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A Phenalenothiophene-Based Organic Dye for Stable and Efficient Solar Cells with a Cobalt Redox Electrolyte Heng Wu, Xinrui Xie, Yayue Mei, Yutong Ren, Zuochun Shen, Sining Li, and Peng Wang ACS Photonics, Just Accepted Manuscript • Publication Date (Web): 02 Apr 2019 Downloaded from http://pubs.acs.org on April 2, 2019
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ACS Photonics
A Phenalenothiophene-Based Organic Dye for Stable and Efficient Solar Cells with a Cobalt Redox Electrolyte
Heng Wu,†,‡ Xinrui Xie,† Yayue Mei,† Yutong Ren,† Zuochun Shen,‡ Sining Li,‡ and Peng Wang*,†
†Center
for Chemistry of Novel & High Performance Materials, Department of Chemistry,
Zhejiang University, Hangzhou 310028, China ‡National
Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of
Technology, Harbin 150001, China
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ABSTRACT: A cobalt based redox electrolyte as a possible alternative to the traditional iodine counterpart has brought forth the remarkable power-conversion-efficiency (PCE) advancement in the past years of dye-sensitized solar cells (DSSCs); however, the decisive issue for practical applications, i.e. long-term stability under the thermal and light dual stress of this type device, remains a huge challenge so far. To address this topic, we herein designed two naphthalene based donoracceptor organic dyes H1 and H2 characteristic of coplanar aromatic polycycles 7H-phenaleno[1,2-b]thiophene (PT) and 7H-benzo[6,7]indeno[1,2-b]thiophene (BIT), in combination with the state-of-the art bis(4-(hexyloxy)phenyl)amine ancillary electron-donor and 4-(benzo[c][1,2,5]thiadiazol-4-yl)benzoic acid (BTBA) electron-acceptor. Dyes H1 and H2 were tested for coadsorbate-free DSSCs with a cobalt redox electrolyte, presenting fairly good PCEs of 9.7% and 10.3%, respectively, at the air mass 1.5 global (AM1.5G) conditions. Encouragingly, the H1-based device was identified to exhibit outstanding durability during full-sunlight soaking at 60 °C for 1,000 h, maintaining 90% of its initial PCE . Systematic photophysical and electrical measurements were performed to probe light-harvesting efficiency and charge carrier kinetics, giving a clue on the experimentally observed variation of photovoltage and photocurrent.
KEYWORDS: solar cell, organic dye, stability, light absorption, charge transfer
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The dye-sensitized solar cell (DSSC) is a potentially low-cost photovoltaic technology for the conversion of abundant solar light to clean electric power.1 Due to the advantage of abundant raw materials and aesthetic effect, metal-free organic dyes have attracted a lot of research interest in comparison with ruthenium polypyridyl and zinc porphyrin complexes.2-5 In the past years, stepwise performance advances of exploiting donoracceptor (DA) organic dyes in combination with the conventional iodide/triiodide redox couple have been achieved by judicious design of dye molecules, fine-tuning electrolyte composition and interfacial optimization.2-26 Recently, a 10%-efficiency and thermostable DSSC with an iodine-based ionic-liquid composite electrolyte was lately reported by co-grafting two robust organic dyes on mesoporous titania.23 However, several drawbacks of the iodine/triiodide redox couple were also widely recognized, such as fast corrosion to many metal electrodes and low open-circuit photovoltage. Therefore, much effort has been devoted to the identification of one-electron, outer-sphere redox couples, such as iron, copper, and cobalt complexes.10-12,27-38 In comparison with the traditional iodine counterpart, a cobalt-based electrolyte normally allows for an improved open-circuit photovoltage of DSSCs if the self-organized dye layer on titania is well controlled. By delicate interface engineering, an impressive power conversion efficiency (PCE) of over 14% at the air mass 1.5 conditions was achieved in 2015 with a cobalt-based electrolyte,39 much higher than the certified record efficiency of 11.9% with an iodine based electrolyte.40 However, an excellent long-term stability of DSSCs based on a cobalt electrolyte is desirable for practical applications.4,5,41-50 We note that the main challenge of using a cobalt redox couple, characteristic of considerably a large hydrodynamic radius, consists in the unavoidable usage of a low-viscosity molecule solvent for electrolyte formulation to meet the mass transport requirement. The presence of molecule solvent is highly possible to desorb organic dye molecules from the surface of titania at an elevated temperature, which has actually also not been identified for the iodine counterpart based DSSC with organic dyes so far.23,51,52 Amongst various aspects needed to be addressed for thermally stable DSSCs, strong binding of dye molecules onto the surface of oxide electrode is very challengeable under the thermal stress, especially in the presence of organic solvents. Combination of a conjugated polycyclic dye (R6) with a propionitrile based cobalt electrolyte lately allowed us to make a DSSC maintaining 90% of its initial
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value (PCE = 9.6% under the 53.5 mW cm–2 sunlight), after 1,000 h of continuous full sunlight soaking at 60 °C.50 This achievement also encouraged us to revisit dye C258 with 4-(benzo[c][1,2,5]thiadiazol-4-yl)benzoic acid (BTBA) as an electron-acceptor for the first time,53 which can also be employed to fabricate a stable DSSC with an initial PCE of 9.4% (Figure S2) when a cobalt based electrolyte is used. In dye C258 4H-cyclopenta[1,2-b:5,4-b']dithiophene (CPDT) is used as the main electron-releasing segment. However, the synthesis of CPDT involves cryogenic temperatures and hazardous organolithium reagents, causing high preparation cost and probably impeding large-scale applications.54 In this work, we aim to develop low-cost naphthalene-based organic dyes H1 and H2 (Figure 1a) characteristic
of
7H-phenaleno[1,2-b]thiophene
(PT)
and
its
isomer
7H-benzo[6,7]indeno[1,2-b]thiophene (BIT). Preliminary theoretical calculations have disclosed that the highest occupied molecular orbital (HOMO) energy levels of PT and BIT (Figure S3) are nearly the same. Attachment of the ancillary electron-releasing unit bis(4-(hexyloxy)phenyl)amine (DPA) to PT and BIT affords two binary electron donors DPA-PT and DPA-BIT (Figure S4) with HOMO energy levels of about 4.86 eV versus vacuum, which are further linked with BTBA. By the joint of photophysical and electrical characterization tools, we will scrutinize the influence of PT and BIT on initial photovoltaic characteristics and the evolution of cell parameters under full sunlight soaking at 60 °C for 1,000 h.
