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 small effect of metal atom in M(Cl8Pc) on the device current and the interfacial energy barrier, a strong effect on 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 heterojunctions formation. In particular, the ambipolar transport regime in Zn(Cl8Pc) based device 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 device demonstrated by 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.

Molecular material, phthalocyanines, ambipolar materials, molecular semiconductor, relative humidity