The substrate material of the silicon cell is P-type monocrystalline silicon or polycrystalline silicon. It uses special cutting equipment to cut monocrystalline silicon or polycrystalline silicon rods (as shown in Figure 1) into silicon wafers with a thickness of about 180μm, and then go through a series of processing procedures.
1. Features of silicon cell
The silicon cell is the main material in the cell assembly, and its appearance is shown in Figure 2. Qualified silicon cells should have the following characteristics.
①It has stable and efficient photoelectric conversion efficiency and high reliability.
②Adopt advanced diffusion technology to ensure the uniformity of conversion efficiency everywhere in the chip.
③Using advanced PECVD film forming technology, the dark blue silicon nitride anti-reflection film is plated on the surface of the cell, and the color is uniform and beautiful.
④ Use high-quality silver and silver-aluminum metal paste to make back field and grid electrode to ensure good conductivity, reliable adhesion and good electrode solderability.
⑤High-precision screen-printed graphics and high flatness make it easy for automatic welding and laser cutting of solar cells.
2. Classification and appearance structure of silicon cells
Silicon cells can be divided into crystalline silicon cells for ground use, crystalline silicon cells for offshore use, and crystalline silicon cells for space use. According to different substrate materials, they are divided into monocrystalline silicon cells and polycrystalline silicon cells. The common specifications of silicon cells are 125mm×125mm, 156mm×156mm, 156.75mm×56.75mm, etc. At present, most of the mainstream applications are 156.75mm×156.75mm, and the cell thickness is generally 180~200μm. As can be seen from Figure 2, there is a blue anti-reflection film on the surface of the cell and silver-white electrode grid lines. Many of the thin grid lines are the leads from the surface electrodes of the cell to the main grid lines. Several wider silver-white wires are the main grid lines, also called electrode lines or upper electrodes (Currently there are 4, 5 or even 12 busbar cells in production).
The back of the cell also has several intermittent silver-white busbars corresponding to the front, called the bottom electrode or the back electrode. The connection between the battery slice and the battery slice is realized by welding the interconnection bar to the main grid line. Generally, the electrode wire on the front is the negative wire of the battery, and the electrode wire on the back is the positive wire of the battery. Regardless of the area of the solar cell (whole piece or cut into small pieces), the peak output voltage between the positive and negative poles of a single piece is 0.52~0.56V, and the area of the cell is proportional to the output current and power generation, the larger the area, the greater the output current and power generation.
3. Analysis of equivalent circuit of silicon cell
The internal equivalent circuit of the silicon cell is shown in Figure 3. For ease of understanding, we can visually regard the inside of a solar cell as a composite of a photovoltaic cell and a silicon diode. That is, a positive-biased diode is connected in parallel at both ends of the photovoltaic cell, and there are series resistance and parallel resistance inside the battery. Due to the existence of the diode, under the action of the external voltage, a leakage current Id through the P-N junction of the diode will be generated. This current is in the opposite direction of the photo-generated current, so it will cancel a small part of the photo-generated current. The series resistance is mainly composed of the bulk resistance of the semiconductor material itself, the lateral resistance of the diffusion layer, the contact resistance between the metal electrode and the cell body, and the resistance of the metal electrode itself. The lateral resistance of the diffusion layer is the main form of series resistance. The series resistance of a normal cell is generally less than 1Ω, and the parallel resistance is also called a bypass resistance. It is mainly due to edge leakage caused by semiconductor crystal defects, battery surface pollution, etc., so that a part of the current that should have passed through the load is short-circuited to form a current of Ir, which is equivalent to a parallel resistor. Therefore, it is equivalent to a parallel resistor in the circuit, and the resistance of the parallel resistor is generally several kiloohms. Analysis shows that the smaller the series resistance of the photovoltaic cell and the larger the bypass resistance, the closer it is to an ideal cell, and the better the performance of the cell.