Oct 21, 2021, CWS

It's clear that variations in the input offset voltage of the commercial electrometers (Keithley, Keysight) are a limitation when used with solar cells that have very low source impedance (like a few hundred ohms in parallel with 1 uF of capacitance). The correlation time for the keysight box is about 1/2 second. Options include:

select solar cells (perhaps the earlier SunPower Gen 2 ones) with highest possible resistance. To do this we need a way to screen them efficiently. I propose an AC measurement using a lock-in amplifier. We can drive a known AC voltage and measure the AC current that is at the drive frequency. This can be done with room lights on, using large signal levels and (for example) a Thor labs or SRS current amplifier.

2. use a charge measurement circuit that has an auto-zero function on the front end. The LTC 1051 is a good potential op amp for this: 10513fa.pdf

3. chop the source at a frequency that allows for correlated double sampling of the photocurrent. This means a chop rate of faster than a few Hz.

4. Custom electronics, with autozeroing front end:

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