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Hanul Moon, Hyunsu Cho, Mincheol Kim, Kazuo Takimiya, and Seunghyup Yoo* Transparent thin-film transistors (TFTs) have attracted great attention as an essential building block in transparent electronic systems, the unique properties of which are expected to greatly extend the portfolio of modern electronic applications.[1–3] For successful realization of transparent TFTs, it is critical to secure transparent semiconductors with suitable electrical properties. Until now, transparent semiconductors and transistors have been based mainly on oxide semiconductors such as ZnO, SnO, In2O3, or their three- or four-element compounds because of their large energy gaps (Eg).[2–4] However, wide-gap oxide semiconductors are predominantly n-type, and thus it is challenging to realize complementary inverters — indispensable elements in modern electronics — with oxide semiconductors alone.[5,6] In that respect, organic semiconductors (OSCs) can play an important role in a hybrid approach using oxide semiconductors for n-channels and OSCs for p-channels[6] or alternatively in a purely organic-based approach. Furthermore, the excellent mechanical flexibility of OSCs can lead to even more versatile electronic systems when combined with optical transparency. For these reasons, several groups have worked on transparent organic TFTs (TOTFTs),[7–10] and have demonstrated transparent functional devices based on TOTFTs, such as inverters, memories, and sensors.[6,11,12] Most of the TOTFTs reported to date, however, are actually translucent and typically have a particular tint of color. This is because the organic channel layers used in those systems have considerable absorption in the visible range; for example, pentacene films exhibit a bluepurple color[6,8,9,11,12] and poly(3-hexylthiophene) (P3HT) films look reddish.[10] This drawback originates from the limited Eg of those OSCs; this Eg is typically in the range of 1.5–2.5 eV, resulting in absorption of photons in a certain spectral band of the visible range. For full transparency, an Eg larger than 3 eV is generally required.[13,14] It should be noted that it is challenging to develop such a wide-gap OSC that simultaneously has excellent electrical performance; this is because the Eg of conjugated organic H. Moon, H. Cho, M. Kim, Prof. S. Yoo Department of Electrical Engineering Korea Advanced Institute of Science and Technology (KAIST) Daejeon, 305–701, Republic of Korea E-mail: [email protected] Prof. K. Takimiya Department of Applied Chemistry Hiroshima University Higashi-Hiroshima, 739–8530, Japan
DOI: 10.1002/adma.201305440
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Towards Colorless Transparent Organic Transistors: Potential of Benzothieno[3,2-b]benzothiophene-Based Wide-Gap Semiconductors
compounds generally becomes small when the number of conjugated units increases, so that π and π* bands become broadened to allow for better electrical properties.[1,15–18] For instance, the Eg of oligoacene with N benzene rings (N = 2, 3,…) scales down with N from 4.84 eV (N = 2) to 2.17 eV (N = 5), according to calculation based on density-functional theory,[16] and thus only naphthalene (N = 2) and anthracene (N = 3) are fully transparent with no color. On the other hand, a carrier mobility exceeding or comparable to that of hydrogenated amorphous silicon (a-Si:H) has been reported only with N ≥ 4.[19,20] Pentacene (N = 5), shown in Figure 1a, is a good example of a low-Eg oligoacene with high carrier mobility.[20] The mobility of anthracene, a representative high-Eg oligoacene, is far below that of pentacene,[19,21] illustrating the challenge in realizing a conjugated organic compound having both high Eg and superior electrical properties. The trend is similar for polymeric materials; relatively wide-gap polymer semiconductors have also been studied for both n-channels and p-channels of TOTFTs,[15,22,23] but Eg of those polymers was not large enough to achieve full transparency in the visible range. In addition, the mobility of those wide-gap polymers was far below that of a-Si:H. With such difficulties in mind, we here try to find a new candidate for fully transparent, colorless channel layers in the OSCs recently developed for environmentally stable, high-performance channel layers. In fact, the strategy used to develop air-stable OSCs relies heavily on finding or synthesizing organic compounds with high Eg, as this can prevent degradation resulting from photo-oxidation. In practice, this goal is the same as that in developing transparent OSCs.[15,16] Benzothieno[3,2-b]benzothiophene (BTBT) derivatives have been among the most successful OSCs developed for such purposes;[24–26] the BTBT core exhibits an Eg greater than 3.5 eV in a dilute solution, although it consists of four aromatic rings like tetracene, whose Eg is approximately 2.5 eV. This makes derivatives based on the BTBT core tend to have a higher Eg than oligoacenes or their derivatives.[16] The fact that the BTBT core has a higher optical gap than tetracene may be attributed to its structural similarity to chrysene, which is a hydrocarbon with four fused aromatic rings like tetracene but which has its aromatic rings arranged in a “zig-zag” fashion. Such a nonlinear molecular arrangement is known to yield higher excitedstate energy than the linear arrangement of the aromatic rings in tetracene.[27] Dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) and 2,7-diphenyl[1]benzothieno[3,2-b]benzothiophene (DPh-BTBT), representative BTBT derivatives shown in Figure 1a, can be regarded as promising candidates for high-performance TOTFTs because they are known to have a mobility on the order of 1 cm2 V−1 s−1
