Electron-deficient N-alkyloyl derivatives of thieno[3,4-c]pyrrole-4,6-dione yield efficient polymer solar cells with open-circuit voltages > 1 v

Article abstract of DOI:10.1021/cm5002303

Poly(benzo[1,2-b:4,5-b′]dithiophene-thieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD) polymer donors yield some of the highest open-circuit voltages (V _OC, ca. 0.9 V) and fill factors (FF, ca. 70%) in conventional bulk-heterojunction (BHJ) solar cells with PCBM acceptors. Recent work has shown that the incorporation of ring substituents into the side chains of the BDT motifs in PBDTTPD can induce subtle variations in material properties, resulting in an increase of the BHJ device V_OC to ～1 V. In this contribution, we report on the synthesis of N-alkyloyl-substituted TPD motifs (TPD(CO)) and show that the electron-deficient motifs can further lower both the polymer LUMO and HOMO levels, yielding device V_OC > 1 V (up to ca. 1.1 V) in BHJ solar cells with PCBM. Despite the high V_OC achieved (i.e., low polymer HOMO), BHJ devices cast from TPD(CO)-based polymer donors can reach power conversion efficiencies (PCEs) of up to 6.7%, making these promising systems for use in the high-band-gap cell of tandem solar cells.

Full text of DOI:10.1021/cm5002303

Article
pubs.acs.org/cm
Electron-Deficient N‑Alkyloyl Derivatives of Thieno[3,4‑c]pyrrole-4,6-
dione Yield Efficient Polymer Solar Cells with Open-Circuit Voltages
> 1 V
Julien Warnan,† Clement Cabanetos,† Romain Bude,† Abdulrahman El Labban,† Liang Li,‡
́
and Pierre M. Beaujuge*,†
‡
†Physical Sciences and Engineering Division, Imaging and Characterization Core Laboratory, King Abdullah University of Science
and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia
S
* Supporting Information
ABSTRACT: Poly(benzo[1,2-b:4,5-b′]dithiophene−thieno-
[3,4-c]pyrrole-4,6-dione) (PBDTTPD) polymer donors yield
some of the highest open-circuit voltages (VOC, ca. 0.9 V) and
fill factors (FF, ca. 70%) in conventional bulk-heterojunction
(BHJ) solar cells with PCBM acceptors. Recent work has
shown that the incorporation of ring substituents into the side
chains of the BDT motifs in PBDTTPD can induce subtle
variations in material properties, resulting in an increase of the
BHJ device VOC to ∼1 V. In this contribution, we report on the synthesis of N-alkyloyl-substituted TPD motifs (TPD(CO)) and
show that the electron-deficient motifs can further lower both the polymer LUMO and HOMO levels, yielding device VOC > 1 V
(up to ca. 1.1 V) in BHJ solar cells with PCBM. Despite the high VOC achieved (i.e., low polymer HOMO), BHJ devices cast
from TPD(CO)-based polymer donors can reach power conversion efficiencies (PCEs) of up to 6.7%, making these promising
systems for use in the high-band-gap cell of tandem solar cells.
