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Resonant Multiple Light Scattering for Enhanced Photon Harvesting in Dye-Sensitized Solar Cells Jihun Kim, Horim Lee, Dong Young Kim, and Yongsok Seo* The scattering property of the electrode film is quite important for the improvement of the light harvesting efficiency of the dye sensitized solar cells (DSSCs). The incident photon-tocurrent conversion efficiency was drastically improved (more than 30%) by the harmonization of the resonant multiple light scattering in the photoelectrode of the hierarchically structured TiO2 aggregates (an average size of 660 nm) with the photon absorption of the novel metal-free organic dye in the visible region. The DSSC yielded an unprecedented high photocurrent density of 20.9 mA cm−2, reaching the efficiency higher than 9% under 100 mW cm−2, AM 1.5 global illumination without an antireflection layer. Using the novel metal-free organic dye, the device also displayed very good long-term stability when aged without a UV filter. Highly efficient dye sensitized solar cells (DSSCs) based on organic optoelectronic materials have attracted significant attention over the past two decades for their promise as economic alternatives to conventional solar cells based on silicon.[1–3] Sensitizers for DSSCs can be classified into two groups: ruthenium(II)-polypyridyl complexes and metal-free organic donor-acceptor dyes.[4–17] The current state-of-the-art DSSCs include ruthenium(II)-polypyridyl complexes as the additive material, and display an overall power conversion efficiency (η) exceeding 11% with an antireflection layer under simulated AM 1.5G irradiation; metal-to-ligand charge transfer (MLCT) plays an important role in the light-harvesting process in these devices.[5,6] Although Ru-complexes are the most efficient sensitizers known so far, their use in large-scale applications is likely to be limited because ruthenium metal is scarce and expensive, and careful, non-ecofriendly and tricky purification steps are required in their syntheses.[7] Thus, researchers have examined metal-free organic dyes to take advantage of their simple synthesis, tunable absorption spectrum, high molar extinction coefficients, and long-term stability.[4] Our approach to increasing the photon conversion efficiency is to improve the light-harvesting capability of a photoelectrode film by utilizing optical enhancement effects,
J. Kim, H. Lee, Prof. Y. Seo Intellectual Textile System Research Center (ITRC) and RIAM School of Materials Science and Engineering College of Engineering Seoul National University Daehakro1, Kwanakgu, Seoul 151–744, Republic of Korea E-mail: [email protected] Dr. D. Y. Kim Optoelectronic Materials Laboratory Korea Institute of Science and Technology (KIST) Hwarangro 14–1, Sungbukku, Seoul 136–791, Republic of Korea
DOI: 10.1002/adma.201400124
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which can be achieved by harmonizing the light scattering in a hierarchically structured TiO2 (HS-TiO2) electrode with the photon absorption spectrum of the dye. Resonant scattering inside a HS-TiO2 electrode can occur if the HS-TiO2 size is comparable to or greater than the wave length of the incident light.[8–10] Since the scattering can extend the photon path length within an electrode film and attenuate the light transmittance, the probability that photons interact with the dye molecules can be further increased.[3] Though both the use of hierarchically structured TiO2 or ZnO as a means to improve light harvesting and the use of molecularly engineered organic dyes to improve light-resistance, light harvesting and charge separation have been reported and demonstrated before,[8,9,18] we believe that the present work may be the first to rationally develop a solar cell that exploits both effects. In this study, we designed a novel metal-free dye with a broad and high intensity absorption spectrum that was harmonized with HS-TiO2 aggregate size to achieve resonant scattering. Also the efficiency and stability of DSSCs based on the novel dye were investigated and compared with the respective properties of DSSCs based on the popular ruthenium(II)-polypyridine dye, N719 (cis-bis(isothiocyanato)-bis(2,2′-bipyridyl-4,4′dicarboxylate) ruthenium (II) bis-tetra-n-butyl ammonium). Although many chromophores have been used for the development of organic dyes suitable for DSSCs, their stability is rarely reported. One empirical but sound design principle for metal-free dyes is to synthesize an electron donor-π-conjugated linker-electron acceptor (D-π-A) structure with an attached group for anchoring to the TiO2 anode which exhibits charge migration from the donor to the acceptor through the π-bridge.[3,11–13] Charge transfer transitions of organic dyes generally have a higher molar extinction coefficient in the visible region than the metalligand charge transition of ruthenium complexes.[6,14] Hence, organic dyes facilitate ultrafast interfacial injection from the excited dye molecules to the TiO2 conduction band and slow the recombination of the injected electrons in TiO2 via the oxidized molecules.[15] Following the sensitizer design principle, we synthesized a novel D-π-A dye. Scheme 1 shows the molecular structures of the synthesized organic dye, 3-(5-(5-(5-(4-(diphenylamino)phenyl)-4-hexylthiophene-2-yl)thieno[3,2-b]thiophene-2-yl)-3-hexylthiophene-2-yl)-2-cyanoacetic acid (denoted as JH-1) and the structure of N719 (see Scheme 2 and SI 1 for synthesis details). The triphenyl amine functions as a strong electron donor group in the push-pull structure that stabilizes the resonance of the oxidized dyes, promoting rapid dye regeneration due to its spatial configuration.[19–22] The π-conjugated linker is composed of 3-hexyl thiophenes and a planar fused thiophene ring that increases the conjugation length to broaden the absorption spectrum and the molar extinction coefficient.
