Solar energy is an inexhaustible renewable energy source for mankind. It is also clean energy and does not produce any environmental pollution. Among the effective utilization of solar energy, solar photovoltaic utilization is the fastest growing and most dynamic research field in recent years, and it is one of the most high-profile projects.
Solar energy is a kind of radiant energy, which must be converted into electrical energy by means of an energy converter. This energy converter that converts solar energy (or other light energy) into electrical energy is called a solar cell.
The working principle of solar cells is based on the "photovoltaic" effect of semiconductor p-n junctions. The so-called photovoltaic effect, simply put, is an effect in which electromotive force and current are generated when an object is exposed to light, and the state of charge distribution in its body changes.
When sunlight or other light irradiates the PN junction of the semiconductor, electron-hole pairs are generated. The carriers generated near the P-N junction inside the semiconductor are not recombined and reach the space charge region. Attracted by the internal electric field, electrons flow into the n In the p-region, holes flow into the p-region, resulting in excess electrons stored in the n-region and excess holes in the p-region. They form a photogenerated electric field near the p-n junction opposite the direction of the potential barrier.
In addition to partially counteracting the effect of the barrier electric field, the photo-generated electric field also makes the p region positively charged and the n region negatively charged, and an electromotive force is generated in the thin layer between the n region and the p region, which is the photovoltaic effect. When energy is added to pure silicon (such as in the form of heat), it causes several electrons to break away from their covalent bonds and leave the atom.
Every time an electron leaves, a hole is left behind. Those electrons would then wander around the lattice, looking for another hole to settle into. These electrons are called free carriers, and they can carry electric current.
This electric field acts as a diode, allowing (and even pushing) electrons to flow from the p-side to the n-side, not the other way around. When light strikes a solar cell in the form of photons, its energy releases electron-hole pairs. Each photon that carries enough energy typically releases exactly one electron, creating a free hole.
If this happens close enough to the electric field, or if free electrons and free holes are just within its sphere of influence, the electric field will send electrons to the N side and holes to the P side. This leads to a further breakdown of electrical neutrality, and if we provide an external current path, the electrons will pass through this path and flow to their original side (P-side), where they merge with the holes sent by the electric field, and in the process of flowing work in. Thereby, a current is formed from the N-type region to the P-type region. A potential difference is then formed in the PN junction, which forms the power supply.
According to the shape, solar cells can be divided into rigid solar cells and flexible solar cells; according to the crystalline state, they can be divided into two categories: crystalline thin film type and amorphous thin film type, and the former can be divided into single crystal form and polycrystalline form; It can be divided into silicon film, compound semiconductor film and organic film; according to the different materials used, it can also be divided into: silicon solar cells, multi-compound thin film solar cells, polymer multilayer modified electrode solar cells, nanocrystalline solar cells batteries, organic solar cells. Among them, silicon solar cells are the most mature and dominant in application.
Silicon solar cells
Silicon solar cells are divided into three types: monocrystalline silicon solar cells, polycrystalline silicon thin film solar cells and amorphous silicon thin film solar cells.
Monocrystalline silicon solar cells
The structure of a single crystal silicon solar cell mainly includes a front comb electrode, an anti-reflection film, an N-type layer, a PN junction, a P-type layer, and a back electrode. Monocrystalline silicon solar cells are widely used in space and on the ground. This solar cell uses high-purity monocrystalline silicon rods as raw materials. The single crystal silicon rod is cut into slices, and a PN junction is formed through a series of semiconductor processes.
Then, the grid lines are made by screen printing, and the back electrode is made by a sintering process, and the single crystal silicon solar cell is made. The single sheet can be assembled into a solar cell module (solar cell panel) by series and parallel methods according to the required specifications to form a certain output voltage and current. Finally, the frame is used for encapsulation, and the solar cell components are assembled into various sizes of solar cell arrays.