Efficient squaraine-based solution processable bulk-heterojunction solar cells

Full text of DOI:10.1021/ja8067879

Published on Web 12/05/2008
Efficient Squaraine-Based Solution Processable Bulk-Heterojunction Solar
Cells
Fabio Silvestri,† Michael D. Irwin,‡ Luca Beverina,† Antonio Facchetti,‡ Giorgio A. Pagani,*,† and
Tobin J. Marks*,‡
Department of Materials Science, UniVersita` di Milano-Bicocca, Via Cozzi 53, 20125, Milano, Italy, and
Department of Chemistry and the Materials Research Center, Northwestern UniVersity,
2145 Sheridan Road, EVanston, Illinois 60208
Received August 27, 2008; E-mail: giorgio.pagani@mater.unimib.it; t-marks@northwestern.edu
Scheme 1. Synthetic Route to Squaraine Donorsa
Organic photovoltaics (OPVs) offer new, renewable sources of
electrical energy, capitalizing on attractions such as low fabrication
cost and easy processing on flexible substrates. In the most
successful embodiment, a soluble n-type fullerene, (6,6)-phenyl C61
butyric acid methylester (PCBM), is blended with conjugated p-type
polymers to form bulk-heterojunction (BHJ) cells.1 An attraction
of these devices, which are processed from solution, is the ability
a (i) DMF, t-BuOK, RBr, 25°. (ii) N,N-Diphenylhydrazine hydrochloride,
to deposit active layer precursors over large areas in a single step,
using techniques ranging from inkjet printing to spin-coating.2 Small
EtOH, 4 h reflux. (iii) Squaric acid, azeotropic mixture, reflux.
molecule donors offer the same facile processing however are easier
to synthesize in many variants and purify (e.g., removal of trace
metal catalysts employed in the synthesis), are intrinsically mono-
disperse, and are often environmentally more stable.3
Squaraine dyes4 have been the subject of many recent investiga-
tions owing to their unique photochemical/photophysical properties.
Applications include photoreceptors in copiers and laser printers,5
and IR absorbers in optical disks and nonlinear optical materials.6
Principal attractions of this dye class include facile synthetic access,
a wide variety of possible structural motifs, and remarkable O2 and
moisture stability, thus enabling active layer deposition under
ambient, in contrast to the inert atmosphere required for P3HT and
Figure 1. (A) Normalized optical absorption spectra: 1 (red line) and 2
(black line) as solutions in CHCl3; 1 (green line) and 2 (blue line) as films
from CHCl3. (B) HOMO/LUMO levels for 1 and 2 vs PCBM.
related BHJ materials. Squaraines were studied as active compo-
nents in single-layer OPVs during the 1970s-80s, affording maxi-
mum power conversion efficiencies (PCEs) of ∼0.02%, and more
recently in dye sensitized-solar cells (DSSCs).7
both molecules, demonstrating that the alkyl chains negligibly affect
In this communication, we report the use of appropriately
the redox properties. The relevant energy level positions (Figure
designed squaraines as effective, long-wavelength donor compo-
1B), absorption coefficients (∼2 × 105 cm mol-1 cm-1), and
nents in BHJ OPVs and the effects of core modification on active
absorption energies suggest that these new squaraines should be
layer film morphology and OPV response. The result is some of
effective donors in BHJ devices.
the highest PCEs reported to date for ambient solution-processed
BHJ OPV cells were fabricated by spin-coating under ambient
small molecule OPVs. New soluble squaraine derivatives 1 and 2
atmosphere squaraine:PCBM (x:y wt:wt ratio, x ) 1,2; y ) 1,3)
blends, first in o-dichlorobenzene (ODCB), then in CHCl3 onto
cleaned tin-doped indium oxide (ITO)-coated glass anodes, modified
were synthesized via a modification of previously reported proce-
dures.8 Both linear and branched alkyl chain substituents allow
manipulating the solubility, but with different effects on the solid
by first spin-coating on PEDOT:PSS [poly(3,4-ethylenedioxythio-
state organization.9 Condensation of the appropriate arylhydrazon-
phene):polystyrenesulfonate] as a hole extraction/electron-blocking
alkylpyrrole precursors with squaric acid in i-PrOH/toluene affords
layer (∼30 nm), affording an active layer thickness of 30-140 nm.