the BHJs and yielding distinct PCEs in optimized solar cells.13
However, it is worth noting that polymer donors involving
INTRODUCTION
■
Relying on donor−acceptor principles, polymers of electron-
BDT(X) motifs all yield relatively high device VOC values,
rich benzo[1,2-b:4,5-b′]dithiophene (BDT) and electron-
independent of the choice of the ring substituent.9,10,13−16 For
deficient thieno[3,4-c]pyrrole-4,6-dione (TPD) motifs (Eopt
∼
example, conventional BHJ solar cells made with PBDT(T)-
TPD and PCBM reach a VOC of up to ∼1 V, while the devices
maintain high PCEs within the range of 6−6.5%.13,17 In
contrast, several other “high-VOC” polymers have been shown
to yield low device photocurrents and poor solar cell
efficiencies in BHJs with PCBM, and the ability to develop
efficient high-VOC systems has remained a matter of debate for
some time.3,18,19 In fact, recent work has emphasized the
detrimental role of inefficient back hole transfer from the
fullerene to the polymer donor in BHJ solar cells with VOC
values over 1 Vexcitons recombining on the fullerene
leading to low internal quantum efficiencies (IQEs), and greatly
reduced device photocurrents and FFs.3
In this report, we show that PCBM-based BHJ solar cells
made from polymers of BDT(T) and N-alkyloyl-substituted
TPD motifs (TPD(CO)) (Chart 1) can combine high PCEs of
up to 6.7% and high VOC > 1 V (up to 1.1 V). Despite their
large IPs and the high VOC achieved in BHJs with PCBM,
optimized solar cells with PBDT(T)TPD(CO) polymers yield
relatively high short-circuit currents (JSC) on the order of 10
1.9 eV) are some of the most efficient polymer donors in bulk-
heterojunction (BHJ) solar cells with fullerene acceptors, such
as phenyl-C61/C71-butyric acid methyl ester (PCBM).1 With
power conversion efficiencies (PCEs) > 8% and fill factors (FF)
of ca. 70% recently reported in conventional BHJ devices,2
PBDTTPDs are outstanding candidates for use in the high-
band-gap cell of tandem solar cells. In PBDTTPD polymers,
the BDT donors set a large ionization potential (IP) of ca. 5.3
eV (i.e., expected low-lying HOMO), which contributes toward
high device VOC values > 0.9 V.3 Importantly, in these systems,
varying the size and branching of the solubilizing side chains
appended to the π-conjugated main chain impacts polymer self-
assembly in thin-film devices, and in turn, BHJ solar cell
efficiency.2,4 Thus, the determining role of side-chain
substituents on polymer performance is currently at the
forefront of solar cell material optimization studies.1,5−8
In recent studies, we and other groups have examined the
effect of ring substituents incorporated into the side chains of
BDT motifs on polymer properties and BHJ efficiency.9−13 In
PBDT(X)TPD polymers for which the heterocyclic substitu-
ents are either furan (X = F), thiophene (X = T), or
selenophene (X = S), the choice of the side group can induce
subtle changes in material properties and molecular interactions
with PCBM acceptors, affecting the thin-film morphologies of
Received: January 21, 2014
Revised: April 2, 2014
Published: April 3, 2014
© 2014 American Chemical Society
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Chart 1. Molecular Structures of PBDTTPD, PBDT(X)TPD
(with X = F, T, or S), and the N-Alkyloyl Derivative
PBDT(X)TPD(CO)
Scheme 1. Syntheses of the BDT(T) and TPD(CO) Motifs
Information. Separate, recent work describes an alternative
route to the preparation of TPD(CO).26 Figure 1 depicts the
molecular conformation of a C3-substituted TPD(CO) motif
(3) resolved by single-crystal X-ray diffraction (see details in
the Supporting Information), indicating that a torsion angle of
20.6° exists between the carbonyl of the N-alkyloyl group and
the molecular plane of TPD (Figure 1c). The thiophene-
substituted BDT(T) motif 6 was synthesized by addition of
lithiated 2-(2-ethylhexyl)thiophene on the precursor 4 in THF.
The trimethyltin moieties were installed by deprotonation with
n-BuLi at −78 °C and subsequent quenching of the lithiated
intermediate with trimethyltin chloride (SnMe3Cl) (Scheme
1b). Similar protocols have been used in recent studies for the
synthesis of various BDT(X) units.9,11,14,16,27−31 Recent work
has emphasized the relevance of thiophene-substituted benzo-
[1,2-b:4,5-b′]dithiophene (BDT(T)) as a motif in the
preparation of polymer donors that yield high VOC ∼ 1 V in
BHJ solar cells with PCBM.9,13 The PBDT(T)TPD(CO)
polymers (Scheme 2) were synthesized by a microwave-assisted
approach (150 °C for 1 h; see details in the Supporting
Information) in order to control polymer growth and molecular
weight (MW), while minimizing reaction times, and were
mA cm−2 and FFs within the range of 55−60%. We find that
solution-processing additives, such as 1-chloronaphthalene
(CN) used in the casting solutions, play a key role in setting
the adequate BHJ morphology for efficient solar cells with
PBDT(T)TPD(CO) polymers.
