Elucidating the structure-property relationships of donor-π-acceptor dyes for dye-sensitized solar cells (DSSCs) through rapid library synthesis by a one-pot procedure

Article abstract of DOI:10.1002/chem.201402093

The creation of organic dyes with excellent high power conversion efficiency (PCE) is important for the further improvement of dye-sensitized solar cells. We wish to describe the rapid synthesis of a 112-membered donor-π-acceptor dye library by a one-pot procedure, evaluation of PCEs, and elucidation of structure-property relationships. No obvious correlations between ε, and the η were observed, whereas the HOMO and LUMO levels of the dyes were critical for η. The dyes with a more positive E_HOMO, and with an E_LUMO<-0.80 V, exerted higher PCEs. The proper driving forces were crucial for a high J_sc, and it was the most important parameter for a high η. The above criteria of E_HOMO and E _LUMO should be useful for creating high PCE dyes; nevertheless, that was not sufficient for identifying the best combination of donor, π, and acceptor blocks. Combinatorial synthesis and evaluation was important for identifying the best dye. Combinatorial chemistry: A 112-membered donor-π-acceptor dye library was rapidly constructed by a one-pot, three-component coupling procedure (see scheme, SM=Suzuki-Miyaura). The evaluation of absorption spectroscopic and electrochemical measurements of all synthesized dyes, and cell performances of the selected 54 dyes were performed. The dyes with a more positive E_HOMO, and with an E _LUMO<-0.80 V, exerted higher power conversion efficiencies.

Full text of DOI:10.1002/chem.201402093

DOI: 10.1002/chem.201402093
Full Paper
&
Solar Cells
Elucidating the Structure–Property Relationships of Donor–p-
Acceptor Dyes for Dye-Sensitized Solar Cells (DSSCs) through
Rapid Library Synthesis by a One-Pot Procedure
Shinichiro Fuse,*[a] Sakae Sugiyama,[a] Masato M. Maitani,[a] Yuji Wada,[a] Yuhei Ogomi,[b]
Shuzi Hayase,[b] Ryuzi Katoh,[c] Tatsuo Kaiho,[d] and Takashi Takahashi[e]
Abstract: The creation of organic dyes with excellent high
power conversion efficiency (PCE) is important for the fur-
ther improvement of dye-sensitized solar cells. We wish to
describe the rapid synthesis of a 112-membered donor-p-ac-
ceptor dye library by a one-pot procedure, evaluation of
PCEs, and elucidation of structure–property relationships. No
obvious correlations between e, and the h were observed,
whereas the HOMO and LUMO levels of the dyes were criti-
cal for h. The dyes with a more positive EHOMO, and with an
E
LUMO <À0.80 V, exerted higher PCEs. The proper driving
forces were crucial for a high Jsc, and it was the most impor-
tant parameter for a high h. The above criteria of EHOMO and
ELUMO should be useful for creating high PCE dyes; neverthe-
less, that was not sufficient for identifying the best combina-
tion of donor, p, and acceptor blocks. Combinatorial synthe-
sis and evaluation was important for identifying the best
dye.
the polypyridyl Ru dyes[3] The creation of high-performance or-
ganic dyes with an excellent PCE would be important for fur-
ther improvement of the cell performance of DSSCs. One pos-
sibility for the design of high-performance dyes is the use of
theoretical calculations. Recent dramatic progress in this field
has made it possible to estimate the properties of dyes, such
as the absorption spectra and the electron distribution of D-p-
A dyes.[4] However, the PCE is influenced by many factors in-
cluding intermolecular interactions and the arrangement of
dyes on TiO2. It remains difficult to rationally estimate these
important properties of dyes.[3c,d] Another method of estima-
tion is the use of dye structure–property information, which is
drawn from extensive past reports. However, the extraction of
precise structure–property information has not been a simple
process, because the procedure for the preparation and evalu-
ation of cells varies depending on the research group.[1d] The
combinatorial approach has been a powerful approach for elu-
Introduction
Dye-sensitized solar cells (DSSCs) have garnered considerable
attention due to a high-power conversion efficiency (PCE),
simple fabrication procedures, and a low production cost.[1]
The dye molecules as sensitizers play a crucial role in
DSSCs.[1b,2] The most common high-performing dyes are the
ruthenium-centered polypyridyls. Metal-free, organic donor-p-
acceptor (D-p-A) dyes comprise a very important class of com-
pounds owing to their large molar absorptivity, design flexibili-
ty, and potential reduction in production costs compared with
[a] Dr. S. Fuse, Dr. S. Sugiyama, Dr. M. M. Maitani, Prof. Dr. Y. Wada
Department of Applied Chemistry, Tokyo Institute of Technology
2-12-1, Ookayama, Meguro-ku, Tokyo 152-8552 (Japan)
Fax: (+81)3-5734-2884
E-mail: sfuse@apc.titech.ac.jp
[b] Dr. Y. Ogomi, Prof. Dr. S. Hayase
Graduate School of Life Science and Systems Engineering
Kyushu Institute of Technology, 2-4, Hibikino
Wakamatsu, Kitakyusyu, Fukuoka 808-0196 (Japan)
[c] Prof. Dr. R. Katoh
Department of Chemical Biology and Applied Chemistry
College of Engineering, Nihon University
Tamura, Koriyama, Fukushima, 963-8642 (Japan)
[d] Dr. T. Kaiho
Kanto Natural Gas Development Co., Ltd.
Brine Resources Research & Technology Division, 661
Mobara, Mobara-shi, Chiba 297-8550 (Japan)
[e] Prof. Dr. T. Takahashi
Yokohama College of Pharmacy, 601, Matana-cho
Totsuka-ku, Yokohama-shi, Kanagawa 245-0066 (Japan)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402093.
Figure 1. One-pot approach for the synthesis of D-p-A dyes based on the
Suzuki–Miyaura coupling reaction.