1 and 2 in 45% and 48% yield, respectively (Scheme 1). All new
After drying, the cells were then completed by sequential thermal
vacuum deposition of LiF and Al as the cathode. All devices were
characterized under the standard AM1.5G 1 Sun test conditions
using instrumentation and analysis procedures described previ-
compounds were characterized by conventional spectroscopic and
analytical methods (see Supporting Information).
Figure 1A shows normalized optical spectra of 1,2-derived films
(from CHCl3) and solutions (in CHCl3). The absorption of the films
ously,10 and PCEs were derived from eq 1
is broad and covers the 550-900 nm region, exactly where the
ηp ) (JscVocFF) ⁄ Po
(1)
solar photon flux is maximum; the absorption maximum is
somewhat red-shifted for squaraine 2. The HOMO and LUMO
energies of 1 and 2, estimated by cyclic voltammetry (see
Supporting Information), are -3.3 and -5.0 eV, respectively, for
where Jsc is the short circuit current, Voc the open circuit voltage, FF
the fill factor, and Po the incident light intensity, in W/area. Before
OPV testing, the squaraine-PCBM blend morphologies were char-
acterized by tapping mode AFM; the images of blends spin-coated
from several solvents reveal smooth morphologies with rms rough-
† Universita` di Milano-Bicocca.
‡ Northwestern University.
9
17640 J. AM. CHEM. SOC. 2008, 130, 17640–17641
10.1021/ja8067879 CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Comparison of Solution-Processed Squaraine:PCBM BHJ
diodes with 1:1 blend ratios spin-coated from ODCB than from
CHCl3, and with lower leakage currents, presumably reflecting the
superior film quality. We also fabricated and evaluated OPV cells
using 2 and PCBM; efficiencies are somewhat lower (∼0.9%) than
devices prepared with 1 (Table 1), however trends are similar.
In summary, we report the fabrication and initial characterization
of BHJ solar cells based, for the first time, on squaraine derivatives
as molecular donors and PCBM as the acceptor. These devices,
solution-processed in air, exhibit some of the highest PCEs reported
for small molecule OPVs (cf., anthradithiophene/fullerene )
1.0%,14a phenyl-core thiophene dendrimers ) 1.3%,14b star-shaped
derivatives ) 1.3%14c). Studies are currently underway to better
define and optimize processing/microstructure/photovoltaic response
relationships.
Photovoltaic Cellsa
active layer
[wt:wt]
solvent/
Ta
d
[nm]
Jsc
[mA/cm2]
Voc
[V]
FF
[%]
η
[%]
1/PCBM (1:1)
1/PCBM (1:1)
1/PCBM (1:1)
1/PCBM (1:1)
1/PCBM (1:1)
1/PCBM (1:1)
1/PCBM (1:3)
1/PCBM (2:1)
1/PCBM (1:1)
1/PCBM (1:3)
2/PCBM (1:1)
2/PCBM (1:1)
2/PCBM (1:3)
ODCB/na
ODCB/70
ODCB/70
ODCB/70
ODCB/na
ODCB/70
ODCB/na
ODCB/na
CHCl3/na
CHCl3/na
ODCB/na
ODCB/70
CHCl3/na
140
140
100
50
30
30
30
140
30
1.85
2.35
2.65
2.93
3.10
3.01
3.89
0.88
3.24
5.70
1.31
1.56
4.72
0.58
0.60
0.60
0.59
0.64
0.61
0.60
0.56
0.62
0.62
0.58
0.59
0.59
31
32
34
35
39
36
32
27
32
35
30
31
32
0.33
0.45
0.54
0.61
0.77
0.66
0.75
0.13
0.64
1.24
0.23
0.29
0.89
30
140
140
30
Acknowledgment. We thank ONR (N00014-08-10923) and the
NSF US-Europe Program (DMR-0353831) for support of this
research, and the Northwestern U. NSF MRSEC (DMR-0520513)
for providing characterization facilities.
a The general structure of devices is ITO/PEDOT:PSS/Sq:PCBMblend/
LiF/Al, with ∼6 mm2 illuminated areas. Ta ) annealing temperature (°C);
na: not annealed. Otherwise annealing is for 1 h.
