Doping Polymer Semiconductors by Organic Salts: Toward High

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Doping Polymer Semiconductors by Organic Salts: Toward HighPerformance Solution-Processed Organic Field-Effect Transistors Yuanyuan Hu, Zachary D. Rengert, Caitlin McDowell, Michael J. Ford, Ming Wang, Akchheta Karki, Alexander T. Lill, Guillermo C. Bazan, and Thuc-Quyen Nguyen ACS Nano, Just Accepted Manuscript • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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ACS Nano

Doping Polymer Semiconductors by Organic Salts: Toward High-Performance SolutionProcessed Organic Field-Effect Transistors Yuanyuan Hu, 1,2 Zachary D. Rengert,1 Caitlin McDowell1, Michael J. Ford1, Ming Wang1, Akchheta Karki1, Alexander T. Lill1, Guillermo C. Bazan1 and Thuc-Quyen Nguyen1* 1

Center for Polymers and Organic Solids, Department of Chemistry and Biochemistry, University of California at Santa Barbara, Santa Barbara, CA 93106, USA

2

Key Laboratory for Micro-Nano Optoelectronic Devices of Ministry of Education, School of Physics and Electronics, Hunan University, Changsha 410082, China

Email: [email protected] Keywords: Organic field-effect transistors (OFETs); organic salts; doping; mobility; contact resistance; complementary circuits;

ABSTRACT Solution-processed organic field-effect transistors (OFETs) were fabricated with the addition of organic salt, trityl tetrakis(pentafluorophenyl)borate (TrTPFB), into thin films of donor-acceptor

ACS Paragon Plus Environment

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copolymer semiconductors. The performance of OFETs is significantly enhanced after the organic salt is incorporated. TrTPFB is confirmed to p-dope the organic semiconductors used in this study and the doping efficiency as well as doping physics was investigated. In addition, systematic electrical and structural characterizations reveal how the doping enhances the performance of OFETs. Furthermore, it is shown that this organic salt doping method is feasible for both p- and n-doping by using different organic salts, and thus can be utilized to achieve high-performance OFETs and organic complementary circuits.

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During the past decades, organic field-effect transistors (OFETs) have been actively investigated due to their compatibility with flexible substrates and low-temperature processing, allowing for exploration into applications which are not accessible with conventional inorganic transistors.1-4 Among them, solution-processed organic field-effect transistors provide an attractive paradigm in device manufacturing through cost-effective spin-coating or printing processes,5-7 and have significant potential to be applied in large area electronics, as well as in wearable electronics and various types of sensors.8-10 The most important figure-of-merit of OFETs is the field-effect mobility (µFET). It has been proposed that the mobility of OFETs must reach approximately 10 cm2/Vs for them to be useful in driving active-matrix OLED (AMOLED) displays.11 Recently, the µFET of solution-processed OFETs has been shown to be 10 or even higher than 20 cm2/Vs,12-14 which is mainly achieved via the design and synthesis of new organic semiconductors (OSCs) and the improvement of the semiconductor molecular orientation, alignment, and thin-film crystallinity. Several methods for improving the polymer backbone chain alignment or film crystallinity have been proposed.15-18 These methods, though effective in enhancing the device performance especially the µFET, generally require delicate setups and careful control over the semiconductor deposition process, which limits their applications for practical large-scale production. Besides, there are a few reports on blending organic semiconductors with other semiconductors or insulating polymers to improve through tuning the morphology or phase separation in the film.19, 20 This method is interesting yet it has not been widely applied for improving the performance of OFETs. Doping organic semiconductors with various dopants provides another way to improve the performance of OFETs.21-25 Doping can induce the formation of more free charge carriers or the passivation of traps in the organic semiconductor, and hence increase the conductivity.26,

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Enhancement of charge carrier mobility has also been observed in some chemically doped organic semiconductors.21, 28 However, organic semiconductor doping is a complex issue because the doping effect is dependent on the structural and electrical properties of both the dopants and the organic semiconductors, and on the interaction between them. Additionally, doping may also introduce unfavorable effects to charge transport in organic semiconductors, such as trapping, Coulomb scattering or causes instability.29-31 The concomitant reduction of on/off ratio (Ion/Ioff) after chemical doping, due to an increase in background carrier density, also makes this method less attractive for OFETs. Due to these challenges, doping has not been widely used in OFETs. Therefore, exploration of more dopants which can be applied for enhancing the performance of OFETs and a deeper understanding of the doping physics are in need. In this work, we report on the performance improvement of donor-acceptor (D-A) type copolymer-based

OFETs

by

doping

semiconductors

with

organic

salts.

