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Analysis of UV-Induced Degradation (UVID) of Solar Cells

2025-04-20

Main Differences Between UV-Induced Degradation (UVID) and Other Degradation Mechanisms

UV-induced degradation (UVID) in solar cells is different from other common degradation types like LID (light-induced degradation), LeTID (light- and elevated temperature-induced degradation), and PID (potential-induced degradation) in the following key ways:

Damage to the Surface Passivation Layer by High-Energy Photons:

Materials like SiNx:H help protect the solar cell surface by using hydrogen atoms to “heal” defects. But UV light can break the Si-H bonds because UV photons have more energy than the bond strength (3.34–3.5 eV). This bond-breaking damages the passivation layer and creates defects on the surface.

Activation of Defects Inside the Silicon Bulk:

UV light can energize electrons to the point that they change the state of metal impurities like iron or copper (e.g., Fe⁰ to Fe⁺), causing these metals to move to defect-prone areas like grain boundaries. This creates new defect centers that reduce performance. While this is similar to what happens in LeTID (where hydrogen-metal bonds break down), UV does it just with light—no heat needed. UV can also trigger defect formation related to oxygen precipitates, whereas traditional LID mostly involves boron-oxygen pairs. There have been studies showing UV can activate such defects in polycrystalline PERC solar cells (see reference [1]).

Hot Carrier Effects:

UV photons generate electron-hole pairs with very high kinetic energy (much higher than in normal conditions). Some of these electrons can overcome the energy barrier at the SiNx/Si interface (~2–3 eV) and crash into the passivation layer, damaging it. These “hot” electrons can also cause secondary damage through tunneling effects (Fowler-Nordheim tunneling) or by knocking other electrons loose, starting a chain reaction of material damage.

Table 1: Characteristics of Different Solar Cell Degradation Types

Degradation Type	Driving Factor	Affected Area	Mechanisms	Solutions
UVID	High-energy photons	Interface/Bulk/Passivation	Bond breakage, defect activation, hot carriers	Optimize passivation, use high-purity materials, barrier engineering
LID	Visible light	Bulk Silicon (B-O pairs)	Boron-oxygen complex formation	Non-boron doping, electrical/optical annealing
LeTID	Light + High heat (>75°C)	Bulk Silicon/Hydrogen-metal	Hydrogen release, metal activation	Low-temp sintering, electrical annealing, impurity absorption
PID	Voltage difference + Humidity	Surface/Encapsulation/Bulk	Sodium migration, defect formation	Anti-PID materials, cell layer optimization

Behavior of UV Degradation in Different Solar Cells

Figure 1 shows the degradation of various types of solar cells under UV exposure. The left graph presents the maximum power UV degradation curves of PERC, TOPCon, and HJT solar cells from different sources. From the graph, it’s clear that PERC solar cells experience relatively low UV degradation, while HJT cells show more significant UV degradation, which is higher than that of PERC cells. TOPCon cells, on the other hand, have a more scattered UV degradation rate. At a UV exposure dose of 120 kWh/m², the maximum power degradation ranges from less than 1% to over 15%.

The right graph shows the iVmpp degradation curves of the same solar cells (PERC, TOPCon, and HJT) under UV exposure. Similar to the left graph, TOPCon cells exhibit a broad range of degradation. At a UV dose of 60 kWh/m², the iVmpp degradation shows two extremes: one cell experiences almost no degradation, while another sees a nearly 40 mV drop. HJT cells also show significant UV degradation, similar to the left graph. However, unlike the left graph, PERC cells show more variation, with iVmpp degradation differing by more than 20 mV across different cells.

A key difference between the left and right graphs is that the degradation curves in the right graph show a clear trend of rapid initial degradation followed by gradual saturation. In contrast, the left graph shows some cells with this trend, while others exhibit a continuous, linear degradation.

Figure 1: Behavior of UV Degradation in Different Solar Cells

Figure 2 shows the change in maximum power of different types of solar cells after 2000 hours of UV exposure at the National Renewable Energy Laboratory. The most significant power loss was observed in the heterojunction (HJT) solar cells, followed by n-PERT and p-type PERC cells. In contrast, the aluminum back surface field (Al-BSF) cells showed little to no power loss. Additionally, the IBC (Interdigitated Back Contact) cells exhibited a slight increase in power.

Figure 2:> the change in maximum power of different types of solar cells after 2000 hours of UV exposure

Table 2 compares the electrical performance of heterojunction (HJT) solar cells before and after UV degradation. It’s clear that UV exposure significantly reduced the minority carrier lifetime and the corresponding iFF of the cells, indicating that UV light negatively impacts the surface passivation of the heterojunction solar cells. Previous reports have shown that when HJT cells are exposed to UV light, high-energy UV photons can break Si-H bonds. This causes a change in the distribution of hydrogen at the amorphous silicon/crystalline silicon interface, increasing the hydrogen concentration near the doped amorphous silicon layer and the intrinsic amorphous silicon layer, while decreasing the hydrogen concentration near the interface and in the crystalline silicon bulk. The reduction in hydrogen concentration observed at the amorphous silicon/crystalline silicon interface suggests that the effective amount of hydrogen, which is crucial for passivating dangling bonds and reducing interface defect density, has decreased.

Table 2:The comparison of electrical characteristics of HJT solar cells before and after UV degradation.

To summarize the UV degradation behavior of the solar cells mentioned above:

Heterojunction (HJT) Solar Cells:

Amorphous silicon passivation plays a key role in the performance of HJT solar cells.
UV exposure causes a noticeable drop in passivation effectiveness.
This results in a significant decrease in the efficiency of silicon heterojunction solar cells.

