# Energy Level Alignment in Organic-Organic Heterojunctions: The TTF-TCNQ
Interface

Juan I. Beltrán<EMAIL_ADDRESS>[  Fernando Flores
<EMAIL_ADDRESS>[  José I. Martínez  [  José Ortega  [

###### Abstract

The energy level alignment of the two organic materials forming the TTF-TCNQ
interface is analyzed by means of a local orbital DFT calculation, including
an appropriate correction for the transport energy gaps associated with both
materials. These energy gaps are determined by a combination of some
experimental data and the results of our calculations for the difference
between the TTFHOMO and the TCNQLUMO levels. We find that the interface is
metallic, as predicted by recent experiments, due to the overlap (and charge
transfer) between the Density of States corresponding to these two levels,
indicating that the main mechanism controlling the TTF-TCNQ energy level
alignment is the charge transfer between the two materials. We find an induced
interface dipole of 0.7 eV in good agreement with the experimental evidence.
We have also analyzed the electronic properties of the TTF-TCNQ interface as a
function of an external bias voltage $\Delta$ between the TCNQ and TTF
crystals, finding a transition between metallic and insulator behavior for
$\Delta\sim 0.5$ eV.

Universidad Autónoma de Madrid]Departamento de Física Teórica de la Materia
Condensada,
Universidad Autónoma de Madrid, E-28049 Madrid, Spain Universidad Autónoma de
Madrid]Departamento de Física Teórica de la Materia Condensada,
Universidad Autónoma de Madrid, E-28049 Madrid, Spain Universidad Autónoma de
Madrid]Departamento de Física Teórica de la Materia Condensada,
Universidad Autónoma de Madrid, E-28049 Madrid, Spain Universidad Autónoma de
Madrid]Departamento de Física Teórica de la Materia Condensada,
Universidad Autónoma de Madrid, E-28049 Madrid, Spain

##  1 Introduction

Figure 1: (color online) TTF-TCNQ bulk structure with its view-plane along the
b and a lattice vectors, which corresponds to $\hat{y}$ and $\hat{x}$ axis,
for the left and right pictures, respectively.

Organic electronics is an area of important research effort where organic
materials provide an immense variety of properties and atomic structures. In
these materials, interfaces play a key role in fundamental phenomena (e.g.
charge separation and charge recombination) for electronic applications such
as solar cells and light emitting diodes, to name a few 1, 2, 3.

TTF/TCNQ (tetrathiofulvalene-7,7,8,8,-tetracyanoquinodimethane) is a very
interesting organic bulk material that was already studied in the early 70’s
showing novel phenomena, like large e- conductivity and superconductivity 4.
TTF/TCNQ shows three phase transitions which have been related to condensed
charge-density waves, due to Peierls distortions, with transition temperatures
of 38, 49 and 54 K 4. Also, the experimental band widths of TTF and TCNQ
crystal-bulks are 0.95 and 2.5 eV respectively, while density functional
theory (DFT) calculations based on local and semi-local exchange-correlation
(XC) functional provide systematically, for TTF and TCNQ, band widths of
around 0.65 and 0.70 eV respectively 5, 6, 7, 8. These intriguing properties
have prompted a lot of experimental and theoretical studies on the properties
of bulk TTF/TCNQ, see e.g. 5, 6, 7, 8, 9, 10; thin films of mixed TTF-TCNQ
have also been studied on different substrates like graphene and Au(111) 11,
12.

However, the large amount of work on bulk and thin film properties of TTF/TCNQ
contrasts with the much lesser attention dedicated to the interface of the two
organic materials. One of the few studies 13 has found that the interface
retains its metallic character due to the charge transfer between the two
molecules. The existence of an interface dipole layer ($IDL$) 14, 15,
generated by charge transfer or/and by polarization effects, and its
corresponding vacuum level shift ($VLS$) 1, shows that the Schottky-Mott limit
is not satisfied at this interface. Careful spectroscopic characterization of
TTF-TCNQ interfaces over different substrates shows a $VLS$ of 0.6 eV and a
$\Delta_{HOMO}$ = HOMO${}_{TTF}-$HOMOTCNQ of either 2.1 or 2.4 eV 16, 17.