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applications where the fractional area of the channel layer is relatively small compared to the total area; this is because the average transmittance spectrum (and thus the perception of color) is given by the sum of the transmittance spectra of participating components weighted by their fractional area. (See Figure 1c for photographs of samples in which pentacene and DNTT layers cover only 30% of the total area and other parts have no layers other than their substrates.) However, as the composition of components and their respective fractional areas may vary over a wide range depending on application, full transparency of active channels will still be beneficial in that it allows a higher degree of freedom in the overall design and processing. For example, the transmittance or areal requirement for signal and power lines will be lessened for a given target average transmittance or color neutrality if TFT parts are highly transparent and colorless. In another example, channel materials may cover a significant portion of the circuit in volume-produced printed electronic devices owing to the limited patterning resolution and registration accuracy; in such a case, full transparency of channel layers will be critical in realizing highly transparent, colorless devices. To utilize DPh-BTBT as channels for TOTFTs, transparent source/drain (S/D) electrodes forming ohmic electrical contacts with these channels are required. While varFigure 1. Pentacene, DNTT, and DPh-BTBT. a) Chemical structures of the molecules. ious transparent S/D electrodes have been b,c) Transmittance as a function of wavelength λ (b) and Photographs (c) of (40 ± 10) nm-thick reported for TOTFTs, such as conducting films deposited on glass substrates. For pentacene and DNTT, photographs are also shown oxides,[6,7,11] conducting polymers,[12] nano(bottom row) for samples having a partial coverage according to the channel pattern shown at [8–10] or thin metal-based multithe bottom right. The fractional area of the organic channels (shown as small orange-colored materials, [28] layers, S/D electrodes achieving all the rectangles) with respect to the rectangle shown by a dashed line is approx. 30%. requirements for transparency, conductivity, injection, and processability are often hard to find. In particular, and Eg (measured in dilute solution) values of 3.0 and 3.2 eV, finding such electrodes is even more challenging considering respectively.[24,25] However, it is noteworthy that the Eg in solidthat DPh-BTBT has a deep highest occupied molecular orbital state films is in general smaller than that in the solution phase (HOMO) level. (5.77 eV from UV photoelectron spectroscopy due to the spectral broadening associated with enhanced inter(UPS) measurement. For comparison, those of DNTT and penmolecular interaction in the condensed phase.[24] Figure 1b,c tacene measured by UPS are reportedly 5.44 eV and 4.85 eV, presents the UV/vis transmittance spectra and photographs of respectively.).[24,29,30] According to our experiment using an (40 ± 10) nm-thick films of pentacene, DNTT, and DPh-BTBT on glass substrates. As expected, both DNTT and DPh-BTBT have opaque top-contact TFT configuration, shown in Figure 2a, even absorption peaks (i.e., transmittance dips) that are significantly Au is not suitable for S/D electrodes owing to its limited injecblue-shifted compared to those of pentacene. From the absorption ability (Figure 2b). Instead, transition-metal-oxide (TMO) tion edge in Figure 1b, the Eg values of these OSCs in the film layers of WO3 or MoO3 turn out to be efficient hole-injection state are estimated to be 1.76, 2.54, and 3.01 eV for pentacene, layers for DPh-BTBT TFTs; TFT devices with Al/TMO bilayer DNTT, and DPh-BTBT, respectively (see Figure S1 in the SupS/D electrodes exhibit much better performance than devices porting Information for details). Note that the solid-state DNTT with Au S/D electrodes, as can be seen in Figure 2b. The satufilm exhibits an Eg significantly below 3 eV and thus looks pale ration mobility (μSat) of the DPh-BTBT TFTs with Al/TMO S/D yellow, making its application as a colorless channel layer limelectrodes and a Cytop (Asahi Glass) dielectric surface is up to ited. On the other hand, DPh-BTBT fulfills the condition of 0.8 cm2 V−1 s−1, which is higher than the value reported previ“Eg ≥ 3 eV” even in the solid-state film, and thus is suitable for ously for the DPh-BTBT TFTs with Au S/D electrodes and an octyltrichlorosilane (OTS)-treated dielectric surface, in which the colorless, fully transparent channels desired for TOTFTs in DPh-BTBT was deposited without substrate heating, as was also a wide range of applications. It is noteworthy that DNTT or even the case for this work.[24] pentacene may also be applicable to some transparent electronic
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extinction coefficient or the imaginary part of the complex refractive index. Such an optical requirement in fact makes Al ineligible. Having a uniform continuous metal film is also critical; otherwise, absorption could be increased owing to unwanted effects such as surface plasmon effects.[35,36] As for the other components in the proposed TFTs, a patterned 150 nm-thick ITO layer is used as a gate electrode, and a 30 nm-thick cross-linked Cytop (C-Cytop) layer prepared by spin coating is adopted as a gate dielectric layer to ensure low-voltage operation and to provide trap-free dielectric interfaces.