Novel Squaraine Cosensitization System of Panchromatic Light

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Novel Squaraine Cosensitization System of Panchromatic LightHarvesting with Synergistic Effect for Highly Efficient Solar Cells Weiwei Zhang, Wenqin Li, Yongzhen Wu, Jingchuan Liu, Xiongrong Song, He Tian, and Wei-Hong Zhu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00674 • Publication Date (Web): 20 Apr 2016 Downloaded from http://pubs.acs.org on April 26, 2016

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ACS Sustainable Chemistry & Engineering

Novel Squaraine Cosensitization System of Panchromatic Light-Harvesting with Synergistic Effect for Highly Efficient Solar Cells Weiwei Zhang,† Wenqin Li,‡ Yongzhen Wu,† Jingchuan Liu,† Xiongrong Song,† He Tian,† and Wei-Hong Zhu*,† †

Shanghai Key Laboratory of Functional Materials Chemistry, Key Laboratory for Advanced

Materials and Institute of Fine Chemicals, Collaborative Innovation Center for Coal Based Energy (i-CCE), School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, P. R. China. E-mail: [email protected]. ‡

School of Urban Development and Environmental Engineering, Shanghai Second Polytechnic

University, Shanghai 201209, P. R. China.

1 Environment ACS Paragon Plus


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ABSTRACT Cosensitization based on two or multiple dyes as “dye cocktails” is a convenient shortcut to construct panchromatic dye sensitized solar cells (DSSCs). Generally, extending absorption bands of D-π-A featured organic sensitizers to long-wavelength and near-infrared (NIR) region through molecular engineering always induces unmatched energy levels (LUMO and HOMO) in DSSCs. In contrast, NIR squaraine dyes commonly exhibit a strong spike-like absorption peak with narrow half-band-width in NIR region, along with weak absorption in visible region, which can bring the relative matched band gap as well as energy levels for DSSCs. It is expected that squaraine dyes are idea cosensitizers to compensate light-harvesting, which can relief the constraint in energy levels for D-π-A featured sensitizers. In this work, the efficient spectralcomplemental couples of D-A-π-A featured organic dye WS-1 with NIR squaraine dyes VG1C8 and HSQ4 are well demonstrated for cosensitized panchromatic solar cells. Interestingly, simultaneous enhancements in both photocurrent and photovoltage are obtained for the cosensitization system with respect to individual WS-1, along with the panchromatic photoresponse covering in visible and NIR region. In comparation with individual WS-1, the optimized cosensitization devices exhibit significant improvements in overall photovoltaic efficiency of 43% and 38%, resulting in notable efficiencies up to 9.0% and 8.7% for WS1/VG1-C8 and WS-1/HSQ4 systems, respectively, which are among the best photovoltaic results for squaraine cosensitized DSSCs. This work demonstrates WS-1/VG1-C8 and WS1/HSQ4 couples are synergistic and efficient cosensitization systems, especially for panchromatic light-harvesting solar cells. KEYWORDS: Cosensitization, Panchromatic, Extending absorption, Squaraine dyes, Simultaneous enhancements, Synergistic


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INTRODUCTION Dye sensitized solar cells (DSSCs)1-8 have attracted extensive attention due to their low cost and relative high conversion efficiency. Organic sensitizers with donor-π-acceptor (D-π-A)9-12 or donor-acceptor-π-acceptor (D-A-π-A)13-18 configuration are ideal lightharvesting materials in DSSCs due to their convenient synthesis and environmental friendly properties from the viewpoints of molecular engineering. Generally, photocurrent and photovoltage are among the main parameters which critically determines the photovoltaic performance and overall efficiency of DSSCs.19-22 As a key factor to photocurrent density, broadening photo-response is quite necessary for an ideal sensitizer.23-31 It is known that almost 53% of solar irradiation energy locates at NIR region, whereas metal-free organic dyes usually show poor light absorption in red/NIR region. Moreover, the pursuit of high-performance organic sensitizers with panchromatic spectral response has encountered with a great obstacle due to the restriction of energy levels (LUMO and HOMO) for organic sensitizers, which are supposed to ensure enough driving force for charge injection to TiO2 conduction band and dye regeneration by electrolyte. In this regard, broadening light response through enhancing the electrondonating ability of donor, π-spacer or the electron-withdrawing ability of acceptor are prone to induce unmatched HOMO or LUMO level in DSSCs. For instances, DPP-II is a typical long-wavelength responsive organic dye, which was designed through incorporating the strong electron-withdrawing unit of DPP.32 Although it exhibited a broad absorption band up to 627 nm, a very limited short circuit current (JSC) of only 2.0 mA cm-2 was deduced due to the unmatched LUMO level. Moreover, G270 designed by



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extending π-bridge exhibited the maximum absorption band up to 612 nm,33 with the unexpected low JSC of 5.0 mA cm-2 due to the unmatched HOMO level. Alternatively, to broaden photo-response and optimize photovoltaic performance, the so-called “dye cocktails” cosensitization of long wavelength or NIR dyes (e.g. ruthenium polypyridine,34 zinc porphyrin complexes35 and squaraine dyes36) with organic dyes has been proved to be an efficient approach. The established “star cosensitizers” zinc porphyrin complexes have been widely explored in cosensitization DSSCs, and resulted in the excellent performance.37-40 In comparison with zinc porphyrin complexes, squaraine dyes that bestow more convenient synthetic procedures and extremely high absorption coefficients in red/NIR region remain relatively unexplored.41-47 Squaraine dyes commonly exhibit strong spike-like absorption band with narrow half-band-width in NIR region. However, the weak light-harvesting in visible region limits the photocurrent improvement for squaraine sensitizers in solar cells. In this context, squaraine dyes can be perceived as a promising cosensitizer for complementary light-harvesting in DSSCs.

