Optical studies of native defects in π-conjugated donor–acceptor copolymers

Abstract

We used multiple spectroscopies such as photoinduced absorption (PIA), magneto photoinduced absorption, and doping induced absorption for studying native defects in π-conjugated donor–acceptor copolymer chains of benzodithio-phene fluorinated benzotriazole. The PIA spectrum contains characteristic photoinduced absorption bands that are due to polarons and triplet exciton species, whose strengths have different dependencies on the modulation frequency, temperature, and laser excitation, as well as magnetic field response. We found that the native defects in the copolymer chains serve as efficient traps that ionize the photoexcited excitons, thereby generating charge carriers whose characteristic optical properties are similar, but not equal to those of intrachain polarons formed by doping. The native defects density is of the order of 10¹⁷ cm⁻³ indicating that most of the copolymer chains contain native defects upon synthesis; however, this does not preclude their used-for photovoltaic applications.

Full text

Optical studies of native defects in π−conjugated donor–acceptor copolymers
Sangita Baniya, Dipak Khanal, Evan Lafalce, Wei You, and Z. Valy Vardeny
Citation: Journal of Applied Physics 123, 161571 (2018);
View online: https://doi.org/10.1063/1.5012995
View Table of Contents: http://aip.scitation.org/toc/jap/123/16
Published by the American Institute of Physics

Optical studies of native defects in p-conjugated donor–acceptor
copolymers
Sangita Baniya,
1
Dipak Khanal,
1
Evan Lafalce,
1
Wei You,
2
and Z. Valy Vardeny
1,a)
1
Department of Physics and Astronomy, University of Utah, Salt Lake City, Utah 84112, USA
2
Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290, USA
(Received 9 November 2017; accepted 3 January 2018; published online 19 January 2018)
We used multiple spectroscopies such as photoinduced absorption (PIA), magneto photoinduced
absorption, and doping induced absorption for studying native defects in p-conjugated donor–
acceptor copolymer chains of benzodithio-phene fluorinated benzotriazole. The PIA spectrum
contains characteristic photoinduced absorption bands that are due to polarons and triplet exciton
species, whose strengths have different dependencies on the modulation frequency, temperature,
and laser excitation, as well as magnetic field response. We found that the native defects in the
copolymer chains serve as efficient traps that ionize the photoexcited excitons, thereby generating
charge carriers whose characteristic optical properties are similar, but not equal to those of intra-
chain polarons formed by doping. The native defects density is of the order of 10
17
cm
3
indicating
that most of the copolymer chains contain native defects upon synthesis; however, this does not
preclude their used-for photovoltaic applications. Published by AIP Publishing.
https://doi.org/10.1063/1.5012995
INTRODUCTION
Low band gap p-conjugated polymers have attracted
substantial interest, especially in the field of organic photo-
voltaic (OPVs) applications because of their extended sun-
light absorption, lightweight, and ease of fabrication.
1–5
In
traditional OPV cells, the donor p-conjugated polymers have
been homopolymers such as polythiophene that absorb in the
visible spectral range of the solar spectrum.
6
Recently, the
homopolymers have been replaced by copolymers as elec-
tron donors whose repeat units consist of alternating donor
(D) and acceptor (A) moieties.
7–16
This chain architecture
substantially reduces the optical gap, and thus, the D-A
copolymers absorb in the near infrared, and this increases the
power conversion efficiency (PCE) of OPV based on copoly-
mer/fullerene blends above 10%.
16,17
However, the open cir-
cuit voltage, Voc of cells based on D-A copolymers, is
relatively low due to the inherent low band gap.
17–19
Thus,
intermediate band gap (2.0 eV) copolymers are still relevant
for use in OPV cells, since they are designed to achieve high
Voc, which is especially important for tandem geometry.
19–21
We also note that despite the increasing interest in device
applications, there is only limited understanding of the molec-
ular doping of D-A p-conjugated copolymers,
22
or the nature
and properties of the charge excitations (polarons) in these
materials, which prevent further improvements in OPV
applications.
23–31
For our present research investigation, we have chosen the
p-conjugated D-A copolymer benzodithio-phene (BnDT) fluo-
rinated benzotriazole (FTAZ); see structure in Fig. 1(a).
19–21
PBnDT-FTAZ is a copolymer having intermediate band-gap
with an optical gap in the visible spectral range, namely at
600 nm [Fig. 1(b)].
19,20
OPVcellsbasedonPBnDT-FTAZ/
([6,6]-phenyl-C61-butyric acid methyl ester) (PCBM) blend
have achieved PCE of up to 7% in an optimized device archi-
tecture.
19,20
Here, we have studied native defects in pristine
PBnDT-FTAZ using photoinduced absorption (PIA) spectros-
copy compared with doping induced absorption (DIA). We
found that the pristine films contain 10
17
cm
3
defects den-
sity that are formed during the synthesis process, which effi-
ciently dissociate the photoexcited excitons and trap positive
FIG. 1. (a) The backbone structure of the PBnDT-FTAZ D-A copolymer
that contains two different moieties. (b) The photoluminescence (PL) and
absorption (O.D.) spectra of the copolymer film at ambient conditions.
a)
Author to whom correspondence should be addressed: val@physics.utah.edu
0021-8979/2018/123(16)/161571/6/$30.00 Published by AIP Publishing.123, 161571-1
JOURNAL OF APPLIED PHYSICS 123, 161571 (2018)

