Solution-processed, molecular photovoltaics that exploit hole transfer from non-fullerene, n-type materials

Article abstract of DOI:10.1002/adma.201305444

Solution-processed organic photovoltaic devices containing p-type and non-fullerene n-type small molecules obtain power conversion efficiencies as high as 2.4%. The optoelectronic properties of the n-type material BT(TTI-n12)₂ allow these devic

Full text of DOI:10.1002/adma.201305444

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Solution-Processed, Molecular Photovoltaics that Exploit
Hole Transfer from Non-Fullerene, n-Type Materials
Jessica D. Douglas, Mark S. Chen,* Jeremy R. Niskala, Olivia P. Lee, Alan T. Yiu,
Eric P. Young, and Jean M. J. Fréchet*
Organic photovoltaics (OPVs) continue to attract research atten-
In pursuit of materials with enhanced purity and reproduc-
tion for their potential to be flexible, lightweight, and efficient
ibility, solution-processable small molecules have been explored
devices for power generation.[
1,2]
Recent work in materials
as both p- and n-type OPV active layer components.
[17,18]
Small
design has primarily focused on the development of p-type
polymers that pair with n-type phenyl-C -butyric acid methyl
molecules rather than polymers are uniquely suited for this
purpose because they can be definitively purified and charac-
61
[
19,20]
ester (PC BM) and PC BM in bulk heterojunction (BHJ)
terized as monodisperse compounds.
Recently, p-type
6
1
71
devices. These polymer:PCBM solar cells have achieved power
molecules have shown remarkable performance improvements
conversion efficiencies (PCEs) beyond 8% in single junction
and achieved PCEs up to 8% when paired with PCBM in BHJ
devices;[
3–6]
however their reproducibility is hindered by the
devices.
[21–23]
Non-fullerene n-type molecules, on the other
[
24–30]
inherent polydispersity of polymer samples and batch-to-batch
hand, have not shown PCEs above 4%.
Despite their lower
[7]
variations in active layer components. Moreover, fullerene
derivatives such as PCBM are costly to synthesize, difficult to
purify, and show little potential for engineering optoelectronic
properties.[
JSC values, these devices reveal two distinct design advantages
that non-fullerene small molecules have over materials like
PCBM. First, the materials have a tunable bandgap that can be
engineered to exhibit a stronger absorption across a broader
spectrum, thereby contributing to increased exciton formation.
Second, the electron affinities of the materials can be lowered
(i.e. higher LUMO energy level) to provide greater VOC values
in devices. If these two design principles can be exploited
without compromising the efficiency of charge splitting, it may
be possible to design a n-type material that performs better in
solar cells than PCBM.
8,9]
Fullerene derivatives are the archetypal n-type materials in
OPVs because they possess chemical properties that promote
[10]
unmatched efficiencies for exciton dissociation and electron
transport.[ Electron affinities of fullerene derivatives, however,
11]
are difficult to tune and can limit device open-circuit voltages
9,12]
(
VOC).[
Additionally, derivatives like PC BM often exhibit
61
[8,13]
low extinction coefficients in the visible spectrum
and have
a tendency to form large crystallites upon annealing in BHJ
Herein, we demonstrate that n-type molecules, when com-
bined with a p-type small molecule, provide solution-processed
BHJ solar cells with efficiencies as high as 2.4%. Compared to
PCBM, our materials exhibit broader-spectrum light absorption
and higher extinction coefficients that enable exciton formation
via n-type photoexcitation. Moreover, their low electron affini-
ties afford high device open-circuit voltages (>0.85 V). Analysis
of our highest performing solar cell with external quantum
efficiency (EQE) and photoluminescence (PL) quenching
shows that the active layer blend favors charge splitting via
hole transfer from excitons generated on the n-type material.
With an open circuit voltage above 1.0 V and a fill factor of
blends.[
14,15]
These characteristics impair device short-circuit
current density (JSC) since poor light absorption limits exciton
[
16]
generation, and phase segregation restricts exciton diffusion.
Although no materials have surpassed fullerene derivatives in
terms of electronic performance, there is clear potential for
improvement in the design of new n-type materials.
J. D. Douglas, M. S. Chen, J. R. Niskala, O. P. Lee,
A. T. Yiu, E. P. Young, J. M. J. Fréchet,
Departments of Chemistry and Chemical
and Biomolecular Engineering
University of California, Berkeley
Berkeley, CA, 94720–1460, USA
0
.60, our optimized OPVs are among the highest performing
solution-processed, non-fullerene, all-molecular devices in the
E-mail: mschen@berkeley.edu
[
31]
literature.
