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.
Contents
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.
Contents
With the current development of renewable energy, the industry of photovoltaic solar 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. Relatively, 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 that depending on the semiconductor material.
The advantages include high efficiency, long usage life, energy saving, tunable, 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 itself has multiple advantages, and the most famous features are:
- High efficiency: high conversion efficiency, current research and development can convert 85% of electric energy into light energy.
- Long usage life: extremely long usage period, usually can reach more than 10,000 hours under the condition of maintaining the junction temperature at 25 degrees.
- Energy saving: save more energy than traditional light sources, which can reduce 80% of energy consumption.
- Tunable: The brightness of LED light sources can be adjusted and can be properly adjusted according to environmental requirements.
- Quick Response on illumination: The illumination speed is very fast, and no need to wait when switching on.
- Eco-friendly: Products of LED 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 different 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 at 25°C. constantly.
- Wide application: Can be used in various plant lighting, artificial intelligence research, optical research, biological research, studio lighting e.g.
- Can simulate various weather conditions such as sunny, cloudy days, etc.
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 conditions and certain conditions of electric current.
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 as the measurement standard. However, the testing is usually carried out in groups of multiple bulbs. When the luminous intensity of more than half of the number of LED bulbs in the same group decays to 70%, the average time is the average usage lifetime of the LED bulb group. But the length of the usage life test is usually measured and evaluated under ideal operating conditions arranged, such as temperature, electric current, and environment must be controlled. Common control conditions include monitoring the luminous intensity and usage life under the junction temperature (Junction Temperature) of 25°C and 2 mA specific conditions of electric current.
In sum, once the environmental conditions of operating do not conform to the measurement standard conditions of the LEDs in the laboratory, it would greatly affect the life of usage.
Using LED as the light source for photovoltaic solar simulators is the primary choice? Disclosure of the Actual disadvantages and undisclosed issues
Theoretically, the higher drive of electrical currents will increase output on the performance of light. Equally, it comes with accelerated power dissipation and loss of light output and efficiency eventually. Moreover, higher temperatures will also cause the LED’s forward voltage to decrease, thus making the power dissipation of the constant electrical current turns higher.
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 relevance between the output of the light source and parameters electrical and thermal.
Electrical, thermal and optical – the three elements all impact an LED’s output characteristics.
Fig.2 explains the interrelation between the output of the light source and parameters of electrical and thermal.
LED lights that are thermal failure easily- output of light source decreases while the temperature rises
According to the literature, AlInGaP quaternary LEDs are the most thermally sensitive. From the testimonials, we can understand that for white-color LEDs to maintain >80% luminous flux, their junction temperature must be kept below 100°C. And in amber-color LEDs, the output luminous flux also plummets with the increase of the 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
TJ increases wavelength or color which caused deviation. The dominant wavelength of an LED depends on junction temperature. Regarding typical values for 1-watt high-brightness LEDs by comparison of color in the below chart. Obviously, 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 on LEDs performance – forward voltage decreases with temperature
The researchers who use LED, need to know that while temperature increases, VF decreases 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 electrical current increases, TJ then continues to increase, causing VF to decrease further, continuously interacting until the balance is reached. Conversely, as 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. And IES LM-80-08 is a standard that specifies how LED manufacturers and luminaire manufacturers should test LED components to determine the 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 relevance between L70 lifespan and junction temperature. It can be observed that as junction temperature increases, the usage life of LED decreases, with usage life below 1200 hours at 85°C. (Reference: https://www.mdpi.com/1996-1073/13/13/3370)
The attained total radiant flux maintenance results of the mid-power blue LEDs, sorted by case temperature and forward current. (Ref. https://www.mdpi.com/1996-1073/13/13/3370)
