  Achieving control over the transport properties of charge-carriers is a
crucial aspect of realizing high-performance electronic materials. In
metal-halide perovskites, which offer convenient manufacturing traits and
tunability for certain optoelectronic applications, this is challenging: The
perovskite structure itself, poses fundamental limits to maximum dopant
incorporation. Here, we demonstrate an organic modifier incorporation strategy
capable of modulating the electronic density of states in halide tin
perovskites without altering the perovskite lattice, in a similar fashion to
substitutional doping in traditional semiconductors. By incorporating organic
small molecules and conjugated polymers into cesium tin iodide (CsSnI3)
perovskites, we achieve carrier density tunability over 2.7 decades, transition
from a semiconducting to a metallic nature, and high electrical conductivity
exceeding 200 S/cm. We leverage these tunable and enhanced electronic
properties to achieve a thin-film, lead free, thermoelectric material with a
near room-temperature figure-of-merit (ZT) of 0.21, the highest amongst all
halide perovskite thermoelectrics. Our strategy provides an additional degree
of freedom in the design of halide perovskites for optoelectronic and energy
applications.