J. D. Douglas, M. S. Chen, J. R. Niskala, O. P. Lee,
A. T. Yiu, J. M. J. Fréchet,
Materials Science Division
Lawrence Berkeley National Laboratory
Berkeley, CA 94720, USA
The n-type small molecules in this report are designed
around a symmetric donor (D)–acceptor (A) motif of A–D–A–
D–A, with solubilizing chains appended to the terminal acceptor
moieties (Scheme 1). Benzothiadiazole (BT) and isothianaph-
thene-nitrile (ITNCN) were chosen as the π-conjugated cores
in order to impart significant electron affinity to the molecules.
BT is a common acceptor monomer that is found in many
Dr. M. S. Chen
Departments of Chemistry and Chemical and Biomolecular Engineering
University of California
Berkeley, Berkeley, CA 94720–1460, USA
[
27,32–35]
Prof. J. M. J. Fréchet
King Abdullah University of Science and Technology (KAUST)
Thuwal 23955–6900, Saudi Arabia
high performing materials,
and ITNCN is a promising
[36]
subunit that we recently used to synthesize a n-type polymer.
Adjacent to thiophene (T) linker units, phthalimide (PI) and
thienoimide (TI) were chosen as the terminal acceptor units for
their electron deficiency and solubilizing ability. In particular,
E-mail: jean.frechet@kaust.edu.sa
DOI: 10.1002/adma.201305444
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Scheme 1. Syntheses and structures of the three n-type molecule classes in this study: BT(TPI) (upper left), ITNCN(TPI) (upper right), and BT(TTI)
2
2
2
(lower left). The structure of p-type DPP-Py is shown in the lower right.
TI was an attractive moiety because of its isomeric relationship
to thienopyrroledione (TPD), which has been incorporated into
UV–vis spectroscopy of the n-type molecules shows that
quinoidal character and backbone coplanarity have strong
effects on thin-film (Figure 1a) and solution (Figure S3, Sup-
porting Information) absorption spectra. With an alternating
[4,37–39]
several high-performing OPV polymers.
Since the extent
of side-chain branching is known to affect OPV device perfor-
mance,[
37,40]
linear n-dodecyl (n12) and branched 2-ethylhexyl
phenyl-thiophenyl backbone structure, the BT(TPI) molecules
2
(
EH) alkyl groups were appended to the small molecules to pro-
have the most blue-shifted absorption onsets (ca. 620 nm) and
the largest optical bandgaps (2.00–2.02 eV). Replacement of PI
vide a range of material processability.
Each molecule was synthesized through
a
convergent
for TI leads to BT(TTI) molecules with slightly lower bandgaps
2
approach that began with the terminal moieties and culmi-
nated in a Suzuki-Miyaura cross-coupling to the electron-defi-
cient core (Scheme 1). The PI (2) and TI (11) end groups were
chosen as the synthetic starting points since their side chains
can impart solubility to subsequent intermediates, unlike the
BT (6) and ITNCN (7) cores. Functionalization of the TPI frag-
ment (3) with either a bromide (4) or boronate ester (5) enabled
cross-coupling with 6 or 7 to provide BT(TPI) or ITNCN(TPI) ,
(1.89–1.92 eV), which may arise from increased backbone copla-
narity, increased effective conjugation length, and stronger π–π
interactions. The ITNCN(TPI) molecules have the most red-
2
shifted absorption onsets (ca. 710 nm) and narrow bandgaps
(1.75 eV), primarily because the isothianaphthene core imparts
[
36]
significant quinoidal character.
Since all of the molecules
exhibit broader absorption spectra and higher extinction coeffi-
cients than PC BM, these non-fullerene, n-type materials have
2
2
71
respectively. The TTI fragment (12) was brominated (13) and
cross-coupled with BT (6) to furnish BT(TTI)2.
the potential to play a much larger role in generating excitons
within the active layer via light absorption (Table 1).
In order to analyze the conformation of our molecules, we
performed density functional theory (DFT) calculations with a
hybrid B3LYP correlation functional and a 6–31G(d) basis set.
