- Acrylic infra-red filters (data sheet)
- There are infra-red sensors but they are what is used in PIR detectors, they are expensive (£2.50 a go) and seem to require fairly complex supporting circuitry
- There are also infra-red receivers for remote controls. They are cheap and combine a sensor (perhaps a phototransistor?) with a pre-amp, and include daylight filtering. Possibly a good bet. Example (data sheet).
- Finally, bare phototransistors. Very cheap. From the datasheet it looks like they might allow more current to pass when exposed to more IR, which might be useful. Example (data sheet).
I tested a small solar panel by exposing it to IR, there was no difference at all in the voltage it produced. Though possibly other panels may work.