Chem. Eur. J. 2014, 20, 10685 – 10694
10685
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Scheme 1. Synthesis of D-p-A dye 5 based on the one-pot approach. dba=
dibenzylideneacetone; Xantphos=9,9-dimethyl-4,5-bis(diphenylphosphi-
no)xanthene; dppf=1,1’-bis(diphenylphosphino)ferrocene; DPEphos=bis(2-
diphenylphosphinophenyl)ether; Pin=pinacol.
cidation of the structure–property relationship.[5] Herein, we
report the rapid combinatorial synthesis of a 112-membered
D-p-A dye library by a one-pot procedure, the evaluation of
PCEs, and an elucidation of the structure–property relation-
ship.
Figure 2. Building blocks for D-p-A dye library synthesis.
Results and Discussion
The construction of a large com-
Table 1. Yields in A) the one-pot SM coupling reaction and B) removal of the tBu group or the Knoevenagel
binatorial library should meet
the following criteria: 1) the mul-
tiple components should be se-
quentially coupled without extra
steps, such as protection and de-
protection; 2) the number of
workup and purification steps
should be minimized; 3) each of
the coupling steps should pro-
duce a good yield and be com-
patible with a variety of func-
tional groups and 4) readily
available, nontoxic and environ-
mentally benign substrates and
reagents should be employed
with no generation of toxic com-
pounds. From this point of view,
Suzuki–Miyaura (SM) coupling is
attractive.[6] Although many
practical syntheses of D-p-A
dyes based on SM coupling have
been reported, they are not
always suitable for the combina-
torial approach.[7] This is because
they usually contain extra steps,
such as halogenation, formyla-
tion, workup, and purification.
We planned to develop a highly
efficient, one-pot SM coupling
procedure with no extra steps,
as shown in Figure 1. In the one-
pot reaction, the remaining sub-
condensation reaction.
Dye
Yield
(A)
[%][a]
Yield
(B)
[%][b]
Dye
Yield
(A)
Yield
(B)
Dye
Yield
(A)
Yield
(B)
Dye
Yield
(A)
Yield
(B)
[%][a]
[%][b]
[%][a]
[%][b]
[%][a]
[%][b]
D1p1A1 60
D1p2A1 56
D1p3A1 78
D1p4A1 66
D1p5A1 16
D1p6A1 64
D1p7A1 68
D1p1A2 63
D1p2A2 60
D1p3A2 69
D1p4A2 67
D1p5A2 30
D1p6A2 68
D1p7A2 57
D1p1A3 41
D1p2A3 53
D1p3A3 59
D1p4A3 38
D1p5A3 36
D1p6A3 36
D1p7A3 37
D1p1A4’ 60
87
90
92
97
D2p1A1 52
D2p2A1 40
D2p3A1 65
D2p4A1 62
82
88
D3p1A1 60
D3p2A1 72
quant. D4p1A1 64
quant. D4p2A1 51
82
9
quant.
79
87
93
74
58
90
quant.
98
79
86
88
96
quant.
97
99
quant.
82
quant. D3p3A1 74
quant. D3p4A1 74
78
88
92
52
93
76
72
89
99
D4p3A1 50
D4p4A1 52
D4p5A1 40
D4p6A1 48
D4p7A1 55
D4p1A2 58
D4p2A2 56
D4p3A2 48
D4p4A2 45
quant. D2p5A1 16
65
78
90
87
83
D3p5A1 52
D3p6A1 78
D3p7A1 69
D3p1A2 65
D3p2A2 44
99
79
98
79
D2p6A1 51
D2p7A1 32
D2p1A2
4
D2p2A2 34
quant. D2p3A2 36
quant. D3p3A2 55
96 D3p4A2 67
quant. D3p5A2 37
quant. D3p6A2 57
82
98
84
quant. D3p3A3 45
89
76
76
79
93
91
75
81
71
65
82
84
D2p4A2 42
D2p5A2 17
D2p6A2 38
D2p7A2 40
D2p1A3 41
D2p2A3 35
D2p3A3 41
D2p4A3 35
D2p5A3 26
D2p6A3 26
quant. D4p5A2 58
97
87
quant. D4p1A3 57
quant. D4p2A3 29
86
95
96
76
96
79
70
77
85
97
53
70
D4p6A2 61
D4p7A2 58
D3p7A2 40
D3p1A3
D3p2A3 53
6
D4p3A3 43
D4p4A3 42
D4p5A3 40
D4p6A3 51
D4p7A3 46
D4p1A4’ 56
D4p2A4’ 38
D4p3A4’ 54
D4p4A4’ 41
D4p5A4’ 34
D4p6A4’ 66
D4p7A4’ 17
D3p4A3 48
D3p5A3 32
quant. D3p6A3 49
quant. D2p7A3 41
99
87
64
72
77
98
69
52
D3p7A3
5
93
77
81
74
70
91
62
64
89
89
65
77
74
99
88
D2p1A4’ 59
D2p2A4’ 27
D2p3A4’ 60
D2p4A4’ 45
D2p5A4’ 37
D2p6A4’ 36
D2p7A4’ 33
D3p1A4’ 64
D3p2A4’ 44
D3p3A4’ 72
D3p4A4’ 64
D3p5A4’ 53
D3p6A4’ 54
D3p7A4’ 20
D1p2A4 20
[c]
D1p3A4
–
D1p4A4’ 49
D1p5A4’ 18
D1p6A4’ 63
D1p7A4’ 52
[a] Isolated yields in the one-pot SM coupling reaction. [b] Isolated yields in acidic removal of tBu group in the
case of A1-, A2-, A3-, or A4-containing dyes, or Knoevenagel condensation reaction in the case of A4’-contain-
ing dyes. [c] It was difficult to isolate the one-pot SM coupling product, D1p3A4, therefore only short-pad
column chromatography was performed, and the obtained compound was used for the next reaction (removal
of tBu group) without further purification.
