Yifan Dong

Two-dimensional hybrid organic-inorganic perovskites (2D HOIPs) have emerged as a promising class of semiconductors for next-generation spin-optoelectronic applications, owing to their structural tunability and rich spin-dependent optoelectronic properties. Their layered organic-inorganic architectures also offer opportunities to control spin relaxation via chirality, layer-number tuning, and symmetry engineering. This work discusses how 2D HOIPs can be engineered to manipulate spin dynamics. The first part focuses on quasi-2D chiral perovskites, in which we vary the layer number and uncover how structural confinement governs chiroptical response, spin relaxation, and charge-carrier mobility. The second part highlights inversion-symmetric 2D perovskites, in which local symmetry breaking within the inorganic layers leads to hidden spin-valley locking via functional non-primary ammonium cations. This local structural asymmetry produces large spin splitting and spin-valley textures, which protect optically generated spin-polarized carriers and reveal room temperature spin lifetimes of hundreds of picoseconds in the Pb-based system.

The combination of semiconducting properties and synthetically tunable chirality in chiral metal halide semiconductors (CMHSs) offers a compelling platform for room temperature control of electronic spin properties, leveraging effects such as chirality-induced spin selectivity (CISS). We report CISS-induced magnetoresistance (CISS-MR) exceeding 100% for spin valves operating at room temperature. The high CISS-MR is attributed to an interfacial tunneling barrier, modulated by chirality and spin, producing current dissymmetry factors ( g _c ) that surpass the limit imposed by the Julliere model, which is governed by the intrinsic spin polarization of the adjacent ferromagnetic (FM) contact. The CISS-MR exhibits a dependence on the CMHS composition, revealing structure-property relationships between CISS and structural chirality. The observed exceptionally large tunneling MR response differs from a subtle anisotropic MR arising from the proximity effect at the FM/CMHS interface in the absence of a tunneling barrier. Our study provides insights into charge-to-spin interconversion in chiral semiconductors, offering design principles to control and enhance the CISS response for functional platforms.

The interplay between carrier density regimes and the various scattering processes is fundamental for understanding charge transport efficiency in high-performance semiconductors. Combining ultrafast time-resolved terahertz spectroscopy (TRTS) and transient microwave photoconductivity (TRMC), we measure carrier mobilities over 7 orders of magnitude (1013–1020 carriers cm–3), reaching a regime where carrier–carrier scattering dominates and mobility begins to roll off. We find the Caughey–Thomas model can be effectively employed for transport modeling in 3D metal halide perovskites (MHPs), and the mobility roll-off near ∼1019 cm–3 can be tuned by MHP composition. Carrier lifetime in MHPs starts decreasing at much lower carrier densities due to interband multi-carrier recombination, while intraband carrier mobility remains unchanged as carrier–carrier scattering is screened by the presence of polarons. The validated Caughey–Thomas model provides a practical input for device simulations, prevents overestimation of diffusion length under high injection, and suggests that MHPs are excellent candidates for unipolar device applications under high carrier densities.

Room-temperature control of spin, coupled with charge and light, remains a long-standing challenge for next-generation spin-optoelectronic applications. Although inversion symmetry breaking offers one path for spin control in semiconductors, strong spin dephasing at higher temperatures remains a persistent challenge. While hidden spin polarization (HSP) without global symmetry breaking provides another promising material design strategy, robust spin stabilization from HSP is yet to be experimentally realized. Here, we demonstrate that 2D hybrid organic-inorganic perovskites can provide spin stabilization at room temperature via hidden spin valley locking. We utilized functional nonprimary ammonium cations (NPACs) that induce giant spin splitting in 2D layers with varying spin-orbit coupling (employing Sn vs Pb) and enhanced dielectric screening, thereby restricting spin relaxation processes. Time-resolved experiments demonstrate optically generated spin-polarized carriers that persist for over four nanoseconds at room temperature without requiring an external magnetic field. Our design paradigm unlocks inversion-symmetric semiconductors with ultralong spin lifetimes, broadening the materials options for light-driven spin control.