[37] The surface of the C-Cytop layer turns out to be very smooth, with root mean square (RMS) roughness as low as 0.8 nm, as shown in its atomic force microscopy (AFM) Figure 2. a) Structure of the conventional opaque DPh-BTBT TFTs with various types of S/D image (Figure 2d). The DPh-BTBT channel layer thickness is set at 30 nm so that it is thin contact. b) Their transfer characteristics. c) Structure of the proposed transparent DPh-BTBT TFTs. The numbers 1–4 indicate four optically distinct positions under consideration. d,e) Surenough to maintain low off current yet thick face morphologies of the C-Cytop layers deposited on ITO electrodes (d) and the DPh-BTBT enough to fully cover the channel region. layers deposited on those C-Cytop layers (e). The AFM image presented in Figure 2e shows densely packed grains, as desired. As can be seen in Figure 2c, the proposed TOTFT comOn the basis of these results, we employed ZnS/Ag/WO3 prises four optically distinct positions with different multilayer (ZAW) trilayers as transparent S/D electrodes, as presented in structures (positions 1–4). Hence the TOTFT structure should Figure 2c, which provides a schematic diagram of the proposed be carefully chosen so that all four positions can have a sufTOTFTs. In the ZAW electrode, an ultrathin Ag layer (a 15 nmficiently high transmittance. To design optimum TOTFTs, we thick Ag layer in this study) enables efficient lateral conduction, used luminous transmittance (TLum), or the average transmitan outer dielectric layer (ZnS) covering the Ag layer enhances the transparency using a destructive interference effect,[31–33] tance weighted for human photopic response,[36] as a figure of and an inner dielectric layer (WO3) provides efficient hole injecmerit for transparency for a given set of WO3 layer thickness tion into the channel from the Ag electrode. It was previously (d WO3 ) and ZnS layer thickness (dZnS). Among the four posireported that hole injection from WO3 or MoO3 was similar tions, the regions including ZAW electrodes (positions 3 and 4) need special attention because they contain Ag layers and thus regardless of the metal layers used together.[34] It is known that may be subject to transmittance reduction due to reflection in order to maximize the destructive interference effect, the and/or absorption. Figures 3a and b, which show the TLum calhigh refractive index of the outer dielectric layer (nDo) is critical;[32] ZnS is chosen because it has a refractive index as high as culated at positions 3 and 4 as a function of dWO3 and dZnS, indi2.4 in the visible range and can be deposited easily by thermal cate that TLum at positions 3 and 4 can indeed vary sensitively evaporation without damaging underlying layers (Figure S2 in over dZnS (and relatively mildly over d WO3 ) in a wide range from the Supporting Information).[31,33] The refractive index of the 30%–50% to over 80%. When one considers TLum at the two inner dielectric layers (nDi) in contact with the organic layers positions 3 and 4, the optimal dZnS and d WO3 leading to maxcan also be important, but the transmittance typically responds imal TLum can be set at 35 ± 7 nm and 50 ± 20 nm, respectively. more sensitively to the thickness of the outer dielectric layer Note that the ZnS layer could be deposited over all the posi(dDo) and can be made sufficiently high as long as nDi is not tions because it is insulating and does not play any electrical role. TLum, calculated as a function of dZnS for positions 1 and 2, much smaller than that of the organic layers.[32,35] The refractive index of WO3 is approximately 2.1 in the visible range however, shows that these positions would be better off without the ZnS layer (Figure 3c). This is because the high refractive (Figure S2, Supporting Information), slightly higher than that index of the ZnS layer makes it function effectively as a highof typical organic layers. In transparent electrodes of this type, reflection (HR) coating when it is deposited on top of dielectric the properties of a metal layer are also very important for both or organic layers that have refractive indexes lower than that of optical and electrical reasons. Note that one generally prefers the ZnS layer.[38] The transmittance dip shown in Figure 3c cora thicker metal layer for lower sheet resistance and a thinner metal layer for lower absorption and thus for higher transmitresponds to the case in which the effect of the HR coating is tance. For example, the optimal range for Ag in these applimaximized owing to constructive interference. cations is typically found to be 10–20 nm. In this case, sheet Figure 3d–f presents the measured and calculated transmitresistance less than 10 Ω sq−1 and transmittance comparable to tance spectra for several (d WO3, dZnS) values near the optimum that of indium tin oxide (ITO) are possible.[31] Another requirecondition. The measured transmittance spectra (shapes) are shown to match well with the calculated results (solid lines) for ment is low refractive index (n). This is because absorption in all the cases under study, indicating that the optical design and a given layer is proportional to its nk-product, where k refers to
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Figure 3. a,b) Calculated TLum of position 3 (a) and position 4 (b) vs. dZnS and d WO 3. c) Calculated TLum of positions 1 and 2 vs. dZnS. d) Measured (shapes) and calculated (solid lines) transmittances (T) of positions 1 and 2 without ZnS layers on top. Inset: Top view of the patterns and a photograph of a sample composed of positions 1 and 2 fabricated on a glass substrate. It contains all the patterns (except for S/D electrodes) including ITO gate electrodes, dielectric layers, and channel layers. e,f) Measured (shapes) and calculated (solid lines) transmittances (T) and photographs of position 3 (e) and position 4 (f) with dZnS of 0, 25, 35, and 45 nm while d WO 3 was fixed at 50 nm.
