Bulk and Interface Charging Mechanisms in Organic Semiconductor-Gate Dielectric Bilayers
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Bulk and Interface Charging Mechanisms in Organic Semiconductor-Gate Dielectric Bilayers H.E. Katz, C. Huang, J. West Department of Materials Science and Engineering and Department of Chemistry, Johns Hopkins University, 102 Maryland Hall, 3400 North Charles Street, Baltimore, MD 21218 Abstract Since the first reports of charge storage in the gate dielectrics of organic semiconductors, several groups have proposed charge-storing dielectrics that become polarized through varied mechanisms, and have offered various explanations for observed charge storage phenomena. These groups were concerned either with nonvolatile memories as an application, or with controlling hysteresis in conventional OFETs. This manuscript describes measurements of surface charging and OFET threshold voltage shift for a case where charge is clearly stored in the dielectric. The magnitude and stability of the charge storage depend on the hydrophobicity of the dielectric and the charge deposition process. We focus on SiO2 as the dielectric and use a thiophene oligomer or hexadecafluoro-copper phthalocyanine as semiconductor. In one case, the phthalocyanine was inverted from electron- to hole-carrying, enabling a complementary device to be made from a single semiconductor. Introduction Organic field-effect transistors (OFETs) based on organic semiconductor (OSC) thin films show promise as building blocks for low-cost, large-area and flexible electronics for applications such as displays, smart cards, radio-frequency identification tags, and sensors1. Most of the OFETs fabricated to date exhibit unipolar conduction with a single carrier type (holes or electrons, p or n respectively)2. Circuits designed for OFETs generally assume that all devices made from a particular OSC have the same mobility and threshold voltage (Vt). For example, to form a complementary inverter, which would be more power-efficient and lower- noise than unipolar analogues3, two different semiconductors are needed because OFETs with very different I-V characteristics are required. For the sake of simplicity, it would be preferable to use only a single OSC. Recent attempts to realize single-component OSC thin-film transistors and complementary inverters relied on OSC-metal electrode interface modifications to control the carrier injection. Single-component inverted OFETs and inverter circuits have been operated in high vacuum either with low work function metals such as calcium employed as the source and drain electrodes4-5 or molecules designed to optimize the higher occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO) energy levels, or band gaps (Eg).6 Here we describe two different types of OSC inverters, one made from unipolar 5,5’-bis(4hexylphenyl)-2,2’-bithiophene (6PTTP6) and the other made from hexadecafluoro-copper phthalocyanine (F16-CuPc) acting in both normal and inversion mode. The properties of the OFETs assembled to make the inverters were set by charging the gate dielectrics prior to deposition of the semiconductors. This
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