Beyond the band edge: Unveiling high-mobility hot carriers in a two-dimensional conjugated coordination polymer

TL;DR

This study demonstrates that solution-processable two-dimensional conjugated coordination polymers can support highly mobile hot carriers, with ultrafast transport and high mobility, challenging the notion that organic materials are unsuitable for hot carrier applications.

Contribution

The paper reveals high-mobility hot carriers in 2D conjugated coordination polymers, showing their potential for optoelectronic applications and providing insights into non-equilibrium charge transport in organic systems.

Findings

Hot carriers achieve mobility of ~2000 cm2 V-1 s-1 in non-equilibrium regime.

Hot carriers traverse 300 nm grain boundaries within a picosecond.

Quasi-equilibrium carriers exhibit mobility of ~400 cm2 V-1 s-1 and diffusion length >1 μm.

Abstract

Hot carriers, inheriting excess kinetic energy from high-energy photons, underpin numerous optoelectronic applications involving non-equilibrium transport processes. Current research on hot carriers has predominantly focused on inorganic materials, with little attention paid to organic-based systems due to their ultrafast energy relaxation and inefficient charge transport. Here, we overturn this paradigm by demonstrating highly mobile hot carriers in solution-processable, highly crystalline two-dimensional conjugated coordination polymer (2D c-CP) Cu3BHT (BHT = benzenehexathiol) films. Leveraging a suite of ultrafast spectroscopic and imaging techniques, we unravel the microscopic charge transport landscape in Cu3BHT films following non-equilibrium photoexcitation across temporal, spatial, and frequency domains, revealing two distinct high-mobility transport regimes. In the non-equilibrium transport regime, hot carriers achieve ultrahigh mobility of ~2,000 cm2 V-1 s-1, traversing grain boundaries up to 300 nm within a picosecond. In the quasi-equilibrium transport regime, free carriers exhibit Drude-type band-like transport with a remarkable mobility of ~400 cm2 V-1 s-1 and an intrinsic diffusion length exceeding 1 micrometer. These findings establish 2D c-CPs as versatile platforms for exploring high-mobility non-equilibrium transport, unlocking new opportunities for organic-based hot carrier applications.