RESULTS AND DISCUSSION Scheme S1 outlines the synthetic routes to DA dyes H1 and H2. Compound 1, already reported in the literature,55 was converted to a diarylamino functionalized napthylboronic acid pinacol ester 2 at satisfactory yield of 87%, via the palladium-catalyzed Miyaura borylation reaction under a mild condition.
Subsequent
Suzuki-Miyaura
cross-coupling
of
2
and
methyl
2-bromothiophene-3-carboxylate produced the key intermediate 3 at excellent yield of 88%. Next, 3 underwent a carbonyl addition reaction with (4-hexylphenyl)magnesium bromide to generate a tertiary alcohol intermidate, which underwent intramolecular Friedel-Crafts cyclization in the presence of an acidic catalyst to simultaneously afford 4 and 5 at remarkable total yield of 90%. The yields for 4 and 5 were almost the same. Electron-rich compounds 4 and 5 were further treated with superbase n-butyllithium and then chlorotrimethylstannane, being converted to their corresponding stannanes, 4 / 28
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which were further cross-coupled with butyl 4-(7-bromobenzo[c][1,2,5]thiadiazol-4-yl)benzoate to form esterified dyes. In the last, the carboxylic esters were treated with strong base KOH and the hydrolyzates were acidified with a diluted hydrochloric acid aqueous solution to give the desired dyes H1 and H2. For details on synthesis and structural characterization, see Supporting Information. To scrutinize the impact of conjugated central units (PT and BIT) on HOMO and lowest unoccupied molecular orbital (LUMO) energy levels of dyes H1 and H2, we performed nonaqueous electrochemical measurements (Figure S6) with tetrahydrofuran (THF) as the electrolyte solvent, in a nitrogen-filled glovebox. In comparison with dye H1 characteristic of PT (HOMO: 5.00 eV; LUMO: 3.31 eV), the corresponding isomeric dye H2 with BIT as the main electron-donating unit displays a destabilized HOMO energy level of 4.98 eV and a stabilized LUMO energy level of 3.32 eV. Our cyclic voltammetric experiments suggest that the use of either PT or BIT as electron-donor leads to a CV similar HOMO/LUMO energy gap (EH/L ). Density functional theory (DFT) calculations also present
the similar energy level alignments and HOMO/LUMO energy gaps for these two dyes (see Table S1). The tiny distinction between HOMO energy levels of isomeric H1 and H2 can be attributed to the disparate substituting pattern (Figure S5b). The solution photos of 150 μM dyes in toluene were taken under either white LED or ultraviolet light at 365 nm (Figure S7). Stationary UV-Vis absorption spectroscopies of dye solutions are shown in Figure 1b. As tabulated in Table 1, the maximum absorption wavelength (a ) of 518 nm for dye H1 is blue-shifted by 16 nm compared to that of 534 nm for dye H2. Time-dependent DFT (TDDFT) calculations nicely replicate the consistent tendency of absorption maxima. Recall that dye H2 possesses almost the same HOMO/LUMO energy gap as dye H1, which should not give rise to a significant difference of absorption maximum. It is noted that the low-energy absorption bands of dyes H1 and H2 are in general ascribed to the S1←S0 vertical electronic transition, from HOMO to LUMO, from HOMO−1 to LUMO, and from HOMO to LUMO+1, however a larger proportion of excitation from HOMO to LUMO is theoretically identified for H2 than H1 (see Table S1), giving a clear explanation for the maximum absorption wavelength distinction. From Figure S5, the feature of intramolecular charge-transfer transition could be explicitly seen from the isodensity surface plots of HOMO, HOMO−1, LUMO, and LUMO+1. Static photoluminescence (PL) spectroscopies in Figure 1b show large Stokes shifts, owing to the vibrational, torsional, and solvation relaxations for the vertically excited states.56 As collected in Table 1, the trends of theoretical PL maxima and Stokes shifts are 5 / 28
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consistent with experimental results. The average time constants ( ) of PL decays of H1 and H2 in toluene were determined by femtosecond fluorescence up-conversion technique.57-61 Processed data are provided in Figure S12S13 and Table S4S5, with analytical details listed in the Supporting Information. From the curves (Figure 1c) probed at different PL wavelengths, it is derived that H1 and H2 are characteristic of an augmentation in
by 1-3 orders of magnitude along with PL wavelength red-shifting. The
values gradually increase, and at a certain long PL wavelength finally become constant, which is considered as the lifetime of equilibrium first excited state (S1eq ) . We have found that the lifetime of S1eq are 1238 ps for H1 and 1253 ps for H2, albeit their different optical energy gaps.
