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DOI: 10.1002/adfm.((please add manuscript number)) Determination of Threshold-Voltage Shifts in Organic Thin-Film Transistors due to Polarized Self-Assembled Monolayers at the Dielectric Surface** By Stefan Possanner, Karin Zojer, Peter Pacher, Egbert Zojer and Ferdinand Schürrer * [*] Prof. F. Schürrer, S. Possanner, K. Zojer

Institute of Theoretical and Computational Physics, Graz University of Technology Petersgasse 16, A-8010 Graz (Austria) E-mail: schuerrer@itp.tugraz.at

Prof. E. Zojer, P. Pacher

Institute of Solid State Physics, Graz University of Technology Petersgasse 16, A-8010 Graz (Austria)

[**] Supporting Information is available online from Wiley InterScience or from the author. Keywords: Organic Semiconductors, Transistors, Self-Assembled Monolayers, Thin Films

We simulate the hole transport in organic thin-film transistors (OTFTs) with dipole layers, i.e., self-

assembled monolayers (SAMs), at the semiconductor/dielectric interface by means of a two-

dimensional drift-diffusion model. Moreover, we analyze the simulated evolution of the electric

potential and the hole density with the help of Gauss law. By means the latter, simple expressions

for the shifts in the threshold voltage and the amount of accumulated charge in the channel are

derived. We demonstrate that the shift in the threshold voltage, that is equal to the potential drop

across the dipole layer, amounts to a few volts for realistically chosen intrinsic dipole moments of

corresponding SAM molecules. Thus, large threshold voltage shifts (> 45 eV) that were observed in

(SiO2/SAM/P3HT) OTFTs upon exposure to ammonia [P. Pacher et al., Adv. Mater., 2008, in

print] cannot be related to changes in dipole moments within a SAM. Furthermore, our simulations

reveal that the capacitance of the gate dielectric does not influence this shift, while it profoundly

influences the accumulated interface charge; thus, a decrease in the thickness of the gate oxide leads

to bigger drain currents. Based on the results of our simulations, we present criteria that any model

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has to fulfill in order to correctly describe the impact of interface dipole layers on the threshold

voltage.

1. Introduction

Over the past few years the organic semiconductor (OSC)/dielectric interface has been the subject

of numerous studies on the device performance of organic thin-film transistors (OTFTs). It is

known that organic transistors usually operate in accumulation mode and that charge transport takes

place mainly in the first few mono-layers of the OSC adjacent to the gate dielectric.[1,2] Therefore, it

appears that transistor performance is greatly influenced by the OSC/dielectric interface properties.

It was shown that the carrier mobility in polycrystalline materials such as pentacene strongly

depends on the morphology and the molecular ordering in the channel, which is governed by the

quality and the reactivity of the oxide surface.[3,4,5,6] Moreover, chemical reactions taking place at

the OSC/dielectric interface were reported to create/annihilate carrier trap states that can shift the

threshold-voltage by some 10 V,[7,8] or even lead to n-type conduction devices.[9] Yoon et al. have

presented a comprehensive experimental study on the correlation between the oxide surface

chemistry and the threshold-voltage, mobility, current on-off ratio and sub-threshold slope for

various p-type, n-type and ambipolar OTFTs.[10]

Recently, several groups reported that the electrical properties of organic transistors can be

controlled via self-assembled mono-layers (SAMs) with different functional groups that align on

the oxide surface. Pentacene as well as polymeric OTFTs with different organosilane-based SAMs

linked covalently to the SiO2 dielectric were investigated.[8,11,12,13] A significant dependence of the

threshold-voltage and the sub-threshold slope on the deposited SAM was observed. To give an

example of the typical behavior, the measured transfer characteristics of an OTFT made of spin-

coated poly(3-hexylthiophene) (P3HT) on an SiO2-dielectric with and without an organosilane-

