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

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 x 10(15) and 3.6 x 10(18) cm(-3) for ambipolar 1, 9.8 x 10(16) and 6.0 x 10(20) cm(-3) for ambipolar 2, and the original hole density (n(h)) of 2.8 x 10(17) and 2.4 x 10(17) 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.