Subscriber access provided by Nottingham Trent University
Article
Cinnamate-Functionalized Natural Carbohydrates as Photopatternable Gate Dielectrics for Organic Transistors Zhi Wang, Xinming Zhuang, Yao Chen, Binghao Wang, Junsheng Yu, Wei Huang, Tobin J. Marks, and Antonio Facchetti Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b02413 • Publication Date (Web): 09 Aug 2019 Downloaded from pubs.acs.org on August 11, 2019
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 11 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Chemistry of Materials
Cinnamate-Functionalized Natural Carbohydrates as Photopatternable Gate Dielectrics for Organic Transistors Zhi Wang,†,‡,# Xinming Zhuang,‡,§,# Yao Chen,‡ Binghao Wang,‡ Junsheng Yu,§ Wei Huang,*,‡ Tobin J. Marks,*,‡ and Antonio Facchetti*,‡,‖ †Research Center for Engineering Technology of Polymeric Composites of Shanxi Province, School of Materials Science and Engineering, North University of China, Taiyuan, 030051, P. R. China ‡Department of Chemistry and the Materials Research Center, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA §State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China ‖Flexterra Inc., 8025 Lamon Avenue, Skokie, IL 60077, USA ABSTRACT: Photolithographic defined films play an important role in modern optoelectronics and are crucial for the development of advanced organic thin-film transistors (OTFTs). Here, we explore a facile photoresist-free photopatterning technique for natural carbohydrates and their use as OTFT gate dielectrics. The effect of the cross-linkable chemical structure on the crosslinking chemistry and dielectric strength of the corresponding films are investigated in cinnamate-functionalized carbohydrates from monomeric (glucose) to dimeric (sucrose) to polymeric (cellulose) backbones. UV-illumination of the cinnamate esters of these carbohydrates leads to [2+2] cycloaddition and thus the formation of robust crosslinked dielectric films in the irradiated areas. Using propylene glycol monomethyl ether acetate as the solvent/developer, patterned dielectric films with micrometer size features can be readily fabricated. P- and N-type OTFTs are successfully demonstrated using unpatterned/patterned crosslinked films as the gate dielectric and pentacene and N,N’-1H, 1H-perfluorobutyl dicyanoperylenecarboxydiimide (PDIF-CN2) as the p- and n-channel semiconducting layers, respectively. These results demonstrate that natural-derived polymer gate dielectrics, which are soluble and patternable using bio-mass derived solvents, are promising for the realization of a more sustainable OTFT technology.
INTRODUCTION Organic thin film transistors (OTFTs) have drawn considerable attention because of their unique attractions including excellent mechanical flexibility, potential for low cost, solutionprocessability, and lightweight.1-5 The combination of these properties makes OTFTs potential candidates for various optoelectronic devices such as displays,6-8 sensors,9 radio frequency identification tags,10, 11 flexible e-skins12-14 and healthcare devices.15, 16 Nevertheless, further expansion of OTFT applications will require the design and realization of new materials, which remains of great interest.17, 18 Furthermore, if some of these electronic materials could be derived from natural products and processed using green solvents, this could greatly accelerate the development of more sustainable green optoelectronic technologies. Among the key OTFT materials components, the gate dielectric plays a fundamental role not only for establishing/stabilizing the conductive channel in the semiconductor region but also for both providing optimal surface chemical and morphological characteristics to suppress charge traps at the semiconductor/dielectric interface as well as, for bottom-gate OTFT structures, promote the growth of an optimal semiconductor film microstructure. Several studies have developed alternative gate dielectrics to the benchmark SiO2, such as SiNx,19 AlOx,20 ZrOx,21 and TaOx,22 due to their outstanding atomic structural and dielectric properties. However, in most cases, high-quality inorganic dielectrics must