To gain more insights into the excited state conformational relaxations, we then compared the optimized geometries of molecules at the ground state (S0 ) and S1eq states via DFT and TDDFT, respectively. According to the Frank-Condon principle, the conformation of vertical excited state (S1vert ) is unconverted upon photo-induced electronic transition, exactly following the ground state
conformation. However, the molecular conformation of S1vert will subsequently relax towards a new equilibrium state conformation by stepwise vibrational and torsional motions. These relaxations will generate significant conformational diversity between S0 and S1eq for DA dyes as demonstrated by theoretical simulation. Obviously, the dihedral angles (Figure 2) between electron-donor and electron-acceptor decrease after excited state structural relaxation.62 The torsinal relaxation occurs in the picosecond time domain, embodied by the slow arisings of kinetic traces and negative amplitudes of small time constants at long PL wavelengths (Table S4 and S5). From the selected bonds in the benzo[c][1,2,5]thiadiazole (BT) unit (Figure S8), the bond lengths of single and double bonds in the S1eq state are different from those of the S0 conformation.63
To evaluate the yield and kinetics of electron injection from photo-excited dye molecules to titania, we compared femtosecond PL traces (Figure S14S17) of H1 and H2 chemically adsorbed on the surface of a mesoporous titania film, with those on alumina. The dyed oxide films were also in contact with the cobalt-based redox electrolyte for device fabrication. The electrolyte composition is detailed in Supporting Information. For a given dye on titania, a blue-shifted PL maximum was observed in contrast to that on alumina, which could be caused by electron injection from the nonequilibrium hot excited state to titania. The up-converted PL traces at three selected wavelengths of 660 nm, 760 nm, and 860 nm for the H1-grafted titania film are shown in Figure S9b, which are remarkedly disparate. 6 / 28
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We globally fitted all PL kinetic traces, with four time constants in Tables S6-9. As Figure 3a and 3b present, the
values of all dyed films increase over two orders of magnitude along with red-shifting
of PL wavelength. Whereafter, we calculated the the PL quenching yield (QY) and the time constant of electron injection ( ei ) , and depicted them in Figure 3c. Both dyes feature lower QY values in nonequilibrium excited states. This could be caused by the two competitive channels of transforming nonequilibrium excited states to both equilibrium excited states and separated charge states. As shown in Figure 3d, along with PL energy variation from 2.0 to 1.3 eV, the ei values increase by over one order of magnitude. This is well consistent with the notion of driving force dependent electron injection kinetics. The S1eq states of H1 and H2 show comparable electron injection time constants of 25 ps and 26 ps, which could be well understood by the similarities of binding modes on titania, reorganization energies, and LUMO energy levels (−2.55 eV for H1 and −2.52 eV for H2) of S1eq , in terms of Marcus electron transfer theory. We then employed a pump-probe absorption spectrometer to measure the nanosecondmillisecond kinetics of reduction reactions of dyes at the oxidized states only with electrons in titania, or with both electrons in titania and electrolyte (cobalt(II) ions).64,65 A multi-exponential function was used to fitting the transient absorption changes probed at 1300 nm for the precise assessment of half-reaction time constants. When dyed titania films were immersed with an inert electrolyte composed of 0.5 M tert-butylpyridine and 0.1 M lithium bis(trifluoromentylsulfonyl)imide, slow kinetic traces (c and d in Figure 4), which are intrinsically connected with the charge transfer reaction between photo-oxidized dye molecules and negatively charged titania, take place in the millisecond time domain with time inert constants (t1/2 ) being 2.0 ms for H1 and 2.9 ms for H2. Furthermore, when a cobalt electrolyte used
for device fabrication was applied, rapid kinetic traces, which are mainly associated with hole injection from photo-oxidized dye molecules to cobalt(II) ions, appear in the microsecond time domain, with Co Co time constants (t1/2 tendency is completely ) being 40 s for H1 and 54 s for H2. The t1/2
consistent with the hole injection driving force. As a result, both H1 and H2 can attain an excellent hole Co inert injection yield (hi (1 t1/2 / t1/2 ) 100%) of 98%.
The photocurrent action spectra of DSSCs fabricated with a bilayer dye-grafted titania film and a cobalt electrolyte are depicted in Figure 5b. Detailed fabrication processes are described in the Supporting Information. The peak values of external quantum efficiency (EQE) are both 90% for H1 and H2, which are in good agreement with the comparable yields of electron injection and hole 7 / 28