charges. We have identified the charges as polarons having
two characteristic absorption bands below the gap
29
that also
show associated IR-active vibration (IRAV) with a huge
oscillator strength. To characterize the various photoexcita-
tion species, we have also used the magnetic field effect to
discriminate between the triplet excitons and polaron
excitations.
EXPERIMENTAL
The PBnDT-FTAZ copolymer powder was synthesized
in Wei You group at North Carolina University. Thin copol-
ymer films were spin cast or drop cast from solution in tri-
chlorobenzene at a concentration of 12 mg/ml onto sapphire
or KBr substrates, depending on the spectral range of inter-
est. The sample films were placed in a cryostat equipped
with transparent windows in the visible to mid-IR, where the
temperature could be regulated from 50 K to 300 K.
The photoluminescence (PL) and absorption spectra of
the pristine films were measured using a steady-state photo-
modulation pump-probe set-up,
32
where the pump was a
diode laser at 486 nm and an intensity of 100 mW/cm
2
. For
PIA, the probe beam was derived from an incandescent
Tungsten lamp (visible spectral range) or glow-bar (mid-IR
range) and passed through a 1
=
4met monochromator opti-
mized at various wavelengths throughout the visible, near-IR
and mid-IR spectral ranges. A variety of semiconductor
detectors such as Si, Ge, and InSb were used to monitor the
transmission, Tthrough the sample film, and the changes, DT
induced by the pump beam. The PIA spectrum was subse-
quently calculated as DT/T. To cover the spectral range of
500–4000 cm
1
, we used an FTIR spectrometer. In this case,
the PIA spectrum was obtained using a shutter to modulate
the laser illumination on the film, and we signal-averaged
the IR absorption spectrum for 5000 scans
33,34
For the DIA
spectrum, we doped the film p-type by exposing it to iodine
vapor for various time durations ranging from a few seconds
to 1 h.
The magneto-PIA (MPA) measurements were performed
using the same cw PIA set-up described above upon the appli-
cation of an external magnetic field provided by an electro-
magnet that produced field, Bup to 1800 Gauss applied
parallel to the film surface. The MPA(B)responseisdefined
by the relation MPA(B)¼[PIA(B)PIA(0)]/PIA(0), where
PIA(B) is the PIA at field strength B, and PIA(0) is the PIA at
B¼0.
35
We measured the MPA response at several wave-
lengths in order to study the existence of correlation among
various bands in the PIA spectrum.
RESULTS AND DISCUSSION
Figure 1shows the absorption and PL spectra of pristine
PBnDT-FTAZ film. Figure 1(a) shows the copolymer back-
bone repeat unit; it is composed of both donor and acceptor
moieties.
19–21
In Fig. 1(b), it is seen that the absorption onset
of the copolymer is at 2 eV, whereas the absorption peaks at
2.15 with maximum slope at 2.1 eV, which set the mean
exciton energy in PBnDT-FTAZ at 2.1 eV. The absorption
spectrum also shows a second pronounced band 180 meV
higher than the first band, which we consider to be a
“vibration replica.” We also note that a third band occurs in
the absorption spectrum at 3.0 eV, which may be due to a
second transition of the copolymer chain.
36,37
The PL spectrum shown in Fig. 1(b) is composed of two
pronounced bands that are in the form of a “mirror image” of
the absorption spectrum close to the band-edge, that are
180 meV apart. The PL main peak is at 1.86 eV having an
apparent “Stokes shift” of 250 meV from the first absorption
peak. However, this large energy difference may be caused
by exciton diffusion to sites on the chain that have the lowest
energy, rather than a natural, intrinsic Stokes shift.
In order to better study the excited states of the copoly-
mer, we used the electro-absorption (EA) spectroscopy,
which is capable of more accurately determining the transi-
tion energies such as van-Hove singularities in the density of
states.
38
Figure 2shows the EA spectrum of pristine PBnDT-
FTAZ film measured at 50 K; the inset shows quadratic
EA voltage dependence that is traditionally obtained in p-
conjugated polymers. The EA feature at the lower photon
energy resembles the first derivative of the absorption spec-
trum at the band-edge; it is therefore caused by Stark shift of
the lowest SE. In this case, the first zero crossing energy at
2.08 eV is therefore assigned to the average singlet exciton
energy, E
SE
of the copolymer.
39,40
The second EA feature is
a derivative-like of the phonon replica
41
(0–1 transition)
180 meV higher than E
SE
. The third EA feature is an
induced absorption band at 2.64 eV, which does not have any
relation with the absorption derivative spectrum. We there-
fore assign it as due to the forbidden exciton, m
1
A
g
state in
the singlet manifold that becomes partially allowed due to
the symmetry breaking caused by the applied electric field
in the PBnDT-FTAZ film.
39,41
The energy difference, D
SE
¼E
mAg
E
SE
¼0.56 eV, is traditionally taken to be the lower
bound of the exciton binding energy, E
b
in p-conjugated pol-
ymers;
41
and we adopt this interpretation here for PBnDT-
FTAZ. We note that E
b
of 0.56 eV is substantially lower
than E
b
values in traditional p-conjugated polymers such as
Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]
FIG. 2. The electroabsorption (EA) spectrum of pristine PBnDT-FTAZ film
measured at 50 K and field strength of 100 kV/cm. The inset shows a qua-
dratic dependence of the EA on the applied voltage.
161571-2 Baniya et al. J. Appl. Phys. 123, 161571 (2018)