Calculations show that the BT core is nearly coplanar (ca. 1°)
with its flanking thiophene units, while the ITNCN core is
twisted 30.3° due to the increased steric interactions of a fused
six- versus five-membered ring (Figure S1, S2, Supporting
Information). The terminal phthalimide moieties found in
BT(TPI) and ITNCN(TPI) also induce significant twisting in
Electrochemical properties of the n-type materials, as meas-
ured by cyclic voltammetry (CV), reveal that the molecular
highest and lowest occupied molecular orbital (HOMO and
LUMO) energy levels are primarily influenced by the central
acceptor subunits, where ITNCN-based materials exhibit nar-
rower bandgaps than BT-based materials (Table 1). The com-
peting aromatic and quinoidal resonance forms of isothian-
aphthene lead to destabilized HOMO and stabilized LUMO
energy levels in ITNCN-based materials, which results in
smaller bandgaps. For identical molecular backbones, side-
chain variation does not significantly affect the electrochem-
ical energy levels. Interestingly, despite their structural differ-
ences, the TPI and TTI end groups hardly affect the molecular
HOMO and LUMO energy levels (variations within 0.1 eV). Yet
2
2
[
41]
the conjugated backbone with torsion angles >20°. Torsion at
the terminal moiety is minimized with thienoimide as is dem-
onstrated by BT(TTI) , which displays a twist of 12.0°. In agree-
2
ment with our design, BT(TTI) exhibits the highest backbone
2
coplanarity since it contains the least sterically-hindered conju-
gated subunits. This molecular planarity is anticipated to pro-
mote strong π–π interactions and tight solid-state packing, and
benefit the electronic performance of the material in thin-film
devices.
despite the electrochemical similarities between BT(TTI) and
2
BT(TPI) , the materials are anticipated to perform differently in
2
solar cells based on their variation in structure and molecular
planarity.
4
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The photovoltaic performance of the non-fullerene materials
was evaluated in all-molecular BHJ devices with the following
architecture: ITO/PEDOT:PSS/DPP-Py:non-fullerene n-type/
Ca/Al (Figure 1b). We chose DPP-Py as the p-type molecule
because it has an absorption spectrum (onset ca. 720 nm, 1.72 eV
bandgap) that is complementary with our n-type molecules
and would effectively broaden the active layer absorption pro-
file. Additionally, we have shown that this diketopyrrolopyrrole-
based molecule can achieve relatively high PCEs in BHJ blends
with PC BM, due to its strong self-assembling properties via
7
1
[
43]
π-stacking of pyrene moieties.
tion at 100 mW cm , devices fabricated with BT(TPI-n12) and
Under AM 1.5 G illumina-
−
2
2
BT(TPI-EH) obtained V values above 1.0 V but were plagued
2
OC
with low short-circuit current densities and PCEs (0.8%
and 0.2%, respectively). Comparatively, devices containing
ITNCN(TPI) provided higher J and PCE values (up to 1.4%),
2
SC
despite a drop in VOC (ca. 0.9 V). Given the twisted structure
of ITNCN(TPI) , it is unlikely that the performance enhance-
2
ments are the result of improved solid-state order. Rather, the
broader absorption profiles and higher extinction coefficients
of ITNCN(TPI)2 versus BT(TPI)2 probably enable the active
layers to absorb more light and generate more excitons. Devices
fabricated from BT(TTI-n12) and BT(TTI-EH) , which exhibit
2
2
the highest extinction coefficients, provided the highest short-
circuit currents and PCEs (2.3% and 1.6%, respectively). These
performance trends suggest that the overall light absorption
of the active layers is strongly dependent on the presence and
properties of our n-type molecules. With a maximum PCE of
2
.4%, our OPVs based on DPP-Py:BT(TTI-n12)2 are among
the highest performing solution-processed, non-fullerene, all-
molecular devices.
In order to correlate OPV performance with nanostruc-
tural order and morphology, DPP-Py:n-type blend films were
analyzed by grazing-incidence X-ray diffraction (GIXD) and
atomic force microscopy (AFM). X-ray diffraction patterns
show that side chains greatly affect intermolecular packing,
where EH-substituted molecules display less order than
those substituted with n12 side chains (Figure 2a). Blend
films containing BT(TTI-n12)2 display the most distinct dif-
Figure 1. a) Thin-film absorption spectra of the n-type molecules cast
from chloroform. b) Characteristic J–V plots for DPP-Py:n-type BHJ
devices fabricated under optimized conditions.
Table 1. Small molecule optoelectronic and device properties.
