Minority Currents in n-Doped Organic Transistors - ACS Applied

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Minority Currents in n‑Doped Organic Transistors Akram Al-shadeedi,† Shiyi Liu,† Chang-Min Keum,† Daniel Kasemann,‡ Christoph Hoßbach,§ Johann Bartha,§ Scott D. Bunge,∥ and Björn Lüssem*,† †

Department of Physics and ∥Department of Chemistry and Biochemistry, Kent State University, Kent, Ohio 44242, United States ‡ Institut für Angewandte Photophysik and §Institut für Halbleiter- und Mikrosystemtechnik, TU Dresden, 01062 Dresden, Germany S Supporting Information *

ABSTRACT: Doping allows us to control the majority and minority charge carrier concentration in organic field-effect transistors. However, the precise mechanism of minority charge carrier generation and transport in organic semiconductors is largely unknown. Here, the injection of minority charge carriers into n-doped organic field-effect transistors is studied. It is shown that holes can be efficiently injected into the transistor channel via Zener tunneling inside the intrinsic pentacene layer underneath the drain electrode. Moreover, it is shown that the onset of minority (hole) conduction is shifted by lightly n-doping the channel region of the transistor. This behavior can be explained by a large voltage that has to be applied to the gate in order to fully deplete the n-doped layer as well as an increase in hole trapping by inactive dopants. KEYWORDS: n-channel transistor, pentacene field-effect transistor, n-type molecular doping, ambipolar transistor, Zener tunneling, minority currents

1. INTRODUCTION

doped pentacene layer, which leads to a strong shift of the threshold voltage.16 Minority currents are as well discussed in ambipolar organic transistors.17−19 However, these transistors almost exclusively consist of intrinsic, i.e., undoped, organic semiconductors only, which makes the definition of minority and majority charge carriers ambiguous. A recent publication of Noh et al.20 is a rare exception. Ambipolar transistors are often discussed as potential building block of a simplified complementary-like logic. However, the ambipolarity leads to increased static power dissipation and lower static noise margins of inverters.21 To reduce minority currents in OFETs, Noh et al. doped the channel region of a transistor based on methanofullerene [6,6]phenyl-C61-butyric acid methyl ester (PCBM) either by the ndopant CsF to reduce hole currents or by the p-dopant F4TCNQ20 to reduce electron currents. Overall, minority currents are reduced, which enabled the authors to build truly complementary logic with low power dissipation. Despite these highly interesting reports on doped organic transistors, the mechanism of minority charge carrier injection and generation is still very poorly understood. To address this gap, minority charge carrier generation in n-doped organic transistors is studied here. It is shown that holes, which are the minority charge carrier in the n-doped channel, can be generated by Zener tunneling at the drain contact.

Doping organic semiconductors has been studied intensively during the last decades.1−3 Both p-4,5 and n-type6,7 dopants were developed, which facilitated highly efficient p-i-n type optoelectronic devices such as organic light-emitting diodes (OLEDs)8,9 or organic solar cells.10,11 However doping can be used not only for organic optoelectronic devices but also for organic field-effect transistors (OFETs) as well. Doping opens a new perspective on organic transistors, as it allows us to distinguish majority from minority charge carriers and to precisely control their equilibrium density. This advantage was used, e.g., to control the majority charge carrier type in organic OFETs. OFETs based on 6,13-bis(triisopropylsilylethynyl)pentacene (TIPSpentacene), usually regarded as a common p-type semiconductor, were turned into n-type transistors by n-doping.12 Similarly, C60, usually seen as an n-type semiconductor, was converted into a p-type semiconductor by blending it with molybdenum trioxide,13 which is known to be a strong pdopant.14,15 In the same way as doping controls the majority charge carrier density, it defines the minority charge carrier density as well. This was used to develop organic inversion-type transistors, which until then have only been known from inorganic semiconductors.16 In inversion-type transistors a doped transistor channel is depleted and inverted, which leads to the creation of a transistor channel of minority charges. For example, a p-type inversion channel can be formed in an n© XXXX American Chemical Society

Received: September 7, 2016 Accepted: October 31, 2016 Published: October 31, 2016 A

DOI: 10.1021/acsami.6b11149 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Furthermore, the influence of doping on the formation of a p(minority type) channel is studied by systematically varying the thickness of the n-doped channel. It is shown that increasing the thickness of the doped channel leads to an increase in the onset voltage of the minority currents. This increase in onset voltage is discussed in terms of an increase in voltage needed to deplete the n-doped channel layer and an increase in minority trapping due to n-doping. In either case, the increase in onset of minority currents provides a means to suppress minority currents in ambipolar organic transistors.

