Our analysis so far has focused on where to find PSs with low effective masses and direct band gaps in the visible. These features are correlated with high μ and high α in an appropriate photon energy range. However, we have not touched upon the third desirable feature mentioned in the Introduction, i.e., a high carrier lifetime τ. Compared to the case of α and μ, it is much more difficult to find simple properties available in materials databases that can be correlated to τ. The carrier lifetime is inversely related to the product of the concentration and capture cross section of crystal defects that are active as Shockley-Read-Hall recombination centers. In addition, the closer a defect state is to mid-gap, the higher the Shockley-Read-Hall recombination rate will tend to be at constant τ. For most compound semiconductors, the dominant defects are not extrinsic impurities, but rather native defects that are thermodynamically favored. Developing general design rules for semiconductors with high τ and low recombination rates ("defect-tolerant") has proved elusive. Nevertheless, there is some consensus that the nature of the electronic states at the band edges plays an important role. Relevant factors include the constituent orbitals, their hybridization, and whether they form bonding or antibonding states.