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based SAM at the oxide surface is depicted in Figure 1 (data taken from Pacher et al.)[8]. The major

changes in the electrical characteristics are explained either by additional interface traps originating

from the interaction of the functional group with the OSC molecules, or by a change of the oxide

surface potential, caused by the intrinsic electric field of the organosilane layer, due to the uniform

arrangement of the dipole moments of the molecules in the SAM. It is known from simulation

studies that energetically distributed interface trap-states could be responsible for the observed

threshold-voltage shifts and changes in the sub-threshold slope.[14,15,16] By contrast, only

speculations exist regarding the possible magnitude of a threshold-voltage shift induced by a dipole

layer at the gate dielectric surface. Our goal is to clarify this situation by time-dependent numerical

simulation of the carrier transport in SAM-modulated OTFTs. We aim at reproducing the curves

from Figure 1, thereby defining possible parameter ranges for an interface dipole density and a

fixed surface charge density that could yield the observed changes. By checking on the evolution of

the electric potential from the case where mobile charges are completely absent (before injection) to

steady state, we are able to determine the threshold-voltage shifts due to dipole layers and fixed

charges at the OSC/dielectric interface from Gauss’ law. Besides, a relation between the upper

bound of accumulated charge in the channel and the gate voltage is obtained for both cases.

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2. Simulation model

The drift-diffusion model was shown to be well applicable to model the carrier transport in organic

devices.[2,14,15,16,18,19,20] In our approach, only p-type conduction is considered and electron transport

is assumed to be negligible. We simulate transistors made of wide band-gap materials with hole

injecting electrodes, i.e., the Fermi-level in the contacts is close to the highest occupied molecular

orbital (HOMO) of the OSC. Our drift-diffusion model is based on solving self-consistently a

system of three equations:

€

dpdt

+1e∇ ⋅ j+G − R = 0 (1)

€

j = epµ(p)∇ϕ − kBTµ(p)e

∇p (2)

€

∇ ⋅ ∇ εrϕ( )[ ] =eε0(NA − p + ntA ,occ − N fix ) (3)

Here, p is the hole density, j hole-current density, e elementary charge, G hole generation rate, R

hole recombination rate, µ(p) carrier concentration-dependent mobility, φ electric potential, kB

Boltzmann constant, T temperature, εr permittivity, ε0 dielectric constant, NA acceptor doping

density, ntA,occ density of occupied electron acceptor traps and Nfix is the density of permanent fixed

charges in the transistor. Equation (1) is the continuity equation for the hole density, (2) indicates

the drift-diffusion equation and the Poisson equation (3) governs the electric potential.

The numerical solution of the system (1)-(3) is computed on a two-dimensional grid with

appropriate geometry and boundary conditions. We focus on the top-contact transistor structure but

confirm our results to be valid for the bottom-contact design too. A schematic view of the

investigated top-contact geometry is depicted in Figure 2. We simulate either a two- or a three-

layer structure consisting of the organic semiconductor as the active material, a gate dielectric and

an optional polarized layer in between the two. Diriclet conditions are used at contact boundaries

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and zero-field Neumann conditions are applied at non-contact surfaces. The contact nodes are

assumed to be in thermal equilibrium, described by non-degenerate statistics, i.e., pcontact=NV exp(-

Δ/kBT), where NV is the effective density of states of the HOMO-level (monomer density) and Δ is

the hole injection barrier. The regions of the gate dielectric and the polarized layer are assumed to

be free of mobile charges. Additionally, no charge transfer is allowed from the organic

semiconductor to the adjacent region, which means that the equations (1) and (2) are solved in the

active domain only. The density of permanently occupied acceptor states NA is assumed to be 1016

cm-2, a value that was confirmed to be small enough not to cause depletion mode operation in the

regarded OTFT structures. The term Nfix in the Poisson equation allows us to place permanent fixed

charges anywhere in the device. The terms G and R in equation (1) together with the term ntA,occ in

equation (3) describe the dynamics of rechargeable, deep, acceptor-like electron traps. Details can

be found in the supporting information. In order to account for the peculiarities of charge transport

in organic materials we implement a carrier density dependent hole mobility. We use a model of

carriers hopping in a Gaussian density of states, elaborated by Pasveer et al.[21] Details are stated in

the supporting information.

