Modulating the Electrical Properties of Organic Heterojunction Devices Based On Phthalocyanines for Ambipolar Sensors

Although ambipolar materials are highly studied in organic electronics, they are rarely used in gas sensors. In the present work, we studied ammonia sensing on organic heterojunctions in a bilayer configuration composed of octachlorinated metallophthalocyanines (M(Cl₈Pc); M: Co, Cu, and Zn) as a sublayer and lutetium bis-phthalocyanine (LuPc₂) as a top layer. Despite the small effect of metal atom in M(Cl₈Pc) on the device current and the interfacial energy barrier, a strong effect on the NH₃ sensing behavior was found such that Co(Cl₈Pc)-, Cu(Cl₈Pc)-, and Zn(Cl₈Pc)-based devices exhibited n-type, p-type, and ambipolar charge carrier transport, respectively. Variable carrier transport has been explained by charges hopping at the interface and subsequent heterojunction formation. In particular, the ambipolar transport regime in Zn(Cl₈Pc)-based devices is triggered by the chemical doping from NH₃ and water when the device is exposed longer under NH₃ at high humidity turning it n-type. Gas sensing studies performed in a wide concentration range of NH₃ at a variable relative humidity (rh) exhibited very high sensitivity of these devices. The best performance is obtained with Co(Cl₈Pc)-based devices demonstrated by a very high relative response (13% at 10 ppm NH₃) and sensitivity (1.47%.ppm^-1), sub-ppm limit of detection (250 ppb), and negligible interference from rh. Such superior sensing characteristics based on a new heterojunction device make it an ideal NH₃ sensor for real application.