Supporting Information Available: Synthetic procedures for 1 and
2, device fabrication and characterization details, optical spectra of
blends, and film AFM images. This material is available free of charge
via the Internet at http://pubs.acs.org.
References
(1) (a) Thompson, B. C.; Frechet, J. M. J. Angew. Chem., Int. Ed. 2008, 47,
58. (b) Yip, H.-L.; Hau, S. K.; Baek, N. S.; Ma, H.; Jen, A. K.-Y. AdV.
Mater. 2008, 20, 2376. (c) Coakley, K. M.; McGehee, M. D. Chem. Mater.
2004, 16, 4533.
(2) (a) Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T. Q.; Dante,
M.; Heeger, A. J. Science 2007, 317, 222. (b) Hoth, C. N.; Choulis, S. A.;
Schilinsky, P.; Brabec, C. J. AdV. Mater. 2007, 19, 3973. (c) Li, G.;
Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Nat.
Mater. 2005, 4, 864.
Figure 2. (A) J-V response of 1:PCBM BHJ OPV devices as a function
of D:A ratio; 1:1 ratio from ODCB (140 nm, black line), 1:1 ratio from
ODCB (30 nm, red line), 1:3 ratio from CHCl3 (140 nm, pink line), and
1:3 ratio from CHCl3 (30 nm, blue line). (B) Dark current measurements
for 1:1 ratio films from ODCB (30 nm, red line), 1:3 ratio from CHCl3
(140 nm, pink line), and 1:3 ratio from CHCl3 (30 nm, blue line).
(3) (a) Brumbach, M.; Placencia, D.; Armstrong, N. R. J. Phys. Chem. C 2008,
112, 3142. (b) Rand, P. B.; Genoe, J.; Heremans, P.; Poortmans, J. Prog.
PhotoVolt: Res. Appl. 2007, 15, 659. (c) Yoo, S.; Potscavage, W. J., Jr.;
Domercq, B.; Han, S.-H.; Li, T.-D.; Jones, S. C.; Szoszkiewicz, R.; Levi, D.;
Riedo, E.; Marder, S. R.; Kippelen, B. Solid-State Electron. 2007, 51, 1367.
(4) Squaraine synthesis reviews: (a) Schmidt, A. H. Synthesis 1980, 961. (b)
Sprenger, G. E.; Ziegenbein, W. Angew. Chem., Int. Ed. Engl. 1968, 7, 530.
(5) (a) Law, K. Y. Chem. ReV. 1993, 93, 449. (b) Law, K. Y.; Bailey, F. C. J.
Org. Chem. 1992, 57, 3278. (c) Law, K. Y. J. Phys. Chem. 1987, 91, 5184.
(6) (a) Chen, C.; Marder, S. R.; Cheng, L. T. J. Am. Chem. Soc. 1994, 116,
3117. (b) Chen, C.-T.; Marder, S. R.; Cheng, L. T. J. Chem. Soc., Chem.
Commun. 1994, 259. (c) Beverina, L.; Crippa, M.; Salice, P.; Ruffo, R.;
Ferrante, C.; Fortunati, I.; Signorini, R.; Mari, C.; Bozio, R.; Facchetti,
A.; Pagani, G. Chem. Mater. 2008, 20, 3242, and references therein.
(7) (a) Yum, J.; Walter, P.; Huber, S.; Rentsch, D.; Geiger, T.; Nu¨esch, F.; De
Angelis, F.; Gra¨tzel, M.; Nazeeruddin, M. K. J. Am. Chem. Soc. 2008,
129, 10320. (b) Liang, K. N.; Law, K. Y.; Whitten, D. G. J. Phys. Chem.
1995, 99, 16704. (c) Piechowski, A. P.; Bird, G. R.; Morel, D. L.; Stogryn,
E. L. J. Phys. Chem. 1984, 88, 934. (d) Merrit, V. Y.; Hovel, H. J. Appl.
Phys. Lett. 1976, 29, 414.