(MEH-PPV) and Poly(3-hexylthiophene-2,5-diyl) (P3HT)
(0.9 eV).
41
The lower E
b
value here may help the exciton
dissociation process, which facilitates charge photogeneration
in this compound.
16
Figure 3(a) shows the steady-state PIA spectrum of pris-
tine PBnDT-FTAZ copolymer which contains several PA
bands. The PA band P1 at 0.4 eV is superposed by several
anti-resonances (AR), and therefore we tentatively identify it
as due to photogenerated polarons.
34
The second PA, namely
band Tat 1.1 eV, is long-lived and neutral (since it is not
correlated with photoinduced IRAVs) and therefore, we
identify it as due to triplet excitons.
35,42
A more detailed PIA
spectrum in the frequency range of the vibrational modes is
shown in Fig. 3(b), where some AR dips are marked with
arrows. These features can be well described by the ampli-
tude mode model,
33,34
which is beyond the scope of the pre-
sent work.
To prove that the two PA bands in the PIA spectrum are
not correlated to each other, and therefore they belong to dif-
ferent photoexcitation species, we compare the frequency,
temperature, and excitation intensity dependencies of the
two PA bands. Figures 4(a) and 4(b) show the frequency
dependent PA strength at 50 K from 30 Hz to 100 kHz for the
Tand P1 bands, respectively. It is seen that the PA lifetime,
determined by the angular frequency x
max
at which the PA
at quadrature gets maximum, is very different for the two
bands. Whereas x
max
5 kHz for the Tband, it is 30 Hz
for the P1 band. To estimate the photoexcitations lifetime,
we use the relation x
max
¼2p/t
0
where t
0
is the mean life-
time. Whereas we estimate the polarons lifetime to be
300 ms at 50 K, the triplet lifetime is about 1 ms. We note
that is not possible to fit the frequency dependence with a
single lifetime, since the disorder in the copolymer gives rise
to a distribution of lifetimes (dispersive recombination); and
this justifies our method to determine the mean lifetime.
More accurate models for the lifetime determination are out-
side the scope of this contribution.
Figures 4(c) and 4(d) show the temperature dependence
of the Tand P1 band, respectively. The Tband decreases
with the temperature rather monotonically from 50 to 300 K,
whereas the polaron band P1 does not decay with the temper-
ature in the range of 50–250 K; however, it vanishes at
300 K. Based on these observations, we conclude that the PA
bands P1 and Tare due to different photoexcitation species.
FIG. 3. (a) The photoinduced absorp-
tion (PIA) spectrum of pristine PBnDT-
FTAZ film. The triplet (T) and polaron
(P1) PA bands are assigned. (b) The
detailed PIA spectrum in the vibrational
spectral range emphasizing the polaron
P1 band and associated vibrational
modes. Various anti-resonances (AR)
are shown by the arrows.
FIG. 4. (a) and (b) PIA frequency
dependence and (c) and (d) tempera-
ture dependence of the PA bands Tand
P1, respectively.
161571-3 Baniya et al. J. Appl. Phys. 123, 161571 (2018)