Scheme 1. Structure of the n-Dopant W2(hpp)4

2. EXPERIMENTAL DETAILS 2.1. Preparation of Transistors and Diodes. OFETs are fabricated at the Institut für Angewandte Photophysik, TU Dresden, by thermal evaporation under high vacuum conditions (base pressure of 10−7 mbar). During deposition the substrate temperature is kept at 60 °C. All layers are deposited on glass substrates with 50 nm Al as a gate and 120 nm Al2O3 as a gate oxide. Al2O3 is deposited by atomic layer deposition (ALD) as described in a previous publication.16 To passivate trap states at the organic/oxide interface, a layer of the insulating long alkane chain tetratetracontane (TTC, C44H90) is deposited on top of Al2O3.22 This deposition is followed by a thin ndoped pentacene layer (doping concentration varies from 1 to 16 wt % and thickness ranges from 0 to 8 nm), an intrinsic pentacene layer (50 nm), n-doped pentacene layers as injection layers underneath the contacts [(50 nm, 2 wt %) or (150 nm, 4 wt %)], and aluminum as source and drain contacts (50 nm). The layers are structured by shadow masks with a channel length of 0.3 mm and a channel width of 20 mm. Doping is realized by thermal coevaporation of matrix and dopant materials. The doping ratio is set by the deposition rates of two independent quartz crystal microbalances (QCMs). Intrinsic pentacene is purchased from Sensient, Wolfen, Germany, and the p-dopant F6-TCNNQ is obtained from Novaled GmbH. The n-dopant W2(hpp)4 is synthesized according to the description in Section 2.2. All OFETs are encapsulated under nitrogen atmosphere before transferring them to air. The p-i-n diode samples are prepared at Kent State University by thermal evaporation as well (base pressure 10−7 mbar). The diodes consist of p-doped pentacene (F6-TCNNQ, 50 nm, 4 wt %), intrinsic pentacene of varying thickness (30, 40, 50, 60, 70, and 80 nm), and ndoped pentacene (W2(hpp)4, 50 nm, 8 wt %). Aluminum is used as anode and cathode material (50 nm). The samples are structured by shadow masks as well (active area is 2.56 mm2). All samples are electrically characterized by a semiconductor parameter analyzer (4200-SCS,Keithley) 2.2. Synthesis of n-Dopant W2(hpp)4. Compared to the extensive list of possible p-dopants3 the number of suitable n-dopants is much smaller. For example, the group of Kahn proposed decamethylcobaltocene (DMC) as an effective n-dopant.23 The Bao group developed the n-dopant (4-(1,3-dimethyl-2,3-dihydro-1Hbenzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI) as a solutionprocessable n-type doping and showed that PCBM can be effectively doped with N-DMBI by solution processing.24 Furthermore, the Marder and Kahn group proposed a series of novel dimer molecules as effective and air-stable n-dopants.25 Aside from these dopants, the compound tetrakis(1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a] pyrimidinate) ditungsten(II) (W2(hpp)4, Scheme 1) was used as an effective n-dopant.26 This molecule, first described by Cotton and co-workers,27 is readily ionized, which allows for an efficient electron transfer to most matrix materials, in particular to pentacene.28 The synthesis of W2(hpp)4 is performed in a slightly modified manner to that first described by Cotton and co-workers.27 A sample of commercially available W2(hpp)4Cl2 (0.2 g, 0.2 mmol) is added to 15 mL of tetrahydrofuran (THF). Potassium metal (0.5 g) is subsequently added and the solution stirred for approximately 1 h until turning a dark red color. The reaction is refluxed (80−85°C) for 3 h. The reaction mixture is then cooled to room temperature, and the THF is removed under vacuum. Toluene is added (10 mL), and the

