With one layer the case is simple. There is a certain light energy at which the conversion of light to current occurs called gap energy. If the light energy is lower than that no conversion can happen and if the light energy is higher the extra energy is converted to heat and only gap energy remains.

Filtering UV would be a loss but a small one.

I would be grateful if you could include the country this applies to in the title.

For everybody else: it’s the US. Who would have thought.