RESULTS AND DISCUSSION
■
Design and Synthesis. Electron-deficient thieno[3,4-
c]pyrrole-4,6-dione (TPD) motifs are promising alternatives
to the thieno[3,4-b]thiophene (TT) motifs20−23 in polymer
donors for BHJ solar cells with PCBM acceptors. Compared to
the 5−7 steps required to prepare electron-deficient TT motifs,
the synthesis of TPD units can be achieved in only 3−4
steps.2,4,24,25 However, TPD motifs have not shown the level of
synthetic modularity achievable with TT analogues thus
far.20−23 On the basis of earlier work with TT motifs appended
with various upper-ring substituents,20 the synthesis and
incorporation of N-alkyloyl-substituted TPD motifs (TPD-
(CO)) into polymer donors could account for lower-lying
LUMO and HOMO levels, affording tunable device VOC values
in BHJ solar cells with PCBM.
Scheme 2. Synthesis of the PBDT(T)TPD(CO)(R1/R2)
Derivatives with Branched 2-Ethylhexyl (2EH) Side Chains
on the BDT(T) Motifs and Linear Alkyl Side Chains of
Varying Length (C1, C3, C7, C11, C13) on TPD(CO)
In recent work,2 we showed that, for polymers with branched
alkyl-substituted BDT motifs, a fine modulation of the number
of aliphatic carbons in linear N-alkyl-substituted TPD motifs (i)
induces significant variations in polymer solubility, and (ii) can
be a contributor to improved material performance. To
examine this effect in PBDT(T)TPD(CO), TPD(CO) motifs
with methyl(C1), propyl(C3), heptyl(C7), undecyl(C11), and
tert-decyl(C13) linear chains were prepared sequentially. The
N-alkyloyl derivatives of TPD (TPD(CO)) (3) were
synthesized in two steps starting from the 4,6-dibromothieno-
[3,4-c]furan-1,3-dione (1) (Scheme 1a). First, the imide
intermediate 2 was prepared by addition of ammonia (NH3)
on the anhydride precursor 1 in anhydrous tetrahydrofuran
(THF), and subsequent condensation in the presence of oxalyl
chloride (yield: ca. 60%). Next, the imide 2 was treated with
sodium hydride (NaH) and subsequently reacted with the
desired acyl chloride (R2COCl). Synthetic details and
characterization data are provided in the Supporting
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purified using established methods.2,4 A batch of PBDT(T)-
TPD(2EH/C8) was also polymerized to serve as a model
polymer in this study (see details in the Supporting
Information). The size exclusion chromatography (SEC)-
estimated polymers’ molecular weights and the thermogravi-
metric analyses (TGA) of the polymers are provided in the
Supporting Information (Table S1, and Figure S3).
2b provides the ionization potentials (IP) of the polymers
measured by photoelectron spectroscopy in air (PESA).
Comparing the spectral absorption of the two polymers, the
absorption onsets and maxima in both solutions and thin films
of PBDT(T)TPD(CO) reflect net bathochromic shifts (ca. 60
nm). With optical gaps (Eopt) estimated from the onsets of the
thin-film absorptions, PBDT(T)TPD(CO)(2EH/C7) is found
to be a lower-optical-gap material: 1.79 vs 1.88 eV for the
model polymer PBDT(T)TPD(2EH/C8). In parallel, from
Figure 2b, it should be noted that the IP of PBDT(T)TPD-
(CO)(2EH/C7), 5.41 eV, is slightly larger than that of its
PBDT(T)TPD(2EH/C8) counterpart, 5.29 eVa result that,
at a first level of approximation, can account for a lower-lying
HOMO. In turn, it can be inferred that the N-alkyloyl
substitution promotes a lower-lying LUMO in PBDT(T)TPD-
(CO)(2EH/C7), which would be the main contributor to a
reduced HOMO−LUMO gap.
Device Testing and Characterization. Thin-film BHJ
solar cells with the standard configuration ITO/PEDOT:PSS/
PBDT(T)TPD(CO):PC71BM/Ca/Al were fabricated and
tested under AM1.5G solar illumination (100 mW/cm2). The
cells with optimized PBDT(T)TPD(CO):PC71BM blend ratios
(1:1, wt/wt) were cast from chloroform (CF); similar device
fabrication and optimization procedures were used for all of the
PBDT(T)TPD(CO) derivatives (see the Supporting Informa-
tion).
