Tailored β-diketones as effective surface passivation for solution processed zinc oxide thin film transistors

Abstract Diketone passivation of defect states at the surface of solution-processed zinc oxide thin film transistors is investigated. Trifluoroacetylacetophenone is used as the basis for passivating molecules due to its easy chemical variability on the phenyl ring. Electron withdrawing or donating substituents can be introduced at the phenyl ring to vary the electron density at the ketone anchoring positions of the passivating molecule. The relative change of electron density is quantified by DFT calculations for different substituents. Experimentally thin zinc oxide films are prepared by aqueous spray pyrolysis, while passivating diketones are deposited in vacuum on the ZnO surface. XPS and AFM measurements confirm the successful deposition of passivating molecules at the zinc oxide surface. Electrical characterization of passivated zinc oxide transistors is carried out for different exposure times. All diketones improve transistor stability for positive and negative bias stress. The passivated transistors show improved electrical properties with increased electron mobility and decreased disorder parameter. Finally, threshold voltages shift systematically to negative values and hysteresis clearly reduces. The experimentally determined electrical properties of passivated zinc oxide transistors show a systematic correlation to the effective electron density at the oxygen anchor atoms of diketones and higher charge results in a better performance.