Semiconductors can be “Direct” or “Indirect”, based on the alignment of the -valley (i.e. largetst)
Phonons are essentially vibrations which increase heat, which lowers efficiency significantly.
More accurately, since we have certain valleys corresponding to alignment and momentum it looks like this:
GaInAs, and GaInAs/InP is specifically really important for Optical Comms, whereas III-N (GaN, AlN, InN) are important for visible light generation.