Question

Why do some LEDs generate a photovoltage in some LEDs but not in other LEDs? Why does white light generate a voltage in all the LEDs?

Answer 1

Why do some LEDs generate a photovoltage in some LEDs but not in other LEDs?

Reason :

• LEDs (Light Emitting Diodes) are designed to emit light when their PN junction is forward biased. PN junction area of LEDs are small and for solar cell it is large. So LEDs have small photovoltaic effect compared to solar cell. When LED is exposed to light source, due to small surface area it will generate photovoltage.
• This photovoltage generated by LED depends upon material used in LED. Different materials are used for producing different colors in LED. All diodes emit photons but LEDs are made of such materials which emit photons whose wavelength falls within visible portion of light spectrum. So LEDs of different colors work at different wavelengths.
• Now consider that LED 1 of particular color is used as a source of light and other LED 2 of particular color is used to observe photovoltaic effect. When LED 2 is exposed to light of LED 1 and if wavelength of color of LED 1 can emit photons in LED 2 then LED 2 will produce photovoltaic voltage. If light of different color (different wavelength) is used for LED 1 which cannot emit photons from LED 2 then LED 2 will not produce photovoltaic voltage.
• So different color LEDs are made of different materials. Gallium, Arsenic, Nitrogen, Phosphorus are materials which are generally used for different color LEDs. Blue light LED is made from GaN, Green light LED from GaP, etc. To produce photovoltage from particular LED, light source of particular wavelength is required. If light source has required wavelength then LED generates photovoltage. Otherwise photovoltage can't be generated. Hence some LEDs generate photovoltage in some LEDs but not in other LEDs.

Why does white light generate a voltage in all the LEDs?

Reason :

• White color is a combination of three basic colors (Red, Green and Blue). These three colors are present in equal amount in white color. White color doesn't have specific wavelength. Other color LEDs have specific wavelength.
• White color is composed of wavelength of different colors. White color has a range of wavelength (400nm-700nm).
• Now if white color is used as light source, then it can emit photons from all type of LED materials. So white LED can generate photovoltage from all the LEDs.

​​​​​​​Hope this answer will help you to solve your queries regarding LEDs of different colors. Thank you!