1. Structural analysis of of poly(3-hexylthiophene-2,5-diyl)
Edward Burt Driscoll∗
Department of Physics, North Carolina State University, Raleigh, North Carolina 27607, USA
(Dated May 7, 2015)
Abstract
Compiling spectroscopic, scattering, and visual data from an array of
sources, conclusions are made on the structure of poly(3-hexylthiophene-
2,5-diyl) (P3HT). With the lack of solid-state data, much of the data and
analysis is based on P3HT exhibiting crystalline structure in solutions and
in thin-films either alone or in heterojunction. Unit cell dimensions of 17 ˚A
and 7.58˚A were found in the apparently crystalline structure of P3HT, with
the majority of the planes being stacked edge-on, with slight variations on
the crystal structure leading to minute face-on stacking. The prominence of
the peaks in both GISAXS and GIWAXS measurements gives evidence to-
wards a very prominent crystal structure. Absorption spectra were found to
point towards an electronic structure with a HOMO-LUMO gap of 2.06 eV,
which in turn yields an average conjugation length of around 10 monomers.
Introduction
Poly(3-hexylthiophene-2,5-diyl), referred to as
P3HT, is a polymer often used as an electron donor
in organic photovoltaics(OPVs)[1][2][3]. A monomer of
P3HT consists of thiophene, which is a heterocyclic ring
consisting of five carbon atoms and one sulfur atom, at-
tached to a hexyl chain at the third ring point. It poly-
merizes at the ring points 2 and 5 with single bonds.
The monomer and possible 2-monomer base unit are
shown in Figure 1.
a. b.
Figure 1: A diagram of P3HT molecules, the first being a singular
3-hexylthiophene (a), and the second being a two-molecule chain
portion of polythiophene (b). The two thiophene pentagons in
(b) make up the focus of the molecules.
Conjugated polymers such as P3HT are vital in
the field of organic electronics, both in semiconductor
diodes and in OPVs, due to their plasticity and their
photoabsorbance[4]. Specifically, their strong photoab-
sorbance is due to their structural and electronic prop-
erties. This is what will be researched in this article.
Research in the morphological and electronic prop-
erties of polymers has been very important in the field
of organic electronics, and is the most important area
of research. Most research on these materials is done
with them in solution or thin film form, rarely in the
solid state. Some smaller molecules, such as tetracene,
rubrene, and pentacene, have been used in solid form
for research as single crystals, but this development is
tedious and often yields still imperfect results.
Significance
The structure of P3HT and other conjugating poly-
mer is very important in the field of organic electronics.
The crystalline structure and plane formation of these
polymers influence charge transfer properties, such as
carrier mobility[5]. Many attempts at correlating mor-
phological characteristics and electronic properties have
been made. As this paper does not take into account
the electronic properties of P3HT, conclusions will not
be made on the correlation of the two.
Further research in the field of organic electronics
is beneficial to the general public in that it is working
to provide cheaper and more cost efficient elements for
electronic devices such as screens, diodes, and solar
cells. OPV development is specifically benefited by
research of P3HT, as it can use P3HT as an electron
donor in a heterojunction system. This will eventually
yield more efficient photovoltaics, and therefore more
∗e-mail: ebdrisco@ncsu.edu
1

2. efficient energy production, and ultimately a lesser re-
liance on nonrenewable resources and pollutants.
Experimental Methods
Due to the lack of solid state measurements, data
was used from P3HT in solution and film form, some
with an addition of other organic materials used in de-
vices. Scattering data used was from small-angle X-
ray scattering (SAXS) and wide-angle X-ray scattering.
Grazing incidence (GI) versions of these two scattering
plots are used to induce Bragg diffraction. Various pa-
pers from throughout academia have been published
about varying aspects of P3HT in different states and
forms, and we will compile the data from these to make
conclusions on the structure of P3HT in the solid state.
GISAXS results are used to make conclusions on
stacking formation of P3HT in the film stage, with more
variation between edge-on and face-on meaning a less
rigid crystal structure, while the opposite would pro-
vide evidence for a more rigid crystal structure. This is
done in the GISAXS data by compiling peaks in 2-D,
following the azimuthal angle of diffraction. Peaks at 0o
and 180o
show examples of face-on stacking, and peaks
at 90o
are created by edge-on stacking.
GIWAXS is largely the most important measure-
ment and data dump for quantifying structural prop-
erties. By converting the originally 2-D data set into
1-D, which is often done in papers[1][6], creates peaks
at single locations in q-space. By identifying each peak
by its effective plane, and converting the q-space loca-
tions into real space, information on the crystal shape
and size, along with unit cell information, is derived.
