Abstract
Recent advances in heterojunction and interfacial engineering of perovskite solar cells (PSCs)
have enabled great progress in developing highly efficient and stable devices. Nevertheless, the
effect of halide choice on the formation mechanism, crystallography and photoelectric
properties of the low-dimensional phase still requires further detailed study. In this work, we
present key insights into the significance of halide choice when designing passivation strategies
comprising large organic spacer salts, clarifying the effect of anions on the formation of quasi2D/3D heterojunctions. To demonstrate the importance of halide influences, we employ novel
neo-pentylammonium halide salts with different halide anions (neoPAX, X = I, Br or Cl). We
find that regardless of halide selection, iodide-based (neoPA)2(FA)(n-1)PbnI(3n+1) phases are
formed above the perovskite substrate, while the added halide anions diffuse and passivate the
perovskite bulk. In addition, we also find the halide choice has an influence on the degree of
dimensionality (n). Comparing the three halides, we find that chloride-based salts exhibit
superior crystallographic, enhanced carrier transport and extraction compared to the iodide and
bromide analogs. As a result, we report high power conversion efficiency in quasi-2D/3D PSCs,
which are optimal when using chloride salts, reaching up to 23.35% and improving long-term
stability.