Sub-picosecond permittivity of carbon nitrides probed with terahertz spectroscopy: revealing high dielectric response and conductivity

Abstract

Organic based semiconductor materials offer emerging and sustainable solutions for solar energy conversion technologies and electronics. However, knowledge of their intrinsic (photo)physical properties is often limited, especially the effect of dielectric properties (ε_r = ε′) on exciton separation and charge generation at sub-picosecond timescales, which corresponds to THz frequencies. Thus, THz Time Domain Spectroscopy (THz-TDS) is used to directly and accurately extract the complex permittivity (ε = ε′ + iε″) and THz conductivity (σ_THz) of organic Carbon Nitrides (CN_x) and other polymers and elucidate the influence of environmental humidities. Overall, the THz dielectric response ε′ of CN_x surpasses other organic and even glycolated materials, and water. For the ionic and 2D carbon nitride K-PHI, complex permittivity ε was observed to be strongly humidity dependent, with both ε′ and σ_THz doubling from dry to humid conditions (ε′ from ∼4 to 8, σ_{THz 75} to 150 S/m, respectively). When compared to other photocatalysts, the THz dielectric response of CN_x and especially humid K-PHI is within range of well-known oxides such as TiO₂ that can efficiently generate charges from excitons, due to low exciton binding energies resulting from high ε′. The importance of dielectric property characterization on functionally relevant frequencies is thus highlighted, especially in the THz gap (0.1 – 10 THz), to understand the photophysical behaviour of organic semiconductors, even in the presence of water and hydrated ions. Such THz complex permittivity determination may also be beneficial for exploring next generation photo(electro)catalysts, electronics or ionotronics, and for computational property predictions that often require knowledge of ultrafast photophysical properties.