Amplifying current for solar cell characterization

[jcpcruz]

Good day!

I'm trying to characterize solar cells. I do this in Labview 8.6 by varying the voltage across the cell using an NI USB-6008 and reading the resulting current to get the IV curve. I measure the current through a 1 ohm shunt resistance. The problem is that these solar cells have very low photocurrents (around 0.005 mA). The NI USB-6008 has trouble reading currents that small (it just looks like noise).

Instead, can I just connect a current-to-voltage converter (like the one shown below) to amplify the photocurrent and convert this to voltage? And then in LabView I'd reconvert the voltage to current, plot this against the photovoltage, and then I have an IV curve?

Thanks for any help and input you can provide.

[johnsold]

Yes.  A current to voltage converter will work fine.

With that circuit you are essentially measuring the short circuit current of the photodiode.  If you want to characterize a solar cell at its maximum power point (where both I and V are non-zero), you will need a more complicated circuit.

Lynn

[jcpcruz]

Thanks. How come it's not possible to find the current and voltage at maximum power with this circuit?

[johnsold]

The maximum power point for a single cell will be at about 0.5 V and some non-zero current.  The circuit you proposed forces the cell to operate at zero volts.  This measures the short circuit current (Isc).  Because the voltage is zero, the delivered power is also zero.  At the maximum power point the current will be less than Isc and the voltage will be less than the open circuit voltage (Voc @ I = 0).  Photovoltaic systems which attempt to operate at the maximum power point do so by adjusting the operating point up and down until the maximum power is found.  These use a feedback system to maintain operation near that point.

Lynn