The lower rather than higher density charge carrier determines the NH3-sensing nature and sensitivity of ambipolar organic semiconductors

Abstract

Despite the extensive studies and great application potentials, the sensing nature of ambipolar organic semiconductor gas sensors still remains unclarified, unlike their inorganic counterparts. Herein, different numbers of thiophenoxy groups are introduced into the phthalocyanine periphery of bis(phthalocyaninato) rare earth semiconductors to continuously tune their HOMO and LUMO energies, resulting in the ambipolar M[Pc(SPh)₈]₂ [M = Eu (1), Ho (2)] and p-type M(Pc)[Pc(SPh)₈] [M = Eu (3), Ho (4)]. An OFET in combination with direct I–V measurements over the devices from the self-assembled nanostructures of 1–4 revealed the original electron and hole densities (n_e and n_h) of 3.6 × 10¹⁵ and 3.6 × 10¹⁸ cm⁻³ for ambipolar 1, 9.8 × 10¹⁶ and 6.0 × 10²⁰ cm⁻³ for ambipolar 2, and the original hole density (n_h) of 2.8 × 10¹⁷ and 2.4 × 10¹⁷ cm⁻³ for 3 and 4, respectively. The comparative studies on the sensing behavior of the self-assembled nanostructures of 1–4 revealed that, towards reducing gas NH₃, the ambipolar 1 and 2 show an n-type sensing behavior, with the response nature determined by the lower n_e rather than higher n_h. Meanwhile, the NH₃ sensor from 1 with much lower n_e than 2 displays higher sensitivity. Nevertheless, also towards NH₃, 3 and 4 exhibit a p-type response, with the lower carrier density device 4 showing higher sensitivity. Consequently, the originally lower density carrier (hole vs. electron) with a faster charge transporting speed in the ambipolar semiconducting layer determines not only the gas sensing response nature but also the sensitivity. This is also true for the p-type organic semiconductor in terms of the gas sensing sensitivity.