The working principle, polarity and classification of solar cell

As an inexhaustible and renewable energy source, solar energy has received extensive attention in recent years, and research and development of solar cells has become increasingly important. A solar cell is simply a semiconductor device made using a semiconductor photovoltaic effect, and is also an application of ionizing radiation effects.

How solar cells work?

When sunlight is applied to a semiconductor, photons with energy greater than the semiconductor band gap will excite electron-hole pairs within the semiconductor. Before the electron-hole pair recombination, if it can pass through the depletion layer, it can be separated under the junction voltage, so that the original built-in electric field barrier is lowered, the electron moves to the n region and the hole moves to the p region to generate photogeneration. The separated electrons and holes make the p region positively charged and the n region negatively charged, so a potential difference is generated between the n region and the p region, which is the photovoltaic effect. The open circuit voltage (Voc) is the potential difference across the p-n junction when the external circuit is open, that is, the maximum value of the photo-generated voltage. The short-circuit current (Jsc) is the current flowing across the p-n junction when the external circuit is short-circuited.

Polarity of solar cell

The electrical properties of a solar cell are related to the properties of the semiconductor material used to make the cell. In the case of sunlight or other illumination, the polarity of the solar cell output voltage is positive for the P-type side electrode and negative for the N-type side electrode. When the solar cell is connected as a power source to an external circuit, the solar cell operates in a forward state. When the solar cell is used in combination with other power sources, if the positive electrode of the external circuit is connected to the P electrode of the battery and the negative electrode is connected to the N electrode of the battery, the external power source provides forward bias to the solar cell; if the positive electrode of the external power source and the N electrode is connected, and the negative electrode is connected to the P electrode, the external power source supplies a reverse bias to the solar cell.

Classification of solar cells

1. crystalline silicon solar cells

(i) Monocrystalline silicon solar cells

Monocrystalline silicon solar cells are very popular and widely used in power plants, road lighting systems, charging systems and split solar power sources, etc., with high photoelectric conversion efficiency, good stability and long service life. At present, the maximum efficiency of monocrystalline silicon solar cells is 25%. The forbidden band width of the single crystal silicon cell is 1.1 eV, and the theoretical maximum open circuit voltage is about 0.7 V, and the corresponding maximum fill factor is 0.84.

(ii) Polycrystalline silicon solar cells

Polycrystalline silicon solar cells are the most widely used, and the fabrication process is similar to that of monocrystalline silicon solar cells. Although its cost is lower than that of monocrystalline silicon cells, its efficiency is much larger than that of monocrystalline silicon solar cells, which is currently up to 21.3%, but for single-junction solar cells, the theoretical efficiency is about 30%. Solar cells have reached 25%, and it is difficult to further improve device efficiency, and there is still room for further improvement in polycrystalline silicon solar cells.

2. Thin film solar cell

(i) amorphous silicon solar cells

After crystalline silicon solar cells, amorphous silicon thin film solar cells began to appear in the 1880s. Since amorphous silicon solar cells are manufactured in a completely different way from crystalline silicon solar cells, the fabrication process is simplified and the amount of materials used is reduced. The cost is much lower, but the main problem currently exists is that the photoelectric conversion efficiency is very low, the highest efficiency is about 13%, and the stability is poor, and the efficiency gradually decreases with the use time.

(ii) compound solar cell

There are many kinds of compound solar cells, but they have not been mass-produced. At present, the research of such cells mainly focuses on II-VI compounds such as CdTe, I-III-VI compounds such as CuInSe₂ (CIS) and CuInSe₂ doped Ga elements (CIGS). III-V group compounds such as GaAs, InP and multi-junction solar cells. The following are the experimental efficiency and future development of compound solar cells.

3. New solar cells

(i) Organic solar cells

The organic solar cell is a new type of solar cell with a photosensitive organic material as a semiconductor. The first organic photoelectric conversion device was prepared by Kearns and Calvin in 1958, but the photoelectric conversion efficiency is very low and the open circuit voltage is only 200mV. At present, the maximum efficiency of organic material devices is 11.5%, and there is still much room for development.

(ii) Dye-sensitized solar cells

Dye-sensitized batteries have a wide variety of materials. The materials used in the early days were mainly liquid electrolytes, and recently developed in solid and colloidal electrolytes. At present, the dye sensitized battery has a verification efficiency of 11.4%. The dye-sensitized battery is simple to manufacture and low in cost, but its stability and efficiency problems are not ideal, so it has not been able to be put into mass production.

Edited by Suzhou Yacoo Science Co., Ltd.