Co-sensitization of the HD-2 complex with low-cost cyanoacetanilides for highly efficient DSSCs

Article abstract of DOI:10.1039/c9pp00381a

The photophysical and electrochemical properties of new targeted 2-cyanoacetanilide-based dyes are illustrated. New cyanoacetanilides SA7-10 were synthesized and employed as co-sensitizers in DSSCs. The chemical structures of these 2-cyanoacetanilides differ according to the substituent at the benzene ring (-H,-Me,-OMe and-NEt₂), with the anchoring moiety being the same, a-COOH group. Furthermore, a density functional theory (DFT) calculation has shown an effective intermolecular charge transfer character, the HOMOs of SA7-10 are mainly located on the corresponding donor part, and their LUMOs are located on carboxylic acid moieties as the acceptor. Interestingly, using photosensitizers SA7-10 as co-sensitizers with HD-2 dye causes an improvement in their photovoltaic performances. Among the dyes, SA10 co-sensitized with HD-2 displayed an overall efficiency of 8.25%, a J_SC of 19.5 mA cm^-2, a V_OC of 0.65 V and an FF of 64.35 compared to 7.46%, 19 mA cm^-2, 0.64 V and 60.54, respectively, of HD-2 only. Moreover, the electrochemical impedance spectroscopy (EIS) data of SA7-10 and HD-2 were found to be in accordance with the obtained photovoltaic parameters. Finally, the results indicated that 2-cyanoacetanilide-based dyes were utilized as promising co-sensitizers due to their easy preparation methods and their relatively small size.

Full text of DOI:10.1039/c9pp00381a

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DOI: 10.1039/C9PP00381A
Co-sensitization of HD-2 complex with low-cost cyanoacetanilides for
highly efficient DSSCs
Mohamed R. Elmorsya*, Luping Lyub, c, Ehab Abdel-Latifa, Safa A. Badawya, Ahmed EL-
Shafeib*, Ahmed A. Faddaa
aDepartment of Chemistry, Faculty of Science, Mansoura University, El-Gomhoria Street,
35516 Mansoura, Egypt.
* Corresponding author. E-mail address: m.r.elmorsy@gmail.com
bPolymer and Color Chemistry Program, North Carolina State University, Raleigh, NC,
27695, USA.
* Corresponding author. E-mail address: ahmed_el-shafei@ncsu.edu
cLinjiang College, Hangzhou Vocational and Technical College, Hangzhou 310018, People’s
Republic of China
Graphical abstract
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DOI: 10.1039/C9PP00381A
Abstract
The photophysical and electrochemical properties of new-targeted 2-cyanoacetanilide-based
dyes are illustrated. The new cyanoacetanilides SA7-10 were synthesized and employed as co-
sensitizers in DSSCs. The chemical structures of these 2-cyanoacetanilides differ according to
the substituent at benzene ring (-H, -Me, -OMe and -NEt2) keeping the same anchoring
moiety –COOH group. Further, density functional theory (DFT) calculation has shown an
effective intermolecular charge transfer character, the HOMOs of SA7-10 are mainly located
on the corresponding donor part, and their LUMOs are on carboxylic acid moieties as
acceptor. Interestingly, using photosensitizers SA7-10 as co-sensitizers with HD-2 dye causes
an improvement in its photovoltaic performances. Among the dyes, SA10 co-sensitized with
HD-2 displayed an overall efficiency of 8.25%, JSC of 19.5 mA.cm-2, VOC of 0.65 V and FF
of 64.35 compared to 7.46%, 19 mA.cm-2, 0.64 V and 60.54 of HD-2 only. Moreover,
electrochemical impedance spectroscopy (EIS) data of SA7-10 and HD-2 were found to be in
accordance with obtained photovoltaic parameters. Finally, the results indicated that 2-
cyanoacetanilide-based dyes were utilized as promising co-sensitizers due to their easily
preparation methods and their relatively small size.
Keywords: 2-cyanoacetanilide; DFT; Co-sensitization; DSSCs.
