Modulating the majority charge carrier type and performance of organic heterojunction ammonia sensors by increasing peripheral fluorination of the silicon phthalocyanine sublayer

Silicon phthalocyanines (R2-SiPcs) are an emerging class of high-performance organic semiconductors which have recently found application in highly sensitive and selective bilayer organic heterojunction devices for ammonia (NH3) sensing. We report bilayer heterojunction devices based on axially-substituted bis(pentafluorophenoxy)silicon phthalocyanines of increasing peripheral fluorination ((F5PhO)2-FXSiPc) as a bottom layer and lutetium bis-phthalocyanine (LuPc2) and demonstrate how increased peripheral fluorination changes device operation from p-type to n-type in response to NH3. Sensors fabricated with (F5PhO)2-F16SiPc exhibits the smallest apparent energy barrier for interfacial charge transport by impedance spectroscopy due to better alignment of the semiconductor molecular orbitals with the semi-occupied molecular orbital of LuPc2. Bilayer heterojunction devices all demonstrated a LOD below 1 ppm with (F5PhO)2-SiPc/LuPc2 yielding an LOD of 307 ppb and a sensitivity of -0.72%·ppm-1. Postdeposition thermal annealing of the (F5PhO)2-SiPc/LuPc2 device is shown to further enhance sensor performance with a two-fold increase in sensitivity to -1.15%·ppm-1 and a LOD of 198 ppb.