Trityl

tetrakis(pentafluorophenyl)borate (TrTPFB) and tetrabutyl ammonium bromide (TBABr) were used to dope the organic semiconductors for enhancing the performance of p-channel and nchannel OFETs, respectively. Specifically, either the device mobility can be increased or the threshold voltage decreased, depending on the host polymer semiconductor. Moreover, the on/off ratios of devices, instead of being sacrificed, remains unchanged or even get better. We confirm that doping occurs when the organic salt is added to the semiconductors and we reveal how this doping enhances the performance of the polymer OFETs by lowering the activation energy or contact resistance through systematic experiments. Finally, we demonstrate the viable applications of organic salt doping by fabricating high-performance organic complementary circuits based on doped OFETs.


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RESULTS AND DISCUSSION Enhancement of OFET performance The organic semiconductor mainly used for this study is the high-mobility copolymer poly[4(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b’]dithiophen-2-yl)-alt-[1,2,5]thiadiazolo[3,4c]pyridine]

(PCDTPT).

The

devices

were

fabricated

by

spin-coating

on

trichloro(octadecyl)silane (ODTS)-passivated SiO2 substrates. As seen in Figure 1(b), the mobility of pristine PCDTPT bottom-gate, bottom-contact (BG-BC) FETs is about 0.64 cm2/Vs. When a small amount of TrTPFB (0.2% wt) is added to the PCDTPT solution, the mobility can be instantly increased to 1.5 cm2/Vs with all other processing conditions the same, as shown in Figure 1(b). By replacing the substrates with nanogrooved SiO2 substrate,16, 33 the mobility of pristine PCDTPT FETs can be further improved, with an average value of 0.77 cm2/Vs, while the mobility of PCDTPT devices with TrTPFB salt (0.2% wt) can be as high as 1.8 cm2/Vs (see Figure S1 in the supporting information). Interestingly, it is observed that other parameters of PCDTPT/TrTPFB (0.2% wt) OFETs such as threshold voltage and on/off ratios, are not varied much after incorporating TrTPFB into the semiconductor, as seen in Table I. These results demonstrate a simple yet effective way to improve the performance of OFETs. It is worth noting that some of the devices in our work show ”double-slope” feature, namely the slope of the square root of the current versus gate voltage in the saturated transfer characteristics is particularly high at small gate voltages (see Figure S1(a)).34-37 However, the mobility values reported here were all extracted from the slope at high gate voltages, which represent the lower limit of the device mobilities.


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Figure 1 (a) The schematic diagram for device structure and molecule structures. (b) The transfer and (c) output characteristics of PCDTPT and PCDTPT/TrTPFB (0.2% wt) FETs (L=50 µm, W=1400 µm) fabricated on ODTS-passivated SiO2 substrates. (d) The transfer and (e) output characteristics of DPPT-TT and DPPT-TT/TrTPFB (0.4% wt) FETs (L=40 µm, W=1400 µm) fabricated on ODTS-passivated SiO2 substrates. (f) The comparison of the mobility and (g) threshold voltage for OFETs fabricated w/o salt. The labeling “normal” indicates ODTSpassivated SiO2 substrates while the labeling “nanogrooved” indicates ODTS-passivated SiO2 substrates with nanogrooves on the surface.

Table 1. Summary of the measured electrical parameters of OFETs w/o TrTPFB salt.

