Marked alkyl- Vs alkenyl-substitutent effects on squaraine dye solid-state structure, carrier mobility, and bulk-heterojunction solar cell efficiency

Article abstract of DOI:10.1021/ja100520q

(Figure Presented) We report two new squaralne dyes substituted at the pyrrolic rings with n-hexyl (squaraine 1) or n-hexenyl (squaraine 2) chains. Although Internal molecular structure variations are minimal, the presence of the terminal double bond results in a much more compact solid-state structure, dramatically affecting charge transport in the thin films; the hole mobility of 2 is ～ 5x that of 1, and the BHJOPV power conversion efficiency (PCE) of 2 is ～2x that of 1. PCEs surpassing 2% for ambient solution-processed devices are demonstrated, the largest so far achieved for squaraine-based organic solar cells. Copyright

Full text of DOI:10.1021/ja100520q

Published on Web 03/05/2010
Marked Alkyl- vs Alkenyl-Substitutent Effects on Squaraine Dye Solid-State
Structure, Carrier Mobility, and Bulk-Heterojunction Solar Cell Efficiency
Diego Bagnis,‡,| Luca Beverina,† Hui Huang,‡ Fabio Silvestri,† Yan Yao,§ Henry Yan,§
Giorgio A. Pagani,*,† Tobin J. Marks,*,‡ and Antonio Facchetti*,‡,§
Department of Materials Science, UniVersita` di Milano-Bicocca, Via Cozzi 53, 20125, Milano, Italy, Department of
Chemistry and the Argonne-Northwestern Solar Energy Research Center, Northwestern UniVersity,
EVanston, Illinois 60208, Polyera Corporation, 8045 Lamon AVenue Skokie, Illinois 60077, Department of CiVil and
EnVironment Engineering, UniVersita` di Perugia, NIPLAB-INSTM, Via Pentima Bassa 21, 05100 Terni, Italy
Received January 27, 2010; E-mail: a-faccchetti@northwestern.edu; giorgio.pagani@mater.unimib.it; t-marks@northwestern.edu
Bulk-heterojunction organic photovoltaic (BHJ-OPV) cells exhibit-
ing high power conversion efficiencies (PCEs) may provide inexpen-
sive, renewable sources of solar electricity via low-temperature
fabrication on flexible substrates, impossible for silicon.1 In such
devices, the photoactive layer consists of a phase-separated donor-
acceptor semiconductor blend that provides interfaces for exciton
splitting and networks for hole and electron transport to the electrodes.2,3
An attractive approach to increasing PCEs includes, besides developing
unconventional device structures and contact/interfacial materials,4
exploring new photoactive semiconductors, particularly BHJ donors.5
In recent years, new molecular polymeric BHJ-OPV donor material
classes have been discovered and implemented with the fullerene
acceptors PC61BM and PC71BM. Their characterization provides
fundamental information on how π-system modifications affect PCE
by altering light absorption, active layer microstructure, exciton
dynamics, and carrier mobility. These results raise the interesting
question of how subtle π-core substituent variations, which marginally
affect solution phase molecular properties, might promote significantly
Figure 1. Molecular and crystal structures of squaraines 1 (right) and 2 (left).
different OPV responses.
We report here the synthesis and remarkable property differences
of two new squaraine dyes, substituted at the pyrrolic nitrogen with
n-hexyl (squaraine 1) or n-hexenyl (squaraine 2) chains (Figure 1).
These derivatives are synthesized via modifying previously reported
procedures,6 condensing the appropriate arylhydrazone-alkylpyrroles
