The power generation principle of photovoltaic cells
The working principle of photovoltaic cells is based on the photoelectric effect, which means that when photons are irradiated onto semiconductor materials, electron hole pairs are generated. These electrons and holes are separated under the action of an electric field, generating an electric current. Its specific working process can be explained as follows:
Photon absorption: When sunlight shines on a photovoltaic cell, semiconductor materials (such as silicon) absorb photons.
Generation of electron hole pairs: When the photon energy is greater than the bandgap energy of the semiconductor, it will excite electrons to transition from the valence band to the conduction band, forming electron hole pairs.
Electric field separation: At the p-n junction of the battery cell, a built-in electric field separates electrons and holes, causing electrons to move towards the n region and holes to move towards the p region.
Current formation: In an external circuit, electrons flow through wires to form current and output electrical energy.
1. What is hidden cracking in photovoltaic cells?
Micro cracks in photovoltaic cells refer to tiny cracks that exist inside or on the surface of the cell, which may not be visible to the naked eye but can have a significant impact on the performance and lifespan of the cell.
a) Reduce effective power generation area: The hidden crack area cannot effectively participate in photoelectric conversion, reducing the effective power generation area of the battery cell. Resulting in a decrease in overall power generation efficiency.
b) Increasing resistance: Hidden cracks can cause local resistance to increase, making it difficult for electrons to flow smoothly in the hidden crack area. Resulting in an increase in series resistance, a decrease in the fill factor (FF) of the battery cells, and a decrease in output power.
c) Current mismatch: Hidden cracks can cause uneven current inside the battery cell, with some areas having lower current. Current mismatch in series connected battery cells can affect the performance of the entire battery string and reduce component output.
d) Hot spot effect: The high resistance in the hidden crack area can cause local overheating, forming hot spots. Hot spots may damage battery cells or cause fires, seriously affecting system safety.
e) Accelerated aging: Hidden cracks can make the structure of battery cells more fragile and susceptible to further damage from external environments such as temperature changes, mechanical stress, etc. The lifespan of battery cells is shortened and their long-term stability decreases.
f) Current leakage: Hidden cracks may cause current to leak from the hidden crack area, forming unnecessary current paths. Resulting in a decrease in the overall efficiency of the battery cells and a reduction in the output of electrical energy.
Reasons for the formation of hidden cracks:
1) Stress during the manufacturing process:
a) During the cutting, welding, and packaging process of battery cells, mechanical stress may be generated, leading to hidden cracks.
b) During the heat treatment process, temperature changes may cause thermal stress, leading to hidden cracks.
2) Vibration and impact during transportation and handling
During transportation and handling, if packaged improperly, battery cells may be subjected to vibration and impact, leading to hidden cracks.
3) Mechanical stress during installation: Improper operation, excessive mechanical compression, and stretching during installation may cause hidden cracks in the battery cells.
4) The impact of environmental conditions
During actual operation, battery cells are affected by environmental factors such as temperature changes, wind loads, and snow loads, leading to hidden cracks.
Testing method:
EL test (electroluminescence test)
Principle: By applying current, the battery cell emits light, and using an infrared camera to detect the light emission of the battery cell. The hidden cracks will appear as dark spots.
Advantages: It can intuitively display the distribution and severity of hidden cracks.
Disadvantage: It needs to be carried out under darkroom conditions and the operation is complex.
Infrared thermal imaging principle: By detecting the infrared thermal imaging image of the battery cell, identify the temperature abnormal area caused by hidden cracks.
Advantages: It can quickly and non destructively detect large-area battery cells.
Disadvantages: High requirements for testing equipment and high cost.
Photoluminescence (PL) testing
Principle: Using laser irradiation to detect the photoluminescence characteristics of battery cells. The luminescence at the hidden crack will show significant changes.
Advantages: Fast detection speed, suitable for production line inspection.
Disadvantage: It has high requirements for detection equipment and environment.
Ultrasonic testing
Principle: Utilizing the reflection and transmission characteristics of ultrasound to detect hidden cracks inside battery cells.
Advantages: High sensitivity for detecting hidden cracks, able to detect smaller hidden cracks.
Disadvantages: The equipment is complex and requires professional skills for operation.
How to prevent mainly from the causes mentioned above
Improve manufacturing processes
Cutting process: Non destructive cutting technology is used to reduce the generation of mechanical stress.
Welding process: Optimize welding parameters to reduce the impact of thermal stress.
Packaging process: Using flexible packaging materials to reduce the impact of mechanical stress.
Optimize transportation and handling
Packaging design: Using shock-absorbing packaging materials to reduce vibration and impact during transportation.
Operation specifications: Develop detailed handling operation specifications to reduce mechanical damage during the handling process.
Precautions during installation
Professional installation: Ensure that installation personnel have received professional training to avoid improper operation leading to hidden cracks.
Mechanical protection: During installation, use protective devices to avoid damage to the battery cells from mechanical stress.
Environmental monitoring and maintenance
Environmental monitoring: Regularly monitor the operating environment of battery cells to avoid damage from extreme environments.
Regular maintenance: Regularly maintain and inspect photovoltaic cells to promptly detect and address hidden cracking issues.