fabrication processes worked as expected. Both positions 1 and 2 exhibit transmittances over 85% and mostly up to 90% over all of the visible range. The inset images of Figure 3d depict the active and gate patterns and provide a photograph of the corresponding sample fabricated on a glass substrate. All regions on the substrate are highly transparent and thus are not distinguishable from one another. To the best of our knowledge, this is the first demonstration of a fully transparent channel region for TOTFTs whose transparency is comparable to that of transparent oxide TFTs.[39] As can be seen in Figure 3e,f, positions 3 and 4 look translucent in dark-gray without ZnS layers and exhibit transmittances of around 50%. With the optimal values of dZnS and d WO3 , the transmittance at positions 3 and 4 dramatically increase to over 70% over most of the visible range and to 80% in the peak region, thus resulting in a highly transparent appearance. The wavelength for the peak transmittance tends to red-shift as dZnS increases from 25 nm to 45 nm; however, the spectral envelope is broad enough to cover the full visible spectral range in all cases, and thus no noticeable interference-induced tint is observed. The transmittance spectra of the fully optimized TOTFT ( d WO3 of 50 nm and dZnS of 35 nm) are shown in Figure 4a. As can be seen in Figure 4a, the resultant TOTFT is colorless and highly transparent, and the patterns, even including that of the S/D region, can hardly be recognized. From an electrical perspective, the proposed TOTFTs exhibit low-voltage operation that is switchable under 3 V with significantly low gate leakage current, saturation mobility of ca. 0.9 cm2 V−1 s−1, threshold voltage of ca. –0.84 V, subthreshold swing of ca. –0.17 V decade−1, and on-to-off drain current ratio
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above 106, as shown in its transfer (Figure 4b) and output characteristics (Figure 4c). Ideal output characteristics without current crowding behavior in the linear region are attributed to low contact resistance between DPh-BTBT channel layers and ZAW S/D electrodes provided by WO3 hole-injecting layers (Figure S3 in the Supporting Information). The low voltage operation and high on/off ratio are attributed to the 30 nm-thick C-Cytop dielectric layers, which show a capacitance density of 55.7 nF cm−2 and leakage current density under 10−8 A cm−2 for a voltage bias under 3 V (Figure 4d). The ideal, hydroxyl-group-free interface[37,40] provided by the C-Cytop dielectric layers leads also to negligible hysteresis in both transfer and output characteristics and to a relatively good operational stability under the constant voltage stress test (CVST) in a diode connection (VG =VD); 85% of the initial ID is preserved after the CVST, with a stress bias (VG.str) of –3 V for 3000 s (Figure 4e). The stress bias used here corresponds to approx. 1 MV cm−1 and was chosen to be large enough to clearly define the “ON”-state. These results are comparable to previously reported stable OTFTs with C-Cytop dielectric layers.[37,40] In conclusion, we have fabricated colorless and highly transparent OTFTs using DPh-BTBT layers as wide-gap organic channels and ZnS/Ag/WO3 layers as transparent S/D electrodes with a good hole injection into the relatively deep HOMO level of the DPh-BTBT layer. The optimum TOTFTs were designed through calculations based on thin-film optics; the resultant devices showed transmittances of over 85% in the channel area and more than 70%–80% in the S/D area over most of the visible range. With the help of the ultrathin C-Cytop dielectric layers and polycrystalline DPh-BTBT channel layers,
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stable, low-voltage operation was also realized with a mobility of ca. 0.9 cm2 V−1 s−1. Together with the superior mechanical flexibility of metal-based multilayer transparent electrodes that had previously been demonstrated,[31] the proposed TOTFTs could serve as a versatile building block in various next-generation electronic devices.
Electrical characterization of devices: The capacitance and the leakage current of the metal–insulator–metal (MIM) devices were measured using a precision LCR meter (HP4284A, Agilent) and a semiconductor parameter analyzer (HP4156A, Agilent), respectively, in ambient air. The electrical characteristics of the TFTs were measured using a semiconductor parameter analyzer (HP4155A, Agilent) in a N2-filled glove box.
Experimental Section
Supporting Information
Fabrication of devices: 150 nm-thick ITO layers precoated on glass substrates were patterned as gate electrodes by conventional photolithography and wet-etching processes. Cytop solution (CTL-809M, Asahi Glass, Chiyoda, Japan) was diluted by mixing it with appropriate solvent (CTsolv-180, Asahi Glass) at 1:4 ratio by volume (CTL-809M : CTsolv-180). Then, cross-linking agent (C6-Si) was added to the diluted Cytop solution (10 µL mL−1), followed by mixing on a hotplate at 100 °C with a stirring bar in a N2-filled glove box. 30 nm-thick crosslinked Cytop (C-Cytop) layers were deposited on the glass substrate with ITO electrodes by spin-coating (500 rpm for 10 s and then 2000 rpm for 20 s); these samples were annealed on a hotplate at 180 °C for an hour in ambient air. The fabrication of the devices was completed by sequential evaporation of the DPh-BTBT layer and the ZnS/Ag/WO3 multilayer stack through appropriate shadow masks under vacuum conditions (10−6 Torr) in a thermal evaporation chamber (HS-1100, Digital Optics & Vacuum, Seoul, Korea). Calculation and measurement of transmittance: The transmittances were calculated using the Essential Macleod (Thin Film Center, Tucson, AZ) program based on thin-film optics; the luminous transmittances were calculated using a customized MATLAB code. The refractive indices (n) and the extinction coefficients (k) of the layers composing the devices were obtained by spectroscopic ellipsometry (Figure S2, Supporting Information).[41,42] The transmittances were measured in ambient air using a UV-vis spectrometer (SV2100, K-MAC, Daejeon, Korea).
Supporting Information is available from the Wiley Online Library or from the author.