with squaric acid in ∼42% (1) and ∼52% (2) yield, respectively. The
new compounds were characterized by conventional spectroscopic/
analytical/X-ray diffraction methodologies (see Supporting Informa-
tion). Although internal molecular structure variations are minimal,
the data reveal that the N-alkenyl substituent affords a more compact
solid-state structure, enhancing charge mobility (thin film transistor
hole mobility is increased ∼5×) and OPV performance (PCE is
enhanced by ∼2×). This study underscores the value of alkene
functionalization and complements recent work by Miyanishi7 on
poly(3-hexenylthiophene) (P3HeT:PC61BM) OPVs. There higher
mobility was observed vs P3HT:PC61BM, and microstructural stabi-
lization could be further enhanced via CdC cross-linking.
Key to understanding the marked properties differences between 1
and 2 are the crystal structures. Crystals of each for diffraction were
obtained by slow diffusion of orthogonal solvents (Figure 1). The
molecular structures feature essentially planar cores extending from
the central squaric rings to two of the four terminal phenyls and are
characterized by trans N-substituted-pyrrolyl ring orientations (Figure
1B). The maximum torsions between the nearly coplanar pyrrolyl and
phenyl rings are ∼10° (1) and 25° (2), whereas the other two phenyl
rings project nearly perpendicular to the molecular planes in both
molecules. Both squaraines crystallize in a cofacial motif (Figure 1),
with minimum interplanar distances of 3.138 and 3.417 Å for 2 and
1, respectively (Figure 1C); the former is smaller than the sum of C · · ·C
(3.30-3.40 Å) van der Waals radii.8 The minimum interstack distance
between cores is also significantly smaller for 2 (7.343 Å) than for 1
(7.809 Å), resulting in denser packing (d ) 1.238 for 2 vs 1.215 g/cm3
for 1). Importantly, this packing also features a very short edge-π
contact between the terminal hexenyl double bond and both phenyl
rings in 2 [<3 Å, comparable to the sum of H · · ·C (2.85-2.90 Å) van
der Waals radii8] vs no contacts and larger distances between the
saturated n-hexyl group and the analogous phenyl rings in 1 (3-4 Å,
Figure 1D).
Figure S1 shows normalized optical spectra of 1- and 2-derived
films cast from CHCl3 solutions, of CHCl3 solutions, and of BHJ blends
with PC61BM and PC71BM. While the solution phase spectra are
identical and exhibit typical sharp squaraine absorptions at 729 nm,
the spectra of the pristine films are broad and span the 550-900 nm
region with λmax of 1 blue-shifted (660 nm) vs 2 (770 nm). Interestingly,
the BHJ film optical spectra resemble those of the corresponding
solutions but are slightly broadened, a signature of film microstructural
details (vide infra). The HOMO and LUMO energies of 1 and 2,
estimated by cyclic voltammetry (Figure S2), are -3.3 and -5.0 eV,
respectively, for both molecules, demonstrating that the alkyl/alkenyl
chains negligibly affect solution redox properties. The HOMO and
LUMO energies (Figure S3), absorption coefficients (∼ 2 × 105
M-1cm-1), and absorption energies suggest that these new squaraines
are promising electron donors for fullerene BHJ-OPVs.
‡ Northwestern University.
| Universita` di Perugia.
† Universita` di Milano-Bicocca.
§ Polyera Corporation.
9
4074 J. AM. CHEM. SOC. 2010, 132, 4074–4075
10.1021/ja100520q  2010 American Chemical Society