Figure 1. (a) Molecular conformation of TPD(CO)(C3) resolved by
single-crystal X-ray; and (b) in-plane conformation. (c) Schematic
illustrating the torsion angle of 20.6° between the N-alkyloyl
substituent and the plane of the imide.
Optical Absorption and Ionization Energies. The
optical absorption spectra of the N-alkyloyl-substituted PBDT-
(T)TPD(CO)(2EH/C7) analogue and that of its counterpart
PBDT(T)TPD(2EH/C8) are shown in Figure 2a, and Figure
As shown in Table 1, “as-cast” BHJ solar cells made from
blends of the PBDT(T)TPD(CO) analogues and PC71BM
achieved moderate to low PCEs, ranging from 3.9% (max.
4.1%) for the (2EH/C1) derivative, down to only 1.5% (max.
1.7%) for the (2EH/C13) analogue. Interestingly, as illustrated
in Figure 3b, the solar cell PCE decreases almost linearly with
increasing side-chain length on the TPD(CO) motifs, mainly
due to a drastic drop in device short-circuit current (JSC) across
the device set (see Table 1, 8.7 → 4.3 mA/cm2). These devices
show modest fill factors (FF) within the range of 37−45%, but
high VOC values > 1 V, in agreement with the large PESA-
estimated IP of the PBDT(T)TPD(CO) polymer analogues
(Figure 2b). Thus, the VOC values achieved with PBDT(T)-
TPD(CO)-based devices are found to be greater than that
obtained with their PBDT(T)TPD counterparts (VOC ∼ 1 V)
by at least 0.04 eV, and by up to 0.09 eV. The current density−
Figure 2. (a) Superimposed UV−vis optical absorption spectra
(solution and film) of PBDT(T)TPD(CO)(2EH/C7) and its
counterpart PBDT(T)TPD(2EH/C8). (b) PESA-estimated ionization
potentials (IP, circles) and optical band gap (Eopt, triangles) estimated
from the onset of the UV−vis absorption spectra (films) for the two
polymer analogues.
a d
,
Table 1. PV Performance of the PBDT(T)TPD(CO) Derivatives in Standard BHJ Devices with PC71BM
polymer analogues
CN, DIO
JSC [mA/cm2]
VOC [V]
FF
avg. PCE [%]
max. PCE [%]
TPD(CO); (2EH/C1)
8.7
10.3
7.8
1.04
1.05
1.09
1.08
1.05
1.05
1.04
1.06
1.06
1.05
1.00
1.00
0.45
0.57
0.45
0.57
0.39
0.60
0.40
0.57
0.37
0.56
0.44
0.58
3.9
5.9
3.6
6.3
2.6
6.5
1.8
4.4
1.5
3.8
1.4
6.1
4.1
6.1
3.8
6.5
2.7
6.7
2.0
4.6
1.7
4.1
1.5
6.5
b
3%
TPD(CO); (2EH/C3)
TPD(CO); (2EH/C7)
TPD(CO); (2EH/C11)
TPD(CO); (2EH/C13)
TPD; (2EH/C8)
b
3%
10.7
6.6
b
3%
10.6
4.6
b
3%
7.7
4.3
b
3%
7.0
3.5
c
3%
11.1
a
b
Optimized devices with a polymer:PC71BM ratio of 1:1 (w/w). All devices were solution-cast from chloroform (CF). Devices prepared from
c
blends containing 3% (v/v) of the processing additive 1-chloronaphthalene (CN). Devices prepared from blends containing 3% (v/v) of the
processing additive 1,8-diiodooctane (DIO). Additional device statistics, including variances, are provided in the Supporting Information (Figure
d
S5).