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Acknowledgements This research was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP)(CAFDC/ Seunghyup Yoo/No. 2007–0056090); a grant (Code No. 2013073183) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Education, Science and Technology, Korea; and the Graphene Research Center Program of KI for the Nanocentury, KAIST. The authors are also grateful to Nippon Kayaku Co. for providing the DNTT and DPh-BTBT materials. Received: November 1, 2013 Revised: December 23, 2013 Published online: February 3, 2014 [1] [2] [3] [4]
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Hanul Moon, Hyunsu Cho, Mincheol Kim, Kazuo Takimiya, and Seunghyup Yoo* Transparent thin-film transistors (TFTs) have attracted great attention as an essential building block in transparent electronic systems, the unique properties of which are expected to greatly extend the portfolio of modern electronic applications.[1–3] For successful realization of transparent TFTs, it is critical to secure transparent semiconductors with suitable electrical properties. Until now, transparent semiconductors and transistors have been based mainly on oxide semiconductors such as ZnO, SnO, In2O3, or their three- or four-element compounds because of their large energy gaps (Eg).[2–4] However, wide-gap oxide semiconductors are predominantly n-type, and thus it is challenging to realize complementary inverters — indispensable elements in modern electronics — with oxide semiconductors alone.[5,6] In that respect, organic semiconductors (OSCs) can play an important role in a hybrid approach using oxide semiconductors for n-channels and OSCs for p-channels[6] or alternatively in a purely organic-based approach. Furthermore, the excellent mechanical flexibility of OSCs can lead to even more versatile electronic systems when combined with optical transparency. For these reasons, several groups have worked on transparent organic TFTs (TOTFTs),[7–10] and have demonstrated transparent functional devices based on TOTFTs, such as inverters, memories, and sensors.[6,11,12] Most of the TOTFTs reported to date, however, are actually translucent and typically have a particular tint of color. This is because the organic channel layers used in those systems have considerable absorption in the visible range; for example, pentacene films exhibit a bluepurple color[6,8,9,11,12] and poly(3-hexylthiophene) (P3HT) films look reddish.[10] This drawback originates from the limited Eg of those OSCs; this Eg is typically in the range of 1.5–2.5 eV, resulting in absorption of photons in a certain spectral band of the visible range. For full transparency, an Eg larger than 3 eV is generally required.[13,14] It should be noted that it is challenging to develop such a wide-gap OSC that simultaneously has excellent electrical performance; this is because the Eg of conjugated organic H. Moon, H. Cho, M. Kim, Prof. S. Yoo Department of Electrical Engineering Korea Advanced Institute of Science and Technology (KAIST) Daejeon, 305–701, Republic of Korea E-mail: [email protected] Prof. K. Takimiya Department of Applied Chemistry Hiroshima University Higashi-Hiroshima, 739–8530, Japan
DOI: 10.1002/adma.201305440
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Towards Colorless Transparent Organic Transistors: Potential of Benzothieno[3,2-b]benzothiophene-Based Wide-Gap Semiconductors
compounds generally becomes small when the number of conjugated units increases, so that π and π* bands become broadened to allow for better electrical properties.[1,15–18] For instance, the Eg of oligoacene with N benzene rings (N = 2, 3,…) scales down with N from 4.84 eV (N = 2) to 2.17 eV (N = 5), according to calculation based on density-functional theory,[16] and thus only naphthalene (N = 2) and anthracene (N = 3) are fully transparent with no color. On the other hand, a carrier mobility exceeding or comparable to that of hydrogenated amorphous silicon (a-Si:H) has been reported only with N ≥ 4.[19,20] Pentacene (N = 5), shown in Figure 1a, is a good example of a low-Eg oligoacene with high carrier mobility.[20] The mobility of anthracene, a representative high-Eg oligoacene, is far below that of pentacene,[19,21] illustrating the challenge in realizing a conjugated organic compound having both high Eg and superior electrical properties. The trend is similar for polymeric materials; relatively wide-gap polymer semiconductors have also been studied for both n-channels and p-channels of TOTFTs,[15,22,23] but Eg of those polymers was not large enough to achieve full transparency in the visible range. In addition, the mobility of those wide-gap polymers was far below that of a-Si:H. With such difficulties in mind, we here try to find a new candidate for fully transparent, colorless channel layers in the OSCs recently developed for environmentally stable, high-performance channel layers. In fact, the strategy used to develop air-stable OSCs relies heavily on finding or synthesizing organic compounds with high Eg, as this can prevent degradation resulting from photo-oxidation. In practice, this goal is the same as that in developing transparent OSCs.[15,16] Benzothieno[3,2-b]benzothiophene (BTBT) derivatives have been among the most successful OSCs developed for such purposes;[24–26] the BTBT core exhibits an Eg greater than 3.5 eV in a dilute solution, although it consists of four aromatic rings like tetracene, whose Eg is approximately 2.5 eV. This makes derivatives based on the BTBT core tend to have a higher Eg than oligoacenes or their derivatives.[16] The fact that the BTBT core has a higher optical gap than tetracene may be attributed to its structural similarity to chrysene, which is a hydrocarbon with four fused aromatic rings like tetracene but which has its aromatic rings arranged in a “zig-zag” fashion. Such a nonlinear molecular arrangement is known to yield higher excitedstate energy than the linear arrangement of the aromatic rings in tetracene.[27] Dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) and 2,7-diphenyl[1]benzothieno[3,2-b]benzothiophene (DPh-BTBT), representative BTBT derivatives shown in Figure 1a, can be regarded as promising candidates for high-performance TOTFTs because they are known to have a mobility on the order of 1 cm2 V−1 s−1