In this communication we consider the interface between TTF and TCNQ crystals,
and analyze the interface energy level alignment and barrier height formation,
using a local orbital DFT-approach in which an appropriate correction is
included to properly describe the transport gaps of both materials. In section
II we present the interface geometries, and in section III we discuss how we
determine the transport energy gaps for TTF and TCNQ and give some details
about our DFT-calculations. In section IV, we present our results. In
particular, our analysis shows that the TTF-TCNQ interface is metallic and
that the energy difference between the TCNQLUMO and TTFHOMO is pinned to
around 0.8 eV. We have also analyzed how the interface properties depend on
the application of an external voltage between the TCNQ and TTF crystals,
finding a transition from a metallic to an insulating interface for a voltage
of $\sim$ 0.5 eV. Finally, the interfacial geometry has also been analyzed by
means of a DFT calculation combined with a semi-empirical parametrization of
the van der Waals interaction.

Figure 2: (color online) Similar to 1 but for the bilayer interface.

##  2 Interface Geometry

At room temperature, TTF/TCNQ bulk presents a monoclinic structure with
lattice parameters a = 12.298 Å, b = 3.819 Å and c = 18.468 Å, and an angle
$\hat{\beta}$ = 104.46° 18 (see 1). However, detailed experimental information
of both the geometry and the energetics at the interface is still missing due
to inherent difficulties in its structural characterization. For the TTF-TCNQ
surface, though, it has been measured by scanning tunneling microscopy 19 and
angle-dependent near-edge X-ray adsorption fine structure 20 that the cleaved
surfaces are highly ordered and retain the periodicity of the bulk. Thus, in
this work we have considered the interfacial structure arising from selecting
the interface geometry between TTF and TCNQ layers from the TTF/TCNQ bulk
structure along the (100) direction (see 2). In the DFT calculations we have
used bulk periodic boundary conditions along b and c vectors, while for the a
direction we have considered two cases: a monolayer interface, with only one
layer of each material, and a bilayer interface, with 2 TTF and TCNQ layers.
This amounts to 2 TTF + 2 TCNQ or 4 TTF + 4 TCNQ molecules in the unit cell,
respectively. In the bilayer interface, the second layer is generated from the
interfacial plane by adding parallel molecules which are shifted in a similar
way as in their independent crystals for both the TTF and TCNQ bulk structures
– see Cambridge Structural Database – and satisfying the periodic boundary
conditions of keeping b and c vectors from the bulk structure (see 2).

Figure 3: (color online) Energy level diagram for the isolated TTF and TCNQ
molecules obtained from $\Delta$-SCF calculations (explained in the main
text).

##  3 TTF-TCNQ Interface Energy Level Alignment

###  3.1 $\Delta$-SCF calculations

As a first step in order to determine the energy level alignment at the TTF-
TCNQ interface, we have performed $\Delta$-SCF calculations for the isolated
TTF and TCNQ molecules. In these calculations, the HOMO and LUMO levels are
determined from total energy DFT calculations 21, 22 for molecules with $N$,
$N+1$ and $N-1$ electrons ($N$ corresponding to the neutral molecule):

$\displaystyle\varepsilon_{HOMO}$ $\displaystyle=$ $\displaystyle
E[N]-E[N-1],$ $\displaystyle\varepsilon_{LUMO}$ $\displaystyle=$
$\displaystyle E[N+1]-E[N],$ (1)

where $E[N_{\alpha}]$ is the total energy for a molecule with $N_{\alpha}$
electrons. The results of these calculations are summarized in 3. We obtain a
transport gap (difference between $\varepsilon_{LUMO}$ and
$\varepsilon_{HOMO}$) of 5.3 eV and 6.3 eV for TCNQ and TTF, respectively,
while the mid-gap positions are found to be located at 5.5 eV (TCNQ) or 2.5 eV
(TTF) below the Vacuum Level ($VL$).

Figure 4: (color online) Left panel: energy level diagram for the TCNQ-TTF
interface obtained from a combination of experimental and theoretical
information. Right panel: energy levels in the DFT calculation for the
periodic interface shown in 2. In this calculation the initial transport gaps
and mid-gap positions have been adjusted to reproduce those given in the left
panel of this Figure (see main text).

