Inspired
by the rapid rise in efficiencies of lead halide perovskite
(LHP) solar cells, lead-free alternatives are attracting increasing
attention. In this work, we study the photovoltaic potential of solution-processed
antimony (Sb)-based compounds with the formula A
3Sb2I9 (A = Cs, Rb,
and K). We experimentally determine bandgap magnitude and type, structure,
carrier lifetime, exciton binding energy, film morphology, and photovoltaic
device performance. We use density functional theory to compute the
equilibrium structures, band structures, carrier effective masses,
and phase stability diagrams. We find the A-site
cation governs the structural and optoelectronic properties of these
compounds. Cs3Sb2I9 has a 0D structure,
the largest exciton binding energy (175 ± 9 meV), an indirect
bandgap, and, in a solar cell, low photocurrent (0.13 mA cm–2). Rb3Sb2I9 has a 2D structure,
a direct bandgap, and, among the materials investigated, the lowest
exciton binding energy (101 ± 6 meV) and highest photocurrent
(1.67 mA cm–2). K3Sb2I9 has a 2D structure, intermediate exciton binding energies
(129 ± 9 meV), and intermediate photocurrents (0.41 mA cm–2). Despite remarkably long lifetimes in all compounds
(54, 9, and 30 ns for Cs-, Rb-, and K-based materials, respectively),
low photocurrents limit performance of all devices. We conclude that
carrier collection is limited by large exciton binding energies (experimentally
observed) and large carrier effective masses (calculated from density
functional theory). The highest photocurrent and efficiency (0.76%)
were observed in the Rb-based compound with a direct bandgap, relatively
lower exciton binding energy, and lower calculated electron effective
mass. To reliably screen for candidate lead-free photovoltaic absorbers,
we advise that faster and more accurate computational tools are needed
to calculate exciton binding energies and effective masses.