However, at least one of the effective masses is greater than the rest mass of the electron m 0 in most of the known ternary PSs (Fig. ). Therefore, the predominance of 3D structures in PSs is generally not sufficient to ensure high μ in this class of compounds. We also note that holes in PSs are generally heavier than electrons, as often seen in semiconductors. Since low effective masses are not fundamentally prohibited by low dimensionality in PSs, we argue that one of the key areas of research in PS semiconductors for optoelectronic energy conversion should be to identify design rules for obtaining low effective masses.