New triphenylamine organic dyes containing dithieno[3,2-b:2′, 3′-d]pyrrole (DTP) units for iodine-free dye-sensitized solar cells

Article abstract of DOI:10.1039/c3cc42121j

Three dithieno[3,2-b:2′,3′-d]pyrrole (DTP) units with different hexyloxyphenyl (HOP) substituents have been developed for triphenylamine organic dyes (XS54-XS56). The introduction of the 4-HOP-DTP unit has resulted in a stronger light harvesting capacity, accounting for the observed photocurrent enhancement in the case of XS54, while the 2-HOP-DTP/2,4-HOP-DTP units induce a strikingly large photovoltage improvement in the cases of XS55 and XS56 due to their higher steric hindrance.

Full text of DOI:10.1039/c3cc42121j

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New triphenylamine organic dyes containing
dithieno[3,2-b:20,30-d]pyrrole (DTP) units for
iodine-free dye-sensitized solar cells†
Zhihui Wang,a Mao Liang,*b Lina Wang,b Yujie Hao,b Chunbo Wang,b Zhe Sunb
and Song Xue*b
Cite this: DOI: 10.1039/c3cc42121j
Received 24th March 2013,
Accepted 7th May 2013
DOI: 10.1039/c3cc42121j
www.rsc.org/chemcomm
Three dithieno[3,2-b:20,30-d]pyrrole (DTP) units with different
hexyloxyphenyl (HOP) substituents have been developed for tri-
phenylamine organic dyes (XS54–XS56). The introduction of the
4-HOP-DTP unit has resulted in a stronger light harvesting capacity,
accounting for the observed photocurrent enhancement in the
case of XS54, while the 2-HOP-DTP/2,4-HOP-DTP units induce a
strikingly large photovoltage improvement in the cases of XS55
and XS56 due to their higher steric hindrance.
During the past two decades, numerous research attempts have
been made to explore new photosensitizers (e.g. Ru-based
complexes,1 zinc porphyrin complexes2 and organic sensitizers1–3
)
and iodine-free redox couples4 as well as to understand in-depth
essential interface processes5 in the development of dye-sensitized
solar cells (DSCs). Amongst the various classes of emerging
photosensitizers, organic sensitizers with robust availability,
ease of structural tuning and generally high molar extinction
coefficients have recently received great attention.
Fig. 1 Chemical structures of XS54–57.
DTP (4-HOP-DTP), 2-hexyloxyphenyl-substituted DTP (2-HOP-DTP)
and 2,4-bis(hexyloxy)phenyl-substituted DTP (2,4-HOP-DTP). XS57
featuring the dihexylcyclopentadithiophene (DH-CPDT) bridge was
prepared as a reference. Device performance characteristics
demonstrate the superiority of employing 4-HOP-DTP relative
to DH-CPDT as the conjugated spacer in this type of organic dyes.
The UV-vis absorption spectra of the dyes in CH2Cl2 solutions
are depicted in Fig. 2a. The optical and electrochemical properties
of the dyes are summarized in Table 1. The dye alteration from
Studies have shown that development of new fused-ring building
blocks as spacers is an effective strategy for obtaining highly efficient
organic sensitizers.1 Representative fused-ring thiophenes reported
in the literature include benzodithiophenes (benzo[1,2-b:4,5-b0]-
dithiophene, BDT1;6 benzo[2,1-b:3,4-b0] dithiophene, BDT26) and
b,b0-bridged bithiophenes (dithieno[3,2-b:20,30-d]silole, DTS;7 cyclo-
pentadithiophene, CPDT;8 dithieno[3,2-b:20,30-d]pyrrole, DTP9). It is
valuable to note that organic dyes featuring the alkyl-functionalized
CPDT bridge hold the record for validated efficiency of over 10%.8
If a new fused-ring thiophene with a superior performance relative to
CPDT is explored, new breakthroughs in the development of organic
dyes would be expected. However, to the best of our knowledge,
there are few reports on this issue.10
In this work, we report three triphenylamine dyes (XS54–XS56,
Fig. 1) containing new DTP units, i.e. 4-hexyloxyphenyl-substituted
a Department of Chemistry, Tianjin University, Tianjin, 300072, P.R. China
b Department of Applied Chemistry, Tianjin University of Technology, Tianjin,
300384, P.R.China. E-mail: liangmao717@126.com, xuesong@ustc.edu.cn;
Fax: +86 22 60214252; Tel: +86 22 60214250
† Electronic supplementary information (ESI) available: Experimental details,
Fig. S1–S6 and Tables S1–S3. See DOI: 10.1039/c3cc42121j
Fig. 2 (a) Absorption spectra of dyes in DCM; (b) IPCEs action spectra for DSCs
employing the cobalt electrolyte.