insoluble material is removed via centrifugation. The toluene solution is brought to dryness under vacuum producing a deep-red solid. We obtain an isolated yield of 0.1 g, 52%. To isolate crystals suitable for single-crystal X-ray diffraction, the solid is dissolved in 5 mL of THF and placed in a freezer at −35 °C. After 1 week, small red blockshaped crystals suitable for X-ray diffraction are collected. The crystallographic data are similar to the previous report:27a = 8.28 Å, b = 9.58 Å, c = 10.08 Å, α = 86.0°, β = 70.96°, γ = 81.06°,V = 747 Å3, and T = 105 K. To show that n-doping pentacene by W2(hpp)4 is effective, the current−voltage characteristics of n-doped pentacene films (Al(50 nm)/pentacene:W2(hpp)4 (100 nm, [1, 2, 4, 8, and 16 wt %)]) on cleaned glass substrates with sample length 1.27 mm and width 4.4 mm (device setup is sketched in Figure 1a) are shown in Figure 1b. The linear trend of the plots indicates that n-doping is indeed functioning. The conductivity of n-doped pentacene films ranges from 2.6 × 10−6 S cm−1 for 1 wt % to 2.5 × 10−4 S cm−1 for 16 wt % as shown in Figure 1c. In comparison to p-doped pentacene by F4TCNQ or F6-TCNNQ, the conductivity is much lower,29 indicating that n-doping is not as efficient as p-doping. Furthermore, the strong increase in conductivity from 1 to 2 wt % can be explained by filling of trap states by doping, seen as well in doped films of C60.30,31

3. RESULTS AND DISCUSSION 3.1. n-Doped Organic Transistors. The design of the ndoped OFETs studied here is shown in Figure 2a. The transistor consists of pentacene as matrix material, Al2O3 (120 nm) as gate oxide, and source drain electrodes made of aluminum. To avoid trapping of electrons at the oxide interface, an aliphatic and insulating layer of TTC is added to the device as a passivation layer.22,32−34 The source and drain regions are doped by the n-dopant W2(hpp)4 (50 nm, 2 wt %). Furthermore, in some transistors a thin layer of n-doped pentacene (1 wt % W2(hpp)4, thickness 0, 2, 4, and 6 nm) is sandwiched between the gate oxide and the intrinsic pentacene layer; i.e., the channel region is lightly n-doped. Figure 2b shows the transfer characteristics of these OFETs. At positive voltages, a quasi-Ohmic contact for majority charge carriers (electrons)35 is formed, leading to efficient electron injection and clear n-type behavior of the transistors. However, at negative gate voltages, minority (hole) conduction is seen as well. Furthermore, doping the channel strongly influences minority conduction of the transistor. As seen in Figure 2b, the onset of hole conduction shifts with increasing thickness of the doped channel layer (cf. Figure 2c as well). B


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Figure 1. (a) Setup of devices used for measuring the conductivity of n-doped films. (b) Current−voltage characteristics of pentacene films doped by W2(hpp)4 (1, 2, 4, 8, and 16 wt %). (c) Conductivity of ndoped pentacene films at various doping ratios.

Figure 2. (a) Structure of OFETs. (b) Transfer characteristics of OFETs with 1 wt % W2(hpp)4 n-doped channel at varying thickness (0, 2, 4, and 6 nm, VDS = 15 V) and 50 nm n-doped contacts with 2 wt % W2(hpp)4. (c) Shift of the onset of hole transport with increasing thickness of n-doped layer. Symbols denote experimental data; the line is a fit to the data using eqs 2 and 3

At first sight, the fact that minority charge carriers are efficiently injected into the transistor channel is counterintuitive: The n-doped injection layer in fact increases the injection barrier for holes and should suppress any hole injection into in the transistor. However, as we will argue in the following, minority charge carrier injection into the transistor

channel can be explained by Zener tunneling underneath the drain contact. 3.2. Injection of Minority Charge Carriers by Zener Tunneling. Charge generation by Zener tunneling is wellC


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ACS Applied Materials & Interfaces known from inorganic semiconductors36,37 and commonly observed in organic p-i-n diodes as well.38 It is used for example in charge generation layers of stacked OLEDs39,40 or AC-driven OLEDs41 and in the injection layer for inverted OLEDs.42 Zener tunneling in an organic p-i-n diode is illustrated in Figure 3. Under reverse bias the applied voltage drops across

Figure 3. Schematic of energy level bending and charge transport via Zener tunneling in organic p-i-n diodes at reverse bias.