PERC and TOPCon Solar Cells:

Degradation patterns vary among different cells of these types.
UV degradation mechanisms are influenced not only by the cell technology but also by the materials and manufacturing processes used.

TOPCon Solar Cells:

UV degradation in TOPCon solar cells has mixed reports.
Some studies report severe UV degradation, while others suggest good UV stability.

3. UV Degradation Mechanism

3.1 UV Degradation Mechanism in Silicon Heterojunction Solar Cells

To quantify the redistribution of hydrogen caused by UV degradation, a secondary ion mass spectrometry (SIMS) method was used in the study. Figure 3 shows the depth distribution of silicon (Si), oxygen (O), and hydrogen (H) elements on the surface of silicon heterojunction solar cell samples after ITO etching, both before and after UV degradation. The results show that after UV degradation, the hydrogen concentration is lower near the amorphous silicon/crystalline silicon interface and in the crystalline silicon bulk material. The decrease in hydrogen near the amorphous silicon/crystalline silicon interface suggests that less hydrogen is available to passivate dangling bonds or reduce interface defect density. This leads to increased carrier recombination at the interface and ultimately results in a decline in passivation performance. This is the main cause of UV degradation in silicon heterojunction solar cells.

Figure 3:Secondary ion mass spectrometry depth distribution chart of silicon, oxygen, and hydrogen elements on the surface of silicon heterojunction solar cell samples after ITO etching, before and after UV degradation

Secondary ion mass spectrometry depth distribution chart of silicon, oxygen, and hydrogen elements on the surface of silicon heterojunction solar cell samples after ITO etching, before and after UV degradation

3.2 UV Degradation Mechanism in PERC Solar Cells

In monocrystalline silicon PERC solar cells, UV-induced degradation mainly results from changes in surface passivation performance. Figure 4 shows the open-circuit voltage degradation curves of PERC solar cells with different refractive index SiNx passivation layers under UV exposure. It’s known that for PECVD-grown SiNx films, the lower the refractive index, the higher the hydrogen content. Similar to the degradation mechanism in heterojunction solar cells, UV exposure causes hydrogen loss in the SiNx films. As a result, with a lower refractive index, the trend of degraded passivation becomes more obvious under UV exposure. On the other hand, in SiNx films with a higher refractive index, the concentration of silicon-hydrogen bonds decreases, and silicon-silicon and silicon-nitrogen bonds are stronger, making them more resistant to UV light and resulting in higher stability.

Figure 4:Open-circuit voltage degradation curves of PERC solar cells with front surface SiNx passivation layers of different refractive indices under UV exposure

$Open-circuit voltage degradation curves of PERC solar cells with front surface SiNx passivation layers of different refractive indices under UV exposure$

3.3 UV Degradation Mechanism in TOPCon Solar Cells

Figure 5 shows the change in IQE curves for different TOPCon solar cells before and after UV exposure at a dose of 60 kWh/m². From the figure, it can be seen that for TOPCon solar cells with high UV degradation, the degradation mainly comes from the front surface. This suggests that it is still primarily related to the deterioration of the front surface passivation.

Figure 6 shows the impact of different passivation structures and post-treatment on passivation stability under UV exposure. For the n/p+ front surface structure of TOPCon solar cells, if there is only a single layer of Al2O3 passivation, annealing at 350°C results in better UV stability for the Al2O3 passivation layer compared to sintering. For the Al2O3 passivation layer, different annealing conditions lead to varying interface state densities (Dit) and fixed charge amounts (Qf), which in turn affect UV stability. For Al2O3/SiNx stacked passivation layers, the SiNx deposition process acts as an additional annealing step, and the involvement of hydrogen during deposition also affects Dit and Qf. For higher Qf, the surface recombination rate is proportional to Dit/Qf². Therefore, different deposition processes, film thicknesses, and post-treatment processes affect these two parameters and the corresponding UV stability. As a result, TOPCon solar cells from different manufacturers often show varying levels of UV stability.

Figure 5: Change in IQE curves of different TOPCon solar cells before and after UV degradation at a dose of 60 kWh/m²

Figure 6: The impact of different passivation structures and post-treatment on passivation stability under UV exposure

4. Methods to Improve UV Stability

(1) SiOx/SiNx Stacked Layer:

Si-O bond energy (~8 eV) is much higher than UV photon energy, so it is relatively stable under UV exposure. This strategy is widely used in the manufacturing of PERC solar cells. Therefore, PERC solar cells with SiOx/SiNx stacked passivation layers, typically produced in later-stage industrialization, generally exhibit good UV stability. However, PERC solar cells with only a single SiNx passivation layer on the front may experience significant UV degradation.

(2) AlOx/SiNx Stacked Layer Optimization:

For TOPCon solar cells with UV degradation issues, UV stability can be improved by optimizing the thickness, coating process, and post-treatment of the AlOx/SiNx stacked layers. As shown in Figure 7, increasing the deposition cycle of aluminum oxide can enhance the UV stability of TOPCon solar cells.

Figure 7:The impact of different aluminum oxide coating deposition cycles on UV stability

(4) Inserting SiOx Layer at the Amorphous Silicon/Silicon Interface in Heterojunction Solar Cells

By adding a natural oxide layer at the amorphous silicon/silicon interface in heterojunction solar cells, the high stability of the Si-O bonds helps reduce the impact of lower hydrogen concentration in the amorphous silicon on surface passivation. This improvement is shown in Table 3, which illustrates the effect of adding an oxide layer at the amorphous silicon/silicon interface on the UV stability of HJT solar cells.

Table 3:The effect of adding an oxide layer at the amorphous silicon/silicon interface on the UV stability of HJT solar cells osition cycles on UV stability

(5) Adding UV Down-Conversion Layers in Cells or Modules

This section refers to specialized literature.

References:

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