Hi Jenny,
I would suggest TSL257 which is a high-sensitivity low-noise light-to-voltage optical converter that combines a photodiode and a transimpedance amplifier on a single monolithic CMOS integrated circuit. This is cheaper than than OPT101. Challenge is getting them in good quantity. RS stocks them regularly.
https://ie.rs-online.com/web/p/photodetector-amplifiers/6424430/