(8) (a) Beverina, L.; Crippa, M.; Landenna, M.; Ruffo, R.; Salice, P.; Silvestri,
F.; Versari, S.; Villa, A.; Ciaffoni, L.; Collini, E.; Ferrante, C.; Bradamante,
S.; Mari, C. M.; Bozio, R.; Pagani, G. A. J. Am. Chem. Soc. 2008, 130,
1894. (b) Beverina, L.; Abbotto, A.; Landenna, M.; Cerminara, M.; Tubino,
R.; Meinardi, F.; Bradamante, S.; Pagani, G. A. Org. Lett. 2005, 7, 4257.
(9) Balakrishnan, K.; Datar, A.; Naddo, T.; Huang, J.; Oitker, R.; Yen, M.;
Zhao, J.; Zang, L. J. Am. Chem. Soc. 2006, 128, 7390.
(10) (a) Irwin, M. D.; Buchholz, B.; Hains, A. W.; Chang, R. P. H.; Marks,
T. J. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 2783. (b) Hains, A. W.;
Marks, T. J. Appl. Phys. Lett. 2008, 92, 023504.
nesses ∼0.35 nm (see Supporting Information). Initial OPV studies
employed squaraine 1, spin-coating the blend from ODCB in air. With
140 nm thick films, Jsc is low, probably reflecting limited charge
mobility, in accord with TFT-derived mobilities, µh ≈ 10-4-10-5 cm2
V-1 s-1.11 By reducing the solution concentration and therefore the
film thickness, OPV performance can be enhanced, with ∼30 nm films
increasing Jsc from 1.85 to 3.10 mA/cm2 and PCE from 0.33% to
0.77% (Table 1, Figure 2A).
The literature suggests that BHJ active layer thermal annealing
can optimize film phase separation/microstructural order and thereby
OPV performance.12 For the present materials, optimum annealing
conditions are 70 °C/1 h as evidenced by the increased phase
separation (AFM Figures S1-S4). As in MDMO-PPV/PCBM solar
cells,13 enhancing carrier mobility by adjusting the donor:acceptor
ratio also contributes to enhanced PCEs. In the present study,
devices fabricated with 1:3 1:PCBM ratios exhibit significant
increases in Jsc (to 3.89 mA/cm2), with PCE ) 0.75%. In contrast,
devices with a 2:1 ratio exhibit decreases in both short circuit current
and PCE (Table 1). Spin-casting the 1:3 1:PCBM blend from CHCl3
rather than ODCB solutions yields a maximum PCE of 1.24%, with
a short circuit current of 5.70 mA/cm2, 1.5 times higher than the
ODCB result, tentatively ascribed to the microstructure evolution
effects of more rapid film growth and drying.
(11) (a) TFT measurement performed in our laboratory on Si-SiO2 substrates
with top Au source/drain contacts. (b) Shirota, Y.; Kageyama, H. Chem.
ReV. 2007, 107, 953.
(12) (a) Padinger, F.; Rittberger, R. S.; Sariciftci, N. S. AdV. Funct. Mater. 2003,
13, 85. (b) Shaheen, S. E.; Brabec, C. J.; Sariciftci, N. S. Appl. Phys. Lett.
2001, 78, 841–843.
(13) Sheenan, S. E.; Brabec, C. J.; Sariciftci, N. S.; Padinger, F.; Fromherz, T.;
Hummelen, J. C. AdV. Funct. Mater. 2001, 14, 865.
That PCE values increase with decreasing thickness is consistent
with the higher observed short circuit currents for thinner films.
The same effect is observed on changing the 1:PCBM ratio. The
EQE (external quantum efficiency) for the most efficient devices
peaks at ∼20% @ 750 nm, averaging ∼10% in the 400-800 nm
region. The dark data in Figure 2B indicate smaller rectification in
(14) (a) Lloyd, M. T.; Mayer, A. C.; Subramanian, S.; Mourey, D. A.; Herman,
D. J.; Bapat, A. V.; Anthony, J. E.; Malliaras, G. G. J. Am. Chem. Soc.
2007, 129, 9144. (b) Kopidakis, N.; Mitchell, W. J.; Lagemaat, J. V. d.;
Ginley, D. S.; Rumbles, G.; Shaheen, S. E. Appl. Phys. Lett. 2006, 89,
103524. (c) Chang, H.; Qingguo, H.; Yuanping, Y.; Guanglong, W.;
Fenglian, B.; Zhigang, S.; Yongfang, L. J. Mater. Chem. 2008, 18, 4085.
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