Figure 2 is going to give you idea. I can give you detail info if you need it. Regards

Hi Victor,

have a look here:

http://iris.elf.stuba.sk/JEEEC/data/pdf/09-10_104-03.pdf

Thank you very much!

The internal resistance of my oscilloscope is 1 MOhm. It is large but still there is some current through the solar cell under test. Thus, I am not sure if I do not measure just the RC constant instead of the Voc decay due to the recombination of photogenerated charge carriers. 

Yes you're wright.

Better to use a high impedance buffer (1 Mohm) as is used for surface photo voltage measurements

Hi Jörg,

Thank you for help! I will try to find the way out next week.

Best,

Victor

You are going to need a high input impedance voltage follower. But you must be careful about offset null voltage. I recommend you JFET or MOSFET input op.amp. Please glance figure.2 at the attached paper. I think It's going to be useful for you.  Regards

Dear Sertap Kavasoglu,

Thank you for useful information. I appreciate your help.

Best,

Victor