Why Do LED Photodetector IV Curves Vary in Different Lighting Conditions?

[jlewis]

I was wondering if anybody could help me understand my measurements. I was interested in measuring the photo current and voltage of a red LED placed in room light and sun light. I measured the voltage across the LED and the current through in both conditions.

In the room measurement, I am measuring the diode while it is being illuminated by the room incandescent lighting. I see voltages anywhere from 30mV to 90mV. The current is too small for my cheap meter to measure.

In the sun measurement, I am measuring the diode while is being illuminated in sunlight. I see correlation to the room measurement in that the room samples with the lowest open circuit voltage have the lowest short circuit current. However the strange thing is that the open circuit voltage measurements in sunlight are larger for the units with lower current.

I was hoping someone could either explain or point me to a good reference. I have enclosed the measurements below.

Thanks in advance,
Jason

Incandescent Sun
Diode Voc (V) Isc (A) Voc (V) Isc (A)
D1 7.80E-02 to low to measure 1.402E+00 9.40E-06
D2 7.20E-02 to low to measure 1.403E+00 9.00E-06
D3 6.60E-02 to low to measure 1.398E+00 8.30E-06
D4 6.30E-02 to low to measure 1.392E+00 8.20E-06
D5 8.90E-02 to low to measure 1.390E+00 9.90E-06
D6 7.90E-02 to low to measure 1.394E+00 9.10E-06
D7 3.10E-02 to low to measure 1.483E+00 6.90E-06
D8 7.30E-02 to low to measure 1.400E+00 8.70E-06
D9 7.70E-02 to low to measure 1.408E+00 9.30E-06
D10 3.90E-02 to low to measure 1.479E+00 6.40E-06

 

[BigJDubs]

From your measurements, it appears that the open circuit voltage of the LED increases when exposed to sunlight, while the short circuit current decreases. This is because the increased illumination raises the forward voltage drop (Vf) of the diode, meaning more of the energy from the light is being converted into electrical energy. The higher Vf reduces the amount of current that can pass through the diode, meaning that the short circuit current decreases with increasing light intensity.

 

[charng]

Hello Jason,

Thank you for sharing your measurements and questions about LED photodetector IV curves. From your data, it seems that your measurements are consistent with what is expected for photodiodes under different lighting conditions.

In the room measurement, the low voltage and current readings are likely due to the low intensity of the room incandescent lighting. Incandescent lights emit a broad spectrum of light, including infrared and ultraviolet, which may not be efficiently absorbed by the red LED. This results in a lower open circuit voltage (Voc) and current (Isc) readings.

In the sun measurement, the higher open circuit voltage readings for diodes with lower current may be due to the higher intensity of sunlight compared to room lighting. This results in more electrons being excited in the diode, leading to a higher Voc. The correlation between Voc and Isc is expected, as a higher intensity of light will result in a higher number of excited electrons and therefore a higher current.

Overall, your measurements appear to be consistent with the expected behavior of photodiodes. If you would like to learn more about the principles behind photodetectors and their IV curves, I recommend consulting a textbook on semiconductor physics or optoelectronics. Additionally, you may find resources on the website of the International Dark-Sky Association, which advocates for responsible outdoor lighting to reduce light pollution.

I hope this helps to clarify your measurements. Keep up the good work in your research!