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Figure 3. Evolution of the figures of merit (JSC, VOC, FF, PCE) for
optimized BHJ solar cells fabricated from the PBD(T)TPD(CO)
derivatives(2EH/C1), (2EH/C3), (2EH/C7), (2EH/C11), and
(2EH/C13)under AM1.5G: (a) Devices cast from CF, no CN
additive; (b) with 3% CN additive (v/v). Additional device statistics,
including variances, are provided in the Supporting Information
(Figure S5).
voltage (J−V) curves of the optimized PBDT(T)TPD(CO)-
based devices and the external quantum efficiency (EQE)
spectra of optimized devices of the five polymer analogues are
shown in Figure S4a (Supporting Information) and Figure 4a.
While comparably broad in the range of 350−680 nm, the EQE
response of the (2EH/C1)-based device is shown to be greater
than that of the device cast with (2EH/C3) by ca. 4%, and
greater than that of the (2EH/C7)-based device by ca. 10%
within this range. In parallel, the EQE response of the (2EH/
C1)-based device averaged over the range of 350−680 nm is
approximately 2 times greater than that of the devices made
with (2EH/C11) and (2EH/C13), suggesting that “as-cast”
BHJs of (2EH/C11) and (2EH/C13) may be hindered by
morphological effects (vide infra) and inefficient charge
separation/extraction. Overall, the EQEs of the (2EH/C11)-
and (2EH/C13)-based solar cells remain under 25% across the
visible spectrum, in agreement with the comparably low device
JSC values obtained with these polymer analogues (<5 mA/
cm2). As illustrated in Figure 4c, it is worth noting that the
EQE response of PBDT(T)TPD(CO) extends beyond that of
its PBDT(T)TPD counterpart by ca. 50 nman observation
that is consistent with the distinct onsets of absorption of the
polymers (Figure 2a). Estimated from the space charge limited
current (SCLC) model, the hole mobilities of PBDT(T)TPD-
(CO)(2EH/C7), 2.0 × 10−4 cm2 V−1 s−1 (see the Supporting
Information, Figure S6), are on the same order of magnitude as
those measured in previous work for its PBDT(T)TPD(2EH/
C8) counterpart, 4.7 × 10−4 cm2 V−1 s−1.13 Thus, other
parameters must come into play to explain the relatively low JSC
Figure 4. Superimposed EQE spectra of optimized BHJ devices
fabricated from the PBDT(T)TPD(CO) derivatives: (a) no CN
additive; (b) with 3% CN (v/v). (c) Superimposed EQE spectra of
optimized BHJ devices of PBDT(T)TPD(CO) and its counterpart
PBDT(T)TPD (model polymer); no CN/DIO additive (empty
symbols), and with 3% CN/DIO (v/v) (full symbols).
and FF values in “as-cast” BHJ solar cells with PBDT(T)TPD-
(CO) polymers.
The BHJ morphologies of optimized devices of PBDT(T)-
TPD(CO)(2EH/C1), PBDT(T)TPD(CO)(2EH/C7), and
PBDT(T)TPD(CO)(2EH/C13) were examined by bright-
field transmission electron microscopy (TEM) (see details in
the Supporting Information); analyses are shown in Figure 5a−
c. While the BHJ thin film cast with the (2EH/C1) derivative
shows a relatively well-mixed BHJ morphology (Figure 5a), the
two other BHJ thin films show a net degree of phase separation
between polymer and fullerene, yet on different length scales:
∼50 nm (avg.) in PBDT(T)TPD(CO)(2EH/C7):PC71BM,
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current increases, leading to net improvements in solar cell
efficiency.33−36 In light of the relatively coarse phase-separated
patterns discussed above for “as-cast” devices, solar cells were
also fabricated from blend solutions containing optimum
concentrations of CN additive (3%, v/v); the results are
summarized in Table 1. As illustrated in Figure 3b, all the BHJ
thin films cast from blends containing CN achieved higher JSC
and greater FFs, yielding significant improvements in device
PCE. While “as-cast” solar cells made with the PBDT(T)TPD-
(CO) derivatives showed only modest FFs in the range of 37−
45% (Figure 3a), the devices cast from CN-containing solutions
reach higher FFs of 56−60% (i.e., +12−21%), while
maintaining their high VOC > 1 V. In particular, JSC values >
10 mA/cm2 can be measured for the shorter-chain polymers:
10.3, 10.7, and 10.6 mA/cm2 with the (2EH/C1), (2EH/C3),
and (2EH/C7) analogues, respectively. Combined with FFs of
ca. 60%, optimized BHJ devices cast with PBDT(T)TPD-
(CO)(2EH/C7) reach 6.7% PCE (avg. 6.5%), which represents
a ca. 2.5-fold increase in PCE relative to “as-cast” BHJ devices.