Chem. Eur. J. 2014, 20, 10685 – 10694
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strates or reagents in the first step should not suffer the fol-
lowing step, but the Pd catalyst is somewhat susceptible to
contaminants. Therefore, the number of the reported one-pot
Pd coupling reactions is surprisingly limited.[8] A p-block I re-
taining two bromo groups was selected as an aromatic scaf-
fold due to its sufficient stability, reactivity, and ready availabili-
ty. We first planned to couple donor block II with the aromatic
scaffold I because the CÀBr bond of the first coupling product
would be less reactive compared with the corresponding CÀBr
bond of the aromatic scaffold I, and, therefore, an undesired
overreaction affording a D-p-D compound would be sup-
pressed. The coupling product IV with tert-butyl (tBu) ester
would retain good solubility and could be easily purified. The
following removal of the tBu group by using volatile acid and
simple purification would afford the desired D-p-A dye V in
good purity.
Scheme 2. Improved synthesis of D-p-A dye 9 with the acceptor A4.
a substantial amount of undesired protodeboronated com-
pounds from 4 were generated. As expected, the removal of
the tBu group by using TFA (trifluoroacetic acid) and a simple
short pad column chromatography afforded the desired dye 5
in high purity.
For the development of the one-pot procedure, a simple
Four donors, D1–D4, seven p-bridges, p1–p7, and four ac-
ceptors, A1–A4 were selected for the construction of the D-p-
and typical D-p-A dye 5[7a,9] was selected as
(Scheme 1). The reaction con-
a target
ditions for the first coupling
Table 2. Absorption maxima (lmax), optical edge, and molar absorption coefficients (e), EHOMO, ELUMO vs. NHE of the
synthesized D-p-A dyes (D1 and D2).
between 1 and 2 were ex-
plored. Combinations of
bases (Na2CO3 and K2CO3), Pd
[a]
[c]
[c]
[a]
[c]
[c]
Dye
lmax
Optical e[b]
EHOMO ELUMO
[LmolÀ1 cmÀ1] [V]
[V]
Dye
lmax
Optical e[b]
EHOMO
[LmolÀ1 cmÀ1] [V]
ELUMO
[V]
[nm] edge
[nm]
[nm] edge
[nm]
catalysts
([Pd(PPh3)4],[10]
Pd(OAc)2,[12]
[Pd2(dba)3],[11]
and [PdCl2(dppf)][13]), phos-
phine ligands (Xantphos,[14]
DPEphos,[14]
D1p1A1 467
D1p2A1 493
D1p3A1 492
D1p4A1 493
D1p5A1 509
D1p6A1 503
D1p7A1 504
D1p1A2 495
D1p2A2 490
D1p3A2 489
D1p4A2 489
D1p5A2 501
D1p6A2 497
D1p7A2 486
580
637
608
586
622
614
648
592
605
587
583
618
593
640
29000
26000
39000
32700
32000
34000
23000
18500
5800
40700
36100
11000
42000
15700
13000
1.32
1.26
1.34
1.33
1.25
1.32
1.31
1.29
À0.81 D2p1A1 503
À0.68 D2p2A1 526
À0.69 D2p3A1 500
À0.79 D2p4A1 489
À0.74 D2p5A1 530
À0.70 D2p6A1 512
À0.60 D2p7A1 512
À0.81 D2p1A2 499
617
616
628
618
648
614
622
622
669
607
626
636
609
627
590
28600
35000
24000
31000
24000
48700
24900
28000
44100
32400
33000
12100
65100
57200
34000
1.28
1.17
1.16
1.15
1.12
1.15
1.15
1.20
1.15
1.16
1.15
1.11
1.16
1.15
1.15
À0.73
À0.84
À0.81
À0.85
À0.79
À0.87
À0.84
À0.79
À0.70
À0.88
À0.83
À0.84
À0.88
À0.82
À0.95
PPh3,
and
P(furyl)3),[15] and solvents (tol-
uene, EtOH, and THF with
H2O) were examined. As
a result, the combination of
1 mol% [Pd2(dba)3], 2 mol%
Xantphos, and Na2CO3 in tol-
uene/H2O afforded the best
result (68% yield). Then, the
one-pot conditions were ex-
plored. After confirming the
completion of the first cou-
pling reaction by TLC analy-
N.D.[d] N.D.[d] D2p2A2 506
1.30
1.31
1.20
1.26
1.23
1.35
À0.81 D2p3A2 501
À0.82 D2p4A2 502
À0.80 D2p5A2 497
À0.83 D2p6A2 510
À0.70 D2p7A2 495
À0.85 D2p1A3 471
D1p1A3 350, 563
440
D1p2A3 447
D1p3A3 447
D1p4A3 343
D1p5A3 477
D1p6A3 460
D1p7A3 470
D1p1A4 512
D1p2A4 513
D1p3A4 505
D1p4A4 505
D1p5A4 536
558
543
538
569
565
570
640
631
617
599
622
24600
21000
37700
26800
23000
20000
32000
30000
39000
39600
68500
15600
1.25
1.34
1.31
1.24
1.35
1.30
1.32
1.27
1.33
1.28
1.22
1.27
À0.97 D2p2A3 472
À0.94 D2p3A3 457
À1.00 D2p4A3 444
À0.94 D2p5A3 492
À0.85 D2p6A3 474
À0.88 D2p7A3 479
À0.61 D2p1A4 526
À0.69 D2p2A4 526
À0.68 D2p3A4 523
À0.79 D2p4A4 524
À0.77 D2p5A4 560
À0.64 D2p6A4 530
603
563
563
606
572
601
638
642
645
649
659
632
31000
26300
24000
25000
30300
30000
23600
40200
30000
39000
63100
33000
1.13
1.18
1.14
1.13
1.16
1.15
1.18
1.18
1.15
1.15
1.13
1.17
À0.92
À1.03
À1.06
À0.92
À1.01
À0.91
À0.77
À0.75
À0.77
À0.76
À0.75
À0.79
sis,
5 mol%
[Pd2(dba)3],
[(tBu3P)H]BF4,[16]
7 mol%
Na2CO3, and 3 were added.