c
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Table 1 Optical and electrochemical properties of the dyes
Table 2 Photovoltaic performance of DSCs employing the cobalt electrolyte
under AM 1.5 irradiationa
max/nm (e/103 Mꢀ1 cmꢀ1
)
E0–0 a/eV
ED/D+
ED*/D+
b
c
Dye
l
Dyes
JSC/mA cmꢀ2
VOC/mV
FF
PCE%
G/10ꢀ7mol cmꢀ2
XS54
XS55
XS56
XS57
338 (53), 520 (79.9)
359 (47), 510 (63.4)
346 (45), 522 (72.5)
344 (43), 529 (66.8)
2.17
2.19
2.17
2.12
0.97
1.00
1.01
0.99
ꢀ1.20
ꢀ1.19
ꢀ1.16
ꢀ1.13
XS54
XS55
XS56
XS57
13.5
11.2
11.6
12.1
850
950
935
907
0.71
0.70
0.69
0.70
8.14
7.45
7.48
7.68
0.76
0.36
0.97
0.46
a
E0–0 values were estimated from the intersections of normalized
b
a
absorption and emission spectra (Fig. S1, ESI). First oxidation poten-
Thin film (5 mm) was used to fabricate the devices with an active
surface area of 0.156 cm2.
tials (vs. NHE) of dye-loaded films internally calibrated with ferrocene
c
(0.63 V vs. NHE). ED*/D+ were calculated from ED*/D+ = ED/D+ ꢀ E0–0
.
short-circuit photocurrent density (JSC) of 13.5 mA cmꢀ2, an
open-circuit photovoltage (VOC) of 850 mV and a fill factor (FF)
of 0.71, affording a PCE of 8.14%. There is 17.8% enhancement
in JSC of XS54 in comparison with XS55 (11.2 mA cmꢀ2) in the
cobalt cells. Moreover, it is also valuable to note that XS54
possesses noticeably enhanced JSC relative to that of its CPDT
congener (XS57, 12.1 mA cmꢀ2), suggesting that the pursuit of
fused-ring thiophene with high electron-donating ability is a
rational strategy for JSC optimization. The JSC enhancement of
XS54, mainly benefiting from its broad and high IPCE action
area (Fig. 2b), can be attributed to its stronger molar absorption
coefficients (Table 1) and higher amounts of the dyes adsorbed
(Table 2). Evidently, the remarkably improved JSC of XS54
significantly contributes to its PCE values. As a result, XS54
sensitized DSCs employing the cobalt electrolyte yield the
highest efficiency amongst these four dyes. Note that there is
no scattering layer used in the fabrication of DSCs, which is one
of the main reasons that the performance of DSCs shown in
this work is not higher than that reported in the literature.
However, in view of the same conditions of the test, our
approach is reliable for fair evaluation of the photovoltaic
performance of the four dyes discussed in this work. Three
batches of cells were fabricated and these results confirm that
for the [Co(II/III)(phen)3](PF6)2/3 redox shuttle, XS54 featuring
the 4-HOP-DTP spacer outperforms the DH-CPDT counterpart,
XS57.
Apart from the photocurrent, the position of hexyloxyl also
has a distinct impact on the photovoltage of the devices.