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injection estimated with time-resolved photophysical measurements. Simultaneously, the red-shifted onset wavelength of photocurrent response for H2 is in good accordance with wavelength dependent light-harvesting efficiencies (LHEs) (Figure 5a) of these two dyes. The current densityvoltage (JV) characteristics (Figure 5c) were measured at an irradiance of 100 mW cm−2, simulated AM1.5G sunlight. The extracted cell parameters are listed in Table 2. H2 exhibits an enhanced short-circuit photocurrent density ( J SC ) of 15.47 mA cm−2, a lower open-circuit photovoltage (VOC ) of 903 mV, and a higher fill factor (FF) of 74.0%, contributing to a higher PCE of 10.3%, in comparison with the corresponding parameters for H1 (J SC = 14.33 mA cm−2, VOC = 931 mV, FF = 72.3%, PCE = 9.7%). The photovoltaic parameters were all averaged from 4 fresh devices, and the small efficiency error (~1%) demonstrated good reproducibility of our devices. With a view to the difficulty in purifying isomers 4 and 5, we also co-grafted dyes H1 and H2 on the surface of mesoporous titania for device fabrication, yielding compromised photovoltaic parameters: J SC = 14.94 mA cm−2, VOC = 912 mV, FF = 73.5%, and PCE = 10.0%. Note that the PCEs of the reference dyes N719, Z907, C101, C106 under the same device condition are 2.1%, 6.9%, 6.7%, 7.1%, respectively. (Figure S11, Table S3). By recording JV curves of cells with H1 and H2 under a series of irradiances attenuated with neutral metal meshes, we could plot VOC as a function of J SC . Obviously, H1 displays an enlarged
VOC at a certain J SC than that of H2 (Figure 5d). We then implemented the charge extraction (CE) and transient photovoltage decay (TPD) measurements66,67 to give a clue to the underlying interfacial energetics and kinetics behind the VOC difference. As Figure 5e shows, the charges (Q CE ) stored in the titania of DSSCs with H1 and H2 are very much similar, implying that the titania conduction-band (CB) edge and distribution of electron trap states below the CB should not be altered. However, the TPD half-lifetime (t1/2 ) at a given Q CE of DSSCs with H1 is longer than cells with H2, interpreting the
VOC variation. The dye load amounts (cm ) were further determined, being 1.6210−8 mol cm−2 μm−1 for H1 and 1.1910−8 mol cm−2 μm−1 for H2. Thereby, the larger VOC for cells with H1 could be explained by a denser molecular layer on the surface of tiania. The stability of DSSCs under heat stress and solar light soaking is indispensable for practical applications. Figure 6 presents photovoltaic parameter evolution of cobalt-based DSSCs covered with a 50 m-thick polyester film (Preservation Equipment Ltd, UK) as a UV cutoff filter, which was irradiated at 60 °C under a Suntest CPS plus lamp (ATLAS GmbH, 100 mW cm−2). It is noteworthy 8 / 28
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that DSSCs with H1 displays an excellent stability with 10% drop of its initial PCE after full sunlight soaking at 60 °C for 1,000 h, much better than the H2 counterparts. The VOC drop in DSSCs with H1 or H2 all exceeds over 100 mV (Figure 6). Interestingly, the J SC value of the H1 cell increases by 0.91 mA cm−2 after aging, while the J SC value of the H2 cell drops by 4.98 mA cm−2. We also performed electrical and photophysical measurements to give a clue on the experimentally observed photovoltage and photocurrent variation under heat stress and light soaking. As Figure 7 shows, the VOC of aged DSSCs with H1 and H2 at a certain J SC are lower than those of fresh ones, and at a certain voltage more charges (Q CE ) are stored in titania for aged ones. Obviously, the aging TPD brings about a downward shift of TiO2 CB edge. In addition, the t1/2 of electrons in titania is
shortened, which further induces lower quasi-Fermi lever of TiO2. Nevertheless, the dissimilar variation patterns of J SC with H1 and H2 could not be explained by almost unaltered photocurrent action spectra (Figure 8a,b). Light-soaking at 60 °C for 1,000 h hardly influenced the light harvesting efficiencies (Figure 8c,d), suggesting that dye desorption should not occur due to a strong binding of H1 and H2 on titania. Transient absorption traces (Figure 8e,f) were also not sensitive to long-term aging, implying unchanged hole injection yields of cobalt-based DSSCs with H1 and H2. Additionally, the accelerated PL decay traces (Figure 8g,h) of aged cells should be relevant to faster electron injection kinetics, owing to the downward-shifted CB edge. Different from the cells with H1, DSSCs with H2 presented a remarkable J SC loss after aging. We have noted that the aged H2 cell has a sub-linear dependence of J SC on irradiance, which is likely correlated with mass transport limitation under strong irradiation.68-70 Theoretical simulations reveal that either single-molecule chemical binding energy on the surface of titania or solvation energy in acetonitrile of dyes H1 and H2 is almost identical, thereby the dye-dye intermolecular interactions should strongly affect the stability of self-assembled dye layer on the titania surface.71,72
CONCLUSIONS In summary, we have synthesized two donoracceptor organic dyes characteristic of conjugated polycycles
phenalenothiophene
and
benzoindenothiophene,
in
combination
with
the
bis(4-(hexyloxy)phenyl)amine ancillary electron-donor and 4-(benzo[c][1,2,5]thiadiazol-4-yl)benzoic acid electron-acceptor. The phenalenothiophene based organic dye has been employed to fabricate coadsorbate-free DSSCs with a cobalt based electrolyte, displaying a fairly good initial PCE of 9.7% 9 / 28
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and an encouraging durability during full-sunlight soaking at 60 °C for 1,000 h. Our work should encourage further exploration on organic dyes for stable and highly efficient dye-sensitized solar cells.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental
details
and
additional
figures,
including
theoretical
calculations,
cyclic
voltammograms, photovoltaic parameters, fluorescence kinetic traces, NMR and mass spectra.
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected]; Telephone: 0086-571-88273217. ORCID Peng Wang: 0000-0002-6018-1515 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by the National Science Foundation of China (nos. 51673165 and 91733302), the National 973 Program (2015CB932204), and the Fundamental Research Fund (no.2017FZA3007) for the Central universities.