The following workup and
silica gel column chromatog-
raphy afforded the desired
product 4 in a 60% yield
(one-pot, two-step yield).
This result indicated that the
remaining compounds in the
first step did not suffer the
following step. The addition
of [Pd2(dba)3] and an elec-
tron-rich and bulky ligand,
[(tBu3P)H]BF4, was crucial for
a good yield. Otherwise, the
reaction was very slow and
D1p6A4 226, 650
518
D1p7A4 524
618
88000
1.25
À0.75 D2p7A4 538
636
14000
1.16
À0.79
[a] Absorption maxima in CH2Cl2. [b] Molar absorption coefficients in CH2Cl2. [c] EHOMO was determined by the cyclic
voltammetry of each dye-sensitizer solution (~0.3 mm) with LiClO4 (0.1m) as a counter electrolyte in DMF (N,N-di-
methylformamide) equipped with Pt plate, Pt wire, and Ag/Ag+ (0.01m of AgNO3 in DMF) as working, counter, and
reference electrodes, respectively. Before and after each measurement, the Ag/Ag+ reference electrode was cali-
brated independently by the redox potential of ferrocene (FeIII/FeII) in DMF referred as +0.853 [V] (vs. NHE).[17]
.
EHOMO–ELUMO gap was determined by the edge of absorption spectra defined by the wavelength for which the ab-
sorbance revealed 1/10 of peak top; that is, EHOMO–ELUMO gap [eV] was calculated by 1240 [nm]/optical edge [nm].
ELUMO was thus calculated by the summation of HOMO potential and EHOMO–ELUMO gap. [d] Not determined (solubili-
ties of the dyes were not sufficient).
Chem. Eur. J. 2014, 20, 10685 – 10694
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Novel rigid and nonplanar
Table 3. Absorption maxima (lmax), optical edge, and molar absorption coefficients (e), EHOMO, ELUMO vs. NHE of the syn-
thesized D-p-A dyes (D3 and D4).
donors D3 and D4 con-
taining
a
norbornane
[b]
[b]
Dye
lmax
Optical e [LmolÀ1 cmÀ1
]
EHOMO ELUMO Dye
lmax
Optical e [LmolÀ1 cmÀ1
]
EHOMO ELUMO
structure were designed
to prevent the undesired
aggregation of dyes.[1c]
The building blocks were
coupled by using the de-
veloped one-pot proce-
dure in a parallel fashion
(Table 1). After the com-
pletion of the one-pot re-
action, the toluene layers
containing desired dyes
were simply removed
from the reaction vessels
by using pipettes and
were loaded onto disposa-
ble silica gel pads. Auto-
mated column purification
was performed to afford
the desired coupling prod-
ucts. In the case of A4, the
one-pot coupling yields
dropped to 20% probably
due to the undesired Pd
poisoning by heteroatoms
in A4.
[nm][a] edge
[nm]
[V][c] [V][c]
[nm][a] edge
[nm]
[V][c] [V][c]
D3p1A1 528
D3p2A1 519
D3p3A1 529
D3p4A1 523
D3p5A1 555
D3p6A1 526
D3p7A1 528
D3p1A2 518
D3p2A2 499
D3p3A2 512
D3p4A2 519
D3p5A2 521
D3p6A2 517
D3p7A2 520
D3p1A3 481
D3p2A3 483
D3p3A3 474
D3p4A3 473
D3p5A3 511
D3p6A3 487
D3p7A3 486
D3p1A4 540
D3p2A4 533
D3p3A4 542
D3p4A4 542
D3p5A4 575
D3p6A4 542
D3p7A4 540
659
653
654
650
686
648
650
633
631
633
630
662
687
620
589
593
584
600
635
596
604
654
650
677
673
704
666
658
30000
18000
36000
33000
31000
20000
81800
14600
25400
24600
31200
35600
21500
12900
14400
22100
18000
20000
29000
20900
19100
20700
42800
49000
40000
16300
19600
11000
1.11 À0.78 D4p1A1 508
1.08 À0.82 D4p2A1 504
1.10 À0.80 D4p3A1 506
1.13 À0.77 D4p4A1 506
1.04 À0.77 D4p5A1 535
1.09 À0.82 D4p6A1 511
1.08 À0.83 D4p7A1 516
N.D.[d] N.D.[d] D4p1A2 505
0.86 À1.11 D4p2A2 504
1.09 À0.87 D4p3A2 489
1.09 À0.87 D4p4A2 500
1.03 À0.84 D4p5A2 535
609
620
604
605
629
610
695
608
657
609
604
632
33500
11000
28400
33000
41800
29600
15800
25300
6550
0.93 À1.10
1.19 À0.81
1.22 À0.83
1.22 À0.83
1.14 À0.83
1.22 À0.82
1.19 À0.60
1.21 À0.83
1.16 À0.73
1.16 À0.88
1.21 À0.85
1.12 À0.84
1.19 N.D.[d]
1.13 N.D.[d]
1.21 À0.99
1.17 À0.93
1.20 À1.01
1.21 À1.03
1.14 À0.89
1.19 À0.87
1.14 À0.99
1.23 À0.76
1.20 À0.79
1.19 À0.80
1.21 À0.79
1.11 À0.76
1.18 À0.72
1.20 À0.77
26100
48300
20300
1.10 À0.71 D4p6A2 N.D.[d] N.D.[d] N.D.[d]
1.06 À0.94 D4p7A2 N.D.[d] N.D.[d] N.D.[d]
1.13 À0.98 D4p1A3 467
1.11 À0.98 D4p2A3 472
1.10 À1.02 D4p3A3 455
1.10 À0.97 D4p4A3 452
1.04 À0.91 D4p5A3 489
1.11 À0.97 D4p6A3 470
1.08 À0.97 D4p7A3 478
1.10 À0.79 D4p1A4 524
N.D.[d] N.D.[d] D4p2A4 560
1.02 À0.81 D4p3A4 522
1.09 À0.75 D4p4A4 526
1.02 À0.74 D4p5A4 558
1.08 À0.78 D4p6A4 526
1.09 À0.79 D4p7A4 525
563
591
560
553
612
600
582
623
623
623
621
663
653
627
33800
42000
16500
22600
37000
44900
15000
14800
48300
38800
14600
46700
16600
16000
Therefore, the readily
available boronic acid pi-
nacol ester 6 (A4’) was
used instead of A4 to im-
[a] Absorption maxima in CH2Cl2. [b] Molar absorption coefficients in CH2Cl2. [c] EHOMO was determined by the cyclic
voltammetry of each dye-sensitizer solution (~0.3 mm) with LiClO4 (0.1m) as a counter electrolyte in DMF equipped
with Pt plate, Pt wire, and Ag/Ag+ (0.01m of AgNO3 in DMF) as working, counter, and reference electrodes, respec-
tively. Before and after each measurement, the Ag/Ag+ reference electrode was calibrated independently by the
redox potential of ferrocene (FeIII/FeII) in DMF referred as +0.853 [V] (vs. NHE).[17] EHOMO–ELUMO gap was determined by
the edge of absorption spectra defined by the wavelength for which the absorbance revealed 1/10 of peak top; that
is, EHOMO-ELUMO gap [eV] was calculated by 1240 [nm]/optical edge [nm]. ELUMO was thus calculated by the summation
of HOMO potential and EHOMO–ELUMO gap. [d] Not determined (solubilities of the dyes were not sufficient).
