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1. Light‐Induced Lattice Coherence and Emission Enhancement in PTM‐Passivated CsSnI ₃ Perovskites

   https://doi.org/10.1002/adom.202502889  

   Adams, Thomas Y; Barrios, Bruce; Ziegenfus, Michael; Cai, Hui; Ghosh, Sayantani (January 2026, Advanced Optical Materials)

   Abstract Metal halide perovskites continue to lead in optoelectronic applications, but the toxicity of lead has driven efforts to identify environmentally benign alternatives. Cesium tin iodide (CsSnI₃) is one such, with a direct bandgap and near‐infrared emission, though its performance is limited by instability. We show that phthalimide (PTM) passivation during single crystal growth enhances optical output and stability. Under continuous excitation, PTM‐passivated microscale crystals show up to one order of magnitude increase in photoluminescence (PL) quantum yield, accompanied by reversible sharpening of a low‐frequency Raman mode associated with Cs⁺ rattling. This reveals dynamic, light‐induced lattice reordering that passivates trap states and enhances radiative recombination. Mechanical grinding yields nanocrystals with redshifted, narrowed PL, consistent with a relaxed polymorph and reduced inhomogeneous broadening. Despite increased surface area, PTM remains effective in preserving near‐infrared emission in nanocrystals. Power‐dependent PL reveals distinct carrier dynamics: microcrystals show redshift due to bandgap renormalization, while nanocrystals show blueshift and elevated carrier temperatures (300–1900 K), consistent with hot‐carrier recombination. Extended illumination reveals reversible optical changes, including PL modulation, reflecting dynamic light–matter interactions and evolving defect landscapes. These results identify PTM‐passivated CsSnI₃as an ideal platform for probing morphology‐dependent carrier relaxation and light‐induced vibrational coherence in lead‐free perovskites. 

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2. Strong Coupling of Self‐Trapped Excitons to Acoustic Phonons in Bismuth Perovskite Cs₃Bi₂I₉

   https://doi.org/10.1002/adom.202300199  

   He, Xing; Tailor, Naveen Kumar; Satapathi, Soumitra; Brgoch, Jakoah; Yang, Ding‐Shyue (May 2023, Advanced Optical Materials)

   To assess the potential optoelectronic applications of metal-halide perovskites, it is critical to have a detailed understanding of the nature and dynamics of interactions between carriers and the polar lattices. Here, we report the electronic and structural dynamics of bismuth-based perovskite Cs3Bi2I9 revealed by transient reflectivity and ultrafast electron diffraction. A cross-examination of these results combined with theoretical analyses allows the identification of the major carrier–phonon coupling mechanism and the associated time scales. It is found that carriers photoinjected into Cs3Bi2I9 form self-trapped excitons on an ultrafast time scale. However, they retain most of their energy and their coupling to Fröhlich-type optical phonons is limited at early times. Instead, the long-lived excitons exert an electronic stress via deformation potential and develop a prominent, sustaining strain field as coherent acoustic phonons in 10 ps. From sub-ps to ns and beyond, a similar extent of the atomic displacements is found throughout the different stages of structural distortions, from limited local modulations to a coherent strain field to the Debye–Waller random atomic motions on longer times. The current results suggest the potential use of bismuth-based perovskites for applications other than photovoltaics to take advantage of carriers’ stronger self-trapping and long lifetime. 

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3. Ultrafast nanometric resolution spatiotemporal measurement of ambipolar diffusivity and carrier-phonon coupling in doped and undoped silicon

   https://doi.org/10.1063/5.0205647  

   Segovia, Mauricio; Xu, Xianfan (May 2024, Applied Physics Letters)

   Understanding and determination of the ambipolar diffusivity of a semiconductor is fundamental for predicting device behavior and optimizing its performance. Ultrafast pump-probe measurements allow for the determination of energy carrier dynamics with sub-picosecond resolution. Due to the inherent diffusive nature of carriers, measurements with a high spatial resolution are needed, in addition to the traditional pump-probe system, to determine not only the carrier dynamics but also the spatial extent caused by carrier diffusion as well. In this work, a spatiotemporal measurement system with ultrafast temporal and nanometric spatial resolution, together with a comprehensive transport model, is used to determine the ambipolar diffusivity and carrier-phonon energy coupling time in both undoped and doped silicon. The results show that as the carrier density increases, the measured ambipolar diffusivity decreases with minimal variation in the carrier-phonon energy coupling time. In general, this work demonstrates an optical-based method for determining ambipolar diffusivity in a semiconductor material. 

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4. Ultrafast Transient Absorption Studies of the Dynamics of Free and Coulombically Trapped Polarons in Doped Conjugated Polymers

   https://doi.org/10.1002/adfm.202507480  

   Wu, Eric C; Vidales, Diego Garcia; Salamat, Charlene Z; Duong, Quynh M; León_Ruiz, Alex; Ready, Austin D; Mehmedović, Zerina; Stanfield, Dane A; Spokoyny, Alexander M; Tolbert, Sarah H; et al (July 2025, Advanced Functional Materials)

   Abstract The relaxation of photoexcited polarons in doped conjugated polymers is studied with ultrafast transient absorption (TA) spectroscopy to examine the effect of polymer morphology and counterion size on polaron mobility. Processing conditions are first used to create F₄TCNQ‐doped (2,3,5,6‐tetrafluoro‐tetracyanoquinodimethane) poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) films with different morphologies and thus free and trapped polarons in different ratios. We find that less crystalline films have a higher fraction of trapped polarons, but, remarkably, that free and trapped polarons have the same relaxation times in all films. Films doped with a large dodecaborane (DDB) cluster‐based dopant are then used to show that trapping is based on Coulomb interactions between polarons and counterions; no trapped polarons are observed in TA due to the reduced Coulomb interaction between the polarons and the DDB counterion. Indeed, the relaxation of polarons in these films is an order of magnitude faster than that in F₄TCNQ‐doped films, consistent with reduced trapping. Finally, the results are used to argue that counterion size has a greater effect on polaron mobility than polymer morphology and crystallinity. All of the experiments show that pump/probe spectroscopy provides a straightforward way to determine the local mobilities and degree of carrier trapping in doped conjugated polymer films. 

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5. Protecting hot carriers by tuning hybrid perovskite structures with alkali cations

   https://doi.org/10.1126/sciadv.abb1336  

   Wang, Ti; Jin, Linrui; Hidalgo, Juanita; Chu, Weibin; Snaider, Jordan M.; Deng, Shibin; Zhu, Tong; Lai, Barry; Prezhdo, Oleg; Correa-Baena, Juan-Pablo; et al (October 2020, Science Advances)

   null (Ed.)

   Successful implementation of hot carrier solar cells requires preserving high carrier temperature as carriers migrate through the active layer. Here, we demonstrated that addition of alkali cations in hybrid organic-inorganic lead halide perovskites led to substantially elevated carrier temperature, reduced threshold for phonon bottleneck, and enhanced hot carrier transport. The synergetic effects from the Rb, Cs, and K cations result in ~900 K increase in the effective carrier temperature at a carrier density around 10 18 cm −3 with an excitation 1.45 eV above the bandgap. In the doped thin films, the protected hot carriers migrate 100 s of nanometers longer than the undoped sample as imaged by ultrafast microscopy. We attributed these improvements to the relaxation of lattice strain and passivation of halide vacancies by alkali cations based on x-ray structural characterizations and first principles calculations. 

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