1. Introduction
Dye-sensitized solar cell (DSSC) is considered as a low-cost solar cell belonging to the
group of thin film solar cells.1,2 The DSSC has many promising properties: simple preparation
method techniques, multi-molecular strategy, high performance under low light intensity and
diffuse-light environments and it has multiple color and design.3,4 In 2014, the DSSC with the
zinc porphyrin dye and Co(II/III) tris(bipyridine)-based redox electrolyte reached an excellent
efficiency record of 13.00% and soon after, the highest efficiency record of 14.30% using a
co-absorber in an unsealed device under the 1.5 global (AM1.5G) condition.5,6 These
breakthroughs further encouraged research studies on developing economical photovoltaic
devices based on DSSCs and rendered DSSCs to be more competitive in the solar cell
markets. Typical DSSCs consists of photosensitizer, counter electrode, working electrode and
redox electrolyte. To effectively improve DSSC performance and reduce its fabrication costs,
various organic materials have been investigated to increase the cell efficiency, enhance the
cell durability, and reduce the production cost.7 For electrolyte, solid electrolyte or quasi-solid
electrolyte is actually a modified form of conventional DSSCs, where a liquid electrolyte has
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Photochemical & Photobiological Sciences
been replaced with a solid state conducting materials or gel electrolytes to improve the VlioewnAgrt-icle Online
DOI: 10.1039/C9PP00381A
term stability of the DSSCs .8-12 For example, the most common polymer matrix that was used
for gel electrolytes is PVDF-based, mainly due its high thermal stability.13,14 For counter
electrode, replacement of Pt was much needed due to its higher cost and less abundance.
Thus, several alternatives have developed to replace Pt in DSSCs such as, carbon materials
and metal sulphides.15,16 In DSSCs, the photosensitizer is a critical component to enhance
their power conversion efficiency (PCE).17 DSSCs can be found in two main categories,
namely, metal-based dyes and metal-free dyes.18,19 First, the majority of the researches use
ruthenium(II) complexes as a type of metal-based dyes because of their exciting
electrochemical and photophysical features.20-22 However, conventional Ru-complexes have
significant drawbacks, such as scarcity, complicated synthesis, and toxicity.17 In contrast,
metal-free organic dyes have been used as alternatives due to their low cost, high molar
extinction coefficient and flexibility in their design.23,24 Nevertheless, metal-free dyes still
provide a low photovoltaic performance compared to Ru-complexes.25,26 Co-sensitization
technique is one of the best alternative approaches to fulfill the limitations of a single dye
through combining two or more.27-32 The higher efficiency of 28.9% was achieved by
combining two judiciously designed sensitizers under ambient light.33 Cyanoacetanilides, as
series of organic compounds, possess attractive features such as distinct anchoring moieties,
highly conjugation and small size allowing for using as a good sensitizer / co-sensitizer in the
applications of DSSCs.34,35 To carry forward the advantages of small organic co-sensitizers in
cocktail DSSC, we have synthesized four small size co-sensitizers SA7-10 based on 2-
cyanoacetanilide moiety and their DSSC performance as co-sensitizers with HD-2 dye was
investigated.
HD-2 is considered as an efficient sensitizer based on Benzodioxan-Stilbazole
ancillary ligand. HD-2 showed intense metal-to-ligand charge-transfer (MLCT) band but with
a decrease in its molar absorptivity. 36 Additionally, there is a shortage in the maximum light
harvesting abilities in the region between 350 and 360 nm. Consequently, the co-sensitization
of SA7-10 along with HD-2 for improving the performance of HD-2 and increasing its light
harvesting ability. The molecular structures of the novel co-sensitizers SA7-10 along with our
efficient dye Ru (II) complex HD-2 are presented in Fig. 1
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DOI: 10.1039/C9PP00381A
N
H3CO
H
H3C
O
O
HN
O
O
HN
HN
HN
CN
CN
CN
CN
COOH
SA10
COOH
SA9
COOH
SA7
COOH
SA8
O
O
O
O
N
N
NCS
Ru
N
HOOC
NCS
N
HOOC
HD-2
Fig 1: Chemical structures of organic sensitizers SA7-10 and HD-2.
