If I have a molecule that is red in colour (in a solar cell), then it absorbs all colours and reflects red (I think).
If I put a filter of the same colour (red) in front of the molecule, then am I correct in my following thinking?
The filter would absorb all colours except for red which it would allow to pass through. So, in theory, the molecule would absorb no light and generate no electricity?
If your filter allows only red light to pass through, and your solar cell reflects all incident red light, then your solar cell will not generate electricity. You are right.
However, take note that there are various kinds of filters. Some transmit selected parts of the spectrum, some reflect selected parts of the spectrum, some absorb specific parts of the spectrum. But, very few filters are perfect. They will almost always pass small portions of the light that falls outside of their nominal spectral pass band. Moreover, a solar cell's output is unlikely to drop completely to zero outside of a given small spectral range. So, in a real case as opposed to an ideal case, your solar cell may generate a small amount of electricity even if you place a perfect red filter in front of it.
Finally, take note that "red" is not a very meaningful term when talking about filters and solar cells. What your eye sees as "red" actually covers a rather broad spectral range.
