Overcoming Charge-Carrier Localization in Metal Chalcohalides

Abstract

Effective charge-carrier transport is a key requirement of next-generation thin-film materials developed for solar cells. Perovskite-inspired materials (PIMs), including metal chalcohalides, show great promise as lead-free solar absorbers. However, intrinsic charge-carrier localization processes have frequently been reported to severely limit their transport properties. Recent research has thus focused on developing a rational understanding of this localization process and identifying strategies to eliminate it. Mixed-metal chalcohalides (A2BCh2X3) may offer promising solutions, combining enhanced chemical stability with promising optoelectronic properties. Here, we demonstrate how charge-carrier localization can be overcome through judicious chemical substitution in this family of materials. Upon changing the M(II) cation on the A-site, the lattice symmetry shifts from the lower-symmetry monoclinic P21/c phase in Pb2SbS2I3 to the higher-symmetry orthorhombic Cmcm phase in Sn2SbS2I3. Crucially, a rapid localization of charge carriers within the first few picoseconds of their generation is observed only for Pb2SbS2I3, whereas Sn2SbS2I3 maintains a longer-lived nanosecond photoconductivity. We attribute this observation to the higher electronic dimensionality of the Cmcm Sn2SbS2I3 structure, whose more symmetric lattice suppresses the charge-carrier localization dominating in the lower-dimensional P21/c Pb-analogue. These findings establish a direct link between structural and optoelectronic properties in metal chalcohalides, demonstrating how facile chemical tuning can be harnessed to overcome charge-carrier localization in PIM absorbers for solar energy harvesting.