Triphenylamine-based tri-anchoring organic dye with enhanced electron lifetime and long-term stability for dye sensitized solar cells

Synthetic Metals 217 (2016) 248–255

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Triphenylamine-based tri-anchoring organic dye with enhanced electron lifetime and long-term stability for dye sensitized solar cells Yong Hui Leea , Hyeong Jin Yunb , Sok Kyun Choia , Yu Seok Yanga , Taiho Parkc , Kwang-Soon Ahna , Thogiti Suresha,* , Jae Hong Kima,* a b c

Department of Chemical Engineering, Yeungnam University, Dae-dong, Gyeongsan, Gyeongbuk 712-749, Republic of Korea Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, United States Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea

A R T I C L E I N F O

Article history: Received 1 March 2016 Received in revised form 2 April 2016 Accepted 12 April 2016 Available online xxx Keywords: Dye-sensitized solar cells Organic photosensitizers Triphenylamine Multi-anchoring dyes Long-term stability

A B S T R A C T

An organic sensitizer with a multi-anchoring system is a versatile methodology for enhancing the stability and conversion efficiency of dye-sensitized solar cells (DSSCs). Triphenylamine dyes (TPA3T1A
3A) containing different numbers of anchoring groups are synthesized to determine the correlation between the number of anchoring groups and photovoltaic properties as photosensitizers for DSSCs. The adsorption properties of the dyes on TiO2 electrode were examined by ATR-FT-IR, which show that a mono-anchoring TPA3T1A system adsorbs in monodentate ester-type mode, and the three carboxylic acids in TPA3T3A adsorbs in bidentate bridging mode. The multi-anchoring dye exhibits strong electronic coupling with TiO2, providing an efficient charge injection rate. Moreover, they increased the electron lifetime significantly by suppressing the charge recombination probability. This synergistic effect enables the fabrication of efficient photovoltaic devices. Furthermore, enhanced long-term stability is also observed in the DSSCs containing a tri-anchoring system compared to the mono- and bianchoring systems. ã 2016 Elsevier B.V. All rights reserved.

1. Introduction Dye sensitized solar cells (DSSCs) have attracted considerable attention as the next-generation of efficient solar cells [1–6], and have shown an overall peak power conversion of approximately 13%. In addition, they have been the focus of an economical solar electricity generation because the raw materials for producing DSSCs are relatively inexpensive, and the manufacturing process is rather simple. The standard structure of a DSSC comprises of an electrochemical cell composed of a dye-adsorbed wide band gap semiconductor electrode, such as TiO2 or ZnO, an electrolyte containing the I/I3 redox couple, and a Pt-coated counter electrode [7–9]. The mechanism of DSSCs is based on the injection of electrons from the photosensitizer into the conduction band of nanocrystalline TiO2 or ZnO. The oxidized photosensitizers are reduced by electron injection from the electrolyte. Therefore, the photosensitizer plays

* Corresponding authors. E-mail addresses: [email protected], [email protected] (T. Suresh), [email protected] (J.H. Kim). http://dx.doi.org/10.1016/j.synthmet.2016.04.009 0379-6779/ ã 2016 Elsevier B.V. All rights reserved.

an important role in capturing photons and generating electron/ hole pairs, as well as transferring them to the interface of the semiconductor and electrolyte, respectively [5]. Ru-complexes are the most widely used photosensitizers for DSSCs because they exhibit high performance and good long-term stability [9,10]. They typically produce electricity with an 11.5% power conversion efficiency under 1 Sun conditions. Ru-complex dyes, however, contain the ruthenium metal ion, which is an expensive precious metal species that increases the cost of DSSCs. Therefore, the development of ruthenium-free organic dyes is essential for improving the commercialization of DSSCs. A range of Ru-free organic photosensitizers have been evaluated as promising alternatives to Ru-complexes because of their many potential advantages [11–15]. Ru-free complexes are much cheaper and have higher molar extinction coefficients (e.g. the emax of squaraine is
3  105 M1 cm1) [16,17] than conventional dyes (e.g. the e of Ru complexes is
1 104 M1 cm1) [18]. They can also absorb photons with long wavelengths in the range, 500–700 nm. In addition, their synthetic process is convenient and their molecular design can be customized easily. Therefore, the development of novel Ru-free dyes for the production of high performance DSSCs will be a great challenge. Organic dyes,