C O M M U N I C A T I O N S
vs 1 squaraine nanofibrils may be due to the denser, more compact 2
microstructure, which provides the principal overall current enhance-
ment. To test this hypothesis, thin-film transistors based on 1 and 2
were also fabricated. These exhibit average field-effect hole mobilities
of (2.7 ( 1.8) × 10-5 and (1.2 ( 0.2) × 10-4 cm2/(V s), respectively,
indicating 2 is a more efficient hole transporter than 1 (Figure S8), as
suggested by the structural analysis.
In summary, we report the fabrication and initial characterization
of BHJ solar cells based on alkyl- and alkenyl-functionalized squaraine
dyes as donors, with PC61BM and PC71BM as acceptors. These devices,
solution-processed in ambient, exhibit the highest PCEs within the
squaraine donor family and surpass those of several other molecular
donor families. More importantly, we demonstrate and rationalize a
new structural strategy, via noncovalent alkenyl-phenyl contacts, to
enhance charge transport efficiency in squaraine-based OPVs. We
believe that this approach can be extended to other molecular and
polymeric semiconductors as well.
Figure 2. Average J-V response of 1- and 2-based BHJ OPVs as a function
of Sqr:PCxxBM ratio, with and without annealing, where PCxxBM is (A)
PC61BM and (B) PC71BM.
Table 1. Comparison of Squaraine:PCxxBM BHJ Photovoltaic Cellsa b
,
c
d
Sqr./PCxxBM
(wt:wt)]
dht
[nm]
Dal
[nm]
Jsc
[mA/cm2]
Voc
[V]
FF
[%]
PCE (PCEmax
[%]
)
1/PC61BM (1:1)
2/PC61BM (1:1)
1/PC61BM (1:3)
2/PC61BM (1:3)
∼50 ∼65
∼50 ∼65
∼75 ∼35
∼75 ∼35
3.83
4.31
4.13
5.15
5.68
6.40
8.21
7.16
9.32
0.49 34 0.63 (0.68)
0.59 34 0.87 (0.99)
0.54 33 0.77 (0.78)
0.56 37 1.10 (1.20)
0.59 39 1.34 (1.45)
0.52 35 1.12 (1.29)
0.56 37 1.72 (1.75)
0.55 37 1.39 (1.47)
0.57 37 1.99 (2.05)
Acknowledgment. We thank DOE (DE-FB02-08ER46536-/
A000) for support of this research, the Northwestern U. MRSEC
(NSF DMR-0520513) for providing characterization facilities, and
Ms. M. Ferrari for sample preparation and helpful discussions.
2/PC61BM (1:3)e ∼75 ∼35
1/PC71BM (1:3)
2/PC71BM (1:3)
∼75 ∼35
∼75 ∼35
Supporting Information Available: Synthetic procedures for 1 and
2; device fabrication and characterization details; optical spectra of 1,
2, and blends; and AFM images of films. This material is available
free of charge via the Internet at http://pubs.acs.org.
1/PC71BM (1:3)e ∼75 ∼50
2/PC71BM (1:3)e ∼75 ∼35
a General device structure is ITO/PEDOT:PSS/Sq:PCXXBM blend/LiF/Al
with ∼6 mm2 illuminated areas. b All devices characterized under the
standard AM1.5G 1 Sun test conditions using instrumentation and analysis
procedures described previously,4c and PCEs are derived from the equation
ηp ) (JscVocFF)/Po, where Jsc ) the short circuit current [mA/cm2], Voc the
open circuit voltage [V], FF the fill factor, and Po the incident light intensity
[mW/cm2]. c Thickness of hole transport layer (PEDOT:PSS). d Thickness
of active layer blend. e Annealed at 50 °C for 30 min.
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BHJ-OPV cells were fabricated by spin-coating, under an ambient
atmosphere, squaraine:acceptor (x:y wt:wt ratio) blends in CHCl3
solution onto cleaned ITO-coated glass anodes, modified by first spin-
coating on a PEDOT:PSS layer as a hole extraction/electron-blocking
layer. After drying, the cells were then completed by sequential thermal
vacuum deposition of LiF and Al as the cathode (Figure S4). Typical
J-V plots are shown in Figure 2, and EQE data for the best cell in
Figure S5. Device optimization was accomplished by varying the
PEDOT:PSS layer thickness, acceptor material (PC61BM or PC71BM),
active layer thickness, and postdeposition thermal annealing (Tables
S1-S3). Optimized OPV performance data are summarized in Table
1, with the PCE for 2 ∼2% vs ∼1.4% for 1. From these data some
general performance trends are clearly discerned: i. Replacing PC61BM
with PC71BM significantly increases PCE (∼ 2 ×). ii. Mild active
layer thermal annealing (50 °C) before device completion enhances
performance by increasing JSC. iii. Hexenyl-substituted squaraine
2-based devices invariably outperform those based on hexyl-substituted
1. Since the Voc (∼0.55 V) and FF (∼35%) of all devices are similar,
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harvesting capacity of this acceptor but also greater microstructural
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negligible donor-acceptor phase separation, with the film morphology
resembling solid solutions (Figures S6B,S7B), whereas the 1-/2-
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embedded in the fullerene matrix (Figures S6C,S7C). Interestingly,
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meaning the enhanced PCE of the former system is not exclusively
morphological in origin. Rather, the enhanced hole transport of the 2
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