lime glass and sodium-free glass after 1000 hours of DH testing. The modules with soda-lime glass showed an 11% drop in maximum power, while those with sodium-free glass only lost about 1%.

The general degradation pattern is illustrated in Figure 4. In silicon heterojunction (SHJ) modules packaged with sodium-containing glass and lacking edge sealant, moisture gradually enters through the edges during DH testing. At the same time, sodium ions leach from the glass and begin to affect the performance of the SHJ cells. As a result, the degradation starts at the module edges. The significant drop in photoluminescence (PL) intensity indicates that the passivation of the SHJ cells is being damaged.

As the DH testing continues, moisture penetrates deeper into the module, and more sodium ions are released, eventually causing a progressive power loss from the edges toward the center of the module.

Figure 4: Qualitative Pattern of DH Degradation in Silicon HJT Solar Cells

So how do moisture and sodium ions individually affect the electrical performance of silicon heterojunction solar modules? Pirot-Berson and colleagues explored this by comparing different encapsulation materials, including soda-lime glass, borosilicate glass, potassium-containing strengthened glass, and organic materials. The various packaging structures are shown in Figure 5, aiming to understand the individual and combined effects of sodium and moisture on silicon heterojunction solar modules.

Figure 5 : Encapsulation Structures of Silicon Heterojunction Solar Cell Modules in Different Groups

Figure 6 shows the changes in I-V parameters of three glass-glass encapsulated silicon heterojunction modules during a 2000-hour DH aging process. The soda-lime glass module showed significant performance degradation after 1500 hours of aging, with a decrease of approximately 57.6%, primarily due to a loss in Isc (-43.4%). Voc and FF decreased by 17.1% and 9.76%, respectively. For the low-sodium and sodium-free glass modules, after 2000 hours of aging, the power loss was only 2.65% and 3.11%, mainly corresponding to FF loss, while Isc and Voc remained almost unchanged throughout the DH testing. These I-V results clearly indicate that the presence of sodium is the root cause of the power degradation.

Figure 6 : Changes in I-V Parameters of Three Glass-Glass Encapsulated Silicon Heterojunction Modules During DH Aging Process

Correspondingly, Figure 7 shows the EL and PL images taken every 500 hours during the DH aging process for the three groups of modules mentioned above. For the soda-lime glass module, starting at 500 hours of aging, a reduction in EL and PL intensity (dark regions) began to appear at the corners and edges of the module. Over the next 500 hours, the degradation spread toward the center of the module, following the penetration path of moisture into the glass-glass encapsulated module. After 1500 hours of aging, the image became completely black. For the low-sodium or sodium-free glass modules, no significant changes in EL and PL intensity were detected after 1500 hours of DH aging.

Figure 7 : EL and PL Images of Three Glass-Glass Silicon Heterojunction Solar Cell Modules Every 500 Hours During DH Aging Process

Excluding the influence of minority carrier lifetime in the silicon wafer itself, the EL image is correlated with both resistance and passivation characteristics, while the PL image is solely related to passivation properties. After 500 hours of DH aging, the decrease in Isc and Voc in the soda-lime glass group is associated with dark regions visible at the edges of the module in the PL images, which corresponds to the degradation of the cell passivation. The changes in the TCO layer alone do not affect the passivation performance of the silicon heterojunction solar cells, indicating that sodium ions may have passed through the TCO layer and reached the amorphous silicon layer of the cell. Additionally, the loss of FF, accompanied by a significant increase in series resistance, could be due to TCO hydration, OH- chemical adsorption at the grain boundaries, or the reduced conductivity of the TCO layer caused by the presence of sodium.

Unlike the soda-lime glass modules, the low-sodium and sodium-free glass modules did not show a decrease in Isc and Voc after 1500 hours of aging, indicating that the passivation of the cells was maintained. This suggests that moisture alone does not significantly degrade the passivation performance of heterojunction solar cells, but it may affect the series resistance and cause a certain degree of FF reduction.

In summary, as shown in Figure 8, in silicon heterojunction solar cell modules with soda-lime glass, as moisture invades, sodium ions leach out of the glass. Assisted by water molecules, these sodium ions can diffuse along the grain boundaries of the TCO layer and reach the amorphous silicon layer. The incorporation of sodium ions into the amorphous silicon layer leads to a decline in the passivation effect. Simultaneously, the presence of water molecules and sodium ions at the TCO grain boundaries also causes a decline in the conductivity of the TCO layer. Additionally, some studies suggest that certain types of sodium salts can significantly degrade the contact resistance between the silver electrode and the TCO layer, leading to a noticeable decrease in FF. These combined factors result in a significant power loss in the module. When only moisture is involved, the degradation of the silicon heterojunction solar cell modules under DH conditions primarily arises from the decrease in FF.

Figure 8 : Schematic of the Degradation Mechanism of Silicon Heterojunction Solar Cell Modules in DH Testing

References:

• https://doi.org/10.1016/j.solmat.2016.12.029
• https://doi.org/10.1016/j.xcrp.2023.101751
• https://doi.org/10.1016/j.solmat.2024.113190
• https://doi.org/10.1016/j.solmat.2024.113325