Jul 13, 2021 solar cell

Testing technology of solar cell’s I-U characteristics/spectral response and minority carrier lifetime

Solar cells are the core components of photovoltaic modules, and their performance directly affects the performance of photovoltaic modules. Improving the conversion efficiency of solar cells and reducing the production cost of solar cells can ultimately reduce the cost of photovoltaic modules and the cost of photovoltaic power generation systems. Observation of the solar cell model
Solar cell is a device that uses photovoltaic effect to convert light energy into electrical energy. It is also called photovoltaic device. It mainly includes single crystal silicon battery and single crystal gallium arsenide battery. Figure 1 ~ Figure 4 show the cross-sectional schematic diagrams of several high-efficiency solar cells.

Figure1 MWT (metal perforated winding technology) high-efficiency solar cell cross-sectional schematic diagram
Figure 2 Schematic diagram of the cross-section of the back electrode high-efficiency solar cell
Figure 3 Schematic diagram of cross-section of HIT high-efficiency solar cell
Figure 4 Schematic diagram of grooved buried grid high-efficiency solar cell

Understand the parameters in the I-U characteristic curve of solar cells
The volt-ampere (I-U) characteristic curve of a solar cell refers to the relationship curve between the current I flowing into the load and the battery terminal voltage U under a certain irradiance, temperature, and different external circuit loads. Figure 5 shows a typical I-U characteristic curve of a solar cell. In the figure, Isc is the short-circuit current, which refers to the output current of the solar cell when the terminal voltage is zero under certain temperature and irradiance conditions; Uoc is the open circuit voltage, which refers to the certain temperature and irradiance conditions, The terminal voltage of the solar cell under no load (open circuit); M point is the maximum power point, which refers to the maximum value of the product of current and voltage on the I-U characteristic curve of the solar cell, that is, the point of maximum power, also called It is the best operating point; UMP and IMP refer to the voltage and current values ​​corresponding to the maximum power point. The ratio of the maximum power to the product of the open circuit voltage and the short circuit current is called the fill factor.

Figure 5 Typical 1-U characteristic curve of solar cell

Solar cell spectral response
The spectral response of a solar cell is a quantitative measure of the size of the photocurrent generated by the solar cell at a certain wavelength of incident light, as shown in Figure 6. Different types of solar cells have different spectral response characteristics, as shown in Figure 7. Spectral response test is divided into relative spectral response test and absolute spectral response test. The absolute spectral response is a function of the short-circuit current generated by the unit irradiance and the wavelength of the incident light; the relative spectral response is the normalized absolute spectral response. Spectral response is the basis of light measurement. It has a wide range of applications in the visual function of the human eye, various photosensitive devices (such as the spectral response of imaging elements CCD, CMOS, etc.), photosensitive films, and light sources. The application of spectral response test in solar cell light attenuation research, solar cell antireflection layer research, and solar cell aluminum back field research is shown in Figure 8-Figure 10.

Figure 6 Spectral response curve of a typical solar cell
Figure 7 Spectral response diagrams of different types of solar cells
Figure 8 Application of spectral response test in the research of solar cell light attenuation
Figure 9 Application of spectral response test in the research of anti-reflective layer of solar cell
Figure 10 Application of spectral response test in the research of solar cell aluminum back field

Understand the lifetime of solar cells
Solar cells have few carriers, also known as unbalanced carriers, minority carriers, or few unbalanced carriers. For p-Si, minority carriers are electrons, and for n-Si, they are holes. Minority carrier lifetime is closely related to the conversion efficiency of solar cells. As shown in Figure 11, it is one of the most important electrical parameters of crystalline silicon. It is used for silicon wafer detection, Fe impurity contamination concentration measurement, surface passivation effect characterization, Characterization of defects in polysilicon and battery failure analysis. The minority births of solar cells can be produced by light or electrical injection, as shown in Figure 12. If there is no continuous light or electrical injection, the non-equilibrium minority carrier will be recombined, and its average survival time is the minority carrier lifetime, denoted by τ. It usually declines exponentially over time
Subtract, as shown in Figure 13.

Figure 11 The relationship between less than life and battery efficiency and the optimal thickness of silicon wafer
Figure 12 Schematic diagram of unbalanced carrier generation
Figure 13 The exponential decline of non-equilibrium carriers over time

There are many methods to test the lifetime of minority carriers, including microwave photoconductivity decay method (m-PCD), surface photovoltage (SPV), direct current photoconductivity method (four probes), electron beam induced current (EBIC), electroluminescence/photoluminescence Wait. These measurement methods all include two basic aspects of unbalanced carrier injection and detection. The most commonly used injection methods are light injection and electrical injection, and there are many ways to detect unbalanced carriers, such as detecting changes in electrical conductivity, detecting changes in microwave reflection or transmission signals, etc. This combination forms many life test methods. Such as: DC photoconductive attenuation, high frequency photoconductive attenuation, surface photovoltage, microwave photoconductive attenuation, etc. For different test methods, the test results may be different, because for different injection methods, the thickness or surface conditions are different, and the detection and algorithms are also different. Therefore, the minority birth life test does not have an absolute concept of accuracy, nor an internationally recognized standard, only the concept of repeatability and resolution. For the same sample, comparison experiments between different test methods are required, but the comparison results are not ideal. Here is a detailed introduction to the microwave photoconductive attenuation method (μ-PCD), as shown in Figure 14 and Figure 15. This method uses a pulsed laser (904nm) to generate electron-hole pairs. When the laser is removed, the electron-hole pairs recombine. By measuring the change in the conductivity of the sample to be tested, the minority carrier lifetime can be obtained, as shown in formula (2-1-1) .

Figure 14 Schematic diagram of microwave photoconductance attenuation method pulsed laser
Figure 15 Schematic diagram of the working principle of the microwave photoconductive attenuation method

In the formula: Teff is the effective life (test life); Tbulk is the body life: Tsd is the life affected by the surface compound. S and S2 are two
The recombination rate of each surface; d is the thickness of the sample; D is the diffusion coefficient.
Compared with other methods, the microwave photoconductive attenuation method (μ-PCD method) has the following characteristics:
① No contact, no damage, fast test.
②Ability to test lower life.
③ Able to test samples with low positive rate (samples with a minimum of 0.1ohm-cm can be tested)
④It can test silicon ingots, silicon rods, silicon wafers or finished batteries
⑤ The sample can be tested directly without passivation treatment
⑥ Both P-type materials and N-type materials can be tested.
⑦There is no strict requirement on the thickness of the test sample.
⑧ This method is the most accepted method of life test for minority births in the market
Minority carrier life test is widely used in the photovoltaic field. For example, in single product growth and slice production, the minority birth life test can be used to adjust the process of single product growth, such as: temperature or speed: control the proportion of recycle materials, head and tail materials or other recycled materials: detect single product rods or single products The factory index of the wafer; in the polycrystalline casting production, the minority carrier life test can be used for the quality control of the silicon ingot process: the head and tail position can be accurately judged according to the minority carrier life distribution; it can be used for the wafer advance inspection and the process in the solar cell production process Contamination control in the process and inspection after the plum process (such as phosphorus diffusion, silicon nitride passivation, metallization, etc.).