The strong sensitivity of organic/polymeric semiconductors to the exposure to O₂ and H₂O atmospheres makes the use of capping layers mandatory for the realization of stable devices based on such materials. Atomic Layer Deposition (ALD) provides smooth and pinhole-free films with a great potential as passivation layer for organic based devices. The use of Al₂O₃ grown by ALD on polymers has been explored by different groups and it was demonstrated that the growth mechanism depends on the nature of the polymer. When ALD oxides are employed to cap organic semiconductor based devices the control of the polymer/oxide interface is critical, since any diffusion of the oxide into the organic semiconductor leads to a degradation of the electrical properties of the device. Moreover the condition for the deposition of the capping layer might cause device degradation.

We show that the deposition of Al₂O₃ on transistors based on poly-3 hexyltiophene (P3HT) allows to obtain air stable devices. Nevertheless the growth of Al₂O₃ directly on the P3HT layer leads to a rough interface and significant intermixing between the oxide and the polymer, resulting in a deterioration of transistor performances. By using polymeric interlayers between the P3HT and the Al₂O₃ it is possible to prevent oxide interdiffusion. The choice of the interlayer is critical, and if a polymer with an high filling factor and proper functional groups is used it is possible to prevent oxide interdiffusion with at a thickness of just 10 nm. We have considered particularly a poly-alcohol such as Poly-vinylphenol (PVP). We have studied in detail the growth mechanism of Al₂O₃ on the PVP. Transistors capped with Al₂O₃/PVP are very stable in air, with no appreciable differences in the electrical characteristics when measured in vacuum or in air. In addition no significant degradation of the transistors electrical properties was detected even after one month of air exposure.

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