Figure 1. Molecular structures of dye WS-1, VG1-C8 and HSQ4.


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In this work, to broaden the light photo-response of known organic dye WS-1,48 two NIR squaraine dyes VG1-C849 and HSQ450 were introduced to build a novel cosensitization system. To be mentioned, the only difference between VG1-C8 and HSQ4 is whether contain the electron-withdrawing group of ethyl cyanoacetate (Figure 1). Interestingly, simultaneous improvements both in photocurrent and photovoltage were obtained for the cosensitization DSSCs with respect to individual WS-1 based devices, resulting in enhancements in overall power conversion efficiency (PCE) of 43% and 38%, respectively. For WS-1/HSQ4 cosensitization system, the incident monochromatic photon-to-current conversion efficiency (IPCE) spectrum was broadened to 800 nm, which is the best performance in cosensitization systems of squaraine dyes with metalfree organic dye to the best of our knowledge.41-47

EXPERIMENTAL SECTION DSSCs Fabrication. The nanocrystalline TiO2 films used in solar devices consisted of double layers of TiO2 (12 µm transparent layer and 4 µm scattering layer). The TiO2 films were prepared by repeating screen printing procedure. Then TiO2 electrodes were sintered at 500 °C for 30 min, followed by treatment with a 40 mM aqueous TiCl4 solution at 70 °C for 30 min and washed with water and ethanol. The TiO2 films were heated again at 500 °C for 30 min. For preparing the counter electrode, the Pt catalyst was deposited on the cleaned FTO glass by coating with two drops of H2PtCl6 solution (0.02 M in 2-propanol solution) and calcination at 400 °C for 15 min. Afterward, a hole (0.8 mm diameter) was predrilled on the counter electrode with a drill press. Then the counter electrodes were cleaned by ultrasound in ethanol for 5 min. The dye solutions



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(0.3 mM WS-1 in chloroform/methanol (v/v, 4/1) and 0.1 mM VG1-C8 or HSQ4 in acetonitrile/tert-butanol (v/v, 1/1) were prepared with 10 mM chenodeoxycholic acid (CDCA). For devices based on individual dyes, the TiO2 electrodes were immersed in aforementioned dye solutions of WS-1, VG1-C8 and HSQ4 for 2 h, 12 h and 4 h, respectively. For cosensitization devices,41,42 the TiO2 electrodes were firstly immersed in dye bath of WS-1 for 2 h, followed by immersion in dye solution containing VG1-C8 or HSQ4 for 3, 4, 5 and 6 h, respectively. As for the assemble procedure of DSSCs, the dye-covered TiO2 electrode and Pt counter electrode were assembled into a sandwich type cell and sealed with a hot-melt gasket of 25 µm thickness. An acetonitrile (AN) solution containing 0.6 M dimethylpropyl-imidazolium iodide (DMPII), 0.05 M I2, 0.1 M LiI, and 0.05 M tert-butylpyridine (TBP) was used as electrolyte. Measurements. The UV-Vis absorption spectra were obtained with CARY 100 spectroscopy. Photocurrent density-voltage (I-V) curves of devices were measured by Keithley 2400 Sourcemeter Instruments under standard AM 1.5 simulated solar irradiation (WXS-155S-10). Monochromatic IPCE spectra were measured by Newport-74125 system (Newport Instruments). Electrochemical impedance spectroscopy (EIS) measurement was performed using a two-electrode system under dark by electrochemical workstation (Zahner IM6e). The magnitude of the sinusoidal perturbations was 5 mV, with the frequency range of 10-1-105 Hz.

RESULTS AND DISCUSSION Complementary Light-Response of Cosensitization Couples. Dye WS-1, VG1-C8 and HSQ4 were synthesized according to the previous reports.48-50 The


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absorption spectra of dyes WS-1, VG1-C8 and HSQ4 were measured in CH2Cl2 and on 4 µm TiO2 film, respectively (Figure 2). The characteristic data are listed in Table 1. The typical intramolecular charge transfer (ICT) band from 350 to 650 nm was observed for D-A-π-A sensitizer WS-1 with a peak located at 495 nm in CH2Cl2 solution (Figure 2a). While squaraine dyes VG1-C8 and HSQ4 display strong compensating absorption in long-wavelength region even to NIR. VG1-C8 exhibits the maximum absorption band (λmax) at 653 nm with extremely high molar extinction coefficient (ε = 3.12 × 105 M-1 cm-1). In contrast, HSQ4 bearing the electron-withdrawing unit of ethyl cyanoacetate red-shifts the maximum absorption band to 712 nm with a lower absorption coefficient (ε = 1.68 × 105 M-1 cm-1) but wider spectral response from 550 to 750 nm. Upon adsorbing onto TiO2 film (Figure 2b), the absorption peak hypochromatically shifted to 490 nm for WS-1, whereas bathochromatically shifted to 655 and 719 nm for VG1-C8 and HSQ4, respectively. Apparently, the panchromatic light absorption can be well realized by complementary effect of WS-1 with squaraine dyes VG1-C8 and HSQ4.



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Figure 2. Absorption spectra of WS-1, VG1-C8 and HSQ4 in CH2Cl2 (a) and on TiO2 film (4 µm, b). Note: the absorption band of WS-1 is magnified by 10 times for clarity.