Based on these observations, we conclude that the PA bands
P1 and Tare due to different photoexcitation species.
Moreover, we compare the pump intensity (I
L
) dependent
of the two PA bands shown in Figs. 5(a) and 5(b),respec-
tively. Both PA bands increase with I
L
. To determine the
recombination kinetics, we plot the PA intensity dependence
in a log –log format, as seen in Figs. 5(c) and 5(d). Both inten-
sity dependencies are in the form of a power law PA (I
L
)
p
,
whereas the exponent p ¼1
=
2for the triplet band T[Fig. 5(c)];
p¼1 for the polaron band P1. We therefore conclude that the
recombination kinetics of the triplet excitons is bimolecular,
probably caused by triplet-triplet annihilation (TTA) process,
whereas the polaron species represented by band P1 is mono-
molecular. This is odd, since the polarons should recombine
in pairs: (P
þ
)þ(P

)!GS, where (P
þ
) is a positive polaron,
(P

) is a negative polaron, and GS is the ground state. We
therefore conclude that the polarons are generated at a charge
defect on the chain, which may be considered as a deep trap
in a Shockley-Reed type recombination. This recombination
process is known to be monomolecular.
Figure 6shows the magneto photoinduced absorption
(MPA) response of the copolymer at 50 K. The MPA(B)
response, where Bis the field strength, is very different for
bands Tand P1. Whereas MPA is positive (increases with B)
for the band T, it decreases for the band P1. Furthermore, the
two MPA responses are not correlated to each other, since
they show different characteristic widths. Whereas the
MPA(B) full width at half maximum due to triplets is 345
G, it is only 180 G for the polaron band. In general, the
MPA originates from the field induced change in the photoex-
citation recombination dynamics that originates from induced
changes in the spin-mixing processes. The different MPA
response thus shows that the two photoexcitations have very
different recombination kinetics, in agreement with the con-
clusion from the PA intensity dependent. The MPA(B)
response for the T band is in agreement with TTA process,
where its width is related to the zero-field splitting parameters
of the triplet excitons. In contrast, the MPA(B) width of the
polaron band is related to the difference, Dgbetween the g-
factors of the hole-polaron and the traps in the copolymer.
FIG. 5. (a) and (b) The PIA spectrum
of band Tand P1 measured at different
pump intensities. The absolute-PA
intensity dependence of bands Tand
P1, plotted in (c) and (d), respectively,
in log –log form.
FIG. 6. Magneto photoinduced absorp-
tion [MPA(B)] response of (a) triplet
and (b) polaron PA bands in the PIA
spectrum, measured at 50 K.
161571-4 Baniya et al. J. Appl. Phys. 123, 161571 (2018)

