Strain Engineering for Tailored Carrier Transport and Thermoelectric Performance in Mixed Halide Perovskites CsPb(I_1–<i>x</i>Br_<i>x</i>)₃

Strain engineering of metal halide perovskites shows promise for better stability and device performance, but the impact on thermoelectric performance remains elusive. We demonstrate that the electronic structures and carrier transport properties in halide perovskites CsPb(I1–xBrx)3 can be tailored synergetically through the practical biaxial strain-engineering strategies. For the pure halide perovskite CsPbI3, the lattice geometry and electronic structures are basically retained under strains (from −6 to 8%), leading to moderately varied transport properties. Interestingly, under a −8% compressive strain, sharp changes in the carrier transport properties are observed in CsPbI3 because of the dramatically increased contribution of iodine electrons to the conduction band minimum. For the mixed halide perovskites, we find that CsPbI3/2Br3/2 is the thermodynamically most stable CsPb(I1–xBrx)3 as determined by the generalized quasi-chemical approximation method. The band gap, carrier effective mass, and other carrier transport properties of CsPbI3/2Br3/2 change dramatically in response to high external strains (≤−6 or ≥6%), accompanied by the ultralow thermal conductivities. Such abnormal phenomena originate from the distorted lattice geometry that is caused by the non-uniform internal stress distribution under high external strains. In addition, external strains can also tailor the optimal carrier concentration needed to achieve the maximum figure of merit (ZT), providing a new avenue to tackle the longstanding challenge in heavy-doping perovskites. Finally, the ZT values are very sensitive to the magnitude of strains, especially for mixed halide perovskites, showing enhanced ZT from ∼0.1 without strain to ∼0.9 under a −6% compressive strain at 300 K. This work provides practical biaxial strain-engineering strategies to enhance the thermoelectric performance and also to optimize the doping process in mixed halide perovskites.