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Effect of doping of zinc oxide on the hole mobility of poly(3-hexylthiophene) in hybrid transistors

Hybrid field effect transistors based on the organic polymer poly(3-hexylthiophene) (P3HT) and inorganic zinc oxide were investigated. In this report we present one of the first studies on hybrid transistors employing one polymeric transport layer. The sol-gel processed ZnO was modified via Al doping between 0.8 and 10 at.%, which allows a systematic variation of the zinc oxide properties, i.e. electron mobility and morphology. With increasing doping level we observe on the one hand a decrease of the electron mobilities by two orders of magnitude,on the other hand doping enforces a morphological change of the zinc oxide layer which enables the infiltration of P3HT into the inorganic matrix. X-ray reflectivity (XRR) measurements confirm this significant change in the interface morphology for the various doping levels. We demonstrate that doping of ZnO is a tool to adjust the charge transport in ZnO/P3HT hybrids, using one single injecting metal (Au bottom contact) on a SiO2 dielectric. We observe an influence of the zinc oxide layer on the hole mobility in P3HT which we can modify via Al doping of ZnO. Hence, maximum hole mobility of 5e-4 cm^2/Vs in the hybrid system with 2 at.% Al doping. 5 at.% Al doping leads to a balanced mobility in the organic/inorganic hybrid system but also to a small on/off ratio due to high off-currents.