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.
Keywords: ambipolar materials; ammonia sensor; molecular material; molecular semiconductor; phthalocyanines; relative humidity.