The charge carrier transport in organic field effect transistors (OFETs) is determined by the
transport properties of the insulator / organic semiconductor interface. We demonstrate that an
adequate treatment of this interface results in a polarity change of the OFET charge carrier
transport properties, without further altering the device structure. Illuminating the utilized
PMMA polymer dielectric, by using UV radiation, leads to the introduction of mainly
electron traps at the dielectric interface. This results in the suppression of the electron
transport for an otherwise n-type pentacene OFET. However, as a consequence of trapped
electrons in the near surface layer of the PMMA dielectric, the hole transport of the device is
enabled though a hole blocking source/drain metallization. This effect, as well as the impact
of the UV irradiation on the PMMA dielectric will be discussed in detail. The UV treatment
yields a PMMA interface rich on polar groups. The influence of these groups on the OFET
characteristics is investigated by studying several polymer dielectrics with varying content of
the emerging groups.
In the present paper a new concept towards O-CMOS technology is presented substantiating the importance of the semiconductor/dielectric interface for charge carrier transport in organic semiconductors. It will be demonstrated that by controlling the interface properties of either SiO2 or PMMA, unipolar p- and n-type OFETs can be realized using a single organic semiconductor and even a single metal for source and drain contacts. Two dielectric/semiconductor interface modifications are considered for the realization of complementary OFETs on the basis of pentacene, otherwise known for its exclusive hole transporting properties. Selective modification of the SiO2 dielectric interface with traces of vacuum deposited Ca, allows for electron transport in pentacene and the realization of complementary pentacene OFETs on a single substrate. By this technique electron traps are removed due to a reaction of atomic Ca with oxygen from available hydroxide groups, resulting in the formation of an oxidized Ca layer. In a second approach, it is demonstrated that by selective UV treatment of a PMMA dielectric surface, unipolar n-type pentacene OFETs can be converted to unipolar p-type by the introduction of electron traps in the form of -OH and -COOH groups at the PMMA interface. Both methods allow for the realization of CMOS organic inverter stages with decent electrical properties.
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