The seeds and homogeneous nucleation of photoinduced nonthermal melting in semiconductors due to self-amplified local dynamic instability

Photoexcitation-induced passivation of SnO2 thin film for efficient perovskite solar cells

A high-quality tin oxide electron transport layer (ETL) is a key common factor to achieve high-performance perovskite solar cells (PSCs). However, the conventional annealing technique to prepare high-quality ETLs by continuous heating under near-equilibrium conditions requires high temperatures and a long fabrication time. Alternatively, we present a non-equilibrium, photoexcitation-induced passivation technique that uses multiple ultrashort laser pulses. The ultrafast photoexcitation and following electron-electron and electron-phonon scattering processes induce ultrafast annealing to efficiently passivate surface and bulk defects, and improve the crystallinity of SnO2, resulting in suppressing the carrier recombination and facilitating the charge transport between the ETL and perovskite interface. By rapidly scanning the laser beam, the annealing time is reduced to several minutes, which is much more efficient compared with conventional thermal annealing. To demonstrate the university and scalability of this technique, typical antisolvent and antisolvent-free processed hybrid organic-inorganic metal halide PSCs have been fabricated and achieved the power conversion efficiency (PCE) of 24.14% and 22.75% respectively, and a 12-square-centimeter module antisolvent-free processed perovskite solar module achieves a PCE of 20.26%, with significantly enhanced performance both in PCE and stability. This study establishes a new approach towards the commercialization of efficient low-temperature manufacturing of PSCs.

Inverse Design of Light Manipulating Structural Phase Transition in Solids

This Perspective focuses on recent advances in understanding ultrafast processes involved in photoinduced structural phase transitions and proposes a strategy for precise manipulation of such transitions. It has been demonstrated that photoexcited carriers occupying empty antibonding or bonding states generate atomic driving forces that lead to either stretching or shortening of associated bonds, which in turn induce collective and coherent motions of atoms and yield structural transitions. For instance, phase transitions in IrTe₂ and VO₂, and nonthermal melting in Si, can be explained by the occupation of specific local bonding or antibonding states during laser excitation. These cases reveal the electronic-orbital-selective nature of laser-induced structural transitions. Based on this understanding, we propose an inverse design protocol for achieving or preventing a target structural transition by controlling the related electron occupations with orbital-selective photoexcitation. Overall, this Perspective provides a comprehensive overview of recent advancements in dynamical structural control in solid materials.

Optically Induced Multistage Phase Transition in Coherent Phonon-Dominated a-GeTe

Ultrafast photoexcitation can decouple the multilevel nonequilibrium dynamics of electron-lattice interactions, providing an ideal probe for dissecting photoinduced phase transition in solids. Here, real-time time-dependent density functional theory simulations combined with occupation-constrained DFT methods are employed to explore the nonadiabatic paths of optically excited a-GeTe. Results show that the short-wavelength ultrafast laser is capable of generating full-domain carrier excitation and repopulation, whereas the long-wavelength ultrafast laser favors the excitation of lone pair electrons in the antibonded state. Photodoping makes the double-valley potential energy surface shallower and allows the insertion of A_1g coherent forces in the atomic pairs, by which the phase reversal of Ge and Te atoms in the ⟨001⟩ direction is activated with ultrafast suppression of the Peierls distortion. These findings have far-reaching implications regarding nonequilibrium phase engineering strategies based on phase-change materials.

Origin of Immediate Damping of Coherent Oscillations in Photoinduced Charge-Density-Wave Transition

In stark contrast to the conventional charge density wave (CDW) materials, the one-dimensional CDW on the In/Si(111) surface exhibits immediate damping of the CDW oscillation during the photoinduced phase transition. Here, we successfully reproduce the experimental observation of the photoinduced CDW transition on the In/Si(111) surface by performing real-time time-dependent density functional theory (rt-TDDFT) simulations. We show that photoexcitation promotes valence electrons from the Si substrate to the empty surface bands composed primarily of the covalent p-p bonding states of the long In-In bonds. Such photoexcitation generates interatomic forces to shorten the long In-In bonds and thus drives the structural transition. After the structural transition, these surface bands undergo a switch among different In-In bonds, causing a rotation of the interatomic forces by about π/6 and thus quickly damping the oscillations in feature CDW modes. These findings provide a deeper understanding of photoinduced phase transitions.