A method of making a CsSnI3 perovskite thermoelectric (TE) material including mixing a fatty acid, a fatty amine, a C8-C30 hydrocarbon, and CsCO3 in a vessel to form a cesium mixture; heating the cesium mixture to a temperature of 400-450 K to form a heated mixture; dissolving SnI2 in an organophosphine to form a tin solution; mixing the tin solution and the heated mixture to form a reaction mixture; cooling the reaction mixture to form a precipitate comprising a CsSnI3 perovskite; enclosing the CsSnI3 perovskite in a chamber; pressurizing the chamber to a hydrostatic pressure of at least 0.1 GPa; and heating the chamber to a temperature of 300-1,000 K to form the CsSnI3 perovskite TE material having a ZT that is at least 0.1.