Optical Probes of Emerging Semiconductors for Light-Harvesting Applications

A plethora of new semiconductors have recently emerged as versatile materials for solar cells and photocatalytic applications. Combinatorial analytical probes have played a pivotal role in uncovering the mechanisms underpinning light-harvesting performance even before device optimisation has been attempted.

Ultrafast optical probes of photoconductivity dynamics are particularly useful here, uncovering the generation, localisation and ultimate recombination of charge carriers following photon absorption. We report on a peculiar ultrafast self-localisation process observed across wide classes of new bismuth-based semiconductors, including bismuth halides and chalcogenides. We have most recently shown such dynamic transitions from large to small polaronic states to dominate the dynamics of charge carriers in Cs₂AgSb_xBi_1–xBr₆ double perovskites,^[1] (AgI)_x(BiI₃)_y Rudorffites,^[2] and AgBiS₂ nanocrystals^[3] and discuss the influence of alloying, cation disorder and stoichiometry on such charge-carrier localization events.

Probing charge-carrier motion in highly anisotropic semiconductors poses particular challenges. We show how such charge transport can be probed successfully layered, two-dimensional (2D) metal halide perovskites that have been found to improve the stability of metal halide perovskite thin films and devices. We show that the 2D perovskites PEA₂PbI₄ and BA₂PbI₄ exhibits an excellent in-plane mobilities and exhibit unexpectedly high densities of sustained populations of free charge carriers, surpassing the Saha equation predictions even at low temperature.^[4] In addition, we examine the transfer of excitations in the direction vertical to the 2D planes, determining anisotropy of transport in these materials.

We further utilize a combination of ultra-low frequency Raman and infrared terahertz time-domain spectroscopies to provide a systematic examination^[5] of the ultra-low frequency vibrational response for a wide range of metal-halide semiconductors: FAPbI₃, MAPbI_xBr_3–x, CsPbBr₃, PbI₂, Cs₂AgBiBr₆, Cu₂AgBiI₆, and AgI. We examine the cause of a frequently reported “central Raman peak” and rule out extrinsic defects, octahedral tilting, cation lone pairs, and “liquid-like” Boson peaks as causes. Instead, we propose that the central Raman response results from an interplay of the significant broadening of Raman-active, low-energy phonon modes that are strongly amplified by a population component from Bose–Einstein statistics toward low frequency. These findings elucidate the complexities of light interactions with low-energy lattice vibrations in soft metal-halide semiconductors emerging for photovoltaic applications.

[1] 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 14, 10340 (2023)
[2] 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, Advanced Functional Materials 34, 2315942 (2024).
[3] M. Righetto, Y. Wang, K. A. Elmestekawy, C. Q. Xia, M. B. Johnston, G. Konstantatos, and L. M. Herz, Advanced Materials 35, 2305009 (2023).
[4] 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. 33, 2300363 (2023).
[5] V. J-Y Lim, M. Righetto, …, L. M. Herz, ACS Energy Letters 9, 4127 (2024).