Atomic layer deposited Al2O3 as a capping layer for polymer based transistors

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. In this paper we explore the realization of inorganic capping layers by atomic layer deposition (ALD) that provides smooth and pinhole-free films with a great potential as passivation layer for organic based devices. We show that the deposition of Al2O3 on transistors based on poly-3 hexyltiophene (P3HT) allows to obtain air stable devices. Whereas the growth of Al2O3 directly on the P3HT layer leads to a rough interface and significant intermixing between the oxide and the polymer, which results in a deterioration of transistor performances, an interlayer of a poly-alcohol such as poly-vinylphenol interposed between the Al2O3 and the P3HT gives a well defined Al2O3/polymer interface without degradation of the hole mobility. 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.