Photovoltaic module degradation is generally divided into light-induced degradation and aging-induced degradation. Currently, a more widely accepted method is PID (Potential Induced Degradation), which has gained considerable acceptance among technical researchers. The former two are currently the most discussed. Light-induced degradation is mainly affected by battery manufacturing processes and battery materials, referring to a significant and rapid decrease in the output power of photovoltaic modules during the initial few days of use, but the output power gradually stabilizes.
I. Theory of Light-Induced Attenuation
Irradiation or current injection causes boron in silicon wafers to combine with oxygen to form boron-oxygen complexes, which further reduces the minority carrier lifetime in the silicon wafer and leads to a decrease in the efficiency of photovoltaic modules. The higher the boron-oxygen content in the silicon wafer, the more boron-oxygen complexes there will be under illumination or current injection conditions. The more complexes there are, the greater the power attenuation of the module. Therefore, low oxygen, low boron, gallium doping, phosphorus doping, and replacing boron with gallium-phosphorus can effectively reduce the attenuation of photovoltaic modules.
Solutions to light-induced degradation in photovoltaic modules: The oxygen and boron content in the silicon wafer determines the degree of light-induced degradation of the module. Therefore, the less boron and oxygen in the silicon wafer, the better the wafer quality and the less light-induced degradation the module will experience. Fundamentally, addressing light-induced degradation in photovoltaic modules starts with the silicon wafer itself.
Method 1: Improving the Quality of Boron-Doped P-Type Czochralski-Grown Monocrystalline Silicon: In China, boron-doped Czochralski-grown monocrystalline silicon is currently the mainstream product in the silicon ingot market. In silicon ingot manufacturing, it is crucial to avoid using low-quality polycrystalline silicon; control the addition of excessive low-resistivity n-type silicon to prevent the production of highly compensated p-type monocrystalline ingots, as extremely high boron and oxygen content will lead to significant light-induced degradation in photovoltaic modules; improve the ingot pulling process to reduce boron and oxygen content, decrease defect density, and improve resistivity uniformity.
Method 2: Replace boron with gallium: No light-induced degradation problem was found with this method.
Method 3: Both magnetron-controlled Czochralski (CZ) monocrystalline silicon and zone-melting monocrystalline silicon processes can alter the quality of silicon wafers. The latter avoids the defect of large amounts of oxygen entering crystalline silicon, thus completely solving the degradation problem of boron-doped silicon wafers and photovoltaic modules.
Method 4: Use p-doped n-type silicon instead of boron-doped p-type silicon wafers. N-type silicon wafers can solve the light-induced degradation problem, but from the perspective of existing technology and processes, they have no advantage in terms of conversion efficiency and manufacturing cost.
Method 5: Improve silicon wafer processing level to improve the consistency of silicon wafer performance, and further improve silicon wafer quality by using silicon wafer sorting machines, such as solar photovoltaic cell module degradation testers.
Method Six: Since the light-induced degradation of photovoltaic modules is caused by the initial light-induced degradation of the cells, the silicon wafer can be photo-induced before the photovoltaic module is manufactured. This can completely control the light-induced degradation within the measurement error and greatly improve the stability of the module's output power.
Summary of initial light-induced degradation tests on photovoltaic modules:
1. When the performance of solar cells degrades, it inevitably leads to a decrease in the output power of photovoltaic modules, which can easily cause hot spot effects in the modules and seriously cause a series of safety hazards in photovoltaic power plants.
2. If the current between battery strings within a module is inconsistent, a stepped curve can be seen on the IV curve of the module connected to the bypass diode.
3. The light-induced degradation phenomenon of the component can be examined by measuring the output characteristics of the component before and after illumination and by infrared imaging analysis.
II. Aging and Decay
In photovoltaic power plants, photovoltaic modules experience slow degradation over long-term use. This degradation is related to the gradual decay of the cells within the module. Generally, the degradation rate of photovoltaic modules is positively correlated with the manufacturing process, encapsulation materials, and the environment in which the module is used. Common issues such as cracking, yellowing, wind and sand abrasion, hot spots, and module aging can all accelerate power degradation. Addressing the aging and degradation problem of photovoltaic modules primarily involves considering the module's manufacturing process, materials, and common quality issues.
III. Potential-Induced Degradation Effect of PID Photovoltaic Modules
Modern researchers generally believe that this component degradation is caused by high voltage between the internal circuitry of the component and its grounded metal frame, which leads to power reduction. Furthermore, potential-induced degradation effects are also present. It is also related to the battery, glass, EVA, temperature, humidity, and voltage.
Attenuation mechanism: Under high voltage, ion migration occurs in the cell encapsulation material and the materials on the upper and lower surfaces of the module, resulting in hot carrier phenomenon in the cell. Charge distribution reduces cell activity.
Finding solutions to potential-induced degradation effects is a future direction for photovoltaic power plant efficiency research, aimed at improving the reduction of component output power loss, enhancing the stability of photovoltaic power generation, and ensuring the lifespan of photovoltaic power plants.