Normal substrates

Nanogrooved substrates

Normal substrates

µsat (cm2/Vs)

Vth (V)

Ion/Ioff

PCDTPT

0.64 (±0.03)

+14.9 (±0.5)

103-104

TrTPFB (0.2% wt)

1.50 (±0.03)

+16.0 (±0.4)

103

PCDTPT

0.77 (±0.13)

+8.9 (±0.7)

106

TrTPFB (0.2% wt)

1.81 (±0.24)

+9.7 (±0.5)

105-106

DPPT-TT

0.38 (±0.02)

-10.8 (±2.0)

102

TrTPFB (0.4% wt)

0.34 (±0.01)

+4.4 (±0.8)

102-103

To see whether the enhancement of OFET performance by adding a small amount of TrTPFB salt into the semiconductor film is a generally valid method, we also tested poly(2,5-bis(2octyldodecyl)-3-(5-(thieno[3,2-b]thiophen-2,5-yl)thiophen-2-yl)-6-(thiophen-2,5-yl)pyrrolo[3,4c]pyrrole-1,4(2H,5H)-dione) (DPPT-TT).38 Figure 1(d) shows the transfer characteristics of


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pristine DPPT-TT and the DPPT-TT/TrTPFB (0.4% wt) FETs. The pristine DPPT-TT exhibits an ambipolar transport feature, which has been reported previously in the literatures.39 With TrTPFB added into the semiconductor film, the electron current is decreased while the hole current is increased significantly. This feature can also be clearly seen in the output curves of the DPPT-TT/TrTPFB (0.4% wt) FETs, from which it is seen that the electron current at low gate (Vg=0 V) is much smaller, whereas the hole mobility is almost unchanged (see Figure 1(d) and (f)). Additionally, the threshold voltage is reduced while the on/off ratio is improved. In Figure 1(g), it is seen that the threshold voltage shifts from -10.8 V in pristine DPPT-TT FETs to be about +4.4 V in DPPT-TT/TrTPFB (0.4% wt) FETs. Noticeably, the on/off ratio is about 5 times higher in the DPPT-TT/TrTPFB (0.4% wt) FETs, which can be attributed to a reduced ambipolar characteristic in these devices. One likely negative effect of incorporating the salt into organic semiconductor films is that the hysteresis may be increased due to trapping or ion drift effects. However, in our work this effect seems trivial, evidenced by a minute amount of hysteresis (Figure S2), probably due to the low weight ratio of the salt in the semiconductor films. Confirmation of organic salt doping To gain insight into the role of the salt in enhancing the OFET performance, UV–vis–NIR absorption

spectroscopy

was

employed

to

characterize

the

pristine

PCDTPT

and

PCDTPT/TrTPFB solutions (Figure 2(a)). The solution absorption spectra of PCDTPT/TrTPFB at various salt weight ratio up to 0.8% is almost the same as that of a pristine PCDTPT solution. However, when the weight ratio of TrTPFB is increased to 3%, a broad absorption in the infrared range (1000-1600 nm) is detected whereas the absorption peak at 868 nm is red-shifted along with a lowering of its amplitude. These features imply the formation of polarons in solution.40 At


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higher salt concentration, the absorption in the infrared range continues to increase and further red-shift of the absorption peak is observed. To confirm that the increased absorption between 1000 and 1600 nm originates from the polaron absorptions, we also inspected the absorption spectra of iodine-doped PCDTPT films. As indicated by the dashed line in Figure 2(a), a broad absorption at λ > 1000 nm is observed. Also the amplitude of the absorption peak is greatly reduced in the iodine-doped film compared to the pristine film. These observations support the claim that the observed absorption increase between 1000 and 1600 nm in the PCDTPT/TrTPFB solutions is from polarons. Similar observations were achieved in the PCDTPT/TrTPFB films (see Figure S3). The absorption spectra of DPPT-TT/TrTPFB solutions and films also show the evidence of polaron formation (see Figure S4). All results of UV–vis–NIR absorption spectra indicate that TrTPFB salt can dope PCDTPT and DPPT-TT. To further confirm that the TrTPFB salt dopes the polymer semiconductors, electron paramagnetic resonance (EPR) measurements were carried out. As seen in Figure 2(c), a PCDTPT/TrTPFB (20% wt) solution shows a very strong paramagnetic signal, consistent with the presence of unpaired electrons.41 Although we just show the EPR results of 20% wt TrTPFB sample, we expect that samples with lower weight ratios of TrTPFB to show similar results but just with weaker paramagnetic signal peaks. For comparison, no peak was observed in the pristine TrTPFB or PCDTPT solutions (see Figure 2(b) and Figure S5 for more details). The EPR results indicate charge transfer (CT) between the cation of TrTPFB and PCDTPT induces effective doping.41


