Reducing anthroprogenic greenhouse gas emissions and mitigating climate change while meeting the ever-increasing demand for energy are among the most important challenges in contemporary science. Achieving these goals requires a range of technologies including improved clean energy sources and supporting energy-storage systems as well as methods for improving the efficiency of energy-intensive processes. The latter is particularly important given that 60% of global energy is currently wasted as heat, and this is projected to remain above 50% into the next decade. One of the leading contenders for improving energy efficiency is thermometric (TE) power, which harnesses the Seebeck effect in a TE material to recover waste heat as electrical energy. Thermoelectric generators (TEGs) are solid-state devices with no moving parts and have diverse applications ranging from established uses in the aerospace industry to prospective uses for powering remote-sensing devices, recovering heat from combustion engine exhaust gases, and repurposing decommissioned oil rigs as geothermal power plants. The performance of a TE material is typically expressed by the dimensionless figure of merit ZT : ZT = S 2 σT κ el + κ latt where S is the Seebeck coefficient, σ is the electrical conductivity, and κ el and κ latt are the electronic and lattice (phonon) components of the thermal conductivity κ. S, σ and κ el are properties of the electronic structure and are usually tightly coupled through the carrier concentration such that the optimum balance is achieved in heavily-doped semiconductors. On the other hand, the κ latt depends on the structure and chemical bonding and is generally minimised in materials formed from heavy elements, with weak chemical bonding, and/or with structural features such as phase transitions and active lone pairs that lead to strongly-anharmonic lattice dynamics. These requirements have historically led to a focus on "heavy" chalcogenide materials such as the current industry standards for low-and high-temperature applications, viz. Bi 2 Te 3 (ZT ≃ 1 from 350-450 K) and PbTe (ZT ≃ 2.2 with endotaxial nanostructuring ). The high performance of PbTe has been linked to "band convergence" at high temperature and to strong intrinsic phonon anharmonicity, although the extent of the latter is disputed. However, the environmental toxicity of Pb and the rarity of Te means both materials are unsuitable for mass-produced TEGs, and considerable effort is being devoted to alternatives including alloys of Bi 2 Te 3 and PbTe with other chalcogens and other chalcogenides such as the flagship TE material SnSe. While chalcogenides have the advantage of favourable electrical properties and low lattice thermal conductivity, a major drawback is that they are prone to oxidation and can degrade or decompose at high temperature. On the other hand, oxide materials, while sought after for their chemical stability and large Seebeck coefficients, typically show undesirable high lattice thermal conductivity. Oxychalcogenides are an emerging class of TE materials that in principle combine the favorable properties of oxides and chalcogenides. Among the most widely-studied oxychalcogenides are the Bi 2 ChO 2 (Ch = S, Se, Te), which have been examined as potential n-type TEs to complement p-type chalcogenides such as SnSe in TEGs. The Bi 2 ChO 2 form layered structures in the tetragonal I4/mmm spacegroup comprising alternating [Bi 2 O 2 ] 2+ oxide and conductive Ch 2 -chalcogen layers (Fig. ). A number of recent studies have been carried out on the thermoelectric performance of Bi 2 SeO 2 , with "headline" ZT values of 0.2 in Br-doped single crystals at ∼ 400 K and ∼ 0.4 at ∼ 800 K in La-and Ta-doped consolidated powders. Bi 2 TeO 2 has been less widely studied, but a bulk ZT = 0.13 at ∼ 600 K was obtained without doping to optimise the carrier concentration. Bi 2 SeO 2 and Bi 2 TeO 2 have also both demonstrated low lattice thermal conductivities of 0.9-1.6 W m -1 K -1 and 0.91 W Figure : Representative structure of Bi 2 ChO 2 (Ch = S, Se, Te). Atoms colours: Bipurple; Ch -gold; O -red. This image was created using the VESTA software. . m -1 K -1 , respectively, at 300 K ). Bi 2 SO 2 has not been studied in experiments, but calculations predict that its larger κ of 2.9 W m -1 K -1 at 300 K can be reduced to 1.85 W m -1 K -1 in a 2D monoloayer, and that such a nanosheet could show ZT > 3 at high temperature. .