be fabricated at high temperatures and/or using vacuum techniques such as RF-magnetron sputtering,23 atomic layer deposition,24 or plasma enhanced chemical vapor deposition.25 Solution-based deposition of these materials has also been investigated,26, 27 however, high-quality inorganic gate dielectrics require high annealing temperatures (typically > 300 oC). Thus, the requirement of using inexpensive flexible plastic substrates and low processing temperatures for device fabrication remain challenging for ceramic dielectrics. In contrast, organic dielectrics have advantages, including mechanical flexibility, electronic tunability lightweight, and most importantly, thin film growth at low temperature by spincoating and printing processes.28-30 Polymers including poly(4vinylphenol) (PVP) ,29, 31 Cytop,32, 33 PMMA,34-36 and polyimides37, 38 have been investigated as gate dielectrics, however, most cannot be easily patterned with eco-friendly solvents and are problematic since they generate considerable organic waste. Therefore, to reduce environmental hazards and increase sustainability, research efforts on environmentallyfriendly electronics, for example, dielectric layers based on natural (by)products and/or nature-inspired materials have increased considerably.39-41 Natural carbohydrates have been investigated as gate dielectric materials in TFT devices.42, 43 Especially, cellulose has been used as dielectric layer44, 45 because of its excellent mechanical properties, environmental stability, raw material availability46 and good dielectric properties.42 Its derivative, trimethylsilyl cellulose (TMSC)47
ACS Paragon Plus Environment
Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
and cyanoethyl pullulan (CEP)48-50 have also been successfully utilized as precursors for the fabrication of cellulose-based high-k dielectric (180 ~ 200 nF/cm2, film thickness = 180 ~ 200 nm, κ = 5.4 ~ 8.4 at 1 kHz) layers in pentacene- and fullerene based OTFTs. CEP dielectric has also been crosslinked, however via thermal annealing, 47 a process which requires the use of a photoresist for film patterning, thus increasing complexity and waste formation. The realization of modern organic electronic circuits requires the efficient patterning of all electronic layers including that of the gate dielectric.51, 52 However, methods to pattern polymers which do not produce byproducts compromising dielectric strength and loss are rather rare.51, 53 For example, a widely used concept for the fabrication of polymer microstructures is based on a UV-promoted photoinduced alteration of the solubility of polymeric materials using photoacid generators.54 Relevant to the research reported here, photo-induced conversion of an acid labile TMSC group to rather insoluble cellulose to form patterned dielectric layer has been investigated.51 However, photoacids can negatively affect charge transport and most importantly bias stress stability because of ionic contaminants. In this contribution we report a family of photo cross-linkable materials derived from natural carbohydrates and provide a strategy for their application as gate dielectric layers (Scheme 1). To explore generality and understand structure-property relationships, we investigate cinnamate (Cin)-functionalized carbohydrates proceeding from a monomer (glucose) to a dimer (sucrose) to a polymer (cellulose) (Scheme 1). The carbohydrate and the cinnamic acid precursors used for the synthesis of these compounds (vide infra) are readily available from sustainable sources such as wood, cotton, corn, beet, and spices.55, 56 Optical absorption measurements demonstrate that the UV light results in a [2+2] cycloaddition converting the soluble precursors into a crosslinked carbohydrate network. These films are characterized by UV-vis, FTIR, and XPS spectroscopies. Impedance spectroscopy shows that the crosslinked films are good dielectrics with 𝑘 ranging 3.1-3.7 (at 1 kHz) and leakage currents of about 1-2×10-6 A/cm2 at 1.5 MV/cm. Pentacene (p-type) and N,N’-1H, 1H-perfluorobutyl dicyanoperylenecarboxydiimide (PDIF-CN2, n-type) TFTs are fabricated as testbeds of the new dielectric films. Finally, we demonstrate that Cin-cellulose can be efficiently patterned at low dosages using a biomass-derived solvent, resulting in stable and high-performance p-and n-type TFTs.
Page 2 of 11
purification by multiple-gradient vacuum sublimation. The organic small-molecule semiconductor PDIF-CN2 was obtained from Flexterra Corp.
