Solution-based silk fibroin dielectric in n-type C60 organic field-effect transistors:
Mobility enhancement by the pentacene interlayer
Li-Shiuan Tsai, Jenn-Chang Hwang, Chun-Yi Lee, Yi-Ting Lin, Cheng-Lun Tsai, Ting-Hao Chang, Yu-Lun Chueh , and Hsin-Fei Meng
Citation: Applied Physics Letters 103, 233304 (2013); doi: 10.1063/1.4841595
View online: http://dx.doi.org/10.1063/1.4841595
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/103/23?ver=pdfcov Published by the AIP Publishing
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Solution-based silk fibroin dielectric in n-type C
60organic field-effect
transistors: Mobility enhancement by the pentacene interlayer
Li-Shiuan Tsai,1Jenn-Chang Hwang,1,a)Chun-Yi Lee,1Yi-Ting Lin,1Cheng-Lun Tsai,1 Ting-Hao Chang,1Yu-Lun Chueh,1and Hsin-Fei Meng2
1
Department of Materials Science and Engineering, National Tsing Hua University, Hsin-Chu 30043, Taiwan
2
Institute of Physics, National Chiao Tung University, Hsin-Chu 30043, Taiwan
(Received 8 October 2013; accepted 17 November 2013; published online 6 December 2013) A pentacene interlayer of 2 nm thick is inserted between fullerene (C60) and the solution-based silk
fibroin dielectric in C60 organic field-effect transistors (OFETs). The pentacene interlayer assists
to improve crystal quality of the C60 layer, leading to the increase of field-effect mobility (lFE)
from 0.014 to 1 cm2V1s1 in vacuum. The lFE value of the C60 OFET is further enhanced to
10 cm2V1s1 when the OFET is exposed to air in a relative humidity of 55%. Generation of mobile and immobile charged ions in solution-based silk fibroin in air ambient is proposed.
VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4841595]
The technology of complementary metal-oxide-semiconductor field-effect transistors (CMOS) is one of the active research areas in the field of organic electronics. One criterion of the organic CMOS technology is to have the field-effect mobility (lFE) of p-type organic field-effect
tran-sistors (OFETs) comparable to that of n-type OFETs. However, two challenging issues exist in the development of organic CMOS. First, the lFE value of p-type OFETs is
approximately one order of magnitude larger than that of n-type OFETs in the past decades.1–5Second, the air stability of n-type OFETs is usually worse than that of p-type OFETs.6–9
Fullerene (C60) is a potential semiconductor for n-type
OFETs because of the reported high lFE value of 1–6
cm2V1s1.10–14The high lFEvalue was achieved by
improv-ing the crystal quality of C60using different fabrication
meth-ods. For instance, a better crystal quality C60was illustrated by
depositing C60 onto the
divinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB) gate dielectric at 250C.11,12 The C60 OFET exhibits a lFE value of 6 cm
2
V1s1. However, the lFEvalue decreases with deposition temperature.
Another method to improve the crystal quality of C60 was
achieved by depositing a very thin pentacene layer on top of the Al2O3gate dielectric at 50C, reported by Itaka et al. in
2006. The lFE value of the C60 OFET was enhanced to
4.9 cm2V1s1with the assistance of the pentacene layer.13 In 2011, Wanget al. reported that the lFEvalue of
pen-tacene OFETs was improved to ca. 23 cm2V1s1using silk fibroin as the gate dielectric.5Silk fibroin is a natural protein, which serves as an excellent gate dielectric for pentacene to deposit on. The fabrication of silk fibroin thin film is an aqueous solution process. Followed with the work of Itaka et al.,13it is of interest to investigate if the pentacene inter-layer effect on the C60OFET with Al2O3gate dielectric also
works for the solution-based silk fibroin dielectric.
In this article, a pentacene layer is inserted between C60
and the solution-based silk fibroin dielectric to fabricate n-type C60OFETs and to illustrate its ability to improve the
lFE value. The C60 OFET with the pentacene interlayer is
abbreviated as the C60/pentacene OFET. The device
charac-teristics of the C60/pentacene OFETs in vacuum and in air
ambient are presented.
The bottom gate configuration of an n-type C60 OFET
with pentacene as the interlayer and silk fibroin as the gate dielectric is schematically shown in Fig. 1. The silk fibroin film was fabricated on the polyethylene naphthalate (PEN) substrate patterned with Au gate electrodes by spin coating using an aqueous solution of silk fibroin. After spin coating, the PEN substrate was cast on a hot plate at 50C for two hours. The preparation of the aqueous solution of silk fibroin can be found in our previous work.5 A pentacene layer of 2 nm (99% Sigma-Aldrich) was thermally evaporated onto the silk fibroin film on PEN at room temperature (25C). A C60layer (99.9% sublimed, Alfa Aesar) of 40 nm thick was
then deposited onto the pentacene interlayer by thermal evaporation at 70C. The morphology of the C60 layer was
measured by using atomic force microscope (AFM, Bruker). Grazing incidence X-ray diffraction (GIXRD) was per-formed to characterize the crystal quality of C60 by using a
diffractometer (Rigaku TTRax III) with CuKa radiation (k¼ 1.54 A˚ ). Au was then deposited through a metal mask to define the source and drain electrodes for the C60OFET. The
channel length and width were 50 lm and 600 lm, respectively.
