In the last decade, metal halide semiconductors have emerged as promising materials for solar cell applications.  While lead halide semiconductors have achieved remarkable power conversion efficiencies, now exceeding 26%, Pb(II) toxicity and alloying-stability issues have raised the urgency of developing nontoxic, stable,  and environmentally friendly alternatives. As a result, a catalogue of bismuth-based semiconductors (e.g., double perovskites, rudorffites, and others) has been the subject of intense investigation. However, record power conversion efficiencies for this new class of materials are currently limited at around ~6%, thus prompting new research efforts to explore and eliminate current limitations to performance. In this talk, I will discuss how ultrafast charge-carrier self-trapping process in fundamentally limits charge-carrier transport in several bismuth-based halides and chalcogenides, and I will present possible strategies to overcome this issue.[1-7] 

Focusing on the archetypal silver-bismuth based double perovskite Cs2AgBiBr6, I will examine how rapid decays in terahertz photoconductivity and their temperature dependence reveal an ultrafast localization of free charge-carriers to a small polaronic state. I will further discuss the implications of the resulting hopping-like transport regime. For instance, alloying Cs2AgBiBr6 with Cs2AgSbBr6 on the trivalent metal site interestingly leads to significantly stronger self-localisation.[2] This effect results from the more local probing of the energetic landscape by small polarons, which turns alloyed low-energy sites – in this case, Sb (III) sites ¬– effectively into traps, and significantly hinders charge-carrier transport. Delving deeper into the causes of this localisation process, I will discuss the role of the electronic structure and the electronic dimensionality. To do so, I will examine the role of Ag and Bi cation-ordering in AgBiS2 nanocrystals thin films.[7] By tuning the Ag/Bi cation arrangement via cation-disorder engineering, I will demonstrate that the combination of a higher electronic dimensionality and a more ordered electronic landscape can help in mitigating the effect of this localisation in silver-bismuth semiconductors.

Overall, the ultrafast localisation of charge carriers emerges as a formidable challenge for the new class of bismuth-based semiconductors and their application in renewable energy applications. The findings presented in this talk explore the underlying causes of such localization and its effects on charge-carrier transport. Furthermore, possible strategies to overcome this issue are presented.