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References (1) O'Regan, B.; Grätzel, M. A Low-Cost, High-efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737740. (2) Wang, P.; Zakeeruddin, S. M.; Moser, J.-E.; Nazeeruddin, M. K.; Sekiguchi, T.; Grätzel, M. A Stable Quasi-Solid-State Dye-Sensitized Solar Cell with an Amphiphilic Ruthenium Sensitizer and Polymer Gel Electrolyte. Nat. Mater. 2003, 2, 402−407. (3) Yu, Q.; Zhou, D.; Shi, Y.; Si, X.; Wang, Y.; Wang, P. Stable and Efficient Dye-Sensitized Solar Cells: Photophysical and Electrical Characterizations. Energy Environ. Sci. 2010, 3, 1722−1725. (4) Yella, A.; Lee, H.-W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.; Nazeeruddin, M. K.; Diau, E. W.-G.; Yeh, C.-Y.; Zakeeruddin, S. M.; Grätzel, M. Porphyrin-Sensitized Solar Cells with Cobalt(II/III)-Based Redox Electrolyte Exceed 12% Efficiency. Science 2011, 334, 629–634. (5) Mathew, S.; Yella, A.; Gao, P.; Humphry-Baker, R.; Curchod, B. F. E.; Ashari-Astani, N.; Tavernelli, I.; Rothlisberger, U.; Nazeeruddin, M. K.; Grätzel, M. Dye-Sensitized Solar Cells with 13% Efficiency Achieved Through the Molecular Engineering of Porphyrin Sensitizers. Nat. Chem. 2014, 6, 342–247. (6) Mishra, A.; Fischer, M. K.; Bäuerle, P. Metal-Free Organic Dyes for Dye-sensitized Solar Cells: From Structure-Property Relationships to Design Rules. Angew. Chem. Int. Ed. 2009, 48, 2474–2499. (7) Clifford, J. N.; Martínez-Ferrero, E.; Viterisi, A.; Palomares, E. Sensitizer Molecular Structure-Device Efficiency Relationship in Dye Sensitized Solar Cells. Chem. Soc. Rev. 2011, 40, 1635–1646. (8) Liang, M.; Chen. J. Arylamine Organic Dyes for Dye-Sensitized Solar Cells. Chem. Soc. Rev. 2013, 42, 3453–13488. (9) Bai, Y.; Zhang, J.; Zhou, D.; Wang, Y.; Zhang, M.; Wang, P. Engineering Organic Sensitizers for Iodine-Free Dye-Sensitized Solar Cells: Red-Shifted Current Response Concomitant with Attenuated Charge Recombination. J. Am. Chem. Soc. 2011, 133, 11442–11445. (10) Nusbaumer, H.; Moser, J.-E.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Grätzel, M. CoII(dbbip)22+ Complex Rivals Tri-iodide/Iodide Redox Mediator in Dye-sensitized Solar Cells. J. Phys. Chem. B 2001, 105, 10461–10464. (11) Hattori, S.; Wada, Y.; Yanagida, S.; Fukuzumi, S. Blue Copper Model Complexes with Distorted Tetragonal Geometry Acting as Effective Electron-Transfer Mediators in Dye-Sensitized Solar Cells. J. 11 / 28
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High-Performance Dye-Sensitized Solar Cells Based on Solvent-Free Electrolytes Produced from Eutectic Melts. Nat. Mater. 2008, 7, 626–630. (23) Wang, P.; Yang, L.; Wu, H.; Cao, Y.; Zhang, J. Xu, N.; Chen, S.; Decoppet, J.-D, Zakeeruddin, S. M.; Grätzel, M. Stable and Efficient Organic Dye-Sensitized Solar Cell Based on Ionic Liquid Electrolyte. Joule 2018, 2, 2145–2153. (24) Zarick, H. F.; Hurd, O. K.; Webb, J. A.; Hungerford, C.; Erwin, W. R.; Bardhan, R. Enhanced Efficiency in Dye-Sensitized Solar Cells with Shape-Controlled Plasmonic Nanostructures. ACS Photonics 2014, 1, 806–811. (25) Zarick, H. F.; Erwin, W. R.; Boulesbaa, A.; Hurd, O. K.; Webb, J. A.; Puretzky, A. A.; Geohegan, D. B.; Bardhan, R. Improving Light Harvesting in Dye-Sensitized Solar Cells Using Hybrid Bimetallic Nanostructures. ACS Photonics 2016, 3, 385–394. (26) Chen, S.; Yang, L.; Zhang, J.; Yuan, Y.; Dong, X. Wang. P. Excited-State and Charge Carrier Dynamics in a High-Photovoltage and Thermostable Dye-Sensitized Solar Cell. ACS Photonics 2017, 4, 165–173. (27) Daeneke, T.; Kwon, T. H.; Holmes, A. B.; Duffy, N. W.; Bach, U.; Spiccia, L. High-Efficiency Dye-Sensitized Solar Cells with Ferrocene-Based Electrolytes. Nat. Chem. 2011, 3, 211–215. (28) Feldt, S. M.; Gibson, E. A.; Gabrielsson, E.; Sun, L.; Boschloo, G.; Hagfeldt, A. Design of Organic Dyes and Cobalt Polypyridine Redox Mediators for High-Efficiency Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2010, 132, 16714–16724. (29) Zhou, D.; Yu, Q.; Cai, N.; Bai, Y.; Wang, Y.; Wang, P. Efficient Organic Dye-Sensitized Thin-Film Solar Cells Based on the Tris(1,10-phenanthroline)Cobalt(II/III) Redox Shuttle. Energy Environ. Sci. 2011, 4, 2030–2034. (30) Zhang, M.; Liu, J.; Wang, Y.; Zhou, D.; Wang, P. Redox Couple Related Influences of π-Conjugation Extension in Organic Dye-Sensitized Mesoscopic Solar Cells. Chem. Sci. 2011, 2, 1401–1406. (31) Tsao, H. N.; Yi, C.; Moehl, T.; Yum, J.-H.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Grätzel, M. Cyclopentadithiophene Bridged Donor–Acceptor Dyes Achieve High Power Conversion Efficiencies in Dye-Sensitized Solar Cells Based on the Tris-Cobalt Bipyridine Redox Couple. ChemSusChem 2011, 4, 591–594. (32) Liu, J.; Zhang, J.; Xu, M.; Zhou, D.; Jing, X.; Wang, P. Mesoscopic Titania Solar Cells with the 13 / 28
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Tris(1,10-phenanthroline)Cobalt Redox Shuttle: Uniped versus Piped Organic Dyes. Energy Environ. Sci. 2011, 4, 3021–3029. (33) Bai, Y.; Yu, Q.; Cai, N.; Wang, Y.; Zhang, M.; Wang, P. High-Efficiency Organic Dye-Sensitized Mesoscopic Solar Cells with a Copper Redox Shuttle. Chem. Commun. 2011, 47, 4376–4378. (34) Cao, Y.; Saygili, Y.; Ummadisingu, A.; Teuscher, J.; Luo, J.; Pellet, N.; Giordano, F.; Zakeeruddin, S. M.; Moser, J. -E.; Freitag, M.; Hagfeldt, A.; Grätzel, M. 11% Efficiency Solid-State Dye-Sensitized Solar Cells with Copper(II/I) Hole Transport Materials. Nat. Commun. 2017, 8, 15390. (35) Kang, S. H.; Jeong, M. J.; Eom, Y. K.; Choi, I. T.; Kwon, M. S.; Yoo, Y.; Kim, J.; Kwon, J.; Park, J, H.; Kim, H. K. Porphyrin Sensitizers with Donor Structural Engineering for Superior Performance Dye-Sensitized Solar Cells and Tandem Solar Cells for Water Splitting Applications. Adv. Energy Mater. 2017, 7, 1602117. (36) Eom, Y. K.; Kang, S. H.; Choi, I. T.; Yoo, Y.; Kim, J.; Kim, H. K. Significant Light Absorption Enhancement