Electrochemical Characterization. Generally, the energy gaps (E0–0) were calculated from the intersection of normalized absorption and photoluminescent spectra or estimated from the absorption edge or thresholds from absorption spectra of dyes. And the longer wavelength the thresholds of absorption spectra is, the narrower energy band gap will be obtained. 49,50,51 It is worthy to note that, D-π-A featured organic sensitizers always endow a wide absorption band from the characteristic ICT, and extending absorption bands of such sensitizers to long-wavelength and NIR region through molecular engineering to afford a broader photo-response always induced unmatched energy levels in DSSCs due to the large red-shift for absorption edge, which brings an excessively


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narrow optical band gap, along with the unsuitable LUMO and HOMO levels for TiO2 and electrolyte.32,33,52 In contrast, NIR squaraine dyes commonly exhibit strong spike-like absorption band with narrow half-band-width in NIR region, which means there is no obvious red-shift of absorption edge with respect to their maximum absorption peak for squaraine dyes, although they show longer absorption band in NIR region than D-π-A featured organic dyes. That is, squaraine dyes can realize both large bathchromatic shift in absorption and minor red-shift in absorption edge to afford relative matched band gap as well as energy levels. Accordingly, squaraine dyes can be an idea cosensitizer to compensate light-harvesting for relieving the constraint of energy levels for D-π-A featured sensitizers. Here, WS-1 displays a broader energy band gap (2.06 eV) with respect to squaraine dyes (1.81 and 1.74 eV, Figure 3 and Table 1). Compared to dye VG1-C8, HSQ4 bearing the electron-withdrawing unit positively shifted the LUMO levels by 0.02 V, meanwhile uplift the HOMO levels by 0.05 V, along with a narrower energy band gap. All three dyes exhibit more negative LUMO levels than TiO2 conduction band (-0.5 V) with 0.66, 0.52 and 0.50 V driving force for efficient charge injection into TiO2, respectively.53 Moreover, the HOMO levels for the three dyes are more positive than energy level of iodine electrolyte (0.4 V), with 0.50, 0.39 and 0.34 V driving force for effective dye regeneration, respectively.53



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Figure 3. Schematic diagram of energy levels (V vs. NHE) of TiO2 conduction band, dyes, and I-/I3- redox couple.

Table 1. Characteristic Data for Dyes WS-1, VG1-C8 and HSQ4. Dyes WS-1 G1-C8 HSQ4

λmaxa (nm) 495 653 712

εa (M cm-1) 17747 311765 167643 -1

λmaxb (nm) 490 655 719

HOMOc (V) 0.90 0.79 0.74

E0-0c (eV) 2.06 1.81 1.74

LUMOc (V) -1.16 -1.02 -1.00

Note: aAbsorption peaks (λmax) and molar extinction coefficient (ε) in CH2Cl2 solution at room temperature; bAbsorption peaks on 4 µm TiO2 films. c Electrochemical properties were obtained from literatures.48-50

Light-Harvesting for Different Adsorption Time. To explore the adsorption conditions of WS-1/VG1-C8 and WS-1/HSQ4 cosensitization, dye WS-1, VG1-C8 and HSQ4 adsorbed on 4 µm TiO2 films were measured under different dipping time. As shown in Figure (4a, 4b), along with increment of dipping time in squaraine dyes, the absorbance of ICT band was decreased for WS-1, whereas increased for VG1-C8 and HSQ4,


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indicating the competitive adsorption of squaraine dyes with WS-1. The growth in absorbance for squaraine dyes became slow down after 4 h and the panchromatic absorption composed by WS-1/VG1-C8 or WS-1/HSQ4 was gradually emerged. Correspondingly, the absorption balance between the increment in long wavelength and decrement in short wavelength can be achieved to optimize the DSSCs performance. On the other hand, the fluorescence spectra of individual WS-1 and dye blend were also measured on TiO2 films and presented in Figure 4c and 4d. A significant red-shift for the emission band of WS-1 can be observed with increasing dipping time in VG1-C8 and HSQ4 from 1 h to 6 h, demonstrating the coadsorption process of WS-1 and squaraine dyes.



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Figure 4. Absorption spectra of WS-1/VG1-C8 (a) and WS-1/HSQ4 (b) adsorbed onto TiO2 films (4 µm) for different dipping time and fluorescence spectra of WS-1/VG1-C8 (c) and WS-1/HSQ4 (d) adsorbed onto TiO2 films (4 µm) for different dipping time.

Optimizing Photovoltaic Performance of Cosensitization DSSCs. The photovoltaic performances of three dyes WS-1, VG1-C8 and HSQ4 as well as cosensitization were evaluated under simulated AM1.5 solar irradiation based on iodine electrolyte. The fabrication of DSSCs was depicted in experimental part, and relevant photovoltaic parameters were summarized in Figure 5, 6 and Table 2. The individual WS-1 based cells show a PCE of 6.3%, with short-circuit current density (JSC) of 13.79 mA cm-2, open circuit voltage (VOC) of 631 mV and fill factor (FF) of 0.72. Meanwhile, the devices based on VG1-C8 display the PCE of 3.5% (JSC = 8.14 mA cm-2, VOC = 608 mV, FF = 0.71). In comparison, DSSCs based on HSQ4 bearing electron-withdrawing unit on the skeleton of structure exhibit an increased JSC of 12.29 mA cm-2 but slightly decreased VOC of 578 mV, with a PCE of 5.0% (Table 2 and Figure 6a, b). Upon cosensitization, interestingly, JSC and VOC were first increased, and then decreased with prolonging immersion time in squaraine solution. For WS-1/VG1-C8 system, the peak value was attained at 5 h, along with the obvious improvements of 3.94 mA cm-2 in JSC and 70 mV in VOC with respect to those based on WS-1 alone, corresponding to an enhanced overall conversion efficiency of 9.0% (JSC = 17.73 mA cm2

, VOC = 701 mV, FF = 0.72). Simultaneously, WS-1/HSQ4 based cosensitization DSSCs

achieved the peak value after 4 h co-adsorption, with a more dramatic improvement of 4.92 mA cm-2 in JSC and a relative minor enhancement in VOC of 26 mV with respect to


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individual WS-1, and an overall PCE of 8.7% was achieved with parameters of JSC = 18.71 mA cm-2, VOC = 657 mV, FF = 0.71 . The relative minor increase in VOC for the WS-1/HSQ4 system compared to that of WS-1/VG1-C8 can be ascribed to the existence of electron-withdrawing unit (ethyl cyanoacetate), which may accelerate charge recombination process of oxidized squaraine dye with electron injected into TiO2 film (discuss later). In comparison, the relative low Jsc for WS-1/VG1-C8 system can be attribute to the absence of additional electron-withdrawing unit, which brings forth relative wide optical band gap, thus narrower IPCE response as well as weaker photocurrent density. Strikingly here, the optimized cosensitized solar cells display a significant improvement in both photocurrent and photovoltage with respect to those of individual dye aforementioned. Note that it was extremely rare to observe such increase in VOC for previous squaraine cosensitization DSSCs, which usually display a VOC between two individual sensitizers based cells.41-47 We herein speculate WS-1/VG1-C8 as well as WS-1/HSQ4 are compact and synergistic cosensitization systems. To our knowledge, the PCE values for these cosensitization systems are among the highest reported for squaraine cosensitized DSSCs.41-47



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Figure 5. J-V curves of DSSCs based on cosensitization of VG1-C8 with WS-1 (a) and cosensitization of HSQ4 with WS-1 (b) for different dipping time.

Table 2. Photovoltaic Performances for DSSCs Based on Individual WS-1, VG1-C8, HSQ4 and Cosensitization of VG1-C8 and HSQ4 with WS-1. Dyes WS-1 VG1-C8 WS-1 (2 h)+VG1-C8 (3 h) WS-1 (2 h)+VG1-C8 (4 h) WS-1 (2 h)+VG1-C8 (5 h) WS-1 (2 h)+VG1-C8 (6 h) HSQ4 WS-1 (2 h)+HSQ4 (3 h) WS-1 (2 h)+HSQ4 (4 h) WS-1 (2 h)+HSQ4 (5 h) WS-1 (2 h)+HSQ4 (6 h)

Jsc (mA cm-2) 13.79 8.14 15.28 17.39 17.73 10.32 12.29 17.38 18.71 14.57 13.36

Voc (mV) 631 608 649 677 701 625 578 642 657 582 579

FF 0.72 0.71 0.70 0.70 0.72 0.75 0.71 0.72 0.71 0.70 0.72

Complementary IPCE Action for Panchromatic Light-Harvesting.


η (%) 6.3 3.5 6.9 8.3 9.0 4.8 5.0 8.0 8.7 6.0 5.5

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Figure 6 shows the current-voltage characteristics and IPCE spectra for DSSCs based on individual WS-1, VG1-C8, HSQ4 as well as optimized cosensitization DSSCs of WS-1/VG1C8 (cosensitized for 5 h in VG1-C8 solution) and WS-1/HSQ4 (cosensitized for 4 h in HSQ4 solution). As shown in Figure 6c, the individual WS-1 sensitized device displays a high IPCE from 300 to 600 nm, but gradually drops to zero beyond 600 nm. Meanwhile, VG1C8 sensitized device shows an intense IPCE action from 600 to 730 nm, with the maximum IPCE at 660 nm. Apparently, WS-1/VG1-C8 can be well organized as a cosensitization couple to compensate IPCE spectra. Under WS-1/VG1-C8 co-adsorption, an impressive light-harvesting from 300 to 730 nm appeared with significantly enhanced photo-response despite that the IPCE onset of cosensitization cells was slightly blueshifted to some extent due to the competitive co-adsorption (Table 3).54 Here, the adsorption amounts for VG1-C8 and HSQ4 were only 0.007 and 0.003 (10-7 mol cm-2) in comparison with that for WS-1 of 1.17 (10-7 mol cm-2). However, the extremely high ε for squaraine dyes can guarantee the efficient light-harvesting of sensitizers, and thus contribute to the enhancement of IPCE. Moreover, ethyl cyanoacetate featuring HSQ4 displays a much broader IPCE action with respect to VG1-C8, bringing forth the maximum IPCE at 510 nm and 718 nm, respectively (Figure 6d). Indeed, DSSCs based on WS-1/HSQ4 cosensitization system exhibit an excellent panchromatic IPCE response from 300 to 800 nm, which is the first report of IPCE spectrum of metal-free organic dye broadening to 800 nm by means of cosensitization with squaraine dyes.41-47



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Table 3 Adsorption Amounts of Dye WS-1, VG1-C8 and HSQ4 on 12 µm Transparent TiO2 Film. Device

Dye

Adsorption time

individual dye

WS-1 VG1-C8 HSQ4 WS-1 VG1-C8 WS-1 HSQ4

2h 5h 4h 2h 5h 2h 4h

Cosensitization of VG1C8 with WS-1 Cosensitization of HSQ4 with WS-1

Adsorption amount (10-7 mol cm-2) 1.31 0.014 0.015 1.17 0.007 1.17 0.003

Figure 6. J-V curves (a, b) and IPCE spectra (c, d) of DSSCs based on individual WS-1, VG1-C8, HSQ4, optimized cosensitization of VG1-C8 with WS-1 (cosensitized in VG1C8 for 5 h) and cosensitization of HSQ4 with WS-1 (cosensitized in HSQ4 for 4 h).

Retarding Charge Recombination through Cosensitization. To gain insights into the distinct variations in VOC arising from cosensitization, electrochemical impedance spectroscopy (EIS)55 analysis was performed in the dark with


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an applied bias of -0.60 V. Figure 7a plots the Nyquist picture of DSSCs based on individual dye WS-1 and cosensitization. The larger middle-frequency semicircle corresponding to charge transfer process at TiO2/dye/electrolyte interface was denoted as Rrec, which can be evaluated by the size of semicircles.56 Generally, a larger Rrec value means a slower charge recombination between photon-generated electron in TiO2 and electrolyte/oxidized dyes. Obviously, there are significant enhancements in Rrec for cosensitization devices with respect to individual WS-1 based devices, along with an order of WS-1 < WS-1/HSQ4 < WS- 1/VG1-C8. That is, the cosensitization process can assist

charge

accumulation

in

TiO2

conduction

band

and

increase

the

VOC

correspondingly. Such an improvement compared to individual WS-1 can be attributed to the octyl chains grafted on both squaraine dyes, which is beneficial to retarding interfacial charge recombination process. In addition, the minor improvement of Rrec for WS1/HSQ4 system compared to that of WS-1/VG1-C8 can be ascribed to the electronwithdrawing group of ethyl cyanoacetate decorated on HSQ4, inducing undesirable charge recapture to some extent. Moreover, the electron lifetime (τ) can be calculated from lower peak frequency (f) in the Bode plots (Figure 7b) using τ = 1 / (2πf).57,58 The calculated electron lifetime was increased in the order WS-1 (5.5 ms) < WS-1/HSQ4 (10.2 ms) < WS-1/VG1-C8 (14.4 ms), which is in line with the improvement of VOC for WS-1 (631 mV) < WS-1/HSQ4 (657 mV) < WS-1/VG1-C8 (701 mV). The EIS analysis demonstrates that the cosensitization process along with the introduction of long alkyl chains into sensitized interface are beneficial to form a compact and efficient adsorbed layer on TiO2 surface, thus contributing to synchronous enhancements in both JSC and VOC.



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Figure 7. Nyquist plots (a) and Bode plots (b) for DSSCs based on WS-1, WS-1+HSQ4 (cosensitized for 4 h in HSQ4 solution) and WS-1+VG1-C8 (cosensitized for 5 h in VG1C8 solution).


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CONCLUSIONS We have demonstrated a synergistic and efficient cocktail cosensitization system of the D-A-π-A featured organic dye WS-1 with two different NIR squaraine dyes VG1-C8 and HSQ4, respectively. Incorporating electron-withdrawing group in HSQ4 efficiently broadens IPCE action spectra and increases photocurrent, with a minor photovoltage loss. Compared to individual WS-1, the present cosensitization systems display obvious improvements of 43% and 38% in PCE, respectively, along with significant enhancements in both JSC and VOC. Notably, VOC for cosensitization devices is enhanced obviously with respect to single dye based devices, a breakthrough for squaraine cosensitization systems. Moreover, WS-1/VG1-C8 based devices show a remarkable improvement in PCE up to 9.0% for squaraine cosensitized DSSCs. WS1/HSQ4 based devices exhibit a superior panchromatic IPCE feature to harvest solar energy up to 800 nm, along with dramatically increasing JSC from 13.79 to 18.71 mA cm-2. Our work paves a novel way to realize the panchromatic light response and high photovoltaic performance DSSCs through cocktail cosensitization. We are currently exploring of more efficient cosensitization systems as well as their internal mechanisms. AUTHOR INFORMATION Corresponding Author * Fax: (+86) 21-6425-2758. E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by NSFC for Creative Research Groups (21421004) and Distinguished Young Scholars (21325625), NSFC/China, Oriental Scholarship, Fundamental Research Funds



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for the Central Universities (WJ1416005 and WJ1315025), Scientific Committee of Shanghai (14YF1410500, 14ZR1409700 and 15XD1501400) and Shanghai Young Teacher Supporting Foundation (No. ZZEGD14011). REFERENCES (1)

O'Regan, B.; Grätzel, M. A low-cost, high-efficiency solar cell based on dye-sensitized

colloidal TiO2 films. Nature 1991, 353, 737−740. (2)

Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Dye-sensitized solar cells.

Chem. Rev. 2010, 110, 6595−6663. (3)

Wang, C.; Hu, J.; Wu, C.; Kuo, H.; Chang, Y.; Lan, Z.; Wu, H.; Wei-Guang Diau, E.; Lin,

C. Highly efficient porphyrin-sensitized solar cells with enhanced light harvesting ability beyond 800 nm and efficiency exceeding 10%. Energy Environ. Sci. 2014, 7, 1392−1396. (4)

Zhang, S.; Yang, X.; Numata, Y.; Han, L. Highly efficient dye-sensitized solar cells:

Progress and future challenges. Energy Environ. Sci. 2013, 6, 1443−1464. (5)

Chou, C.; Hu, F.; Yeh, H.; Wu, H.; Chi, Y.; Clifford, J. N.; Palomares, E.; Liu, S.; Chou,

P.; Lee, G. Highly efficient dye-sensitized solar cells based on panchromatic ruthenium sensitizers with quinolinylbipyridine anchors. Angew. Chem., Int. Ed. 2014, 53, 178−183. (6)

Xu, B.; Sheibani, E.; Liu, P.; Zhang, J.; Tian, H.; Vlachopoulos, N.; Boschloo, G.; Kloo,

L.; Hagfeldt, A.; Sun, L. Carbazole-based hole-transport materials for efficient solid-state dyesensitized solar cells and perovskite solar cells. Adv. Mater. 2014, 26, 6629−6634. (7)

Higashino, T.; Fujimori, Y.; Sugiura, K.; Tsuji, Y.; Ito, S.; Imahori, H. Tropolone as a

high-performance robust anchoring group for dye-sensitized solar cells. Angew. Chem., Int. Ed. 2015, 127, 9180−9184. (8)

Zhou, N.; Prabakaran, K.; Lee, B.; Chang, S. H.; Harutyunyan, B.; Guo, P.; Butler, M. R.;


Page 21 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Timalsina, A.; Bedzyk, M. J.; Ratner, M. A.; Vegiraju, S.; Yau, S.; Wu, C.; Chang, R. P. H.; Facchetti, A.; Chen, M.; Marks, T. J. Metal-free tetrathienoacene sensitizers for highperformance dye-sensitized solar cells. J. Am. Chem. Soc. 2015, 137, 4414−4423. (9)

Kim, S.; Lee, J. K.; Kang, S. O.; Ko, J.; Yum, J. H.; Fantacci, S.; De Angelis, F.; Di

Censo, D.; Nazeeruddin, M. K.; Grätzel, M. Molecular engineering of organic sensitizers for solar cell applications. J. Am. Chem. Soc. 2006, 128, 16701−16707. (10)

Eom, Y. K.; Choi, I. T.; Kang, S. H.; Lee, J.; Kim, J.; Ju, M. J.; Kim, H. K. Thieno[3,2-

b][1]benzothiophene derivative as a new π-bridge unit in D-π-A structural organic sensitizers with over 10.47% efficiency for dye-sensitized solar cells. Adv. Energy Mater. 2015, 5, 1500300. (11)

Yang, L.; Zheng, Z.; Li, Y.; Wu, W.; Tian, H.; Wang, Z. N-annulated perylene-based

metal-free organic sensitizers for dye-sensitized solar cells. Chem. Commun. 2015, 51, 4842−4845. (12)

Wang, Z.; Koumura, N.; Cui, Y.; Takahashi, M.; Sekiguchi, H.; Mori, A.; Kubo, T.;

Furube, A.; Hara, K. Hexylthiophene-functionalized carbazole dyes for efficient molecular photovoltaics: tuning of solar-cell performance by structural modification. Chem. Mater. 2008, 20, 3993−4003. (13)

Wu, Y. Z.; Zhu, W. H. Organic sensitizers from D-π-A to D-A-π-A: Effect of the internal

electron-withdrawing units on molecular absorption, energy levels and photovoltaic performances. Chem. Soc. Rev. 2013, 42, 2039−2058. (14)

Wu, Y. Z.; Zhu, W. H.; Zakeeruddin, S. M.; Grätzel, M. Insight into D-A-π-A structured

sensitizers: A promising route to highly efficient and stable dye-sensitized solar cells. ACS Appl. Mater. Interfaces 2015, 7, 9307−9318. (15)

Chaurasia, S.; Hung, W.; Chou, H.; Lin, J. T. Incorporating a new 2H-[1,2,3]triazolo[4,5-



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 29

c]pyridine moiety to construct D-A-π-A organic sensitizers for high performance solar cells. Org. Lett. 2014, 16, 3052−3055. (16)

Kang, X.; Zhang, J.; O’Neil, D.; Rojas, A. J.; Chen, W.; Szymanski, P.; Marder, S. R.; El-

Sayed, M. A. Effect of molecular structure perturbations on the performance of the D-A-π-A dye sensitized solar cells. Chem. Mater. 2014, 26, 4486−4493. (17)

Zhu, H. B.; Li, W. Q.; Wu, Y. Z.; Liu, B.; Zhu, S. Q.; Li, X.; Ågren, H.; Zhu, W. H.

Insight into benzothiadiazole acceptor in D-A-π-A configuration on photovoltaic performances of dye-sensitized solar cells. ACS Sustainable Chem. Eng. 2014, 2, 1026−1034. (18)

Wu, Y. Z.; Marszalek, M.; Zakeeruddin, S. M.; Zhang, Q.; Tian, H.; Grätzel, M.; Zhu, W.

H. High-conversion-efficiency organic dye-sensitized solar cells: Molecular engineering on D-Aπ-A featured organic indoline dyes. Energy Environ. Sci. 2012, 5, 8261−8272. (19)

Yao, Z.; Zhang, M.; Wu, H.; Yang, L.; Li, R.; Wang, P. Donor/Acceptor indenoperylene

dye for highly efficient organic dye-sensitized solar cells. J. Am. Chem. Soc. 2015, 137, 3799−3802. (20)

Dai, P.; Yang, L.; Liang, M.; Dong, H.; Wang, P.; Zhang, C.; Sun, Z.; Xue, S. Influence of

the terminal electron donor in D-D-π-A organic dye-sensitized solar cells: dithieno[3,2-b:2’,3’d]pyrrole versus bis(amine). ACS Appl. Mater. Interfaces 2015, 7, 22436−22447. (21)

Zhao, B.; Wang, J.; Li, H.; Xu, Y.; Yu, H.; Jia, X.; Zhang, X.; Hao, Y. Solar-to-electric

performance enhancement by titanium oxide nanoparticles coated with porous yttrium oxide for dye-sensitized solar cells. ACS Sustainable Chem. Eng. 2015, 3, 1518−1525. (22)

Liu, B.; Liu, Q.; You, D.; Li, X.; Naruta, Y.; Zhu, W. H. Molecular engineering of

indoline based organic sensitizers for highly efficient dye-sensitized solar cells. J. Mater. Chem. 2012, 22, 13348−13356.


Page 23 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(23)

Shiu, J.; Chang, Y.; Chan, C.; Wu, H.; Hsu, H.; Wang, C.; Lin, C.; Diau, E. W.

Panchromatic co-sensitization of porphyrin-sensitized solar cells to harvest near-infrared light beyond 900 nm. J. Mater. Chem. A 2015, 3, 1417−1420. (24)

Lu, X.; Lan, T.; Qin, Z.; Wang, Z.; Zhou, G. A near-infrared dithieno[2,3-a:3’,2’-

c]phenazine-based organic co-sensitizer for highly efficient and stable quasi-solid-state dyesensitized solar cells. ACS Appl. Mater. Interfaces 2014, 6, 19308−19317. (25)

Zhang, Y.; Bao, X.; Xiao, M.; Tan, H.; Tao, Q.; Wang, Y.; Liu, Y.; Yang, R.; Zhu, W.

Significantly improved photovoltaic performance of the triangular-spiral TPA(DPP-PN)3 by appending planar phenanthrene units into the molecular terminals. J. Mater. Chem. A 2015, 3, 886−893. (26)

Tao, Q.; Xia, Y.; Xu, X.; Hedström, S.; Bäcke, O.; James, D. I.; Persson, P.; Olsson, E.;

Inganäs, O.; Hou, L.; Zhu, W.; Wang, E. D-A1-D-A2 copolymers with extended donor segments for efficient polymer solar cells. Macromolecules 2015, 48, 1009−1016. (27)

Tan, W.; Wang, R.; Li, M.; Liu, G.; Chen, P.; Li, X.; Lu, S.; Zhu, H. L.; Peng, Q.; Zhu,

X.; Chen, W.; Choy, W. C. H.; Li, F.; Peng, J.; Cao, Y. Lending triarylphosphine oxide to phenanthroline: A facile approach to high-performance organic small-molecule cathode interfacial material for organic photovoltaics utilizing air-stable cathodes. Adv. Funct. Mater. 2014, 24, 6540−6547. (28)

Maeda, T.; Arikawa, S.; Nakao, H.; Yagi, S.; Nakazumi H. Linearly π-extended squaraine

dyes enable the spectral response of dye-sensitized solar cells in the NIR region over 800 nm. New J. Chem. 2013, 37, 701−708. (29)

Shi, J.; Chai, Z.; Tang, R.; Hua, J.; Li, Q.; Li, Z. New triphenylamine-based sensitizers

bearing double anchor units for dye-sensitized solar cells. Sci. China Chem. 2015, 58,



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 29

1144−1151. (30)

Cheng, M.; Yang, X.; Li, J.; Zhang, F.; Sun, L. Co-sensitization of organic dyes for

efficient dye-sensitized solar cells. ChemSusChem 2013, 6, 70−77. (31)

Ganesan, P.; Yella, A.; Holcombe, T. W.; Gao, P.; Rajalingam, R.; Al-Muhtaseb, S. A.;

Grätzel, M.; Nazeeruddin, M. K. Unravel the impact of anchoring groups on the photovoltaic performances of diketopyrrolopyrrole sensitizers for dye-sensitized solar cells. ACS Sustainable Chem. Eng. 2015, 3, 2389−2396. (32)

Qu, S.; Wu, W.; Hua, J.; Kong, C.; Long, Y.; Tian, H. New diketopyrrolopyrrole (DPP)

dyes for efficient dye-sensitized solar cells. J. Phys. Chem. C 2010, 114, 1343−1349. (33)

Gao, P.; Tsao, H. N.; Yi, C.; Grätzel, M.; Nazeeruddin, M. K. Extended π-bridge in

organic dye-sensitized solar cells: The longer, the better? Adv. Energy Mater. 2014, 4, 1301485. (34)

Han, L.; Islam, A.; Chen, H.; Malapaka, C.; Chiranjeevi, B.; Zhang, S.; Yang, X.;

Yanagida, M. High-efficiency dye-sensitized solar cell with a novel co-adsorbent. Energy Environ. Sci. 2012, 5, 6057−6060. (35)

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 percent efficiency. Science 2011, 334, 629−634. (36)

Choi, H.; Kim, S.; Kang, S. O.; Ko, J.; Kang, M.; Clifford, J. N.; Forneli, A.; Palomares,

E.; Nazeeruddin, M. K.; Grätzel, M. Stepwise cosensitization of nanocrystalline TiO2 films utilizing Al2O3 layers in dye-sensitized solar cells. Angew. Chem., Int. Ed. 2008, 47, 8259−8263. (37)

Wang, Y.; Chen, B.; Wu, W.; Li, X.; Zhu, W. H.; Tian, H.; Xie, Y. Efficient solar cells

sensitized by porphyrins with an extended conjugation framework and a carbazole donor: From molecular design to cosensitization. Angew. Chem., Int. Ed. 2014, 53, 10779−10783.


Page 25 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(38)

Wang, C.; Shiu, J.; Hsiao, Y.; Chao, P.; Wei-Guang Diau, E.; Lin, C. Co-sensitization of

zinc and free-base porphyrins with an organic dye for efficient dye-sensitized solar cells. J. Phys. Chem. C 2014, 118, 27801−27807. (39)

Xie, Y.; Tang, Y.; Wu, W.; Wang, Y.; Liu, J.; Li, X.; Tian, H.; Zhu, W. H. Porphyrin

cosensitization for a photovoltaic efficiency of 11.5%: A record for non-ruthenium solar cells based on iodine electrolyte. J. Am. Chem. Soc. 2015, 137, 14055−14058. (40)

Wu, H.; Ou, Z.; Pan, T.; Lan, C.; Huang, W.; Lee, H.; Reddy, N. M.; Chen, C.; Chao, W.;

Yeh, C.; Diau, E. W. Molecular engineering of cocktail co-sensitization for efficient panchromatic porphyrin-sensitized solar cells. Energy Environ. Sci. 2012, 5, 9843−9848. (41)

Kuang, D.; Walter, P.; Nüesch, F.; Kim, S.; Ko, J.; Comte, P.; Zakeeruddin, S. M.;

Nazeeruddin, M. K.; Grätzel, M. Co-sensitization of organic dyes for efficient ionic liquid electrolyte-based dye-sensitized solar cells. Langmuir 2007, 23, 10906−10909. (42)

Yum, J.; Jang, S.; Walter, P.; Geiger, T.; Nuesch, F.; Kim, S.; Ko, J.; Grätzel, M.;

Nazeeruddin, M. K. Efficient co-sensitization of nanocrystalline TiO2 films by organic sensitizers. Chem. Commun. 2007, 4680−4682. (43)

Lin, R. Y.; Yen, Y.; Cheng, Y.; Lee, C.; Hsu, Y.; Chou, H.; Hsu, C.; Chen, Y.; Lin, J. T.;

Ho, K.; Tsai, C. Dihydrophenanthrene-based metal-free dyes for highly efficient cosensitized solar cells. Org. Lett. 2012, 14, 3612−3615. (44)

Delcamp, J. H.; Shi, Y.; Yum, J.; Sajoto, T.; Dell'Orto, E.; Barlow, S.; Nazeeruddin, M.

