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3 Things You Should Know When Selecting LED Light Source Solar Simulators. Pros and Cons of LED Solar Simulators for IV Measurement of Perovskite Solar Cells.
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With the current development of renewable energy, the photovoltaic solar industry is growing rapidly. To test the power generation efficiency of solar cells, solar simulators are needed for indoor simulation. LED light sources have become one of the mainstream light sources for solar simulators due to their advantages such as energy saving, long usage life, etc. However, there are some disadvantages and limitations when using LED light sources. This article will discuss the pros and cons analysis of LED solar simulators when testing perovskite solar cells.
What is LED?
LED (Light Emitting Diode) is a semiconductor lighting device that converts electrical energy into light energy. It is made of a semiconductor material that emits light when current flows through it. The emitted light can be red, yellow, green, blue, or white, depending on the semiconductor material.
The advantages include high efficiency, long usage life, energy saving, tunability, fast illumination, and eco-friendliness. As a result, LEDs have been widely used in various lighting, display, and communication systems.
The LED (Light Emitting Diode) light source has multiple advantages, the most notable of which are:
- High efficiency: It has a high conversion efficiency, with current research and development able to convert 85% of electrical energy into light energy.
- Long lifespan: It has an extremely long usage period, typically reaching more than 10,000 hours when the junction temperature is maintained at 25 degrees.
- Energy-saving: It saves more energy than traditional light sources, reducing energy consumption by 80%.
- Tunable: The brightness of LED light sources can be adjusted according to environmental requirements.
- Quick response: The illumination speed is very fast, eliminating the need to wait when switching on.
- Eco-friendly: LED products do not contain toxic substances and are harmless to the environment.
What are the advantages of using LED as the light source for solar simulators?
According to the characteristics of LED light sources, solar simulator manufacturers usually emphasize that using LEDs as the light source for solar simulators has the following 7 advantages:
- Adjustable color temperature: The color temperature can be adjusted according to different wavelengths to simulate various lighting conditions.
- High controllability: Brightness and color temperature can be adjusted according to different simulation needs.
- Energy-saving: Lower power consumption than traditional lamp light sources.
- Eco-friendly: LED light sources do not contain toxic substances and are harmless to the environment.
- Long usage life: The claimed usage life of LED light sources can last much longer than other light sources, which can be advertised to reach more than 10,000 hours, but the premise is that the junction temperature must remain constantly at 25°C.
- Wide application: Can be used in various fields such as plant lighting, artificial intelligence research, optical research, biological research, and studio lighting.
- Weather simulation: Can simulate various weather conditions such as sunny and cloudy days.
But are LED lights really faultless? The definition and myth of long usage life
The LED usage life refers to the time when the luminous intensity is maintained at least 70% of the original brightness under certain temperature and electric current conditions.
The calculation method is based on the time required for the luminous intensity of the light-emitting diode to decay to 70% of the original brightness, which is the measurement standard. However, testing is usually conducted in groups of multiple bulbs. When the luminous intensity of more than half of the LED bulbs in the same group decays to 70%, the average time is considered the average usage lifetime of the LED bulb group. The length of the usage life test is usually measured and evaluated under ideal operating conditions, such as controlled temperature, electric current, and environment. Common control conditions include monitoring the luminous intensity and usage life under the junction temperature (Junction Temperature) of 25°C and specific conditions of electric current at 2 mA.
In summary, if the environmental operating conditions do not conform to the measurement standard conditions of the LEDs in the laboratory, it could greatly affect the usage life.
Is using LED as the light source for photovoltaic solar simulators the primary choice? Disclosure of actual disadvantages and undisclosed issues
Theoretically, a higher drive of electrical currents will increase the performance of light output. However, this also comes with accelerated power dissipation and a loss of light output and efficiency over time. Moreover, higher temperatures will cause the LED’s forward voltage to decrease, thus increasing the power dissipation of the constant electrical current.
Therefore, the dominant wavelength, light output, and forward voltage of the LED interact as listed below. (Reference: **https://canada.newark.com/lig-article-thermal-consideration**)
The relationship between the output of the light source and electrical and thermal parameters is significant.
Electrical, thermal, and optical – these three elements all impact an LED’s output characteristics.
Fig.2 explains the interrelation between the output of the light source and the parameters of electrical and thermal.
LED lights that fail thermally easily - output of the light source decreases as the temperature rises
According to the literature, AlInGaP quaternary LEDs are the most thermally sensitive. From the testimonials, we understand that for white-color LEDs to maintain >80% luminous flux, their junction temperature must be kept below 100°C. Also, in amber-color LEDs, the output luminous flux significantly decreases with the increase in junction temperature.
The above figure shows the relationship between junction temperature and luminous flux.
LED lights that easily change color with temperature – dominant wavelength (color change) varies with temperature
An increase in TJ causes a deviation in wavelength or color. The dominant wavelength of an LED depends on the junction temperature. The comparison of color in the chart below shows typical values for 1-watt high-brightness LEDs. Evidently, the amber color is the most sensitive because the deviation could be 0.09nm/°C.
Assuming an indoor lighting scenario, with ambient temperatures ranging from 10 to 40°C, the dominant wavelength deviation for amber color over a 30°C temperature range is 2.7 nanometers (40 – 10 * 0.09).
The hotter the condition, the more decay in LED performance – forward voltage decreases with temperature
Researchers using LEDs need to know that as temperature increases, VF decreases by 2mV/°C. Although when LEDs are connected in series because they are driven with a constant electrical current, VF variation should not be a serious issue. However, if the LEDs are in parallel, VF will decrease with increasing temperature, causing the electrical current to increase. As the electrical current increases, TJ then continues to increase, causing VF to decrease further, continuously interacting until balance is reached. Conversely, low temperatures increase VF, which can make it difficult to achieve the required fixed luminosity under the constant voltage operation of LEDs.
Declare strike LEDs when overheated – the usage life decreases with temperature
LED Reliability is a Direct Function of Junction Temperature, with higher junction temperatures often decreasing the usage life of the LED. IES LM-80-08 is a standard that specifies how LED manufacturers and luminaire manufacturers should test LED components to determine lumen maintenance over time. The L70 lifespan of an LED is defined as the time it takes for the LED lumen output to decrease from 100% to 70% under 25°C conditions (see figure below).
LM-80-08 reports are used to predict various temperatures and drive electrical current operating environments. The figure below explains the relationship between L70 lifespan and junction temperature. It can be observed that as junction temperature increases, the usage life of the LED decreases, with usage life below 1200 hours at 85°C. (Reference: https://www.mdpi.com/1996-1073/13/13/3370)
The total radiant flux maintenance results of the mid-power blue LEDs, sorted by case temperature and forward current, are presented below. (Reference: https://www.mdpi.com/1996-1073/13/13/3370)