Scheme 1. Synthesis of cinnamate esters of the indicated carbohydrates and corresponding UV-promoted crosslinking process. The degree of hydroxyl functionalization with R = Cin is > 85% for all carbohydrates. Transistor Fabrication and Electrical Characterization. Silicon wafers used as substrates were cleaned in isopropyl alcohol with ultrasonication and then in an O2 plasma for 5 min before use. Dielectric precursor solutions were spin-coated onto the substrates at 1500 rpm for 40 s and then dried on a hot plate (40 oC) for 30 min. The films were then cured with a UV flood exposure system (Inpro Technologies F300S) for 0 - 180 s with/without a photomask. Patterned films were developed in PGMEA for 10 s. After that, the dielectric films were annealed at 110 oC for 30 min. Next, 30 nm pentacene or 30 nm PDIFCN2 were vacuum-deposited at the rate of ~ 0.05 nm s-1 at ~ 5 × 10-6 Torr. Finally, gold source/drain electrodes (channel length = 100 μm; channel width = 1000 μm) were deposited by thermal evaporation through a shadow mask. To test the dielectric properties of the pristine/crosslinked carbohydrate cinnamate films, metal-insulator-silicon (MIS) devices were fabricated on n++-Si substrates in the configuration, Si/insulator/Au. TFT characterization was performed in air (~20%RH) in the dark on probe station. All electrical parameters were measured with an Agilent 1500 semiconductor parameter analyzer. The carrier mobility (μ) was evaluated in saturation region by using eq. 1 where 𝐼𝐷 is the drain current, 𝐶𝑖 is the capacitance
EXPERIMENTAL SECTION Material Synthesis and characterization. NMR spectra were obtained using an Inova 500 (500 MHz) NMR spectrometer. Elemental analyses were performed by Midwest Microlab, LLC. Cellulose cinnamate, sucrose cinnamate, and glucose cinnamate were synthesized according to Scheme 157-60 and detailed synthetic procedures can be found in the Supporting Information. FTIR spectra were measured on a Nexus 870 spectrometer (Thermo Nicolet) with a single reflection horizontal ATR accessory having a diamond ATR crystal fixed at an incident angle of 45o. UV-vis spectroscopy was performed on a Varian Cary 50 spectrophotometer for carbohydrate cinnamate films spin-coated on quartz substrates. Precursor Solutions. Cellulose cinnamate (Cin-Cell), sucrose cinnamate (Cin-Suc), and glucose cinnamate (Cin-Glu) were dissolved in propylene glycol monomethyl ether acetate (PGMEA, Sigma-Aldrich, ≥99.5%) to achieve a concentration of 60 mg mL-1. The precursor solutions were stirred for 8 h before use. Pentacene (Sigma-Aldrich, ≥99%) was used after
𝐼𝐷 =
𝜇𝐶𝑖𝑊 2𝐿
(𝑉𝐺𝑆 ― 𝑉𝑇)2
(1)
per unit area of dielectric, 𝑉𝑇 is the threshold voltage, and 𝑉𝐺𝑆 is gate voltage. W and L are the channel width and length, respectively. The dielectric constant, 𝑘, is calculated by eq. 2 where 𝜀0 is the vacuum 𝑘
𝐶𝑖 = 𝜀0𝑑
(2)
permittivity, and 𝑑 is the thickness of the dielectric. Film Characterization. The advancing aqueous contact angle of the dielectric surfaces was obtained using a Phoenix Series, S. E. O Co. Ltd. contact angle measurement system. The thickness of the cured cellulose cinnamate films was measured
ACS Paragon Plus Environment
2
semiconductor (MIS) device structures (Figure 2a). These devices were fabricated on heavily doped silicon wafers (n++, ρ < 0.02 Ω) on which Cin-Carb solutions (60 mg/mL in PGMEA) were spin-coated. The resulting films were first dried at 40 oC for 30 min and UV-cured for 180 s. Next, the dielectric films were annealed at 110 oC for 30 min. Finally, these devices were completed by thermal evaporation of Au electrodes through a shadow mask (pixel size 200 x 200 m2).
