Wow, I just now checked, and the price on spectrometers has really come down a lot! Something like this: https://www.adafruit.com/product/3779 together with something similar for infrared might possibly be "good enough", provided that you worked out the conversion factors for the solar cell. Nonetheless... nearly $20 more than taking measurements from the solar cell directly. It's the cheapest I've found so far. For something with better quality, the prices can be a lot more: e.g. https://www.digikey.com/product-detail/en/seeed-technology-co-ltd/114991292/114991292-ND/7622897