Solar cell imaging technology
Solar cell imaging technology has a wide range of applications in the observation of surface microstructures, the inspection of thermal effects, CCD imaging positioning, CCD surface color detection, and EL and PL light-emitting defect detection. Here is a detailed introduction to EL light-emitting defect detection technology and infrared thermal imaging technology.
① EL imaging technology
Electroluminescence utilizes the radiative recombination effect of excited carriers between bands in solar cells. The components for electroluminescence research work like light-emitting diodes. The radiation recombination effect is detected by a sensitive silicon charge-coupled device camera with a wavelength range of 300 ~ 100m. An external current Isc less than its short-circuit current is applied to the solar cell and the camera is used. Record the image of the photon emission. The damaged area is darker or less bright than the undamaged area. Electroluminescence imaging can be used to detect a variety of defects in crystalline silicon and thin-film silicon cells. High-resolution electroluminescence imaging makes the detection of defects more accurate than infrared imaging.
EL is the abbreviation of “etroluminescence” in English, translated as electroluminescence or electroluminescence. At present, EL testing technology has been applied by many crystalline silicon solar cell and module manufacturers, and is used for finished product inspection or online products of crystalline silicon solar cells and modules. Quality Control.
In solar cells, the diffusion length of minority carriers is much larger than the width of the barrier. Therefore, the probability of electrons and holes disappearing due to the recombination surface when passing through the barrier zone is small, and they continue to diffuse to the guard dispersion zone. Under the forward bias voltage, minority carriers are injected into the barrier region and diffusion region of the PN junction. These unbalanced minority carriers continue to recombine with the majority carriers to emit light. This is the basic principle of electroluminescence in solar cells. . Luminescence imaging effectively utilizes the radiative recombination effect of excited electron carriers in the solar cell. A forward bias is added to both ends of the solar cell, and the photons emitted by it can be obtained by a sensitive CCD camera, that is, a composite radiation distribution image of the solar cell is obtained. However, the electroluminescence intensity is very low, and the wavelength is in the near-infrared region, requiring the camera to have high sensitivity and very little noise at 900 ~ 1100m.
The EL test process is to apply a forward bias voltage to the crystalline silicon solar cell, and the DC power supply injects a large number of unbalanced carriers into the silicon solar cell. The solar cell recombines continuously with a large number of unbalanced carriers injected from the diffusion zone. Glowing and emitting photons is the reverse process of the photovoltaic effect; these photons are captured by a CCD camera, processed by a computer and displayed in the form of images. The whole process is carried out in a dark room.
The image brightness of the EL test is proportional to the minority carrier lifetime (or minority carrier diffusion length) and current density of the solar cell. In the defective part of the solar cell, the minority carrier diffusion length is lower, so the displayed image brightness is darker. The analysis of EL test images can clearly find hidden defects in solar cells and modules. These defects include silicon material defects, diffusion defects, printing defects, sintering defects, and cracks in the module packaging process.
The electroluminescence brightness of a solar cell is proportional to the minority carrier diffusion length and is proportional to the current density. The analysis of EL images can effectively find possible problems in each link of silicon wafer, diffusion, passivation, screen printing and sintering, which play an important role in improving the process, increasing efficiency and stabilizing production, as shown in Figure 1~Figure 10. Show.
The black chips show concentric circles that gradually brighten from the center to the edge of the cell in the EL image. They are produced in the production stage of silicon materials and are related to the solubility of oxygen and the segregation coefficient during the production of silicon rods. This material defect will inevitably lead to a decrease in the minority carrier concentration of the crystalline silicon solar cell, resulting in weak or no luminescence during the EL test.
In the cell production process, when the sintering process parameters are not good or the sintering equipment is defective, the produced cell will be displayed as a large area track mark during the EL test. In actual production, the problem of mesh belt printing can be effectively eliminated through targeted tooling transformation. For example, in the EL test chart, only a few black dots can be seen in the battery sheet produced by the thimble-type exhibition belt, and there is no large-area track mark.
The broken grid of solar cells is mainly caused by poor grid line printing of the cell itself or irregular welding of the cells. In the EL test chart, it appears as a dark line along the main grid line of the cell, as shown in Figure 5. This is because after the thin grid line of the battery is broken, the current injected from the main grid line of the battery during the EL test has a very small or zero current density near the broken grid, which causes the broken grid of the battery to emit light. The intensity is weak or does not emit light.
②Infrared thermal imaging technology
Infrared measurement can be performed by using an external current or light source. In dark field measurement, no light is added, and an external current Isc less than or equal to the forward short-circuit current is applied to the photovoltaic module. In order to avoid thermal damage to the thin-film module, it must be ensured that the short-circuit current of the module cannot be exceeded. In light measurement, incident light (such as sunlight) will generate current, which leads to different heat radiation. In order to obtain more accurate defect detection, the experiment also carried out thermal imaging of light, and made comparisons under different conditions such as short circuit, open circuit and maximum power point. Some defects can be identified by changing the circuit load to obtain a specific I-U characteristic curve. The fever can be identified with a suitable infrared camera and compared with the measurement results of electroluminescence. The infrared thermal imaging tool used in this study is a portable uncooled infrared camera, and the wavelength range of its infrared detector is 8~14μm. Figure 11 shows the EL image of the photovoltaic module and the infrared image under the condition of forward short-circuit current.
Standard solar cell
The response of the solar cell is related to the wavelength of the incident light. The spectral distribution of natural light varies with geographic location, climate, season and time; the spectral distribution of solar simulators varies with its type and working status. If a thermopile radiometer that is non-selective to the spectrum is used to measure the radiance, the change in the spectral distribution will bring a fraction of an error to the measured conversion efficiency.
In order to reduce this error, it is necessary to select a standard solar cell with basically the same spectral response as the battery under test to measure the irradiance of the light source. The relationship between the short-circuit current of this standard solar cell and the irradiance of the light source to be measured is called the calibration value.