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applications where the fractional area of the channel layer is relatively small compared to the total area; this is because the average transmittance spectrum (and thus the perception of color) is given by the sum of the transmittance spectra of participating components weighted by their fractional area. (See Figure 1c for photographs of samples in which pentacene and DNTT layers cover only 30% of the total area and other parts have no layers other than their substrates.) However, as the composition of components and their respective fractional areas may vary over a wide range depending on application, full transparency of active channels will still be beneficial in that it allows a higher degree of freedom in the overall design and processing. For example, the transmittance or areal requirement for signal and power lines will be lessened for a given target average transmittance or color neutrality if TFT parts are highly transparent and colorless. In another example, channel materials may cover a significant portion of the circuit in volume-produced printed electronic devices owing to the limited patterning resolution and registration accuracy; in such a case, full transparency of channel layers will be critical in realizing highly transparent, colorless devices. To utilize DPh-BTBT as channels for TOTFTs, transparent source/drain (S/D) electrodes forming ohmic electrical contacts with these channels are required. While varFigure 1. Pentacene, DNTT, and DPh-BTBT. a) Chemical structures of the molecules. ious transparent S/D electrodes have been b,c) Transmittance as a function of wavelength λ (b) and Photographs (c) of (40 ± 10) nm-thick reported for TOTFTs, such as conducting films deposited on glass substrates. For pentacene and DNTT, photographs are also shown oxides,[6,7,11] conducting polymers,[12] nano(bottom row) for samples having a partial coverage according to the channel pattern shown at [8–10] or thin metal-based multithe bottom right. The fractional area of the organic channels (shown as small orange-colored materials, [28] layers, S/D electrodes achieving all the rectangles) with respect to the rectangle shown by a dashed line is approx. 30%. requirements for transparency, conductivity, injection, and processability are often hard to find. In particular, and Eg (measured in dilute solution) values of 3.0 and 3.2 eV, finding such electrodes is even more challenging considering respectively.[24,25] However, it is noteworthy that the Eg in solidthat DPh-BTBT has a deep highest occupied molecular orbital state films is in general smaller than that in the solution phase (HOMO) level. (5.77 eV from UV photoelectron spectroscopy due to the spectral broadening associated with enhanced inter(UPS) measurement. For comparison, those of DNTT and penmolecular interaction in the condensed phase.[24] Figure 1b,c tacene measured by UPS are reportedly 5.44 eV and 4.85 eV, presents the UV/vis transmittance spectra and photographs of respectively.).[24,29,30] According to our experiment using an (40 ± 10) nm-thick films of pentacene, DNTT, and DPh-BTBT on glass substrates. As expected, both DNTT and DPh-BTBT have opaque top-contact TFT configuration, shown in Figure 2a, even absorption peaks (i.e., transmittance dips) that are significantly Au is not suitable for S/D electrodes owing to its limited injecblue-shifted compared to those of pentacene. From the absorption ability (Figure 2b). Instead, transition-metal-oxide (TMO) tion edge in Figure 1b, the Eg values of these OSCs in the film layers of WO3 or MoO3 turn out to be efficient hole-injection state are estimated to be 1.76, 2.54, and 3.01 eV for pentacene, layers for DPh-BTBT TFTs; TFT devices with Al/TMO bilayer DNTT, and DPh-BTBT, respectively (see Figure S1 in the SupS/D electrodes exhibit much better performance than devices porting Information for details). Note that the solid-state DNTT with Au S/D electrodes, as can be seen in Figure 2b. The satufilm exhibits an Eg significantly below 3 eV and thus looks pale ration mobility (μSat) of the DPh-BTBT TFTs with Al/TMO S/D yellow, making its application as a colorless channel layer limelectrodes and a Cytop (Asahi Glass) dielectric surface is up to ited. On the other hand, DPh-BTBT fulfills the condition of 0.8 cm2 V−1 s−1, which is higher than the value reported previ“Eg ≥ 3 eV” even in the solid-state film, and thus is suitable for ously for the DPh-BTBT TFTs with Au S/D electrodes and an octyltrichlorosilane (OTS)-treated dielectric surface, in which the colorless, fully transparent channels desired for TOTFTs in DPh-BTBT was deposited without substrate heating, as was also a wide range of applications. It is noteworthy that DNTT or even the case for this work.[24] pentacene may also be applicable to some transparent electronic