Lastly, absorption spectroscopy is used to identify
characteristics of the electronic structure of P3HT. This
paired up with electron orbit modeling gives insight on
this particular structure of P3HT. Like many other con-
jugated polymer, the electronic structure affected by
photoabsorbance is associated with the carbon atoms
in the main chain, and in this case the thiophene car-
bons. Their p-orbitals become π-orbitals, and allow the
electron energies and orbits to be simplified into a two-
dimensional particle-in-the-box problem.
Using these three data sources, conclusions will be
made to the best of our ability on the structure of
P3HT in the solid state. The limited amount of data
and past interpretation of the data will also limit these
conclusions, but even at a minimal point some conclu-
sions can be made.
Results
In a pure P3HT film, scattering peaks are visible at
0.045 ˚A−1
for qy in GISAXS measurements, and Bragg
peaks at 0.39 ˚A−1
and 0.78 ˚A−1
in qz. The qy peak
shifts to lower values with the addition and increase
in polymer concentration, while the qz peaks remain
constant[1]. Exact crystal structure cannot be decided
using these measurements, but approximations of ori-
entation of stacking can made. A conversion between
q-space and real space can be made:
d = 2π
q (1)
The small qy value exemplifies very large distances
in face-on stacking, that is 140 ˚A while the qz values
are associated with edge-on stacking, and the two val-
ues represent the unit cell dimensions, which will be
discussed next, of 16.1 ˚A and 8.1 ˚A with slight discrep-
ancies of peak location.
Figure 2[6]: GIWAXS measurements for P3HT ribbons, along
with P3HT powder. Extraction temperature and ribbon width
were both varied.
GIWAXS of P3HT ribbons yields scattering inten-
sity peaks around 0.37 ˚A−1
for [100], 0.75 ˚A−1
for [200],
1.12 ˚A−1
for [300], 1.22 ˚A−1
for [111], 1.36 ˚A−1
[121],
and 1.66 ˚A−1
for [020][6]. The scattering data devel-
oped by the use of a 2-D to 1-D program is shown in
Figure 2.Interpreting these plane definitions to be true,
we can use the relation between q-space and real space
in Equation 1 to find the actual dimensions of the crys-
tal structure of the P3HT ribbons.
Equation 1 applied to the scattering peaks gives
us dimensions of 17 ˚A in the [100] plane, 8.38 ˚A for
[200], 5.61 ˚A for [300], 5.15 ˚A for [111], 4.62 ˚A for [121],
and 3.79 ˚A for [020]. Two of the more important re-
sults from this are the [100] and [020] plane dimensions.
Since when polymers such as P3HT stack they form
overlaps that are not as monotonously crystallined like
small molecules and inorganics, these dimensions are
not those inside molecule chains, but instead between
chains. The 17 ˚A distance in the ˚A plane applies to
the lateral in-plane stacking of P3Ht at a distance of
17 ˚A ˙The [020] plane gives us the distance in between
nearest neighboring chains in π − π stacking, where the
true unit cell encompasses parts of three stacked planes,
with the one in between offset. The distance between
each of these three planes is 3.79 ˚A meaning a true unit
cell length of 7.58 ˚A.
2

3. Figure 3: Stacking between nearest neighbors of P3HT chains,
effectively creating a crystalline pattern. The plane-plane stack-
ing (labeled with distance 3.79 ˚A) represents a slight offset of the
thiophene chain, as is in 3-D honeycomb lattices.
Using multiple solvents, UV-vis absorption spectra
for P3HT in a thin film have reported absorption peaks
at 525, 555, and 603 nm[1][4][7] ˙These peaks are visible
in Figure 4. Using the Planck-Einstein relation for the
phonon:
E = hc
λ
(2)
where h is the reduced Planck constant and c is
the speed of light, yields photon energies of 2.06,
2.23, and 2.36 eV. The lowest energy peak, 2.06 eV,
can be assumed to be the smallest energy jump from
P3HT’s ground state, which is the difference between
the HOMO and LUMO energies.
Figure 4[7]: Absorbance spectra of P3HT and associated tri-
block copolymers in film films, using a solvent of chloroform
(CHCl3). A double peak is visible at wavelengths of 525 and
555 nm, while a shoulder peak at 603 nm is also very visible.
Peak location does not vary significantly with the addition of
copolymers, only relative intensity.
