A solar panel is a device that absorbs sunlight and converts solar radiation energy directly or indirectly into electrical energy through the photoelectric effect or photochemical effect. It is one of the important components of a solar power generation system, and the solar cell is a crucial factor determining the conversion efficiency and lifespan. Today, we'll discuss the principles of solar panels and their characteristics.
I. Structural Components of Solar Panels
1. Tempered glass
Its function is to protect the main body of power generation (such as battery cells). There are requirements for its light transmittance. One is that the light transmittance must be high (generally above 91%), and the other is that it must be ultra-white tempered.
2. EVA
Used to bond and fix tempered glass and power generation components (such as solar cells), the quality of transparent EVA material directly affects the lifespan of the module. EVA exposed to air is prone to aging and yellowing, which affects the light transmittance of the module and thus the power generation quality. In addition to the quality of the EVA itself, the lamination process of the module manufacturer also has a significant impact. For example, if the EVA adhesiveness is not up to standard, or if the bonding strength between EVA and tempered glass or backsheet is insufficient, it will cause the EVA to age prematurely and affect the lifespan of the module.
3. Battery cells
Its main function is power generation. The mainstream solar cells on the market are crystalline silicon solar cells and thin-film solar cells, each with its own advantages and disadvantages. Crystalline silicon solar cells have relatively low equipment costs, but high consumption and cell costs. However, they also have high photoelectric conversion efficiency and are suitable for generating electricity outdoors under sunlight. Thin-film solar cells have relatively high equipment costs, but very low consumption and cell costs. Their photoelectric conversion efficiency is only slightly more than half that of crystalline silicon solar cells, but they have excellent low-light performance and can generate electricity even under ordinary light, such as the solar cells on calculators.
4. Back panel
The backsheet serves to seal, insulate, and waterproof (it is generally made of materials such as TPT and TPE that must be resistant to aging; most module manufacturers offer a 25-year warranty. Tempered glass and aluminum alloy are generally fine; the key is whether the backsheet and silicone meet the requirements).
5. Aluminum alloy
The protective laminate serves a certain degree of sealing and support.
6. Junction box
The junction box acts as a current relay station to protect the entire power generation system. If a module short-circuits, the junction box automatically disconnects the short-circuited battery string to prevent damage to the entire system. The most critical component of the junction box is the selection of diodes; different diodes are used depending on the type of battery cells within the module.
7. Silicone
It serves a sealing function, used to seal the junctions between components and aluminum alloy frames, and between components and junction boxes. Some companies use double-sided tape or foam instead of silicone, but silicone is widely used in China due to its simple, convenient, and low-cost process.
The core component of a solar panel is the solar cell. Generally speaking, the electrical characteristics of the cells used in each panel should be basically the same; otherwise, a so-called hot spot effect will occur on cells with poor electrical performance or those that are shaded (problem cells).
To prevent hot spots, a bypass diode should be connected in parallel with each cell. When a cell malfunctions or is blocked, the current generated by other cells that is greater than that of the problematic cell will be bypassed by the bypass diode.
In reality, it's impractical to connect a diode in parallel with every single cell. Typically, solar cell modules have 18 cells (for modules with 36 or 54 cells connected in series) or 24 cells (for modules with 72 cells connected in series) connected in series, followed by a diode in parallel.
It is conceivable that when the current generated in these 18 or 24 batteries is inconsistent, that is, when there is a faulty battery, the current passing through this string of batteries will cause hot spots on the faulty battery.
The laminate structure of a solar cell module (in order of manufacturing process):
1. Tempered glass: Its function is to protect the main power generation unit (battery cells). There are requirements for its selection. The light transmittance must be high (generally above 91%); ultra-white tempered treatment.
2. EVA: Used to bond and fix tempered glass and the main power generation unit (solar cells). The quality of transparent EVA material directly affects the lifespan of solar cell modules. EVA exposed to air is prone to aging and yellowing, which affects the light transmittance of the module and thus the power generation quality of the module. In addition to the quality of the EVA itself, the lamination process of the module manufacturer also has a great impact. For example, if the EVA adhesive viscosity is not up to standard, or if the bonding strength between EVA and tempered glass and backsheet is insufficient, it will cause EVA to age prematurely and affect the lifespan of the module.
3. Power Generation Components: Their main function is to generate electricity. The mainstream power generation components on the market are crystalline silicon solar cells and thin-film solar cells. Each has its advantages and disadvantages. Crystalline silicon solar cells have relatively low equipment costs, but high consumption and cell costs. However, they also have high photoelectric conversion efficiency and are more suitable for generating electricity under outdoor sunlight. Thin-film solar cells have relatively high equipment costs, but very low consumption and cell costs. However, their photoelectric conversion efficiency is only slightly more than half that of crystalline silicon solar cells. They have excellent low-light performance and can generate electricity even under ordinary lighting conditions.