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extinction coefficient or the imaginary part of the complex refractive index. Such an optical requirement in fact makes Al ineligible. Having a uniform continuous metal film is also critical; otherwise, absorption could be increased owing to unwanted effects such as surface plasmon effects.[35,36] As for the other components in the proposed TFTs, a patterned 150 nm-thick ITO layer is used as a gate electrode, and a 30 nm-thick cross-linked Cytop (C-Cytop) layer prepared by spin coating is adopted as a gate dielectric layer to ensure low-voltage operation and to provide trap-free dielectric interfaces.[37] The surface of the C-Cytop layer turns out to be very smooth, with root mean square (RMS) roughness as low as 0.8 nm, as shown in its atomic force microscopy (AFM) Figure 2. a) Structure of the conventional opaque DPh-BTBT TFTs with various types of S/D image (Figure 2d). The DPh-BTBT channel layer thickness is set at 30 nm so that it is thin contact. b) Their transfer characteristics. c) Structure of the proposed transparent DPh-BTBT TFTs. The numbers 1–4 indicate four optically distinct positions under consideration. d,e) Surenough to maintain low off current yet thick face morphologies of the C-Cytop layers deposited on ITO electrodes (d) and the DPh-BTBT enough to fully cover the channel region. layers deposited on those C-Cytop layers (e). The AFM image presented in Figure 2e shows densely packed grains, as desired. As can be seen in Figure 2c, the proposed TOTFT comOn the basis of these results, we employed ZnS/Ag/WO3 prises four optically distinct positions with different multilayer (ZAW) trilayers as transparent S/D electrodes, as presented in structures (positions 1–4). Hence the TOTFT structure should Figure 2c, which provides a schematic diagram of the proposed be carefully chosen so that all four positions can have a sufTOTFTs. In the ZAW electrode, an ultrathin Ag layer (a 15 nmficiently high transmittance. To design optimum TOTFTs, we thick Ag layer in this study) enables efficient lateral conduction, used luminous transmittance (TLum), or the average transmitan outer dielectric layer (ZnS) covering the Ag layer enhances the transparency using a destructive interference effect,[31–33] tance weighted for human photopic response,[36] as a figure of and an inner dielectric layer (WO3) provides efficient hole injecmerit for transparency for a given set of WO3 layer thickness tion into the channel from the Ag electrode. It was previously (d WO3 ) and ZnS layer thickness (dZnS). Among the four posireported that hole injection from WO3 or MoO3 was similar tions, the regions including ZAW electrodes (positions 3 and 4) need special attention because they contain Ag layers and thus regardless of the metal layers used together.[34] It is known that may be subject to transmittance reduction due to reflection in order to maximize the destructive interference effect, the and/or absorption. Figures 3a and b, which show the TLum calhigh refractive index of the outer dielectric layer (nDo) is critical;[32] ZnS is chosen because it has a refractive index as high as culated at positions 3 and 4 as a function of dWO3 and dZnS, indi2.4 in the visible range and can be deposited easily by thermal cate that TLum at positions 3 and 4 can indeed vary sensitively evaporation without damaging underlying layers (Figure S2 in over dZnS (and relatively mildly over d WO3 ) in a wide range from the Supporting Information).[31,33] The refractive index of the 30%–50% to over 80%. When one considers TLum at the two inner dielectric layers (nDi) in contact with the organic layers positions 3 and 4, the optimal dZnS and d WO3 leading to maxcan also be important, but the transmittance typically responds imal TLum can be set at 35 ± 7 nm and 50 ± 20 nm, respectively. more sensitively to the thickness of the outer dielectric layer Note that the ZnS layer could be deposited over all the posi(dDo) and can be made sufficiently high as long as nDi is not tions because it is insulating and does not play any electrical role. TLum, calculated as a function of dZnS for positions 1 and 2, much smaller than that of the organic layers.[32,35] The refractive index of WO3 is approximately 2.1 in the visible range however, shows that these positions would be better off without the ZnS layer (Figure 3c). This is because the high refractive (Figure S2, Supporting Information), slightly higher than that index of the ZnS layer makes it function effectively as a highof typical organic layers. In transparent electrodes of this type, reflection (HR) coating when it is deposited on top of dielectric the properties of a metal layer are also very important for both or organic layers that have refractive indexes lower than that of optical and electrical reasons. Note that one generally prefers the ZnS layer.[38] The transmittance dip shown in Figure 3c cora thicker metal layer for lower sheet resistance and a thinner metal layer for lower absorption and thus for higher transmitresponds to the case in which the effect of the HR coating is tance. For example, the optimal range for Ag in these applimaximized owing to constructive interference. cations is typically found to be 10–20 nm. In this case, sheet Figure 3d–f presents the measured and calculated transmitresistance less than 10 Ω sq−1 and transmittance comparable to tance spectra for several (d WO3, dZnS) values near the optimum that of indium tin oxide (ITO) are possible.[31] Another requirecondition. The measured transmittance spectra (shapes) are shown to match well with the calculated results (solid lines) for ment is low refractive index (n). This is because absorption in all the cases under study, indicating that the optical design and a given layer is proportional to its nk-product, where k refers to
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Figure 3. a,b) Calculated TLum of position 3 (a) and position 4 (b) vs. dZnS and d WO 3. c) Calculated TLum of positions 1 and 2 vs. dZnS. d) Measured (shapes) and calculated (solid lines) transmittances (T) of positions 1 and 2 without ZnS layers on top. Inset: Top view of the patterns and a photograph of a sample composed of positions 1 and 2 fabricated on a glass substrate. It contains all the patterns (except for S/D electrodes) including ITO gate electrodes, dielectric layers, and channel layers. e,f) Measured (shapes) and calculated (solid lines) transmittances (T) and photographs of position 3 (e) and position 4 (f) with dZnS of 0, 25, 35, and 45 nm while d WO 3 was fixed at 50 nm.