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π−π* transitions of the conjugated molecule. The molar extinction coefficient (ε) of JH-1 was 50 000 M−1 cm−1 at 450 nm in ethanol, nearly three times the value of N719 (ε = 15 000 M−1 cm−1) at 550 nm. A high molar extinction coefficient is highly desirable as it enables the use of thinner nanocrystallite TiO2 layers, thereby facilitating charge collection while maintaining the light-harvesting capabilities and photocurrent generation in the DSSCs.[20] Decreasing the film Scheme 1. Chemical structures of (a) the synthesized dye (JH-1) and (b) N719. thickness also augments the open-circuit photovoltage of the cell by reducing the dark current.[17] The maximum possible photocurrent is determined The long alkyl chains on the thiophene rings prevent charge recombination of the injected electrons or dye aggregation.[15] by the overlap of two spectra, the absorption spectrum of the sensitized TiO2 layer and the solar flux spectrum.[3] Although Most DSSC photoelectrodes use nanocrystalline (nc-) TiO2 particles because small nc-TiO2 particles enhance dye adsorpthe organic dye is highly efficient for converting absorbed photons into free electrons in the TiO2, only photons absorbed tion and exciton generation due to their large surface areas; however, small particles do not always provide the best perforby the dye ultimately produce the current. Typically used dye mance because they increase the number of grain boundaries molecules generally have poorer absorption in the red part of and defects on the particle surfaces, which can delay electron the spectrum than the silicon; hence fewer of the photons in transport, as revealed by the electron diffusion coefficient sunlight are usable for current generation by a DSSC.[25] Inter[ 8–10 ] measurements (see Figure 3(B) below). estingly, HS-TiO2 photoelectrode sensitized by JH-1 shows Typically, polydisperse aggregates increase the irregularities in the film assembly more broad absorption than nc-TiO2 photoelectrode because of resulting in light scattering over a broad spectral range.[8] The the additional light scattering occurring in HS-TiO2 photoelecHS-TiO2 electrode was prepared by electro-spraying nc-TiO2 trode. More details are discussed later. particles, 21 nm in diameter, which formed larger filled-spheres The electrochemical properties of JH-1 are summarized in of less grain boundaries rather than hollow spheres due to the Table 1. The electron-rich tri-phenyl amine moiety lifted the ultrafast evaporation of the solvent at the surfaces of the fine HOMO level whereas the thienothiophene and thiophene units solution droplets containing nc-TiO2 particles.[8] Figure 1 shows conjugated with the acceptor unit adjusted the LUMO level to a suitable value. The HOMO level of the dye (−5.54 eV) was a schematic figure of the resonant light scattering process and sufficiently more negative than the iodine/tri-iodide redox an SEM image of the HS-TiO2 electrode consisting of polydispotential (−4.9 eV, corresponding to 0.44 eV vs NHE (the perse aggregates with an average size of 660 nm which scatters normal hydrogen electrode)),[26] leading to rapid dye regeneramost of the light in the visible range and can potentially lead to light trap formation for optical confinement (see SI 4 for partion and limiting charge recombination between the oxidized ticle size distribution and pore size distribution in the HS-TiO2 dye molecules and the electrons photoinjected into the titania film. The oxidized dyes formed after electron injection into electrode).[23,24] the conduction band of TiO2 could thermodynamically accept Figure 2(A) shows the absorption spectra of JH-1 and N719 in ethanol or attached to different TiO2 electrodes. The electrons from I− to provide a driving force sufficient for effiabsorption spectra of JH-1 included strong peaks at 450 nm cient regeneration of the neutral sensitizer state.[17] The LUMO in ethanol or at 455 nm on the HS-TiO2 electrode due to the level of the novel dye (−3.21 eV) was sufficiently more positive than the conduction band energy level of TiO2 (−4.01 eV), thereby facilitating efficient charge injection from the dye in the excited state to the conduction band of TiO2 and regeneration of the oxidized dye. Figure 2(B) and 2(C) show the absolute and normalized incident photon-to-current conversion efficiency (IPCE) spectra of JH-1 and N719. The IPCE spectra were measured under white bias light and in AC mode with the frequency of 4 Hz.[27,28] The spectra covered the whole visible region over the range 300 – 800 nm. For nc-TiO2 electrode, JH-1 based DSSC shows a maximum absorption peak of 74.8% at 460 nm while that for HS-TiO2 electrode was 93% at 460 nm. On the other hand, N719 based DSSC with nc-TiO2 electrode had a maximum Scheme 2. Synthesis route of JH-1.
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because of the large absorption coefficient. The enhanced light harvesting characteristics of HS-TiO2 are evident if the IPCE is normalized (Figure 2(C)). The IPCE spectra of HS-TiO2 with both JH-1 and N719 based cells show more broad absorption over all wavelength region. This result indicates that polydisperse HS-TiO2 aggregates induce additional light scattering over wide wavelength range of incident photon, which is consistent with UV absorption spectra Figure 1. A schematic figure of the resonant multiple light scattering and SEM image of a TiO2 in Figure 2(A). In our previous study, we reported that HS-TiO2 DSSCs showed electrode made of polydisperse aggregates (HS-TiO2). higher IPCE values than that of nc-TiO2 DSSCs (Table 2), especially in the long wave length region, though they had almost the same amount of absorption peak of 75% at 550 nm and 82% at 550 nm with adsorbed dye.[8] This means that multiple reflections of phoHS-TiO2 electrode. This high and broad IPCE value of the JH-1 sensitized DSSC in the visible region was attributed tons in HS-TiO2 electrodes of JH-1 based DSSC results in to the increased conjugation length and the segment plaincreased light absorption to produce higher short-circuit narity resulting from the π-conjugated linker composed of photocurrent density as shown in Table 2. Figure 2(D) compares the photovoltaic characteristics of the JH-1 and N719 thiophenes and fused thiophene ring.[23,29] In the long wave DSSCs based on HS-TiO2 electrodes. The measured openlength region (>650 nm, low energy wavelength region), N719 based DSSCs show higher IPCE values than JH-1 based ones circuit photovoltage (Voc), short-circuit photocurrent density due to the metal-to-ligand charge transfer (MLCT) transition. (Jsc), fill factor (FF), and solar-to-electric conversion efficiency On the low energy side in the long wavelength, a significant (η) of these solar cells are listed in Table 2. Because of its part of the incident radiation penetrates the layer due to the higher molar extinction coefficient, the increase of JSC (and low absorption coefficient of N719 dye, while photons in the η) in JH-1 sensititized DSSC with HS-TiO2 electrode is larger visible region can be easily absorbed by JH-1based DSSC than that of N719 with HS-TiO2.
Figure 2. (A) Absorption spectra of the JH-1 and N719 dyes in ethanol (gray), on the nc-TiO2 films (dashed), and on the HS-TiO2 films (solid). The spectra in ethanol were extinction coefficient values downsized to fit in the graph whereas those on the TiO2 films were normalized to the maxima of JH-1 for comparison. (B) Absorption spectra of JH-1 (black) and N719 (gray) based DSSCs with the HS-TiO2 electrodes (solid) or the nc-TiO2 electrodes (dashed). (C) Normalized IPCE spectra of the DSSCs: JH-1-based DSSC (black) and N719-based DSSC (gray) with the HS-TiO2 electrodes (solid) or the nc-TiO2 electrodes (dashed). (D) I-V curves of the JH-1-(solid) and N719-(dashed) based solar cells with HS-TiO2 photoelectrodes. The TiO2 layer thickness was 12 µm.
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Dye
λmax/nm
εmax/M−1 cm−1
JH-1
450
50 000
−5.54
−3.21
2.33
15 000
−5.6
−3.0
2.6c)
N719
550
EHOMO/eVa) ELUMO/eV EHOMO-LUMO gap/eVb)
c)
a)Evaluated
by cyclic voltammetry (see SI 3); absorption spectrum; c)See ref. [11].