In order to measure accurate gate dielectric capacitance of silk fibroin, two issues in our measurements were consid-ered. First, Au/silk fibroin/Au of 600 lm 400 lm in size was fabricated next to each OFET as the metal/insulator/ metal (MIM) structure for capacitance measurements in order to get the same film thickness of silk fibroin. The thick-ness of the silk fibroin film was also determined to be ca. 1 lm by using AFM. Second, the quasi-static capacitance (QSC) method was applied to measure the capacitance of the Au/silk fibroin/Au MIM structure using Agilent B1500A. In the QSC method, the gate voltage was swept step by step from the “start” to the “stop” voltage. The sweep rates were set at 0.8 V/s in vacuum and at 0.18 V/s in air ambient, which were the same as those in the transfer curve measurements in order to simulate the real operation conditions.
a)Author to whom correspondence should be addressed. Electronic mail:
The output and transfer characteristics of the OFETs were measured using HP4155C (Agilent) in air ambient and Agilent B1500A in vacuum. The relative humidity is 55% in air ambient in all the measurements. All the samples was pumped down to ca. 1.5 102Torr and waited for ca. 2 h prior to electrical measurements. The water resided in the solution-based silk fibroin thin film is considered to be removed after 2 h pumping since all the electrical data no longer change with time after 2 h in vacuum.
A typical C60OFET with silk fibroin as the gate
dielec-tric exhibits output characteristics with pinch-off and current saturation in vacuum as shown in Fig.2(a). The field-effect mobility in the saturation regime (lFE,sat) was obtained from
the transfer characteristics in Fig.2(b)using the equation
ID;sat¼ Cð ilW=2LÞ Vð G–VTÞ 2
;
where ID,sat, Ci, VG, VT, W, and L denote the drain current in
the saturation regime, gate capacitance, gate voltage, thresh-old voltage, channel width, and channel length, respectively. The Ci value of the Au/silk fibroin/Au MIM structure is ca.
10 nF/cm2as shown in Fig.2(c). The lFE,satvalue is derived
to be 0.014 cm2V1s1. The transfer characteristics in Fig. 2(b) exhibit an off-current of ca. 2 109A, an on/off current ratio of ca. 5 102, and a threshold voltage of ca. 25 V.
The lFE,sat value of 0.014 cm 2
V1 s1 is about two orders of magnitude lower than that reported for the C60
OFETs with Al2O3 as the gate dielectric. The low lFE,sat
value is attributed to the polar bonds of amino acid residues in silk fibroin. Silk fibroin is a protein consisting of the recur-rent amino acid sequence of glycine (Gly), serine (Ser), Gly, alanine (Ala), Gly, and Ala.15 N-H and C¼O are the polar bonds in the amino acid residues Gly, Ser, and Ala. The O-H polar bond appears in the side chain of the amino acid resi-due Ser. The N-H, C¼O, and O-H polar bonds may act as scattering centers to slow down the mobility of electrons along the channel near the C60/silk fibroin interface.
16
The device performance of the C60OFET can be greatly
improved by inserting a pentacene layer of 2 nm thick between C60 and silk fibroin. The C60/pentacene OFET
exhibits output characteristics with pinch-off and current sat-uration in vacuum as shown in Fig. 3(a). The drain current increases greatly from 8.5 107A (Fig.2(a)) to 4.5 105 A at VG¼ 60 V with the assistance of 2 nm pentacene
inter-layer. The corresponding transfer characteristics are shown in Fig. 3(b), which exhibit an off-current of ca. 1 107A,
FIG. 1. Schematic showing the structure of a n-type C60OFET with
penta-cene as the interlayer and silk fibroin as the gate dielectric.
FIG. 2. Electrical characteristics of a typical C60OFET and the Au/silk fibroin/Au MIM structure in vacuum. (a) Output characteristics. (b) Transfer
character-istics. The inset is the ID1/2versus VGplot for the determination of lFE,sat. (c) Quasi-static capacitance versus voltage curve taken at a sweep rate of 0.8 V/s.
FIG. 3. Electrical characteristics of the C60/pentacene OFET and the Au/silk fibroin/Au MIM structure in vacuum. (a) Output characteristics. (b) Transfer
char-acteristics. The inset is the ID1/2versus VGplot for the determination of lFE,sat. (c) Quasi-static capacitance versus voltage curve taken at a sweep rate of
0.8 V/s.
an on/off current ratio of ca. 3 102and a threshold voltage
of ca. 36 V. The Ci value of the Au/silk fibroin/Au MIM
structure is ca. 12 nF/cm2, as shown in Fig.3(c). The lFE,sat
value is derived to be 1 cm2V1s1, that is, about two orders of magnitude larger than that of the C60OFET without 2 nm
pentacene interlayer.
