We predict all four n-doped Pnma phases, i.e. GeS/GeSe as well as SnS/SnSe, to show larger ZT max than can be obtained with ptype doping, whereas p-doped R3m GeS and GeSe are predicted to show larger ZT max than with n-type doping. For Pnma SnS and SnSe, we predict n-type doping to lead to 28% and 15% larger ZT max than p-type doping, whereas the ZT max of GeS and GeSe are predicted to be 77% and 59% larger with n-type doping. Pnma GeSe and SnSe are predicted to have superior TE performance to the corresponding sulfides with 77% and 49% larger ZT max with p-type doping and 59% and 34% larger ZT max with n-type doping. This is due to a combination of a generally larger σ and comparable or larger S, leading to comparable or larger PFs, and lower total thermal conductivity. On the other hand, we predict p-type R3m GeS to have an 8% higher ZT max than R3m GeSe due to the larger predicted κ latt of the selenide outweighing the larger PF and comparable κ el .