Interfaces dipole

The interface dipole in organic-organic junctions is negligible with the exception of strong donor-acceptor interfaces where a barrier of 0.2-0.3 eV may exist due to the charge transfer process. 

FIGURE 3.3 Schematic of an organic-metal interface energy diagram (a) without and (b) with an interface dipole and (c) UPS spectra of metal and organic. (From Hung, L.S. and Chen, C.H., Mater. Sci. Eng., R39, 143, 2002. With permission.)... 

Fig. 4-6. llie inner potential, 4, and the outer potential, tf, of two condensed phases A and B before and after their contact d4 )= inner (outer) potential difference between two contacting phases o = surface or interface charge dip = surface or interface dipole. 

Duhm S, Heimel G, Salzmarm I, Glowatzki H, Johnson RL, Volhner A, Rabe JP, Koch N (2008) Orientation-dependent ionization energies and interface dipoles in ordered molecular assemblies. Nat Meter 7 326... 

W. A. Harrison and J. Tersoff, Tight-binding theory of heterojunction band lineups and interface dipoles, J. Vac. Sci. and Technol. B4, 1068 (1986). 

Figure 16.2 Sdiematic representation showing metal / organic interface enagy transfer whidi is equivalent to the near-field coupling of the interface dipole to sur ce plasmons in the plane metal. The energy transfer across the interface could cause quenching of the interface dipole at metal / organic interface.

In conclusion, the open circuit voltage is limited primarily by molecular energy levels ( oc < LUMOacceptor-HOMOdonor)> with potential secondary limitations from the contacts (compare with Fig. 17), which themselves depend critically on the possible formation of interface dipoles, which can lead to substantial deviations from that simple relationship. 

Veenstra SC, Heeres A, Hadziioannou G, Sawatzky GA, Jonkman HT (2002) On interface dipole layers between Cgo and Ag or Au. Appl Phys A 75 661... 

As first noted by Seki and coworkers [12] and later confirmed by Kahn and coworkers [13] even in the most simple cases this approximation does not hold. When saturated hydrocarbons, which are among the least reactive molecules in organic chemistry, are deposited onto the most inert metal, Au, a reduction of the work function on the order of 1 eV can be seen. It is interesting to note that this work function lowering has about the same amount encountered when an electro-positive element like Cs is deposited on a metal surface. These changes of the work-function even in the absence of any chemical interaction (like charge transfer or bond formation) are attributed to the formation of an interface dipole located between the molecule and the metal substrate. 

C. W611, Vacuum level alignment at organic/metal junctions Cushion effect and the interface dipole, Appl. Phys. Lett. 87, 263502 (2005). 

P. S. Bagus, V. Staemmler, and C. Woll, Exchange-like effects for closed-shell adsorbates Interface dipole and work function, Phys. Rev. Lett. 89(9), 0961041-3 (2002). 

P. S. Bagus, K. Hermann and C. Woll, The Interaction of QHg and CgHj2 with Noble Metal Surfaces Electronic Level Alignment and the Origin of the Interface Dipole, J. Chem. Phys. 123, 184109(2005). 

Capacitance-based chemical sensors are in the class of devices that transduce analytes into electrical currents. Such sensors are typically comprised of a dielectric, chemically-sensitive film coated onto a substrate electrode these films pass low conduction current, making amperometric or conductimetric measurements less sensitive or attractive for signal transduction. To detect an analyte, changes in the chemically-sensitive film s capacitive properties (associated with its dielectric constant, charge uptake, or formation of interface dipole layers) are measured when an active species is present or generated. 

Instead, the interface exhibits an additional dipole barrier A that shifts the vacuum level upward by more than 1 eV, hence increasing the barrier height by the same amount. The rather large interface dipole is explained by the fact that the electron density at a metal surface presents a tail that extends from the metal free surface into vacuum, thus forming a dipole pointing at the metal bulk. Molecules deposited on the metal tend to push back this tail, thus reducing the surface dipole and decreasing the work function of the metal. 

The energy barrier for hole injection at the metal-poymer interface is determined by the vacuum work function of the metal contact and the ionization potential Ip of the polymer. For conjugated polymer films spin-coated onto hole injecting metal electrodes, it has been reported that as long as is smaller than a critical value characteristic of the polymer, no interface dipole is formed [104]. In this case, the barrier for hole injection can be estimated simply by aligning the vacuum levels of the metal and the polymer (Mott-Schottky limit) the measured work function of the metal with the polymer deposited on top increases linearly with with a slope of one (see Figure 2.3.11). 

However, when exceeds said critical valne a significant interface dipole can be formed. Positive charges are transferred from the metal to the semiconductor and the position of the Fermi level at the interface becomes pinned at an energy level interpreted as the hole polaron/bipolaron energy level in the polymer semiconductor. This simple picture suggests that, at least in the case of solntion-deposited polymers on common hole-injecting contacts, chemical interactions between the metal and... 

FIGURE 2.4.2 (a) Simple band line-up diagram for a metal-organic semiconductor interface assuming that the Mott-Schottky rule holds and that the vacuum levels for the metal and semiconductor are aligned, (b) Application of a positive bias to the metal can result in hole injection into the semiconductor by thermionic emission over the barrier, (c) Band line-up diagram in the case where an interface dipole is present, causing a shift (A) in the vacuum levels across the junction. 

Kera, S. et al.. Impact of an interface dipole layer on molecular level alignment at an organic-conductor interface studied by ultraviolet photoemission spectroscopy, Phys. Rev. B, 70, 085304, 2004. 

During processing there are a number of opportunities to create shallow states in the semiconductor which can degrade the device transconductance and shift the threshold voltage through the formation of a charged interface dipoles or bulk trap states and release of mobile charges. 

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