Doping and thermoelectric properties of the zero-dimensional inorganic halide perovskite derivative, Cs₃Cu₂I₅

Halide perovskites have been considered as promising thermoelectric materials due to their unusual combination of good charge mobility and ultralow thermal conductivity. Low dimensional halide perovskite derivatives (0D, 1D and 2D) have been predicted to have high thermoelectric figure of merit due to quantum confinement effects, but this class of metal halides has been under-explored experimentally. Here, we investigate the thermoelectric properties of the all-inorganic 0D halide perovskite, Cs₃Cu₂I₅, doped with barium. Ba-doped pellets were fabricated by solid state synthesis. An enhancement of electrical conductivity by >3 orders of magnitude was achieved upon doping and extended X-ray absorption fine structure measurements indicate that Ba substituted Cs on the smaller of the two A-sites. We show that thermal conductivity is in the ultralow regime and decreases with doping, consistent with increased scattering from defects. The positive Seebeck coefficient of + 2400 ± 60 μV.K^-1 for Cs₃Cu_2(1--x)Ba_xI₅ (x = 0.1) confirmed p-type doping. To our knowledge, this is the first study on substitutional doping of a 0D halide perovskite to improve the thermoelectric figure of merit.