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3. Results and discussion

3.1. Operation regimes of OTFTs

It is well known that organic transistors like the one described in Figure 2 work in p-accumulation

mode and that, in the gradual-channel approximation, the drain current ID (above threshold)

depends on the gate-source voltage VGS and the drain-source voltage VDS as[18]

€

ID =µWLCox (VGS −Vth −

VDS

2)VDS for VDS < VGS −Vth

µW2L

Cox (VGS −Vth )2 for VDS > VGS −Vth

(4)

where µ is a constant hole mobility, W channel width, L channel length, Cox=ε0εr/dox oxide

capacitance per unit area and Vth is the threshold-voltage. The first condition in (4) defines the linear

regime and the second one the saturation regime of OTFT-operation. These equations are widely

used by experimenters to obtain threshold voltages and carrier mobilities from measured transfer

characteristics (ID or ID1/2

vs. VGS at a fixed VDS). In an ideal p-accumulation device (no space

charges) with little unintentional acceptor doping (so that depletion operation is not possible), Vth

should in principle be the flat-band voltage VFB, that is the difference between the gate work

function and the HOMO-level of the OSC.[1] Applying VFB at the gate leads to a flat transport-level

and no channel is formed at the OSC/dielectric interface, i.e. the transistor is turned off. Possible

space charges and dipole layers at the OSC/dielectric interface will lead to a shift ΔVth of the

threshold voltage so that

€

Vth =VFB + ΔVth (5)

In order to determine the magnitude of such shifts we start with the investigation of the ideal case

ΔVth=0, i.e. no interface dipole layers and no permanent surface charge densities in the device. We

simulate a top-contact OTFT as depicted in Figure 2 with channel length L=10 µm, an organic layer

of thickness dosc=30 nm and a gate oxide with dox=100 nm. The hole-mobility is set to µ=4.0×10-4

cm2V-1s-1, which is a common value in polymeric organic materials. We chose the monomer density

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as the effective density of states, i.e. NV=2.0×1021 cm-3, and assume a hole-injection barrier of

Δ=0.1 eV between the contacts and the OSC. The flat-band voltage is set to zero, which in this case

leads to Vth=0. The dielectric constants of the OSC and the oxide are chosen as εr,osc=3.2 and

εr,ox=3.9, respectively. For all simulations we choose room temperature, T=298.15 K. The source

potential is set to VS=0 and the drain potential is held at VD=-20 V. The solutions of the Laplace

equation, i.e. (3) with the bracketed expression on the right-hand side being zero in the whole

device, are depicted for four different gate voltages in Figure 3. The images display the situation at

a time before charge carriers are injected into the device, i.e. shortly after setting the gate from flat-

band condition (0 V in this case) to the specific gate voltage. It is interesting to note that in all cases,

there is no electric field between source and drain at that stage. Instead, we find a large domain in

the active region (0<x<0.03 µm) where the potential is completely flat, thereafter referred to as the

zero-field domain. This is a consequence of the big aspect ratio of the regarded cross-section in

OTFTs, here L/d=10/0.13≈77. From figure 3d) we derive that the transistor is turned off when the

zero-field domain lies above the source potential, since then no holes can overcome the resulting

barrier and flow from source to drain. In the linear regime, depicted in 3a), charges will enter the

channel from both source and drain until a steady source-drain electric field has been established.

By contrast, in the saturation regime shown in 3b), the region under the drain contact immediately

gets depleted and charges are only injected from the source electrode. The steady state of the

electric potential and the hole density for both regimes is shown in Figure 4. The pinch-off

(depletion under the drain contact) in the saturation regime is clearly visible in the figures 4c) and

4d).

