- Solar cell structure and principle
At present, the most common and most productive solar cells are crystalline silicon semiconductor solar cells, referred to as crystalline silicon solar cells. Generally, crystalline silicon solar cells are mainly composed of five parts: a positive electrode, an anti-reflection layer, an emitter region, a base region, and a back electrode. As shown in Figure 1.
Crystal solar cell wafers usually use P-type crystalline silicon semiconductor material as the base region of semiconductor solar cells, with a thickness of 150-400 μm, and the doping impurities are phosphorus atoms with a concentration of 1015cm-3. The emitter region is generally an N-type crystalline silicon semiconductor with a thickness of more than ten microns, and the doping impurity is chlorine atoms, with a concentration of 1016cm-3~1017cm-3. In this way, the emitter region and the base region together form a P-N junction at the interface. In order to increase the absorption of sunlight, an anti-reflection film is generally coated on the surface of the emission area. According to the characteristics and technology of crystalline silicon solar cells, the material of the anti-reflection film is SiO2 or Si3N4. The upper surface of the anti-reflection layer has grid-shaped metal electrodes, which are usually called grid lines. The wider metal electrodes are called main grids, and those with thinner metal electrodes are called fine grids.
Generally, there are two busbars on the surface of the solar cell, which are evenly distributed on the surface of the solar cell; there are dozens of fine grids, which are evenly distributed on the surface of the solar cell perpendicular to the busbar. The front surface grid lines are generally made of high-purity silver paste by screen printing. The back of the silicon wafer is a metal back electrode, and the back electrode is completely covered on the back of the solar cell. The positive electrode and the back electrode respectively form ohmic contact with the emitter region and the base region through sintering, so as to reduce the contact resistance between the electrode and the contact surface.
When someone radiates light into the solar cell through the anti-reflection coating, photons with energy greater than or equal to the forbidden band width of the semiconductor will excite the generation of electron-hole pairs. The generated electron-hole pairs will be separated by the internal) electric field of the space charge region, and the electron-hole pairs generated in the N region or P region near both sides of the space charge region will enter through diffusion, and then be separated in the end region, and photogenerated and carried. As long as the carriers can cross the space charge region before recombination, they will effectively convert light energy into electrical energy. The separated photogenerated carriers, in which electrons move to the N region by diffusion, and holes move to the P region by diffusion, so that oppositely charged charges accumulate on both sides of the solar cell (both sides of the P-N junction) to form Photovoltaic voltage, which is the photovoltaic effect.
For crystalline silicon solar cells, different regions absorb different wavelengths of photons. The emission area near the solar cell is sensitive to short-wavelength violet light and ultraviolet light, accounting for 5% to 10% of the total light source current; the P-N junction space charge is sensitive to visible light, and the photogenerated current accounts for about 5%; Infrared light is sensitive and generates 80%~90% of the photogenerated current, which is the main component of the photogenerated current. The solar cell forms an accumulated potential at the two poles through the photovoltaic effect. After the load is applied to the two poles, the photocurrent will flow from the P region to the N region through the load under the action of the accumulated potential, and the load will output electric power.
- Electrical performance parameters of solar cells
There are five main parameters that describe the electrical properties of solar cells: open circuit voltage VOC, short circuit current ISC (or short circuit current density JSC), maximum output power Pm, fill factor FF and photoelectric conversion efficiency η.
(1) Open Circuit Voltage VOC
The open circuit voltage is the voltage value output by the solar cell at both ends of the open circuit when the solar cell is placed under standard test conditions (light intensity of 1000W/m2, air quality of AM1.5, and ambient temperature of 25°C). In the case of low precision requirements, the open circuit voltage of the solar cell can be approximately measured by a DC millivoltmeter with high internal resistance.
(2) Short Circuit Current ISC
The solar cell is placed under standard test conditions. When the solar cell is short-circuited, the current flowing through the solar cell is the short-circuit current. In the case of low precision requirements, an ammeter with a small enough internal resistance (generally <1Ω) can be directly connected to both ends of the solar cell to approximately measure the short-circuit current.
(3) Maximum output power Pm
The maximum output power varies with the load resistance value, and the operating voltage and current of the solar cell will also vary accordingly. Use loads with different resistance values to connect to the solar cell, and measure the I-V curve drawn by the corresponding working voltage and current value, that is, the volt-ampere characteristic curve of the solar cell. The product of the corresponding voltage and current at each point of the I-V curve is the output power, and its maximum value is the maximum output power (Pm). Current (Im), the relationship is as follows:
(4) Fill Factor FF
The fill factor FF is the ratio of the maximum output power of the solar cell to the product of the open-circuit voltage and the short-circuit current. The fill factor FF is one of the important parameters reflecting the electrical performance of the solar cell. Its formula expression is as follows:
It can be seen from formula (1-2) that since the maximum voltage and current are always smaller than the open-circuit voltage and short-circuit current, the value of the fill factor is always <1, and the closer the value is to 1, the greater the possible output power of the solar cell. The better the electrical performance of the solar cell. From the practical application point of view, the actual value of the solar cell fill factor is lower than the value calculated by the formula (1-2), which is mainly affected by the series resistance and parallel resistance of the solar cell. The series resistance mainly affects the short-circuit current value. When the series resistance increases, the short-circuit current decreases and the fill factor decreases accordingly. The parallel resistance mainly affects the open-circuit voltage value. The smaller the parallel resistance, the lower the open-circuit voltage and the smaller the fill factor. . In addition, from the point of view of the I-V curve, the larger the FF is, the closer the I-V curve is to a rectangle, and the higher the conversion efficiency of the solar cell is.
(5) Photoelectric conversion efficiency η of solar cell When the solar cell is placed under standard test conditions and the optimal load is connected to the solar cell, the maximum energy conversion efficiency of the solar cell is defined as the conversion efficiency of the solar cell, and its value is equal to the output power of the solar cell and The ratio of the incident power of sunlight to the surface of the solar cell:
In the formula, S is the solar cell area (m2); Pm is the incident solar power (W/m2).
When the air mass is AM 1.5, the solar irradiance is 1000W/m2, and the ground temperature is 25℃, the maximum output power of the solar cell is defined as the peak wattage (WP) of the solar cell. The photoelectric conversion efficiency of solar cells is closely related to the forbidden band width of the semiconductor materials used in the manufacture of solar cells. Only photons whose photon energy is greater than or equal to the forbidden band width can generate photo-generated electron-hole pairs in the semiconductor, which will be removed before recombination. The built-in electric field is effectively separated to form a photo-generated current. However, the forbidden band width of solar cell materials is not as small as possible, because the photon energy with energy greater than the forbidden band width will be converted into thermal energy, which not only reduces the utilization rate of photon energy, but also reduces the efficiency of solar cells due to the generation of thermal energy. . The band gap also directly affects the open circuit voltage. The larger the band gap, the higher the open circuit voltage (and vice versa), and the smaller the reverse saturation current (or vice versa). Therefore, choosing a battery material with an appropriate band gap is very important for solar cells. The theoretical efficiency has a greater impact. The photoelectric conversion efficiency of solar cells is closely related to the structure, junction characteristics, material properties, operating temperature, radiation damage of radioactive particles and environmental changes, and is an important parameter to measure the quality and technical level of the battery.