Hi @kwible,
It sounds like 2 Hz may be enough for your I-V solar cell capture. You should check out the downsample_logging.py example I mentioned above. You can find more info in this post: Joulescope data logging
Supporting downsampling in the UI and recording to other file formats is a planned future feature for the Joulescope UI. However, it is not yet implemented. Running from python is hopefully pretty easy, but let me know if you run into issues.