Efficient p-Type Doping of Tin Halide Perovskite via Sequential Diffusion for Thermoelectrics

Metal halide perovskites (MHPs) hold great potential in thermoelectric (TE) applications, thanks to their regular and soft lattice in nature. However, the poor electrical conductivity caused by low charge carrier density (<10¹⁴ cm^-3 for lead-based MHPs) strongly impedes its TE development. In this scenario, tin halide perovskites (THPs) emerge as promising TE candidates owing to their high background hole densities (>10¹⁹ cm^-3). However, further electrical doping remains challenging, originating from the limited capability of accommodating heterogeneous dopants and the heavy compensation in THPs. Herein, a novel diffusion-mediated doping approach is demonstrated to prominently increase the p-type doping level of THPs by a sequence of air exposure and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TCNQ) surface treatments. In paradigm photovoltaic THP materials-CH(NH₂)₂SnI₃ (namely FASnI₃), the electrical conductivity is dramatically increased by 300× from 0.06 to 18 S cm^-1 in thin films, leading to a remarkable enhancement of power factor by 25× up to 53 μW m^-1 K^-2. In contrast, only a slight variation of thermal conductivity is observed after F₄TCNQ deposition, which is in accordance with the increase in electrical conductivity, indicating that the lattice structures of FASnI₃ remain intact after doping. This study paves an illuminating way to ameliorate TE properties in halide perovskites.