c)
b)Evaluated
from the onset of the
Table 2. Photovoltaic parameters of the DSSCs based on the JH-1 and N719 dyes (all TiO2 electrodes were 12 µm thick). Dye JH-1
N719
Jsc/mA cm−2
Electrode
Voc/V
FF (%)
η (%)
HS-TiO2
20.9
0.687
63.9
9.18
HS-TiO2 (Masked)
15.1 (14.9 (integrated))
0.686
67.6
7.01
nc-TiO2
14.5
0.680
65.2
6.40
nc-TiO2 (Masked)
11.8 (11.4 (integrated))
0.684
68.6
5.56
HS-TiO2
19.5
0.767
65.4
9.77
HS-TiO2 (Masked)
15.1 (14.9 (integrated))
0.769
68.0
7.91
nc-TiO2
16.1
0.773
66.8
8.30
nc-TiO2 (Masked)
13.4 (12.9 (integrated))
0.768
68.2
7.03
Intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) were used to gain an insight into the charge transport characteristics in the TiO2 layer and charge recombination at the DSSC's titania/electrolyte interface. Figure 3 shows plots of the recombination lifetimes and electron diffusion coefficients as a function of the incident photon flux (light intensity). For a given TiO2 electrode of 12 µm thick, which was the optimum thickness of JH-1 with HS-TiO2 photoelectrode consistent with diffusion length calculated by IMPS and IMVS measurement (see SI 5 for the optimum electrode thickness), recombination was 7 to 10 times slower in the N719 cell than in the JH-1 based cell, indicating that the back-reaction rate of the N719 cell was slower; hence the charge collection efficiency of the N719 cell was higher. The shorter recombination lifetime of JH-1 would, therefore, reduce VOC in the JH-1 sensitized photoelectrode.[30] The measured
recombination lifetimes were in accords with the VOC values in Table 2. The accuracy of the measurement can be checked by comparison of measured Jsc values with those from calculated ones (Jsc = ∫ q F(λ) IPCE(λ)dλ where q is electron charge and F(λ) is the incident monochromatic photon flux density of AM 1.5 global solar spectrum).[3,28] In Table 2, all masked samples show good agreement with those calculated values within experimental error range. Voc values are not much different from each other while ff values were slightly varied due to the I-V curve shape change. On the other hand, large increase of Jsc values was observed when the masks were removed. For nc-TiO2 electrode DSSC which did not scatter lights due to its small size (∼21 nm), the increase of Jsc for unmasked ones was the same for JH-1 and N719 based DSSCs (2.7 mA cm−2) because this was due to lateral scattering.[2] For nc-TiO2 electrode DSSCs, N719 sensitizer DSSC shows a little bit larger Jsc than that of JH-1 dye DSSC because of higher IPCE values of N719 sensitizer DSSC at the wavelength longer than 500 nm. For masked HS-TiO2 electrode DSSCs, both (JH-1 and N719 based) DSSCs show Jsc increase compared to masked nc-TiO2 electrode DSSCs (3.3 mA cm−2, 28% increase for JH-1 based DSSC and 1.7 mA cm−2, 13% increase for N719 based DSC). This increase is attributed to photon scattering effect; HS-TiO2 particles scatter photons due to its size polydispersity, increasing the photon pass length and absorption by sensitizers. DSSCs with unmasked HS-TiO2 electrodes show 6.4 mA cm−2 (44% increase) and 3.4 mA cm−2 (21% increase) increase of Jsc for JH-1 and N719 based cells respectively, compared to those with unmasked nc-TiO2 electrode DSSCs. JH-1 based DSSC shows much higher Jsc value than that of N719 based DSSC due to both higher molar extinction coefficient and resonant photon scattering in visible region as mentioned before. All sensitized DSSCs with the HS-TiO2 electrodes yielded higher photocurrent densities and better efficiencies compared to DSSCs using the electrodes of nc-TiO2 particles of ∼21 nm size. Though N719 sensitizer shows higher IPCE in long wave length (>700 nm) (Figure 2(B) and (C)), its effect was not that great because of weak light energy in that frequency range. When the mask was removed for HS-TiO2 electrode DSSCs, JH-1 based DSSC showed surprisingly high Jsc value
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Table 1. Absorption spectrum and electrochemical properties of JH-1 and N719
Figure 3. Plots of the electron recombination lifetime (A) and the electron diffusion coefficient (B). The nc-TiO2 electrode was composed of ∼21 nm particles whereas the HS-TiO2 electrode was composed of the aggregates of the same nc-TiO2 particles with an average diameter of 660 nm. The lines are guides for the eyes.
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of 20.9 mA cm−2 while that of the N719 based solar cell was 19.5 mA cm−2. To the best of our knowledge, this is the highest photocurrent density reported thus far for metal-free dye sensitized DSSCs. The electron diffusion coefficients (D) and the recombination life times (τ) were used to calculate the relative electron diffusion lengths, Ln ≈ (Dτ)1/2, of the DSSCs.[17] The diffusion length is the average distance an injected electron travels through the conduction band of TiO2 before recombination. A shorter electron diffusion length (Ln) in the JH-1 based device (∼11 µm) compared to a N719 based device (∼21 µm) indicates that an electron injected into JH-1 sensitized photoelectrode travels a much shorter distance than in a N719 sensitized photoelectrode. The optimum thickness of JH-1 with HS-TiO2 photoelectrode (∼12 µm) is consistent with diffusion length calculated by IMPS and IMVS measurement (see SI 5). Despite the shorter electron diffusion length (lower charge collection efficiency) due to the faster recombination reaction, a higher photocurrent density was measured in the JH-1-based DSSC. The high photocurrent density of JH-1 based DSSC with HS-TiO2 electrode was not only because of the JH-1 dye characteristics including the high molar extinction coefficient, appropriate energy level matching between the LUMO of the dye and the conduction band of TiO2[23] and the HS-TiO2 characteristics including faster electron diffusion properties than nc-TiO2, but also to the resonant multiple light scattering from the HS-TiO2 electrode. As a DSSC sensitized by a metal-free organic dye, the conversion efficiency of the JH-1 dye (9.18%) using HS-TiO2 electrode without an antireflection layer was quite promising. Under the same conditions, the N719 based solar cell yielded a conversion efficiency of 9.77%. Therefore, it is clear that the
harmonization of the resonant multiple scattering by tuning the size of TiO2 aggregates and the absorption spectrum of the dye is a very effective approach to achieving a high photocurrent density and thus, high energy conversion efficiency. Although enhanced electron diffusion in HS-TiO2 partly contributed to the increase of photocurrent density in much the same way as the lateral scattering also helped the photon harvesting, they equally contribute in the N719 based solar cells.[31] The long-term stability of any new photovoltaic technology is just as important as the photon conversion efficiency for practical large scale applications.[16,17] We submitted our cells without a UV cut-off filter to 800 h of accelerated testing at 60°C under a light intensity of 100 mW cm−2. The JH-1 DSSC exhibited very good properties over 400 h, with weak attenuation of the device upon further illumination (Figure 4) due to the loss of sensitizer and/or penetration of I3− into the TiO2 surface.[16] The hydrophobic alkyl moieties of the JH-1 dye also contributed to the high stability.[32] After 800 h, the JH-1 sensitized DSSC exhibited an efficiency of 7.44% (81% of its initial efficiency) whereas that using N719 dye was 5.27% (54% of its initial efficiency). This result is quite encouraging as it brings the JH-1 based DSSC performance within the current standards required for practical applications. In summary, we have set a new bench mark for DSSC performances by fabricating a very efficient device with a high photocurrent density and good stability. This bench mark was accomplished by harmonizing the absorption spectrum of the dye with the average size of the aggregates in the TiO2 electrode. The resultant resonant multiple scattering enhanced the light harvesting efficiency and charge collection yield. The high
Figure 4. Evolution of the photovoltaic parameters for the JH-1 based HS-TiO2 DSSC (solid) and the N719 based HS-TiO2 DSSC (dashed) aged under illumination of 100 mW cm−2 at 60°C without a UV filter.
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Experimental Section Materials: The synthetic route for JH-1 is outlined in Scheme 2 and the details are described in SI 1. Device Fabrication: The photoanodes composed of hierarchically structured TiO2 (HS-TiO2) nanospheres and nanocrystalline TiO2 particles (nc-TiO2, P25 powder from Degussa) were prepared using the electro-spraying method.[8] Details of device fabrication are described in SI 2. Other experimental measurements are fully explained in SI 3 (UV, cyclic voltammetry, and SEM).
Supporting Information Supporting Information for details of dye synthesis and device fabrication as well as other experimental measurements is available from the Wiley Online Library or from the author.
Acknowledgements This study was supported by the Korea National Research Foundation through the RIAM [Basic Research Program (RIAM 0427–20100020)], the ITRC [Basic Science Research Program (R11–2005–065)], by the Ministry of Knowledge and Economy [Original Material Technology Program (RIAM-AC14–10) and Fundamental R&D Program for Core Technology of Materials (RIAM 0427–20100043)], and by KIST (KIST Institutional Programs (2E24610)). The photograph in the artwork by Youngwook P. Seo is greatly acknowledged. Received: January 9, 2014 Revised: April 15, 2014 Published online: May 28, 2014 [1] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Patterson, Chem. Rev. 2010, 110, 6595. [2] A. Mishra, M. K. R. Fischer, P. Bäuerle, Angew. Chem. Int. Ed. 2009, 48, 2474. [3] K. Hara, H. Arakawa, Chapter 15 in Handbook of Photovoltaic Science and Engineering (A. Luque, S. Hegedus (Eds)) John Wiley and Sons Ltd. UK 2003. [4] J. Yum, J, M. K. Nazeeruddin, Chapter 3 in Dye-Sensitized Solar Cells (K. Kalyanasudaram (Ed.)), EPFL Press, Lausanne 2010. [5] M. Grätzel, J. Photochem. Photobiol. A. 2004, 164, 3. [6] F. Gao, Y. Wang, D. Shi, J. Zhang, M. Wang, X. Jing, R. Humphry-Baker, P. Wang, S. Zakeeruddin, M. Grätzel, J. Am. Chem. Soc. 2008, 130, 10720.
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[7] G. Zhang, H. Bala, Y. Cheng, D. Shi, X. Lv, Q. Yu, P. Wang, Chem. Commun. 2009, 16, 2198. [8] D. Hwang, H. Lee, S.-Y. Jang, S. M. Jo, D. Kim, Y. Seo, D. Y. Kim, ACS. Appl. Mater. Interfaces. 2011, 3, 2719. [9] a) Q. Zhang, T. P. Chou, B. Russo, S. A. Jenekhe, G. Cao, Adv. Funct. Mater. 2008, 18, 1654; b) Q. Zhang, G. Cao, Nano Today. 2011, 6, 91; c) T. P. Chou, Q. Zhang, G. E. Fryxell, G. Cao, Adv. Mater. 2007, 19, 2588. [10] F. Huang, D. Chen, X. L. Zhang, R. A. Caruso, Y.-B. Cheng, Adv. Funct. Mater. 2010, 20, 1301. [11] S. Hwang, J. H. Lee, C. Park, H. Lee, C. Kim, C. Park, M.-H. Lee, W. Lee, J. Park, K. Kim, N.-G. Park, C. Kim, Chem. Commun. 2007, 21, 4887. [12] H. Tian, X. Yang, J. Pan, R. Chen, M. Liu, Q. Zhang, A. Hagfeldt, L. Sun, Adv. Funct. Mater. 2008, 18, 3461. [13] T.-H. Kwon, V. Armel, A. Nattestad, D. R. MacFarlane, U. Bach, S. J. Lind, K. C. Gordon, W. Tang, D. J. Jones, A. B. Holmes, J. Org. Chem. 2011, 76, 4088. [14] R. Argazzi, C. A. Bignozzi, J. Phys.Chem. B. 1997, 101, 2591. [15] G. Li, Y.-F. Zhou, X.-B. Cao, P. Bao, K.-J. Jiang, Y. Lin, L.-M. Yang, Chem. Commun. 2009, 16, 2201. [16] K. Jiang, S. Yanagida, Chapter 4 in Dye-Sensitized Solar Cells (K. Kalyanasudaram (Ed.)), EPFL Press, Lausanne 2010. [17] D. Shi, N. Pootrakulchote, R. Li, J. Guo, Y. Wang, S. M. Zakeeruddin, M. Grätzel, P. Wang, J. Phys. Chem. C. Lett. 2008, 112, 17046. [18] H. M. Ko, H. Choi, S. Paek, K. Kim, K. Song, J. K. Lee, J. Ko, J. Mater. Chem. 2011, 21, 7248. [19] S. G. Esteban, P. Cruz, A. Aljarilla, L. M. Arellano, F. Langa, Org. Lett. 2011, 13, 5362. [20] J. H. Yum, D. P. Hagberg, S. J. Moon, K. M. Karlsson, T. Marinado, A. Hagfeldt, M. K. Nazeeruddin, M. Grätzel, Angew. Chem. Int. Ed. 2009, 48, 1576. [21] M. Liang, W. Xu, F. Cai, P. Chen, B. Peng, J. Chen, Z. Li, J. Phys. Chem. C. 2007, 111, 4465. [22] J. Xu, L. Zhu, D. Ffang, B. Chen, L. Liu, L. Wang, W. Xu, ChemPhysChem. 2012, 13, 3320. [23] Q. Zhang, T. P. Chou, B. Russo, S. A. Jenekhe, G. Cao, Angew. Chem. Int. Ed. 2008, 47, 2402. [24] H. Cao, J. Y. Xu, D. Z. Zhang, S.-H. Chang, S. T. Ho, E. W. Seelig, X. Liu, R. P. H. Chang, Phys. Rev. Lett. 2000, 84, 5584. [25] K. Sayama, K. Hara, N. Mori, M. Satsuki, S. Suga, S. Tsukagoshi, Y. Abe, H. Sugihara, H. Arakawa, Chem. Commun. 2000, 24, 1173. [26] X. Zhan, Z. Tan, E. Zhou, Y. Li, R. Misra, A. Grant, B. Domercq, X.-H. Zhang, Z. An, X. Zhang, S. Barlow, B. Kippelen, S. R. Marder, J. Mater. Chem. 2009, 19, 5794. [27] 2006 ASTM Standard E1021–06, Test method for spectral responsivity Measurements of Photovoltaic Devices (West Conschohocken, PA : ASTM International) Vol.12.02, doi :10.1520/E1021–06. [28] G. Xue, X. Yu, C. Bao, J. Zhang, H. Huang, Z. Tang, Z. Zou, J. Phys. D : Appl. Phys. 2012, 45, 425104. [29] A. Hagfeldt, M. Grätzel, Chem. Rev. 1995, 95, 49. [30] Q. Wang, J.-E. Moser, M. Grätzel, J. Phys. Chem. B. 2005, 109, 14945. [31] F. Sauvage, D. Chen, P. Comte, F. Huang, L.-P. Heiniger, Y.-B. Cheng, R. A. Caruso, M. Grätzel, ACS Nano 2010, 4, 4420. [32] P. Wang, S. M. Zakeeruddin, J. E. Moser, M. K. Nazeeruddin, T. Sekiguchi, M. Grätzel, Nature Mater. 2003, 2, 402.
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and robust photovoltaic performance (with an initial efficiency of 9.18% and an efficiency of 7.44% after 800 h of irradiation with a light intensity of 100 mW cm−2) of the JH-1 DSSC prepared without an antireflection layer demonstrated the promise of this novel sensitizer for large scale applications. Our result should stimulate further investigations into novel dyes with similar structures and we expect that our method may be readily applicable to practical DSSC systems.
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Resonant Multiple Light Scattering for Enhanced Photon Harvesting in Dye-Sensitized Solar Cells Jihun Kim, Horim Lee, Dong Young Kim, and Yongsok Seo* The scattering property of the electrode film is quite important for the improvement of the light harvesting efficiency of the dye sensitized solar cells (DSSCs). The incident photon-tocurrent conversion efficiency was drastically improved (more than 30%) by the harmonization of the resonant multiple light scattering in the photoelectrode of the hierarchically structured TiO2 aggregates (an average size of 660 nm) with the photon absorption of the novel metal-free organic dye in the visible region. The DSSC yielded an unprecedented high photocurrent density of 20.9 mA cm−2, reaching the efficiency higher than 9% under 100 mW cm−2, AM 1.5 global illumination without an antireflection layer. Using the novel metal-free organic dye, the device also displayed very good long-term stability when aged without a UV filter. Highly efficient dye sensitized solar cells (DSSCs) based on organic optoelectronic materials have attracted significant attention over the past two decades for their promise as economic alternatives to conventional solar cells based on silicon.[1–3] Sensitizers for DSSCs can be classified into two groups: ruthenium(II)-polypyridyl complexes and metal-free organic donor-acceptor dyes.[4–17] The current state-of-the-art DSSCs include ruthenium(II)-polypyridyl complexes as the additive material, and display an overall power conversion efficiency (η) exceeding 11% with an antireflection layer under simulated AM 1.5G irradiation; metal-to-ligand charge transfer (MLCT) plays an important role in the light-harvesting process in these devices.[5,6] Although Ru-complexes are the most efficient sensitizers known so far, their use in large-scale applications is likely to be limited because ruthenium metal is scarce and expensive, and careful, non-ecofriendly and tricky purification steps are required in their syntheses.[7] Thus, researchers have examined metal-free organic dyes to take advantage of their simple synthesis, tunable absorption spectrum, high molar extinction coefficients, and long-term stability.[4] Our approach to increasing the photon conversion efficiency is to improve the light-harvesting capability of a photoelectrode film by utilizing optical enhancement effects,
J. Kim, H. Lee, Prof. Y. Seo Intellectual Textile System Research Center (ITRC) and RIAM School of Materials Science and Engineering College of Engineering Seoul National University Daehakro1, Kwanakgu, Seoul 151–744, Republic of Korea E-mail: [email protected] Dr. D. Y. Kim Optoelectronic Materials Laboratory Korea Institute of Science and Technology (KIST) Hwarangro 14–1, Sungbukku, Seoul 136–791, Republic of Korea
DOI: 10.1002/adma.201400124
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which can be achieved by harmonizing the light scattering in a hierarchically structured TiO2 (HS-TiO2) electrode with the photon absorption spectrum of the dye. Resonant scattering inside a HS-TiO2 electrode can occur if the HS-TiO2 size is comparable to or greater than the wave length of the incident light.[8–10] Since the scattering can extend the photon path length within an electrode film and attenuate the light transmittance, the probability that photons interact with the dye molecules can be further increased.[3] Though both the use of hierarchically structured TiO2 or ZnO as a means to improve light harvesting and the use of molecularly engineered organic dyes to improve light-resistance, light harvesting and charge separation have been reported and demonstrated before,[8,9,18] we believe that the present work may be the first to rationally develop a solar cell that exploits both effects. In this study, we designed a novel metal-free dye with a broad and high intensity absorption spectrum that was harmonized with HS-TiO2 aggregate size to achieve resonant scattering. Also the efficiency and stability of DSSCs based on the novel dye were investigated and compared with the respective properties of DSSCs based on the popular ruthenium(II)-polypyridine dye, N719 (cis-bis(isothiocyanato)-bis(2,2′-bipyridyl-4,4′dicarboxylate) ruthenium (II) bis-tetra-n-butyl ammonium). Although many chromophores have been used for the development of organic dyes suitable for DSSCs, their stability is rarely reported. One empirical but sound design principle for metal-free dyes is to synthesize an electron donor-π-conjugated linker-electron acceptor (D-π-A) structure with an attached group for anchoring to the TiO2 anode which exhibits charge migration from the donor to the acceptor through the π-bridge.[3,11–13] Charge transfer transitions of organic dyes generally have a higher molar extinction coefficient in the visible region than the metalligand charge transition of ruthenium complexes.[6,14] Hence, organic dyes facilitate ultrafast interfacial injection from the excited dye molecules to the TiO2 conduction band and slow the recombination of the injected electrons in TiO2 via the oxidized molecules.[15] Following the sensitizer design principle, we synthesized a novel D-π-A dye. Scheme 1 shows the molecular structures of the synthesized organic dye, 3-(5-(5-(5-(4-(diphenylamino)phenyl)-4-hexylthiophene-2-yl)thieno[3,2-b]thiophene-2-yl)-3-hexylthiophene-2-yl)-2-cyanoacetic acid (denoted as JH-1) and the structure of N719 (see Scheme 2 and SI 1 for synthesis details). The triphenyl amine functions as a strong electron donor group in the push-pull structure that stabilizes the resonance of the oxidized dyes, promoting rapid dye regeneration due to its spatial configuration.[19–22] The π-conjugated linker is composed of 3-hexyl thiophenes and a planar fused thiophene ring that increases the conjugation length to broaden the absorption spectrum and the molar extinction coefficient.
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π−π* transitions of the conjugated molecule. The molar extinction coefficient (ε) of JH-1 was 50 000 M−1 cm−1 at 450 nm in ethanol, nearly three times the value of N719 (ε = 15 000 M−1 cm−1) at 550 nm. A high molar extinction coefficient is highly desirable as it enables the use of thinner nanocrystallite TiO2 layers, thereby facilitating charge collection while maintaining the light-harvesting capabilities and photocurrent generation in the DSSCs.[20] Decreasing the film Scheme 1. Chemical structures of (a) the synthesized dye (JH-1) and (b) N719. thickness also augments the open-circuit photovoltage of the cell by reducing the dark current.[17] The maximum possible photocurrent is determined The long alkyl chains on the thiophene rings prevent charge recombination of the injected electrons or dye aggregation.[15] by the overlap of two spectra, the absorption spectrum of the sensitized TiO2 layer and the solar flux spectrum.[3] Although Most DSSC photoelectrodes use nanocrystalline (nc-) TiO2 particles because small nc-TiO2 particles enhance dye adsorpthe organic dye is highly efficient for converting absorbed photons into free electrons in the TiO2, only photons absorbed tion and exciton generation due to their large surface areas; however, small particles do not always provide the best perforby the dye ultimately produce the current. Typically used dye mance because they increase the number of grain boundaries molecules generally have poorer absorption in the red part of and defects on the particle surfaces, which can delay electron the spectrum than the silicon; hence fewer of the photons in transport, as revealed by the electron diffusion coefficient sunlight are usable for current generation by a DSSC.[25] Inter[ 8–10 ] measurements (see Figure 3(B) below). estingly, HS-TiO2 photoelectrode sensitized by JH-1 shows Typically, polydisperse aggregates increase the irregularities in the film assembly more broad absorption than nc-TiO2 photoelectrode because of resulting in light scattering over a broad spectral range.[8] The the additional light scattering occurring in HS-TiO2 photoelecHS-TiO2 electrode was prepared by electro-spraying nc-TiO2 trode. More details are discussed later. particles, 21 nm in diameter, which formed larger filled-spheres The electrochemical properties of JH-1 are summarized in of less grain boundaries rather than hollow spheres due to the Table 1. The electron-rich tri-phenyl amine moiety lifted the ultrafast evaporation of the solvent at the surfaces of the fine HOMO level whereas the thienothiophene and thiophene units solution droplets containing nc-TiO2 particles.[8] Figure 1 shows conjugated with the acceptor unit adjusted the LUMO level to a suitable value. The HOMO level of the dye (−5.54 eV) was a schematic figure of the resonant light scattering process and sufficiently more negative than the iodine/tri-iodide redox an SEM image of the HS-TiO2 electrode consisting of polydispotential (−4.9 eV, corresponding to 0.44 eV vs NHE (the perse aggregates with an average size of 660 nm which scatters normal hydrogen electrode)),[26] leading to rapid dye regeneramost of the light in the visible range and can potentially lead to light trap formation for optical confinement (see SI 4 for partion and limiting charge recombination between the oxidized ticle size distribution and pore size distribution in the HS-TiO2 dye molecules and the electrons photoinjected into the titania film. The oxidized dyes formed after electron injection into electrode).[23,24] the conduction band of TiO2 could thermodynamically accept Figure 2(A) shows the absorption spectra of JH-1 and N719 in ethanol or attached to different TiO2 electrodes. The electrons from I− to provide a driving force sufficient for effiabsorption spectra of JH-1 included strong peaks at 450 nm cient regeneration of the neutral sensitizer state.[17] The LUMO in ethanol or at 455 nm on the HS-TiO2 electrode due to the level of the novel dye (−3.21 eV) was sufficiently more positive than the conduction band energy level of TiO2 (−4.01 eV), thereby facilitating efficient charge injection from the dye in the excited state to the conduction band of TiO2 and regeneration of the oxidized dye. Figure 2(B) and 2(C) show the absolute and normalized incident photon-to-current conversion efficiency (IPCE) spectra of JH-1 and N719. The IPCE spectra were measured under white bias light and in AC mode with the frequency of 4 Hz.[27,28] The spectra covered the whole visible region over the range 300 – 800 nm. For nc-TiO2 electrode, JH-1 based DSSC shows a maximum absorption peak of 74.8% at 460 nm while that for HS-TiO2 electrode was 93% at 460 nm. On the other hand, N719 based DSSC with nc-TiO2 electrode had a maximum Scheme 2. Synthesis route of JH-1.
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because of the large absorption coefficient. The enhanced light harvesting characteristics of HS-TiO2 are evident if the IPCE is normalized (Figure 2(C)). The IPCE spectra of HS-TiO2 with both JH-1 and N719 based cells show more broad absorption over all wavelength region. This result indicates that polydisperse HS-TiO2 aggregates induce additional light scattering over wide wavelength range of incident photon, which is consistent with UV absorption spectra Figure 1. A schematic figure of the resonant multiple light scattering and SEM image of a TiO2 in Figure 2(A). In our previous study, we reported that HS-TiO2 DSSCs showed electrode made of polydisperse aggregates (HS-TiO2). higher IPCE values than that of nc-TiO2 DSSCs (Table 2), especially in the long wave length region, though they had almost the same amount of absorption peak of 75% at 550 nm and 82% at 550 nm with adsorbed dye.[8] This means that multiple reflections of phoHS-TiO2 electrode. This high and broad IPCE value of the JH-1 sensitized DSSC in the visible region was attributed tons in HS-TiO2 electrodes of JH-1 based DSSC results in to the increased conjugation length and the segment plaincreased light absorption to produce higher short-circuit narity resulting from the π-conjugated linker composed of photocurrent density as shown in Table 2. Figure 2(D) compares the photovoltaic characteristics of the JH-1 and N719 thiophenes and fused thiophene ring.[23,29] In the long wave DSSCs based on HS-TiO2 electrodes. The measured openlength region (>650 nm, low energy wavelength region), N719 based DSSCs show higher IPCE values than JH-1 based ones circuit photovoltage (Voc), short-circuit photocurrent density due to the metal-to-ligand charge transfer (MLCT) transition. (Jsc), fill factor (FF), and solar-to-electric conversion efficiency On the low energy side in the long wavelength, a significant (η) of these solar cells are listed in Table 2. Because of its part of the incident radiation penetrates the layer due to the higher molar extinction coefficient, the increase of JSC (and low absorption coefficient of N719 dye, while photons in the η) in JH-1 sensititized DSSC with HS-TiO2 electrode is larger visible region can be easily absorbed by JH-1based DSSC than that of N719 with HS-TiO2.
Figure 2. (A) Absorption spectra of the JH-1 and N719 dyes in ethanol (gray), on the nc-TiO2 films (dashed), and on the HS-TiO2 films (solid). The spectra in ethanol were extinction coefficient values downsized to fit in the graph whereas those on the TiO2 films were normalized to the maxima of JH-1 for comparison. (B) Absorption spectra of JH-1 (black) and N719 (gray) based DSSCs with the HS-TiO2 electrodes (solid) or the nc-TiO2 electrodes (dashed). (C) Normalized IPCE spectra of the DSSCs: JH-1-based DSSC (black) and N719-based DSSC (gray) with the HS-TiO2 electrodes (solid) or the nc-TiO2 electrodes (dashed). (D) I-V curves of the JH-1-(solid) and N719-(dashed) based solar cells with HS-TiO2 photoelectrodes. The TiO2 layer thickness was 12 µm.
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Dye
λmax/nm
εmax/M−1 cm−1
JH-1
450
50 000
−5.54
−3.21
2.33
15 000
−5.6
−3.0
2.6c)
N719
550
EHOMO/eVa) ELUMO/eV EHOMO-LUMO gap/eVb)
c)
a)Evaluated
by cyclic voltammetry (see SI 3); absorption spectrum; c)See ref. [11].
c)
b)Evaluated
from the onset of the
Table 2. Photovoltaic parameters of the DSSCs based on the JH-1 and N719 dyes (all TiO2 electrodes were 12 µm thick). Dye JH-1
N719
Jsc/mA cm−2
Electrode
Voc/V
FF (%)
η (%)
HS-TiO2
20.9
0.687
63.9
9.18
HS-TiO2 (Masked)
15.1 (14.9 (integrated))
0.686
67.6
7.01
nc-TiO2
14.5
0.680
65.2
6.40
nc-TiO2 (Masked)
11.8 (11.4 (integrated))
0.684
68.6
5.56
HS-TiO2
19.5
0.767
65.4
9.77
HS-TiO2 (Masked)
15.1 (14.9 (integrated))
0.769
68.0
7.91
nc-TiO2
16.1
0.773
66.8
8.30
nc-TiO2 (Masked)
13.4 (12.9 (integrated))
0.768
68.2
7.03
Intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) were used to gain an insight into the charge transport characteristics in the TiO2 layer and charge recombination at the DSSC's titania/electrolyte interface. Figure 3 shows plots of the recombination lifetimes and electron diffusion coefficients as a function of the incident photon flux (light intensity). For a given TiO2 electrode of 12 µm thick, which was the optimum thickness of JH-1 with HS-TiO2 photoelectrode consistent with diffusion length calculated by IMPS and IMVS measurement (see SI 5 for the optimum electrode thickness), recombination was 7 to 10 times slower in the N719 cell than in the JH-1 based cell, indicating that the back-reaction rate of the N719 cell was slower; hence the charge collection efficiency of the N719 cell was higher. The shorter recombination lifetime of JH-1 would, therefore, reduce VOC in the JH-1 sensitized photoelectrode.[30] The measured
recombination lifetimes were in accords with the VOC values in Table 2. The accuracy of the measurement can be checked by comparison of measured Jsc values with those from calculated ones (Jsc = ∫ q F(λ) IPCE(λ)dλ where q is electron charge and F(λ) is the incident monochromatic photon flux density of AM 1.5 global solar spectrum).[3,28] In Table 2, all masked samples show good agreement with those calculated values within experimental error range. Voc values are not much different from each other while ff values were slightly varied due to the I-V curve shape change. On the other hand, large increase of Jsc values was observed when the masks were removed. For nc-TiO2 electrode DSSC which did not scatter lights due to its small size (∼21 nm), the increase of Jsc for unmasked ones was the same for JH-1 and N719 based DSSCs (2.7 mA cm−2) because this was due to lateral scattering.[2] For nc-TiO2 electrode DSSCs, N719 sensitizer DSSC shows a little bit larger Jsc than that of JH-1 dye DSSC because of higher IPCE values of N719 sensitizer DSSC at the wavelength longer than 500 nm. For masked HS-TiO2 electrode DSSCs, both (JH-1 and N719 based) DSSCs show Jsc increase compared to masked nc-TiO2 electrode DSSCs (3.3 mA cm−2, 28% increase for JH-1 based DSSC and 1.7 mA cm−2, 13% increase for N719 based DSC). This increase is attributed to photon scattering effect; HS-TiO2 particles scatter photons due to its size polydispersity, increasing the photon pass length and absorption by sensitizers. DSSCs with unmasked HS-TiO2 electrodes show 6.4 mA cm−2 (44% increase) and 3.4 mA cm−2 (21% increase) increase of Jsc for JH-1 and N719 based cells respectively, compared to those with unmasked nc-TiO2 electrode DSSCs. JH-1 based DSSC shows much higher Jsc value than that of N719 based DSSC due to both higher molar extinction coefficient and resonant photon scattering in visible region as mentioned before. All sensitized DSSCs with the HS-TiO2 electrodes yielded higher photocurrent densities and better efficiencies compared to DSSCs using the electrodes of nc-TiO2 particles of ∼21 nm size. Though N719 sensitizer shows higher IPCE in long wave length (>700 nm) (Figure 2(B) and (C)), its effect was not that great because of weak light energy in that frequency range. When the mask was removed for HS-TiO2 electrode DSSCs, JH-1 based DSSC showed surprisingly high Jsc value
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Table 1. Absorption spectrum and electrochemical properties of JH-1 and N719
Figure 3. Plots of the electron recombination lifetime (A) and the electron diffusion coefficient (B). The nc-TiO2 electrode was composed of ∼21 nm particles whereas the HS-TiO2 electrode was composed of the aggregates of the same nc-TiO2 particles with an average diameter of 660 nm. The lines are guides for the eyes.
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of 20.9 mA cm−2 while that of the N719 based solar cell was 19.5 mA cm−2. To the best of our knowledge, this is the highest photocurrent density reported thus far for metal-free dye sensitized DSSCs. The electron diffusion coefficients (D) and the recombination life times (τ) were used to calculate the relative electron diffusion lengths, Ln ≈ (Dτ)1/2, of the DSSCs.[17] The diffusion length is the average distance an injected electron travels through the conduction band of TiO2 before recombination. A shorter electron diffusion length (Ln) in the JH-1 based device (∼11 µm) compared to a N719 based device (∼21 µm) indicates that an electron injected into JH-1 sensitized photoelectrode travels a much shorter distance than in a N719 sensitized photoelectrode. The optimum thickness of JH-1 with HS-TiO2 photoelectrode (∼12 µm) is consistent with diffusion length calculated by IMPS and IMVS measurement (see SI 5). Despite the shorter electron diffusion length (lower charge collection efficiency) due to the faster recombination reaction, a higher photocurrent density was measured in the JH-1-based DSSC. The high photocurrent density of JH-1 based DSSC with HS-TiO2 electrode was not only because of the JH-1 dye characteristics including the high molar extinction coefficient, appropriate energy level matching between the LUMO of the dye and the conduction band of TiO2[23] and the HS-TiO2 characteristics including faster electron diffusion properties than nc-TiO2, but also to the resonant multiple light scattering from the HS-TiO2 electrode. As a DSSC sensitized by a metal-free organic dye, the conversion efficiency of the JH-1 dye (9.18%) using HS-TiO2 electrode without an antireflection layer was quite promising. Under the same conditions, the N719 based solar cell yielded a conversion efficiency of 9.77%. Therefore, it is clear that the
harmonization of the resonant multiple scattering by tuning the size of TiO2 aggregates and the absorption spectrum of the dye is a very effective approach to achieving a high photocurrent density and thus, high energy conversion efficiency. Although enhanced electron diffusion in HS-TiO2 partly contributed to the increase of photocurrent density in much the same way as the lateral scattering also helped the photon harvesting, they equally contribute in the N719 based solar cells.[31] The long-term stability of any new photovoltaic technology is just as important as the photon conversion efficiency for practical large scale applications.[16,17] We submitted our cells without a UV cut-off filter to 800 h of accelerated testing at 60°C under a light intensity of 100 mW cm−2. The JH-1 DSSC exhibited very good properties over 400 h, with weak attenuation of the device upon further illumination (Figure 4) due to the loss of sensitizer and/or penetration of I3− into the TiO2 surface.[16] The hydrophobic alkyl moieties of the JH-1 dye also contributed to the high stability.[32] After 800 h, the JH-1 sensitized DSSC exhibited an efficiency of 7.44% (81% of its initial efficiency) whereas that using N719 dye was 5.27% (54% of its initial efficiency). This result is quite encouraging as it brings the JH-1 based DSSC performance within the current standards required for practical applications. In summary, we have set a new bench mark for DSSC performances by fabricating a very efficient device with a high photocurrent density and good stability. This bench mark was accomplished by harmonizing the absorption spectrum of the dye with the average size of the aggregates in the TiO2 electrode. The resultant resonant multiple scattering enhanced the light harvesting efficiency and charge collection yield. The high
Figure 4. Evolution of the photovoltaic parameters for the JH-1 based HS-TiO2 DSSC (solid) and the N719 based HS-TiO2 DSSC (dashed) aged under illumination of 100 mW cm−2 at 60°C without a UV filter.
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Experimental Section Materials: The synthetic route for JH-1 is outlined in Scheme 2 and the details are described in SI 1. Device Fabrication: The photoanodes composed of hierarchically structured TiO2 (HS-TiO2) nanospheres and nanocrystalline TiO2 particles (nc-TiO2, P25 powder from Degussa) were prepared using the electro-spraying method.[8] Details of device fabrication are described in SI 2. Other experimental measurements are fully explained in SI 3 (UV, cyclic voltammetry, and SEM).
Supporting Information Supporting Information for details of dye synthesis and device fabrication as well as other experimental measurements is available from the Wiley Online Library or from the author.
