Semiconductor photovoltaic effect and solar cells

Semiconductor photovoltaic effect and solar cells

Solar cells are the core components of photovoltaic power generation systems, which can directly convert light energy into electrical energy. The working principle and technological progress are summarized below.

Silicon is one of the most abundant elements on the earth. After “purification” and “growth”, it becomes a crystalline semiconductor and is the basic material for solar cells. The unique electrical characteristics of solar cells are the doping of certain elements (such as phosphorus or boron, etc.) into crystalline silicon to cause permanent imbalance in the molecular charge of the material, forming a semiconductor material with special electrical properties. Semiconductor devices with photoelectric conversion characteristics are usually combined by two materials called P-type semiconductor and N-type semiconductor. When light is irradiated on the PN junction, electron-hole pairs are generated, which are generated near the internal junction of the semiconductor. Currents are attracted by the built-in electric field to reach the space charge region. 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 has excess holes, forming a photo-generated electric field in the opposite direction to the barrier near the PN junction. In addition to partially offsetting the effect of the barrier electric field, the photogenerated electric field also makes the P zone positively charged and the N zone negatively charged. An electromotive force is generated in the thin layer between the N zone and the P zone. This is the photovoltaic (or called photovoltaic) )effect. At this time, if the external circuit is short-circuited, a photocurrent that is proportional to the incident light energy flows in the external circuit, and this current is called the short-circuit current. On the other hand, if the two ends of the PN junction are opened, the electrons and holes flow into the N zone and the P zone respectively, so that the Fermi energy level of the N zone is higher than the Fermi energy level of the P zone. There is a potential difference V between the energy levels. This value can be measured and is called the open circuit voltage.

Solar cell is a kind of semiconductor device with photovoltaic effect (referred to as photovoltaic device), which directly converts sunlight into direct current and is the most basic unit of photovoltaic power generation. The solar cell is composed of two layers of semiconductor materials, the thickness of which is about 0.25mm, forming two areas, namely a positively charged area and a load-bearing area. The negative zone is located in the upper layer of the battery, where phosphorus is forced to penetrate and adhere to silicon. The positive area is placed under the surface of the battery, and the positive and negative interface area is called the PN junction. The PN junction is given constant characteristics when manufacturing the battery. When sunlight hits the electrons that remain loose in the solar cell, these electrons close to the PN junction flow toward the surface of the cell. The metal wire connects the front of each cell in the photovoltaic module with the back of the next cell, so that the electrons establish the series voltage of all cells through many PN junctions. The voltage at the PN junction of each battery increases by an electromotive force of approximately 0.5V, and this battery voltage has nothing to do with the size of the battery. The current is affected by the battery area and solar irradiance. A battery with a larger area can generate a stronger current.

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