4. The backsheet serves to seal, insulate, and waterproof (TPT, TPE, etc. are generally used). The material must be resistant to aging. Nowadays, module manufacturers offer 25-year warranties. Tempered glass and aluminum alloy are generally fine. The key is whether the backsheet and silicone can meet the requirements.
It provides sufficient mechanical strength to withstand the stresses caused by impacts and vibrations during transportation, installation, and use, and can withstand the impact of hail; it has good sealing properties, preventing wind, water, and corrosion of the solar cells under atmospheric conditions; it has good electrical insulation properties; it has strong UV resistance; and its operating voltage and output power are designed according to different requirements, providing multiple wiring methods to meet different voltage, current, and power output requirements.
5. The efficiency loss caused by the series and parallel combination of solar cells is small;
6. Solar cell connection*;
7. Long service life, requiring solar cell modules to be usable for more than 20 years under natural conditions;
8. Under the aforementioned conditions, the packaging cost should be as low as possible.
A solar cell is a device that responds to light and converts light energy into electricity. Many materials can produce the photovoltaic effect, such as monocrystalline silicon, polycrystalline silicon, amorphous silicon, gallium arsenide, and copper indium selenide. Their power generation principles are basically the same; we will use a crystalline silicon cell as an example to describe the photovoltaic process. P-type crystalline silicon can be doped with phosphorus to obtain N-type silicon, forming a PN junction.
When sunlight shines on the surface of a solar cell, some photons are absorbed by the silicon material. The energy of the photons is transferred to the silicon atoms, causing electrons to migrate and become free electrons. These free electrons accumulate on both sides of the PN junction, creating a potential difference. When an external circuit is connected, a current flows through the external circuit under the influence of this voltage, generating a certain output power. Essentially, this process converts photon energy into electrical energy. The basis of solar cell energy conversion is the photovoltaic effect of the junction.
When light shines on a pn junction, electron-hole pairs are generated. The charge carriers generated near the junction inside the semiconductor are not recombinated and reach the space charge region. Attracted by the built-in electric field, electrons flow into the n-region, and holes flow into the p-region. As a result, the n-region stores excess electrons, and the p-region stores excess holes. These create a photogenerated electric field near the pn junction, opposite to the direction of the potential barrier.
In addition to partially counteracting the effect of the potential barrier electric field, the photogenerated electric field also makes the p-region positively charged and the N-region negatively charged, thus generating an electromotive force in the thin layer between the N-region and the P-region. This is the photovoltaic effect.
At this point, if the external circuit is short-circuited, a photocurrent proportional to the incident light energy will flow through it; this current is called the short-circuit current. Conversely, if the PN junction is open-circuited, electrons and holes flow into the N-region and P-region respectively, causing the Fermi level of the N-region to be higher than that of the P-region, creating a potential difference between these two Fermi levels. This value can be measured and is called the open-circuit voltage. Since the junction is forward-biased at this time, the aforementioned short-circuit photocurrent is equal to the forward current of the diode, and the value of the potential difference can be determined accordingly.
Basic characteristics of solar cells
The basic characteristics of solar cells include three fundamental properties: polarity, performance parameters, and current-voltage characteristics. These are explained in detail below:
1. Polarity of solar cells
Silicon solar cells are generally fabricated with a P+/N type structure or an N+/P type structure. P+ and N+ represent the conductivity type of the semiconductor material in the front-side light-illuminating layer of the solar cell; N and P represent the conductivity type of the semiconductor material in the back-side substrate of the solar cell. The electrical performance of a solar cell is related to the characteristics of the semiconductor materials used in its manufacture.
2. Performance parameters of solar cells
The performance parameters of a solar cell consist of open-circuit voltage, short-circuit current, maximum output power, fill factor, and conversion efficiency. These parameters are indicators of the performance of a solar cell.
3. Current-voltage characteristics of solar cells
A PN junction solar cell comprises a shallow PN junction formed on the surface, a strip-shaped and finger-shaped front ohmic contact, a back ohmic contact covering the entire back surface, and an anti-reflective layer on the front. When the cell is exposed to the solar spectrum, photons with energy less than the bandgap Eg do not contribute to the cell's output. Only photons with energy greater than the bandgap Eg contribute energy Eg to the cell's output; energy less than Eg is dissipated as heat. Therefore, the impact of this heat on the cell's stability and lifespan must be considered during the design and manufacturing of solar cells.
What are the main components of a solar cell?
Solar cells are made of elemental silicon. Elemental silicon is often tested in tests on chips. Silicates are ceramics, glass, cement, and silicon dioxide are often tested in tests on optical fibers and crystals.