3.2 Fixed surface charges at the OSC/dielectric interface

In Figure 5 simulated transfer characteristics of the regarded transistor are displayed for different

permanent surface charge densities σif at the OSC/dielectric interface (x=x’ in figure 2). The

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simulations confirm the expressions (4) to be valid in principle, even though the macroscopic

mobility extracted from the slope of linear fits of ID or ID1/2 vs. VGS can differ from the microscopic

input mobility by about 20 %. We explain this by the omission of diffusion currents in the

derivation of the equations (4), but a sophisticated analysis is yet to be done. The threshold voltages

were determined in the saturation regime, as the intersection of a linear fit of ID1/2 vs. VGS with the

x-axis (indicated as the dashed line in figure 5). From figure 5 we obtain that permanent negative

interface charges have two effects: a) they shift the threshold voltage to positive values and b) they

increase the drain current in both the saturation and the linear regime. In order to understand this we

investigate the potential profile of the OTFT with σif=-6×1012 ecm-2 at VGS=0, depicted in Figure 6.

In the absence of mobile charges, the zero-field domain is found to be at a voltage that is different

from the one applied to the gate. We call it the effective gate voltage Veff. It can be derived from

Gauss’s law by taking into account that the displacement current D in the zero-field domain is

constant in the y-z-plane:

€

dDx

dx= ρ (6)

where Dx=εEx is the x-component of the displacement current and ρ is the charge density. Assuming

ρ=σif×δ(x-x’), where δ is the Dirac delta function and x’ is the position of the surface charge, and

integrating equation (6) over a small distance around x’, we obtain

€

Dox −Dosc =σ if (7)

Here, Dox and Dosc are the constant x-components of the displacement currents in the oxide and the

OSC, respectively. Since the simulation yields Dox=εox (VG-Veff)/dox in the oxide region and Dosc=0

in the zero-field domain due to the Neumann boundary condition at the transistor surface, we obtain

for the effective gate voltage

€

Veff =VG −σ if

Cox

(8)

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where Cox=εox/dox is the capacitance per unit area of the gate oxide. If VG is set to the flat-band

voltage, negative interface charges cause the transistor still to be on, because then, according to

equation (8), the zero-field domain is at a voltage below flat-band condition and charges are able to

enter the channel region from the source. The figures 4b) and 4d) reveal that the amount of

accumulated mobile charges at the OCS/dielectric interface reaches its maximum σmax under the

source contact. Above threshold, it is given by

€

σmax =VeffCox (9)

From the equations (8) and (9) we derive that, in case of permanent interface charges σif, the

occurring threshold voltage shift and the dependence of the maximally accumulated charges on the

gate voltage (above threshold) are described by

€

ΔVth = −σ if

Cox

(10)

€

σmax =VGCox −σ if (11)

The threshold voltage shifts predicted from equation (10) agree outstandingly well with the

simulated results from figure 5, as can be seen in Table 1. In organic transistors the magnitude of

fixed interface charges will in most cases not be constant, but will change with threshold voltage

due trapping/de-trapping of charge carriers into/from localized states in the transport gap. This

process and the meaning of equation (10) in this case will be addressed in another work.

3.3 Dipole layers at the OSC/dielectric interface

We simulate OTFTs with the same geometry and material parameters as in section 3.1, but now

assuming a polarized layer (e.g. a SAM) with different dipole densities γ at the OSC/dielectric

interface. The layer is realized by implementing two surface charge densities of the same magnitude

σ and opposite signs at x’ and x’’, respectively (see figure 2). The resulting dipole density γ=ddip×σ

is related to the dipole moment µdip of a single molecule of the SAM via

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γ = ddipσ =µdip

A (12)

where A is the area of a single molecule of the SAM. From equation (6), Gauss’ law, the potential

drop Δφdip over a dipole layer with a dipole density as in (12) and a permittivity εdip is calculated as