2. Experimental
2.1. Materials and methods
The chemicals and solvents were purchased from Alfa Aesar, Sigma-Aldrich and
o
TCI-America and used as received. The melting points were measured in C on Gallenkamp
apparatus and are uncorrected. The FTIR spectra were recorded with Nicolet iS10 Thermo
Scientific spectrometer and UV-visible spectra were measured with a pg T180
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DOI: 10.1039/C9PP00381A
1
spectrophotometer. HNMR spectra were recorded with 500 MHz spectrometer using
DMSO-d6 as a solvent. 13C NMR spectra were recorded at 125 MHz spectrometer.
2.2. Synthesis of cyanoacetanilides 1a-c
Derivatives 1a-c has been previously investigated according to known procedure.37
2.3. Synthesis of 2-cyano-p-(diethylamino)acetanilide (1d )
A mixture of N,N-diethyl-p-phenylenediamine (0.82 g, 5 mmol) and 1-(cyanoacetyl)-3,5-
dimethylpyrazole (0.82 g, 5 mmol) was boiled in dioxane (20 ml) for 4 h. The solid that
formed during heating was filtered off and dried.
Black crystals; m.p. 145-146 °C; yield = 90%. IR (KBr) νmax.(cm-1): 3388 (N-H), 3163 (C≡N),
1667 (C=O). 1H NMR (DMSO-d6, ppm): δ 1.04 (t, J = 6.9 Hz, 6H), 3.27 (q, J = 6.9 Hz, 4H),
13
3.78 (s, 2H), 6.60 (d, J = 8.9 Hz, 2H), 7.29 (d, J = 9.6 Hz, 2H), 9.91 (s, 1H, NH). C NMR
(DMSO-d6, ppm): δ 159.85, 144.41, 126.91, 121.15 (2C), 116.20, 111.77 (2C), 43.71, 26.30,
12.37.
2.4. General synthesis of cyanoacetanilide sensitizers SA7-10
4-formylbenzoicacid (0.45 g, 3 mmol) was dissolved in 20 mL absolute ethanol and
added to 3 mmol of corresponding cyanoacetanilides 1a-d (namely, 2-cyanoacetamide 2-
cyano-p-methylacetanilide, 2-cyano-p-methoxacetanilide and 2-cyano-p-(diethylamino)-
acetanilide). The reaction components were refluxed for 3 h, the obtained solid was collected
by filtration and then recrystallized by ethanol.
2-(p-carboxybenzylidene)-2-cyanoacetanilide (SA-7)
Yellow crystals; m.p. = 312-314 °C; yield = 65%. IR (KBr) νmax.(cm-1): 3332 (N-H), 2220
(C≡N), 1722 (C=O), 1661 (C=O). 1H NMR (DMSO-d6, ppm): δ 7.15 (t, 1H, J = 7.2 Hz), 7.37
(t, 2H, J = 6.7 Hz), 7.67 (d, 2H, J = 7.6 Hz), 8.05 (d, 2H, J = 8.2 Hz), 8.10 (d, 2H, J = 8.2 Hz),
8.33 (s, 1H, C=CH), 10.48 (s, 1H, NH), 13.34 (s, 1H, COOH). 13C NMR (DMSO-d6, ppm): δ
166.54, 160.20, 149.60, 138.15, 135.76, 133.63, 130.03 (2C), 129.97 (2C), 128.80 (2C),
124.47 (2C), 120.56, 115.85, 109.41.
2-(p-carboxybenzylidene)-2-cyano-p-methylacetanilide (SA-8)
Yellow crystals; m.p. above 300 °C; yield = 80%. IR (KBr) νmax.(cm-1): 3339 (N-H), 2531 (O-
H), 2202 (C≡N), 1682 (C=O). 1H NMR (DMSO-d6, ppm): δ 2.27 (s, 3H), 7.17 (d, 2H, J = 8.2
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Hz), 7.54 (d, 2H, J = 8.2 Hz), 8.04 (d, 2H, J = 8.9 Hz), 8.10 (d, 2H, J = 8.9 Hz),D8O.I3: 110.1(0s3,9/1CH9P,P00381A
C=CH), 10.40 (s, 1H, NH), 13.36 (s, 1H, COOH). 13C NMR (DMSO-d6, ppm): δ 166.56,
159.97, 149.42, 135.75, 135.61, 133.70, 133.55, 130.00 (2C), 129.94 (2C), 129.18 (2C),
120.57 (2C), 115.89, 109.43, 20.51.