by
a
Single
Heterocyclic
Unit
Change
in
the
π-Bridge
Moiety
from
Thieno[3,2-b]Benzothiophene to Thieno[3,2-b]Indole for High Performance Dye Sensitized and Tandem Solar Cells. J. Mater. Chem. A 2017, 5, 2297–2308. (37) Liu, Y.; Cao, Y.; Zhang, W.; Stojanovic, M.; Dar, M. I.; Péchy, P.; Saygili, Y.; Hagfeldt, A.; Zakeeruddin, S. M.; Grätzel, M. Electron-Affinity-Triggered Variations on the Optical and Electrical Properties of Dye Molecules Enabling Highly Efficient Dye-Sensitized Solar Cells. Angew. Chem. Int. Ed. 2018, 57, 14125–14128. (38) Cao, Y.; Liu, Y.; Zakeeruddin, S. M.; Hagfeldt, A.; Grätzel, M. Direct Contact of Selective Charge Extraction Layers Enables High-Efficiency Molecular Photovoltaics. Joule, 2018, 2, 1108–1117. (39) Kakiage, K.; Aoyama, Y.; Yano, T.; Oya, K.; Fujisawa, J. Hanaya, M. Highly-Efficient Dye-Sensitized Solar Cells with Collaborative Sensitization by Sily-Anchor and Carboxy-Anchor Dyes. Chem. Commun. 2015, 51, 15894–15897. (40) Komiya, R; Fukui, A; Murofushi, N; Koide, N; Yamanaka, R; Katayama, H. Improvement of the Conversion Efficiency of a Monolithic Type Dye Sensitized Solar Cell Module. Technical Digest, 21st International Photovoltaic Science and Engineering Conference 2011, 2C–5O–08. (41) Miettunen, K.; Saukkonen, T.; Li, X.; Law, C.; Sheng, Y. K.; Halme, J.; Tiihonen, A.; Barnes, P. R. F.; Ghaddar, T.; Asghar, I.; Lund, P.; O'Regan, B. C. Do Counter Electrodes on Metal Substrates Work with Cobalt Complex Based Electrolyte in Dye Sensitized Solar Cells? J. Electrochem. Soc. 14 / 28
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2012, 160, H132–H137. (42) Kashif, M. K.; Nippe, M.; Duffy, N. W.; Forsyth, C. M.; Chang, C. J.; Long, J. R.; Spiccia, L.; Bach, U. Stable Dye-Sensitized Solar Cell Electrolytes Based on Cobalt(II)/(III) Complexes of a Hexadentate Pyridyl Ligand. Angew. Chem. Int. Ed. 2013, 52, 5527–5531. (43) Xiang, W.; Huang, W.; Bach, U.; Spiccia, L. Stable High Efficiency Dye-Sensitized Solar Cells Based on a Cobalt Polymer Gel Electrolyte. Chem. Commun. 2013, 49, 8997–8999. (44) Jiang, R.; Anderson, A.; Barnes, P. R. F.; Law, C.; O'Regan, B. 2000 Hours Photostability Testing of Dye Sensitized Solar Cells Using a Cobalt Bipyrdine Electrolyte. J. Mater. Chem. A. 2014, 2, 4751– 4757. (45) Gao, J.; Achari, M. B.; Kloo, L. Long-Term Stability for Cobalt-Based Dye-Sensitized Solar Cells Obtained by Electrolyte Optimization. Chem. Commun. 2014, 50, 6249–6251. (46) Gao, J.; Yang, W.; Pazoki, M.; Boschloo, G.; Kloo, L. Cation-Dependent Photostability of Co(II/III)-Mediated Dye-Sensitized Solar Cells. J. Phys. Chem. C 2015, 119, 24704–24713. (47) Yao, Z.; Zhang, M.; Li, R.; Yang, L.; Qiao, Y.; Wang, P. A Metal-Free N-Annulated Thienocy -clopentaperylene Dye: Power Conversion Efficiency of 12% for Dye-Sensitized Solar Cells. Angew. Chem. Int. Ed. 2015, 54, 5994–5998. (48) Giribabu, L.; Bolligarla, R.; Panigrahi, M. Recent Advance of Cobalt(II/III) Redox Couples for Dye-Sensitized Solar Cell. Chem. Rec. 2015, 15, 760–788. (49) Yang, W.; Hao, Y.; Ghamgosar, P.; Boschloo, G. Thermal Stability Study of Dye-Sensitized Solar Cells with Cobalt Bipyridyl-Based Electrolytes. Electrochim. Acta 2016, 213, 879–886. (50) Ren, Y.; Sun, D.; Cao, Y.; Tsao, H. K.; Yuan, Y.; Zakeeruddin, S. M; Wang, P.; Grätzel, M. A Stable Blue Photsensitizer for Colar Palette of Dye-Sensitized Solar Cells Reaching 12.6% Efficiency. J. Am. Chem. Soc. 2018, 140, 2405–2408. (51) Choi, H.; Kim, S.; Kang, S. O. K.; Ko, JJ.; Kang, M. -S.; 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. (52) Qin, H.; Wenger, S.; Xu, M.; Gao, F.; Jing, X.; Wang, P.; Zakeeruddin, S. M.; Grätzel, M. An Organic Sensitizer with a Fused Dithienothiophene Unit for Efficient and Stable Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2008, 130, 9202–9203. (53) Zhang, M.; Wang, Y.; Xu, M.; Ma, W.; Li, R.; Wang, P. Design of High-Efficiency Organic Dye 15 / 28
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for Titania Solar Cells Based on the Chromophoric Core of Cyclopentadithiophene-Benzothiadiazole. Energy Envron. Sci. 2013, 6, 2944–2949. (54) Coppo, P.; Turner, M. L. Cyclopentadithiophene Based Electroactive Materials. J. Mater. Chem. 2005, 15, 1123–1133. (55) Chen, Y.-C.; Chen, Y.-H. Chou, H.-H.; Chaurasia, S.; Wen, Y. S.; Lin, J. T.; Yao, C.-F.; Napthyl and Thienyl Units as Bridges for Metal-Free Dye-Sensitized Solar Cells. Chem. Asian J. 2012, 7, 1074–1084. (56) Adamo, C.; Jacquemin, D. The Calculations of Excited-State Properties with Time-Dependent Density Functional Theory. Chem. Soc. Rev. 2013, 42, 845–856. (57) Rehm, J. M.; Mclendom, G. L.; Nagasawa, Y.; Yoshihara, K.; Moser, J.; Grätzel, M. Femtosecond Electron-Transfer Dynamics at a Sensitizing Dye-Semiconductor (TiO2) Interface. J. Phys. Chem. 1996, 100, 9577–9578. (58) Tachibana, Y.; Rubtsov, I. V.; Montanari, I.; Yoshihara, K.; Klug, D. L.; Durrant, J. R. Transient Luminescence Studies of Electron Injection in Dye Sensitized Nanocrystalline TiO2 Films. J. Photochem. Photobiol. A 2001, 142, 215–220. (59) Luo, L.; Lo, C.-F.; Lin, C.-Y.; Chang, I.-J.; Diau, E. W.-G. Femtosecond Fluorescence Dynamics of Porphyrin in Solution and Solid Films: The Effects of Aggregation and Interfacial Electron Transfer between Porphyrin and TiO2. J. Phys. Chem. B 2006, 110, 410–419. (60) Martín, C.; Ziółek, M.; Marchena, M.; Douhal, A. Interfacial Electron Transfer Dynamics in a Solar Cell Organic Dye Anchored to Semiconductor Particle and Aluminum-Doped Mesoporous Materials. J. Phys. Chem. C 2011, 115, 23183–23191. (61) Fakis, M.; Hrobárik, P.; Yushchenko, O.; Sigmundová, I.; Koch, M.; Rosspeintner, A.; Sththatos, E.; Vauthey, E. Excited State and Injection Dynamics of Triphenylamine Sensitizers Containing a Benzothiazole Electron-Accepting Group on TiO2 and Al2O3 Thin Films. J. Phys. Chem. C 2014, 118, 28509–28519. (62) Grabowski, Z. R.; Rotkiewicz, K. Rettig, W. Structural Changes Accompanying Intermolecular Electron Transfer: Focus on Twisted Intramolecular Charge-transfer States and Structures. Chem, Rev. 2003, 103, 3899–4031. (63) Kim, P.; Park, K. H.; Kim, W.; Tamcchi, T.; Iyoda, M.; Kim, D. Relationship between Dynamic Planarization Processes and Exciton Delocalization in Cyclic Oligothiophenes. J. Phys. Chem. Lett. 16 / 28