Y.H. Lee et al. / Synthetic Metals 217 (2016) 248–255

however, have limited applications, due mainly to their inferior energy conversion efficiency and stability compared to Rucomplexes dyes. A novel strategy was recently introduced to the design of Rufree organic dyes for the fabrication of efficient DSSCs [13]. Organic dyes containing multiple-electron acceptors/anchors are more efficient than single-electron acceptor/anchor types because the former provides more electron extraction paths between the photosensitizing dye and TiO2, resulting in stronger electronic coupling with TiO2. These types of DSSCs, however, are unstable compared to Ru-complexes because they can be desorbed easily from the TiO2 surface in a liquid electrolyte. In this study, a highly efficient photosensitizing system capable of operating with good stability was fabricated and optimized. In particular, a series of electron donor-acceptor (D-A) chromophores containing a triphenylamine unit as the donor and different numbers of cyanoacrylic acids as the anchors/acceptors with thiophene bridges in the chromophore for the DSSCs were designed and synthesized. The dyes contained a different number of anchoring moieties in their chromophores. Organic dyes containing multiple-electron acceptors/anchors are expected to be bound strongly to the Ti atoms on the surface of TiO2, which improves the stability of the DSSCs significantly. The effect of the number of anchoring groups on the photovoltaic performance was investigated systemically by comparing their photophysical properties. Their photovoltaic performance was examined by measuring the photocurrent density-voltage (J-V) characteristics under irradiation with simulated solar light and the incident photon to current efficiency (IPCE). The interfacial electron transfer processes were analyzed by measuring the electrochemical impedance and open-circuit voltage decay (OCVD). The long-term stability of each DSSC was also examined by measuring the photovoltaic performance over a 1000 h period.

2. Experimental 2.1. Synthesis of organic dyes Unless stated otherwise, all commercially available starting materials and solvents were purchased from Aldrich, TCI or Acros Co., and used as received. High performance liquid chromatography grade toluene and tetrahydrofuran (THF) were obtained from Samchun Chemical and distilled from CaH2 immediately before use. Triphenylamine-based organic dyes, TPA3T1A, TPA3T2A and TPA3T3A, were prepared using the synthesis method reported elsewhere [19]. Fig. 1 shows the chemical structures.

249

2.2. Instrumental analyses The 1H-nuclear magnetic resonance (NMR, Advance NMR 300 MHz, Bruker) spectra were recorded using CDCl3 and DMSOd6 solvents purchased from Cambridge Isotope Laboratories, Inc. The Fourier transform infrared (FT-IR, Perkin Elmer) spectra were obtained using a Miracle single bounce diamond ATR cell from PIKET Technologies. 2.3. Assembly and characterization of the DSSCs The transparent conducting glass substrates (FTO; TEC8, Pilkington, 8 V/cm2) were cleaned in ethanol with ultrasonication. The TiO2 pastes (particle size, ca. 20–30 nm in diameter) were prepared using ethyl cellulose (Aldrich), lauric acid (Fluka), and a-terpineol (Aldrich). Pre-cleaned glass substrates were coated with the prepared TiO2 paste using the doctor blade method, followed by calcination at 500  C for 30 min. The film thickness was measured using a surface profiler (Alpha-step IQ surface profiler, KLA Tencor). A scattering layer consisting of rutile TiO2 particles (250 nm in a size) was deposited onto the mesoporous TiO2 films. The layers were dipped into an aqueous solution of TiCl4 (0.04 M) at 70  C for 30 min. For dye adsorption, the TiO2 electrodes were immersed into a dye solution (0.5 mM of dye in DMF) at 50  C for 3 h. Pt counter electrodes were prepared by thermal reduction of the films dip-coated in H2PtCl6 (7.0 mM) in 2-propanol at 400  C for 20 min. The dye-adsorbed TiO2 and Pt counter electrodes were sandwiched between the 60 m-thick Surlyn (Dupont 1702), which was used as a bonding agent and spacer. A liquid electrolyte was introduced to the Pt counter electrode through a pre-punched hole and finally sealed. The electrolyte was composed of 3-propyl-1methyl-imidazolium iodide (PMII, 0.7 M), lithium iodide (LiI, 0.2 M), iodine (I2, 0.05 M), and t-butylpyridine (TBP, 0.5 M) in acetonitrile/valeronitrile (85:15). The active area of the dyeadsorbed TiO2 films was estimated using a digital microscope camera using image-analysis-software (Moticam 1000). The photovoltaic I–V characteristics of the prepared DSSCs were measured under a 1 sunlight intensity (100 mW cm2, AM1.5), which was confirmed using an AIST-calibrated Si-solar cell (PECL11, Peccell Technologies, Inc.). The incident monochromatic photon-to-current efficiencies (IPCEs) were plotted as a function of the wavelength of light using an IPCE measurement instrument (PEC-S20, Peccell Technologies, Inc.). The long-term stability was assessed under 1 Sun illumination at 30  C for 1100 h. The impedance spectra (EIS) and open-circuit photovoltage decay (OCVD) curves were acquired using an electrochemical impedance analyzer (IVIUMSTAT, IVIUM). EIS is performed over the frequency range, 100 kHz and 100 MHz, with an AC voltage amplitude of