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Figure 2 (a) The absorption spectra of PCDTPT solutions and the iodine-doped films. (b) EPR spectra (100 K) of PCDTPT/TrTPFB (20% wt) solutions. (c) The device structure diagram of Mott-Schottky diodes. Capacitance versus reverse bias plot of (d) PCDTPT and (e) DPPT-TT Mott-Schottky diodes. The charge carrier density is derivated from the slope according to equation (1).

The carbocation of TrTPFB is capable of accepting an electron to form a carbon radical. This provides the polymer an opportunity to donate an electron to a trityl cation, creating a radical cation on the polymer backbone (p-doping) and a neutral trityl radical. It is known that the trityl radical is quite stable, presumably avoiding unwanted side reactions occurring subsequent to electron

transfer,

which

may

interfere

with

charge

transport

processes.

The


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tetrakis(pentafluorophenyl)borate anion of TrTPFB will then be Coulombically attracted to the radical-cation on the polymer backbone to maintain charge compensation. This series of events is analogous to a molecular dopant, such as F4-TCNQ, with the exception that the salt provides a counter anion to electrostatically interact with the p-doped polymer backbone, instead of the resulting radical anion of a molecular dopant. It is currently unclear how important of a role the counter anion plays in favoring the electron transfer process, but it is possible that bulky nature of the counter anion can decrease the Coulombic binding energy of the complex, aiding in charge delocalization and thus charge transport. Conversely, because ions in a semiconductor film have an inherent mobility, just as charges do, it is possible that the movement of these ions could hamper charge transport processes. As previously mentioned, ion motion does not seem to play an important role in the systems studied here, as there is minimal hysteresis in the transfer curves of these OFETs (Figure S2). In order to see how efficiently TrTPFB can dope organic semiconductors, we conducted experiments to extract the background charge carrier density in pristine and doped semiconductor films. Mott-Schottky diodes with a structure of glass/PEDOT:PSS/Active layer/Al (see Figure 2(d)) were fabricated for this purpose. Al is used as a blocking contact to create a depletion zone. The well-known relation between the variation of the depletion capacitance Cd with bias voltage V for a given ionized dopant density, ND, is:27

= −

( )/

(1)

The dependence of the depletion capacitance (Cd) on the applied bias can be extracted using impedance spectroscopy measurements, and the resulting dependence of Cd-2 versus bias V is shown in Figure 2(e). The extracted charge carrier density for pristine PCDTPT and 0.2% wt TrTPFB doped films are 1.3×1016 and 2.2×1016 cm-3, respectively. With film density (about 1.02


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g/cm3 according to X-ray reflectivity measurements, Figure S6) and the molecular weight (50,000 Da) of PCDTPT known, the density of TrTPFB dopants and PCDTPT monomers in the film can be estimated, which are about 1.3×1018 cm-3 and 4.0×1020 cm-3, respectively. This gives a doping ratio (number of dopants per semiconductor monomer) of 3.4×10-3 and thus a doping efficiency (i.e., the ratio of the number of free charge carriers to the number of dopant molecules) of ～ 0.7% for TrTPFB in PCDTPT. Similar work was carried out on DPPT-TT films, and the corresponding doping ratio as well as doping efficiency is 4.3×10-3 and 1.5%, respectively. Though the doping efficiency is low, these values are still comparable to the doping efficiency of some common dopants for organic semiconductors such as F4-TCNQ, MoOx, etc.25, 26, 31, 42, 43