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2015, 6, 451–456. (64) Tachibana, Y.; Moser, J. E.; Grätzel, M.; Klug, D. R.; Durrant, J. R. Subpicosecond Interfacial Charge Separation in Dye-Sensitized Nanocrystalline Titanium Dioxide Films. J. Phys. Chem. 1996, 100, 20056–20062. (65) Plete, S.; Moser, J.-E.; Grätzel, M. Cooperative Effect of Adsorbed Cations and Iodide on the Interception of Back Electron Transfer in the Dye Sensitization of Nanocrystalline TiO2. J. Phys. Chem. B 2000, 104, 1791–1795. (66) Duffy, N. W.; Peter, L. M.; Rajapakse, R. M. G.; Wijajyantha, K. G. U. A Novel Charge Extraction Method for the Study of Electron Transport and Interfacial Transfer in Dye Sensitized Nanocrystalline Solar Cells. Electrochem. Commun. 2000, 2, 658–662. (67) O'Regan, B.; Scully, S.; Mayer, A. C.; Palomares, E.; Durrant, J. The Effect of Al2O3 Barrier Layers in TiO2/Dye/CuSCN Photovoltaic Cells Explored by Recombination and DOS Characterization Using Transient Photovoltage Measurements. J. Phys. Chem. B 2005, 109, 4616–4623. (68) Nelson, J. J.; Amick, T. J.; Michael Elliott, C. Mass Transport of Polypyridyl Cobalt Complexes in Dye-Sensitized Solar Cells with Mesoporous TiO2 Photoanodes. J. Phys. Chem. C 2008, 112, 18255– 18263. (69) Heiniger, L.; Giordano, F.; Moehl, T.; Grätzel, M. Mesoporous TiO2 Beads Offer Improved Mass Trasport for Cobalt-Based Redox Couples Leading to High Efficiency Dye-Sensitized Solar Cells. Adv. Energy Mater. 2014, 4, 1400168. (70) Jiang, R.; Boschloo, G. The Impact of Non-Uniform Photogeneration on Mass Transport in Dye-Sensitised Solar Cells. J. Mater. Chem. A. 2018, 6, 10264–10276. (71) Domenico, J.; Foster, M. E.; Spoerke, E. D.; Allendorf, M. D.; Sohlberg, K. Effect of Solvent and Substrate on the Surface Binding Mode of Carboxylate-Functionalized Aromatic Molecules. J. Phys. Chem. C 2018, 122, 10846−10856. (72) Marotta, G.; Lobello, M. G.; Anselmi, C.; Consiglio, G. B.; Calamante, M.; Mordini, Al.; Pastore, M.; De Angelis, F. An Integrated Experimental and Theoretical Approach to the Spectroscopy of Organic-Dye-Sensitized TiO2 Heterointerfaces: Disentangling the Effects of Aggregation, Solvation, and Surface Protonation. ChemPhysChem 2014, 15, 1116–1125.
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Figure 1. (a) Chemical structures of donoracceptor organic dyes H1 and H2 with 7H-phenaleno[1,2-b]thiophene (PT, painted in cyan) and 7H-benzo[6,7]indeno[1,2-b]thiophene (BIT, painted in magenta) as the main electron-releasing units. Herein bis(4-(hexyloxy)phenyl)amine is employed as the auxiliary electron-releasing unit, and 4-(benzo[c][1,2,5]thiadiazol-4-yl)benzoic acid (BTBA) as the electron-acceptor. (b) Steady-state UV-Vis absorption (solid lines) and photoluminescence (PL) (dot lines) spectra of dyes in toluene. The PL excitation wavelength: 475 nm. (c) Plots of average time constants ( ) of PL decay traces of dyes in toluene. The solid lines are displayed as a guide to the eyes.
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Table 1. Electronic Absorption and Photoluminescence Properties of Dyes in Toluenea Dye
aTD-MPW1K [nm]
a [nm]
PLTD-MPW1K [nm]
PL [nm]
PL [%]
H1
526
518
645
672
53
H2
545
534
655
690
63
aTheoretical
absorption maxima
(aTD-MPW1K ) and PL maxima
TD-MPW1K (PL )
are calculated at the
TD-MPW1K/6-311G(d,p) level for dye molecules in toluene. Experimental absorption maxima (a ) and fluorescence maxima ( PL ) are obtained from the spectra in Figure 1b. PL is the absolute PL quantum yield measured with an integrating sphere system.
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Figure 2. (ad) Optimized geometries in toluene of the S1eq states of (a) H1 and (b) H2, and the S0 states of (c) H1 and (d) H2. The large aliphatic chains were replaced with ethyl to improve the computational efficiency. Selected dihedral angles are shown on the side of a molecular skeleton, and are evidently distinct at the S1eq and S0 states for H1 and H2. Aromatic units are filled with different colors for clarity of presentation.
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E [eV]
(a)
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1
10
10
H1@alumina 1
[ps]
[ps]
E [eV]
(b)
2
H1@titania
H2@alumina
10
H2@titania
0
640
720
800
[nm]
880
960
1.4
1.3
E [eV]
(c) 100
1.9
1.8
1.7
1.6
1.5
10 640
720
800
[nm]
880
960
1.4
1.3
E [eV]
(d)
1.9
1.8
1.7
1.6
1.5
90 1
10 80
ei [ps]
QY [%]
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
ACS Photonics
H1
70
H1
H2
60 640
H2 0
720
800
[nm]
880
960
10 640
720
800
[nm]
880
960
Figure 3. Plots of wavelength-dependent average time constants ( ) of dyes grafted on alumina (a) and titania (b). The solid lines are displayed as a guide to the eyes. (c) PL quenching yields (QY) and (d) time constants of electron injection ( ei ) plotted as a function of up-converted PL wavelengths
( ) . The solid lines are also included as a guide to the eyes. Pump wavelength: 530 nm.
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(a) 1.0
(b) 1.0 a
c
A [a.u.]
A [a.u.]
