Publication date: 15th December 2025
Suboptimal charge-carrier transport remains a major bottleneck for advancing efficient perovskite-inspired material (PIM) solar cells. Even in defect-tolerant systems such as metal chalcohalides, where deep traps are less detrimental, intrinsic charge-carrier localisation can still strongly limit transport. Understanding this localisation process and learning how to suppress it is therefore key for progress in PIMs. Mixed-metal chalcohalides (A₂BCh₂X₃) have recently emerged as promising candidates, offering enhanced chemical stability alongside favourable defect-tolerant optoelectronic properties, with Sn₂SbS₂I₃ showing the highest recorded PCE for this material class. [1]
We will show how charge-carrier localisation can be mitigated in this material family and explain how these behaviours arise through a structural–optoelectronic relationship. By substituting the M(II) cation (Pb, Sn), we tune the lattice from the lower-symmetry and more electronically confined P2₁/c structure in Pb₂SbS₂I₃ to the higher-symmetry Cmcm structure in Sn₂SbS₂I₃, which supports greater electronic dimensionality. This increase in symmetry has pronounced consequences for charge transport: Pb₂SbS₂I₃ exhibits a higher initial mobility (µdeloc = 4.7 cm²/Vs, measured by optical-pump terahertz-probe) but undergoes ultrafast localisation within a few picoseconds. In contrast, Sn₂SbS₂I₃, despite its larger static lattice distortions, shows lower initial mobility (µ = 2.51 cm²/Vs) yet sustains photoconductivity on nanosecond timescales.
The higher electronic dimensionality in the Sn analogue, combined with the reported high-Z ns² electronic contribution, mitigates defect-mediated localisation. These results demonstrate that lower symmetry and reduced electronic dimensionality promote rapid localisation, whereas higher symmetry and greater dimensionality can sustain transport. [2] This provides a clear pathway for compositionally tuning and ultimately overcoming charge-carrier localisation in mixed-metal chalcohalide PIMs.
The authors gratefully acknowledge funding from the Engineering and Physical Sciences Research Council (EPSRC) UK.
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