Not sure if you have seen this video about "crystal cells" or not but your setup reminds me of this video that I watched many many years ago and had decent results with when I made them myself. I never actually tested the power output, but lighting LEDs and powering a small motor seemed to be no problem. They do however degrade/corrode over time and will become unusable after a few years. It may give you a few more ideas to try/experiment with.