Understanding Solar Cell Specifications: Voltage, Current, and Efficiency

[fisico30]

Hello Forum,
I am trying to perform a solar cell experiment. I went to several websites and they list their small solar cells panels as having an output of 0.5 volts, 100mA.
The cell size is 1 3/4" X 1"...

Does 0.5 V represent the open circuit voltage, when the cell receives the max energy from the sun?
his is the max voltage that the cell can produce. What if we placed the solar cell in front of something stronger that the sun, with a higher irradiance? the open circuit voltage would be higher...

Does anyone know at what wavelength a silicon solar cell has the highest efficiency?

Does 100mA represent the short circuit current (when the load resistance is almost zero and the sun is giving the max energy)?

Some cells have the same output voltage but higher output current? Why? For instance, 0.5 V and 300 mA or 0.5 V and 400 mA...

Thanks,
fisico30

 

[fisico30]

Or are they the voltage and current when max power is emitted?

 

[fisico30]

maybe the voltage and current that give the max power under 1000 W/m^2 solar illumination and 25 degrees temperature?

 

[PaulS1950]

The silicon junction has a voltage drop of .5 VDC. A silicon solar cell will deliver no more than .5 VDC when open. It will produce 100 ma of current when exposed to a rated light source at a rated load at close to .5 VDC.
Cells made from other materials produce different voltages and power levels. GaAr cells typically produce .3 VDC.

 

[cybhunter]

Does anyone know at what wavelength a silicon solar cell has the highest efficiency?


the energy of a photon is E=hf where h is planks constant (6.63*10^-34J/S) and f is the freq of the photon (f=c/λ)

In modern physics, the longest wavelength that can liberate an electron from a metal is known as the 'threshold wavelength' , and is denoted by KEmax=hf-phi with phi being the work function in electron volts. For silicon, it has a Work function of 4.52eV (from http://environmentalchemistry.com/yogi/periodic/Si.html)

with that the longest wavelength is 2.74*10^-7m (f=1.09*10¹⁵hz)

 

[NascentOxygen]

Some cells have the same output voltage but higher output current? Why? For instance, 0.5 V and 300 mA or 0.5 V and 400 mA...

Bigger area of exposed silicon contribute more current. If you want higher voltage, you have to add them in series and expose them to the same intensity of sunlight.

 

[fisico30]

Silicon solars are said to be able to produce 0.5V and can supply up to 0.1 A when exposed to bright light.

For a silicon cell, the open circuit voltage is 0.5 V and the short circuit current is 50 mA.

The maximum power the solar cell can emit is the product of the current and voltage at the max power point. The current and voltage needed for the max power are not the short circuit current and open circuit current...
Both the voltage and current change depending on the resistance of the load...

So when they say that the current they can supply is 0.1 A they assume that the load has resistance close to zero...

If you gain in current you lose in voltage. You cannot have both high...

 

[Mike_In_Plano]

As I recall, there was on open voltage rating and a shorted current rating. You could multiply the two, but it wouldn't give you the max power. That was described by the peak power number.

A solar cell has a family of curves showing current versus voltage. Each curve is somewhat rectangular, with the corner rounded off a bit. Thus, they do move towards having P = V_rating x I_rating, but they fall just a bit short.

The different curves are based on operating temperature with the output voltage dropping as the temperature increases. Thus a cell that's really hot has less efficiency.

Then, you have to compute the captured sunlight versus time of day and sum those effects, since the cells typically don't get direct light in the morning and evening. Take the sum of all that energy and divide over the number of hours in the day, and the poor cell doesn't get much energy overall.

 

[fisico30]

Hello Mike,
thanks for your reply. I agree with what you say, but when we go online and look for a solar cell, they always give you a voltage (0.5 V) and a current rating...

I am still wondering what that current rating is...Max current allowed, is that the current needed achieve the max power (for V=0.5 V)...?