Fig. 1. Chemical structures of the synthesized organic dyes TPA3T1A, TPA3T2A, and TPA3T3A.

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10 mV at 0 V vs. Voc. The impedance spectra are interpreted by a nonlinear least-square fitting procedure using commercial software (ZVIEW 2TM). Three solar cells are fabricated under each condition. The solar cell characteristics are obtained from the solar cell that showed medium efficiency among the three samples tested. The time-resolved photocurrent-voltage decay curves were acquired using a picoammeter and oscilloscope. A HeNe laser (633 nm, 21.0 mW, THOR LAB) was used as the excitation source with the irradiation of white light modified by a ND filter (OD of 0.6, 1.0, 2.0, and 3.0). 3. Results and discussion 3.1. Molecular design and synthesis of the organic dyes containing tri-anchors/acceptors Organic dyes containing multi-electron acceptors have been reported to be promising photosensitizers for DSSCs compared to dyes with a single-electron acceptor because of the larger number of electron extraction paths from the electron donor and higher molar extinction coefficients [13]. The cyanoacrylic acid moiety of

the organic dyes exhibited strong electron-withdrawing ability as well as abundant electronic coupling at the interface of the TiO2 particles, leading to efficient electron injection from the LUMO level of the dye to the conduction band of TiO2 [20]. A series of electron donor-acceptor (D-A) chromophores containing a triphenylamine unit as the donor and different numbers of cyanoacrylic acids as the anchors/acceptors, and with thiophene bridges in the chromophore for the DSSCs were designed and synthesized. 3.2. Optical properties of the organic dyes Fig. 2 shows the UV–vis absorption spectra of the organic dyes in DMF solution and on the TiO2 thin films. Table 1 lists the optical properties of the organic dyes. The maximum absorption wavelength (lmax) of TPA3T1A, which has been assigned to the p-p* transitions in the conjugated molecules, was observed at 419 nm with a molar extinction coefficient (e) of 3.84  104 M1 cm1. The lmax and e value of TPA3T2A with bi-anchors/acceptors in the chromophore were 421 nm and 5.12  104 M1cm1, respectively, which is similar to TPA3T1A. The TPA3T3A containing the tri-anchors/acceptors in the chromophore exhibited a bathochromic shift in lmax compared to TPA3T1A and TPA3T2A because of the symmetric arrangement of tri-acceptors in the chromophore. The molecular extinction coefficient of TPA3T3A is the highest among the samples tested (7.86  104 M1 cm1) because of the symmetrical conjugation of p-electrons in the chromophore, which can improve the light harvesting efficiency of the organic dye in DSSCs. The dye absorption band on the TiO2 film can be shifted due to dye aggregation through molecular p-p stacking [21–25]. As shown in Fig. 2(A), the absorption peak of the organic dye, TPA3T1A, on the TiO2 thin film was shifted significantly toward a longer wavelength (Dlmax = 18 nm) compared to that in the solution state. The absorption peaks of TPA3T2A and TPA3T3A were red-shifted slightly by 14 and 6 nm, respectively. This red shift in the absorption spectra of the organic dyes on the TiO2 surface was attributed mainly to the formation of organic dye aggregates. This means that TPA3T1A tends to aggregate more on the electrode surface than TPA3T2A or TPA3T3A. 3.3. Adsorption properties of organic dyes on TiO2

Fig. 2. Absorption spectra of (A) TPA3T1A, (B) TPA3T2A and (C) TPA3T3A in DMF solution state and on TiO2 films.

