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

Modulation of interfacial conductivity in organic heterostructures is a highly promising strategy to improve the performance of electronic devices. In this endeavor, the present work reports the fabrication of a bilayer heterojunction device, combining octafluoro copper phthalocyanine (CuF₈Pc) and lutetium bis-phthalocyanine (LuPc₂) and tunes the charge transport at the Cu(F₈Pc)-(LuPc₂) interface by aryl electrografting on the device electrode to improve the device NH₃-sensing properties. Dimethoxybenzene (DMB) and tetrafluoro benzene (TFB) electrografted by an aryldiazonium electroreduction method form a few-nanometer-thick organic film on ITO. The conductivity of the heterojunction devices formed by coating a Cu(F₈Pc)/LuPc₂ bilayer over the aryl-grafted electrode strongly varies according to the electronic effects of the substituents in the aryl. Accordingly, DMB increases while TFB decreases the mobile charges accumulation at the Cu(F₈Pc)-(LuPc₂) interface. This is explained by the perfect alignment of the frontier molecular orbitals of DMB and Cu(F₈Pc), facilitating charge injection into the Cu(F₈Pc) layer. On the contrary, TFB behaves like a strong acceptor and reduces the mobile charges accumulation at the Cu(F₈Pc)-(LuPc₂) interface. Such interfacial conductivity variation influences the device NH₃-sensing properties, which increase because of DMB grafting and decrease in the presence of TFB. DMB-based heterojunction devices contain four times higher active sites for NH₃ adsorption and could detect NH₃ down to 1 ppm with limited interference from humidity, making them suitable for real environment NH₃ detection.