2-(p-carboxybenzylidene)-2-cyano-p-methoxyacetanilide (SA-9)
Orange crystals; m.p. 300-302 °C; yield = 70%. IR (KBr) νmax.(cm-1): 3328 (N-H), 2549 (O-
1
H), 2224 (C≡N), 1692 (C=O), 1671 (C=O). H NMR (DMSO-d6, ppm): δ 3.74 (s, 3H), 6.94
(d, 2H, J = 8.9 Hz), 7.57 (d, 2H, J = 8.2 Hz ), 8.04 (d, 2H, J = 8 Hz), 8.10 (d, 2H, J = 8 Hz),
8.31 (s, 1H, C=CH), 10.35 (s, 1H, NH), 13.34 (s, 1H, COOH). 13C NMR (DMSO-d6, ppm): δ
166.53, 159.75, 156.08, 149.31, 135.81, 133.54, 131.10, 129.99 (2C), 129.94 (2C), 122.21
(2C), 115.89, 113.90 (2C), 109.42, 55.20.
2-(p-carboxybenzylidene)-2-cyano-p-(diethylamino)acetanilide (SA10)
Black powder; m.p. = 289-290 °C; yield = 60%. IR (KBr) νmax.(cm-1): 3600 (N-H), 2215
1
(C≡N), 1684 (C=O). H NMR (DMSO-d6, ppm): δ 1.06 (t, 6H, J = 6.9 Hz), 3.30 (q, 4H, J =
7.1 Hz), 6.64 (d, 2H, J = 8.9 Hz), 7.43 (d, 2H, J = 8.9 Hz), 8.03 (d, 2H, J = 8.2 Hz), 8.09 (d,
2H, J = 8.2 Hz), 8.26 (s, 1H, C=CH), 10.15 (s,1H, NH), 13.34 (s, 1H, COOH). 13C NMR
(DMSO-d6, ppm): δ 166.53, 159.22, 148.82, 144.76, 135.94, 133.38, 129.94 (2C), 129.92
(2C), 126.44, 122.40 (2C), 116.01, 111.52 (2C), 109.61, 43.71 (2C), 12.40 (2C).
3. Result and discussion
3.1. Synthesis and structure characterization
1-cyanoacetyl-3,5-dimethylpyrazole has been utilized an effective reagent for
cyanoacetylation of different primary aromatic amines to furnish the conforming
cyanoacetanilides. The synthesis of cyanoacetanilides 1a-c has been achieved by refluxing
aniline, 4-toluidine and/or 4-anisidine with 1-cyanoacetyl-3,5-dimethylpyrazole as previously
described in the literature.37 Cyanoacetanilide of the type 1d was prepared by
cyanoacetylation of N,N-diethyl-p-phenylenediamine with 1-cyanoacetyl-3,5-
dimethylpyrazole by refluxing in dioxane as shown in Scheme 1. Then, SA7-10 sensitizers
were obtained by applying the conditions of Knoevenagel reaction between various
cyanoacetanilides 1a-d and 4-formylbenzoic acid. The condensation reaction was carried out
in boiling solution of EtOH and in the presence of piperidine as a catalyst to afford
cyanoacetanilide sensitizers SA7-10 (Scheme 1).
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DOI: 10.1039/C9PP00381A
N
N
Dioxane
N
+
N
C
HN
CN
CN
O
NH2
O
1d
R
O
R
O
CN
4-formylbenzoic acid
EtOH / pip.
N
CN
N
H
H
1a-c
COOH
1a, R = H
1b, R = OCH3
1c, R = CH3
SA7, R = H
SA8, R = OCH3
SA9, R = CH3
SA10, R=N(Et)2
1d, R = N(Et)2
Scheme 1: Synthesis of cyanoacetanilide sensitizers SA7-10.