Non-Fullerene n-Type Molecule
Optoelectronic Properties
Device Propertiesd)
HOMOa)
eV]
LUMOa)
[eV]
Egb)
[eV]
c)
VOC
[V]
JSC
[mA cm−2]
FF
PCEe)
[%]
α
[× 10 cm−1]
4
[
BT(TPI-n12)2
5.93
5.86
5.81
5.85
5.99
5.96
3.47
3.55
3.96
3.89
3.53
3.61
2.02
2.00
1.75
1.75
1.89
1.92
5.81
8.68
8.45
10.1
18.5
11.0
1.01
1.07
0.92
0.89
1.05
0.95
−1.44
−0.46
−1.77
−3.13
−3.72
−3.37
0.54
0.41
0.47
0.46
0.60
0.49
0.76 ± 0.04 (0.85)
0.19 ± 0.02 (0.23)
0.72 ± 0.06 (0.82)
1.28 ± 0.09 (1.44)
2.33 ± 0.05 (2.40)
1.55 ± 0.08 (1.71)
BT(TPI-EH)2
ITNCN(TPI-n12)2f)
ITNCN(TPI-EH)2f)
BT(TTI-n12)2
BT(TTI-EH)2
a)
Measured by cyclic voltammetry, potential onsets vs. Fc/Fc at −5.13 eV;[42] b)Determined by absorption onset of thin films; c)Measured at λmax of thin films. PC71BM was
+
measured to have an extinction coefficient of 3.26 × 10 cm−1; Films composed of DPP-Py:n-type were spun from chloroform solutions in a 1:2 blend ratio and annealed
at 130 °C, unless otherwise noted; Mean average values are determined from at least 8 devices, the data in parentheses are maximum values; Films were spun from
4
d)
e)
f)
chloroform solutions in a 1:1 blend ratio of DPP-Py:n-type.
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mean square (RMS) roughness (Figure 2b).
Overall, these data provide strong evidence
that the superior fill factors, short-circuit
current densities and PCEs of BT(TTI)2
blend devices are
a result of favorably
smooth films that consist of nanoscale,
ordered domains.
Since one of the goals of designing a non-
fullerene, n-type molecule is to participate
in light absorption, we sought to investi-
gate whether our materials were involved in
exciton formation. In a BHJ device where
blend components exhibit complementary
absorptions, external quantum efficiency
(
EQE) analysis can reveal how each compo-
nent contributes to the generation of free
charge carriers. In agreement with the poor
JSC values of BT(TPI)2 devices, BT(TPI)2
blend films also demonstrated low EQE
spectra and quantum efficiencies below
1
5% (Figure S6, Supporting Information).
In contrast, BT(TTI)2 devices that displayed
the highest JSC values demonstrated EQEs
of 15–25% in the spectral region that corre-
sponds to the absorption profile of BT(TTI)2
molecules (400–550 nm, Figure 3a). This
overlap between EQE and absorption spectra
suggests that excitation of the n-type, rather
than p-type, material generates free charge
carriers more efficiently. This result may
be attributed to the higher extinction coef-
ficient of BT(TTI-n12) versus DPP-Py
2
(
1.9 × 105 cm vs 6.3 × 104 cm ), but it
−1
−1
may also be indicative that charge splitting
is more favorable from an n-type-localized
exciton. Although photocurrent originating
from the excitation of n-type material is often
not considered in organic solar cells, it has
been observed in a few polymer-fullerene
Figure 2. Characterization of active layer blend films through: a) two-dimensional grazing inci-
dence X-ray diffraction (GIXD) and b) atomic force microscopy (AFM, height images).
[
45,46]
blend systems.
In such cases, free holes
and electrons are generated by the disso-
ciation of n-type-localized excitons through a
fraction peaks, which suggests that these small molecules
generate the most well-oriented domains in films. Nanostruc-
tural order is especially crucial to non-fullerene materials
since they lack the three-dimensional structure of PCBM,
which enables isotropic charge delocalization and enhanced
transport properties. The crystallinity of BT(TTI-n12)2 fi lms
may result from close packing via strong π–π interactions
of a highly coplanar conjugated backbone. Achieving these
orbital interactions is essential for films of conjugated mate-
rials since molecular ordering facilitates bulk charge transport
hole transfer mechanism.
In general, bound excitons dissociate into free charge carriers
through two possible mechanisms: electron or hole transfer.
Since OPV materials are often designed to generate excitons
through p-type excitation, electron transfer from the LUMOp-type
to the LUMO
is the major pathway for producing free holes
n-type
47]
[
and electrons. If the n-type material is capable of absorbing light
and forming an exciton, then charge carriers can also be gener-
[
48]
ated by hole transfer from the HOMOn-type to the HOMOp-type
With either mechanism, stronger driving force for
electron or hole transfer is anticipated from greater
LUMOp-type − LUMOn-type ( ΔELUMO) or HOMO − HOMOn-type
.
a
[
44]
via carrier hopping.
