Modulation of the organic heterojunction behavior, from electrografting to enhanced sensing properties

Abstract

The energy barrier of an organic heterojunction built on ITO electrodes and made from a low conductive sublayer (Cu(F16Pc)) covered by a highly conductive semiconductor (LuPc2) is modulated by electrografting of organic layers before depositing the sublayer. Impedance spectroscopy clearly demonstrates the increase of the energy barrier at the ITO – sublayer interface. Additionally, the electrografting is a versatile and promising method for the tuning of heterojunctions. The I(V) characteristics of the heterojunctions are highly modified by the electrografting. The same electromodifications of electrodes carried out on LuPc2 resistors lead to a modification of their transport properties too. The effect of the grafting of four different aromatic moieties bearing electron-donating and electron-withdrawing substituents is studied. One important feature is that the sensing properties are highly improved compared to the unmodified devices. Thus, the electrografting of dimethoxybenzene doubles the relative response of the heterojunction towards 90 ppm NH3, as well as the sensitivity in the range 1–9 ppm. This electrografting allows attaining a limit of detection as good as 140 ppb. The modified heterojunctions favorably compete other conductometric transducers for the detection of ammonia, at room temperature and in a broad range of relative humidity.