For instance, Lüssem et al. reported the efficiency of doping Meo-TPD with F4-TCNQ

is about 3-4% when the doping ratio is 3.24×10-2.26 Hamwi et al. reported a doping efficiency of 1.5% on MoO3 doped 4,4′,4′′-Tris[2-naphthyl(phenyl)amino]triphenylamine with a doping ratio of 5.6×10-2.42 Tietze et al. reported that the doping efficiency in pentacene/C60F36 thin-films is about 0.4% at a doping ratio of 4×10-3.31 Mechanism of the enhancement of OFET performance With the charge transfer doping to organic semiconductors by TrTPFB confirmed, it is expected that the charge transport or charge injection in the OFETs will be affected. Generally, doping organic semiconductors would result in favorable charge injection, thus improving the device performance of OFETs.21 Also, the charge carriers induced from doping could fill the tail states of organic semiconductors, and so passivate the traps and/or increase charge carrier mobility. In the following, we seek to understand how the TrTPFB salt doping enhances the performance of OFETs in our work.


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First, we inspected the influence of doping on contact resistance in the devices. The contact resistance was extracted by using the transfer line method (TLM),44 which is a common way for extracting contact resistance in OFETs (see Figure S8). Figure 3(a) shows the contact resistance of the PCDTPT and PCDTPT/TrTPFB (0.2% wt) FETs extracted at different Vg. It is seen that the contact resistance of PCDTPT/TrTPFB (0.2% wt) devices is almost unvaried compared to that of the pristine PCDTPT devices. Again, we also inspected the contact resistance in DPPTTT and DPPT-TT/TrTPFB (0.4% wt) FETs. In contrast to PCDTPT devices, it is found that the contact resistance in DPPT-TT/TrTPFB (0.4% wt) devices is significantly reduced compared to the pristine devices, as shown in Figure 3(b).


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Figure 3 Comparison of the contact resistance of (a) PCDTPT and PCDTPT/TrTPFB (0.2% wt) FETs; and (b) DPPT-TT and DPPT-TT/TrTPFB (0.4% wt) FETs. Temperature-dependent mobilities of (c) PCDTPT and PCDTPT/TrTPFB (0.2% wt) FETs; and (d) DPPT-TT and DPPTTT/TrTPFB (0.4% wt) FETs. The mobilities were all extracted in saturation regime (Vd = −80 V).


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Furthermore, temperature-dependent measurements were carried out to probe the effect of doping on charge transport. The temperature-dependent mobilities of both pristine PCDTPT FETs and PCDTPT/TrTPFB (0.2% wt) FETs are shown in Figure 3(c). The activation energy

was extracted according to Arrhenius equation: = (− ), in which is a prefactor and EA is the activation energy. As indicated in Figure 3(c), the PCDTPT/TrTPFB (0.2% wt) FETs have an activation energy that is about 10 meV lower compared to the pristine FETs (41.5 meV). In contrast, we find that the mobility of DPPT-TT/TrTPFB(0.4% wt) devices is very comparable to or only slightly higher than that of pristine DPPT-TT devices at low temperatures, and the activation energies in DPPT-TT and DPPT-TT/TrTPFB (0.4% wt) FETs are actually the same or very close (~ 39.6 meV), as seen in Figure 3(d). The very comparable or even lower activation energy in doped devices suggests that doping at such low ratios does not introduce enough energetic disorder to notably disturb charge transport in the semiconductors. In addition, the introduction of TrTPFB salt may induce film morphology or structure changes in the polymer semiconductor films, which can affect the device performance. To understand how the film morphology or structure of the polymer semiconductor is influenced by the TrTPFB salt, atomic force microscopy (AFM) and Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS) characterizations were carried out. AFM data shows that the PCDTPT/TrTPFB (0.2% wt) film is a little more grainy compared to the pristine film. The 0.2% wt TrTPFB doped PCDTPT film has a root mean square (RMS) of 2.30 nm while the RMS of pristine PCDTPT film is only 1.17 nm. For the DPPT-TT films, the morphology of the salt doped film (0.4% wt TrTPFB) is comparable with that of the pristine DPPT-TT film (Figure S9). The RMS of the pristine DPPT-TT film and the 0.4% wt TrTPFB doped film are 1.46 and 1.56 nm, respectively. GIWAXS results show that neither the alkyl chain stacking nor the pi-pi


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stacking is notably changed by the incorporation of the TrTPFB salt (Figure S10). Also the crystallite correlation lengths (CCL) of the films with TrTPFB salt are very comparable to that of the pristine films for both PCDTPT and DPPT-TT (see Figure S11 and Table S1 in the supporting information). The GIWAXS results therefore suggest that the higher performance of OFETs with TrTPFB is unlikely to be caused by the crystallinity improvement in the films. The results also imply that the grainy structures we see in the PCDTPT/TrTPFB (0.2% wt) films may be just bundles of small grains, which have no oriented alignment over long distance.