It is likely that the presence of these
well-ordered domains, formed from the assembly of planar
molecules, play a key role in enabling DPP-Py:BT(TTI-n12)2
p-type
[
49,50]
(ΔEHOMO) energy offsets, respectively.
Based on our pre-
devices to achieve the highest J values in the series, up to
vious report of DPP-Py, the CV-determined HOMO energy
level of our p-type molecule is calculated to be –5.53 eV (rela-
SC
−
2
3
.72 mA cm . AFM height images of the blend films reveal
+
that active layers containing BT(TTI) exhibit the finest inter-
tive to Fc/Fc = −5.13 eV), which is offset from the HOMO of
2
[
43]
mixing of p- and n-type components, based on their low root
BT(TTI-n12) by 0.46 eV (ΔEHOMO). Meanwhile, the LUMO
2
4
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Figure 4. Possible mechanisms for exciton dissociation in DPP-
Py:BT(TTI-n12) blends to generate free carriers. a) Electron transfer from
2
DPP-Py-localized excitons is dependent on material LUMO level offsets.
b) Hole transfer from BT(TTI) -localized excitons is dependent on mate-
2
rial HOMO level offsets. Orbital energy levels are based on cyclic voltam-
metry and absorption spectroscopy measurements.
photoluminescence (PL) quenching of DPP-Py:BT(TTI-n12)2
films at different blend ratios (Figure 3b,c). Excitation of either
BT(TTI-n12) or DPP-Py was achieved with relative selectivity
2
by exploiting their complementary absorption spectra and
irradiating blend films at 470 or 625 nm, respectively. Pristine
films of BT(TTI-n12) show a strong emission near 750 nm,
2
while DPP-Py emits near 920 nm. From overlaid plots of PL
emission it is apparent that the BT(TTI-n12)2 excited state
energy transfers efficiently to DPP-Py, even at blend ratios as
low as 1:10 (Figure 3b). Conversely, the PL of excited DPP-Py
only shows significant quenching when BT(TTI-n12) is pre-
2
sent in films at ratios equal to or greater than 1:1 (Figure 3c).
Since PL from BT(TTI-n12) is more effectively quenched by
2
DPP-Py and not vice-versa, in combination with the EQE meas-
urement, it suggests that hole transfer is more efficient than
electron transfer in these material blends (Figure 4). These
data provide evidence that DPP-Py:BT(TTI-n12)2 blend OPVs
are greatly reliant on the formation of n-type localized excitons
and subsequent dissociation into free charge carriers. Not only
do these n-type molecular materials promote charge splitting
and electron transport, like PCBM, they also contribute signifi-
cantly to exciton generation within the active layers.
In conclusion, we report the synthesis and performance of
six non-fullerene, n-type molecules in all-molecular, organic
solar cells. Through a combination of DFT calculations, absorp-
tion spectroscopy and cyclic voltammetry, we have correlated
the optoelectronic properties of each material with their degrees
of molecular planarity and quinoidicity. As the most planar
molecule in the series, BT(TTI-n12) exhibits a V above 1.0 V,
Figure 3. a) External quantum efficiency (EQE) plot of the active layer
blend overlaid with the absorption spectra of DPP-Py and BT(TTI-n12)2.
2
OC
a FF of 0.60, and a device efficiency as high as 2.4% in BHJ
blends with DPP-Py. Film analyses by GIXD and AFM reveal
nanoscale morphologies, where finely intermixed, ordered
domains promote efficient exciton diffusion and dissociation.
EQE and PL quenching studies demonstrate that n-type photo-
excitation is more efficient at generating free charge carriers in
our system, since hole transfer has a greater thermodynamic
driving force than electron transfer. The high performance of
these all-molecular devices and their explicit demonstration of
photocurrent generation via hole transfer highlight the poten-
tial for light absorbing, non-fullerene materials to serve as
viable components for BHJ OPVs.
b,c) Plots of active layer photoluminescence (PL) emission upon excita-
tion at 470 nm (b) or 625 nm (c).
(
EHOMO + Egoptical) energy levels of DPP-Py and BT(TTI-n12) are
2
estimated at –3.81 and –4.10 eV, respectively. By direct compar-
ison of ΔEHOMO (0.46 eV) versus ΔELUMO (0.29 eV) in electro-
chemical potentials, this active layer blend would be expected to
favor hole transfer as the mechanism for charge splitting from a
bound electron–hole pair.
In order to evaluate the efficacy of hole and elec-
tron transfer pathways in our devices, we measured the
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