fabrication processes worked as expected. Both positions 1 and 2 exhibit transmittances over 85% and mostly up to 90% over all of the visible range. The inset images of Figure 3d depict the active and gate patterns and provide a photograph of the corresponding sample fabricated on a glass substrate. All regions on the substrate are highly transparent and thus are not distinguishable from one another. To the best of our knowledge, this is the first demonstration of a fully transparent channel region for TOTFTs whose transparency is comparable to that of transparent oxide TFTs.[39] As can be seen in Figure 3e,f, positions 3 and 4 look translucent in dark-gray without ZnS layers and exhibit transmittances of around 50%. With the optimal values of dZnS and d WO3 , the transmittance at positions 3 and 4 dramatically increase to over 70% over most of the visible range and to 80% in the peak region, thus resulting in a highly transparent appearance. The wavelength for the peak transmittance tends to red-shift as dZnS increases from 25 nm to 45 nm; however, the spectral envelope is broad enough to cover the full visible spectral range in all cases, and thus no noticeable interference-induced tint is observed. The transmittance spectra of the fully optimized TOTFT ( d WO3 of 50 nm and dZnS of 35 nm) are shown in Figure 4a. As can be seen in Figure 4a, the resultant TOTFT is colorless and highly transparent, and the patterns, even including that of the S/D region, can hardly be recognized. From an electrical perspective, the proposed TOTFTs exhibit low-voltage operation that is switchable under 3 V with significantly low gate leakage current, saturation mobility of ca. 0.9 cm2 V−1 s−1, threshold voltage of ca. –0.84 V, subthreshold swing of ca. –0.17 V decade−1, and on-to-off drain current ratio
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above 106, as shown in its transfer (Figure 4b) and output characteristics (Figure 4c). Ideal output characteristics without current crowding behavior in the linear region are attributed to low contact resistance between DPh-BTBT channel layers and ZAW S/D electrodes provided by WO3 hole-injecting layers (Figure S3 in the Supporting Information). The low voltage operation and high on/off ratio are attributed to the 30 nm-thick C-Cytop dielectric layers, which show a capacitance density of 55.7 nF cm−2 and leakage current density under 10−8 A cm−2 for a voltage bias under 3 V (Figure 4d). The ideal, hydroxyl-group-free interface[37,40] provided by the C-Cytop dielectric layers leads also to negligible hysteresis in both transfer and output characteristics and to a relatively good operational stability under the constant voltage stress test (CVST) in a diode connection (VG =VD); 85% of the initial ID is preserved after the CVST, with a stress bias (VG.str) of –3 V for 3000 s (Figure 4e). The stress bias used here corresponds to approx. 1 MV cm−1 and was chosen to be large enough to clearly define the “ON”-state. These results are comparable to previously reported stable OTFTs with C-Cytop dielectric layers.[37,40] In conclusion, we have fabricated colorless and highly transparent OTFTs using DPh-BTBT layers as wide-gap organic channels and ZnS/Ag/WO3 layers as transparent S/D electrodes with a good hole injection into the relatively deep HOMO level of the DPh-BTBT layer. The optimum TOTFTs were designed through calculations based on thin-film optics; the resultant devices showed transmittances of over 85% in the channel area and more than 70%–80% in the S/D area over most of the visible range. With the help of the ultrathin C-Cytop dielectric layers and polycrystalline DPh-BTBT channel layers,
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stable, low-voltage operation was also realized with a mobility of ca. 0.9 cm2 V−1 s−1. Together with the superior mechanical flexibility of metal-based multilayer transparent electrodes that had previously been demonstrated,[31] the proposed TOTFTs could serve as a versatile building block in various next-generation electronic devices.
Electrical characterization of devices: The capacitance and the leakage current of the metal–insulator–metal (MIM) devices were measured using a precision LCR meter (HP4284A, Agilent) and a semiconductor parameter analyzer (HP4156A, Agilent), respectively, in ambient air. The electrical characteristics of the TFTs were measured using a semiconductor parameter analyzer (HP4155A, Agilent) in a N2-filled glove box.
Experimental Section
Supporting Information
Fabrication of devices: 150 nm-thick ITO layers precoated on glass substrates were patterned as gate electrodes by conventional photolithography and wet-etching processes. Cytop solution (CTL-809M, Asahi Glass, Chiyoda, Japan) was diluted by mixing it with appropriate solvent (CTsolv-180, Asahi Glass) at 1:4 ratio by volume (CTL-809M : CTsolv-180). Then, cross-linking agent (C6-Si) was added to the diluted Cytop solution (10 µL mL−1), followed by mixing on a hotplate at 100 °C with a stirring bar in a N2-filled glove box. 30 nm-thick crosslinked Cytop (C-Cytop) layers were deposited on the glass substrate with ITO electrodes by spin-coating (500 rpm for 10 s and then 2000 rpm for 20 s); these samples were annealed on a hotplate at 180 °C for an hour in ambient air. The fabrication of the devices was completed by sequential evaporation of the DPh-BTBT layer and the ZnS/Ag/WO3 multilayer stack through appropriate shadow masks under vacuum conditions (10−6 Torr) in a thermal evaporation chamber (HS-1100, Digital Optics & Vacuum, Seoul, Korea). Calculation and measurement of transmittance: The transmittances were calculated using the Essential Macleod (Thin Film Center, Tucson, AZ) program based on thin-film optics; the luminous transmittances were calculated using a customized MATLAB code. The refractive indices (n) and the extinction coefficients (k) of the layers composing the devices were obtained by spectroscopic ellipsometry (Figure S2, Supporting Information).[41,42] The transmittances were measured in ambient air using a UV-vis spectrometer (SV2100, K-MAC, Daejeon, Korea).
Supporting Information is available from the Wiley Online Library or from the author.
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Acknowledgements This research was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP)(CAFDC/ Seunghyup Yoo/No. 2007–0056090); a grant (Code No. 2013073183) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Education, Science and Technology, Korea; and the Graphene Research Center Program of KI for the Nanocentury, KAIST. The authors are also grateful to Nippon Kayaku Co. for providing the DNTT and DPh-BTBT materials. Received: November 1, 2013 Revised: December 23, 2013 Published online: February 3, 2014 [1] [2] [3] [4]
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