###  3.2 Energy levels at the interface

The energy level values for the isolated molecules have to be corrected due to
the interaction with the other molecules in the crystal and at the interface.
The effect of this ‘environment’ is three-fold: (a) broadening of the energy
levels into bands; (b) relative shift of the TTF and TCNQ molecular levels due
to electrostatic and Pauli exclusion effects; (c) Polarization or screening
effects, shifting in different directions occupied and empty states. Point (b)
gives rise to a Vacuum Level Shift ($VLS$), or interface potential, between
both crystals, while the effect of point (c) is a reduction of the transport
gaps at the interface (or in the crystals) as compared with the values for
isolated TTF or TCNQ molecules, $E_{g}^{T}=(E_{g}^{T})^{0}-\delta U$. For
example, for a molecule interacting with a metal, image potential effects
reduce the transport gap as follows: $E_{g}^{T}=(E_{g}^{T})^{0}-eV_{IM}$,
where $V_{IM}$ is the image potential 23. In the case of an organic crystal
(or interface) a similar effect is present, due to the screening/polarization
of the other molecules.

In order to obtain the energy level alignment for the TTF-TCNQ interface, we
have used a combination of experimental and theoretical information:

  * (1)

The $VLS$ at the interface is 0.6 eV, as determined experimentally 17. In
these experiments it is also found that
$\Delta\varepsilon_{HOMO}=\varepsilon_{HOMO}(TTF)-\varepsilon_{HOMO}(TCNQ)=$
2.1 eV 17.

  * (2)

The mid-gap energies obtained in the $\Delta$-SCF calculations mentioned above
are 5.5 eV (TCNQ) and 2.5 eV (TTF) below their corresponding Vacuum Level
($VL$) positions.

  * (3)

As discussed below, we find that
$\varepsilon_{LUMO}(TCNQ)-\varepsilon_{HOMO}(TTF)\approx$ 0.8 eV, a result
that is related to the experimental observation that the interface is metallic
14, see below.

Using this information, we can deduce that at the interface the transport gap
is 2.9 eV for TCNQ, and 3.5 eV for TTF, i.e. screening/polarization effects
reduce the transport gaps by 2.4 eV (TCNQ) and 2.8 eV (TTF). 4 (left panel)
summarizes the resulting energy level alignment for the TCNQ-TTF interface.

Figure 5: (color online) A) VIDIS vs. $\Delta$ (initial shift of the TCNQ
levels w.r.t. the TTF levels, see text) for the monolayer and bilayer
interfaces; $\Delta$ = 0 corresponds to the realistic interface configuration.
The experimental $VLS$ (or VIDIS) value, 0.6 eV, is included as a dashed
horizontal line; the screening parameter for each of the two regimes is also
shown; B) Transfer of charge vs. $\Delta$ for the monolayer interface; and C)
Value of [$\varepsilon_{HOMO}$(TTF)$-\varepsilon_{LUMO}$(TCNQ)] vs. $\Delta$
for the monolayer interface.

###  3.3 DFT calculations

We have performed DFT calculations for the monolayer and bilayer periodic
interface structures presented in section II (see 2). In these calculations we
have used a local-orbital DFT code 24, and the initial transport gaps for TCNQ
and TTF have been adjusted to the values given in 4 (left panel) using a
scissor operator; besides, the initial TTF and TCNQ molecular levels are
shifted in such a way that their initial $VL$ positions have the same value,
and the initial mid-gap positions correspond to the values given in 4 (left
panel) (see e.g. Refs. 25, 26, 27 and references therein for details). Other
technical details are the use of the local density approximation (LDA) and an
optimized basis set of $s$ (H) and $sp^{3}d^{5}$ (C, N and S) Numerical
Atomic-like Orbitals 28, with the following cut-off radii (in a.u.): $s$=4.1
for H, and $s$=4.0, 3.6, 4.2, $p$=4.5, 4.1, 4.7, and $d$=5.4, 5.2, 5.5 for C,
N and S, respectively.

Figure 6: (color online) Density of states projected on either the TCNQ or TTF
of the monolayer interface for $\Delta$ = 0.

