Optical Probes of Emergent Low-Dimensional Semiconductors

Low-dimensional semiconductors have emerged as attractive materials for light-emitting diodes and solar cells, with lead halide perovskites emerging as particularly high-performing materials. The search for less toxic ingredients has led to the emergence of a plethora of new bismuth-based semiconductors, including bismuth halides and chalcogenides. Rapidly improving power conversion efficiencies have been realised for such materials, triggering new research efforts to explore and eliminate current limitations to performance. We examine thin films of AgBiS₂ nanocrystals as a function of Ag and Bi cation-ordering,^[1]which is modified via thermal-annealing. Local Ag-rich and Bi-rich domains formed during hot-injection synthesis are transformed to induce homogeneous disorder (random Ag-Bi distribution). Such cation-disorder engineering results in a sixfold increase in the charge-carrier mobility, reaching 2.7 cm² V^-1 s^-1 in AgBiS₂ nanocrystal thin films. We reveal an ultrafast charge-carrier self-trapping process that reduces charge-carrier mobilities on an ultrafast timescale, similar to previous observations for other bismuth-based semiconductors.^[2-4] We show that homogeneous cation disorder reduces such charge-carrier localization, most likely because cation-disorder engineering flattens the disordered electronic landscape, removing tail states that would otherwise exacerbate Anderson localization of small polaronic states.^[1]<br/>We further discuss the charge-carrier photoconductivity dynamics in layered, 2D perovskites that have been found to improve the stability of metal halide perovskite thin films and devices.^[5]We show that the 2D perovskite PEA₂PbI₄exhibits an excellent long-range mobility of 8.0 cm² (V s)^–1, ten times greater than the long-range mobility determined for a comparable 3D material FA_0.9Cs_0.1PbI₃. These values shows that the polycrystalline 2D thin films already have single-crystal-like qualities. We further demonstrate that PEA₂PbI₄ and BA₂PbI₄ exhibit unexpectedly high densities of sustained populations of free charge carriers, surpassing the Saha equation predictions even at low temperature.^[6]These findings provide new insights into the temperature-dependent interplay of exciton and free-carrier populations in 2D MHPs. Furthermore, such sustained free charge-carrier population and high mobilities demonstrate the potential of these semiconductors for applications such as solar cells, transistors, and electrically driven light sources. In addition, we examine the transfer of excitations in the direction vertical to the 2D planes, examining anisotropy of transport in these materials.<br/><i>[1] M. Righetto, Y. Wang, K. A. Elmestekawy, C. Q. Xia, M. B. Johnston, G. Konstantatos, and L. M. Herz</i>, Advanced Materials <b>35</b>, 2305009 (2023).<br/>[2] M. Righetto, S. Caicedo-Davila, M. T. Sirtl, V. J.-Y. Lim, J. B. Patel, D. A. Egger, T. Bein, L. M. Herz, JPC Letters <b>14</b>, 10340 (2023)<br/><i>[3] S. Lal, M. Righetto, B. W. Putland, H. C. Sansom, S. G. Motti, H. Jin, M. B. Johnston, H. J. Snaith, and L. M. Herz</i>, Advanced Functional Materials <b>34</b>, 2315942 (2024).<br/>[4] L.R.V. Buizza, H. C. Sansom, A. D. Wright, A. M. Ulatowski, M. B. Johnston, H. J. Snaith, L. M. Herz, Advanced Functional Materials <b>32</b>, 2108392 (2022).<br/>[5] <i>M. Kober-Czerny, S. G. Motti, P. Holzhey, B. Wenger, J. Lim, L. M. Herz, and H. J. Snaith</i><i>,</i><br/>Adv. Func. Mater. <b>32</b>, 2203064 (2022).<br/>[6] S. G. Motti, M. Kober-Czerny, M. Righetto, P. Holzhey, J. Smith, H. Kraus, H. J. Snaith, M. B. Johnston, and L. M. Herz Adv. Func. Mater. <b>33</b>, 2300363 (2023).