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
0.5
0.0 -2 10
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b
d
0.5
0.0 1
10 t [s]
4
10
1
10 t [s]
4
10
Figure 4. Normalized transient absorption traces upon femtosecond laser pulse excitation of (a,c) H1and (b,d) H2-grafted titania films soaked with (a,b) an inert electrolyte composed of 0.5 M tert-butylpyridine and 0.1 M lithium bis(trifluoromentylsulfonyl)imide in acetonitrile and (c,d) a Co-bpy electrolyte. The excitation wavelengths were selected according to a 0.5 optical density of dye-grafted tiania films to obtain a similar distribution profile of vertically excited states in our testing samples. Excitation wavelength: 625 nm for H1 and 642 nm for H2. Pulse fluence: 35 μJ cm2. Probe wavelength: 1300 nm. Experimental data were further fitted by a sum of multi-exponential function as light gray solid lines.
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(d)
(a) 100 60
VOC [V]
LHE [%]
80 H1 H2
40
0
1.0
H1 H2 0.9
0.8
20 400
500
600
700
800
0.5
1
(e) VOC [V]
80
EQE [%]
10
JSC [mA cm ]
(b) 100 60 H1 H2
40
5 2
[nm] 1.0
H1 H2
0.9 0.8
20 0
0.7 400
500
600
700
800
5
10
15 10
H1 H2
5 0 0.0
100
[C]
3
(f)
10 10
TPD
2
Q
t1/2 [ms]
(c)
50 CE
[nm]
J [mA cm ]
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
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2
H1 H2
1
10
0
0.2
0.4
0.6
0.8
1.0
10
5
10
50 CE
Q
V [V]
100
[C]
Figure 5. Cell characterization. (a) Plots of light-harvesting efficiency (LHE) as a function of for dye-grafted mesoporous titania films (7.0 m-thick), which were immersed in a Co-bpy electrolyte. (b) Plots of external quantum efficiency (EQE) as a function of . (c) Currentvoltage (JV) curves at an irradiance of the 100 mW cm–2, AM1.5G conditions. (d) Plots of open-circuit photovoltages (VOC ) as a function of short-circuit photocurrents
( J SC ) . (e) Plots of charges
(Q CE )
extracted from
TPD dye-grafted titania films as a function of VOC . (f) Plots of half-lifetimes (t1/2 ) of electrons stored in
titania as a function of Q CE .
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Table 2. Averaged Photovoltaic Parameters of 4 Cells with a Co-bpy Electrolyte Measured under Irradiation of 100 mW cm2, AM1.5G Conditionsa Device
J SCEQE [mA cm2]
J SC [mA cm2]
VOC [mV]
FF [%]
PCE [%]
H1
14.40±0.06
14.33±0.09
931±6
72.3±0.3
9.7±0.1
H2
15.32±0.04
15.47±0.05
903±4
74.0±0.3
10.3±0.1
a
J SCEQE was computed by wavelength integral of the product of the EQEs measured at the short-circuit
and the standard AM 1.5G emisssion specturum (ASTM G173-03). Note that the errors of photovoltaic parameters are calculated from the equation
1 4 (X i )2 , where , X i , and represent 4 i 1
the standard deviation, observed value, and mean value of photovoltaic parameter, respectively.
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2
JSC [mA cm ]
VOC [V]
1.2 H1
H2
H1
H2
0.8
24 16 8
H1
H2
H1
H2
FF
1.0
0.5
PCE [%]
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
ACS Photonics
12 6 0
200
400
600
800
1000
t [h]
Figure 6. Evolution of photovoltaic parameters measured at the AM1.5G conditions for DSSCs made with H1 and H2 in contact with a Co-bpy electrolyte, during aging under the full sunlight at 60 °C for 1,000 h.
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(b) 1.0 VOC [V]
VOC [V]
(a) 1.0 0.8 fresh aged
0.6
0.8
1
0.8
1
0.8
fresh aged
0.6
2
10
1
10
Q
CE
2
10 CE Q [C]
[C]
10
(f)
(e)
2
10
2
10
t1/2 [ms]
1
10
TPD
TPD
10 -2 JSC [mA cm ]
(d) 1.0 fresh aged
VOC [V]
VOC [V]
(c) 1.0
0.6
fresh aged
0.6
1 10 -2 JSC [mA cm ]
t1/2 [ms]
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
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fresh aged
0
10
0
10
1
10
1
2
10
10
Q
CE
[C]
fresh aged 1
10 CE Q [C]
2
10
Figure 7. (a,b) Comparison of open-circuit photovoltages (VOC ) at given short-circuit photocurrent densities ( J SC ) of DSSCs before and after aging under the full sunlight at 60 °C for 1,000 h: (a) H1 and (b) H2. (c,d) Plots of charges (Q CE ) extracted from the titania layer against (VOC ) of DSSCs before and after aging under the full sunlight at 60 °C for 1,000 h: (c) H1 and (d) H2. (e,f) Plots of TPD electron half-lifetimes (t1/2 ) against Q CE of DSSCs before and after aging under the full sunlight at
60 °C for 1,000 h: (e) H1 and (f) H2.
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(a) 100
(b) EQE [%]
EQE [%]
75 fresh aged
50 25 0
400
500
600
700
[nm]
(d) fresh aged
50 25 0
400
500
600
700
[nm]
800
(f)
1.0
OD [a.u.]
OD [a.u.]
(e)
LHE [%]
LHE [%]
75
fresh aged
0.5 0.0 0
1
2 10 100 1000 t [s]
(g)
fresh
100 75 fresh aged
50 25 0
800
(c) 100
400
500
600
[nm]
2
4
t [ps]
10
100
700
800
fresh aged
50 25 0
400
500
600
[nm]
1.0 0.5
fresh aged
0.0 0
1
2
t [s]
10 100 1000
(h)
fresh aged 800 nm
860 nm
860 nm
0
800
75
IPL [a.u.]
800 nm
700
100
aged
IPL [a.u.]
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
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0
2
4
t [ps]
10
100
Figure 8. (a,b) External quantum efficiencies (EQEs) of DSSCs with a bilayer TiO2 film fresh (cyan lines) and aged (magenta lines) aging under the full sunlight at 60 °C for 1,000 h: (a) H1 and (b) H2. (c,d) Light-harvesting efficiencies (LHEs) of DSSCs with a transparent TiO2 film: (c) H1 and (d) H2. (e,f) Normalized transient absorption traces upon femtosecond laser pulse excitation of DSSCs with a transparent TiO2 film: for H1 (e) and for H2 (f). (g,h) Normalized kinetic traces at the PL wavelength of 800 nm, and 860 nm of DSSCs with a transparent TiO2 film: (g) H1 and (h) H2.
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TOC
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