3.2. Photophysical properties
Figure 2 showed the UV-Vis absorption spectra of the four novel sensitizers SA7-10
(2 × 10-5 M) at DMF and their corresponding data are listed in Table 1.
Table 1 Absorption data for cyanoacetanilides SA7-10.
Sensitizer
λ abs.
Ɛ (104 M-1 cm-1)
E0-0
SA7
SA8
412
421
440
464
0.72
0.88
0.95
1.03
2.46
2.41
2.26
2.18
SA9
SA10
The targeted cyanoacetanilide dyes SA7-10 showed two distinct absorption bands. The
first peak in the region from 280 to 350 nm is related to the π-π* electronic transition, while
the second peak in the region from 380 to 520 nm is attributed to the internal charge transfer
(ICT) from the donor parts (-C6H4CH3, -C6H4OCH3 and -C6H4N(Et)2) to the acceptor moiety
(-C6H4COOH). Moreover, energy band gaps (E0-0) for all cyanoacetanilide dyes are
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determined from the UV-Vis spectra by recording the lowest energy onset forDOeIa: c10h.10d39y/Ce9.3P8P00381A
SA10 is found to be more red shifted than other products because of the relatively strong
donating ability of -C6H4N(Et)2 group. Additionally, sensitizer SA10 showed the smallest
band gap of 2.18 eV with highest extinction coefficient of 1.03 ×104 M-1 cm-1, indicating
stronger light harvesting ability compared to other corresponding dyes SA7-9. The UV-Vis
spectra of the main sensitizer HD-2 was given in the supplementary data.
5
SA7
4
SA8
3
SA9
2
SA10
1
0
280
330
380
430
480
530
580
Wavelength/nm
Fig 2: UV-Vis spectra of SA7-10 recorded in DMF (2 x10-5 M).
3.3. Cyclic voltammetry (CV) experiments
Cyclic voltammetry measurements were studied on SA7-10 to estimate their ground
state oxidation potential (GSOP/HOMO) and excited state oxidation potential (ESOP). These
values suggest a better explanation of the possibility of electron transfer to the TiO2
conduction band edge, and the regeneration process of the dyes. From CV traces showed in
Figure 3, the onset oxidation peak can be detected and from equation 1 and GSOP values can
be evaluated.
GSOP = - [Oxidation onset + 4.7]
Eq. (1)
Also, from the values of calculated (GSOP) and the measured E0-0, (ESOP) can be
calculated through equation 2.
ESOP = [(GSOP - E0–0] eV
Eq. (2)
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DOI: 10.1039/C9PP00381A
0.04
0.03
0.02
0.01
0
SA7
SA8
SA9
SA10
0
0.5
1
1.5
2
Ewe/V vs. Ag/Ag+
Fig 3: Cyclic voltammetry (CV) graphs for cyanoacetanilides SA7-10.
Furthermore, the estimated GSOP values of sensitizers SA7 (-6.19 eV), SA8 (-5.98
eV), SA9 (-5.55 eV) and SA10 (-5.33 eV) were found to be lower than the redox potential of
(I3/I-) electrolyte system (-5.2 eV). Additionally, ESOP values of the newly dyes, SA7 (-3.73
eV), SA8 (-3.57 eV), SA9 (-3.29 eV) and SA10 (-3.15 eV), which are higher than the
potential of CB (conduction band) of TiO2 (-4.2 eV) for an efficient electron injection to the
TiO2 edge.39 The results obtained from photophysical and CV studies are depicted in the form
of energy level diagram in Figure 4. It’s clear from the result that co-sensitizer SA10 carrying
carboxylic acid as anchoring group and N,N-diethyl benzene as strong donor would be more
efficient for electron efficient than other dyes. On the other hand, for SA7, the electron
injection free energy is not enough to guarantee thermodynamic charge injection. Therefore, it
is expected to display a poor photovoltaic performance according to its electrochemical
property.
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DOI: 10.1039/C9PP00381A
Fig 4: Energy level diagram for cyanoacetanilides SA7-10.