##  4 Results and Discussion

The right panel of 4 shows the TCNQ-TTF interface energy level diagram
obtained in our DFT calculations (explained in third subsection of previous
section) with the initial conditions as explained above. This self-consistent
result is quite similar to the energy level diagram obtained in second
subsection of previous section, shown in the left panel of 4. The band-gaps
and interface dipole are quite similar in both panels of 4, however the mid-
gap positions move slightly upwards by around 0.3-0.4 eV. This shift is due to
the Pauli repulsion (related to the Pauli exclusion principle).

In order to analyze the mechanism responsible for the energy level alignment
at the TTF-TCNQ interface, we have performed DFT calculations as described in
third subsection of previous section, in which we shift by $\Delta$ the
initial (before self-consistency) position of the molecular levels of TCNQ
with respect to the molecular levels of TTF; $\Delta$ = 0 corresponds to equal
initial vacuum level positions, and $\Delta>$ 0 corresponds to a
destabilization of the TCNQ levels, w.r.t. the TTF levels. This shift
simulates the application of a bias potential of value $\Delta$ between the
two crystals. 4 (right panel) represents the final levels (after self-
consistency) for the $\Delta$ = 0 case. A change $\Delta$ in the initial
relative position of the TCNQ and TTF energy levels will influence the induced
dipole and the rest of the electronic properties at the interface. The final
$VLS$ will depend on the initial misalignment setup $\Delta$ (e.g. the
external bias between the two crystals) and on the induced potential after
electronic self-consistency, $V_{IDIS}$, in such a way:

$VLS=\Delta+V_{IDIS}.$ (2)

In 5A we depict $V_{IDIS}$ as a function of $\Delta$ for the mono and bilayer-
interfaces. In both graphs two different regimes are clearly observed, split
by a crossing point located at $\Delta$ = 0.2 (0.45) eV for the monolayer
(bilayer) case. For both regimes, we see that:

$\delta V_{IDIS}=-(1-S)\delta\Delta,$ (3)

$S$ being a screening parameter 23, taking the value 0.03 for $\Delta<$ 0.2
(0.45) eV and 0.93 for $\Delta>$ 0.2 (0.45) eV. The case of $\Delta<$
0.2(0.45) eV corresponds to a metallic phase in which the
$\varepsilon_{LUMO}(TCNQ)$ and $\varepsilon_{HOMO}(TTF)$ Densities of States
(DOS) are overlapping, as shown in 6 and 7; in this case, the difference
between $\varepsilon_{LUMO}(TCNQ)$ and $\varepsilon_{HOMO}(TTF)$ is almost
constant ($\approx$ 0.8 eV), due to the transfer of charge between these two
levels. For the other phase, $\Delta>$ 0.2(0.45) eV, we find a typical
heterojunction with small screening and a small induced potential, $V_{IDIS}$.

Figure 7: (color online) DOS of the monolayer and bilayer interfaces projected
on each material for $\Delta$ = 0. The PDOS intensity of the monolayer
interface has been scaled according with the number of atoms included in the
bilayer case.

This is illustrated in 5B-C, where we depict, for the monolayer interface, the
charge transfer (positive if electron charge flows from TTF to TCNQ) and the
value of $[\varepsilon_{HOMO}(TTF)-\varepsilon_{LUMO}(TCNQ)]$ vs. $\Delta$.
For both plots the crossing point between the two different regimes is located
at the same position than for $V_{IDIS}$, close to $\Delta=$ 0.2 eV, dividing
the charge transfer plot into a regime with a large increase of charge
transfer as $\Delta$ is reduced, for $\Delta<$ 0.2 eV, and another in which
the transfer of charge is small and changes rather slowly as $\Delta$ is
increased, for $\Delta>$ 0.2 eV. In 5C we see that for $\Delta>$ 0.2 eV the
value of $[\varepsilon_{HOMO}(TTF)-\varepsilon_{LUMO}(TCNQ)]$ vs. $\Delta$
presents a slope close to -1, while for $\Delta<$ 0.2 eV
$[\varepsilon_{HOMO}(TTF)-\varepsilon_{LUMO}(TCNQ)]$ is approximately
constant, with a value of -(0.7-0.8) eV. This value
($\varepsilon_{HOMO}(TTF)-\varepsilon_{LUMO}(TCNQ)\approx$ 0.8 eV) has been
used in second subsection of previous section (see left panel of 4) to deduce
the TCNQ and TTF energy level positions at the interface. It is worth
mentioning that zero charge transfer corresponds to the case of $\Delta=$ 3.12
eV; for this alignment $V_{IDIS}$ is not, however, zero, but takes the values
0.10 and 0.11 eV for the mono and the bilayer cases, respectively. These
induced potentials are created by the polarization effects of the molecule.

