I think the efficiency of these sensors is quite good, latest generation full frame camera sensor operate well at lower ligh t than you can see in, for example.

Fitting more of them into tiny arrays and maintaining exposed surface is more of a problem, and noise properties could be better.

At the end of the day though, there are limits to what can ever be achieved with tiny sensors like we currently see in most cel phones; you can pack things in more tightly but you can't avoid the optical physics.