IV curve with Keithley 2400 sourcemeter

[BrunoPuntzJones]

Hello,

I use a Keithley 2400 sourcemeter to trace I-V curves for solar cells under illumination (the keithley sources voltage, measures current).

I've found that if the current compliance level for the Keithley is set much higher than the current I'm actually measuring, the I-V curve will have a strange shape, where the current dips down between the short circuit side and the knee of the curve. As long as I have this compliance level (or maximum current setting) set to a value slightly above the highest current in my sweep, everything looks normal.

I don't really need to fix the problem, but I'm just looking to understand what is causing this measurement artifact. I should add I'm not an electrical engineer, I'm a material scientist that just occasionally has to deal with this stuff, so forgive me my ignorance and use small words. %-)

 

[IRstuff]

Do you have a picture?

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[VEBill]

"...under illumination..."

That'll complicate things. The voltage and current from the illuminated solar cell probably isn't "expected" by the instrument.

 

[IRstuff]

Oh, I missed that part. It's likely that the illumination is causing the measurement circuit to freak out, since photocurrent the illumination could be construed to come from a negative resistance.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize