Molecular Engineering of 2-Quinolinone Based Anchoring Groups for

Article pubs.acs.org/JPCC

Molecular Engineering of 2‑Quinolinone Based Anchoring Groups for Dye-Sensitized Solar Cells Paramaguru Ganesan,†,‡ Aravindkumar Chandiran,† Peng Gao,*,† Renganathan Rajalingam,*,‡ Michael Graẗ zel,† and Mohammad Khaja. Nazeeruddin*,† †

Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland ‡ School of Chemistry, Bharathidasan University, Tiruchirappalli 620024, Tamilnadu, India S Supporting Information *

ABSTRACT: Six new donor−π spacer−acceptor (D-π-A) organic sensitizers based on the 2-quinolinone moiety as acceptor and anchoring groups were synthesized and tested for their performance in dye-sensitized solar cells (DSSCs). The systematic tuning of acceptor strength and anchoring modes lead to a variation of the photoelectrochemical properties and device performances. Characterization of the molecular structure and electronic and optical properties of the dyes, as well as their photovoltaic performance in DSSCs, was accomplished by means of NMR, UV spectroscopy, electrochemistry, quantum chemical, Fourier transform infrared spectroscopy, and transient photovoltage decay techniques. Thereby, significant dependence of DSSC performance on the substituents and anchoring groups was observed. In general, the optoelectronic properties of the sensitizers were mainly governed by the 2-quinolinone acceptor segment. Different types of anchoring modes were employed in this new type of acceptor segment, and their significance in the solar conversion efficiency was analyzed. Amide N,O-chelating anchoring mode and bianchor-type anchoring mode show higher short circuit current density among the various anchoring groups. This new type of anchoring mode will emerge as a promising alternative anchoring group to the widely utilized cyanoacrylic acid group.

1. INTRODUCTION Dye sensitized solar cells (DSSCs) are deemed to be promising photovoltaic technology to meet ever-increasing energy demands because of their low cost, efficient solar energy conversion, and easy fabrication methods.1,2 DSSCs utilize dye molecules to harvest solar energy, a process that mimics the natural photosynthetic principles.3 Upon light absorption, photoinduced electron injection occurs from the excited sensitizer into the conduction band of the semiconductor TiO2 anode. Further, the presence of redox electrolyte regenerates the oxidized sensitizer. Generally, the dye molecules bind covalently to the semiconductor surface, which enables strong electronic coupling between them and facilitates an efficient electron injection process.4−6 Highly efficient metal-based ruthenium polypyridyl complexes and zinc porphyrin sensitizers that exhibited 11% and 13% photovoltaic power conversion efficiency, respectively, employ carboxylate anchoring group.7−9 Donor-π-acceptor (D-π-A)-type metal-free organic dyes have been paid more attention because of their low cost, tunable structural arrangement, high molar extinction coefficient, and environmental friendliness.10−14 The efficiencies of organic dyes have been improving continuously and to date have achieved up to 12.8%.15,16 Most of the D-π-A organic molecules utilize the cyanoacrylic acid anchoring moiety because of the presence of an electron withdrawing (EW) © 2014 American Chemical Society

cyano group near the anchoring carboxylic acid group that enhances the spectral response through intramolecular charge transfer (ICT) and good electron injection properties.17−19 Alternatively, the rhodanine moiety is also used as an EW anchoring group in various sensitizers.20 Numerous efforts have been devoted to tuning the donor and π spacer groups for efficient solar energy conversion by keeping the cyanoacrylic acid anchoring moiety as electron acceptor.21 In recent years, great attention has been paid to developing anchoring groups such as pyridine, pyridine N-oxide, hydroxylpyridium, 8hydroxylquinoline, phosphonic acid, etc. as an alternative to the carboxylate anchoring groups.22−27 Tian et al. have employed 2-(1,1-dicyanomethylene)rhodanine (DCRD) type acceptors which result in an efficiency higher than that of cyanoacrylic acid as electron acceptor.28 Several reports emphasize the significance of multiple anchoring groups that enhance the binding strength and electron injection properties.29−31 Hence, the quest of developing new anchoring groups focuses on strong binding affinity with TiO2, good spectral Special Issue: Michael Grätzel Festschrift Received: January 14, 2014 Revised: April 1, 2014 Published: April 3, 2014 16896

dx.doi.org/10.1021/jp5004352 | J. Phys. Chem. C 2014, 118, 16896−16903

The Journal of Physical Chemistry C

Article

Scheme 1. Structures of 2-Quinolinone Acceptor-Based D-π-A Sensitizers

2. RESULTS AND DISCUSSION Synthesis and Characterization of 2-QuinolinoneBased D-π-A Sensitizers. Schemes S1−S5 of Supporting Information display the various synthetic routes employed for the synthesis of quinolinone acceptor-based D-π-A sensitizers. The synthesis of acceptor fragments Br-Q1 and Br-Q2 started with the acetylation of 3-bromoaniline, yielding 3-bromoacetanilide (Br-AA). Br-AA, upon treatment with the Vilsmeier reagent (DMF−POCl3), gave 2-chloro-3-formyl quinoline (BrQ1) which undergoes oxidation in acetic acid/water medium, affording the 2-quinolinone type acceptor Br-Q2 (Scheme S1). Reaction pathways for synthesizing the donor-π spacer group are based on literature reports33,34 and are shown in Scheme S2. Alkylation of commercially available cyclopentadithiophene 1 using 1-bromohexane yields the alkylated π spacer 2. To couple this π spacer with the donor part, 2 is converted to its mono stannous compound 3, which undergoes Stille coupling with the donor fragment triphenyl amine 4, affording the donor π spacer part 5. Compound 5 is then converted to its stannous analogue 6 for convenient coupling with the acceptor fragments. As depicted in Scheme S3, Stille coupling of 6 with Br-Q1 and Br-Q2 yielded the aldehyde precursors PQ1 ALD and PQ2 ALD, respectively. The final dyes were synthesized from these two aldehyde precursors (Schemes S4 and S5). Knoevenagel condensation of PQ1 ALD and PQ2 ALD with cyanoacetic acid afforded the final dyes PQ1 and PQ2, respectively. PQ2 ALD upon knoevenagel condensation with malononitrile yields PQ3. Lindgren oxidation of PQ1 ALD

response, and most importantly higher power conversion efficiencies. In this context, we have employed a new type of 2quinolinone based acceptor segment and investigated their significance in DSSCs. 2-quinolinone type molecules have interesting structural and optoelectronic properties.32 The choice of this acceptor is motivated by its advantages, which include easy modification of anchoring groups, tunable electron withdrawing strength, and multiple anchoring sites. Scheme 1 displays the various types of 2-quinolinone acceptor-based D-πA sensitizers under investigation. We have employed a common donor and π spacer group so that the effect of variations in the acceptor part can be clearly understood. PQ1 and PQ2 have common cyanoacrylic acid type anchoring moiety in which the 2-chloro substituent in the former is replaced with an oxo group in the latter. Because of the presence of an oxo group, PQ2 has another possible amide N,O-chelating anchoring mode. The strong EW cyanoacrylic acid anchoring groups in PQ1 and PQ2 are replaced with mild accepting carboxylic acid anchoring groups for PQ5 and PQ4, respectively. PQ4 can utilize the carboxylic acid and carbonyl group in bianchor mode. PQ3 utilizes an amide-type of anchoring mode with the presence of EW cyano groups which mimics the DCRD type of anchoring segment reported in the literature.17 All the new dyes were successfully synthesized and characterized. The significance of these variations in their optoelectronic properties and energy conversion efficiencies were investigated through optical, electrochemical, and photovoltaic methods. 16897



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Figure 1. Absorption (A) and emission (B) spectra of the sensitizers measured in dichloromethane.

and PQ2 ALD using sodiumchlorite and sulfamic acid afford the carboxylate type anchoring dyes PQ5 and PQ4, respectively. All the intermediates and the final compounds were characterized by 1H NMR, 13C NMR, and mass spectroscopy. The observed isotopic pattern M, M+2, and M +4 peaks in the mass spectrum of Br-Q1 is due to the presence of one chlorine and one bromine atom. The aldehyde proton of Br-Q1 appears at 10.5 ppm. The presence of M and M+2 peaks at m/z for Br-Q2 (Figure S20 of Supporting Information) in the ratio of 100:94 corresponds to the isotopic pattern of one bromine atom. The signal at 11 ppm in the 1H NMR of Br-Q2 can be assigned to the −NH proton in the quinolinone framework (Figure S4 of Supporting Information). The −CH2 proton present near the oxygen atom in the donor π intermediate 5 appears at 3.9 ppm. The aldehyde proton signal for PQ1 ALD and PQ2 ALD appears around 10.5 ppm, and the respective compounds carbon signal appears around 190 ppm. The observed NMR and mass data are very well-correlated with the expected final compounds. Electronic Properties. UV−visible absorption spectral measurements were carried out to understand the lightharvesting properties of these D-π-A sensitizers by varying the electron-withdrawing anchoring segments. The absorption spectra of all the sensitizers measured in dichloromethane are shown in Figure 1A, and their spectral data are summarized in Table 1. All the sensitizers exhibit strong and broad absorption band in the range of 400−600 nm and a high-energy band around 300−400 nm. The longer-wavelength absorption region is assigned to the intramolecular charge-transfer transition from the triphenyl amine donor to the acceptor segments, whereas the shorter wavelength region is attributed to the π → π* transition of the donor.33−35 Because all the compounds have common donor and π spacer segments, the changes in their absorption behaviors will indicate the strength of the various acceptor parts under investigation. The absorption maxima of PQ1 and PQ5 appear at 477 and 473 nm, respectively. Their structures differ in their anchoring segments: the former sensitizer has a strong EW cyanoacrylic acid anchoring group, whereas the latter has a mild EW carboxylic acid group. As observed in the literature, replacing the strong EW cyanoacrylate part with a mild acceptor resulted in blue-shifting the absorption maxima by 50 nm.36−38 However, this phenomena is not observed for PQ1 and PQ5 sensitizers. Interestingly, when we look over the absorption behavior of PQ2 and PQ4, which have similar differences in

Table 1. Absorption, Emission, and Electrochemical Properties of All the Sensitizers sensitizer PQ1 PQ2 PQ3 PQ4 PQ5 PQ2 ALD

λabsa (nm)

εa (×104 M−1 cm−1)

λemia (nm)

Eoxb (V)

E(0−0)c (eV)

Eox*d (V)

477 469 470 504 473 518

5.41 3.38 4.36 2.92 2.1 5.9

588 583 586 587 576 597

0.77 0.74 0.74 0.74 0.81 0.75

2.31 2.33 2.33 2.25 2.39 2.21

−1.54 −1.59 −1.59 −1.46 −1.51 −1.58

a

Absorption and emission spectra were measured in dichloromethane; λabs, maximum absorption wavelength; λemi, maximum emission wavelength; ε, molar extinction coefficient. bEox: ground-state oxidation potential. The oxidation potential of the dyes was measured under the following conditions: glassy carbon as working electrode; platinum as counter electrode; electrolyte, 0.1 M tetrabutylammonium hexafluoro phosphate, in dichloromethane; scan rate, 0.1 V/s. Potentials measured versus Fc+/Fc were converted to normal hydrogen electrode (NHE) by adding +0.63 V.34 cE(0−0): the zero− zero excitation energies are estimated from the intercept of the normalized absorption and emission spectra. dEox*: the excited state oxidation potentials were derived from the equation Eox* = Eox − E(0−0).

their acceptor parts as discussed above, PQ4 with carboxylic acid as anchoring group shows absorption maximum in a wavelength region higher than that of PQ2 with cyanoacrylic acid. Furthermore, replacing the carboxylic acid group in PQ2 with one more EW cyano group in PQ3 results in little change in their absorption behavior. The presence of aldehyde as EW group in PQ2 ALD shows the maximum absorption wavelength (518 nm) compared to those of all other sensitizers. All these observations suggest that the strength of the EW anchoring groups has minimal effect in tuning the absorption behavior of these sensitizers. The other possible factor determining the absorption behavior lies in the quinolinone framework, where PQ1 and PQ5 possess a 2-chloro quinoline type segment, whereas PQ2, PQ3, and PQ4 possess a 2-quinolinone type framework. During photoexcitation, the different quinoline framework also acts as an EW part, which affects the strength of the anchoring groups in their EW behavior and results in different absorption maxima. The observed results reveal the significance of the quinolinone framework as acceptor group in tuning the absorption behavior despite the strength of anchoring groups. Fluorescence properties of all the sensitizers 16898



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Figure 2. Absorption spectra of the sensitizers in dichloromethane solution (●) and adsorbed on TiO2 films (▼).

Supporting Information. Good spatial separation of the frontier molecular orbitals was observed for all the sensitizers. In the ground state, the electron density of the HOMO in the dyes is mainly populated over the triphenylamine and CPDT spacer unit, whereas LUMO is delocalized through the quinolinone and various anchoring segments. This further supports that the quinolinone segment will act as an electron acceptor moiety. To some extent LUMO is more localized on the EW cyano groups than the quinolinone segment in PQ1 and PQ3, signifying the accepting strength of the cyano group. This spatially directed separation of the frontier orbitals strongly promotes intramolecular charge separation and hence favors efficient electron injection from the excited state of the dye into the semiconducting oxide, limiting charge recombination processes. In addition, the hole localization on the triphenylamine fragment facilitates the approach of the electron donor to the redox electrolyte, promoting the fast dye regeneration. Analyzing Binding Modes. The anchoring of sensitizers onto the TiO2 surface is observed from the color change of the TiO2 as displayed in Figure S30 of Supporting Information. The sensitization of PQ3 and PQ2 ALD confirms the binding ability of a new type of amide anchoring group with the TiO2 surface and reveals their potential application in DSSC. This new type of amide anchoring mode will be an alternative to the carboxylic acid type anchoring group. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy of the dyes in solid state and in adsorbed state on to TiO2 have been analyzed to further understand the anchoring mode of these molecules to the TiO2 surface. The FTIR spectra of free dyes and dyes on TiO2 surface are displayed in Figures S31−S34 of Supporting Information. The fingerprint peaks of the free sensitizers around 1660 and 1615 cm−1 in PQ3 correspond to the amide −CO stretching and N−H bending vibration, respectively. When the sensitizers bind to the TiO2, both these bands disappeared, supporting the binding of PQ3 using amide-type anchoring mode with TiO2. Recent reports

were measured in dichloromethane to investigate the acceptor role in their excited state of the dyes. The corresponding spectrum is displayed in Figure 1B, and the data are listed in Table 1. The observed emission maxima is insensitive to the nature of the electron-withdrawing groups, similar to their ground-state absorption properties, and ranges from 576−597 nm for the various sensitizers under investigation. Electrochemical Properties. Electrochemical behavior was analyzed to determine the oxidation potentials of the dyes in the ground (Eox) and excited states (Eox*). Figure S28 of Supporting Information shows the cyclic voltammogram of the sensitizers, and the corresponding data are listed in Table 1. Two oxidation peaks were observed for all the sensitizers in the cyclic voltammogram. The first oxidation potential of the sensitizers, which corresponds to their highest occupied molecular orbital (HOMO) energy levels, is found to not vary so much among the various sensitizers despite the difference in their EW acceptor. This is due to the fact that the HOMO orbital is mainly localized on the triphenyl amine segment which is common among all the sensitizers.39 The observed ground-state oxidation levels are more positive than that of the iodide/triiodide redox couple (0.4 V vs NHE),39,40 ensuring a sufficient driving force for the dye regeneration. The excited-state oxidation potential Eox* of the dyes was calculated from the zero−zero transition energy and their oxidation potential energy levels. E0−0 is determined from the intercept of the absorption and emission spectra. The estimated excitedstate oxidation potential levels of the dyes are found to be more negative than the TiO2 conduction band (−0.5 V vs NHE),41 indicating that the excited state of these sensitizers can inject the electron efficiently into the conduction band of the TiO2. DFT calculations at the B3LYP/6-31G (d,p) level of theory were carried out to analyze further the distribution of HOMO and lowest unoccupied molecular orbital (LUMO) energy levels of the sensitizers. The isodensity surface plots of HOMO and LUMO of all the sensitizers are presented in Figure S29 of 16899



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are evaluated under simulated 1 Sun illumination (AM 1.5 G irradiation at 100 mW cm−2). The photocurrent density voltage (J−V) curves are shown in Figure 3, and their corresponding

reveal that new types of anchoring modes, such as N,O atoms of 8-hydroxylquinoline and the N,O atoms of amide-type anchoring in rhodanine, can chelate to TiO2.25,28 On the basis of these literature reports and FTIR observations, it is predicted that this amide N,O-chelating-type anchoring group in quinolinone based compounds will emerge as a novel anchoring mode for DSSC application. Whereas in PQ2 ALD the amide −CO and N−H bending frequency appears around 1680 and 1590 cm−1, respectively, these peaks disappeared on binding with TiO2, which further supports the amide-type anchoring mode. The PQ2 sensitizer shows complication because it has both amide and −COOH anchoring modes. There are two peaks in PQ2 around 1660 and 1650 cm−1, which may correspond to the −CO stretching of amide carbonyl and carboxylic acid, respectively. N−H bending vibration appears around 1590 cm−1. The disappearance of 1590 and 1660 cm−1 peaks and the appearance of one single broad peak around 1650 indicate that PQ2 also binds through amide anchoring mode. A strong signal around 1722 and 1645 cm−1 disappears in PQ4 upon binding with TiO2, and a new signal appears around 1670 cm−1. The 1722 and 1630 cm−1 peaks correspond to the carbonyl peak of −COOH and the amide carbonyl, respectively. The disappearance of these peaks implies that the PQ4 can bind with both carboxylic acid and amide carbonyl in bianchor-type mode. PQ4 and PQ2 ALD have almost the same color in the solution state; however, when the sentsitizer binds with the TiO2 surface, the color of the PQ2 ALD appears to be blueshifted, whereas PQ4 retains its color as it is observed from the sensitized cells (Figure S30 of Supporting Information). Similar type blue-shifted sensitization is observed for PQ2 and PQ3. These observations reveal that the binding through amide-type anchoring mode shifts the absorption in the blue region and further support the conclusion that the PQ2 prefers binding through amide-type anchoring mode rather than carboxylic mode. To further scrutinize the changes in the absorption behavior of the dyes on binding with TiO2, we have measured the absorption behavior of all the sensitizers in the TiO2 film; their corresponding spectra are displayed in Figure 2. The absorption spectra reveal that PQ1 and PQ4 have similar absorption maxima in both the solution state and TiO2 film, whereas a large blue shift is observed for PQ2 and PQ5 upon binding with TiO2. As described in the literature, blue shift in the absorption of PQ2 and PQ5 indicates the H aggregation of these dyes on the TiO2 surface.42,43 Retaining the absorption behavior as in solution state suggests that the aggregation of PQ1 and PQ4 is minimal on the surface of TiO2. The light-harvesting ability of PQ2 and PQ5 in the visible region upon binding with TiO2 is relatively low. The presence of bulky chlorine near the anchoring carboxylic acid group hinders the uniform binding of PQ5 on the TiO2 surface and results in aggregation-type absorption behavior. However, the cyanoacrylic acid anchoring mode in PQ1 and bianchor mode in PQ4 reduce the aggregation in these quinoline based acceptor dyes. The new type of amide anchoring mode in PQ3 and PQ2 ALD shows a blue shift, revealing the presence of a small degree of aggregation, but this is common in D-π-A dyes having a cyanoacrylate anchoring group. The impact of these modifications in anchoring modes on solar energy conversion behavior is investigated in the following section. Photovoltaic Properties. All the dyes were fabricated into DSSC devices for understanding their photovoltaic performance by varying the acceptor groups. The DSSC of all the dyes

Figure 3. J−V curves of the all the sensitizers using iodide/triiodide redox mediator.

Table 2. Photovoltaic Performance of DSSCs of 2Quinolinone Acceptor-Based Sensitizers Using Iodide/ Triiodide Redox Mediatora sensitizer

VOC (V)

JSC (mA cm−2)

FF

η (%)

PQ1 PQ2 PQ3 PQ4 PQ5 PQ2 ALD

0.618 0.593 0.637 0.600 0.515 0.584

2.85 1.02 6.81 7.61 0.72 0.98

0.724 0.780 0.732 0.662 0.731 0.771

1.30 0.77 3.23 3.08 0.27 0.45

TiO2 film has a 8 μm scattering layer and a 5 μm transparent layer. Electrolyte composition: 1.0 M DMII (1,3-dimethylimidazolium iodide), 0.03 M iodine, 0.025 M NaI, 0.5 TBP, 0.1 M guanidinium thiocyanide, and acetonitrile solvent. a

photovoltaic data are listed in Table 2. Among the various parameters that account for solar cell performance, much discrepancy has been observed in short circuit photocurrent density (JSC) of various sensitizers. For instance, PQ3 and PQ4 exhibit higher JSC of 6.81 and 7.61 mA cm−2, respectively. Lower currents were obtained with other sensitizers; for example, JSC is 2.85 mA cm−2 for PQ1 and sinks to 0.72−1.02 mA cm−2 for other sensitizers. In general, JSC is mainly governed by the light-harvesting efficiency of the sensitizers in the visible region and their efficient electron injection into the conduction band of TiO2.44 The light-harvesting efficiency of PQ2 and PQ5 in the visible region upon binding with TiO2 is relatively lowered, which accounts for their very low JSC. Although the light-harvesting efficiency of PQ3 and PQ2 ALD in the visible region is almost the same, their strength in JSC is abridged from 6.81 to 0.98 mA cm−2, respectively. Consideration of the structural difference between these two sensitizers reveals that the presence of strong electronwithdrawing dicyano group near the anchoring moiety facilitates better electron injection into the conduction band 16900



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Figure 4. (A) Distribution of trap states as a function of bias voltage. (B) Dependence of recombination rate as a function of capacitance.

of TiO2 and results in JSC higher than that of PQ2 ALD. This indicates the importance of EW groups near the amide-type anchoring moiety for increasing the short circuit current. PQ3 possess higher open circuit potential VOC of 637 mV; it is slightly lowered to 618 and 600 mV for PQ1 and PQ4. VOC ranges from 580 to 600 mV for PQ2 and PQ2 ALD and is lowered to 515 mV for PQ5. Although PQ4 has higher JSC, its fill factor (FF) of 0.662 is the smallest among all the sensitizers. There is no drastic change in the FF of other sensitizers, and it ranges from 0.724 to 0.780. Overall, PQ3 exhibits higher solar energy conversion efficiency of 3.23% with a new type of amide anchoring group. Although PQ4 has a JSC higher than that of PQ3, the loss in VOC and FF lowered its efficiency to 3.0%, whereas PQ1 exhibits 1.3% efficiency. The very low JSC of other sensitizers greatly affected their conversion efficiencies and ranges between 0.27 to 0.77%. The open circuit potential of a dye-sensitized solar cells is determined by three factors: (a) the distribution of the trap states of the conduction band of the semiconductor, (b) recombination of the photogenerated electrons from the semiconductor to the redox mediator, and (c) the redox potential of the electrolyte species. In this study, the redox mediator is held constant; hence, factor (c) is excluded from the discussion.45−47 Using charge extraction measurements,48 the surface and bulk trap states of the sensitized TiO2 surfaces are probed. Figure 4a shows the distribution of traps as a function of bias voltage. The surface sensitized with dye PQ4 exhibited deeper DOS distribution. With the sensitization of PQ3, the defects are negatively shifted and are further shifted up by PQ2. This variation in the trap states could be attributed to the difference in the dipole moment of the adsorbing dye molecules and the extent of the surface protonation. For PQ2ALD dye, the charge extraction measurement did not lead to a conclusive measurement possibly because of the excessive recombination of the electrons from TiO2 to the oxidized redox species and low transient currents. The recombination rate of electrons is studied using transient photovoltage decay technique, and the plot is presented in Figure 4b as a function of measured capacitance. At any given charge density (or film capacitance), the rate of the back electron flow is low for the devices sensitized with PQ3 dye.

PQ2 and PQ4 exhibit a similar rate profile, and the recombination rate is higher than that of PQ3. The solar cells with dye PQ2 ALD displayed the highest recombination rate. For a given charge density and trap state distribution, the increase in the recombination decreases the steady-state electron density in the TiO2 film, and in turn it lowers the open circuit potential of the device. The trend observed is consistent with the obtained VOC of the corresponding devices. Incident photon to current efficiency (IPCE) measurements will provide a clear picture of the ability of the sensitizers to convert the absorbed light energy into current. Photocurrent action spectra of all the sensitizers are measured with their corresponding photovoltaic devices, and the plot of IPCE as a function of wavelength is displayed in Figure 5. Among all the sensitizers, the IPCE of PQ3 and PQ4 is broadened in the visible region with onset around 700 nm. PQ3 exhibits 70% maximum IPCE, whereas PQ4 exhibits 56%; it lowers further to 24% for PQ1 around 450−550 nm. The broadened IPCE of

Figure 5. IPCE spectra of various sensitizers using iodide/triiodide redox mediator. 16901



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these sensitizers accounts for their higher JSC in photovoltaic measurements. The ability of photon conversion is limited to 10% for PQ2 and PQ2 ALD, which have lower JSC.

voltaic Materials, Department of Material Chemistry, Korea University, Chungnam, Korea, funded by the Ministry of Education, Science and Technology through the National Research Foundation of Korea (R31-2008-000-10035-0).

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3. CONCLUSIONS The 2-Quinolinone moiety having different anchoring modes was introduced for the first time as an electron acceptor in donor−π-conjugated bridge−acceptor systems for dye-sensitized solar cells. Photophysical and electrochemical properties of these D-π-A systems with different anchoring groups have been systematically investigated. The absorption was tuned from 469 to 518 nm by keeping the same donor-π group and varying the electron-withdrawing anchoring segments. The observed optical properties imply the significance of quinolinone fragments as acceptor groups and in determining the absorption behavior despite the variation in the strength of electron-withdrawing anchoring groups. All the different types of anchoring groups bind well with TiO2. Among the different types of anchoring groups, a new type of amide N,O-chelating anchoring mode and bianchor-type anchoring mode shows JSC of 6.81 and 7.61 mA cm−2 with an overall conversion efficiency of 3.23 and 3.28%, respectively. The energy conversion efficiencies of all the sensitizers were found to be mainly governed by the short circuit current density. Incident photon to current conversion efficiency of the above-mentioned new type of anchoring modes shows a maximum of 70.56% and 56.3%,respectively, whereas other anchoring modes lie below 25%. Overall, a new type of 2-quinolinone acceptor segment which can be tuned to different anchoring modes was successfully employed for DSSC. We envisage that the observed results will pave the way for designing new types of sensitizers with 2-quinolinone based anchoring moieties for better solar cell performance. Introducing this new type of quinolinone based anchoring groups in highly colored porphyrin, diketopyrrolopyrrole based molecules and their significance in energy conversion efficiencies are under investigation in our laboratory.

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(1) O’Regan, B.; Grätzel, M. A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature (London, U.K.) 1991, 353, 737−740. (2) Grätzel, M. Photoelectrochemical Cells. Nature (London, U.K.) 2001, 414, 338−344. (3) Grätzel, M. Solar Energy Conversion by Dye-Sensitized Photovoltaic Cells. Inorg. Chem. 2005, 44, 6841−6851. (4) Snaith, H. J.; Grätzel, M. Enhanced Charge Mobility in a Molecular Hole Transporter via Addition of Redox Inactive Ionic Dopant: Implication to Dye-Sensitized Solar Cells. Appl. Phys. Lett. 2006, 89, 262114 (1−3). (5) Park, H.; Bae, E.; Lee, J.-J.; Park, J.; Choi, W. Effect of the Anchoring Group in Ru−Bipyridyl Sensitizers on the Photoelectrochemical Behavior of Dye-Sensitized TiO2 Electrodes: Carboxylate versus Phosphonate Linkages. J. Phys. Chem. B 2006, 110, 8740− 8749. (6) Bae, E.; Choi, W. Effect of the Anchoring Group (Carboxylate vs Phosphonate) in Ru-Complex-Sensitized TiO2 on Hydrogen Production under Visible Light. J. Phys. Chem. B 2006, 110, 14792−14799. (7) Komiya, R.; Fukui, A.; Murofushi, N.; Koide, N.; Yamanaka, R.; Katayama, H., Improvement of the Conversion Efficiency of a Monolithic Type Dye-Sensitized Solar Cell Module. Technical Digest, 21st International Photovoltaic Science and Engineering Conference, Fukuoka, Japan, November 2011, 2C-50-08. (8) Yella, A.; Lee, H.-W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.; Nazeeruddin, M. K.; Diau, E. W.-G.; Yeh, C.-Y.; Zakeeruddin, S. M.; Grätzel, M. Porphyrin-Sensitized Solar Cells with Cobalt (II/III)Based Redox Electrolyte Exceed 12% Efficiency. Science 2011, 334, 629−634. (9) Mathew, S.; Yella, A.; Gao, P.; Humphry-Baker, R.; Curchod, B. F. E.; Ashari-Astani, N.; Tavernelli, I.; Rothlisberger, U.; Nazeeruddin, M. K.; Grätzel, M. Dye-Sensitized Solar Cells with 13% Efficiency Achieved through the Molecular Engineering of Porphyrin Sensitizers. Nat. Chem. 2014, 6, 242−247. (10) Hirata, N.; Lagref, J.-J.; Palomares, E. J.; Durrant, J. R.; Nazeeruddin, M. K.; Gratzel, M.; Di Censo, D. Supramolecular Control of Charge-Transfer Dynamics on Dye-Sensitized Nanocrystalline TiO2 Films. Chem.Eur. J. 2004, 10, 595−602. (11) Clifford, J. N.; Palomares, E.; Nazeeruddin, M. K.; Grätzel, M.; Nelson, J.; Li, X.; Long, N. J.; Durrant, J. R. Molecular Control of Recombination Dynamics in Dye-Sensitized Nanocrystalline TiO2 Films: Free Energy vs Distance Dependence. J. Am. Chem. Soc. 2004, 126, 5225−5233. (12) Yum, J.-H.; Chen, P.; Grätzel, M.; Nazeeruddin, M. K. Recent Developments in Solid-State Dye-Sensitized Solar Cells. ChemSusChem 2008, 1, 699−707. (13) Kay, A.; Grätzel, M. Artificial Photosynthesis. 1. Photosensitization of Titania Solar Cells with Chlorophyll Derivatives and Related Natural Porphyrins. J. Phys. Chem. 1993, 97, 6272−6277. (14) Hara, K.; Dan-oh, Y.; Kasada, C.; Ohga, Y.; Shinpo, A.; Suga, S.; Sayama, K.; Arakawa, H. Effect of Additives on the Photovoltaic Performance of Coumarin-Dye-Sensitized Nanocrystalline TiO2 Solar Cells. Langmuir 2004, 20, 4205−4210. (15) Yum, J.-H.; Holcombe, T. W.; Kim, Y.; Rakstys, K.; Moehl, T.; Teuscher, J.; Delcamp, J. H.; Nazeeruddin, M. K.; Grätzel, M. BlueColoured Highly Efficient Dye-Sensitized Solar Cells by Implementing the Diketopyrrolopyrrole Chromophore. Sci. Rep. 2013, 2446. (16) Zhang, M.; Wang, Y.; Xu, M.; Ma, W.; Li, R.; Wang, P. Design of High-Efficiency Organic Dyes for Titania Solar Cells Based on the Chromophoric Core of Cyclopentadithiophene-Benzothiadiazole. Energy Environ. Sci. 2013, 6, 2944−2949. (17) Wiiberg, J.; Marinado, T.; Hagberg, D. P.; Sun, L.; Hagfeldt, A.; Albinsson, B. Effect of Anchoring Group on Electron Injection and

ASSOCIATED CONTENT

S Supporting Information *

Experimental and synthetic details, NMR and mass characteristic data, cyclic voltammogram, FT-IR and theoretical optimized geometries. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*P.G.: e-mail, peng.gao@epfl.ch. *R.R.: e-mail, [email protected]. *M.K.N.: e-mail, mdkhaja.nazeeruddin@epfl.ch. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.R. thanks the Department of Science and Technology (DST) (SR/S1/PC-12/2011, DT: 20.09.2011) and (SR/NM/NS-26/ 2013/19.02.2014) for the project. G.P. thanks the Swiss Government Scholarship (2012.0795/India/OP.) P.G. thanks the European Union Seventh Framework Programme (FP7/ 2007-2013) under grant agreement “ENERGY-261920, ESCORT” and 308997 of the NANOMATCELL project. M.K.N. acknowledges the World Class University programme, Photo16902



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Molecular Engineering of 2-Quinolinone Based Anchoring Groups for

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