3.4. Theoretical calculations
DFT simulations on SA7-10 were performed with the Guassian09 software.40 In the
first step, the optimized geometry structures of the co-sensitizers and their related electronic
density delocalization in FMO energy levels were performed at DFT level via B3LYP
function and 6-31G* as basis set in DMF as a solvent. Furthermore, GSOP, ESOP and E0-0
obtained values were summarized in Table 2, which agree with CV data. As shown in Fig. 4,
it is obvious that there is effective charge separation between GSOP and ESOP of the dyes. As
evidenced from their HOMO energy levels, the electron cloud is predominantly localized on
donor moieties of all the dyes. However, at their LUMO levels, the electron cloud is shifted
clearly from the electron donor to the electron acceptor indicating better charge transfer and
effective electron injection to TiO2.
Table 2 E0-0, GSOP and ESOP for SA7-10.
Experimental (eV)
aGSOP
dTheoretical (eV)
Sensitizer
bE0-0
cESOP
E0-0
GSOP
ESOP
SA7
SA8
2.46
2.41
-6.19
-5.98
-3.73
-3.57
2.48
2.45
-6.22
-6.01
-3.74
-3.56
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SA9
2.26
2.18
-5.55
-5.33
-3.29
-3.15
2.28
2.22
-5.59
-5.37
D-O3I:.3101.1039/C9PP00381A
SA10
-3.15
[a] GSOPs were measured in dimethyl formamide using 0.1 m [TBA][PF6] with scan rate of 50 mV s-1. [b] E0-0
was measured through the energy onset determining by the absorption spectra DMF. [c] ESOP = GSOP- E0–0. [d]
DFT calculations using Guassian09 package.
3.5. TiO2 electrode preparation and device fabrication
The preparation of TiO2 along with the fabrication procedure are described in the
supplementary information.
3.6. Photovoltaic parameters and EIS measurements
To study the effect of co-sensitization of SA7-10 in a device fabricated with an
efficient Ru(II) complex based HD-2 sensitizer, the DSSCs were fabricated with 0.2 mM HD-
2 dye alone. Next, they were co-sensitized with co-sensitizers SA7-10 in the presence of
chenodeoxycholic acid (20 mM CDCA) as a co-adsorbent on the TiO2 layer.41 The Doctor
Blade method was used for fabrication process and its details are given in the electronic
supplementary information. Figures 5a and 5b show the J-V and IPCE characteristic curves
of the fabricated cells with HD-2 alone and co-sensitized with dyes SA7-10. Their subsequent
photovoltaic parameters are presented in Table 3.
Fig 5: a) J-V spectra and b) IPCE spectra of co-sensitizers SA7-10 along with HD-2.
One of the merits of our newly co-sensitizers SA7-10 is their small sizes making them
an ideal co-sensitizer in DSSCs. As small dyes can easily fill the gap between the bulky
complex (HD-2), resulting in an improvement in the TiO2 area coverage, decrease in the
aggregation of the dye and subsequent enhancement of the photovoltaic properties.
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Table 3. Photovoltaic measurements data of “fabricated DSSCs”. average values of all the
photovoltaic data are based on five replicate measurements.
JSC
Sensitizer Co-sensitizer CDCA
VOC (V)
FF(%)
Ƞ(%)
(mA cm-2)
19
(0.2 mM)
(0.2 mM)
(mM)
-------
SA7
SA8
SA9
SA10
SA7
SA8
SA9
SA10
20
20
20
20
20
10
10
10
10
0.64
0.68
0.65
0.68
0.65
0.476
0.5
61.35
66.28
65.96
66.71
65.09
62.21
67.70
70.85
70.39
7.46
7.23
16.04
17.7
HD-2
7.59
18.16
19.5
8.24
8.25
-------
-------
-------
-------
0.422
0.83
0.125
0.281
0.756
1.38
0.55
0.58
1.94
3.38
As shown from the results, a co-sensitization process enhanced the photovoltage
values of HD-2. Such increase in the VOC values can be attributed to the maximum coverage
of the dye to TiO2 surface. Interestingly, SA10 dye showed the best efficiency (8.25%) on co-
sensitization with HD-2. This can be attributed to the increasing in the values of both VOC
(0.65 V) and JSC (19.5 mA cm -2). IPCE measurements offered further indication of the best
effect of the dye SA10 as a co-sensitizer with HD-2. SA10 showed broad spectra from the
range of 380-790 nm with IPCE of 70%. The intense light harvesting ability of SA10 as a co-
sensitizer along with HD-2 can be attributed to the highest JSC value which comes from the
best injection of the electron to TiO2 edge.