the intrinsic layer resulting in large electric fields. If the electric field exceeds the breakdown field, electrons can tunnel from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of neighboring molecules. Effectively, electron and holes are generated within the junction, which are transported to the n- and p-doped layer, respectively, by the applied electric field. Zener tunneling in organic diodes was thoroughly studied in the past. It was found that the tunneling current depends exponentially on the thickness of the intrinsic layer and only weakly on the temperature, confirming the tunneling nature of the current.38,43,44 Furthermore, it was found that the breakdown voltage increases for increasing intrinsic layer thickness and lower doping concentrations.38 Zener tunneling is effective in pentacene homojunctions as well. Figure 4b displays I−V curves of pentacene p-i-n diodes with different intrinsic interlayer thicknesses (30 to 80 nm) prepared as described in the experimental information and as sketched in Figure 4a. In forward direction a sharp increase in current at about 2 V is seen. In accordance to other reports on organic diodes, the current onset in forward direction does not depend on the interlayer thickness of pentacene.38 In reverse direction we observe an exponential rise in current at large reverse voltages. In contrast to the forward direction, the onset of the backward current, i.e., the breakdown voltage of the Zener diode, depends strongly on the interlayer thickness and shifts by approximately −1.5 V for each additional 10 nm of intrinsic interlayer. The dependency of the backward current on the intrinsic interlayer thickness is shown in Figure 4c. A purely exponential dependency is found, which is in accordance to the results of Kleemann et al.38 The almost linear increase in breakdown voltage leads to an approximately constant breakdown field shown in Figure 5. Overall, a fairly constant breakdown field of around 1.2−1.33 MV cm−1 is observed. Zener tunneling as observed in the pentacene p-i-n homodiodes can explain the observed minority injection into

Figure 4. (a) Structure of p-i-n diodes. (b) Current−voltage characteristics of organic p-i-n diodes with different intrinsic interlayer thicknesses. (c) Reverse current density at −7 V vs intrinsic interlayer thickness. The p-i-n device consists of Al(50 nm)/pentacene:F6TCNNQ (4 wt %, 50 nm)/intrinsic pentacene (x nm)/pentacene:W2(hpp)4 (8 wt %, 50 nm)/Al (50 nm).

the transistor channel as shown in Figure 6. At negative gate voltages a large voltage drops between the gate and drain electrode. If the electric field in the intrinsic pentacene layer D


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where dintr denotes the thickness of the intrinsic pentacene layer. Without channel doping, hole conduction sets in at VGS = 0.5 V (OFETs with 0 nm n-doped channel) (cf. Figure 2b), resulting in an electric field strength in the range 1.19 MV cm−1 (Cins = 3.7 × 10−8 F cm−2; a dielectric constant εr = 3 is assumed for organic layers), close to the breakdown electric field shown in Figure 5 confirming the assumption that Zener tunneling is responsible for minority injection. 3.3. Influence of Doping on Minority Channel Formation. Only transistors without channel doping were discussed in the previous section. However, as seen in Figures 2b and c, doping the transistor channel has a strong influence on the formation of the hole (minority) channel as well. The onset of hole conduction shifts by approximately 4.5 V due to an increase in thickness of the doped channel by 6 nm. Two mechanisms can cause this strong shift in onset of minority conduction. First of all, the injection mechanism itself can explain the observed dependency. Hole injection by Zener tunneling can only be efficient if the n-doped layer at the insulator/semiconductor interface is fully depleted by the applied negative gate voltage. Otherwise, all holes generated inside the intrinsic pentacene layer will recombine with electrons still remaining in the n-doped channel. Hence, an increase in the doped layer thickness leads to an increase in the voltage needed to fully deplete the layer from electrons, which in turn increases the onset voltage as seen in Figures 2b and c. The voltage shift resulting from an increase in thickness of the doped channel layer can be estimated by straightforward arguments. The voltage across the insulator and doped layer needed to fully deplete an n-doped layer at the insulator− semiconductor interface can be estimated as45,46

Figure 5. Electric field at breakdown for increasing intrinsic interlayer thickness. Without loss of generality the breakdown field is defined as the electric field needed to reach a reverse current of −0.1 mA cm−2. The device consists of: Al(50 nm)/pentacene:F6-TCNNQ (4 wt %, 50 nm)/intrinsic pentacene varied thickness/pentacene:W2(hpp)4 (8 wt %, 50 nm)/Al (50 nm).

