What is the Efficiency of APS-C Sensor Photo Sites for Gathering Photons?

I would expect the answer to this question would give us the answer to many people's questions (including mine) regarding the future of megapixel count, tonal range, ISO limit(s), dynamic range etc.

I would expect current efficiency is low (20% of photons recorded).  This is just a non-engineer/non-physicist guess.  That is based on the currrent silicon/copper sensors we have now.

If the efficiency is low then it would be interesting to know what it would take to get it up.  I am sure sensor manufacturers are constantly working on this question though there must be a general understanding of photon gathering efficiency of current sensors.