Increased carrier mobility in end-functionalized oligosilanes

Article abstract of DOI:10.1039/c4sc03274h

We show that a class of oligosilane-arene σ, π-hybrid materials exhibits distinct and enhanced solid-state electronic properties relative to its parent components. In the single crystal structure, the σ-conjugation axis of one molecule points towards the

Full text of DOI:10.1039/c4sc03274h

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Increased carrier mobility in end-functionalized
oligosilanes†
Cite this: Chem. Sci., 2015, 6, 1905
S. Surampudi,a M.-L. Yeh,ab M. A. Siegler,a J. F. Martinez Hardigree,b T. A. Kasl,a
a
H. E. Katzab and R. S. Klausen
*
We show that a class of oligosilane–arene s, p-hybrid materials exhibits distinct and enhanced solid-state
electronic properties relative to its parent components. In the single crystal structure, the s-conjugation
axis of one molecule points towards the p-face of a neighboring molecule due to an unusual gauche
conformation. This organization is hypothesized to be beneficial for charge transport. We show that
solution-deposited crystalline films of the hybrid materials show up to a 100-fold increase in space-
charge limited current (SCLC) mobility relative to literature reports of photoinduced hole transport in
oligosilane films. The discovery that s, p-hybrids are more than the sum of their parts offers a design
opportunity for new materials.
Received 25th October 2014
Accepted 29th December 2014
DOI: 10.1039/c4sc03274h
www.rsc.org/chemicalscience
molecular silicon with functionalized p-conjugated organic
groups.
Introduction
The ordering of individual molecules in a solid is as important
for efficient charge transport as the electronic properties of the
molecule itself. This manuscript describes a new, close-packed
molecular crystal form exhibited by oligosilanes terminated
Results and discussion
with cyanovinyl groups. We report record-setting oligosilane Design and synthesis
charge carrier mobilities in their crystalline thin lms.
Tetra- and hexasilanes were selected for initial study on the
Crystalline silicon is the preeminent semiconductor. While
molecular variants of other extended materials are well devel-
oped for electronic applications (for example, the pentacene1–3
core is a substructure of the graphitic carbon lattice), oligosi-
lanes are not broadly studied as small molecule semi-
conductors. Nonetheless, oligosilanes have attractive properties
that merit consideration. Permethylsilanes (SinMe2n+2, Fig. 1)4
are the simplest organosilanes and are bench-stable, process-
able in organic solvents and absorb light strongly in the ultra-
basis of structure–property relationships identied in photo-
induced hole transport studies of oligosilanes7 and polysilanes8
(m ¼ 10ꢀ4 to 10ꢀ3 cm2 Vꢀ1 sꢀ1). Okumoto et al. in a time-of-ight
study of polycrystalline SinMe2n+2 (n ¼ 8–12) lms showed that
mobility is lower in odd-numbered than even-numbered silane
lms and that mobility increases with oligosilane length
(Si8Me18, m y 7.5 ꢁ 10ꢀ4 cm2 Vꢀ1 sꢀ1).7 Kepler et al. observed
similar magnitude hole mobilities in poly(SiMePh) and
violet region due to delocalized
s
molecular orbitals.5,6
Molecular silanes retain the practical attractions of silicon
itself, like earth abundance and low toxicity. Lastly, the poten-
tial for structural variation is enormous. As part of a program
dedicated to identifying the structural requirements for facile
charge transport in silanes, here we focus on the integration of
aDepartment of Chemistry, Johns Hopkins University, 3400 N. Charles St, Baltimore,
MD 21218, USA. E-mail: klausen@jhu.edu
bDepartment of Materials Science and Engineering, Johns Hopkins University, 3400 N.