prove
the
yields
(Scheme 2). After silica gel
column purification, Knoe-
venagel condensation of
the obtained coupling
A dye library (Figure 2). Except for D3 and D4, all blocks are
frequently found in high-performance dyes for DSSCs.[3c,d]
product 7 with 8 was performed to afford the intended dye 9.
Thus, the total number of reaction steps was not changed. Al-
Figure 3. a) The average values for EHOMO vs. NHE of D1-containing 27 dyes, D2-containing 28 dyes, D3-containing 26 dyes, and D4-containing 28 dyes. b) The
average values for ELUMO vs. NHE of D1-containing 27 dyes, D2-containing 28 dyes, D3-containing 26 dyes, and D4-containing 26 dyes. c) The average values
for EHOMO vs. NHE of p1-containing 15 dyes, p2-containing 14 dyes, p3-containing 16 dyes, p4-containing 16 dyes, p5-containing 16 dyes, p6-containing 16
dyes, and p7-containing 16 dyes. d) The average values for ELUMO vs. NHE of p1-containing 15 dyes, p2-containing 14 dyes, p3-containing 16 dyes, p4-contain-
ing 16 dyes, p5-containing 16 dyes, p6-containing 15 dyes, and p7-containing 15 dyes. e) The average values for EHOMO vs. NHE of A1-containing 28 dyes, A2-
containing 26 dyes, A3-containing 28 dyes, and A4-containing 27 dyes. f) The average values for ELUMO vs. NHE of A1-containing 28 dyes, A2-containing 24
dyes, A3-containing 28 dyes, and A4-containing 27 dyes. The values were expressed as means of Æstandard deviation (SD).
Chem. Eur. J. 2014, 20, 10685 – 10694
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good
to
excellent
yields
Table 4. Photovoltaic properties of solar cells prepared by using 33 randomly selected dyes.
(Table 1). These results clearly in-
dicate the robustness of our de-
veloped synthetic procedure.
Dye
EHOMO ELUMO
[V][a] [V][a]
h
[%][b]
FF[b] Voc
[V][b] [mAcmÀ2
Jsc
Dye
EHOMO ELUMO
[V][a] [V][a]
h
[%][b]
FF[b] Voc
[V][b] [mAcmÀ2
]
Jsc
[b]
[b]
]
The absorption spectra and
electrochemical properties of all
synthesized dyes were measured
as shown in Tables 2 and 3. As
donors influenced the EHOMO of
the dyes, the EHOMO became
more positive on the order of
D3<D2ꢀD4<D1 (Figure 3a).
On the other hand, acceptors in-
fluenced the ELUMO of the dyes,
and therefore the ELUMO became
more negative on the order of
D3p3A4 1.02
D3p5A1 1.04
D3p5A3 1.04
D3p2A1 1.08
D3p6A1 1.09
D3p4A4 1.09
D3p3A1 1.10
D3p1A1 1.11
D4p5A2 1.12
D2p2A3 1.13
D4p5A1 1.14
D2p1A3 1.15
D2p6A1 1.15
D2p7A1 1.15
D2p7A2 1.15
D4p3A2 1.16
D2p1A4 1.18
À0.81 1.4 0.70 0.57 3.4
À0.77 0.6 0.53 0.52 2.2
À0.91 1.6 0.54 0.59 5.2
À0.82 2.8 0.60 0.58 8.0
À0.82 2.7 0.55 0.59 8.2
À0.75 1.6 0.69 0.58 4.0
À0.80 4.0 0.65 0.64 9.7
À0.78 3.5 0.58 0.61 10.3
À0.84 0.9 0.61 0.49 3.0
À0.92 3.4 0.63 0.64 8.6
À0.83 3.4 0.62 0.61 9.1
À0.95 4.0 0.65 0.66 9.4
À0.87 4.1 0.66 0.64 9.7
À0.84 3.3 0.65 0.58 8.9
À0.82 2.3 0.65 0.55 6.3
À0.88 4.9 0.64 0.66 11.9
À0.77 1.1 0.64 0.51 3.5
D4p3A4 1.19
D1p5A2 1.20
D2p1A2 1.20
D4p2A4 1.20
D4p4A4 1.21
D1p5A4 1.22
D4p4A1 1.22
D1p5A1 1.25
D2p1A1 1.28
D1p7A3 1.30
D1p4A2 1.31
D1p6A1 1.32
D1p3A4 1.33
D1p4A1 1.33
D1p3A1 1.34
D1p6A3 1.35
À0.80 3.1 0.72 0.61 7.3
À0.80 4.2 0.65 0.61 10.5
À0.79 3.2 0.68 0.58 8.1
À0.79 2.4 0.69 0.54 6.3
À0.79 2.7 0.72 0.62 5.9
À0.77 1.7 0.62 0.54 5.2
À0.83 5.2 0.66 0.71 11.3
À0.74 3.3 0.69 0.60 7.9
À0.73 2.7 0.63 0.57 7.6
À0.88 4.8 0.69 0.67 10.2
À0.82 4.6 0.69 0.68 9.8
À0.70 4.6 0.64 0.65 11.2
À0.68 1.9 0.75 0.63 4.1
À0.79 4.6 0.72 0.70 9.0
À0.69 4.9 0.71 0.71 9.6
À0.85 4.5 0.68 0.71 9.5
A4ꢀA1<A2<A3
(Figure 3f).