Acknowledgements This study was supported by the Korea National Research Foundation through the RIAM [Basic Research Program (RIAM 0427–20100020)], the ITRC [Basic Science Research Program (R11–2005–065)], by the Ministry of Knowledge and Economy [Original Material Technology Program (RIAM-AC14–10) and Fundamental R&D Program for Core Technology of Materials (RIAM 0427–20100043)], and by KIST (KIST Institutional Programs (2E24610)). The photograph in the artwork by Youngwook P. Seo is greatly acknowledged. Received: January 9, 2014 Revised: April 15, 2014 Published online: May 28, 2014 [1] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Patterson, Chem. Rev. 2010, 110, 6595. [2] A. Mishra, M. K. R. Fischer, P. Bäuerle, Angew. Chem. Int. Ed. 2009, 48, 2474. [3] K. Hara, H. Arakawa, Chapter 15 in Handbook of Photovoltaic Science and Engineering (A. Luque, S. Hegedus (Eds)) John Wiley and Sons Ltd. UK 2003. [4] J. Yum, J, M. K. Nazeeruddin, Chapter 3 in Dye-Sensitized Solar Cells (K. Kalyanasudaram (Ed.)), EPFL Press, Lausanne 2010. [5] M. Grätzel, J. Photochem. Photobiol. A. 2004, 164, 3. [6] F. Gao, Y. Wang, D. Shi, J. Zhang, M. Wang, X. Jing, R. Humphry-Baker, P. Wang, S. Zakeeruddin, M. Grätzel, J. Am. Chem. Soc. 2008, 130, 10720.
Adv. Mater. 2014, 26, 5192–5197
[7] G. Zhang, H. Bala, Y. Cheng, D. Shi, X. Lv, Q. Yu, P. Wang, Chem. Commun. 2009, 16, 2198. [8] D. Hwang, H. Lee, S.-Y. Jang, S. M. Jo, D. Kim, Y. Seo, D. Y. Kim, ACS. Appl. Mater. Interfaces. 2011, 3, 2719. [9] a) Q. Zhang, T. P. Chou, B. Russo, S. A. Jenekhe, G. Cao, Adv. Funct. Mater. 2008, 18, 1654; b) Q. Zhang, G. Cao, Nano Today. 2011, 6, 91; c) T. P. Chou, Q. Zhang, G. E. Fryxell, G. Cao, Adv. Mater. 2007, 19, 2588. [10] F. Huang, D. Chen, X. L. Zhang, R. A. Caruso, Y.-B. Cheng, Adv. Funct. Mater. 2010, 20, 1301. [11] S. Hwang, J. H. Lee, C. Park, H. Lee, C. Kim, C. Park, M.-H. Lee, W. Lee, J. Park, K. Kim, N.-G. Park, C. Kim, Chem. Commun. 2007, 21, 4887. [12] H. Tian, X. Yang, J. Pan, R. Chen, M. Liu, Q. Zhang, A. Hagfeldt, L. Sun, Adv. Funct. Mater. 2008, 18, 3461. [13] T.-H. Kwon, V. Armel, A. Nattestad, D. R. MacFarlane, U. Bach, S. J. Lind, K. C. Gordon, W. Tang, D. J. Jones, A. B. Holmes, J. Org. Chem. 2011, 76, 4088. [14] R. Argazzi, C. A. Bignozzi, J. Phys.Chem. B. 1997, 101, 2591. [15] G. Li, Y.-F. Zhou, X.-B. Cao, P. Bao, K.-J. Jiang, Y. Lin, L.-M. Yang, Chem. Commun. 2009, 16, 2201. [16] K. Jiang, S. Yanagida, Chapter 4 in Dye-Sensitized Solar Cells (K. Kalyanasudaram (Ed.)), EPFL Press, Lausanne 2010. [17] D. Shi, N. Pootrakulchote, R. Li, J. Guo, Y. Wang, S. M. Zakeeruddin, M. Grätzel, P. Wang, J. Phys. Chem. C. Lett. 2008, 112, 17046. [18] H. M. Ko, H. Choi, S. Paek, K. Kim, K. Song, J. K. Lee, J. Ko, J. Mater. Chem. 2011, 21, 7248. [19] S. G. Esteban, P. Cruz, A. Aljarilla, L. M. Arellano, F. Langa, Org. Lett. 2011, 13, 5362. [20] J. H. Yum, D. P. Hagberg, S. J. Moon, K. M. Karlsson, T. Marinado, A. Hagfeldt, M. K. Nazeeruddin, M. Grätzel, Angew. Chem. Int. Ed. 2009, 48, 1576. [21] M. Liang, W. Xu, F. Cai, P. Chen, B. Peng, J. Chen, Z. Li, J. Phys. Chem. C. 2007, 111, 4465. [22] J. Xu, L. Zhu, D. Ffang, B. Chen, L. Liu, L. Wang, W. Xu, ChemPhysChem. 2012, 13, 3320. [23] Q. Zhang, T. P. Chou, B. Russo, S. A. Jenekhe, G. Cao, Angew. Chem. Int. Ed. 2008, 47, 2402. [24] H. Cao, J. Y. Xu, D. Z. Zhang, S.-H. Chang, S. T. Ho, E. W. Seelig, X. Liu, R. P. H. Chang, Phys. Rev. Lett. 2000, 84, 5584. [25] K. Sayama, K. Hara, N. Mori, M. Satsuki, S. Suga, S. Tsukagoshi, Y. Abe, H. Sugihara, H. Arakawa, Chem. Commun. 2000, 24, 1173. [26] X. Zhan, Z. Tan, E. Zhou, Y. Li, R. Misra, A. Grant, B. Domercq, X.-H. Zhang, Z. An, X. Zhang, S. Barlow, B. Kippelen, S. R. Marder, J. Mater. Chem. 2009, 19, 5794. [27] 2006 ASTM Standard E1021–06, Test method for spectral responsivity Measurements of Photovoltaic Devices (West Conschohocken, PA : ASTM International) Vol.12.02, doi :10.1520/E1021–06. [28] G. Xue, X. Yu, C. Bao, J. Zhang, H. Huang, Z. Tang, Z. Zou, J. Phys. D : Appl. Phys. 2012, 45, 425104. [29] A. Hagfeldt, M. Grätzel, Chem. Rev. 1995, 95, 49. [30] Q. Wang, J.-E. Moser, M. Grätzel, J. Phys. Chem. B. 2005, 109, 14945. [31] F. Sauvage, D. Chen, P. Comte, F. Huang, L.-P. Heiniger, Y.-B. Cheng, R. A. Caruso, M. Grätzel, ACS Nano 2010, 4, 4420. [32] P. Wang, S. M. Zakeeruddin, J. E. Moser, M. K. Nazeeruddin, T. Sekiguchi, M. Grätzel, Nature Mater. 2003, 2, 402.
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and robust photovoltaic performance (with an initial efficiency of 9.18% and an efficiency of 7.44% after 800 h of irradiation with a light intensity of 100 mW cm−2) of the JH-1 DSSC prepared without an antireflection layer demonstrated the promise of this novel sensitizer for large scale applications. Our result should stimulate further investigations into novel dyes with similar structures and we expect that our method may be readily applicable to practical DSSC systems.
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