Charles St, Baltimore, MD 21218, USA
† Electronic supplementary information (ESI) available: Synthetic procedures,
tabulated characterization data, single crystal X-crystallography, differential
scanning calorimetry thermograms, optical and force microscopy, optical and
electronic characterization and device fabrication procedures. CCDC
1030984–1030987. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4sc03274h
Fig. 1 (a) Representative s-conjugated, p-conjugated and hybrid
materials. (b) Synthesis of cyanovinylsilanes. (i) n-BuLi, Et2O, 0 ꢂC;
Cl(SiMe2)nCl (82–96%). (ii) n-BuLi, Et2O, 0 ꢂC; DMF, (iii) RO2CCH2CN,
piperdine, benzene, 80 ꢂC (72–89%, 2 steps).
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poly(Si(n-Pr)2) lms (m ¼ 10ꢀ4 cm2 Vꢀ1 sꢀ1), but poly(SiMePh)
devices were more photostable.8
To further elucidate substituent effects, we installed terminal
functionalized arenes, which also increase crystallinity. We
particularly focused on cyanovinyl groups, an archetypal electron
acceptor moiety.9,10 Thienyloligosilanes and polymers have
previously been described.11,12 a,u-Dichlorooligosilanes of
dened length were synthesized and then arylated with 4-bro-
mophenyllithium according to Tamao's literature procedure
(Fig. 1b).13,14 Subsequent lithium–halogen exchange, formylation
with N,N-dimethylformamide, and Knoevenagel condensation
with methyl 2-cyanoacetate yields cyanovinylsilanes Si4C1 and
Si6C1. Use of n-hexyl 2-cyanoacetate in the Knoevenagel conden-
sation yields Si4C6 and Si6C6. Gratifyingly, all products are crys-
talline solids at room temperature. Products are exclusively the
(E)-olen isomer, as determined by 2-D NMR spectroscopy and
single crystal X-ray diffraction (XRD).
Unusual gauche-conformation in crystal structure
The single crystal XRD structures of Si4C1 and Si6C1 show that
one terminal arene is gauche relative to the silane chain
(Fig. 2a), a highly unusual conformation. Most acyclic a,u-
arylmethylsilanes in the Cambridge Crystallographic Database
adopt a fully extended, all-trans structure and a more limited
number have eclipsed conformations.14,15 A complete list of
references to relevant crystal structures can be found in the
ESI.† A single crystal XRD structure of an acyclic per-
methylsilane has not been reported, but the powder XRD
pattern of a vacuum-deposited Si12Me26 thin lm shows a d-
Fig. 2 Cyanovinylsilane crystal packing. (a) Birds-eye view of Si6C1
molecule highlighting the gauche conformation at Si1. (b) Unit cell of
Si6C1. (c) Crystal packing of Si6C1 extended along the c direction
illustrating the short (3.44 A˚) intermolecular distance between
carbonyl carbons. (d) Powder XRD pattern of Si6C1 thin film. Inset
shows higher resolution image. (e) Powder XRD pattern of Si6C6 thin
film. Cu Ka ¼ 1.54 A˚. Displacement ellipsoids are shown at 50%
probability. Carbon ¼ gray; silicon ¼ yellow; nitrogen ¼ blue; oxygen ¼
red. Hydrogens omitted for clarity.
˚
spacing (d ¼ 25.9 A) consistent with all-anti-silanes oriented
Increased charge carrier transport
perpendicular to the substrate.16
In the Si6C1 structure, the gauche conformation allows for a
close-packed structure and the intermolecular spacing in the c-
Two molecules of gauche-Si6C1 pack such that cyanovinylar-
ene groups co-facially localize and the resultant dimer consti-
tutes the unit cell (Fig. 2b). The distance between carbonyl
carbons in neighboring dimers along the c direction is short
˚
axis direction is short (3.44 A). Charge transport in the c-axis
direction could therefore be particularly efficient as the c-axis is
also aligned with the s-conjugation axis. We fabricated devices
to characterize electronic properties (Fig. 3a). Gold electrodes
(50 nm) were thermally evaporated onto clean glass slides with a
25 mm tungsten wire shadow mask. The wires were removed and
a rectangular pattern (1 ꢁ 2.5 mm) dened on the electrodes
with Novec™ 1700 Electronic Grade Coating. A 1,2-dichloro-
benzene solution of oligosilane was drop-cast onto the Novec™
pattern, then sequentially dried in ambient conditions and in a
vacuum oven. A transparent, crystalline lm covers the 25 mm
channel (Fig. 3b). Current–voltage characteristics were
measured and the charge carrier mobility extracted from the
space-charge limited current (SCLC) regime of the J1/2–V plot
using the Mott–Gurney law (Fig. 3c and ESI†).17 We observe, as
did Okumuto et al.,7 that the mobility increases with silane
length.