On the basis of above experimental results, we now discuss the mechanism of performance enhancement in TrTPFB salt doped OFETs. For PCDTPT, it has been seen that the salt doping reduces the activation energy of devices by about 10 meV. This activation energy reduction can potentially account for the mobility increase in the TrTPFB-doped devices, even though the contact resistance is marginally affected. As to DPPT-TT, the reduction of contact resistance after doping may explain the lowering of threshold voltages in those DPPT-TT/TrTPFB (0.4% wt) devices. Previously, it has been reported that ions can inhibit the torsion of the branching alkyl chains, which leads to improvement of the degree of edge-on lamellar packing of the alkyl side chains and interchain pi-pi stacking.45 However, we do not see detectable changes in the lamellar or pipi stacking according to GIWAXS results in our systems. Although the effect of ions on inhibiting the torsion of the branching alkyl chains may still exist in our systems, which can be beneficial to charge transport,46 this effect alone cannot explain why the hole-current is enhanced while the electron-current is deteriorated, as observed in the DPPT-TT/TrTPFB (0.4% wt) OFETs. Therefore, we argue that doping is the main mechanism, if not the only, for improving


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the p-type performance of OFETs. Only in this way can it be explained why the current of one charge carrier is increased while the other is decreased in TrTPFB-doped OFETs. Another intriguing question to answer is why doping results in different behaviors between PCDTPT and DPPT FETs. Specifically, in PCDTPT/TrTPFB (0.2% wt) FETs the mobility is increased, but the threshold voltage is almost unchanged, while in DPPT-TT/TrTPFB (0.4% wt) FETs the mobility is not notably affected but instead the threshold voltages are greatly reduced. One likely explanation is that this is caused by the difference in the electronic structures of the two organic semiconductors. Specifically, the density of traps states or bandgap states might be low in PCDTPT and so the Fermi level can be shifted closer to the HOMO or transport band as extra carriers are introduced (See Figure 4). This Fermi level shift can then explain the increase of mobility and also the decrease of activation energy as seen in the TrTPFB-doped FETs according to the theory of multiple trapping and release (MTR) model.47-49 In fact, we used Kelvin probe force microscopy (KPFM) to characterize the Fermi level in the PDCTPT films with different TrTPFB doping concentrations, and we do see that the Fermi level shifts closer to HOMO as the concentration of TrTPFB increases (see Figure S12). Conversely, DPPT-TT may have a high density of trap states and thus that the Fermi level pinning occurs when the doped charge carriers fill the trap states. Consequently, the mobility is not improved much after doping. But this trap filling can still reduce the threshold voltage and so the contact resistance of OFET devices.29 In any case, the holes induced by doping can get recombined by electrons, which reduces the electron current and thus results in a higher on/off ratio.


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Figure 4 The diagram of density of states (DOS) in the PCDTPT and DPPT-TT. For PCDTPT, the density of tail states is low and so the Fermi level can be shifted closer to HOMO when extra holes fill the DOS. In comparison, the DPPT-TT has a higher density of tail states and so the Fermi level is pinned.

Organic salt doping for complementary circuits In addition to enhancing the p-channel performance of OFETs, we also tried to enhance the nchannel performance of OFETs by using this organic salt doping method. For this purpose, another salt tetra-n-butylammonium bromide (TBABr) was selected as it was previously reported to be an electron dopant.41 We take PCDTPT as the host semiconductor again for n-doping since donor-acceptor conjugated polymers are generally ambipolar in nature.50,

51

Also we are

interested in seeing if we can achieve both the p- and n-channel performance-enhanced OFETs just by doping the same semiconductor material with different dopants. As seen in Figure 5(a) and (b), TBABr doping can enhance the n-channel performance while deteriorating the pchannel performance of PCDTPT FETs. When suitable dielectric and electrodes are selected, the 1%wt TBABr doped PCDTPT FETs can exhibit electron-dominant transport with an electron mobility of 0.2 cm2/Vs (see more in the supporting information, Figure S13).