a
5s 10 s
1.0 0.8
Cin-Cell
30 s
0.6 0.4
180 s
0.2 0.0 200
240
280
Si
320
360
Wavelength (nm)
400
40 Cin-Glu Cin-Suc Cin-Cell
0
40
80
120
Time (s)
160
10-8 10-9
10-10 0
cCin-Glu
cCin-Glu cCin-Suc cCin-Cell
1 2 Electric Field (MV/cm)
cCin-Glu cCin-Suc cCin-Cell
104 105 Freq (Hz)
106
cCin-Cell
cCin-Suc
1μm
1μm
Table 1 reports details of film morphological and dielectric properties. After cross-linking, the films of the three cCin-Carbs exhibit almost identical thicknesses (~ 300 nm). Current density-Voltage (J-V) and Capacitance-Frequency (C-F) plots are reported in Figures 2b and 2c, respectively. The leakage current density at 1.5 MV/cm of the crosslinked cCin-Carb films remains at a low level of 1.3 – 2.1×10-6 A/cm2. Note, these ~ 300 nm thick films do not exhibit breakdown upon application of the maximum bias of our instrumentation (100 V). Thus, to access breakdown characteristics we fabricated ~100 nm thick films, which represent the minimum thickness to achieve reliable data for dielectric films fabricated outside a clean room. As shown in Figure S3, the breakdown fields of cCin-Glu and cCin-Suc are 4.1 MV/cm and 5.1 MV/cm, respectively, while the cCin-Cell film does not breakdown even under an electric field as high as ~ 10 MV/cm. Such high breakdown electric fields, particularly for the cCin-Carb dielectric, demonstrate the excellent dielectric strengths of these materials. From the C-F plots of the ~ 300 nm thick films, the areal capacitances of the dielectrics (at 1 kHz) are 8.85 nF/cm2 (Cin-Glu), 9.73 nF/cm2 (Cin-Suc), and 10.5 nFcm-2 (Cin-Cell), thus, the resulting dielectric constant increases from 3.1 in cCin-Glu to 3.3 and 3.5 in cCin-Suc and cCin-Cell, respectively. Moreover, to access the capacitance values approaching quasi-static conditions, cCin-Carb capacitors were measured at even lower frequencies (1 Hz 103 Hz). As shown in Figure S4, cCin-Carb capacitances remain in a narrow range of 8.85-9.50 nF/cm2, 9.73-11.1 nF/cm2, and 10.5-11.8 nF/cm2 for cCin-Glu, cCinSuc, and cCin-Cell, respectively, demonstrating stable dielectric response. Note, the small change in capacitance under different frequencies indicates that the calculated FET parameters are reliable (vide infra). The dielectric constant variation going from the small-molecule to the polymer-based cCin-Carb films is currently not fully understood, however, it could possibly reflect the different degrees of substitution, degrees of crosslinking, and changes in film density of the
60
0
10-7
12 10 8 6 4 2 0 3 103
Figure 2. (a) MIS device structure used in this study. (b) Leakage current density vs. electric field and (c) capacitance vs. frequency plots for the indicated cCin-Carb films ~ 300 nm thick. (d) AFM images of the cCin-Carb films.
80
20
10-6
1μm
b 100 Conversion (%)
0s
1.2
Cin-Carb
d
c
10-5
Au
RESULTS AND DISCUSSION We first discuss the preparation of the crosslinked thin films of the subject materials followed by characterization of their dielectric properties. Next, OTFTs are fabricated demonstrating that the crosslinked cinnamate-carbohydrate (cCin-Carb) films can be good gate dielectric materials. Finally, we explore the photopatterning characteristics of these materials demonstrating the superiority of the patterned cCin-Cell films for OTFT applications. Synthesis and Crosslinking of Cinnamate-Carbohydrates. Cin-Cell, Cin-Suc, and Cin-Glu were synthesized according to small modifications of the literature procedures57-60 and were obtained in 84.3%, 80.1% and 77.5% yields, respectively, after purification (Scheme 1). The degrees of substitution (DS) of the hydroxyl group of these molecules is found 2.55, 7.92, and 4.89, respectively (theory = 3 - 8), based on both elemental analysis and 1H NMR spectroscopy (see SI and Figure S1).
a 1.4
b
C (nF/cm2)
with a Tencor P10 Profilometer, and averaged based on five measurements. Atom force microscopy (AFM) topographies were imaged with a Veeco Dimension Icon scanning probe microscope in the tapping mode. Grazing incidence X-ray diffraction (GIXRD) measurements were performed with a Rigaku SmartLab Thin Film Diffraction Workstation using a high intensity 9 kW copper rotating anode X-ray source with multilayer optics. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Scientific ESCALAB 250Xi instrument.