Applying a simplified electronic structure of P3HT,
and pretty much any polythiophene, each thiophene
ring contributes four p-orbitals into π-orbitals, one or-
bital from each carbon atom. Assuming that the size
of the thiophene ring varies only slighty during P3HT
synthesis and polymerization, the size of the thiophene
ring can be used to estimate energy states, using a 2-D
particle in the box approximation:
E = ¯h2
π2
2me
n2
L2
x
+ m2
L2
y
(3)
where ¯h is the reduced Planck constant, me is the
mass of an electron, n and m are quantum numbers,
and Lx and Ly are the dimensions of the thiophene
chain. Since polythiophene conjugates in only one di-
mension, it is safe to say that one of the dimensions,
which we choose to be labeled as Ly, has the dimension
of the ”height” of the polythiophene ring, that is, be-
tween the sulfur atom and the C-C bond opposite the
sulfur atom. This length is known to be 2.2 ˚A, while
the width of thiophene is 2.4 ˚A. And due to the diyl at-
tachments between single molecules, the length of each
thiophene ring in conjugation is 2.4 ˚A + 1.5 ˚A = 3.9 ˚A.
The following table gives the results of possible electron
energy states for a single thiophene molecule.
n m ’Energy’
1 0 6.5 eV
0 1 7.8 eV
1 1 14.3 eV
2 0 26.1 eV ’HOMO’
0 2 31.1 eV ’LUMO’
However, in a large conjugation chain of a polymer
such as P3HT, it is safe to assume that Lx >> Ly,
which turns our focus on the quantum number n. This
turns Equation 3 into
E = ¯h2
π2
n2
2meL2
x
(4)
And with a HOMO-LUMO gap of 2.06 eV, where
nLUMO = nHOMO + 1, we can solve for Lx using the
electron density of 4Lx
3.9˚A
:
∆E = ¯h2
π2
2meL2
x
4Lx
3.9˚A
+ 1
2
− 4Lx
3.9˚A
2
= 2.06eV
Lx = 37.9˚A
This conjugation length of 37.9 ˚A can be converted
to chain length by dividing by thiophene’s Lx, giving
us a conjugation length of around 10.
Discussion
Since the data compiled was from past scientific re-
search articles, the peak locations can be confirmed by
them also. Most articles, however, did not take the next
step in interpreting these peaks as crystal structure and
unit cell dimensions.
GISAXS and WISAXS results are not surprising.
The dimensions are to scale with nanostructures in or-
ganic materials. The similarities between GISAXS and
WISAXS q-values, and between papers is remarkable.
Although P3HT seems like a dynamic molecule, with
many different forms, it’s crystalline structure stays
similar, limited to some kinks. This being said, the
3

4. structure of solid state P3HT will exemplify a similar
structure, most likely.
The HOMO-LUMO gap derived in the electronic
structure of P3HT, 2.06 eV, is in close agreement with
past results[9]. This gap is affected mainly by con-
jugation length, so the mode of the material should
not change this much. It is reasonable to make the
assumption that the HOMO-LUMO gap and conju-
gation length derived can be applied to P3HT in the
solid state. Unfortunately, for many of the assumptions
used in deriving the conjugation length, it being only
10 monomers may prove these assumptions false to an
extent, but the magnitude of this length is more than
likely similar to the actual average conjugation length.
Conclusion
With limited data, many conclusions were made of
the structure, both interplane and intraplane, of P3HT.
The SAXS data revealed a strong edge-on stacking with
a [100] plane, with a very minute face-on impurity.
WAXS revealed structural dimensions of 17 ˚A for the
[100] plane and 3.79 ˚A for the [020] plane. The π − π
offset stacking causes the [020], in between the two lined
up polythiophenes separated by 7.58 ˚A. UV-vis absorp-
tion revealed a HOMO-LUMO gap of 2.06 eV, which
then yielded, using a particle-in-a-box approximation
and eventually a 1-D approximation, an average conju-
gation length of 37.9 ˚A, or around 10 monomers. The
conclusions made from SAXS and WAXS are very much
applicable to the solid state crystal, while the absorp-
tion data and interpretations would most likely change
compared to this solution and film data, but would most
likely be similar.
Future Work
Much research could go in to researching polymers,
and other organic materials used in organic electron-
ics, in their solid state. Electronic properties may be
different, along with structural. Although some stud-
ies of single crystal simple organic materials have been
done[10], these are few and far between, and usually
focus on electric not structural properties. These of-
ten yield uncertain results due to material and struc-
tural impurities, that could instead be studied and in-
terpreted using methods discussed and used in this ar-
ticle.
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