The synthesized organic dyes containing different numbers of anchors/acceptors in their chromophore were used to fabricate DSSCs and their photovoltaic performance was assessed. For dye adsorption, the TiO2 electrodes were immersed in each of the dye solutions in DMF (0.5 mM) at 50  C for 3 h. The interaction and adsorption mechanism of the anchoring groups in the dyes on the TiO2 surface have direct effects on the electron transfer and performance of the resulting DSSCs [26]. ATR-FT-IR spectroscopy was performed to examine the adsorption mechanism of the organic dyes on the TiO2 surface, as shown in Fig. 3. The CN frequencies in the organic dyes (
2200 cm1) were not changed in the spectra before and after the dye had been adsorbed on the TiO2 surface, suggesting that no interaction occurred between the cyano moieties and TiO2 during the adsorption process. The carbonyl peak at
1700 cm1 in the neat sample disappeared or shifted after dye adsorption. Therefore, the anchoring of organic dyes on the TiO2 surface occurs through the carboxylate group. The carboxylate peak of TPA3T1A shifts from 1700 cm1 to a higher wavenumber region (1730 cm1), indicating the presence of a monodentate binding mode of dye molecules on the TiO2 surface [27,28]. The peak (1730 cm1 in TPA3T1A) was either small or not observed in TPA3T2A and TPA3T3A, respectively. The symmetrical and asymmetrical stretching band of carboxylate was observed at 1438 cm1 and 1586 cm1, respectively, which suggests that two and three carboxylic acids in the TPA3T2A and TPA3T3A dyes

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Table 1 Optical properties of TPA3T1A, TPA3T2A and TPA3T3A obtained from the UV-vis absorbance spectra. Dyes

emaxa(M1 cm1)

lmax (nm) in solution

lmax (nm) on TiO2b

lemc (nm)

E0–0d (eV)

TPA3T1A TPA3T2A TPA3T3A

3.84  104 5.12  104 7.86  104

419 421 436

437 435 442

557 557 555

2.55 2.21 2.18

a b c d

Extinction coefficient at the maximum absorption of the dyes in a DMF solution. Absorption maxima of the dyes adsorbed onto the TiO2 films. Fluorescence maxima of the dyes. E0–0 (band gap) was determined from the absorption edge of the absorption spectra in a DMF solution.

adsorb mainly in bidentate carboxylate mode on the TiO2 surface, as shown in Scheme 1. Therefore, TPA3T3A can cover the TiO2 surface more effectively than the other organic dyes, which can affect the photovoltaic properties of the DSSCs. In addition, TPA3T1A aggregates more easily on the TiO2 surface than on the other dyes.

3.4. Photovoltaic performance and long-term stability of the DSSCs with the multi-anchoring organic dyes Fig. 4(A) shows the photocurrent-voltage (I–V) curves of the DSSCs with the mono-, bi-, and tri-anchoring organic dyes, and Table 2 summarizes the corresponding parameters. The overall conversion efficiency (h) of a DSSC is determined by the short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF) of the cell, and intensity of incident light (Is) [5].

h¼

(c)

Intensity (A. U.)

(A)

(b) (a)

500 1000 1500 2000 2500 3000 3500 4000 -1

Wavenumber (cm )

(B) Intensity (A. U.)

(c) (b) (a)

1000

1500

2000

2500

3000

-1

Intensity (A. U.)

Wavenumber (cm )

(C)

(c) (b) (a)

1000

1200

1400

1600

1800

-1

Wavenumber (cm ) Fig. 3. ATR-FT-IR spectra of (a) TPA3T1A, (b) TPA3T2A, and (c) TPA3T3A obtained (A) in neat conditions, (B) on the TiO2 surface adsorbed, and (C) in high wavelength region on TiO2 adsorbed.