˚
(3.44 A, Fig. 2c). We prepared solution-deposited crystalline
lms by drop-casting a 1,2-dichlorobenzene oligosilane solu-
tion onto a Si/SiO2 substrate. The XRD pattern of the Si6C1 thin
lm is consistent with a polycrystalline lm in which silanes are
oriented both perpendicular and parallel to the substrate. The
most intense peak in the Si6C1 powder XRD pattern (Fig. 2d, 2q
ꢂ
˚
¼ 4.67 , d ¼ 18.9 A) is consistent with the c-axis of the Si6C1 unit
˚
cell (20.4 A) and is therefore assigned to reꢂection from the
˚
(001) plane. A lower intensity peak (2q ¼ 7.71 , d ¼ 11.5 A) is
˚
assigned to reection from the (010) plane (b-axis ¼ 11.8 A).
While we were unsuccessful in obtaining a single crystal XRD
structure of Si6C6 or Si4C6, the thin lm XRD patterns are not
consistent with a fully extended conformation. The intense
reection at 2q ¼ 6.47ꢂ corresponds to a d-spacing of 13.7 A,
˚
signicantly shorter than the predicted fully extended molec-
˚
More unusually, we observe an increase in mobility with
increasing the length of the ester alkyl chain (Si6C1 < Si6C6 and
Si4C1 < Si4C6, Fig. 3d). This is in distinct contrast to Okumoto's
study showing a decrease in mobility in anti-silane lms with
longer alkyl groups due to increased interlayer distances.18 That
ular length of >41.0 A (see ESI† for more details). Peaks in the
Si6C6 powder XRD pattern are much more intense than in the
Si6C1 lm, suggesting that Si6C6 forms the more well-ordered
lm with larger domains, a hypothesis supported by optical
microscopy and atomic force microscopy (vide infra).
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between 0.01 and 0.06 cm2 Vꢀ1 sꢀ1 were calculated, consistent
with the SCLC values (see ESI†).
The carrier mobilities in this study are 1–2 orders of
magnitude greater than previous reports, an especially
notable result given that our oligosilanes (n ¼ 4–6) are
shorter than those previously characterized (n $ 8) and
mobility increases with silane length. The observed carrier
mobilities likely represent the lower limit of the carrier
mobility because of the polycrystalline nature of the thin
lms investigated and the hypothesized anisotropy in charge
transport.
Alkyl-linked cyanovinylarenes are less effective charge
transport materials
Alkane control molecules (Fig. 5a) strongly point to an essential
structural and electronic role for the silane moieties in the
hybrid materials. We nd that the parent arene 1 adopts a slip-
˚
stacked structure in which centroids are separated by 4.00 A
(Fig. 5b), a very different packing structure from gauche-silanes.
XRD analysis of a solution-deposited thin lm of 1 shows high
intensity, narrow peaks consistent with
a polycrystalline
Fig. 3 (a) Device architecture. (b) Optical microscope image of Si6C6
crystalline thin film. (c) Representative J1/2–V plot of Si6C6 device. A least
squares fit (red line) to the SCLC regime gives y ¼ ax + b (a ¼ 5.7 ꢁ 10ꢀ4; b
¼ 5.4 ꢁ 10ꢀ3; R2 ¼ 0.98). (d) Tabulated SCLC mobility data for cyanovi-
nylsilane materials. Each mobility is the average of at least five devices and
error bars represent standard deviations. J ¼ current density; V ¼ voltage;
Novec™ ¼ 3M™ Novec™ 1700 Electronic Grade Coating.
we observe the opposite trend is supportive of the hypothesized
benecial effect of the gauche-conformation.