K.; Marder, S. R.; Grätzel, M. The role of π bridges in high-efficiency DSCs based on unsymmetrical squaraines. Chem.−Eur. J. 2013, 19, 1819−1827. (45)

Chang, J.; Lee, C.; Kumar, D.; Chen, P.; Lin, L.; Thomas, K. R. J.; Ho, K. Co-

sensitization promoted light harvesting for organic dye-sensitized solar cells using



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 29

unsymmetrical squaraine dye and novel pyrenoimidazole-based dye. J. Power Sources 2013, 240, 779−785. (46)

Lin, L.; Yeh, M.; Lee, C.; Chang, J.; Baheti, A.; Vittal, R.; Justin Thomas, K. R.; Ho, K.

Insights into the co-sensitizer adsorption kinetics for complementary organic dye-sensitized solar cells. J. Power Sources 2014, 247, 906−914. (47)

Hua, Y.; Lin Lee, L. T.; Zhang, C.; Zhao, J.; Chen, T.; Wong, W.; Wong, W.; Zhu, X. Co-

sensitization of 3D bulky phenothiazine-cored photosensitizers with planar squaraine dyes for efficient dye-sensitized solar cells. J. Mater. Chem. A 2015, 3, 13848−13855. (48)

Zhu, W-H.; Wu, Y.; Wang, S.; Li, W.; Li, X.; Chen, J.; Wang, Z.; Tian, H. Organic D-A-π-

A solar cell sensitizers with improved stability and spectral response. Adv. Funct. Mater. 2011, 21, 756−763. (49)

Park, J.; Barolo, C.; Sauvage, F.; Barbero, N.; Benzi, C.; Quagliotto, P.; Coluccia, S.; Di

Censo, D.; Grätzel, M.; Nazeeruddin, M. K.; Viscardi, G. Symmetric vs. asymmetric squaraines as photosensitisers in mesoscopic injection solar cells: A structure-property relationship study. Chem. Commun. 2012, 48, 2782−2784. (50)

Qin, C.; Numata, Y.; Zhang, S.; Yang, X.; Islam, A.; Zhang, K.; Chen, H.; Han, L. Novel

near-infrared squaraine sensitizers for stable and efficient dye-sensitized solar cells. Adv. Funct. Mater. 2014, 24, 3059−3066. (51)

Yang, J.; Ganesan, P.; Teuscher, J.; Moehl, T.; Kim, Y. J.; Yi, C.; Comte, P.; Pei, K.;

Holcombe, T. W.; Nazeeruddin, M. K.; Hua, J.; Zakeeruddin, S. M.; Tian, H.; Grätzel, M. Influence of the donor size in D-π-A organic dyes for dye-sensitized solar cells. J. Am. Chem. Soc. 2014, 136, 5722−5730. (52)

Xie, Y.; Wu, W.; Zhu, H.; Liu, J.; Zhang, W.; Tian, H.; Zhu, W-H. Unprecedentedly


Page 27 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


targeted customization of molecular energy levels with auxiliary-groups in organic solar cell sensitizers. Chem. Sci. 2016, 7, 544-549. (53)

Daeneke, T.; Mozer, A. J.; Uemura, Y.; Makuta, S.; Fekete, M.; Tachibana, Y.; Koumura,

N.; Bach, U.; Spiccia, L. Dye regeneration kinetics in dye-sensitized solar cells. J. Am. Chem. Soc. 2012, 134, 16925−16928. (54)

Zhang, W.; Wu, Y.; Zhu, H.; Chai, Q.; Liu, J.; Li, H.; Song, X.; Zhu, W-H. Rational

molecular engineering of indoline-based D-A-π-A organic sensitizers for long-wavelengthresponsive dye-sensitized solar cells. ACS Appl. Mater. Interfaces 2015, 7, 26802−26810. (55)

Chen, B.; Chen, D.; Chen, C.; Hsu, C.; Hsu, H.; Wu, K.; Liu, S.; Chou, P.; Chi, Y.

Donor-acceptor dyes with fluorine substituted phenylene spacer for dye-sensitized solar cells. J. Mater. Chem. 2011, 21, 1937−1945. (56)

Koide, N.; Islam, A.; Chiba, Y.; Han, L. Improvement of efficiency of dye-sensitized

solar cells based on analysis of equivalent circuit. J. Photochem. Photobiol., A 2006, 182, 296−305. (57)

van de Lagemaat, J.; Park, N. G.; Frank, A. J. Influence of electrical potential

distribution, charge transport, and recombination on the photopotential and photocurrent conversion efficiency of dye-sensitized nanocrystalline TiO2 solar cells: A study by electrical impedance and optical modulation techniques. J. Phys. Chem. B 2000, 104, 2044−2052. (58)

Fan, S. H.; Lu, X. F.; Sun, H.; Zhou, G.; Chang, Y. J.; Wang, Z. S. Effect of the co-

sensitization sequence on the performance of dye-sensitized solar cells with porphyrin and organic dyes. Phys. Chem. Chem. Phys. 2016, 18, 932−938.



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Table of Contents


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For Table of Contents Use Only Novel Squaraine Cosensitization System of Panchromatic Light-Harvesting with Synergistic Effect for Highly Efficient Solar Cells Weiwei Zhang,† Wenqin Li,‡ Yongzhen Wu,† Jingchuan Liu,† Xiongrong Song,† He Tian,† and WeiHong Zhu*,† Cosensitization is a convenient shortcut to construct panchromatic solar cells. Herein the impressively high efficiencies and panchromatic IPCE are successfully achieved by two different squaraine cosensitization couples.

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