Surprisingly, the superior performance of 2-HOP-DTP as a
p-linker relative to 4-HOP-DTP in terms of the VOC is parti-
cularly noteworthy. Compared with XS54, the introduction of
the 2-HOP group into the DTP unit in XS55 successfully realizes
a significant increase in VOC by 100 mV. The positive impact of
2-HOP-DTP on the VOC of devices can also be found in the case
of XS56, which exhibits a high VOC of 935 mV. The photovoltaic
characteristics of XS56 show a slightly decreased JSC value as for
XS57, while the VOC is 28 mV higher, resulting in a comparable
PCE value of 7.48%.
XS55 (510 nm) to XS54 (520 nm) has caused a lmax red-shift of
10 nm along with enhanced maximum molar absorption coeffi-
cients (e), indicating that the electron density of 4-HOP-DTP is
higher than that of 2-HOP-DTP. This observation can be under-
stood from molecular modeling studies of XS54 and XS55 (Fig. S2,
ESI†). Theoretical computation shows that the ground state
structure of XS54 possesses a 491 twist between the 4-HOP and
the DTP skeleton, while the corresponding dihedral angle in XS55
is larger, with the value being 621. Therefore, the red shift of the
absorption band of XS54 is predominantly caused by a more
planar conjugating system. Although XS54 displays a slight blue
shift of lmax as compared to XS57, a higher maximum molar
absorption coefficient is observed for the former, which is desir-
able for thin-layer DSCs.
Cyclic voltammetry (CV) was carried out in a typical three-
electrode electrochemical cell with TiO2 film stained with
sensitizers as the working electrode (Fig. S3, ESI†). XS54 exhi-
bits the lowest HOMO level of 0.97 V vs. NHE among the three
dyes, which is even lower than that of XS57 (0.99 V vs. NHE),
indicating the stronger electron-donating capability of the
4-HOP-DTP unit. The driving forces for dye regeneration of
XS54–XS56 are around 0.37 eV, which is close to that of XS57.
On the other hand, the driving forces for electron injection of
XS54–XS56 (0.66–0.7 eV) are larger than that of XS57 (0.63 eV).
As shown in Fig. 3, we recorded the photocurrent density–
voltage (J–V) curves of the devices employing a tris(1,10-
phenanthroline)cobalt(II/III) redox shuttle under AM 1.5 irradia-
tion (100 mW cmꢀ2), and the detailed parameters are collected
in Table 2. The cobalt electrolyte is composed of 0.25 M
[Co(II)(phen)3](PF6)2, 0.05 M [Co(III)(phen)3](PF6)3, 0.5 M 4-tert-
pyridine (TBP) and 0.1 M lithium bis-(trifluoromethanesulfonyl)-
imide (LiTFSI) in acetonitrile.11 A typical cell based on XS54 has a
To clarify the origin of the observed VOC improvement upon
the employment of the photosensitizers featuring a 2-HOP-DTP/
2,4-HOP-DTP, we focus on the dyes with the most contrasting
photovoltages, i.e. XS54 and XS55. As commonly understood,
VOC is intimately correlated to the conduction band (CB) position
and the charge recombination rate.12,13 Fig. 4a shows the
relationship between VOC and extracted charge density (Q) at
open circuit potential. At a fixed Q, the higher VOC value of
XS54 relative to XS55 indicates a negative shift of the CB.14
Fig. 3 J–V characteristics of DSCs employing the cobalt electrolyte.
c
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In summary, three DTP based triphenylamine sensitizers
have been designed and synthesized. Despite the slight blue
shift of absorption, XS54 has a relatively high IPCE mainly due
to stronger molar absorption coefficients and higher amounts
of the dyes adsorbed on TiO2. In conjunction with the cobalt
electrolyte, XS54 featuring the 4-HOP-DTP spacer yields a PCE
of 8.14%, outperforming the analogous device constructed
using XS57. Furthermore, electron lifetime studies show that
charge recombination rates are suggested to be determined by
the direction of alkyl chains rather than the number of the alkyl
chains. These findings will considerably encourage further
molecular engineering of low-cost organic dyes for DSCs.
The authors acknowledge financial support from the National
Natural Science Foundation of China (21003096, 21072152,
21103123).
Fig. 4 Charge density at open circuit potential (a) and electron lifetime (b) as a
function of voltage for DSCs.
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c
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Chem. Commun.