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Figure 5 The transfer (a) and output (b) characteristics of PCDTPT and PCDTPT/TBABr (1% wt) FETs fabricated in a bottom-gate, botttom-contact (BG-BC) configuration. ODTS-treated SiO2 substrates were used for device fabrication and Au were used for source/drain electrodes. (c) The transfer characteristics of DPPT-TT, DPPT-TT/TrTPFB (0.4% wt) (top) and N2200/TBABr (1% wt) (bottom) FETs fabricated in a top-gate, bottom-contact (TG-BC) configuration with CYTOP as the dielectric. (d) The transfer and gain curves of the inverter composed of pristine DPPT-TT and N2200 OFETs. The inset shows a schematic diagram of an


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inverter comprising of a p-channel and an n-channel FET. (e) The transfer and gain curves of the inverter composed of DPPT-TT/TrTPFB (0.4% wt) and N2200/TBABr (1% wt) OFETs.

These results indicate that salt doping is a generally valid method for both p-channel and nchannel OFETs. This method is especially applicable for the recently highly-researched donoracceptor copolymers because the performance of one carrier can be improved while the other is inhibited concomitantly, resulting in a high on/off ratio in the doped OFETs. More importantly, the results indicate that the method of controlling the polarity of an individual organic semiconductor by p- or n-doping to realize complementary organic circuit is promising. With this idea in mind, we employed the salt doping method to fabricate simple complementary inverters,52 which are expected to have improved performance due to the enhanced performance of each component OFET. DPPT-TT and N2200 were selected as p- and n-channel semiconductors, respectively, and correspondingly TrTPFB and TBABr were used for p- and n-doping, respectively. Figure 5(c) shows the comparison of electrical properties of the DPPT/TrTPFB (0.4% wt) and N2200/TBABr (1% wt) FETs which were fabricated in top-gate, bottom-contact (TG-BC) configuration. Apparently, the p-channel performance in the TrTPFB doped DPPT-TT FET is much better compared to the pristine DPPT-TT FET. The electron current is greatly inhibited in the TrTPFB-doped devices, leading to almost unipolar transport and a better on/off ratio. As to N2200 FETs, the electron current in N2200/TBABr (1% wt) devices is one order of magnitude higher compared to the pristine N2200 device. Additionally, it is observed that the off-current in the N2200/TBABr (1% wt) devices is reduced dramatically due to the decreased hole current. It is worth noting again that this performance enhancement in


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the doped devices is mainly caused by the shift of threshold voltage rather than the increase of mobility. Figure 5(d) and (e) shows the performance of a complementary inverter based on pristine OFETs and salt-doped OFETs, respectively. Obviously, the performance of the inverter is much improved in terms of the Vout range and gain values when each component transistor is doped. The results demonstrate that doping organic semiconductors with suitable dopants for achieving high-performance p- and n-channel OFETs towards CMOS technology is a very promising method.

CONCLUSIONS We have shown a method for enhancing the performance of solution-processed OFETs by incorporating an organic salt into the semiconductor films. It was found that organic salts such as TrTPFB and TBABr, can greatly enhance the p-channel and n-channel performance of OFETs, respectively, in terms of mobility increase or threshold voltage reduction. Systematic characterizations have revealed that TrTPFB can dope organic semiconductors, and the doping efficiency is comparable to the common dopants, such as F4-TCNQ. Organic salt doping is confirmed to be the major reason for enhancing OFET performance, since doping can lower the activation energy or contact resistance of OFET devices and thus increase the mobility or decrease the threshold voltages, respectively. Especially, the ambipolarity of organic semiconductors that are commonly seen in low-bandgap D-A copolymers can also be inhibited with this effect. This organic salt doping method is simple and completely compatible with solution-processing method, which provides a convenient route to enhance the performance of solution-processed OFETs. Our work has demonstrated that it is promising to boost the


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performance of OFETs by exploring other organic salts or the combination of polymer semiconductors and organic salts .