In 5 we observe that from the monolayer to the bilayer case the crossing point
moves around 0.25 eV to positive values of $\Delta$. This is explained in 7,
that shows, for the monolayer and bilayer interfaces, the DOS projected on
each material for a value of $\Delta$ close to the crossing point. We notice
that there is a broadening for the bilayer case of the $LUMO(TCNQ)$ DOS, which
shifts by 0.25 eV the position where this DOS starts to overlap with the DOS
corresponding to the $HOMO(TTF)$, as compared with the monolayer case.

Figure 8: (color online) Energy of the monolayer-interface as a function of
the distance between the two planes.

Finally, as a check of the interface geometry used (see 2), we have calculated
the total energy as a function of the distance between the TTF and TCNQ layers
at the interface. 8 shows the total energy curves for the LDA calculation and
the van der Waals (vdW) interactions energy; this energy is included in a
semiempirical fashion as an atractive $-f_{D}(R)C_{6}/R^{6}$ atom-atom term.
$f_{D}(R)$ is the Grimme damping expresion 29 and $C_{6}$=0.7$\times
C_{6}^{\prime}$, where $C_{6}^{\prime}$ is the interatomic parameter
calculated using the London dispersion relation as a function of the atomic
polarizability and the first ionization potential 30; 0.7 is a constant
correction calculated from a detailed analysis of the C–C interaction 31,
which is applied to every $C_{6}^{\prime}$ parameter. In order to prevent for
the over-counting that would appear including both, the exchange-correlation
energy provided by a conventional LDA and the correlation energy associated
with the long-range vdW interaction, we have also performed a “corrected LDA”
calculation in which the exchange-correlation energy for the complete system
is calculated as the sum of the exchange-correlation energies for each
subsystem, taken each one independently, neglecting in this way the effect of
the overlapping densities in the exchange-correlation energy 32. In 8 this
interaction energy is labeled weak chemical interaction (WCI). The minimum of
the WCI+vdW total energy curve is around 2.5 Å, which is close to the
experimental value for the distance between TTF and TCNQ layers in bulk
TTF/TCNQ; notice, however, the very flat energy profile (within 0.02 eV) from
2.2 to 2.7 Å. This result suggests that starting from the bulk structure to
generate the interface geometry is a good approximation.

##  5 Conclusions

In conclusion, we have presented a DFT analysis of the energy level alignment
in the TTF/TCNQ interface, introducing in a self-consistent fashion
appropriate transport gaps for both materials as determined from a combination
of experimental and theoretical information. In these calculations we have
also analyzed the metal-insulator phase-transition occurring in this interface
when applying a bias voltage between the two materials. We find that at zero
bias the interface is metallic while for bias larger than $\sim$ 0.5 eV
(shifting TTF towards higher binding energies w.r.t. TCNQ) the system becomes
an insulator. Our results have been favorably compared with the experimental
data, indicating that the main mechanism controlling energy level alignment in
the TTF/TCNQ interface is the charge transfer between both organic
semiconductors.

##  6 Acknowledgements

This work is supported by Spanish MICIIN under contract FIS2010-16046, the CAM
under contract S2009/MAT-1467 and the European Project MINOTOR (Grant
FP7-NMP-228424). JIB gratefully acknowledges financial support by the European
Project MINOTOR. JIM acknowledges funding from Spanish MICIIN through Juan de
la Cierva Program. Special thanks to Dr. Daniel González for the development
of the Xeo software package, which has been extensively used for the data
analysis.

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