Chiral metal halide perovskite semiconductors (CMHS) are fascinating semiconductors with unique chiroptical properties and spin-polarized charge transport. Achieving long spin lifetimes and high carrier mobility concurrently is essential to realize the true potential of CMHS in manipulating charge, spin, and light. While conventional monolayer n = 1 CMHS possess appreciable anisotropy factors of circular dichroism (g_CD) and photoluminescence (g_lum), imparting chirality to quasi-2D CMHS (n > 1) with enhanced carrier mobilities is underexplored. Herein, we systematically investigate the layer number (n-value) dependence and emergent trade-offs in chiroptical properties, spin-relaxation times, and carrier mobilities in chiral quasi-2D (R/S-MPEA)₂MA_n-1Pb_nI_3n+1 single crystals and thin films (R/S-MPEA: R/S-β-methylphenylethylammonium; MA: methylammonium; n = 1-3). Films with n = 2 exhibited the highest g_CD of 8 × 10^-3, an order of magnitude larger than their n = 1 and n = 3 counterparts. On the other hand, n = 3 films demonstrated enhanced spin lifetimes up to 15 ps along with increased carrier mobility up to 11.6 cm² V^-1 s^-1. As a result, photodiode-type photodetectors based on n = 3 CMHS reveal high specific detectivity and superior discrimination of circularly polarized light, outperforming n = 1 and 2. These findings highlight the potential of quasi-2D CMHS as tunable, high-performance platforms with longer spin lifetime and diffusion length, enabling new functionalities.

Chirality-induced spin selectivity (CISS) phenomena arise from an interplay among structural chirality, electron spin orientation, and charge current. Steady-state observations such as magnetoresistance offer little insight into the timescales that govern the spin-charge interconversion and often conflate interfacial and bulk phenomena. By contrast, inverse CISS involves the conversion of spin to a charge current. Using terahertz (THz) emission spectroscopy, we directly measured an ultrafast charge current due to inverse CISS with picosecond time resolution. Polarity and polarization analysis of the THz emission map the induced charge current direction upon spin injection. We found that a charge current is generated along the spin orientation that changes direction with stereochemical configuration. These observations directly demonstrate the inherent coupling between spin and charge currents in chiral systems, offering key insights into their fundamental dynamics.

The ability to induce and modulate chirality in metal halide perovskite semiconductors (MHPs) using chiral additives expands the compositional design space and offers a means of tuning chiroptical properties. Motivated by the ability of phosphonic acids to interact with metal ions, we designed three chiral phosphonic acids (CPAs) to impose chirality in otherwise achiral 2D MHP, phenylammonium tin iodide (PA₂SnI₄). We found that both the position of the phosphonic acid relative to the bond between the two naphthalene rings (i.e., the chiral axis) and the distance between the phosphonic acid and the binaphthyl chiral units significantly impact the transfer of structural chirality into the MHP lattice. The compound with a phosphonic acid directly bound to one of the naphthalene rings at the carbon adjacent to the chiral axis resulted in the largest circular dichroism dissymmetry factor of the three phosphonic acids. Furthermore, optical pump-terahertz probe measurements reveal an increase in the charge carrier mobility in the MHPs following the addition of CPAs. This dual functionality of CPAs in inducing chirality and improving charge transport properties in MHPs is promising for chiral-optoelectronic applications.

Singlet fission (SF) is a multielectron process in which one singlet exciton S converts into a pair of separated triplet excitons T. SF is widely studied as it may help overcome the Shockley-Queisser efficiency limit for semiconductor photovoltaic cells. To elucidate and control the SF mechanism, great attention has been given to the identification of intermediate states in SF materials, which often appear elusive due to the complexity and fast time scales of the SF process. Here, we apply 14 fs-1 ms transient absorption techniques to high-purity rubrene single crystals to disentangle the intrinsic fission dynamics from the effects of defects and grain boundaries and to identify reliably the fission intermediates. Our data demonstrates that above-gap excitation directly generates a hybrid vibronically assisted mixture of singlet state and triplet-pair multiexciton [S/TT], which rapidly (<100 fs) and coherently branches into pure singlet or triplet excitations. The relaxation of [S/TT] to S is followed by a relatively slow and temperature-activated (48 meV activation energy) incoherent fission process. The SF competing pathways and intermediates revealed here unify the observations and models presented in previous studies of SF in rubrene and offer alternative strategies for the development of SF-enhanced photovoltaic materials.