How solar battery work

Posted by sere on November 30th, 2020

With the global energy shortage, solar energy has been vigorously developed as a new type of energy. Among them, solar batterys are the most used in our lives. Solar batterys are mainly based on semiconductor materials. Photoelectric materials are used to absorb light energy and then undergo photoelectric conversion to make it generate current. What is the working principle of solar battery?

In the past, from an electrical point of view, the silicon we used was all neutral. The excess electrons are neutralized by the excess protons in the phosphorus. The missing electrons (holes) are neutralized by the missing protons in boron. When holes and electrons are mixed at the junction of N-type silicon and P-type silicon, neutrality is destroyed. Will all free electrons fill all holes? will not. If so, then the whole preparation is meaningless. However, at the junction, they do mix to form a barrier, making it increasingly difficult for electrons on the N side to reach the P side. Eventually an equilibrium state will be reached, so that we have an electric field separating the two sides.

This electric field is equivalent to a diode, allowing (or even pushing) electrons to flow from the P side to the N side, not the other way around. It is like a mountain-electrons can easily slide down the hill (to the N side), but cannot climb up (to the P side).

In this way, we get an electric field that acts as a diode, in which electrons can only move in one direction. Let's take a look at what happens when sunlight hits the battery.

When light hits a solar battery in the form of photons, its energy causes electron-hole pairs to be released.

Each photon with enough energy usually releases exactly one electron, thereby creating a free hole. If this occurs close enough to the electric field, or free electrons and free holes are just within its influence, the electric field will send electrons to the N side and holes to the P side. This will cause the electrical neutrality to be further destroyed. If we provide an external current path, electrons will pass through the path and flow to their original side (P side), where they merge with the holes sent by the electric field and flow in the process. In doing work. The flow of electrons provides current, and the electric field of the battery produces voltage. With current and voltage, we have power, which is the product of the two.

How much sunlight energy can our photovoltaic cells absorb? Unfortunately, the absorption rate of the solar energy of the simple battery introduced here is at most about 25%, and the usual absorption rate is 15% or lower. Why is the absorption rate so low?

Visible light is only part of the electromagnetic spectrum. Electromagnetic radiation is not single-frequency—it consists of a series of different wavelengths (and thus a series of energy levels). (For a detailed introduction to the electromagnetic spectrum, please refer to the basic principles of special relativity.)

Light can be divided into different wavelengths, and we can see this through the rainbow. Because the photon energy range of the light hitting the battery is very wide, some photons do not have enough energy to form electron-hole pairs. They just pass through the battery, as if the battery is transparent. But the energy of some other photons is very strong. Only when a certain amount of energy is reached-in electron volts (eV), determined by the battery material (about 1.1 eV for crystalline silicon)-can electrons escape. We call this energy value the band gap energy of the material. If the energy of the photon is more than the required energy, the excess energy will be lost (unless the energy of the photon is twice the required energy and multiple sets of electron-hole pairs can be created, but this effect is not important). These two effects alone will cause about 70% of the radiant energy loss in the battery.

Why don't we choose a material with a very low band gap in order to use more photons? Unfortunately, the band gap also determines the electric field strength (voltage). If the band gap is too low, while increasing the current (by absorbing more electrons), a certain amount of voltage will be lost. Remember, power is the product of voltage and current. The optimal band gap energy must be able to balance these two effects. For a battery made of a single material, this value is approximately 1.4 electron volts.

We have other energy losses. Electrons must flow from one side of the battery to the other through an external circuit. We can plate a layer of metal on the bottom of the battery to ensure good conductivity. But if we completely plate the top of the battery with metal, the photons will not be able to pass through the opaque conductor, which will lose all current (in some batteries, only the upper surface is used instead of transparent conductors in all locations). If we only set contacts on both sides of the battery, the electrons need to travel a long distance (for electrons) to reach the contact point. You know, silicon is a semiconductor, and its performance in transmitting current is not as good as metal. Its internal resistance (called series resistance) is quite high, and high resistance means high loss. In order to minimize these losses, the battery is covered with a metal contact net, which can shorten the distance the electrons move while covering only a small part of the battery surface. Even so, some photons will be blocked by the grid, and the grid cannot be too small, otherwise its own resistance will be too high.

Before actually using the battery, there are several other steps. Silicon is a shiny material, which means it has good reflective properties. The reflected photons cannot be used by the battery. For this reason, using an anti-reflective coating on the top of the battery can reduce the reflection loss to less than 5%.

The last step is to install the glass cover to separate the battery from the components to protect the battery. Photovoltaic modules are composed of multiple batteries (usually 36) in series and parallel to provide usable voltage and current levels. These batteries are placed in a sturdy frame with positive and negative terminals led out at the rear, and a glass cover Seal the board.

Monocrystalline silicon is not the only material used in photovoltaic cells. Polycrystalline silicon is also used in battery materials. Although the cells produced in this way are not as efficient as monocrystalline silicon cells, they can reduce costs. In addition, amorphous silicon without a crystalline structure is also used, which is also to reduce costs. Other materials used include gallium arsenide, indium copper selenide and cadmium telluride. Because different materials have different band gaps, they seem to be "tuned" to photons of different wavelengths or different energies. One way to improve efficiency is to use two or more layers of different materials with different band gaps. Materials with higher band gaps are placed on the surface and absorb higher-energy photons; materials with lower band gaps are placed below and absorb lower-energy photons. This technology can greatly improve efficiency. Such batteries are called multi-junction batteries, and they can have multiple electric fields.