This article describes a new principle of transduction involving an heterojunction between a Molecular Semiconductor and a Doped Insulator (MSDI). Herein, we report on an MSDI-based sensor featuring an heterojunction between a lutetium bisphthalocyanine (LuPc2), which acts as Molecular Semiconductor (MS) and a thin film of Doped Insulator (DI) made of substituted or fluorinated copper phthalocyanine (CuFnPc, where n = 0, 8, 16). Previously, we reported the peculiar effect of the heterojunction on the MSDI’s electronic behavior, suggesting this device as a new kind of transducer for gas chemosensing. Indeed, of particular significance was the key role of modulator played by the nature of the doped insulator sub-layer. While the MS thin film remains the only layer of the sensor exposed to gas atmosphere, the DI’s ability to tune the electronic characteristics of the organic heterojunction allows it to drastically affect the nature of the effective charge carriers. In particular, an increase in fluorination of the doped insulator can cause an inversion of the LuPc2 response toward electron accepting (ozone, ppb level) or donating (ammonia, ppm level) gases. The present work focuses on the structural, electronic and electrical properties of the MSDI heterojunction, which have been studied by UV-vis spectroscopy, atomic force microscopy, current-voltage measurements and chemical doping, in order to shed some light on this phenomenon. The unique ambipolar nature of LuPc2 is suggested to be the main property responsible for the MSDI’s unique behavior.