Jsc  V oc  FF Is

ð1Þ

The DSSCs with TPA3T3A containing the tri-anchors/acceptors showed the highest photovoltaic performance among the DSSCs tested with Jsc, Voc and FF values of 13.6 mA/cm2, 0.65 V and 0.72, respectively, giving a h value of 6.3%. In particular, the TPA3T3Asensitized cells exhibited a comparatively high Jsc value, which was similar to that of the N3 sensitized solar cells. h tends to decrease with decreasing number of anchoring groups of the photosensitizer. The enhanced cell efficiency of the DSSCs with the multianchoring organic dyes might be due to the more efficient electron extraction paths from the electron donor and the higher molar extinction coefficients of the dye chromophore. Multiple-anchoring leads stronger electronic coupling between the photosensitizers and TiO2. This enhancement in electron coupling leads to the injection of photo-generated holes in a photosensitizer to the conduction band of TiO2 with rapid charge transfer kinetics, leading to the generation of a high photo-current in DSSCs. Moreover, the molecular extinction coefficient tended to increase with increasing number of anchoring groups. The effective solar energy harvesting performance of TPA3T3A was attributed to the synergistic effect of enhanced electronic coupling and a high extinction coefficient. IPCE measurements were conducted to further understand the photovoltaic cell performance of DSSCs. The IPCE is calculated by measuring the photocurrent of each cell at different levels of monochromatic excitation. Fig. 4(B) shows the incident photon-tocurrent efficiency (IPCE) spectra of the DSSCs with different organic dyes. All the solar cells prepared generate electricity by the irradiation of visible light with long wavelengths between 300 and 650 nm. The IPCE performance of the DSSCs with TPA3T2A and TPA3T3A was improved considerably compared to the monoanchoring TPA3T1A, which is in good agreement with the increased short-circuit currents shown in Fig. 4(A). The dye containing the tri-anchors/acceptors (TPA3T3A) was not desorbed from the TiO2 surface, even in a concentrated KOH aqueous solution, suggesting that the multi-anchoring groups of the dyes strengthen the adsorption properties of the dye to the TiO2 surface and cover the electrode surface. Therefore, the DSSCs containing TPA3T3A can show an enhanced electron lifetime and stability compared to those of the other dyes. The impedance spectra and long-term stability of the DSSCs containing organic dyes were measured to investigate the electrode kinetics and interfacial transfer process in DSSCs.

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Scheme 1. Schematic diagram of the adsorption modes for anchoring groups of each organic dye on TiO2 electrode surface. (A) Monodentate ester type of TPA3T1A; (B) bidentate bridging mode of TPA3T2A; and (C) bidentate bridging mode of TPA3T3A.

The overall charge transfer resistance can be estimated by electrochemical impedance spectroscopy (EIS), which is useful for quantifying the interfacial electrochemical behavior of DSSCs [29– 32], QDSSCs [33–35] and photoelectrochemical cells [36]. EIS was performed under light illumination conditions, as shown in Fig. 5, and the results are summarized in Table 3. Fig. 5 presents the Nyquist plots for each DSSC. The ohmic serial resistance (R1) corresponds to the electrolyte and FTO resistance, and the resistances, R2 and R3, correspond to the charge transfer process occurring at the Pt counter electrode and TiO2/dye/electrolyte interface, respectively. Three semicircles were observed for each Nyquist plot, which were assigned to interfacial electrochemical charge transfer on the cathode (at higher frequency), anode (at medium frequency) and mass transfer in the liquid electrolyte

Jsc (mA/cm2)

15

TPA3T1A TPA3T2A TPA3T3A

(A)

12 9 6 3 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Voc (V) 100

TPA3T1A TPA3T2A TPA3T3A

(B) IPCE (%)

