Molecular Engineering of Porphyrin-Tapes/Phthalocyanine Heterojunctions for a Highly Sensitive Ammonia Sensor

Modulating the interfacial charge alignments by molecular engineering in an organic heterojunction device is a smart strategy to improve its conductivity, which can be exploited in high-performance gas-sensor development. Herein, the fabrication of new organic heterojunction devices based on porphyrin tapes and phthalocyanines and their potentiality in ammonia sensing at different relative humidity (rh) are investigated. The devices are built using dry approach relying on oxidative chemical vapor deposition for simultaneous synthesis, doping, and deposition of the porphyrin tape layer and physical vapor deposition of phthalocyanine layer. The association of the porphyrin tapes with copper phthalocyanine (CuPc) or its perfluorinated analogue (Cu(F16Pc)) in a bilayer device configuration reveals a nonlinear current-voltage characteristic, assigned to the formation of organic heterojunction at the bilayer interface. Cu(F16Pc)-based devices reveal higher response, faster sorption kinetics, stable baseline, and less interference from rh fluctuations toward ammonia than CuPc-based devices, which are attributed to more conducting interface in the former. Moreover, depending on the porphyrin tape associated, Cu(F16Pc)-based devices exhibit sensitivity values 6.7 or 0.4% ppm(-1) and detection limit of 1 ppm or 228 ppb allowing to obtain among the best NH3 sensors reported.