The enhancement of lFE,satfrom 0.014 to 1 cm2V1s1
is attributed to the improvement of the crystal quality of C60.
Note that small C60grains of ca. 50 nm in size are randomly
distributed all over the silk fibroin surface as shown in the AFM image in Fig.4(a). With the assistance of the 2 nm pen-tacene interlayer, small C60grains are connected to form an
irregular body surrounding with some valleys as shown in the AFM image in Fig.4(b). The crystal qualities of these C60 grains are further characterized using GIXRD as shown
in Figs.4(c)and4(d). Without the 2 nm pentacene interlayer, the crystal quality of small C60grains is very poor since both
(002) and (004) peaks of C60are very weak. When the 2 nm
pentacene layer is inserted, the C60layer becomes highly
ori-ented along [002] direction since only C60 (002) peak at
2h¼ 10.7 is greatly enhanced.13 The connected C
60 grains
are highly oriented along [002] based on the GIXRD spec-trum in Fig. 4(b). From the view of material science, the crystal defects at the grain boundaries of the C60 layer are
greatly reduced when small C60 grains are highly oriented
along a fixed direction. The reduction of the trap state den-sity across the highly oriented C60 grains is expected. The
lFE,satvalue is thus enhanced.
A peculiar phenomenon occurs when the C60/pentacene
OFET is operated in air ambient. The C60/pentacene OFET
can be operated at low voltage as shown in the output characteristics in Fig. 5(a). The 2 nm pentacene interlayer results in the appearance of ambipolar characteristics at VG
less than 2 V. This is a typical phenomenon for the solution-based silk fibroin dielectric in n-type C60 OFETs
with the pentacene interlayer. The n-type output characteris-tics with saturation of the C60OFET are clearly observed at
VGhigher than 4 V. The corresponding transfer
characteris-tics in Fig. 5(b) exhibit an off-current of ca. 3 109A, an on/off current ratio of 1 103 and a threshold voltage of 5.9 V. The Civalue of the Au/silk fibroin/Au MIM structure
slightly depends on the gate voltage especially at VGlower
than 2 V. The Civalues of ca. 21 nF/cm 2
at VGranging from
4 to 8 V is used for the derivation of lFE,satsince the
satura-tion region of the output characteristics occurs at VGhigher
than 4 V as shown in Fig. 5(c). The lFE,sat value of the
n-type C60/pentacene OFET is derived to be 10 cm 2
V1s1. Note that the Ci value of the Au/silk fibroin/Au MIM
structure increases from ca. 12 nF/cm2 in vacuum to 21 nF/cm2in air ambient using the QSC method. The increase of Cican be explained by generation of charged ions in silk
fi-broin due to the interaction of water and an amino acid residue in air ambient, similar to what was reported for pentacene OFETs with gelatin or PVP as the gate dielectric.17–19Water may react with the O-H polar bonds in the amino acid residue Ser in silk fibroin and form negatively charged Oions in the side chain and H3Oþions by the reaction
The negatively charged amino acid residue Ser is immobile since it is connected to the main chain of silk fibroin. In con-trast, H3Oþions are mobile. When the gate electrode is
posi-tively biased, H3Oþions would migrate toward the opposite
electrode and form an electric double-layer. This may result in the increase of the Civalue.
The enhancement of the lFE,sat value from
1 cm2V1s1 in vacuum to 10 cm2V1s1 in air ambient
FIG. 4. AFM images of the C60layer of 40 nm thick. (a) On silk fibroin. (b)
On 2 nm pentacene/silk fibroin. GIXRD spectra of the C60layer of 40 nm
thick. (c) On silk fibroin. (d) On 2 nm pentacene/silk fibroin.
FIG. 5. Electrical characteristics of the C60/pentacene OFET and the Au/silk fibroin/Au MIM structure in air ambient. The relative humidity is 55%. (a) Output
characteristics. (b) Transfer characteristics. The inset is the ID1/2versus VGplot for the determination of lFE,sat. (c) Quasi-static capacitance versus voltage
does not result from the crystal quality of highly oriented C60 grains since the crystal quality is the same in vacuum
and in air ambient. The enhancement of lFE,sat can be
explained by the increase of the gate capacitance of silk fi-broin in air ambient that helps to accumulate more electrons to fill some trap states near the C60/pentacene interface. The
lFE,satvalue is enhanced consequently.
In summary, with the assistance of the 2 nm pentacene interlayer, the lFE,sat value of the n-type C60/pentacene
OFET is enhanced from ca. 0.014 to 1 cm2V1s1 in vac-uum. This is attributed to the better crystal quality of C60due
to the insertion of the 2 nm pentacene layer between C60and
the solution-based silk fibroin dielectric. The water absorbed in silk fibroin in air ambient results in the generation of mo-bile ions and immomo-bile charged side chains. This further increases the lFE,sat value to 10 cm
2
V1s1 when the C60/pentacene OFET is operated in air ambient.
The authors like to thank for the financial support from National Science Council, Republic of China, through the projects E-007-094-MY3 and NSC100-2221-E-007-067-MY3.
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