Two-dimensional covalent organic frameworks (2D COFs) represent a family of crystalline porous polymers with a long-range order and well-defined open nanochannels that hold great promise for electronics, catalysis, sensing, and energy storage. To date, the development of highly conductive 2D COFs has remained challenging due to the finite π-conjugation along the 2D lattice and charge localization at grain boundaries. Furthermore, the charge transport mechanism within the crystalline framework remains elusive. Here, time- and frequency-resolved terahertz spectroscopy reveals intrinsically Drude-type band transport of charge carriers in semiconducting 2D COF thin films condensed by 1,3,5-tris(4-aminophenyl)benzene (TPB) and 1,3,5-triformylbenzene (TFB). The TPB–TFB COF thin films demonstrate high photoconductivity with a long charge scattering time exceeding 70 fs at room temperature which resembles crystalline inorganic materials. This corresponds to a record charge carrier mobility of 165 ± 10 cm 2 V –1 s –1 , vastly outperforming that of the state-of-the-art conductive COFs. These results reveal TPB–TFB COF thin films as promising candidates for organic electronics and catalysis and provide insights into the rational design of highly crystalline porous materials for efficient and long-range charge transport.

MXenes are emerging layered materials that are promising for electrochemical energy storage and (opto-)electronic applications. A fundamental understanding of charge transport in MXenes is essential for such applications, but has remained under debate. While theoretical studies pointed to efficient band transport, device measurements have revealed thermally activated, hopping-type transport. Here we present a unifying picture of charge transport in two model MXenes by combining ultrafast terahertz and static electrical transport measurements to distinguish the short- and long-range transport characteristics. We find that band-like transport dominates short-range, intra-flake charge conduction in MXenes, whereas long-range, inter-flake transport occurs through thermally activated hopping, and limits charge percolation across the MXene flakes. Our analysis of the intra-flake charge carrier scattering rate shows that it is dominated by scattering from longitudinal optical phonons with a small coupling constant (α ≈ 1), for both semiconducting and metallic MXenes. This indicates the formation of large polarons in MXenes. Our work therefore provides insight into the polaronic nature of free charges in MXenes, and unveils intra- and inter-flake transport mechanisms in the MXene materials, which are relevant for both fundamental studies and applications.

Bond-free integration of two-dimensional (2D) materials yields van der Waals (vdW) heterostructures with exotic optical and electronic properties. Manipulating the splitting and recombination of photogenerated electron−hole pairs across the vdW interface is essential for optoelectronic applications. Previous studies have unveiled the critical role of defects in trapping photogenerated charge carriers to modulate the photoconductive gain for photodetection. However, the nature and role of defects in tuning interfacial charge carrier dynamics have remained elusive. Here, we investigate the nonequilibrium charge dynamics at the graphene−WS 2 vdW interface under electrochemical gating by operando optical-pump terahertz-probe spectroscopy. We report full control over charge separation states and thus photogating field direction by electrically tuning the defect occupancy. Our results show that electron occupancy of the two in-gap states, presumably originating from sulfur vacancies, can account for the observed rich interfacial charge transfer dynamics and electrically tunable photogating fields, providing microscopic insights for optimizing optoelectronic devices.

Linear conjugated polymers have attracted significant attention in organic electronics in recent decades. However, despite intrachain π‐delocalization, interchain hopping is their transport bottleneck. In contrast, two‐dimensional (2D) conjugated polymers, as represented by 2D π‐conjugated covalent organic frameworks (2D c‐COFs), can provide multiple conjugated strands to enhance the delocalization of charge carriers in space. Herein, we demonstrate the first example of thiophene‐based 2D poly(arylene vinylene)s (PAVs, 2DPAV‐BDT‐BT and 2DPAV‐BDT‐BP, BDT=benzodithiophene, BT=bithiophene, BP=biphenyl) via Knoevenagel polycondensation. Compared with 2DPAV‐BDT‐BP, the fully thiophene‐based 2DPAV‐BDT‐BT exhibits enhanced planarity and π‐delocalization with a small band gap (1.62 eV) and large electronic band dispersion, as revealed by the optical absorption and density functional calculations. Remarkably, temperature‐dependent terahertz spectroscopy discloses a unique band‐like transport and outstanding room‐temperature charge mobility for 2DPAV‐BDT‐BT (65 cm2 V−1 s−1), which far exceeds that of the linear PAVs, 2DPAV‐BDT‐BP, and the reported 2D c‐COFs in the powder form. This work highlights the great potential of thiophene‐based 2D PAVs as candidates for high‐performance opto‐electronics.

Large polarons are known to form in lead halide perovskites (LHPs). Photoinduced isolated polarons at low densities have been well-researched, but many-body interactions at elevated polaron densities, exceeding the Mott criterion (i.e., Mott polaron density), have remained elusive. Here, employing ultrafast terahertz spectroscopy, we identify a stable Mott polaron state in LHPs at which the polaron wavefunctions start to overlap. The Mott polaron density is determined to be ∼1018 cm–3, in good agreement with theoretical calculations based on the Feynman polaron model. The electronic phase transition across the Mott density is found to be universal in LHPs and independent of the constituent ions. Exceeding the Mott polaron density, excess photoinjected charge carriers annihilate quickly within tens to hundreds of picoseconds, before reaching the stable and long-lived Mott state. These results have considerable implications for LHP-based devices and for understanding exotic phenomena reported in LHPs.