V = VFB +

Vins =

exceeds the threshold for Zener tunneling, free electrons and holes are generated. Electrons will drift back to the n-doped drain electrode, whereas holes are injected into the transistor channel. To show that the electric field inside the intrinsic pentacene layer underneath the drain electrode is indeed in the range of the breakdown field, we estimate the voltage drop across the intrinsic pentacene layer by modeling this part of the transistor by a simple series connection of the capacitance of the gate insulator (Cins, 120 nm of Al2O3 and 25 nm of TTC) and the intrinsic pentacene layer (Cintr 50 nm). Hence, for transistors without channel doping, the electric field in the intrinsic pentacene layer Eintr becomes 1 d intr

C ins (VDS − VGS) C intr + C ins

(2)

where VFB is the flatband voltage, e the elementary charge, N+D the density of ionized, i.e., active, dopants, ddop the thickness of the doped layer, εP5 the dielectric constant of pentacene, dins the thickness of the gate insulator, and εins the dielectric constant of the gate insulator. Assuming that the capacitance of the depleted doped channel layer Cdep is much larger than the capacitance of the gate insulator Cins and the intrinsic pentacene layer Cintr we can rearrange eq 1 and solve for Vins, i.e., the voltage dropping across the insulator

Figure 6. Equivalent circuit of the region of the OFET underneath the drain electrode at reverse bias. If the electric field in the intrinsic pentacene layer exceeds the breakdown field, electrons and holes are generated by Zener tunneling.

E intr =

2 ⎛ eND+ddop d ε ⎞ ⎜⎜1 + 2 ins P5 ⎟⎟ 2ε0εP5 ⎝ ddop εins ⎠

C intr (VDS − VGS) C intr + C ins

(3)

Using eqs 2 and 3 we can model the shift in onset of hole conduction Δ(VDS − VGS) for a given concentration of free electrons in the doped layer N+D. A good agreement to the experimental data is reached for a density of active dopants N+D of approximately 9.5 × 1017 cm−3, which is shown as a straight line in Figure 2c. This electron density corresponds to a doping efficiency defined as the ratio of active dopants N+D to the total density of dopants ND of 11%. However, we would like to stress that this number is only valid under the assumption that the shift in the onset voltage is caused by an increasingly more difficult depletion of a thicker doped channel. The other mechanism that can explain the shift in onset voltage of minority conduction is an increase in hole trapping due to an increase in the number of n-dopants. For most

(1) E


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ACS Applied Materials & Interfaces dopant/host combinations a low doping efficiency was observed, which means that most dopant molecules do not donate an electron to the LUMO of host material. However, these states are not entirely inactive. Although they are not sufficiently strong to donate an electron to the LUMO of the host material, they still represent trap states for holes located in the HOMO of the matrix. Hence, an increase in the thickness of the doped layer will increase the number of traps Nt at the gate insulator/pentacene interface, which in turn will shift the onset of hole conduction according to ΔV =

Nteddop C ins

(4)

Following eq 4 and the experimental result shown in Figure 2c (ΔV = 4.5 V), a density of traps Nt in the range of 1.75 × 1018 cm−3 can be calculated. 3.4. Influence of Channel Doping on Majority Charge Carriers - Determination of the Doping Efficiency. Two mechanisms were discussed in the previous section to explain the shift of the onset of hole conduction: an increase in onset voltage due to the additional voltage needed to fully deplete the doped channel layer or, alternatively, an increase in onset voltage due to stronger trapping of holes by inactive n-dopants. Both mechanisms can explain the experimental results well but show a different dependency on the doping efficiency. While a high doping efficiency increases the density of free charge carriers in the doped layer, which will make depletion more difficult, a low doping efficiency will leave more dopant molecules inactive resulting in increased trapping. Therefore, to determine which mechanism is dominating, we determine the density of free charge carriers in the doped channel and hence the doping efficiency. We can make use of the influence of doping on the majority charge carriers, i.e., on electron transport visible at positive gate voltages. It is known that doping leads to a shift in threshold voltage of majority charge carriers according to16 Vth = VFB +

eND+ddop C ins

Figure 7. (a) Transfer characteristics of doped OFETs with 8 nm doped channel, different n-doping concentrations (1, 2, 4, and 16 wt %), and 150 nm n-doped contacts (4 wt % W2(hpp)4). (b) Shift of threshold voltage at an n-doping concentration of 1, 2, 4, and 16 wt %.