In solution, Si6C1 and Si6C6 have identical optical and elec-
tronic properties. The dependence of device mobility on alkyl
chain length is therefore attributed to the increased crystallinity
of the thin lm as detected by XRD analysis (Fig. 2d and e).
Optical and atomic force microscopy images conrm the
superior lm structure. The drop cast Si6C1 lm contains both
relatively large (100–150 mm wide) needles and precipitated
islands (Fig. 4a and c). In contrast, the optical microscope
image of the Si6C6 thin lm shows a highly oriented smooth and
continuous polycrystalline lm (Fig. 4b and d). The Si6C6 thin
lm was further characterized by atomic force microscopy
(AFM) (Fig. 4e and f). Both the AFM height and phase images
show oriented, smooth regions and are consistent with a
lamellar sheet morphology. Additional images are available in
the ESI.†
The combination of silane length and desirable lm
Fig. 4 (a and c) Optical microscope image of drop-cast Si6C1 thin film
morphology makes the Si6C6 SCLC mobility (m ¼ 0.023 cm2 Vꢀ1 on Si/SiO2 showing discontinuous structure and large needles. Images
sꢀ1) the highest mobility measured in this system or in any
molecular silane. High carrier mobility is also observed in
are at different resolutions. (b and d) Optical microscope image of
drop-cast Si6C6 thin film on Si/SiO2 showing continuous and highly
oriented film structure. Images are at different resolutions. (e) AFM
height image of Si6C6 thin film. (f) AFM phase image of Si6C6 thin film.
preliminary thin lm transistor studies. Three devices with a
Si6C6 active layer and width-to-length ratios between 10 and 20
showed transistor-like output curves, from which mobilities images are consistent with a lamellar sheet morphology.
Both the height and phase images show oriented smooth regions. The
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structure (Fig. 5c). However, electronic characterization of optimized geometry. One low energy conformation (Fig. 6b) has
devices fabricated in the same manner as the cyanovinylsilanes a gauche conformation within the silane chain itself (u ¼ 57.7ꢂ),
did not show a SCLC region and charge carrier mobility could a conformation different from both the Si6C1 crystal structure
not be determined (see the ESI† for more details). C6C6 (Fig. 5a), and the all-anti C6C1 optimized geometry. Other low energy
an analog of Si6C6, was synthesized and characterized as well. conformations of Si6C1 have an all-anti relationship. We inter-
Widely displaced and separated aryl rings are observed in the pret the computational data to mean that rotation about the Si–
C6C6 crystal structure (Fig. 5d). Current–voltage characteriza- Si bond in the gas phase is facile. We suggest that in the bulk
tion does not show a SCLC region in devices prepared from material, the exible oligosilane chain can easily adjust to
C4C1, C6C1, nor C6C6.†
accommodate stabilizing interactions with neighbouring func-
tional groups, which drive the preference for the gauche-cya-
novinylsilane conformation in the crystal structure. Fig. S4†
highlights some of the intermolecular short contacts between
Si6C1 molecules in the unit cell.
Density functional theory calculations
The structural role of the silane was further investigated by
quantum chemical calculations (B3LYP/6-31G) of C6C1 and
Si6C1. The optimized gas phase structure of an isolated C6C1
molecule is different from its crystal structure conformation
(Fig. 6a). In the gas phase, linker chains adopt an all-anti
conformation. The terminal arenes are in line with the linker
axis, but the arene planes are perpendicular. Clearly, the kink in
the alkane linker observed in the C6C6 crystal structure is sup-
ported by intermolecular interactions. Intermolecular CH–p
interactions involving the alkane linker are found in the C6C6
crystal structure and are illustrated in Fig. S3 (see the ESI† for
more details).
