  Silver bismuth sulfide (AgBiS$_2$) nanocrystals have emerged as a promising
eco-friendly, low-cost solar cell absorber material. Their direct synthesis
often relies on the hot-injection method, requiring the application of high
temperatures and vacuum for prolonged times. Here, we demonstrate an
alternative synthetic approach via a cation exchange reaction. In the
first-step, bis(stearoyl)sulfide is used as an air-stable sulfur precursor for
the synthesis of small, monodisperse Ag2S nanocrystals at room-temperature. In
a second step, bismuth cations are incorporated into the nanocrystal lattice to
form ternary AgBiS$_2$ nanocrystals, without altering their size and shape.
When implemented into photovoltaic devices, AgBiS$_2$ nanocrystals obtained by
cation exchange reach power conversion efficiencies of up to 7.35%,
demonstrating the efficacy of the new synthetic approach for the formation of
high-quality, ternary semiconducting nanocrystals.
