Solar cells are semiconductor devices that convert solar energy into electrical energy. From the perspective of application and research, its photoelectric conversion efficiency, output volt-ampere characteristic curve and parameters must be measured, and this measurement must be measured in the specified standard solar It is meaningful to proceed under the light. If the characteristics of the test light source are very different from the sunlight, the measured data cannot represent the real situation when it is used under the sun, or even can not be converted to the real situation, considering that the sun itself changes with time and place Therefore, a standard sunlight condition must be specified so that the measurement results can be compared with each other, and the performance parameters of the solar cell in actual application can be estimated based on the test data under the standard sunlight.
(1) Introduction to solar radiation. The sun is not a black body with a constant temperature, but a radiator that emits and absorbs many layers of different wavelengths. When applying solar energy systems, it is usually regarded as a black radiator with a temperature of 600K, as shown in Figure 1. The relationship between monochromatic radiation density and wavelength and temperature is determined according to Planck’s law. The radiation density in the entire wavelength range is determined by Stephen Boltzmann’s law.
When the earth orbits the sun, the distance between the earth and the sun does not change much. The intensity of solar radiation reaching the upper boundary of the earth’s atmosphere is almost constant, which is represented by the solar constant AMO. The value of the solar constant refers to the solar energy received per unit time per unit area surface of the upper boundary of the earth’s atmosphere perpendicular to the sun’s rays at the average distance between the sun and the earth. The solar constant value AMO = 1350W/m2 measured in recent years, the influence caused by the change of the distance between the sun and the earth does not exceed 3.4% of the soil. Common ground solar constant value AM1.5 = 844W/m2
When solar radiation passes through the earth’s atmosphere, it is not only scattered by air molecules, water vapor and dust in the atmosphere, but also absorbed and reflected by molecules such as oxygen, ozone, water vapor and carbon dioxide in the atmosphere, making the solar radiation reaching the ground significant attenuation.
① X-rays and other shorter-wavelength radiations in the solar spectrum cannot pass through the atmosphere to reach the surface because they are strongly absorbed by nitrogen, oxygen and other atmospheric molecules in the ionosphere.
②Most ultraviolet rays are absorbed by ozone.
③The weakening of visible light energy is mainly caused by the strong scattering of the earth’s atmosphere.
④The weakening of infrared spectrum energy is mainly due to the selective absorption of solar radiation by water vapor.
⑤Radiation with a wavelength of more than 2500μm is inherently very low in the upper boundary of the atmosphere. Coupled with the strong absorption of carbon dioxide and water, the energy that can reach the ground is even smaller.
Therefore, for the solar energy reaching the ground, only the radiation of 290 ~ 2500pum should be considered.
(2) The basic characteristics of solar radiation.
① Irradiance: usually called “light intensity”, that is, the light power that a person hits on a unit area, the unit is W/m2 or mW/cm2.
For space applications, the specified standard irradiance is 1367W/m2, and for ground applications, the specified standard irradiance is 100W/m2. In fact, the ground sunlight is related to many complex factors. This value can only be obtained at a specific time and under ideal climate and geographical conditions. The more common irradiance on the ground is in the range of 600 ~ 900W/m2. In addition to the irradiance value range, one of the characteristics of solar radiation is its uniformity, which ensures that each point on the same solar cell array The irradiance is the same.
②Spectral distribution: solar cells have different responses to light of different wavelengths, that is to say, light with the same irradiance but different spectral components irradiates the same solar cell with different effects. Sunlight is a combination of various wavelengths. Light, its spectral components constitute a spectral distribution curve, and its spectral distribution is also the same with the difference of location, time and other conditions. The situation outside the atmosphere is very simple. The solar spectrum is almost equivalent to the blackbody radiation spectrum of 6 000K, which is called the AMO spectrum. On the ground, part of the sunlight is absorbed after passing through the atmosphere. This absorption is related to the thickness and composition of the atmosphere, so it is selective absorption, resulting in a very complex spectral distribution, and changes with the sun’s zenith angle , The sunlight transmission path is different, the absorption situation is also different, so the spectrum of the ground sunlight changes at any time. Therefore, from the perspective of testing, it is necessary to specify a standard terrestrial solar spectrum distribution. In clear weather conditions, when the sun travels through the atmosphere to reach the ground 1.5 times the thickness of the atmosphere, its spectrum is the standard terrestrial solar spectrum, referred to as the AM1.5 standard solar spectrum. At this time, the sun’s zenith angle is 48.19°, because this situation is more representative on the ground. The solar radiation spectrum near the ground is shown in Figure 2.
③Luminous intensity: referred to as light intensity, the international unit is candela candela), abbreviated as cd. lcd refers to the light of a monochromatic light source (frequency is 540×1012Hz, wavelength is 0.55um), the unit solid angle in a given direction The intensity of the light emitted from the inside, when the light source radiates uniformly. Then the light intensity is I=F/Ω, Ω is the solid angle, the unit is steradian (sr), F is the luminous flux, the unit is lumens, and for point light sources I=F/4.
The standard sunlight on the ground has an irradiance of 100W/m2, a solar spectrum of AMI. 5, and sufficient uniformity and stability. There are few opportunities for such standard sunlight to be found outdoors, and solar cells must be here. Therefore, the only way is to use artificial light sources to simulate sunlight, the so-called solar simulator.
