Strong absorption and ultrafast localisation in NaBiS2 nanocrystals with slow charge-carrier recombination

Abstract

I-V-VI2 ternary chalcogenides are gaining attention as earth-abundant, nontoxic, and air-stable absorbers for photovoltaic applications. However, the semiconductors explored thus far have slowly-rising absorption onsets, and their charge-carrier transport is not well understood yet. Herein, we investigate cation-disordered NaBiS2 nanocrystals, which have a steep absorption onset, with absorption coefficients reaching >105 cm−1 just above its pseudo-direct bandgap of 1.4 eV. Surprisingly, we also observe an ultrafast (picosecond-time scale) photoconductivity decay and long-lived charge-carrier population persisting for over one microsecond in NaBiS2 nanocrystals. These unusual features arise because of the localised, non-bonding S p character of the upper valence band, which leads to a high density of electronic states at the band edges, ultrafast localisation of spatially-separated electrons and holes, as well as the slow decay of trapped holes. This work reveals the critical role of cation disorder in these systems on both absorption characteristics and charge-carrier kinetics.

Frenkel Excitons in Vacancy-Ordered Titanium Halide Perovskites (Cs2TiX6)

“…For example, in the diamond-cubic crystal family, research moved from group IV elements Si and Ge to II–VI compounds like CdTe, to yield direct rather than indirect electronic band gaps, and then further splitting into the I–III–VI 2 (e.g., CuInSe 2 ) and I 2 –II–IV–VI 4 families (e.g., Cu 2 ZnSnS 4 ), to give earth-abundant compositions. While strategies such as dimensional modification and disorder engineering , have recently gained in popularity, elemental substitution remains the prevailing design approach.…”

“…Another consequence is that, in contrast to the electron masses, the hole effective masses are actually larger for Cs 2 SnX 6 than for Cs 2 TiX 6 (Table ). Unlike conventional perovskites and many other “perovskite-inspired” materials that retain the partially oxidized, filled valence subshell of the B cation (yielding antibonding character at the VBM ,, ), the fully oxidized B 4+ in A 2 BX 6 means we have a less dispersive, nonbonding VBM, yielding heavier hole masses (particularly for X = Br or Cl) and aiding carrier localization. In contrast, the conduction band of the Sn analogues is relatively disperse with low electron effective masses (Table ) due to strong mixing and delocalization of the Sn s and X p states, while extremely flat bands are found for B = Ti due to weak Ti d –X p mixing and localized, isolated Ti d states.…”

“…1,39,40 ), the fully oxidized B 4+ in A 2 BX 6 means we have a less dispersive, nonbonding VBM,4 yielding heavier hole masses (particularly for X = Br or Cl) and aiding carrier localization. In contrast, the conduction band of the Sn analogues is relatively disperse with low electron effective masses (Table2) due to strong mixing and delocalization of the Sn s and X p states, while extremely flat bands are found for B = Ti due to weak Ti d−X p mixing and localized, isolated Ti d states.…”

Interplay of static and dynamic disorder in the mixed-metal chalcohalide Sn₂SbS₂I₃

“…The fluctuations, in combination with changes to the electronic structure driven by sample inhomogeneity from the solution synthesis, may reduce the bandgap further. 50,51 The results demonstrate the sensitivity of the electronic bandgap to static cation disorder in these materials, potentially allowing for the optimisation of the material properties for photovoltaic operations. Cation disorder can be induced at relatively low temperatures (for example compared to the widely-investigated II-IV-N 2 nitrides with typical order-disorder transitions in the range 2000-3000 K), allowing for disorder engineering under less energyintensive conditions.…”

Frenkel Excitons in Vacancy-ordered Titanium Halide Perovskites (Cs₂TiX₆)

“…Cu 2 ZnSnS 4 ) -to give earth-abundant compositions. While strategies such as dimensional modification 2 and disorder engineering 3,4 have recently risen in popularity, elemental substitution remains the prevailing design approach.…”