Although co-sensitizer SA7 showed the best VOC value (0.68 V) due to its smaller size,
which can fill the gap in between the bulky HD-2 dye, it showed the lowest (PCE) value of
7.23%, which can be attributed to poor photocurrent value, JSC of 16.04 mA cm-2 due to the
absence of the donor moiety in its structure. Moreover, it was reported that the co-sensitizers
employed in the cells may decrease the dye loading up to 60%, which may considerably
diminish the photocurrent density.42 The aforementioned reduction in dye loading has been
observed in the co-sensitized cells SA7-9, resulting in a reduced light harvesting ability ( JSC).
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To further confirm of the over aforementioned results, we also fabricated tDhOeI:c1e0.l1l0s39w/Ci9tPhP00381A
SA7-10 alone and measured their photovoltaic performance (Table 3). Their J-V plots and
IPCE curves are shown in Figure 6. We found that SA10 possessing N,N-diethyl benzene as a
donor moiety showed the best photon-to-current efficiency (PCE) of 1.38%.
Fig 6: (a) J-V spectra sensitizers SA7-10 and (b) IPCE spectra of sensitizers SA7-10.
Better efficiency of SA10 compared to other dyes may be related to the higher JSC of
3.38 mA cm -2. This increasing of the JSC value can be attributed to the higher value of molar
absorptivity (Ɛ), which translated to more electron injection to the TiO2 surface.
Furthermore, EIS studies were performed to study the rate of recombination resistance
process between the dye / TiO2 / electrolyte interface. Moreover, EIS measurements give us an
indication about electron lifetime, which is related to the photovoltage.43
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DOI: 10.1039/C9PP00381A
Fig 7: EIS Nyquist plots for SA7-10 co-sensitized with HD-2.
From Nyquist plot displayed in Figure 7, SA7 co-sensitized with HD-2 showed the
−
biggest semicircle radius, indicated the highest recombination resistance from TiO2 to I3 /I−
electrolyte, which translated to maximum VOC value (0.68 V). On the other hand, HD-2 alone
showed the smallest semicircle, so which is in well agreement with the lowest VOC (0.64 V).
The observed results coincide with the achieved PCE values of SA7-10 based DSSCs.
14
12
HD-2
10
HD-2 + SA7
8
HD-2 + SA8
6
4
2
0
HD-2 + SA9
HD-2 + SA10
0
1
2
3
4
5
6
log (freq/Hz)
Fig: 8 EIS Bode plots for SA7-10 co-sensitized with HD-2.
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From Bode plot displayed in Figure 8, electron lifetime of the dyes canDbOeI: 1d0e.1t0e39c/tCe9dP.P00381A
The more shifted the peak to lower frequency means higher electron lifetime value (τeff =
1/2πf), which translated to higher VOC value.44 VOC value of SA7-10 was found to be in the
order of SA7, SA9 > SA8, SA10 > HD-2. The data indicated well matching between the
measured data with experimental date.
4. Conclusions
We have designed, synthesized and well described a promising series of 2-
cyanoacetanilide derivatives SA7-10. All dyes SA7-10 satisfy pre-requisites to act as
sensitizers and exhibited promising results over co-sensitization with the effective HD-2 dye.
Furthermore, SA10 as co-sensitizer showed the best η of 8.25%, attributed to its highest value
of JSC (19.5 mA cm -2), and so translated to best electron injection to TiO2 surface.
Additionally, because of the small size of our new series of 2-cyanoacetanilides, they can
effectively fill the gap on TiO2 surface and reduce charge recombination over co-sensitization
with HD-2. EIS measurements were found to be in well agreement with the photovoltaic
measurements. Finally, DFT calculation provided a better explanation of the effective charge
transfer of the electron through their optimized structures.
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