The current density−voltage (J−V) curves of the optimized
PBDT(T)TPD(CO) devices cast from CN-containing blends
and the external quantum efficiency (EQE) spectra of
optimized devices of the five polymer analogues are shown in
Figure S4b (Supporting Information) and Figure 4b. The EQE
responses of the (2EH/C1)-, (2EH/C3)-, and (2EH/C7)-
based devices are comparable in the range of 350−680 nm,
averaging ca. 50% across this range. In contrast, the EQEs of
the (2EH/C11)- and (2EH/C13)-based solar cells remain
within the range of 30−40% across the visible spectrum,
correlating with lower device JSC values (<8 mA/cm2). As
shown in Figure 5d−f, in examining the thin-film morphologies
of the solar cells cast from the (2EH/C1), (2EH/C7), and
(2EH/C13) analogues in blends with PC71BM and CN (3%), it
should be noted that the relatively coarse network of polymer-
and PCBM-rich domains visible earlier in the (2EH/C1)- and
(2EH/C7)-based BHJ thin films is no longer apparent. The
much finer patterns observed for these two derivatives point to
a more intimately mixed polymer:PC71BM blend morphology.
In parallel, optimized BHJ solar cells of the (2EH/C1) and
(2EH/C7) analogues with CN tend to yield higher JSC values
(see Table 1, 8.7 → 10.3 mA/cm2 and 6.6 → 10.6 mA/cm2,
respectively). Meanwhile, Figure 5f shows that BHJ thin films
of the (2EH/C13) derivative retain a certain degree of apparent
phase separation, which may be at the origin of the lower
photocurrents and EQE of the solar cells.
Overall, BHJ solar cells fabricated from PBDT(T)TPD-
(CO)(2EH/C7) achieved figures of merit comparable to those
of the model polymer PBDT(T)TPD(2EH/C8) (Table 1), yet
it is worth noting from Figure 4c that the EQE of the
PBDT(T)TPD(CO)(2EH/C7)-based device does not match
that of its PBDT(T)TPD(2EH/C8) counterpart in the short-
wavelength region (350−550 nm). Excitons recombining on
the fullerene could explain the lower EQE of the device in this
range dominated by PCBM absorption.3 Comparing the texture
of PBDT(T)TPD(CO)(2EH/C7) and PBDT(T)TPD(2EH/
C8) by grazing incidence X-ray scattering (GIXS) (see the
Supporting Information, Figure S8a: neat polymer films), the
two analogues show comparable scattering patterns, with a
partial arc at q ≈ 1.72 Å−1 along the out-of-plane direction (qxy
≈ 0), indicating the presence of “face-on”-oriented π-aggregates
(stacking “out-of-plane”, π−π spacing: ca. 3.65 and 3.68 Å, in
PBDT(T)TPD(CO)(2EH/C7) and PBDT(T)TPD(2EH/
C8), respectively). While “face-on” polymer orientations have
Figure 5. Bright-field TEM images of the thin-film morphologies in
optimized BHJs of (a) PBDT(T)TPD(CO)(2EH/C1), no CN
additive; (b) the (2EH/C7) analogue, no CN; (c) the (2EH/C13)
analogue; no CN; (d) PBDT(T)TPD(CO)(2EH/C1), 3% CN (v/v);
(e) the (2EH/C7) analogue, 3% CN (v/v); and (f) the (2EH/C13)
analogue; 3% CN (v/v). Dark areas: PCBM-rich domains; clear areas:
polymer-rich domains.