Absorbance
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Chemistry of Materials
Jleak (A/cm2)
Page 3 of 11
200
Figure 1. (a) UV-vis spectra of Cin-Cell films on quartz substrates after different UV curing times. (b) Conversion degree vs. curing time of Cin-Carbs.
Figures 1a and S2 shows the UV-vis spectra of Cin-Carb films on quartz substrates after different UV curing times (0-180 s). The uncured films exhibit characteristic absorption at max = ~270 nm due to the cinnamoyl ester moiety. Upon exposure to UV light, this peak intensity decreases due to the photoinduced [2+2] cycloaddition process (see Scheme 1). The degree of crosslinking can be calculated by integrating the 270 nm peak area and is ~33%, ~48%, and ~65% for 10 s, 30 s, and 60 s exposure for Cin-Cell as shown in Figure 1b. In comparison, the crosslinking of Cin-Glu and Cin-Suc films is more efficient and achieves ~81% and 72% crosslinking, respectively, for 60 s curing (Figures 1b and S2). Enhanced conversion at lower dosage for the cinnamate monomer- and dimeric-carbohydrate based films vs. that of the cellulose is doubtless due to enhanced molecular motion favoring the [2+2] cycloaddition reaction in the solid state.61, 62 Moreover, an almost complete crosslinking, 91% (Cin-Glu), 93% (Cin-Suc), and 89% (Cin-Cell), can be obtained on sample overexposure in a 180 s curing time. Characteristics of Cross-linked Cinnamate-Carbohydrate Films. The dielectric properties of the 180 s crosslinked cinnamate-functionalized carbohydrate (cCin-Carb, 180 s UV exposure) films were assessed in metal-insulator-
ACS Paragon Plus Environment
3
Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 11
Table 1. Dielectric properties of UV-cured Cin-Carb films. Dielectric
Degree of curing (at 180 s) (%)*
Thickness (nm)**
Ci (nF/cm2)
𝒌 (at 1 kHz)
Jleak (at 1.5 MV/cm) (A/cm2)
RMS (nm)
cCin-Glu
~91
310±12
8.85±0.08
3.1±0.15
1.31±0.10×10-6
0.16
cCin-Suc
~93
297±9
9.73±0.21
3.3±0.1
2.07±0.18×10-6
0.19
3.5±0.2
1.42±0.14×10-6
0.21
cCin-Cell
~89
295±9
10.5±0.3
*from UV-Vis; **before developing
polymer versus the small molecule-based precursors.63, 64 Particularly, small variation in the OH content can greatly affect dielectric characteristics because of the substantial dipolar polarizability of this functional group. AFM images of the cCinCarb films indicate similar smooth surface with a RMS roughness (RMS) of ~ 0.2 nm and an absence of major topological irregularities and cracks from the curing process (Figure 2d). The morphology data combined with the dielectric properties indicate that all Cin-Carb films after UV curing are potentially good dielectrics for organic thin-film transistors. Pentacene TFTs Based on the Cin-Carb Dielectric Films. To investigate how the properties of the Cin-Carb films affect charge transport in OTFTs, bottom-gate top-contact OTFTs with pentacene (p-type) as active layer and Au as source/drain electrodes were first fabricated on unpatterned cCin-Carb dielectric films (curing time = 180 s; Figure 3-left). Table 2 summarizes the major pentacene TFT parameters and representative transfer and output curves are shown in Figure 4. These plots demonstrate typical p-channel TFT behavior with off-currents in the range of 10-9 A, which is typical of unpatterned pentacene devices. Importantly, the transport characteristics also indicate that there is no obvious I-V hysteresis, which demonstrate a low interfacial trap density likely originating from the low density of residual OH groups (vide infra).65-67 Table 2 evidences a steady increase of mobility from 0.020 cm2/Vs with cCin-Glu, to 0.10 cm2/Vs with cCinSuc, and to 0.19 cm2/Vs with cCin-Cell is obtained, along with a positively shifting VT from -17.4 V (cCin-Glu) to -15.7 V (cCin-Suc), and -11.3 V (cCin-Cell), respectively.
