Interface dipoles
A lot of methods have been applied to tune the work function of elec-trodes. Examples of such kind of methods include:
1. Creating interfaces with COMs having an intrinsic dipole moment [86,87].
2. Use of strong molecular acceptors or donors in order to induce charge transfer at the interface and thus creating an interface dipole [63–65,88,89].
3. Use of self assembled monolayers (SAMs) with intrinsic dipole mo-ments attached on the surface of the electrode [90,91].
The interface dipoles discussed above create a shift inEvac, that can be calculated using Helmholtz’s equation:
∆Φ = ∆Evac = q n µ
0
, (2.16)
with∆Φbeing the work function change,∆Evacthe shift in vacuum level, q the elementary charge, n the dipole surface density,µthe dipole moment perpendicular to the surface, the dielectric permittivity of the organic material and 0 the vacuum permittivity. The assumptions taken for this equation to be valid is that there is no dipole-dipole interaction and po-larisability of the COMs. The depolarisation effect is usually of the order of 100 meV, allowing for the Helmholtz equation to be used as a good ap-proximation. A better model that does take into account the polarisability of the molecules is the Topping model [92], that includes the interaction between neighbouring dipoles. Instead of using this model though, an ef-fective coverage-dependent dielectric constant can be used [93] together with Helmholzt’s equation.
By controlling the magnitude of the dipole moment µ, together with the dipole areal density, one can tune the work function according to eq.
(2.16).
Despite the fact that all the methods can achieve the adjustment of the work function, in the current work, the use of molecular acceptors
Figure 2.14: Schematic illustration showing the work function adjustment by the use of interface dipoles to a) increase and c) decrease the work function. b) and d) show the corresponding decrease of the hole injection barrier (HIB) and electron injection barrier (EIB) between the molecular electrode and the corresponding hole or electron transport material (HTL or ETL). Adapted from [79].
or donors is preferred. The reasons for selecting this route instead of the other methods is the fact that first, molecules with intrinsic dipoles are harder to control and can result in random antiferroelectric assemblies on the surface and secondly, SAMs have to be anchored on the surface by the formation of chemical bonds [94], which would result in the disrup-tion of the electronic network of graphene. In contrast, the use of strong molecular acceptors/donors provides an effective way for controlling the direction of the dipole moment. Electron acceptors will result in a dipole moment having the vertical component facing towards the surface plane, whereas electron donors will result in a dipole moment facing away from the surface plane. The addition of such dipoles by increasing the material on the surface usually yields a continuous increase or decrease of the work function, until the first monolayer is completed, as shown in Fig. 2.14.
Surface charge transfer doping
Doping of inorganic semiconductors refers to the manipulation of their charge carrier density and conductivity by introduction of impurities in their crystal structure and is a significant technological tool in the field of (opto)electronic devices based on semiconductors [95].
Conventional doping is achieved by bombardment of the semiconduc-tor with energetic ions that are incorporated into the lattice of the host semiconductor by ion implantation. According to the type of ion, the introduction of negative charge carriers can occur when the ion donates electrons into the conduction band of the semicondutor (n-type doping) or positive charge carriers when the ion accepts electrons from the valence band, leaving behind positive holes (p-type doping) [95].
In the search for novel and more gentle doping techniques that could be applied in nanoelectronics, the doping at the surface/near-surface re-gion by using organic molecules that can act as electron acceptors or elec-tron donors has been developed [56, 96, 97]. With this method, the ex-change of electrons between the semiconductor and the surface dopant leads to an effective surface doping. The mechanism that drives the dop-ing is the Fermi level pinndop-ing phenomenon. As described above, upon the formation of a solid/solid interface, if theEF of the semiconductor lies above the LUMO or below the HOMO, the system comes out of
thermo-Figure 2.15: Schematic illustration showing how the Fermi level, EF of graphene can be altered upon surface charge transfer doping, resulting in an electron en-riched or electron poor graphene sheet. ED is the energy of the Dirac point.
Adapted from [47].
dynamic equilibrium, thus, charge transfer spontaneously occurs in order to bring the system back into equilibrium and Fermi level pinning occurs, as described in the previous section.
Consequently, the surface region or near-surface region would be doped by charge carriers [98]. The surface CT doping model was first introduced to explain the high surface conductivity that was found in di-amond, a high band gap insulator [56,96,97,99] and it was only recently invoked as a way to manipulate the conductivity of nanomaterials. This technique was further exploited in order to dope graphene epitaxially grown on SiC(0001) [100] using the strong molecular acceptor F4TCNQ.
For the case of graphene, Fig. 2.15 shows schematically the process. A COM that acts as a n-dopant shifts the EF of graphene upwards, thus enriches the graphene sheet with electrons, whereas a COM that acts as a p-dopant shifts the EF downwards, thus depletes electrons from the graphene sheet.
The following equation (2.17) is derived from the Dirac energy disper-sion near the K-points (eq. (2.7)) and it can be used as an approximation to the doping of the graphene by introducing charge carriers due to the
adsorbed molecule [101]:
∆EF =n2π2~uF , (2.17) with n being the density of charge carriers,uF the Fermi velocity and∆EF
the shift of the Fermi level EF with respect to the Dirac point (located at the energyED).
Hence, in the case of graphene, the resulting work function change can be separated into two contributions: (a) shift of the vacuum level (∆Evac) due to formation of interface dipoles and (b) shift of the Fermi level due surface CT doping that induces a shift in the EF with respect to the ED (∆EF). Thus the work function change (∆Φ) of graphene follows the equa-tion:
∆Φ = ∆EF + ∆Evac. (2.18) As a conclusion, the precoverage of the graphene electrode with a molecular acceptor or donor that finely tailors the surface potential, will result in reduced charge injection barriers (HIB or EIB) for the further de-posited hole or electron transport layer, resulting to a better performance for the final organic electronics device.