€

Δϕdip =ddipσεdip

=σCdip

(13)

where Cdip=εdip/ddip is the capacitance per unit area of the dipole layer. The simulations yield

threshold voltage shifts in very good agreement with the potential drop from equation (13), as is

demonstrated in Table 2. These results become obvious by examining the potential profile of an

OTFT with a dipole layer of γ=1.92×1016 Dcm-2 at the OSC/dielectric interface at VGS=0 in the

absence of mobile charges and in steady state, as depicted in Figure 7. The zero-field domain is

almost exactly at the potential Δφdip=31.4 V, calculated from equation (13), below the source

potential. Additionally, it is revealed that the accumulation of mobile charges at the interface does

not influence the potential drop over the dipole layer. The dependence of the maximally

accumulated interface charge σmax on the gate voltage can be derived from the curve at Δt=0.0 in

figure 8a). According to Gauss’ law, σmax relates to the interface potential V’ (above threshold) as

€

σmax =V '(Cosc + Cox ) (14)

where Cosc=εosc/dosc is the capacitance per unit area of the active organic layer. From Δφdip=V’’-V’

and V’Cosc=Dosc=Dox=(VG-V’’)Cox we derive that

€

V '= Cox

Cosc + Cox

(VG −Δϕdip ) (15)

By combining the equations (13), (14) and (15), we state that, in case of a SAM with a dipole

charge σSAM and a capacitance per unit area CSAM at the OSC/dielectric interface, the occurring

threshold voltage shift and the dependence of the maximally accumulated charges on the gate

voltage (above threshold) are described by

€

ΔVth =σ SAM

CSAM

(16)

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€

σmax = (VG −σ SAM

CSAM

)Cox (17)

where for σSAM, the dipole charge closer to the gate must be taken to yield the correct sign. In

contrast to the threshold voltage shift, the charge σmax and therefore the drain current do depend on

the oxide capacitance. This fact could be used in experiments to distinguish between influences of

dipole layers and surface charges on the electrical characteristics of an OTFT. Simulated transfer

characteristics of transistors with different oxide and organic layer thicknesses are displayed in

Figure 9. The relations (16) and (17) are confirmed.

3.4 Simulation of measured transfer characteristics

We aim at reproducing the experimental data from Pacher et al.,[8] displayed in figure 1. The

regarded top-contact OTFTs are made of 10 nm regioregular poly(3-hexylthiophene) (rr-P3HT) as

the active material deposited on an approximately 1 nm thick organosilane based SAM covalently

linked to 170 nm of SiO2 as the gate dielectric. The channel length was L=25 µm and the width was

W=7 mm. The other input parameters were chosen as in section 3.1, representing values describing

the above mentioned materials. The intrinsic dipole moments of organosilane molecules are known

to be of the order of some Debye. As was shown in the preceding section, this cannot be the reason

for the observed threshold voltage shift (see table 2).

In Figure 10 the result of a simulation with a constant mobility µ=4.0×10-4 cm2V-1s-1 and a

permanent negative interface charge σif=-5×1012 ecm-2 is displayed along with the experimental

curve and the most accurate simulation result. The latter is obtained by comprising rechargeable

acceptor interface trap states, which allows the regulation of the off-current and the sub-threshold

slope, and a hole density dependent mobility, which gives the non-linear ID(VG)-dependence in the

linear regime. The best value for the acceptor interface traps is a density of ntA=1014 cm-2 positioned

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at ΔE=0.25 eV above the HOMO level. In the Pasveer-model for the mobility, we chose µ0=1.0×10-

8 cm2V-1s-1 and the broadness of the Gaussian density of states to be ŝ=3.5 kBT.

4. Conclusions

We developed a drift-diffusion simulator for organic transistors and investigated the effect of

surface charges and dipole layers at the OSC/dielectric interface on the electrical characteristics of

OTFTs. The simulation results revealed that one can apply Gauss’ law to determine the occurring

threshold voltage shifts and the amount of accumulated charge at the interface. We found that the

dipole moments of SAM molecules are by far too small to cause the experimentally observed

threshold voltage shifts.