microstructure was investigated by AFM and XRD (Figure 5). AFM images indicate that the average pentacene grain size increases from 0.12 um2 (cCin-Glu) to 0.14 um2 (cCin-Suc) to 0.30 um2 (cCin-Cell) while the corresponding RMS also increases from 5.70 nm to 6.07 and to 6.33 nm, respectively. Furthermore, the XRD plots support enhanced texturing of the pentacene films on going from Cin-Glu to Cin-Cell, thus increasing the molecular mass of the dielectric film precursor (Figure 5). Thus, morphological changes corroborate charge transport variation in Table 2 as seen previously in the literature for pentacene based TFTs.68 These results demonstrate that all three cCin-Carb materials can be used as the gate dielectric layer after almost complete crosslinking.
Figure 4. TFT transfer (a) and output (b) characteristics of the indicated Pentacene/cCin-Carb based OTFTs. (Blue dash curves are the gate leakage currents)
Table 2. Performance metrics of pentacene TFTs based on cCin-Carb (curing time = 180 s) gate dielectrics. Performance Ci Figure 3. Illustration of the OTFT device fabrication process (left: without patterned dielectric and semiconductor; right: with patterned dielectric and semiconductor).
To understand those factors which underlie the TFT performance variation, the pentacene film morphology and
(nF/cm2)
Cin-Glu
Cin-Suc
Cin-Cell
8.85
9.73
10.50
μ (cm2/Vs)
0.020±0.003
0.10±0.02
0.19±0.01
Vth (V)
-17.4±2.5
-15.7±2.1
-11.3±1.6
Ion/Ioff
104
105
105
Crosslinking and Patterning of Cinnamate Cellulose Films. Since cCin-Cell based TFTs exhibit the best performance in this
ACS Paragon Plus Environment
4
dielectric class, we investigated Cin-Cell material crosslinking properties in more detail. Thus, the cured Cin-Cell films for different doses were studied by a combination of techniques including FTIR, contact angle measurements, AFM and XPS. First the [2+2] cycloaddition reaction in the solid state of CinCell was also monitored by FTIR spectroscopy (Figure 6a). The IR band at 1710 cm-1 assigned to the C=O stretching mode of the cinnamoyl ester group connected to the -C=C- bound shifts to higher frequency (1716 cm-1) after curing for 30 s and shifts even further (1726 cm-1) after curing for 180 s (Figure 6b).69, 70 By deconvoluting these peaks, the conversion of Cin-Cell is ~35% after UV curing for 10 s, ~60% for 30 s, and 88% for 180 s, which is in agreement with the optical absorption data (Figure 6c). Interestingly, FTIR also indicates a reduction of the -OH group content, assigned at 3500 cm-1, along with enhanced degree of crosslinking (Figure S5a). This result may be due to enhanced film hydrophobicity (vide infra), preventing the absorption of water, rather than suppressing the residual -OH due to incomplete cinnamoyl functionalization of the cellulose precursor hydroxyl groups. Regardless of the mechanism, these data are in agreement with the negligible I-V hysteresis observed, and thus minimum amounts of surface/bulk charge traps.71 Finally, the cCin-Cell film hydrophilicity was assessed by aqueous contact angle (CA) measurements. As shown in Figure S5b, the CA increases with increasing the UV curing time [~83o (0 s), ~91o (10 s), ~92o (20 s)], demonstrating that the film surface becomes more hydrophobic with curing time up to 20 s. It is known that hydrophobic gate dielectrics favor charge transport in organic thin-film transistors.72 However, longer curing times (> 30 s) strongly depress the CA to ~77o (30 s) and ~57o (60 s -180 s) likely due to photo-oxidation of the dielectric surface since the curing process was done in air.
1000
1μm
500 10
15
20
2 (degrees)
c
25
1500 1000
1μm
500 0
30
5
10
15
20
2 (degrees)
25
1μm
500 5
10
15
20
2 (degrees)
25
Wavenumbers (cm )
c 100
1500
0
0s
-1
2000
1000
b 180 s
4000 3500 3000 2500 2000 1500 1000
30
Conversion (%)
5
Intensity (a.u.)
0
a
2000
30
80
FTIR
60
UV-vis
40 20 0 0
Figure 5. XRD scans and AFM images of P5 films deposited on 180 s cured (a) cCin-Glu, (b) cCin-Suc, and (c) cCin-Cell films.
