Solar cells convert only a tiny portion of solar rays to electricity because they only operate on a very narrow band of light. The 20% figure probably represents 20% conversion within the narrow band that the solar cells operate in. Now if you can widen the bandwidth then you can easily get 9 times as much energy.
I don’t have a source that addresses this question directly, but it makes no sense to calculate efficiency on a narrow band. The whole effort to improve efficiency is to widen the conversion band.