where T h and T c are the hot-and cold-side temperatures, respectively, and ZT is the average ZT over the temperature range T c → T h . The continuous increase in the ZT with temperature results in a ZT that increases continuously up to ∼1.9 and 2.3 at T h = 800 K with the two doping levels (Fig. (b)). In contrast, the slower rise in ZT with temperature in Pnma SnSe means ZT > 1 is only achieved when T h ≥ 650 K. Our results therefore show that n-type π SnSe, given a suitably-high doping level, could display viable thermoelectric performance for a wide range of low-to-mid temperature applications. A similar contrast is seen between π and Pnma SnS, i.e. the π phase performs better at low T and therefore shows a higher ZT over a wide temperature range. However, the generally lower ZT means the threshold of ZT = 1 is not reached until T h = 675 K (500 K with n = 10 20 cm -3 ). However, for larger T h = 880 K we predict reasonable ZT of 1.2 and 1.5 at the two doping levels. For applications where sulphide-based TEs are preferred, exploring doping strategies to maximise the n of π SnS may be facile.