Suppressed excitonic effects enable high mobility, high-yield photoconductivity in a two-dimensional polymer crystal with axial pyridine coordination

Two-dimensional polymers (2DPs) and their layer-stacked covalent organic frameworks (2D COFs) offer modular, atomically precise platforms for organic optoelectronics, yet their photoconductive responses remain fundamentally constrained by strong excitonic effects and localized charge transport. Here, we demonstrate that a diyne-linked 2DP crystal with axial pyridine coordination overcomes this limitation, enabling simultaneous efficient free-carrier generation and band-like transport. Introducing pyridine ligands that axially coordinate to Cu-porphyrin nodes transforms weak van der Waals stacking into a pyridine-bridged architecture with pronounced interlayer band dispersion and substantially reduced carrier effective masses. The resulting strong interlayer electronic coupling suppresses the exciton binding energy to well below thermal energy, such that optical excitation directly populates delocalized electronic states. Time-resolved terahertz spectroscopy reveals Drude-type photoconductivity with room-temperature mobilities approaching 500 cm^2 V^-1 s^-1 and a photon-to-free-carrier conversion ratio of ~0.4, yielding a photoconductive response that exceeds that of state-of-the-art organic and many inorganic photoactive materials. These results establish interlayer coordination as a powerful strategy for mitigating excitonic effects and accessing inorganic-like charge transport in organic 2D crystals, opening a pathway toward highly efficient photo-to-electricity conversion in organic-based systems.

💡 Research Summary

This paper presents a groundbreaking strategy to overcome the fundamental “exciton bottleneck” that has long limited the photoconversion efficiency of organic two-dimensional (2D) crystals, such as 2D polymers (2DPs) and covalent organic frameworks (COFs). While these materials can achieve high charge carrier mobility through their ordered structures, their strong excitonic effects typically prevent efficient generation of free carriers upon light absorption.

The researchers designed a model system: a diyne-linked, AB-stacked copper-porphyrin 2DP (DY2DP). The key innovation was the introduction of axial pyridine coordination into the intrinsic interlayer voids of this structure. They synthesized two variants: pyridine-coordinated DY2DP (PI-DY2DP) and pyridine-free DY2DP (PF-DY2DP) as a control.

Structural characterizations confirmed that both materials share the same in-plane lattice, but only PI-DY2DP features axial Cu-N(pyridine) coordination bonds between adjacent layers. This interlayer coordination dramatically altered the electronic structure. Density functional theory (DFT) calculations revealed that while both materials have similar intralayer band dispersion, PI-DY2DP exhibits pronounced interlayer band dispersion along the stacking direction, leading to significantly reduced electron and hole effective masses (0.74 m₀ and 0.24 m₀, respectively). In contrast, PF-DY2DP showed nearly flat interlayer bands. Charge density difference analysis visualized enhanced electron delocalization across the pyridine-bridged layers in PI-DY2DP. Crucially, the calculated exciton binding energy (Eb) for PI-DY2DP was suppressed to well below thermal energy at room temperature (~25 meV), whereas PF-DY2DP retained a large Eb of ~130 meV.

The profound impact of this electronic structure modification was directly probed using time-resolved terahertz spectroscopy (TRTS). PF-DY2DP showed no detectable photoconductivity, even at high photoexcitation densities, indicating that photogenerated species remain as tightly bound excitons. In stark contrast, PI-DY2DP exhibited an immediate, strong Drude-type photoconductivity response at low excitation densities. The analysis yielded a remarkably high room-temperature charge carrier mobility of nearly 500 cm² V⁻¹ s⁻¹ and an exceptionally high photon-to-free-carrier conversion yield (φ) of approximately 0.4 (40%). This performance surpasses that of many state-of-the-art organic and even some inorganic photoactive materials.

In conclusion, the work demonstrates that engineered interlayer coordination—transforming weak van der Waals stacking into a coherent, electronically coupled architecture—serves as a powerful and previously underexplored design parameter. It simultaneously suppresses excitonic effects and preserves band-like transport, thereby directly addressing the core challenge in organic optoelectronics. This strategy opens a new pathway for developing highly efficient organic-based photovoltaics, photodetectors, and other light-to-electricity conversion devices.