synthesized by Suzuki polycondensation in toluene using Pd(PPh3)4
as catalyst from dibromo-monomer 1 and diboronated triphenyl-
1
9
amine (TPA) monomer. PTPA-co-DTDPP has good solubility in
common organic solvents such as chloroform, toluene, and chloro-
benzene owing to its long branched side chains. Molecular weight and
polydispersity index (PDI) of PTPA-co-DTDPP are determined by
gel permeation chromatography (GPC) analysis with a polystyrene
standard calibration. PTPA-co-DTDPP has a high number-averaged
ꢁ1
n
molecular weight (M ) of 14.9 kg mol with a PDI of 1.39.
The UV-Vis spectra of the copolymer in dilute chloroform solution
and thin film are shown in Fig. 1a. In the solution, PTPA-co-DTDPP
exhibits two absorption bands. The first peak is in the wavelength
range of 300–400 nm and the second is in the range of 500–700 nm.
The low energy band is due to ICT between the electron-donating
TPA and electron-withdrawing DPP blocks. The absorption
maximum (lmax) of the PTPA-co-DTDPP in solution is observed
between 600 and 640 nm with a somewhat vibronic feature, indi-
cating the rigid-rod nature of the polymer main backbone, as a result
of the coplanar structure and strong polarity of the DPP units. In
contrast, the absorption of PTPA-co-DTDPP keeps almost
unchanged from the solution to the film, suggesting that the p–p
stacking along the polymer chains is limited due to the presence of 3D
propeller-like TPA units and two bulky side substituents on the DPP
Fig. 2 (a) Chemical structure of PTPA-co-DTDPP. (b) Schematic
representation of OFETs. (c) Transfer and (d) output characteristics of
OFET device with PTPA-co-DTDPP (L ¼ 50 mm, W ¼ 1.5 mm).
opt
units. The optical band gap (Eg ), estimated from the absorption
edge of the thin film absorption spectrum, is 1.66 eV, much smaller
than that (1.9 eV) of widely used rr-P3HT.
saturation. The OFET mobilities are calculated in the saturation
2
regime using the following equation: Ids ¼ (W/2L)mC (V ꢁ V ) ,
i
gs
th
To estimate HOMO and LUMO energy levels of PTPA-co-
DTDPP, we have studied electrochemical properties using cyclic
voltammetry of films drop cast onto a platinum carbon working
electrode with a platinum-wire auxiliary electrode, a Ag wire pseudo-
where W and L are the channel width and length, respectively, C is
i
the capacitance per unit area of the insulation layer. Linear plots of
1
/2
gs ds gs
Ids vs. V , deduced from the I vs. V measurements, yielded a hole
mobility of m ¼ 3.3 ꢀ 10ꢁ cm V s .
3
2
ꢁ1 ꢁ1
+
reference electrode, and Fc/Fc as the external standard (Fig. 1b).
The variation of mobility values from rr-P3HT OFET perfor-
mance is very large because of its various degrees of crystallinity as
induced by the fabrication conditions. For example, hexamethyldi-
PTPA-co-DTDPP shows one quasi-reversible oxidation peak and
one quasi-reversible reduction peak. According to the empirical
equation E(HOMO)/(LUMO) ¼ [(E(ox)/(red) ꢁ E(ferrocene)) + 4.8] eV, the
HOMO and LUMO energy levels were estimated as ꢁ5.07 and
7,13
20,21
silazene (HMDS),
octadecyltrichlorosilane (OTS),
or other
20
silane or alkyl treatments have been used to provide a well-defined,
ordered surface for subsequent semiconductor deposition, which is
attributed to significant improvements in the performance of poly-
thiophenes due to the perpendicular orientation of molecules to the
ꢁ
3.42 eV for PTPA-co-DTDPP. An excellent agreement is found for
the optical band gap (1.66 eV) and the electrochemical band gap
1.65 eV from CV).
(
7,13
Fig. 2 shows the OFET characteristics fabricated using PTPA-co-
DTDPP as an active semiconductor. PTPA-co-DTDPP reveals
a typical p-type semiconductor characteristic operating in accumu-
lation mode. Importantly, a markedly high on/off current ratio of
substrate. Our experimental results obtained from rr-P3HT devices
also exhibit the massive variation of mobility values upon various
modified substrates even on the same fabrication condition, leading
to the differences of over two order of magnitude in the OFET
performance (see Fig. 3). Notably, in contrast to rr-P3HT, the
mobilities of PTPA-co-DTDPP OFETs are almost no change when
5
approximately 10 is not only obtained from the drain-source current
(I ) versus the gate voltage (V ) graph, but also the output charac-
ds G
teristics indicate highly stable device performance with reliable
going from bare SiO to HMDS or OTS treated substrates. In order
2
to obtain further additional evidence on a morphological dependence
free in the OFETs, we subsequently decide to study the annealing
temperature dependence of the mobilities. Fig. 4 shows the mobility
data as a function of annealing temperatures under various modified
2
substrates (SiO , HMDS, and OTS), respectively. It is clearly revealed
that various annealing temperatures are sufficient to induce signifi-
cant morphological changes in the case of rr-P3HT OFETs, resulting
in large deviations for the measured mobilities, which is consistent
22,23
with other literatures.
However, the contribution from the
temperature dependence of the PTPA-co-DTDPP OFETs is slightly
negligible in the temperature range studied (Fig. 4b). Therefore,
PTPA-co-DTDPP can be ideal to be OFETs with a variety of
dielectrics in both top and bottom gate configurations and isolate
effects relating to morphology.
Fig. 1 (a) UV-Vis absorption spectra of PTPA-co-DTDPP in dilute
3
CHCl solution and thin film on quartz plate. (b) Cyclic voltammogram
4 6
of PTPA-co-DTDPP thin film on the Pt electrode in 0.1 M n-Bu NPF
acetonitrile solution at room temperature.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 8528–8531 | 8529