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_[1] T. Li, J. W. Balk, P. P. Ruden, I. H. Campbell, D. L. Smith, J. Appl. Phys., 2002, 91(7),

4312

_[2] M. A. Alam, A. Dodabalapur, M. R. Pinto, IEEE Trans. Electron. Devices, 1997, 44(8),

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Chang, C. Chang, M. Joo, C. Y. Ryu, K. Cho, Adv. Func. Mater., 2005, 15(1), 77

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_[7] A. Wang, I. Kymissis, V. Bulovic, A. I. Akinwande, IEEE Trans. Electron. Devices, 2006,

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_[8] Peter

_[9] L.-L. Chua, J. Zaumseil, J.-F. Chang, E. Ou, P. Ho, H. Sirringhaus, R. H. Friend, Nature,

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Yoshimoto, S. Ogawa, Y. Iwasa, Nature Mater. 2004, 3, 317

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Rashid, G. Schitter, J. Appl. Phys. 2004, 96(11), 6431

_[13] C. Huang, H. E. Katz, J. E. West, Langmuir, 2007, 23, 13223

_[14] A. Bolognesi, A. Di Carlo, J. Comp. Electron., 2003, 2, 297

_[15] Scheinert 1

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_[16] S. Scheinert, K. P. Pernstich, B. Batlogg, G. Paasch, J. Appl. Phys., 2007, 102, 104503-1

_[17] A. Rolland, J. Richard, J. P. Kleider, D. Mencaraglia, J. Electrochem. Soc., 1993, 140(12),

3679

_[17] E. J. Meijer, C. Tanase, P. Blom, E. van Veenendaal, B.-H. Huisman, D. M. de Leeuw, T. M.

Klapwijk, Appl. Phys. Lett., 2002, 80(20), 3838

_[18] S. Scheinert, G. Paasch, Phys. Stat. Sol. A, 2004, 201(6), 1263

_[19] N. Tessler, Y. Roichman, Appl. Phys. Lett., 2001, 79(18), 2987

_[20] T. Li, P. P. Ruden, I. H. Campbell, D. L. Smith, J. Appl. Phys., 2003, 93(7), 4017

_[21] W. F. Pasveer, J. Cottaar, C. Tanase, R. Coehoorn, P. A. Bobbert, P. W. M. Blom, D. M. de

Leeuw, M. A. J. Michels, Phys. Rev. Lett., 2005, 94(20), 206601-1

_[22] M. Schwoerer, H. C. Wolf, in Organic Molecular Solids, Wiley-VCH, Weinheim, Germany

2007, Ch. 8.

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Figure 1. Semi-log plot of the measured drain current vs. gate voltage (transfer characteristics) of a P3HT-based OTFT with and without an organosilane-based SAM at the OSC/dielectric interface. Data is taken from Pacher et al.[8]

Figure 2. Schematic view of the simulated top-contact transistor structure. L is the channel length and d is the thickness of the device. Boundary conditions for the electric potential ϕ and the current density j are indicated. The regions of the gate dielectric and the polarized layer are assumed to be free of mobile charges. The latter is realized by two surface charge densities of opposite sign at x’ and x’’, respectively.

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Figure 3. Simulated electric potential profiles of a top-contact transistor with L=10 µm, dosc=30 nm and dox=100 nm before charge carriers (holes) enter the device (charge-neutrality). The source potential is set to 0 V and the drain potential to -20 V. The device is free of permanent space-charges. The flat-band voltage is assumed to be VFB=0. The gate potential is chosen to represent a) the linear regime (-50 V), b) the saturation regime (-12 V), c) the switch-on/off condition (0 V) and d) the off-state (+14 V).