(5)

Figure 7a shows the transfer characteristics of a series of transistors with 8 nm thick doped channel and increasing doping concentrations (1, 2, 4, and 16 wt %). A shift of the nthreshold voltage is clearly seen and summarized in Figure 7b. Overall, the threshold voltage shifts from approximately 7 V at low doping concentrations to 4 V at 16 wt %. As seen as well, an increase in n-doping concentration in the channel not only increases the n-threshold voltage by increasing the density of free electrons but also suppresses the p-type behavior. This behavior is in accordance to the observation that the onset of p-conduction increases with increasing thickness of the doped layer and is consistent with the observation of Noh et al.21 Using eq 5 we can deduce the density of free electrons in the doped channel N+D. We obtain a density of charge carriers of 6.7 × 1016 cm−3 at a doping concentration of 1 wt %. Combined with the known density of dopants in the film this results in a doping efficiency of 0.8% only, much lower than the doping efficiency estimated from eq 2, i.e., by assuming that the depletion of the n-doped transistor channel is limiting hole conduction.

The low doping efficiency observed here therefore indicates that trapping of holes by inactive n-dopants in the channel is causing the strong shift in the onset of hole transport. Adding more dopants by increasing the thickness of the doped layers leads to an increase in trapping and consequently a shift in the transfer characteristic. Furthermore, the low doping efficiency explains why no shift in n-threshold voltage is observed at the low concentration used in Figure 2 and why the overall conductivity of n-doped pentacene films is much lower compared to p-doped pentacene films (Figure 1).

4. CONCLUSION Doping organic transistors adds a new perspective on charge transport in organic transistors as it allows us to define and control the majority and minority charge carrier type. Although a few publications have already taken advantage of this benefit of organic doping, the precise mechanisms of minority charge generation and minority channel formation are still unknown. F



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Here, we have studied the influence of doping on minority charge carriers in n-doped OFETs. We show that, despite ndoped source and drain electrodes, holes can efficiently be injected into the transistor channel leading to an ambipolar transfer characteristic. We rationalize this behavior by Zener tunneling inside the intrinsic pentacene layer underneath the drain electrode and are able to show that the electric field in this layer is indeed in the range of the breakdown field of pentacene-based p-i-n Zener homodiodes. Furthermore, the onset of minority (hole) conduction can be shifted by lightly n-doping the channel. Two mechanisms can lead to this behavior: first, the shift in the onset voltage can be explained by a larger voltage that has to be applied to the gate in order to fully deplete the n-doped layer, which is a prerequisite for efficient minority injection. Second, an increase in hole trapping by inactive dopants will shift the onset voltage as well. These two mechanisms can be distinguished by the doping efficiency of our particular doping system. While a high doping efficiency will benefit the first mechanism (depletion of the doped channel), a low doping efficiency will make the latter mechanism (trapping) more likely. By calculating the doping efficiency from the shift of the threshold voltage of majority charge carriers (electrons), we can conclude that the shift of the onset voltage of minority conduction is dominated by an increase in hole trapping due to inactive n-dopants. Overall, the experiments discussed here provide a better understanding of minority charge carrier injection and the influence of doping on minority charges in the transistor channel. Furthermore, doping provides a means to turn ambipolar OFETs into unipolar OFETs, which are in some cases preferred as they allow for complementary circuits operating at low power losses and large noise margins.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b11149. I−V and C−V characteristics of OFETs with different TTC layer thickness. Mobilities of hole and electron for different doped channel thickness and doping concentration (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +1 (330) 672 9129. Fax: +1 (330) 672 2959. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Institut für Angewandte Photophysik, TU Dresden, for preparing OFET samples. AA was supported by The Higher Committee For Education Development in Iraq. Funding from the Start-Up funds of Kent State University and from the Binational Science Foundation (grant no. 2014396) is greatly acknowledged. Characterization of samples was partially done at the Characterization Facility of the Liquid Crystal Institute, Kent State University. CK and BL acknowledge funding from the Kent State University Internal Post-Doctoral Competition. G


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Minority Currents in n-Doped Organic Transistors - ACS Applied

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