METHODS Bottom-gate, bottom-contact OFETs: Highly doped silicon wafers (0.001–0.005 Ω cm−1) with 300 nm of thermally grown SiO2 were used as substrates. Standard lithography procedures were used to pattern the wafers with gold electrodes of 50 nm using 5 nm of nickel as an adhesion layer.

The

substrates

were

then

treated

by

oxygen

plasma

and

passivated

by

trichloro(octadecyl)silane to reduce the traps. All semiconductor materials and organic salts were dissolved in chloroform and then mixed to make a solution of 2.5 mg/ml. The semiconductor films were deposited by spin-coating at 2000 rpm in a nitrogen glovebox. Following that the devices were annealed before testing. The thermal annealing temperatures for PCDTPT, DPPTTT, and N2200 (P(NDI2OD-T2)) are 200 oC, 200 oC, 150 oC, respectively. Top-gate, bottom-contact OFETs: Bottom-contacts and semiconductor films were prepared in the same way as mentioned above. CYTOP was spin-coated on top of the semiconductors and then annealed at 90 oC for 20 minutes to form the dielectrics. Finally, gate electrodes were defined by evaporating 30 nm Al through a shadow mask. Mott-Schottky diodes and Hole-only diodes: Mott-Schottky diodes were fabricated using a structure of ITO (140nm)/PEDOT:PSS (35 nm)/active layer (about 150 nm)/Al (100 nm). Holeonly devices were fabricated with the structure of ITO (140nm)/PEDOT:PSS (35 nm)/active layer (about 150 nm)/Au (30 nm).


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Device characterization: Electrical properties of OFETs were characterized in dry nitrogen glove box using a Keithley 4200. Temperature dependent measurement was carried out in a Lakeshore cryo probe station. UV-vis-NIR absorption and EPR: UV–vis–NIR absorption spectroscopy was measured using a Lambda 750 UV/VIS Spectrometer (Perkin Elmer). EPR experiments were carried out in a Bruker EMXplus EPR Spectrometer. The measurements were done at 100K. Impedance spectroscopy: Impedance spectroscopy measurement was carried out using a Solartron 1260 impedance analyzer, applying an AC frequency of 100Hz, with DC biases from 1.5 to -2 V. The amplitude dependence of the real and imaginary components of impedance was then used to determine the spectral capacitance. AFM and KPFM: The film morphology was measured by Innova AFM operated in a tapping mode. KPFM measurements were done by a MFP-3D Infinity AFM system (Asylum Research). A Pt/Ir coated tip with resonant frequency f≈275 kHz was used for surface potential measurement. Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS):Films were cast directly onto silicon substrates with approx. 200 nm native oxide layer, which were prepared as the ITO-glass described above. Measurements were performed at the Advanced Light Source at Lawrence Berkeley National Lab on the 7.3.3 beamline. Samples were scanned for 2 to 10 s at an incidence angle of 0.12° or 0.14°, and a photon energy of 10 keV (λ = 1.24 Å), while under a helium environment to minimize beam damage and reduce air scattering. The width of the incident x-ray beam is about 1 mm and silver behenate was used to calibrate the lengths in the reciprocal space. A 2D detector (PILATUS 2M from Dectris) with a sample-to-detector distance of 305.4 mm was


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used to collect the images. The Nika software package for Igor (by Wavemetrics) was used to process the images.

ASSOCIATED CONTENT Supporting Information Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author E-mail:[email protected]

ACKNOWLEDGMENT We thank the National Science Foundation (NSF-DMR#1411349) for the support. Z. D. R. thanks the National Science Foundation Graduate Research Fellowships Program under Grant No. 1650114. We thank B. Yurash for help with X-ray reflectivity measurements and S. Walker for help with EPR measurements. EPR was conducted in the MRL Shared Experimental Facilities at UCSB, which are supported by the MRSEC Program, a member of the NSF-funded Materials Research Facilities Network, under grant NSF DMR 1121053.

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Doping Polymer Semiconductors by Organic Salts: Toward High

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