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Figure 5. Square root and logarithm of the drain current vs. the gate voltage of the regarded top-contact OTFT with different permanent surface charge densities σif at the OSC/dielectric interface. The drain-source voltage is -20 V. The dashed line indicates the linear fit of the square root of the drain current in the saturation regime made to determine the threshold voltage.

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0246810121400.03

0.060.09

0.12

!5

!10

!20

!25

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source

y [µm]

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x [µm]

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0246810121400.03

0.060.09

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source

y [µm]

gate dielectric

drain

gate

x [µm]

! [V

]

b)

OSCinterface

steadC state

Figure 6. Simulated electric potential of the regarded top-contact OTFT with a fixed surface charge of σif=-6×1012 ecm-2 at the OSC/dieletric interface at zero gate voltage a) in absence of mobile charges and b) in steady state. The source potential is set to 0 V and the drain potential is -20 V.

0246810121400.03

0.060.09

0.12!30

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0

10

20

source

y [µm]

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gate

drain

x [µm]

! [V

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0246810121400.03

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Figure 7. Simulated electric potential a) in the absence of mobile charges and b) in steady state of the regarded top-contact OTFT with a polarized layer of γ=1.92×1016 Dcm-2 at the OSC/dieletric interface. Source and gate potentials are set to 0 V and the drain potential is -20 V. The layer was realized by two surface charge densities σ=±5.0×1014 ecm-2 of opposite sign at a distance ddip=0.8 nm.

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0 0.02 0.04 0.06 0.08 0.1 0.12

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Figure 8. Simulated evolution of the electric potential perpendicular to the source-drain direction a) under the source contact and b) in the middle of the channel of the regarded OTFT with a dipole layer of γ=1.92×1016 Dcm-2 at the OSC/dieletric interface. Source and gate potentials are set to 0 V and the drain potential is -20 V.

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Figure 9. Simulated transfer characteristics of an OTFT with L=10 µm and a polarized layer with γ=8.1×1015 Dcm-2 at the OSC/dieletric interface for different thicknesses of the OSC and the gate oxide. We obtain that, in contrast to the drain current ID, the threshold voltage is independent of the oxide capacitance.

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best simulationexperiment

Figure 10. Comparison of the measured transfer characteristics of an OTFT with a SAM at the OSC/dielectric interface from Pacher et al. with simulation results.[8] The squared line is obtained with µ=4.0×10-4 cm2V-1s-1 and σif=-5×1012 ecm-2. The best simulation is obtained by comprising rechargeable interface traps and a carrier density dependent hole mobility. Table 1. Comparison of the threshold voltage shifts ΔVth_sim obtained from simulated transfer characteristics as shown in figure 5 with ΔVth_gau calculated from Equation (10) for an OTFT with dox=100 nm and εr,ox=3.9 and for different fixed surface charge densities σif at the OSC/dielectric interface. σif [1012 ecm-2] ΔVth_sim [V] ΔVth_gau [V]

+6.0 -28.6 -28.3

+4.0 -19.0 -18.9

+2.0 -9.3 -9.4

0.0 0.1 0.0

-2.0 9.5 9.4

-4.0 19.1 18.9

-6.0 28.7 28.3

Table 2. Comparison of the threshold voltage shifts ΔVth_sim obtained from simulated transfer characteristics of the regarded SAM-modulated OTFT with the potential drop Δφdip over a dipole density γ calculated from Equation (13). The absolute value of the corresponding dipole moment µdip of a single molecule of the layer is obtained from equation (12) assuming A=25 Å2. γ [1015 Dcm-2] ΙµdipΙ [D] Δφdip [V] ΔVth_sim [V]

19.19 47.97 -31.4 -31.1

3.84 9.59 -6.3 -6.2

1.92 4.80 -3.1 -3.0

-1.92 4.80 3.1 3.2

-3.84 9.59 6.3 6.3

-19.19 47.97 31.4 31.6

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Supporting Information should be included here (for submission only; for publication, pleae provide

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