Our results are consistent with the body of theoretical
studies on oligosilane structure that show a at conformational
prole.4,19,20 Studies by West and others report that both gauche
andꢀa1nti conformations are energy minima and differ by <1 kcal
mol . The calculated barrier to rotation is small (ꢃ1 kcal
molꢀ1).19
Because of the well-known dependence of oligosilane elec-
tronic structure on conformation,21,22 it is difficult to compute a
reliable Si6C1 electronic structure for comparison with C6C1 in
view of the similar energies of multiple Si6C1 conformations.
Nevertheless, we present one possible pairwise comparison
here. In C6C1, the HOMO and LUMO are localized on opposite
arenes and no orbital density is found on the alkane chain
(Fig. 6c). In gauche-Si6C1, we nd that the LUMO is also localized
on one arene, while the HOMO is delocalized across the
Quantum chemical calculations of Si6C1 are computationally
intensive because of the six heavy elements and the additional
degrees of freedom introduced by the silane methyl substitu-
ents. The calculations have difficulty converging on a single
Fig. 6 (a) DFT optimized ground state structure of C6C1 (B3LYP/6-
31G). All-anti alkane linker is highlighted. Carbon ¼ gray; nitrogen ¼
blue; oxygen ¼ red. Hydrogens omitted for clarity. (b) One of several
low energy structures calculated for Si6C1. The torsional angle defined
by Si1–Si2–Si3–Si4 is 57.7ꢂ, a gauche relationship. Carbon ¼ gray;
silicon ¼ yellow; nitrogen ¼ blue; oxygen ¼ red. Hydrogens omitted
Fig. 5 (a) Chemical structure of alkyl cyanovinyl materials. (b) Crystal for clarity. (c) DFT calculated highest occupied and lowest unoccupied
packing of 1 along the a direction highlighting slip-stacked structure. molecular orbitals of C6C1. Orbital density is localized on opposite
Distances are given for arene centroid to centroid. (c) Powder XRD arenes. Carbon ¼ gray; blue ¼ nitrogen; red ¼ oxygen; white ¼
diffraction pattern of solution-deposited thin film of 1. Cu Ka ¼ 1.54 A˚. hydrogen. (d) DFT calculated highest occupied and lowest unoccupied
(d) Structure of two neighboring molecules of C6C6. Distances are molecular orbitals of Si6C1. The LUMO is localized on one arene, while
given for arene centroid to centroid. Displacement ellipsoids are the HOMO is delocalized on the hexasilane and partially onto one
shown at 50% probability. Carbon
oxygen ¼ red. Hydrogens are omitted for clarity.
¼
gray; nitrogen
¼
blue; arene. Carbon ¼ gray; pale green ¼ silicon; blue ¼ nitrogen; red ¼
oxygen; white ¼ hydrogen.
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hexasilane (Fig. 6d). The alternating nodal pattern reproduces
the electronic structure of simple permethyl and
perhydrohexasilanes.4,23
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685, 9.
The difference in the Si6C1 and C6C1 HOMO energies
supports an electronic role for silicon. This is also strongly
suggested by a visual comparison of the alkyl and silyl mate-
rials: the silanes are bright yellow in color, while the alkyl
materials are white. A donor–acceptor interaction between the
silane and the cyanovinyl group is consistent with intermolec-
ular charge transfer from cyclosilanes to tetracyanoethylene
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Conclusions
We report here that the combination of s- and p-conjugated
units is greater than the sum of its parts. We observe electronic
and structural properties in hybrid materials that cannot be
achieved by either unit alone. This observation is expected to be
of general interest to the electronic materials community. Our
results indicate that further structural optimization of molec-
ular silicon and hybrid materials could yield even greater
advances in electronic properties.
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Acknowledgements
S. S., M.-L. Y., T. A. K., and R. S. K. thank Johns Hopkins
University for start-up funds. H. E. K. was supported by the
National Science Foundation Division of Materials Research
(DMR) (Grant 1005398). We thank the National Science Foun-
dation Division of Materials Research (DMR), MRSEC program,
grant number 1121288 for the AFM study. We thank Dr Michael
L. Steigerwald for helpful discussions. We thank Jiawang Zhou
and Justin DeFrancisco of Johns Hopkins University for assis-
tance with DFT calculations.
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