40
80
120
Time (s)
180 s
Transmittance
1500
patterning, the 5 s cured films are fully dissolved by the developing solvent, while the 10 - 30 s curing times lead to almost complete film retention with thicknesses of ~274 nm and ~290 nm, respectively. The latter thicknesses (30 s cured film) remains almost identical in patterned (~290 nm) and unpatterned (~300 nm) films. Further film UV curing does not significantly change the film thickness characteristics upon patterning. The PGMEA patterned films at the transition curing time from soluble to insoluble films (10-30 s) were further investigated by AFM, impedance spectroscopy, and XPS measurements (Figure 7). The AFM image of the patterned films cured for 10 s exhibit an obvious etched surface morphology characterized by a large RMS of 0.98 nm (Figure 7a). By increasing the curing time to 20 s and 30 s the morphology of the patterned films improves in uniformity and becomes smoother (σRMS = 0.56 nm and 0.26 nm, respectively). Note that the latter value is practically identical to that of the 180 s cured and unpatterned/patterned Cin-Cell films (vide infra, Figures 2d and S6). Figure 7b shows the film areal capacitance as a function of frequency. The areal capacitance of patterned cCin-Cell films increases with the curing time, from ~8.1 nF/cm2 (20 s) to 9.1 nF /cm2 (30 s) to 11.1 nF/cm2 (180 s) at 1 kHz. Current densityVoltage (J-V) plots of Cin-Cell films cured at different times are shown in Figure 7c. The leakage current density at 1.5 MV/cm of crosslinked Cin-Cell films remain almost the same range (~1.4×10-6 A/cm2) after different curing times. Therefore, compared with the unpatterned films cured for 180 s, there are no obvious changes in dielectric properties, demonstrating the excellent patternability of the crosslinked Cin-Carb films.
Transmittance
b
2000
Intensity (a.u.)
a Intensity (a.u.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Chemistry of Materials
160
200
30 s 0s
1710
1800
d 330 Thickness (nm)
Page 5 of 11
1600
1400
1200
Wavenumbers (cm-1)
320
1000
Before patterning
310 300 290
After patterning
280 270
0
40
80
120
Time (s)
160
200
Figure 6. (a) Full FTIR spectrum and (b) partial FTIR spectrum in the range of 1800 cm-1 ~ 1000 cm-1 of Cin-Cell films cured for the indicated times. (c) Conversion degree vs. curing time based on FTIR and UV-vis measurements. (d) Film thickness vs. curing time of cured Cin-Cell films before and after patterning.
The crosslinked cellulose cinnamate films exposed through a mask cannot only be processed with PGMEA but also efficiently patterned with this solvent. To determine what is the minimum curing time (UV dose) enabling efficient patterning, the film thicknesses versus curing time for the unpatterned and patterned films were measured and are compared in Figure 6d. Thus, upon increasing the curing time from 0 s to 30 s, the thickness of the unpatterned Cin-Cell films decreases from ~320 nm to ~300 nm and then remains almost constant at ~295 nm with longer curing times. Clearly, by increasing the crosslinking density, the film thickness decreases due to enhanced densification.73-75 In contrast, upon PGMEA film
XPS measurements were also carried out to monitor the compositional changes of the patterned films upon increasing exposure times (Figures 7d and S7). Note the penetration depth of X-ray here is less than 10 nm, thus XPS mainly gives the information on the top surface of the dielectrics. The highresolution XPS C1s spectra mainly indicate the presence of three types of carbon species, specifically C=C/C-C (C1, ~284.9 eV), C-O (C2, ~286.9 eV), and O-C=O (C4, ~289.0
5 ACS Paragon Plus Environment