80 60

(at low frequency), respectively. The radii of the former two semicircles are strongly related to the Faradaic Red/Ox charge transfer resistance of each electrode. No significant change in the radii of the semicircles at the higher and lower frequencies was observed within the Nyquist plot for each photovoltaic cell because the Pt coated counter electrode and iodine electrolyte were used for all experiments. On the other hand, R3 decreased remarkably from 39.32 to 27.23 V cm2 with increasing electronic coupling, suggesting a decrease in charge transfer resistance. This also suggests that the multi-anchored dyes retarded backward electron transfer from TiO2 to the electrolyte and facilitated forward electron transport from TiO2 to the FTO substrate. The open-circuit photo voltage decay (OCVD) curves were also measured to confirm the electron lifetimes of the samples [37–39]. The OCVD technique is a method that involves turning off the illumination in the steady state and monitoring the subsequent photo voltage decay, Voc, as shown in Fig. 6. The electron life times were calculated from the OCVD curves using Eq. (2).

te ¼ 




kB T dVoc e

1 ð2Þ

dt

where KBT is the thermal energy, e is the positive elementary charge, and dVoc/dt is the derivative of the open circuit voltage transient. The photovoltage decay rate is related directly to the electron lifetime because, as illumination of the DSSC at the open circuit is interrupted, the excess electrons are removed through recombination [39]. The TPA3T3A dye exhibited a longer electron lifetime compared to the other dyes. Therefore, TPA3T3A provides much faster electron transport and an enhanced electron lifetime, which gives rise to an improved photocurrent. This tendency is also observed in the electrochemical impedance spectroscopy (EIS) results performed under dark conditions (Fig. S1). To estimate the diffusion coefficient (De), electron lifetime (te) and diffusion length (Le), stepped light-induced transient measurements of photocurrent and voltage (SLIM-PCV) measurements [40–42] are conducted (Fig. S2), which has a simpler set-up and shorter measurement time compared to pulsed-laser-induced current transients and intensity-modulated photovoltage

40 Table 2 Photoelectrochemical parameters of each DSSC under the simulated solar light illumination (AM 1.5, 100 mW/cm2).

20 0 300

400

500

600

700

Wavelength (nm) Fig. 4. (A) Current density-voltage characteristics for each DSSC under simulated solar light illumination (AM 1.5, 100 mW/cm2). (B) IPCE curves for each DSSC.

Dyes

Jsc (mA cm2)

Voc (V)

FF

h (%)

TPA3T1A TPA3T2A TPA3T3A N3

10.6 12.9 13.6 14.8

0.65 0.66 0.65 0.73

0.71 0.71 0.71 0.70

4.9 6.1 6.3 7.5

Y.H. Lee et al. / Synthetic Metals 217 (2016) 248–255

253

Fig. 5. Nyquist plot for each DSSC under simulated solar light illumination (AM 1.5, 100 mW/cm2). Fig. 6. Electron lifetimes of each DSSCs derived from Eq. (1) as a function of Voc.

spectroscopy. A HeNe laser is used as the excitation source and white light with different irradiance are applied at various light biases. Their light irradiance was controlled by changing the ND filter prior to irradiation of each sample. The photocurrent and voltage decay with the excitation laser chopped. Their decay can be fitted to the exponential decay function. The lifetime of a photogenerated electron can be estimated by fitting to the theoretical exponential equation, as follows: V oc ðtÞ ¼ a  et=t

ð4Þ

where Voc (t), a and t are the photo-voltage decay, weight parameter and the lifetime of photo-generated electron. t of the photo-generated electron in the TPA3T1A-sensitized solar cell was estimated to be 29.6 ms, which increased with increasing number of anchoring moieties. In particular, TPA3T3A exhibits the longest electron lifetime of approximately 88.3 ms, which is almost 3 times longer than the photosensitizer with a single moiety. These results are due to a decrease in the probability of charge recombination with increasing number of anchors. As verified by impedance experiments, TPA3T3A exhibits large charge transfer resistance for the recombination reaction. The adsorption configuration of TPA3T3A can be reduced the recombination kinetics between photoanode and electrolyte significantly, consequently increasing the lifetime of the photo-generated electron. The photocurrent decay can also fitted be using the following function: Jsc ðtÞ ¼ b  et=tc

ð5Þ

where Jsc (t), b, and tc are the photo-current decay, weight parameter and the time constant. Using the fitted constant of tc, De can be obtained from the following equation: De ¼

L2 2:77t c

ð6Þ

where L is the thickness of the electrode. Fig. 7(A) and (B) show the electron diffusion coefficient and lifetime in each DSSCs,