and within range of 100−150 nm in PBDT(T)TPD(CO)-
(2EH/C13):PC71BM blend films. In bright-field TEM, darker
areas can be attributed to the PCBM-rich regions, here
interconnected via a network of polymer-rich boundaries. It is
worth noting that (i) none of these thin-film morphologies
compare to the type of fibrillar networks achieved in efficient
BHJ devices cast from PBDTTPD(2EH/C8),32 and (ii) the
coarsely phase-separated morphology in PBDT(T)TPD(CO)-
(2EH/C13):PC71BM blend films (see higher-magnification
TEM images in the Supporting Information, Figure S7) is likely
to limit the efficiency of exciton diffusion to the donor−
acceptor domain boundaries, in turn, yielding low device
photocurrents and poor BHJ solar cell efficiencies. These
observations are consistent with the respectable PCE of
PBDT(T)TPD(CO)(2EH/C1)-based devices (avg. 3.9%;
max. 4.1%), and with the notably lower PCE of the BHJ
solar cells made with the (2EH/C13) derivative (avg. 1.5%;
max. 1.7%).
Small-molecule additives mixed in the polymer:PCBM blend
solution, such as 1-chloronaphthalene (CN) and 1,8-
diiodooctane (DIO), have previously been shown to help
mitigate the thin-film morphology in BHJ solar cells.33−36 The
optimized BHJ morphologies can result in remarkable photo-
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been suggested to promote BHJ solar cell efficiency,4,37−41 the
scattering intensity at q ≈ 1.72 Å−1 in the optimized BHJ thin
films of the PBDT(T)TPD(CO) derivatives and PCBM (see
the Supporting Information, Figure S8b: BHJ thin films) is not
as pronounced as in the neat films, suggesting that the presence
of oriented π-aggregates is not a determining factor in
PBDT(T)TPD(CO)-based BHJ device performance.
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CONCLUSIONS
■
In summary, we have shown that electron-deficient N-alkyloyl-
substituted TPD motifs (TPD(CO)) can lower both the
LUMO and the HOMO levels of the polymer donor
PBDT(T)TPD, yielding device VOC > 1 V (up to ca. 1.1 V)
in standard BHJ solar cells with PC71BM. Despite the high VOC
achieved (i.e., low polymer HOMO), BHJ solar cells cast with
PBDT(T)TPD(CO) polymers in blend solutions containing
the small-molecule additive CN (3%) can reach JSC values > 10
mA/cm2, FFs on the order of 60%, and PCEs of up to 6.7%.
The significant improvements in JSC, FFs (up to +21%), and
overall PCE obtained with optimized devices cast from CN-
containing blends appear to result from finer BHJ morpholo-
gies compared to that of “as-cast” solar cells. With their
relatively wide optical band gaps (Eopt ∼ 1.8 eV), PBDT(T)-
TPD(CO) analogues are promising systems for use in the high-
band-gap cell of tandem solar cells.
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ASSOCIATED CONTENT
* Supporting Information
■
(18) Bijleveld, J. C.; Verstrijden, R. A. M.; Wienk, M. M.; Janssen, R.
A. J. Appl. Phys. Lett. 2010, 97, 073304−073303.
S
Synthetic details, monomer and polymer characterizations,
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higher-magnification TEM images, and GIXS patterns. This
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AUTHOR INFORMATION
Corresponding Author
*E-mail: pierre.beaujuge@kaust.edu.sa.
■
(22) Hou, J.; Chen, H.-Y.; Zhang, S.; Chen, R. I.; Yang, Y.; Wu, Y.;
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Notes
́ ́
(24) Zou, Y.; Najari, A.; Berrouard, P.; Beaupre, S.; Reda Aich, B.;
The authors declare no competing financial interests.
Tao, Y.; Leclerc, M. J. Am. Chem. Soc. 2010, 132, 5330−5331.
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ACKNOWLEDGMENTS
■
The authors acknowledge financial support under Baseline
Research Funding from King Abdullah University of Science
and Technology (KAUST). The authors thank KAUST
Analytical Core Laboratories for mass spectrometry, SEC
measurements, and elemental analyses, and Sandra Seywald
(MPIP − Mainz, Germany) for additional SEC measurements.
The authors thank the Advanced Imaging and Characterization
Laboratories at KAUST for technical support. Portions of this
research were carried out at the Stanford Synchrotron
Radiation Lightsource user facility, operated by Stanford
University on behalf of the U.S. Department of Energy, Office
of Basic Energy Sciences.
́
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