(3) Classification of solar simulators. The solar simulator is a device that simulates sunlight. Due to the small size of the solar simulator itself, the test process is not affected by factors such as environment, climate, time, etc., thus avoiding the limitations of various factors of outdoor measurement. In the photovoltaic field, the solar simulator is equipped with electronic load, data acquisition and computer equipment, which can be used to test the electrical performance of photovoltaic devices (including solar cells, solar cell modules, etc.), such as Pmax, Imax, Umax, ISC , UOC, FF, Eff, Rs, Rsh and IU curves, etc.
Solar simulators are usually divided into two types: steady-state solar simulators and pulsed solar simulators. Steady-state solar simulators are solar simulators with stable output irradiance during operation; while pulsed solar simulators are not Discontinuous light emission, which only emits light in a short period of time (usually below the order of milliseconds) in pulse form. The types and characteristics of solar simulators are shown in Table 1.
|Types of||definition||advantage||Disadvantage||Scope of application|
|Steady state||The output irradiance is stable and unchanging when working||Continuous irradiation;|
|The optical system and power supply system are complex and huge||Manufacturing small area solar simulator|
|pulse||Millisecond pulse luminescence||High instantaneous power||Complex acquisition system||Large area measurement|
Commonly used light sources of solar simulators include halogen tungsten lamps, tritium lamps, etc. (see Table 2). The performance indicators of the light source include four aspects: total irradiance, spectral matching, uniformity and irradiance stability.
|Halogen light||Halogen lamp |
with water film
|There is a big difference between the spectrum and sunlight, |
the infrared content is large, the ultraviolet content is less, and the color temperature is 2300K
|The 3cm water film filters out part of the infrared rays and |
cannot supplement the ultraviolet rays
|Cold light||Tungsten halogen|
lamp plus dielectric film
|The reflector is transparent to infrared rays and reflects other light. |
The color temperature is 3 400K
|The lamp life is short, 50h||Simple type|
|Xenon lamp||Xenon lamp |
|The spectrum is close to daylight, |
but there are more infrared rays. Use a filter to filter out
|The spot is uneven, the circuit is complicated, the price is expensive,|
the optical integration equipment is complicated, and the effective area is difficult to enlarge
|Precision solar simulator|
|Pulsed xenon lamp||Pulsed xenon lamp||Short-term light intensity, better spectral characteristics than steady-state xenon lamp,|
large area uniform spot can be obtained
①Total irradiance. The simulator must be able to reach the standard irradiance of 1000W/m2 (measured with a standard battery) on the test plane, and can adjust the irradiance above and below the standard irradiance value as needed.
②Spectral matching. The optical harmonic irradiance distribution of the simulator should match the standard spectral irradiance distribution. The matching degree of grade A is 0.75-1.25. The matching degree of grade B is 0.6-1.4. The matching degree of grade C is 0.4-2.0.
③ Uniformity. On the test plane, the irradiance in the designated test area should reach a certain uniformity, and the irradiance should be measured with a suitable detector. The uniformity of irradiation of grade A is not more than ±2%, the uniformity of irradiation of grade B is not more than ±5%, and the uniformity of radiation of grade C is not greater than ±10%.
For the test of single battery and battery string, the maximum size of the detector should be less than half of the minimum size of the battery.
For components, the size of the detector should not be larger than the size of the single battery in the component.
Unevenness = ±[(maximum irradiance-minimum irradiance)/(maximum irradiance + minimum irradiance)] x100%
Among them, the maximum irradiance and the minimum irradiance refer to the measured value of the detector at any specified point within the specified range.
④ Irradiation stability. During the data collection period, the irradiance should have a certain degree of stability. The stability of grade A is not greater than ±2%, the stability of grade B is not greater than ±5%, and the stability of grade C is not greater than ±10%.
Irradiation instability = ±[(maximum irradiance﹣minimum irradiance)/(maximum irradiance+minimum irradiance)] x100%
Among them, the maximum irradiance and the minimum irradiance are the measured values of the detector at any designated point in the test plane during data collection.
Although the solar simulator has a wide range of applications in solar cell testing, it deviates from the real sunlight after all. For solar cells, the most credible test conditions are sunny days with no visible clouds in a large area around the sun, and light intensity conditions greater than 800W/m2. At this time, the spectral deviation is less than 5%, which is suitable for testing and calibrating various solar cells. It is the best condition for making standard solar cells (or photovoltaic modules).
Correction of test temperature
Solar cells are required to be measured under standard test conditions, but in the actual test process, the temperature of the solar cells will fluctuate with the ambient temperature, which will affect the accuracy of the measurement. In accordance with the “Correction Method of Temperature and Irradiance Specified in the CB/T 6495. 4 Standard”, the measured I-U characteristics are corrected to the standard test conditions according to the following formula:
Where: I1 and U1 are the coordinates of the measured characteristic points: I2 and U2 are the coordinates of the corresponding points of the correction characteristic: ISC is the measured short-circuit current of the sample: IMR is the measured short-circuit current of the standard solar cell. When measuring IMR, if any If necessary, the temperature of the standard battery should be corrected: ISR is the short-circuit current of the standard solar battery under standard or other desired irradiance: T1 is the measured temperature of the sample: T2 is the standard temperature, or other desired temperature : a and β are the temperature coefficients of current and voltage of the sample under the standard or other desired irradiance: and in the temperature range of interest (β is a negative value): Rs is the internal series resistance of the sample: K Is the curve correction coefficient.
Here, the current temperature coefficient of the solar battery refers to the change in the short-circuit current of the solar battery for each change in the measured solar battery temperature under the specified test conditions, usually expressed by a: and the voltage temperature of the solar ground The coefficient refers to the change value of the open circuit voltage of the solar cell for every 1°C change in the temperature of the solar cell under the specified test conditions, usually expressed by β.