Chemistry of Materials eV).76-79 From the data in Table 3 it is clear that for relatively short exposure times ( 30 s is there a statistical reduction of the unoxygenated C species and an increase of those which are oxygenated. However, the fully cured films (180 s) exhibit a substantially different elemental composition of the surface, with a considerable increase of oxidized products. This is conclusion is further supported by examining the carbon and oxygen atomic ratio from the C 1s and O 1s peaks (Table S1). Moreover, these data agree with the CA results, where short curing time (10 s – 20 s) correlates with higher CA due to cross-linking and negligible newly generated oxygen species and long curing times (> 30 s) result in lower CA due to oxidized products. Thus, since the films cured for 20 – 30 s can already be patterned by PGMEA and exhibit good dielectric characteristics (Figures 6d and 7b), XPS analysis confirms that moderate curing times enables sufficient crosslinking, good patternability, and a lower content of chargetrapping oxygenated groups. Note that this elemental composition evolution upon curing has been observed for other photocurable materials.78
a 10 s
20 s
c Jleak (A/cm2)
Cap (nF/cm2)
d
10 s 20 s 30 s 180 s 30 s (unpatterned)
10-4 10-6
20 s 30 s 180 s 30 s (unpatterned)
104 105 Freq (Hz)
0s
1μm
1μm
b12 10 8 6 4 2 0 3 10
OTFTs Based on Patterned cCin-Cell Dielectric Films. From the morphological and dielectric data combined with the need to achieve excellent dielectric films patterned at minimum dosage (curing time), the 30 s cCin-Cell films were selected for the fabrication of patterned OTFTs and compared with those fabricated using the overexposed (180 s) patterned films. Note, cCin-Cell dielectric films can be photopatterned to features as small as 5 um using PGMEA as the developing solvent (see Figure S8). To demonstrate generality, we fabricated TFTs using both p- and n-type semiconductors. These OTFTs have the same bottom-gate top-contact structure as reported above but with patterned gate islands (2.56 mm2) and patterned semiconductor areas which consisted of thermally evaporated 30 nm channel films of pentacene (p-type) or PDIF-CN2 (ntype) semiconductors through a second shadow mask.
30 s
1μm
Intensity (a. u.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 6 of 11
10-8
106
10-10 0
1 2 3 Electric Field (MV/cm)
10 s
30 s
Figure 8. Transfer characteristics of (a) pentacene and (b) PDIFCN2 based TFTs with patterned Cin-Cell dielectrics and semiconductors. Note, cCin-Cell is cured for 30 s or 180s as indicated in the figure.
Table 4 summarizes the major pentacene and PDIF-CN2 TFT performance parameters and representative transfer and output I-V characteristics for selected devices are shown in Figure 8. Typical p-channel and n-channel TFT behaviors are observed for pentacene and PDIF-CN2 based TFTs, respectively. The hole mobility of pentacene (μh) is found to be ~0.32 cm2/Vs and ~0.34 cm2/Vs for the 30 s and 180 s cured dielectric films, respectively (Figure 8a). We consider these data to be statistically identical considering that these devices were not fabricated in a cleanroom nor fabricated rigorously stringent photolithography methodologies. The μh values are comparable to literature data when using different dielectric materials (μh = 0.2 ~ 1.0 cm2/Vs).80-82 Moreover, the μh obtained with cCin-Cell is almost identical to those obtained using conventional SiO2 (~0.3 cm2/Vs) which further corroborates the good dielectric property of cCin-Cell. Note the VT remains in a narrow range [10 V (30 s) and -8 V (180 s)] as does the off currents (~10-10 10-9 A). Furthermore, both positive and negative gate bias stress measurements were conducted on the pentacene TFTs (Figure
292 288 284 292 288 284 292 288 284 Binding energy (eV) Binding energy (eV) Binding energy (eV)
Figure 7. (a) AFM images, (b) capacitance vs. frequency plots, (c) leakage current density vs. voltage, and (d) C1s XPS spectra for patterned cCin-Cell films cured for the indicated times.
Table 3. XPS carbon compositions of Cin-Cell films cured for the indicated times. Carbon compositi on (%)
Binding Energy (eV)
0s
10 s
20 s
30 s
180 s
C-C/C=C
284.9±0.1
61.0
57.8
61.2
48.4
33.0
C-O
286.9±0.1
20.4
25.6
25.1
32.7
40.0
O-C=O
289.0±0.1
18.6
16.6
13.8
19.0
27.1
6 ACS Paragon Plus Environment
Page 7 of 11 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Chemistry of Materials
S9), demonstrating excellent stability with minimal shift of the transfer curves (ΔVT
