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(ClPc); M: Co, Cu, and Zn) as a sublayer and lutetium bis-phthalocyanine (LuPc) as a top layer. Despite the small effect of metal atom in M(ClPc) on the device current and the interfacial energy barrier, a strong effect on the NH sensing behavior was found such that Co(ClPc)-, Cu(ClPc)-, and Zn(ClPc)-based devices exhibited -type, -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(ClPc)-based devices is triggered by the chemical doping from NH and water when the device is exposed longer under NH at high humidity turning it -type. Gas sensing studies performed in a wide concentration range of NH at a variable relative humidity (rh) exhibited very high sensitivity of these devices. The best performance is obtained with Co(ClPc)-based devices demonstrated by a very high relative response (13% at 10 ppm NH) and sensitivity (1.47%.ppm), 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 NH sensor for real application.