Post corrected. BTW, it's partially your fault for my long post. Those Rigol scope captures are what got me thinking. :)
I would be interested to see that setup, how you attached the resistor, what value, and whether you got adequate speeds?
I didn't post this (but will now). I did test a Casio calculator solar cell, and the results were not good:
The calculator was disassembled (read destroyed). The solar cell was somehow sonic welded into the case, couldn't remove it with out a lot of cutting, so just left it in. I did attach the rotating potentiometer to the solar cell, but regardless of whether the resistor value was 0 or 10K, the waveform was the same.
The J5 Tactical V1 Pro on low, DC coupled. The solar cell is very slow with a fall time of 1.2ms. In addition, the cell has a constant DC bias, which the waveform rides on.
J5 on low, AC coupled. Slow fall time of 1.5ms
Convoy S2+, at 10% brightness. DC coupled. Again, the solar cell has a DC bias, which the waveform rides on top.
Convoy S2+, at 10% brightness. AC coupled.
CONCLUSION. Although technically you can measure the higher frequencies of a Convoy S2+, the duty cycle and waveform are highly skewed.
Are there faster solar cells that might do better?