Wow, I just now checked, and the price on spectrometers has really come down a lot! Something like this: 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.