The HOMO and LUMO levels of
all the synthesized dyes met the
minimum requirement for a dye-
sensitizer in DSSCs, that is, EHOMO
and ELUMO located more positive
[a] EHOMO and ELUMO were estimated by using the potential and an optical edge of dye-sensitizers by means of
electrochemical measurements and UV/Vis absorption spectra (see Tables 2 and 3). [b] Average values from
three independent experiments. FF, fill factor; Voc, open-circuit voltage; Jsc, short-circuit current.
though decreased yields were observed in the case of p5
(average one-pot yield 34%) due to the slow oxidative addi-
tion of an electron-rich CÀBr bond in p5, other combinations
generally afforded good yields (average one-pot yield 50%).
Following either acidic removal of the tBu group or Knoevena-
gel condensation, all the desired compounds were afforded in
than iodine/iodide redox potential (+0.4 V vs. normal hydro-
gen electrode (NHE)), and more negative than the conduction
band edge of TiO2 (À0.5 V vs. NHE), respectively.
Table 5. Photovoltaic properties of solar cells prepared by using D149
and 21 dyes that fulfil the requirements of HOMO and LUMO levels.
[b]
Dye
EHOMO [V][a] ELUMO [V][a] h [%][b] FF[b] Voc [V][b] Jsc [mAcmÀ2
]
D149
–
–
6.1
2.7
2.4
1.9
4.1
2.8
4.2
2.9
2.6
6.2
4.4
2.7
4.2
5.0
4.6
4.8
4.0
4.6
4.9
5.0
3.9
3.4
0.68 0.69
0.68 0.59
0.62 0.56
0.64 0.54
0.66 0.64
0.70 0.62
0.63 0.74
0.65 0.57
0.69 0.57
0.65 0.67
0.65 0.76
0.63 0.60
0.67 0.59
0.71 0.70
0.71 0.69
0.66 0.64
0.65 0.57
0.70 0.66
0.73 0.76
0.57 0.71
0.72 0.76
0.71 0.66
12.9
6.9
7.0
5.5
9.2
6.5
8.9
7.8
6.6
14.3
8.9
7.2
10.5
10.0
9.5
11.5
10.8
9.9
D2p3A2 1.16
D2p6A2 1.16
D2p6A3 1.16
D2p2A1 1.17
D2p3A3 1.18
D4p3A3 1.20
D4p1A2 1.21
D4p1A3 1.21
D4p4A2 1.21
D4p4A3 1.21
D4p6A1 1.22
D4p3A1 1.22
D1p5A3 1.24
D1p2A3 1.25
D1p6A2 1.26
D1p1A2 1.29
D1p3A2 1.30
D1p4A3 1.31
D1p1A1 1.32
D1p3A3 1.34
D1p1A3 1.35
À0.88
À0.88
À1.01
À0.84
À1.03
À1.01
À0.83
À0.99
À0.85
À1.03
À0.82
À0.83
À0.94
À0.97
À0.83
À0.81
À0.81
À1.00
À0.81
À0.94
À0.85
8.9
12.6
7.2
7.2
[a] EHOMO and ELUMO were estimated by using the potential and an optical
edge of dye-sensitizers by means of electrochemical measurements and
UV/Vis absorption spectra (see Tables 2 and 3). [b] Average values from
three independent experiments.
Figure 4. Correlations between a) e and h, b) lmax and h, and c) optical edge
and h of the 54 dyes.
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Figure 5. Correlations between e and Jsc of a) D1-containing 19 dyes (red),
D2-containing 13 dyes (blue), D3-containing 8 dyes (green), and D4-contain-
ing 14 dyes (yellow), b) p1-containing 10 dyes (red), p2-containing 5
dyes (blue), p3-containing 12 dyes (green), p4-containing 8 dyes (yellow).
p5-containing 8 dyes (orange), p6-containing 8 dyes (dark green). p7-con-
taining 3 dyes (brown), and c) A1-containing 18 dyes (red), A2-containing 13
dyes (blue), A3-containing 15 dyes (green), A4-containing 8 dyes (yellow).
Figure 6. Correlations between lmax and Jsc of a) D1-containing 19 dyes (red),
D2-containing 13 dyes (blue), D3-containing 8 dyes (green), and D4-contain-
ing 14 dyes (yellow), b) p1-containing 10 dyes (red), p2-containing 5
dyes (blue), p3-containing 12 dyes (green), p4-containing 8 dyes (yellow).
p5-containing 8 dyes (orange), p6-containing 8 dyes (dark green). p7-con-
taining 3 dyes (brown), and c) A1-containing 18 dyes (red), A2-containing 13
dyes (blue), A3-containing 15 dyes (green), A4-containing 8 dyes (yellow).
Photovoltaic properties were evaluated for the solar cells
that were prepared using 33 randomly selected dyes (Table 4).
The EHOMO and ELUMO of the dyes were critical for cell per-
formance. Five of the dyes, D4p3A2, D4p4A1, D1p7A3,
D1p4A2, and D1p6A3, with EHOMO > +1.15 V and ELUMO
<
À0.80 V, exerted a higher cell performance (average of h=
4.8Æ0.3%). On the other hand, the other 28 dyes exerted
a lower cell performance (average of h=2.9Æ1.2%).