Table 3 R1, R2, and R3 from the fitted results of the EIS spectra for each DSSC under the simulated solar light illumination (AM 1.5, 100 mW/cm2). The experimental EIS spectra were fitted using ZView 2 software. R1 is the FTO interface resistance. R2 is the faradaic charge transfer resistance over the counter electrode. R3 is the faradaic charge transfer resistance occurring over each dye-sensitized photoanode. Dyes

R1 (V)

R2 (V)

R3 (V)

TPA3T1A TPA3T2A TPA3T3A

26.68 27.78 28.78

8.456 8.137 8.352

39.32 28.38 27.23

Fig. 7. (A) De, (B) te, and (C) Le according to the Jsc for each DSSC, which are estimated from SLIM-PCV measurements.

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respectively, according to the stationary Jsc controlled by changing light irradiance. TPA3A3T exhibits the highest value of De and te. These values tend to decrease with decreasing anchoring moieties. As mentioned above, TPA3T3A shows much better electronic coupling with TiO2. This strong coupling leads to rapid photogenerated electron transfer from the photosensitizer to TiO2. Therefore, the mass transfer rate of a single electron become high, yielding a high De value. In addition, TPA3T3A also shows a lower probability of charge recombination at the interfacial region of photoanode and electrolyte. This enhances the lifetime of the photo-generated electron. Le can be estimated as follows: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð7Þ L e ¼ De  t e The results are presented in Fig. 7(C). The Le of TPA3T1A, TPA3T2A and TPA3T3A are 9.29, 10.72 and 18.82 mm, respectively. The longest Le of TPA3T3A can be attributed to the synergistic effect of strong electronic coupling and long electron lifetime. Therefore, more photo-generated electrons can move to the current collector rapidly with a lower probability of charge recombination, resulting in significant enhancement of the photovoltaic performance. Fig. 8 shows the long-term stability curves of the DSSCs with (A) TPA3T2A and (B) TPA3T3A, respectively, which were measured under one sun light illumination for 1100 h. Even bi-anchoring TPA3T2A provided low stability, showing a drastic decrease in Jsc and h after 5 days (Fig. 8(A)). The tri-anchoring TPA3T3A, however, exhibited superior long-term stability to the bi-anchoring TPA3T2A. The long-term stability of the DSSCs was influenced by more by Jsc than the other factors, such as the open-circuit potential and fill factor. This suggests that the degraded stability of the TPA3T2A-DSSC might be due to the formation of unstable radicals under UV-containing light soaking conditions and dye desorption from the TiO2 nanoparticle surfaces. The significant improvement in the stability of the DSSCs with the TPA3T3A dye can be attributed to the stronger adsorption properties on the TiO2 electrode surface provided by tri-anchoring groups within the dye.

4. Conclusions Organic photosensitizers based on triphenylamine with trianchors/acceptors (TPA3T3A) were synthesized and applied to DSSCs. The TPA3T1A and TPA3T2A organic dyes with the monoand bi-anchoring groups, respectively, were prepared for comparison. The multi-anchoring organic dyes (TPA3T2A and TPA3T3A) provided efficient electron extraction pathways from the electron donor with the strong adsorption properties of multi–anchoring chromophores. In addition, TPA3T3A leads to a significant increase in electron lifetime by suppressing the charge recombination probability. This synergistic effect enables the fabrication of efficient photovoltaic devices. In particular, the DSSCs with TPA3T3A showed enhanced long-term stability under one sun light illumination at 30  C. This was attributed to the strong adsorption properties provided by the tri-anchoring groups attached to the TiO2 surface. These results provide good insight into improving the long-term stability of organic dyes for different applications, including DSSCs, organic photovoltaics and dyesensitized photoelectrochemical water-splitting cells. Acknowledgments This study was supported by a grant from the Fundamental R&D program for Core Technology of Materials (10050966) funded by the Ministry of Knowledge Economy, Republic of Korea. This study was also supported by “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea. (No. 20154030200760). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. synthmet.2016.04.009. References [1] [2] [3] [4] [5] [6] [7]

[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] Fig. 8. Long-term stability test results of (A) TPA3T2A- and (B) TPA3T3A-sensitized solar cells under 1 sun light soaking at 30  C for 1100 h.

[22]

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