Molecular Engineering of Silicon Phthalocyanine to Improve the Charge Transport and Ammonia Sensing Properties of Organic Heterojunction Gas Sensors

Abstract

Novel organic heterostructures fabricated with a bilayer consisting of an axially substituted silicon phthalocyanine (R 2 -SiPc) derivative and lutetium bis-phthalocyanine (LuPc 2 ) are investigated for their ammonia sensing properties. Surface and microstructure characterization of the heterostructure films reveal either compact or highly porous surface topography in (345F) 2 -SiPc and Cl 2 -SiPc-based heterostructures, while electrical characterization reveals a strong influence of the axial substituent in R 2 -SiPc on NH 3 sensing capabilities. Electrical characterization further demonstrates an apparent energy barrier for interfacial charge transport, which is higher in the (345F) 2 -SiPc/LuPc 2 heterojunction device. In-depth charge transport studies by impedance spectroscopy further reveal a resistive interface in (345F) 2 -SiPc/LuPc 2 and faster bulk and interfacial charge transport in Cl 2 -SiPc/LuPc 2 heterojunction devices. Different interfacial charge transport capabilities and surface topographies affect NH 3 sensing properties of the two heterojunction devices, in which (345F) 2 -SiPc/LuPc 2 reveals a fast and non-linear response with a limit of detection (LOD) of 310 ppb, while Cl 2 -SiPc/LuPc 2 exhibits a slow, and linear response to NH 3 with LOD of 100 ppb. Finally, different metrological parameters of the two sensors are correlated to the respective gas-material interactions, in which adsorption and diffusion regimes are modulated by the surface topography and hydrophobicity of the sensing layer.

π-Extended Porphyrin–Phthalocyanine Heterojunction Devices Exhibiting High Ammonia Sensitivity with a Remarkable Light Effect

“…47 Even though double-layer heterojunctions are less sensitive than double lateral heterojunctions, 48,49 they exhibit better LOD due to lower noise. Thus, AM6/LuPc 2 exhibits a LOD comparable to the best ever reported for heterojunction devices, i.e., 100 ppb, based on fluorinated silicon phthalocyanine, 36 and even better under illumination. It is worth mentioning that the present sensors favorably compete with previously reported devices and operate in a humid atmosphere and at room temperature.…”

“…This difference could be due to morphological differences between the heterojunctions, as already demonstrated with silicon phthalocyanine-based gas sensors. 36 Here, we observe an opposite response to humidity (current increase) and NH 3 (current decrease). So, it is difficult to predict the variation of the response to NH 3 upon changing the RH value.…”

“…Up to now, 100 ppb has been reported with a silicon phthalocyanine-based heterojunction device, and between 100 and 500 ppb in a few examples. 36 Such response enhancement under light has already been reported, but only with inorganic conductometric gas sensors. 45 Two phenomena can occur in the present molecular material-based sensors.…”

“…Such features were characteristic of bilayer organic heterojunction devices, originating from the accumulation of mobile charges at the interface. 11,36 Both sublayers are highly resistive compared to LuPc 2 . 37 Due to the poorly conductive sublayer, the injected charges from the electrode experienced high resistance, resulting in a smaller current at a lower voltage.…”

Tuning of Interfacial Charge Transport in Organic Heterostructures via Aryl Electrografting for Efficient Gas Sensors

“…25 In majority of the cases, the top layer was lutetium bis-phthalocyanine (LuPc 2 ) because of its radical nature, imparting high charge carrier density, except in few cases when porphyrin tapes were coated as an alternative to LuPc 2 . 19 Hosts of low-conducting MPc, MP, and similar macrocycles were used as a sublayer, investigating the role of the central metal atom, 26 the peripheral substituents, 27 and the axial ligands 28 on the device charge transport properties and finally the sensing behavior toward NH 3 . MSDI heterojunction devices displayed high sensitivity toward NH 3 with limits of detection going down to 250 ppb in the case of the Co(Cl 8 Pc)/LuPc 2 heterojunction.…”