Photovoltaic properties of the solar cells prepared by using
22 dyes, which fulfilled the requirement of EHOMO > +1.15 V,
and ELUMO <À0.80 V, were evaluated (Table 5). Not surprisingly,
most of the evaluated dyes exerted high cell performance.
These results clearly show that the driving forces of forward in-
terfacial electron transfer are highly important for a high h
with respect to organic D-p-A dyes. To our delight, the dye
D4p4A2, which contained our originally designed D4, exerted
the highest efficiency at 6.2%, which was comparable to that
of the commercially available high-performance dye, D149 (h=
6.1%).[3b]
Unexpectedly, no obvious correlation between the e and the
h (R2 value=0.014) were observed (Figure 4a). Only a very
weak correlation between the lmax and the h (R2 value=0.19)
was observed (Figure 4b), and the h tended to decrease as the
lmax became longer. A week correlation between the optical
edge and h (R2 value=0.31) was observed (Figure 4c), and the
h tended to decrease as the optical edge became longer. It is
conceivable that this result also indicated the importance of
the proper driving forces.
Figure 7. Correlations between optical edge and Jsc of a) D1-containing 19
dyes (red), D2-containing 13 dyes (blue), D3-containing 8 dyes (green), and
D4-containing 14 dyes (yellow), b) p1-containing 10 dyes (red), p2-contain-
ing 5 dyes (blue), p3-containing 12 dyes (green), p4-containing 8 dyes (yel-
low). p5-containing 8 dyes (orange), p6-containing 8 dyes (dark green). p7-
containing 3 dyes (brown), and c) A1-containing 18 dyes (red), A2-contain-
ing 13 dyes (blue), A3-containing 15 dyes (green), A4-containing 8 dyes
(yellow).
tions were not observed in general, correlations between lmax
and Jsc (R2 value=0.56), and optical edge and Jsc (R2 value=
0.53) were observed in the case of p5-containing dyes. Among
all the dyes, p5-containing dyes retained the longest lmax and
For a more detailed discussion, the correlations between
lmax, e, optical edge, and Jsc for each of the structural compo-
nents were examined (Figures 5–7). Although strong correla-
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Figure 9. The average values for a) h, b) FF, c) Voc, and d) Jsc, of the 10 dyes
retaining p1, the 5 dyes retaining p2, the 12 dyes retaining p3, the 8 dyes
retaining p4, the 8 dyes retaining p5, the 8 dyes retaining p6, and the 3
dyes retaining p7. The values were expressed as means of ÆSD.
Figure 8. The average values for a) h, b) FF, c) Voc, and d) Jsc, of the 19 dyes
retaining D1, the 13 dyes retaining D2, 8 dyes retaining D3, and the 14 dyes
retaining D4. The values were expressed as means of ÆSD.
optical edge. This means that the driving forces of p5-contain-
ing dyes are not sufficient. Therefore, Jsc tended to decrease as
the lmax and optical edge became longer in the case of p5-
containing dyes.
In terms of the D structure-device parameter (h, FF, Voc, and
Jsc) relationships, all the parameters became higher in the
order of D3<D2<D4<D1 (Figure 8). This tendency was con-
sistent with the EHOMO (Figure 3). Namely, the dyes with a more
positive EHOMO exerted higher PCEs. The results corroborated
the importance of the driving force.
In terms of the p structure–device parameter relationships,
roughly all the parameters became higher in the order of p5<
p1ꢁp2ꢁp3ꢁp6ꢁp7<p4. Similar to the previous report,[18]
p-bridges with long alkyl chains (p3 and p4) contributed to
the high Voc (Figure 9c) and high FF (Figure 9b). On the other
hand, the electron-donating p5 decreased in all the parame-
ters. This contrasted with recently reported D–A-p-A dyes,
which exerted excellent PCEs.[3g]
In terms of the A structure-device parameter relationships,
A1–A3 exerted a similar level of h and Jsc, whereas they were
decreased for A4 (Figure 10a and d). Among A1–A4, only the
average value for ELUMO of A4-containing dyes (À0.75Æ0.05 V)
was less negative than À0.80 V. Therefore, it is conceivable
that the driving force was not sufficient in the case of A4.[19]
Although the average values for the ELUMO of A1–A3-contain-
ing dyes (A1=À0.79Æ0.09, A2=À0.83Æ0.08, A3=À0.96Æ
0.06 V) were different from each other (Figure 3f), this did not
influence the PCEs. It seemed that the key to high PCEs was
the fulfillment of the previously described requirement of
Figure 10. The average values for a) h, b) FF, c) Voc, and d) Jsc, of the 18 dyes
retaining A1, the 13 dyes retaining A2, 15 dyes retaining A3, and the 8 dyes
retaining A4. The values were expressed as means of ÆSD.
the order of A4ꢀA2ꢀA1<A3. Tian and co-workers pointed
out the possibility that the LUMO level is correlated with Voc.[1c]
It is conceivable that a high Voc of A3-containing dyes can be
attributed to their most negative LUMO, and thus the low Voc
E
LUMO <À0.80 V. No obvious correlation between FF and the A
structure was observed (Figure 10b). The Voc became higher in
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Figure 12. Correlation between a) Jsc and the average values for the EHOMO of
D1-, D2-, D3-, and D4 containing dyes and b) Jsc and the average values for
the ELUMO of A1-, A2-, A3-, and A4-containing dyes.
a weak or no obvious correlation between spectroscopic prop-
erties (lmax, and e) and cell performance, and, therefore, the in-
jection efficiency seemed to be the most important factor for
high PCEs with respect to the organic D-p-A dyes. The injec-
tion efficiency was correlated with the HOMO and LUMO levels
of dyes, and these were influenced by D and A structures, re-
spectively (Figure 3a and f).