Next, we identify the P1 band as due to polarons from
the similarity with the lower energy of the doping induced
absorption (DIA) band. Figure 7(a) shows the complete
DIA spectrum of the PBnDT-FTAZ copolymer upon doping
with iodine. It is seen that, in fact, the DIA spectrum of the
charge excitations contains two bands, namely P1 peaked at
0.4 eV and P2 with a peak at 1.3 eV (Refs. 33 and 42)
where 2E(P1) þE(P2) E
g
, the copolymer optical gap. We
therefore identify the charges on the copolymer chains as
polarons since these excitation species have two allowed
absorption bands, as seen in many DIA and PIA spectra of
homopolymers,
34
and calculated for the copolymers.
29
From
the photon energy of the P1 band maximum, we estimate the
polaron relaxation energy in PBnDT-FTAZ copolymer upon
doping to be 0.3 eV. Importantly, we note that apart from
the photon energy of the maximum absorption of the P1
band in the PIA and DIA spectra, they are quite similar to
each other. This validates our interpretation that the P1 band
in the PIA spectrum is due to photogenerated polarons. The
red-shift of P1 upon photogeneration relative to that formed
upon doping can be readily explained as due to smaller
charge pinning for the photogenerated polarons.
It is known that in p-conjugated polymers, the optical
cross section, rfor polarons, is estimated to be r10
16
cm
2
. Thus, the photogenerated polaron density, N, can be
obtained from the PA strength using the relation: DT
T
¼Dad¼Nrd:
43
Here, dis the film thickness, and DT
Tis
the PA strength. Using this relation, rfor polarons, and d
100 nm, we estimate the photoinduced steady-state polaron
density, N, in the pristine film to be N¼10
17
cm
3
. Direct
photogeneration of polarons in DA-copolymers is not a via-
ble process because, as we determined above, the exciton
binding energy in these compounds is 0.56 eV, much larger
than k
B
Tat room temperature. We thus conclude that there
are native defects in the copolymer chains that aid exciton
dissociation; in this case, the photogenerated polaron density
is a lower limit for the defects density on the copolymer
chains. These native defects may efficiently ionize the photo-
excited excitons into electron-hole pair, while trapping the
holes in the form of positive polarons.
Figure 7(b) shows the evolution of the IR-absorption spec-
trum in PBnDT-FTAZ upon iodine doping for various time
durations. We note that doping of PBnDT-FTAZ occurs rela-
tively easy here, similar to doping of many homopolar poly-
mers, and in contrast to other copolymers discussed in the
literature.
22
We can divide the DIA spectrum in PBnDT-FTAZ
into two regions, namely the vibrational spectral range up to
1500 cm
1
and the electronic spectral range at higher photon
energy. The absorption strength in both spectral ranges intensi-
fies similarly with the iodine exposure time, showing that they
are correlated. We identify the strong vibrations as IR-active
vibrations (IRAVs) related to the added charges onto the copol-
ymer chains, which form an extra IR absorption activity to the
otherwise inactive Raman modes.
44
It is interesting that the
IRAVs oscillator strengths are similar (but weaker overall) to
that of the electronic absorption band associated with
the charge excitation. The immense oscillator strength of the
polaron related vibrations indicates that the added charges are
“easy to move,” pointing to a relatively small effective kinetic
mass
33,44
despite the relaxation energy associated with the
added charges on the chain, namely the “polaronic effect.”
SUMMARY
In summary, we identified the native defects in PBnDT-
FTAZ copolymer chains with a density of 10
17
cm
3
using
the photoinduced absorption technique. This is possible since
direct polaron photogeneration is not a viable process in
these copolymers, and therefore, the existence of polaron
bands in the PIA spectrum of the pristine copolymer indi-
cates that defects are responsible for the photogeneration
of these photoexcitation species. Magneto photoinduced
absorption, frequency dependent temperature, and excitation
intensity dependence of photoinduced absorption allow us to
identify the two PA bands in the PIA spectrum as due to trip-
let excitons and polarons, respectively.
ACKNOWLEDGMENTS
The work at the University of Utah was supported by the
AFOSR under Award No. FA9550-16-1-0207 (E.L.; PA
spectroscopy); the NSF-MRSEC Program No. DMR 1121252
FIG. 7. (a) The doping induced absorption (DIA) spectrum of PBnDT-
FTAZ film. The polaron bands P1 and P2, and the IRAV are assigned. (b)
The evolution of the PIA spectrum upon exposure to iodine vapor measured
at various time durations, as given. P1 polaron band and related IRAVs are
assigned.
161571-5 Baniya et al. J. Appl. Phys. 123, 161571 (2018)

(S.B.; PA spectroscopy), and the NSF Grant No. DMR-
1701427 (D.K.; MPA spectroscopy). The authors declare no
competing financial interest.
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