As shown in Figure 12a and b, the dyes with a more positive
EHOMO, and with an ELUMO <À0.80 V, exerted a higher Jsc. Based
on the obtained structure–property information, the combina-
tions of D1, p3, or p4 and A1, A2, or A3 were supposed to
exert a high h, because the average value for EHOMO of D1 was
the most positive. The p3 and p4 contributed to the high FF
and Voc. The average values for ELUMO of A1, A2, and A3 were
either equal to, or more negative than, the value of À0.80 V. In
accordance with speculation, most of the combinations actual-
ly exerted a high h (Table 4 and 5). However, it was interesting
to note that the best dye was not found in the supposed best
combinations of D, p, and A blocks. The combination of D4,
p4, and A2 exerted the highest PCE. Here, we considered the
reason for the high h of D4p4A2. The photocurrent-voltage
curve and the incident photon-to-current conversion efficiency
(IPCE) spectra of D4p4A2 were compared with those of the
simplest structure D-p-A dye (D1p1A1), and D149 as referen-
ces (Figure 13). D4p4A2 efficiently converted UV/Vis light to
a photocurrent in the region from 350 to 660 nm, similar to
D149. Compared with D1p1A1, the IPCE of D4p4A2 in the
region from 300 to 590 nm was lower, and that of in the
region from 590 to 750 nm was higher. An advantage of
D4p4A2 seemed to be that the dye exerted a somewhat high
IPCE in the wide region from 350 to 660 nm.
Figure 11. Correlations between a) FF and h, b) Voc and h, c) Jsc and h, d) Jsc
and IPCE area of the 54 dyes.
For the evaluation of the electron injection efficiency of
D4p4A2, and D1p1A1 in the film, a time-resolved microwave
conductivity (TRMC) measurement was carried out (see the
Supporting information for details). TRMC is a type of transient
absorption spectroscopy that measures the conductivity s of
mobile charges produced by pulsed-light excitation.[21] The ob-
tained results indicated that the electron injection efficiency
for D4p4A2 was lower than that for D1p1A1 (electron injec-
tion efficiencies, N719: 1,[21b] D1p1A1: 0.9, D4p4A2: 0.7).
We have described the importance of the proper driving
forces (HOMO and LUMO levels) for smooth forward interfacial
electron transfer for the creation of high PCE dyes in this
of the A4-containing dyes was attributed to their least-nega-
tive LUMO.
Reportedly, Voc is the most important factor to achieve high
PCEs with organic dyes.[1c,20] However, it is noteworthy that Jsc
strongly correlated (R2 value=0.87) with h (Figure 11c). The Jsc
also correlated (R2 value=0.74) with IPCE (Figure 11d). On the
other hand, only a weak correlation (R2 value=0.38) between
Voc and h (Figure 11b) was observed. No obvious correlation (R2
value=0.073) between FF and h (Figure 11a) was observed.
Generally, Jsc is correlated with the sensitizer absorption capa-
bility and the electron injection efficiency. There was only
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were critical for cell performance. The dyes with a more posi-
tive EHOMO, and with an ELUMO <À0.80 V, exerted higher PCEs.
This indicated that the proper driving forces were crucial for
smooth forward interfacial electron transfer, which directly in-
fluences the Jsc. The Jsc was the most important parameter for
h in the case of organic D-p-A dyes. The above criteria of EHOMO
and ELUMO should be useful for creating high PCE dyes. Howev-
er, is interesting that the best dye, D4p4A2 that contained our
originally designed, rigid, and nonplanar D4 was not found in
the supposed combinations of D, p, and A blocks based on
the above criteria of EHOMO and ELUMO. This clearly indicated the
importance of combinatorial synthesis and evaluation for iden-
tifying the best dye.
Experimental Section
Experimental details are given in the Supporting Information.
These include synthetic, photophysical and electrochemical proce-
dures, and the characterization of the synthesized compounds.
Acknowledgements
We thank Global COE Program “Education and Research
Center for Emergence of New Molecular Chemistry”, MEXT,
Japan for financial support. We also thank Keitaro Hashimoto,
Tokyo Institute of Technology and Akihiro Namekawa, Nihon
University for TRMC measurements.
Keywords: combinatorial chemistry · cross-coupling · dyes/
pigments · dye-sensitized solar cells · one-pot reactions
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d) H. J. Snaith, Nat. Photonics 2012, 6, 337–340.
Figure 13. Photocurrent-voltage curves obtained with DSSCs based on
D4p4A2, D1p1A1, and D149 under standard AM1.5 solar conditions (a) and
in the dark (b). c) Incident photon-to-current conversion efficiency spectra
&
*
*
for DSSCs based on D4p4A2, D1p1A1, and D149. : D4p4A2; : D149;
:
[2] J. N. Clifford, E. Martinez-Ferrero, A. Viterisi, E. Palomares, Chem. Soc.
Rev. 2011, 40, 1635–1646.
D1p1A1.
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manuscript. However, the electron injection efficiency of the
best dye, D4p4A2, was not the highest among the synthesized
dyes. In fact, the electron injection efficiency of D4p4A2 was
lower than that of D1p1A1. These results indicated that the
criteria of the more positive EHOMO, and ELUMO <À0.80 V should
be useful for creating high PCE dyes. Nevertheless, that was
not sufficient for identifying the best combination of D, p, and
A blocks. Our results indicated that combinatorial synthesis
and evaluation were important for identifying the best dye.
Conclusion
In summary, a large-scale (112-membered) D-p-A dye library
was rapidly constructed by a one-pot, three-component SM
coupling procedure. The evaluation of absorption spectroscop-
ic and electrochemical measurements of all synthesized dyes,
and cell performance of the selected 54 dyes were performed.
No obvious correlations between e, and the h were observed.
On the other hand, the HOMO and LUMO levels of the dyes
Chem. Eur. J. 2014, 20, 10685 – 10694
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Received: February 9, 2014
Published online on June 20, 2014
Chem. Eur. J. 2014, 20, 10685 – 10694
www.chemeurj.org
10694
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim