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Low-energy-gap organic photosensitizers with phenalenothiophene and benzoindenothiophene as primary electron-donors for durable dye-sensitized solar cells
Journal of Power Sources 451 (2020) 227748
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Low-energy-gap organic photosensitizers with phenalenothiophene and benzoindenothiophene as primary electron-donors for durable dye-sensitized solar cells Heng Wu a, b, Xinrui Xie b, Jing Zhang b, Sining Li a, *, Zuochun Shen a, Yi Yuan b, ** a b
National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, Harbin, 150001, China Department of Chemistry, Zhejiang University, Hangzhou, 310028, China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Two low-energy-gap organic dyes with long excited-state lifetimes were prepared. � Both dyes with stabilized LUMO still ensure efficient electron-injection. � A organic dye achieves an impressive efficiency of 10.9% without any coadsorbate. � 7H-phenaleno[1,2-b]thiophene based dye solar cell presents excellent durability.
A R T I C L E I N F O
A B S T R A C T
Keywords: Solar cell Photosensitizer Low energy gap Charge carrier dynamics
Reducing the optical energy gap of a photosensitizer to broaden the spectral response is of paramount importance to improve the power conversion efficiency of dye-sensitized solar cells (DSSCs). Herein, we report two isomeric photosensitizers with planar aromatic polycycles 7H-phenaleno[1,2-b]thiophene (PT) and 7H-benzo[6,7]indeno [1,2-b]thiophene (BIT) as the kernel units of electron-donors and 4-(benzo[c][1,2,5]thiadiazol-4-ylethynyl) benzoic acid (BTEBA) as the electron-acceptor. The efficient electron-acceptor BTEBA can bring forth an obvious downward shift of the lowest unoccupied molecular orbital energy level of an organic photosensitizer and thus shrink the energy gap significantly. Femtosecond fluorescence decay and nanosecond transient absorption measurements suggest that the two low-energy-gap photosensitizers present efficient charge separation yields in DSSCs. These two photosensitizers are used to make coadsorbate-free, high-photovoltage-output DSSCs in combination with a tris(2,20 -bipyridine)cobalt based redox electrolyte. DSSCs with the BIT based photosensitizer achieves up to 10.9% power conversion efficiency at the 100 mW cm-2, simulated AM1.5G conditions. The cell using the PT based photosensitizer presents an excellent stability under full sunlight soaking at 60 � C for 1000 h, retaining 88% of its initial efficiency.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Li), [email protected] (Y. Yuan). https://doi.org/10.1016/j.jpowsour.2020.227748 Received 5 September 2019; Received in revised form 9 January 2020; Accepted 12 January 2020 Available online 30 January 2020 0378-7753/© 2020 Elsevier B.V. All rights reserved.
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Journal of Power Sources 451 (2020) 227748
Fig. 1. (a) Chemical structures of D-A organic dyes H3 and H4 with 4-(benzo[c] [1,2,5]thiadiazol-4-ylethynyl)benzoic acid (BTEBA, marked in red) as the electron-acceptor. (b) Theoretical LUMO energy levels (values above color bars), HOMO energy levels (values under color bars), and HOMO/LUMO energy gaps ðΔEB3LYP H=L Þ of H3 and H4 in THF. (c) Steady UV–Vis absorption spectra (solid lines) and fluorescence spectra (short dash) of dyes in PhMe and THF. (d) Scatter plots of averaged time constants ðτÞ of fluorescence decay traces for dyes in PhMe and THF. The solid lines are displayed only as a guide to the eyes. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Table 1 Electronic absorption and fluorescence properties of dyes in THF and PhMe.a. Dye
MPW1K λTD [nm] ABS;MAX
λMEAS ABS;MAX [nm]
εMEAS ABS;MAX [mM
H3/THF H4/THF H3/PhMe H4/PhMe
553 573 559 580
531 550 538 558
27.7 29.0 27.1 30.4
a
1
cm 1]
MPW1K [nm] λTD FL;MAX
λMEAS FL;MAX [nm]
ΔνTD
710 724 667 678
767 809 690 704
4.00 3.64 2.90 2.55
MPW1K
[103 cm
1
]
ΔνMEAS [103 cm
1
]
5.79 5.82 4.09 3.72
MPW1K MPW1K Theoretical absorption maxima ðλTD Þ and fluorescence maxima ðλTD Þ were calculated at the TD-MPW1K/6-311G(d,p) level for dye molecules in PhMe ABS;MAX FL;MAX
MEAS MEAS and THF. Experimental absorption maxima ðλMEAS Þ were ABS;MAX Þ and fluorescence maxima ðλFL;MAX Þ were acquired from the spectra in Fig. 1c, and stokes shifts ðΔν derived from the electronic absorption and fluorescence spectra of dye dissoved in PhMe and THF.
1. Introduction
DSCs is the main factor influencing the charge collection efficiency. Meanwhile, continuous explorations on electron-rich donors and electron-deficient acceptors have been performed to prolong the excited-state lifetime of D-A organic photosensitizers by constructing exotic polycyclic heteroaromatics (PHAs) [7,8]. It is also necessary to attach the flexible side chains onto the photoactive backbone to alleviate the adverse intermolecular π-π stacking of dye molecules grafted on the surface of titania, which is normally considered to engender dissipative exciton annihilation. In this context, some organic dyes have been prepared and achieved impressive power conversion efficiencies (PCEs) of over 12% in conjunction with a cobalt-based redox electrolyte, such as perylene dye C275 [9], C281 [10], anthracene dye R6 [7], silyl-anchor dye AKDEKA-1 [11], and triazatruxene dye ZL003 [12]. In spite of the gradual improvement in efficiency, the commercial application of DSSCs based on a cobalt electrolyte is heavily hindered by the long-term sta bility issue [13,14]. Recently, we prepared two naphthalene based D-A organic dyes H1
Solar energy conversion is one of the most promising approaches to solve the problems of fossil fuel depletion and environmental pollution. In this regard, dye-sensitized solar cells (DSSCs) have been proposed as one of the next generation photovoltaic technologies due to the low-cost and facile manufacture procedure [1]. The exploration of photosensi tizers including ruthenium-polypyridine [2,3] and zinc-porphyrin complexes [4] as well as metal-free organic dyes [5,6] has been inten sively carried out in the past decades. Donor–acceptor (D-A) organic sensitizers are characteristic of abundant raw materials, elastic molec ular designs, and gorgeous colors. In general, the structure of a sensitizer as the kernel component of DSSCs plays a pivotal role in dominating light absorption and regulating the compactness of self-assembling molecular layer on the surface of wide-band-gap titania. To improve the harvesting yield of near-infrared (NIR) solar photons, the common strategy is to design a low optical energy gap sensitizer. Interface recombination relative to charge transport through the photoanodes in 2
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Journal of Power Sources 451 (2020) 227748
Fig. 2. Reduced density gradient (RDG) isosurfaces (s ¼ 0.5 au.) for the S0 and Seq 1 states of H3 and H4 in PhMe. The surfaces are colored on a blue-green-red scale, according to the values of the electron density multiplied by the sign of the second Hessian eigenvalue ðsignðλ2 ÞρÞ, ranging from 0.04 to 0.02 au. Blue represents strong attractive interactions, and red represents strong nonbonded overlap. The S0 state: (a) H3; (c) H4. The Seq 1 state: (e) H3; (g) H4. Plots of the RDG as a function of signðλ2 Þρ. The S0 state: (b) H3; (d) H4. The Seq 1 state: (f) H3; (h) H4. The hexyl chain was substituted with ethyl for the sake of computational efficiency. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 3. (a, b) Wavelength-dependent average time constant ðτÞ of fluorescence decay traces for H3 and H4 grafted oxide films in conjunction with a cobalt electrolyte: (a) alumina; (b) titania. Pump wavelength: 530 nm. (c) Plots of fluorescence quenching yield ðQYÞ as a function of fluorescence wavelength ðλÞ for H3 and H4. (d) Plots of time constants of electron injection as a function of λ. (e,f) Normalized transient absorption traces at 1300 nm of dye-grafted titania films with (e) H3 and (f) H4, which are also in contact with an inert electrolyte composed of 0.5 M 4-tert-butypyridine and 0.1 M lithium bis(trifluoromethanesulfonyl)imide in acetonitrile or a cobalt electrolyte. Pulse fluence: 25 μJ cm 2. The excitation wavelength was selected at a 0.5 optical density of dye-grafted titania films to yield a similar dis tribution profile of vertically excited states in the testing samples. Excitation wavelength: 618 nm for H3; 640 nm for H4.
3
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Journal of Power Sources 451 (2020) 227748
Fig. 4. (a) EQE curves of DSSCs with a cobalt electrolyte. (b) LHE curves of dye-sensitized transparent titania films (7.0 μm-thick). (c) Plots of electron diffusion length (Ln ) as a function of density of states (DOS). The short dash line represents thickness of the mesoporous titania layer of DSSCs.
Fig. 5. (a) J V curves of H3 and H4 based DSSCs measured at the 100 mW cm 2, AM 1.5G condition. (b) Charge extracted from the dye-grafted titania films ðQCE Þ as a function of VOC : (C) Plots of half-lifetime ðt TPD 1=2 Þ of electrons in the titania films versus.QCE :
and H2 featuring the coplanar aromatic units 7H-phenaleno[1,2-b] thiophene (PT) and 7H-benzo [6,7]indeno[1,2-b]thiophene (BIT) [15]. The H1-based device exhibits an excellent stability during full-sunlight soaking at 60 � C for 1000 h, retaining 90% of its initial PCE. In this work, we employ an efficient electron-acceptor 4-(benzo[c] [1,2,5] thiadiazol-4-ylethynyl)benzoic acid (BTEBA) [10] to replace electron-acceptor 4-(benzo[c] [1,2,5]thiadiazol-4-yl)benzoic acid (BTBA) [16], constructing two simple low-energy-gap organic dyes H3 and H4 (Fig. 1a). Preliminary theoretical calculations show that the introduction of more electron-deficient acceptor BTEBA does not impact the highest occupied molecular orbital (HOMO) energy levels, but re sults in an obviously downward displacement of the lowest unoccupied molecular orbital (LUMO) energy levels. The influence of electron-acceptor BTEBA on light-harvesting, excited state dynamics, and photovoltaic characteristics of these two dyes will be further sys tematically investigated.
Table 2 Photovoltaic parameters of cells with H3, H4, and C268 measured at the 100 mW cm 2, AM1.5G condition. Cell
2 JEQE SC [mA cm ]
JSC [mA cm
H3 H4 C268
15.16 16.53 17.95
15.21 16.58 16.33
2
]
VOC [mV]
FF [%]
PCE [%]
926 902 826
73.1 72.6 74.4
10.3 10.9 10.0
a EQE JSC
was calculated by wavelength integation of the product of the standard AM 1.5G emission spectrum (ASTM G173-03) and the EQE curve measured at the short-circuit.
before use. The synthetic details on H3 and H4 and structure charac terization with 1H NMR, 13C NMR, MALDI-TOF, and elemental analysis are elaborated in the Electronic supplementary information.
2. Experimental
2.2. Fabrication and characterization of DSSCs
2.1. Materials
A bilayer titania film was screen-printed on fluorine-doped tin oxide (FTO) glass (1.6 mm, white glass, NSG) and employed as the negative electrode (photoanode) of a DSSC. The bilayer film is composed of a 4.0μm-thick mesoporous layer formed with small anatase particles (Dyesol, 30NR-D), and a 3.0-μm-thick macroporous layer formed with large anatase particles (paticle size, 350–450 nm, Dyesol, WER4-0). The de tails on film and cell manufacture were elaborated in a former paper [17]. Dye-loading was carried out by dipping a titania film in a dye solution overnight. The dyes (150 μM) were dissolved in a solvent
Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), 1-ethyl-3methylimidazolium bis(trifluoromethanesulfonyl)imide (EMITFSI), decamethylferrocene (DMFc), ferrocene (Fc), 4-tert-butylpyridine (TBP), bis(triphenylphosphine)palladium(II) chloride (Pd(PPh3)2Cl2), n-butyl lithium (n-BuLi), trimethyltin chloride (Me3SnCl), potassium hydroxide (KOH), and phosphoric acid were purchased from Energy Chemical. Toluene (PhMe) and tetrahydrofuran (THF) were dried and distilled 4
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Journal of Power Sources 451 (2020) 227748
2.4. Theoretical calculations Quantum calculations were performed by using the 6-311G(d,p) basis set in the Gaussian 09 software package. The conductor-like polarized continuum model (C-PCM) [20] was applied to simulate the solvent (PhMe and THF) effect. The universal B3LYP exchange-correlation functional [21] was selected to optimize the ground-state geometries. The TD-MPW1K hybrid functional, which features 42% Hartree-Fock exchange, was employed to compute the vertical electron transitions and excited-state geometries [22,23]. The analysis of reduced density gradient non-covalent interaction (RDG-NCI) was conducted by a wavefunction software Multiwfn [24]. 2.5. Photophysical measurements and data analysis The steady UV–Vis spectra were measured by the G1103A spec trometer (Agilent). The static fluorescence spectra were tested with a QEPro spectrometer (Ocean Optics). Femtosecond fluorescence upconversion traces were implemented with a time-resolved fluorescence spectrometer (Halcyone Fire, Ultrafast). The fundamental pulses were generated with a Ti: Sapphire laser system (Astrella, 800 nm, 35 fs, 7 mJ per pulse, 1 kHz repetition rate, Coherent Inc.). The output of femto second pulses at 800 nm was divided into two parts: the large portion was sent to one optical parametric amplifier (OPA) to generate the pump pulse (530 nm), and the small portion was sent to another OPA to generate the gate pulse (1200 nm). Both the wavelength-dependent fluorescence emitted from the randomly moving sample and the 1200 nm gate pulse were focused onto a BBO crystal to generate a sum fre quency light, which then passed through a monochromator and detected by a photomultiplier tube (PMT). This setup is very useful to monitor the broad fluorescence emission of DSSC dyes in the infrared region. Subnanosecond transient absorption (TA) traces were measured using a pump-probe transient absorption spectrometer (EOS, Ultrafast System). The time-dependent fluorescence intensities ðIFL Þ at all detected wavelengths of each dye were globally fitted with several time constants according to Eq. (1)
Fig. 6. Evolution of photovoltaic parameters measured at the 100 mW cm 2, AM1.5G condition of cells with H3 and H4, which were aged under full sunlight at 60 � C.
n X
IFL ∝
t = τi Þ � IRF;
Ai expð
(1)
i¼1
mixture of chloroform and ethanol at a volume ratio of 1/19 for both H3 and H4. A Surlyn O-ring (thickness, 25 μm) was used to stick a dyed titania electrode and a platinized FTO electrode under a proper me chanical pressure at 130 � C. Electrical measurements were performed as reported in previous papers [18,19]. The composition of a cobalt based redox electrolyte employed in the device manufacture is listed as fol lows: 0.25 M tris(2,20 -bipyridine)cobalt(II) di[bis(tri fluoromethanesulfonyl)imide, 0.10 M tris(2,20 -bipyridine)cobalt(III) tris [bis(trifluoromethanesulfonyl)imide], 0.5 M TBP, and 0.1 M LiTFSI in acetonitrile.
where Ai is the amplitude, t and τi represent the delay time and the time constant, respectively, and IRF is the instrumental response function. The amplitude-weighted average lifetimes ðτÞ of fluorescence decays can be derived from Eq. (2) , ! n n X X τ¼ Ai τ i Ai Ai > 0 : (2)
2.3. Electrochemical measurements
where τðT; λÞ and τðA; λÞ represent the average lifetimes of fluorescence decay traces for dyes on titania and alumina, respectively. The calcu lation of time constant of electron injection ðτei ðλÞÞ is based on Eq. (4)
i¼1
i¼1
The calculation of quenching yield ðQYðλÞÞ is based on Eq. (3) � � � � τðT; λÞ QY λ ¼ 1 � 100%; τðA; λÞ
A CHI660C electrochemical workstation was used to measure cyclic voltammograms in combination with a three-electrode electrochemical cell equipped with a glass carbon as working electrode, a platinum gauze as counter electrode, and a silver wire as auxillary electrode. Impedance spectroscopy measurements of DSSCs were performed under illumina tion of a white light-emitting diode on an IM6ex electrochemical workstation, with a frequency range from 50 mHz to 1 MHz and a po tential modulation of 20 mV. A potential bias (V) was applied to equal the open-circuit photovoltage ðVOC Þ at each irradiation intensity, meeting the requirement of zero current. The obtained impedance spectra were fitted with the Z-view software (v2.80, Scribner Associates Inc.).
τei ðλÞ ¼
1 1=τðT; λÞ
: 1=τðA; λÞ
(3)
(4)
The transient absorption traces at 1300 nm (Fig. 3c and d) were fitted with a multi-exponential function for easy determination of the halfreaction time constants. The calculation of hole injection yield ðφhi Þ is based on Eq. (5) ! tCo 1=2 φhi ¼ 1 inert � 100%; (5) t1=2 5
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Journal of Power Sources 451 (2020) 227748
Fig. 7. (a, b) EQE curves of fresh (solid symbols) and aged (open symbols) cells: (a) H3; (b) H4. The cells were aged under full sunlight at 60 � C for 1000 h. (c, d) LHE curves of DSSCs with a 4 μm-thick TiO2 film: (c) H3; (d) H4. (e, f) Normalized kinetic traces of transparent DSSCs with a 4 μmthick TiO2 film at the fluorescence wave lengths of 800 nm and 880 nm: (e) H3 and (f) H4. (g, h) Normalized transient absorp tion traces upon femtosecond laser pulse excitation of transparent DSSCs with a 4 μmthick TiO2 film: (g) H3; (h) H4.
bear a similar HOMO/LUMO energy gap ðECV H=L Þ: Our experiments have
where tCo 1=2 represents the half-reaction time constant of transient ab
further proved the benefit of using BTEBA instead of BTBA for a lowenergy-gap DSSC dye [8,10]. Density functional theory (DFT) calcula tions at the B3LYP/6-311G(d,p) level shown in Fig. 1b could well simulate the relative tendencies of electrochemical HOMO and LUMO energy levels of H3 and H4.
sorption trace of a transparent DSSCs in combination with a cobaltbased electrolyte, and t inert 1=2 represents the half-reaction time constant of transient absorption trace of a transparent dummy cell using an inert electrolyte without the cobalt based redox couple. 3. Results and discussion
3.2. Photophysical properties
3.1. Energy levels
Fig. 1c shows the steady-state UV–Vis absorption and fluorescence spectra of dyes H3 and H4 dissolved in PhMe and THF. Key spectro scopic data are collected in Table 1. The maximum absorption wave lengths ðλMEAS ABS;MAX Þ of both dyes in PhMe display red-shifted absorption
To evaluate the influence of electron acceptors (BTEBA versus BTBA) on the HOMO and LUMO energy levels of PT and BIT based dyes, cyclic voltammograms (CVs) of H3 and H4 dissolved in tetrahydrofuran (THF) were measured under nitrogen and also compared with those of H1 and H2. In our experiments, EMITFSI was used as the supporting electrolyte and DMFc was added to the electrolyte as an internal reference. The potential was further calibrated with respect to the standard redox couple ferrocene/ferrocenium (Fc/Fcþ). The measured redox potential of Fc is 488 mV more positive than that of DMFc. The HOMO and LUMO energy levels can be obtained from CV curves shown in Fig. S3 by use of equation E ¼ 4:88 eEonset ; where Eonset represents the redox potential of a ground state dye molecule in THF. In contrast to dyes H1 and H2 with electron acceptor BTBA, H3 and H4 with BTEBA feature ~100 meV stablized LUMO energy levels and comparable HOMO levels. H3 and H4
peaks compared with those in THF. In contrast to dye H3, the λMEAS ABS;MAX values of dye H4 are red-shifted, by 20 nm in PhMe and 19 nm in THF. The maximum molar absorption coefficients ðεMEAS MAX Þ in PhMe are 27.7 � 103 M-1 cm-1 for dye H3 and 30.4 � 103 M-1 cm-1 for dye H4, similar to those in THF (27.1 � 103 M-1 cm-1 for H3 and 29.0 � 103 M-1 cm-1 for H4). Time-dependent DFT (TDDFT) calculations show that the relative MPW1K tendency of theoretical λTD values is in good agreement with that ABS;MAX of experimental ones. The S1←S0 vertical electronic excitations of dyes H3 and H4 are mainly due to the transition from HOMO and HOMO-1 to LUMO (Table S1). An apparent intramolecular charge-transfer feature 6
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Journal of Power Sources 451 (2020) 227748
Fig. 8. (a) Charge extracted from the dye-grafted titania films ðQCE Þ as a function of VOC for DSSCs before and after aging under full sunlight at 60 � C for 1000 h. (b) CE Plots of half-lifetime ðt TPD of fresh and aged cells. (c, d) Pictorial illustration of the self-assembling films of H3 and H4 1=2 Þ of electrons in the titania films versus Q
grafted on the surface of titania to mitigate the adverse contact between titania and electrolyte, and to control the diffusion and permeation of lithium ions: (c) H3; (d) H4.
can be perceived from the contour plots of HOMO and LUMO in Figs. S5 and S7. The fluorescence maximum ðλMEAS FL;MAX Þ of either H3 or H4 in THF is red-shifted compared with that in PhMe, owing to the more stabilized excited state in the polar solvent. Moreover, dye H4 possesses a redshifted fluorescence peak in contrast to that of H3. As tabulated in Table 1, H3 and H4 in either THF or PhMe show large Stokes shift ðΔvMEAS Þ. The large energy losses upon vertical excitation have indicated the presence of vibration, torsional, and solvation relaxation. A larger ΔvMEAS in THF could be detected against that in PhMe for each dye. In addition, the tendencies of measured fluorescence maxima and Stokes shifts are in good accord with those based on DFT and TD-DFT calculations. Noncovalent interactions (NCI) analysis [25] is calculated for dyes H3 and H4 at the ground state ðS0 Þ and the first equilibrium excited state ðSeq 1 Þ. The H⋯S, N⋯H, H⋯S, and H … π intramolecular interactions of both dyes are identified by the reduced density gradient (RDG) iso surfaces shown in Fig. 2. Note that the value (ρsignλ2 ) of the electron density multiplied by the sign of the second Hessian eigenvalue is considered to be an indicator of interaction strength, and the sign of ρsignλ2 represents the interaction type in which the negative shown by blue or green color in RDG isosurfaces is indicative of attractive inter action, and the positive shown by red color is indicative of nonbonding. The spikes in the negative ρsignλ2 region (Fig. 2) for H3 and H4 at the S0 state are observed to move to larger negative values at the Seq 1 state, demonstrating that the noncovalent interactions between the benzo[c] [1,2,5]thiadiazole (BT) unit and the adjacent thiophene unit are enhanced after conformation relaxation of the vertical excited states. This is consistent with the improved planarity of the conjugated back bones, as revealed by the significantly reduced dihedral angle between thiophene (green) and BT (red) from the S0 state to the Seq 1 state (Figs. S6 and S8). To integrally acquire the excited-state properties of H3 and H4 in
PhMe and THF, we measured femtosecond fluorescence up-conversion traces [26,27] in a broad wavelength band range with a 1200 nm gate pulse. Note that very few studies [15] have been performed on the excited state properties of DSSCs dyes with this technique. All kinetic traces can be globally fitted by five time constants and the processed data are provided in Fig. S15 S18 and Table S2 S5, with analytical details listed in experimental section. As Fig. 1d presents, the average time constant (τ) of both dyes are raised by 1–3 orders of magnitude with the red-shifting of fluorescence wavelength. Eventually, the τ reaches a steady value at long fluorescence wavelengths interpreted as the lifetime eq of Seq 1 for each dye molecule. The S1 lifetime in PhMe is 868 ps for H3 and 1055 ps for H4, which are slightly reduced compared to those of H1 and H2 (Fig. S4), owing to the HOMO/LUMO energy gap shrinkage [28]. In THF, H4 features a Seq 1 lifetime of 224 ps, which is about twice longer than H3 (111 ps). The shorter Seq 1 lifetime of each dye in THF compared with that in PhMe is attributed to the acceleration of non-radiative recombination. Although dye H4 bears a smaller optical energy gap, its equilibrium excited state lasts longer, especially in a polar solvent, which is important for sufficient charge separation in the high dielectric environment of DSSCs. 3.3. Photoinduced charge transfer and photocurrent action spectra of solar cells As presented in Fig. S9a, there is a blue-shifted fluorescence maximum of either H3 or H4 on titania in contrast to that on alumina, which could be associated with the occurrence of electron injection from the nonequilibrium hot excited state to titania and thereby reduced emission from the quilibrium excited state. Note that in our experiments the dyed oxide films are in contact with a cobalt-based electrolyte for device fabrication. To further ascertain the yield ðφei Þ and kinetics ðτei Þ of electron injection, we also executed femtosecond fluorescence up7
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Journal of Power Sources 451 (2020) 227748
mA cm-2, an open-circuit photovoltage ðVOC Þ of 926 mV, and a fill factor (FF) of 73.1%, yielding a PCE of 10.3%. Meanwhile, the H4 cell has an improved PCE of 10.9%, generated from the obviously enhanced JSC of 16.58 mA cm-2, the VOC of 902 mV, and the FF of 72.6%. The higher JSC value of H4 is attributed to the broader photocurrent response compared with that of H3. Note that the PCE of the device based on the highperformance C268 dye [8] under the same condition is 10.0% ðJSC ¼ 16.33 mA cm 2, VOC ¼ 826 mV, FF ¼ 74.4%). Furthermore, the PCEs of DSSC with the well-known dyes N719 and Z907 under the same device condition are 2.1% and 6.9% [15]. The high photovoltage output of low-energy-gap organic dyes H3 and H4 contribute to the impressive power conversion efficiencies. We then carried out charge extraction (CE) and transient photo voltage decay (TPD) measurements [32,33] to gain a deep insight into the inherent interfacial energetics and kinetics behind the VOC variation. As Fig. 5b shows, the charges ðQCE Þ stored in titania of the H3 and H4 cells were similar at the same potential bias. However, the H3 cell CE than that of the H4 presents a longer half-lifetime ðtTPD 1=2 Þ at a fixed Q
conversion measurements (Fig. S19 S22) on dyed films. All fluores cence decay traces can be globally fitted by four time constants, and the fitting parameters are listed in Table S6 S9. The three fluorescence decays at 660 nm, 760 nm, and 860 nm (Fig. S9b) for a H3 dyed titania film are obviously different. It is apparent that the τ values of decay traces for the dyed films (Fig. 3a and b) increase by 1–3 orders of magnitude along with the red-shifting of fluorescence wavelength. The τ at longer fluorescence wavelength is regarded as the Seq 1 lifetime, being about 300 ps for each dye on alumina, which is much larger than that (~40 ps) of the dyed titania samples. The Seq 1 lifetimes of H3 and H4 on alumina are shorter than those in THF, which can be rationalized by the joint contribution of dye aggregation and high dielectric environment of electrolyte. Fluorescence quenching yields ðQYÞ and time constants ðτei Þ of electron injection were depicted in Fig. 3c and d. On account of vibrational, torsional, and solvent relaxtions of the nonequilibrium excited states, the lower fluorescence QY values at short fluorescence wavelengths are detected for both dyes. The fluorescence QY values of the equilibrium excited states are 89% for H3 and 87% for H4. As pre sented in Fig. 3d, electron injection time constants of both dyes gradu ally increase by several times along with the red-shift of fluorescence wavelength, which can be understood by the driving force dependent electron injection kinetics. The τei values of electron injection for H3 and H4 at the Seq 1 states are almost the same owing to their nearly identical LUMO energy levels. Next, pump-probe transient absorption experiments were performed to measure the traces at 1300 nm, in order to evaluate nano second millisecond kinetics of interfacial charge-transfer reactions of oxidized dye molecules only with electrons in titania, or with both electrons in titania and cobalt(II) ions in the redox electrolyte [29,30]. We employed a multi-exponential function to fit transient decay traces for the accurate assessment of half-reaction time constants ðt1=2 Þ. When dyed titania films are in contact with an inert electrolyte composed of 0.5 M TBP and 0.1 LiTFSI, slow kinetic traces (Fig. 3e and f) which are intrinsically connected with the interfacial charge recombination of photo-oxidized dye molecules with electron in titania occur in the millisecond time region, with time constants ðt inert 1=2 Þ being 2.1 ms for the
cell (Fig. 5c). Considering the same titania conduction band (CB) edge, the H3 cell stored more charge and possessed higher quasi-Fermi-level than the H4 cell, nicely interpreting the VOC distinction. The dye load amounts ðcm Þ on the surface of titania were further measured to be 1.47 � 10 8 mol cm 2 μm 1 for dye H3 and 1.26 � 10 8 mol cm 2 μm 1 for dye H4. Accordingly, the larger VOC value for the H3 cell may originate from a more compact and well-organized molecular layer on the surface of tiania. 3.5. Long-term light and thermal stability of DSSCs
To test the long-term stability of DSSCs under heat stress and continuous one-sun illumination, the cobalt-based DSSCs covered with a 50 μm-thick polyester film (Preservation Equipment Ltd, UK) as a UV cutoff filter were irradiated at 60 � C under a Suntest CPS plus lamp (ATLAS GmbH, 100 mW cm 2) for 1000 h. The evolution of photovoltaic parameters (VOC ; JSC ; FF, and PCE) is presented in Fig. 6. The H3 cell shows an excellent stability, retaining 88% of its original PCE after full sunlight soaking at 60 � C for 1000 h, much better than that of 79% for the H4 cell. The open-circuit voltage drop after accelerated thermal aging is a general plight in DSSCs. The VOC drops are 107 mV for the H3 cell and 144 mV for the H4 cell. Simultaneously, the JSC value of the H3 cell is increased by 0.39 mA cm 2, while that of the H4 cell is reduced by 0.44 mA cm 2. The reduced ratios of FF for the H3 and H4 cells are about ~3%. In order to uncover the cause of JSC variation observed for the aged cells, we first measured external quantum efficiencies of the aged H3 and H4 cells (Fig. 7a and b). The EQE maxima of both aged cells are increased from 85% to 90%. As depicted in Fig. 7c and d, the LHEs of the aged cells with H3 and H4 were nearly not altered by full-sunlight soaking and thermal stress, suggesting the strong binding of dye mole cule on the surface of mesoporous titania and the photo-stability of dye molecules. As shown in Fig. 7g and h, the fresh and aged cells with H3 and H4 keep the similar transient absorption decay traces, implying that hole injection kinetics of the H3 and H4 cells after aging were not changed. Nevertheless, the femtosecond fluorescence kinetic traces (Fig. 7e and f) of the aged H3 and H4 cells have suggested faster electron injection kinetics due to the negative shift of the titania CB edge, which is related to the increased EQE maxima. Obviously, the electron injec tion acceleration was the main reason for the JSC augmentation for the H3 cell. However, the aged H4 cell presents a sublinear response in photocurrent under one sun illumination. We deduce that the thermal and light stress may induce insufficient mass transport of Co(III) species to the platinum electrode [34]. In consideration of DSSCs with the same redox electrolyte, the VOC drop after 1000 h aging under dual stress should stem from the shift of the quasi-Fermi-level ðEF;n Þ of the mesoporous titania film, which is
H3 sample and 1.8 ms for H4. Moreover, in the presence of a cobalt electrolyte for cell fabrication, accelerated kinetic traces due to hole injection from photo-oxidized dye molecules to electrolyte occur in the microsecond time domain, with time constants ðtCo 1=2 Þ being 28 μs for the
H3 sample and 21 μs for H4. Overall, the hole injection yields ðφhi Þ for H3 and H4 are 99%. We further measured photocurrent action spectra of dyed titania films in combination with a tris(2,20 -bipyridine)cobalt(II)/(III) based redox couple (for details see Fabrication and characterization of DSSCs), and the photos of DSSCs with H3 and H4 are shown in Fig. S13. As depicted in Fig. 4a, the H3 and H4 cells display a comparable external quantum efficiency (EQE) peak value of 85%. In addition, the H4 cell possesses higher EQE values in the wavelength range from 610 nm to 800 nm. In comparison to H3, H4 grafted on titania presents augmented light-harvesting efficiencies (LHEs) at longer wavelengths (Fig. 4b), which nicely interprets the broadened photocurrent response of the H4 cell. As shown in Fig. 4c, the electron diffusion length ðLn Þ plotted as a function of density of states (DOS) in DSSCs are comparable for the H3 and H4 cells, implying that both cells should have similar charge collection yields [31]. The Ln values of both cells are larger than the thickness of nanocrystalline TiO2 layer, suggesting close-to-unity charge collection yield. 3.4. Photovoltaic performance of DSSCs Typical photocurrent density-voltage (J-V) curves (Fig. 5a) of the H3 and H4 cells were measured under the illumination of 100 mW cm-2, simulated AM1.5G sunlight. Cell parameters are compiled in Table 2. The H3 cell affords a short-circuit photocurrent density ðJSC Þ of 15.21 8
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inherently related to the conduction band edge ðEc Þ and free electron density ðnc Þ of titania. As Fig. 8a presents, the aged H4 cell stores more charges than that of the aged H3 cell at a fixed voltage, suggesting that the downward Ec movements for the aged H3 and H4 cells should be different. As shown in Fig. 8b, the aged H3 cell displays an evidently longer half-lifetime ðt TPD 1=2 Þ of titania electrons compared to the aged H4
[5] Y.S. Yen, H.H. Chou, Y.C. Chen, C.Y. Hsu, J.T. Lin, Recent developments in molecule-based organic materials for dye-sensitized solar cells, J. Mater. Chem. 22 (2012) 8734–8747. [6] H. Fan, R. Huang, G. Yan, Regulation of energy levels and kinetics in dye-sensitized solar cells: synergistic effect of N,N-bis(9,9-dimethyl-fluoren-2-yl)-aniline and 3,4ethylenedioxythiophene, J. Power Sources 397 (2018) 196–203. [7] Y. Ren, D. Sun, Y. Cao, H.K. Tsao, Y. Yuan, S.M. Zakeeruddin, P. Wang, M. Gr€ atzel, A stable blue photosensitizer for color palette of dye-sensitized solar cells reaching 12.6% efficiency, J. Am. Chem. Soc. 140 (2018) 2405–2408. [8] P. Wang, L. Yang, H. Wu, Y. Cao, J. Zhang, N. Xu, S. Chen, J.-D. Decoppet, S. M. Zakeeruddin, M. Gr€ atzel, Stable and efficient organic dye-sensitized solar cell based on ionic liquid electrolyte, Joule 2 (2018) 2145–2153. [9] Z. Yao, M. Zhang, H. Wu, L. Yang, R. Li, P. Wang, Donor/acceptor indenoperylene dye for highly efficient organic dye-sensitized solar cells, J. Am. Chem. Soc. 137 (2015) 3799–3802. [10] Z. Yao, H. Wu, Y. Li, J. Wang, J. Zhang, M. Zhang, Y. Guo, P. Wang, Dithienopicenocarbazole as the kernal module of low-energy-gap organic dyes for efficient conversion of sunlight to electricity, Energy Environ. Sci. 8 (2015) 3192–3197. [11] K. Kakiage, Y. Aoyama, T. Yano, K. Oya, J. Fujisawa, M. Hanaya, Highly-efficient dye-sensitized solar cells with collaborative sensitization by sily-anchor and carboxy-anchor dyes, Chem. Commun. 51 (2015) 15894–15897. [12] L. Zhang, X. Yang, W. Wang, G.G. Gurzadyan, J. Li, X. Li, J. An, Z. Yu, H. Wang, B. Cai, A. Hagfeldt, L. Sun, 13.6% efficient organic dye-sensitized solar cells by minimizing energy losses of the excited state, ACS Energy Lett. 4 (2019) 943–951. [13] J. Gao, M.B. Achari, L. Kloo, Long-term stability for cobalt-based dye-sensitized solar cells obtained by electrolyte optimization, Chem. Commun. 50 (2014) 6249–6251. [14] W. Yang, Y. Hao, P. Ghamgosar, G. Boschloo, Thermal stability study of dyesensitized solar cells with cobalt bipyridyl-based electrolytes, Electrochim. Acta 213 (2016) 879–886. [15] H. Wu, X. Xie, Y. Mei, Y. Ren, Z. Shen, S. Li, P. Wang, Phenalenothiophene-based organic dye for stable and efficient solar cells with a cobalt redox electrolyte, ACS Photonics 6 (2019) 1216–1225. [16] M. Zhang, J. Zhang, Y. Fan, L. Yang, Y. Wang, R. Li, P. Wang, Judicious selection of a pinhole defect filler to generally enhance the performance of organic dyesensitized solar cells, Energy Environ. Sci. 6 (2013) 2939–2943. [17] P. Wang, S.M. Zakeeruddin, P. Comte, R. Charvet, R. Humphry-Baker, M. Gr€ atzel, Enhance the performance of dye-senesitized solar cells by co-grafting amphiphilic sensitizer and hexadecylmalonic acid on TiO2 nanocrystals, J. Phys. Chem. B 107 (2003) 14336–14341. [18] J. Liu, R. Li, X. Si, D. Zhou, Y. Shi, Y. Wang, X. Jing, P. Wang, The structureproperty relationship of organic dyes in mesoscopic titania soalr cells: only one double-bond different, Energy Environ. Sci. 3 (2010) 1924–1928. [19] N. Cai, Y. Wang, M. Xu, Y. Fan, R. Li, M. Zhang, P. Wang, Improving the photovoltage of dithienopyrrole dye-sensitized solar cells via attaching the bulky bis(octyloxy) biphenyl moiety to the conjugation π-linker, Adv. Funct. Mater. 23 (2013) 1846–1854. [20] M. Cossi, N. Rega, G. Scalmani, V. Barone, Energies, structures, and electronic properties of molecules in solution with the C-PCM solvation model, J. Comput. Chem. 24 (2003) 669–681. [21] A.D. Becke, A new mixing of Hartree-Fock and local density-functional theories, J. Chem. Phys. 98 (1993) 1372–1377. [22] B.J. Lynch, P.L. Fast, M. Harris, D.G. Truhlar, Adiabatic connection for kinetics, J. Phys. Chem. A 104 (2000) 4811–4815. [23] M. Pastore, E. Mosconi, F. De Angelis, M. Gr€ atzel, A computational investigation of organic dye-sensitized soalr cells: benchmark, strategies, and open issues, J. Phys. Chem. C 114 (2010) 7205–7212. [24] T. Lu, F. Chen, Multiwfn: a multifunctional wavefunction analyzer, J. Comput. Chem. 33 (2012) 580–592. [25] H. Huang, L. Yang, A. Facchetti, T.J. Marks, Organic and polymeric semiconductors enhanced by noncovalent conformational locks, Chem. Rev. 117 (2017) 10291–10318. [26] C. Martín, M. Zi� ołek, M. Marchena, A. Douhal, Interfacial electron transfer dynamics in a solar cell organic dye anchored to semiconductor particle and aluminum-doped mesoporous materials, J. Phys. Chem. C 115 (2011) 23183–23191. [27] M. Fakis, P. Hrob� arik, O. Yushchenko, I. Sigmundov� a, M. Koch, A. Rosspeintner, E. Sththatos, E. Vauthey, Excited state and injection dynamics of triphenylamine sensitizers containing a benzothiazole electron-accepting group on TiO2 and Al2O3 thin films, J. Phys. Chem. C 118 (2014) 28509–28519. [28] R. Englman, J. Jortner, Energy gap law for radiationless transitions in large molecules, Mol. Phys. 18 (1970) 145–164. [29] Y. Tachibana, J.E. Moser, M. Gr€ atzel, D.R. Klug, J.R. Durrant, Subpicosecond interfacial charge separation in dye-sensitized nanocrystalline titanium dioxide films, J. Phys. Chem. 100 (1996) 20056–20062. [30] S. Pelet, J.-E. Moser, M. Gr€ atzel, 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 104 (2000) 1791–1795. [31] Q. Wang, S. Ito, M. Gr€ atzel, F. Fabregat-Santiage, I. Mora-Ser� o, J. Bisquert, T. Bessho, H. Imai, Characteristics of high efficiency dye-sensitized solar cells, J. Phys. Chem. B 110 (2006) 25210–25221. [32] N.W. Duffy, L.M. Peter, R.M.G. Rajapakse, K.G.U. Wijayantha, A novel charge extraction method for the study of electron transport and interfacial transfer in dye sensitized nanocrystalline solar cells, Electrochem. Commun. 2 (2000) 658–662.
cell, which interprets the VOC trend. It is known that lithium ions could downward shift the CB edge of metal oxide semiconductors [35]. Taking account of the optimized molecular geometry, the dye loading amount, and the electrical measurements, we assume that the H3 dye layer could possess less permeation channel with respect to the H4 dye layer as demonstrated in pictorial scheme (Fig. 8c and d). The self-assembling molecular layer on the surface of titania with H3 is more compact and better organized than that with H4, retarding the andante penetration of lithium ions into titania and mitigating the interfacial charge recombination. 4. Conclusions In summary, we have used 4-(benzo[c] [1,2,5]thiadiazol-4-yle thynyl)benzoic acid as the electron-acceptor to prepare two narrow-energy-gap isomeric photosensitizers, in combination with coplanar aromatic units of phenalenothiophene and benzoindenothio phene for electron-donors. In contrast to the phenalenothiophene based photosensitizer, the benzoindenothiophene counterpart has achieved a higher power conversion efficiency owing to the broadened spectral response. However, the phenalenothiophene based dye has presented a better retention ratio of power conversion efficiency under full-light soaking at 60 � C for 1000 h, due to the smaller drop of photovoltage and the improved output of photocurrent. Our studies have suggested that a joint effort on wise design of organic photosensitizers and inter face control should be taken into consideration to further enhance both efficiency and stability of colorful dye-sensitized solar cells. Notes The authors declare no competing financial interest. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was supported by the National Science Foundation of China (Grants 51673165 and 91733302), the National 973 Program (Grant 2015CB932204), and H. Glass. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2020.227748. References [1] B. O’Regan, M. Gr€ atzel, A low-cost, high-efficiency solar cell based on dyesensitized colloidal TiO2 films, Nature 353 (1991) 737–740. [2] C.-C. Chou, K.-L. Wu, Y. Chi, W.-P. Hu, S.J. Yu, G.-H. Lee, C.-L. Lin, P.-T. Chou, Ruthenium(II) sensitizers with heteroleptic tridentate chelates for dye-sensitized solar cells, Angew. Chem. Int. Ed. 50 (2011) 2054–2058. [3] Y. Huang, W.-C. Chen, R. Ghadari, X.-P. Liu, X.-Q. Fang, T. Yu, F.-T. Kong, Highly efficient ruthenium complexes with acetyl electron-acceptor unit for dye sensitized solar cells, J. Power Sources 396 (2018) 559–565. [4] M. Urbani, M. Gr€ atzel, M.K. Nazeeruddin, T. Torres, Meso-substituted porphyrins for dye-sensitized solar cells, Chem. Rev. 114 (2014) 12330–12396.
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Journal of Power Sources 451 (2020) 227748 [34] R. Jiang, G. Boschloo, The impact of non-uniform photogeneration on mass transport in dye-sensitised solar cells, J. Mater. Chem. A. 6 (2018) 10264–10276. [35] P. Qu, G.J. Meyer, Proton-controlled electron injection from molecular excited states to the empty states in nanocrystalline TiO2, Langmuir 17 (2001) 6720–6728.
[33] B. O’Regan, S. Scully, A.C. Mayer, E. Palomares, J. Durrant, 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 109 (2005) 4616–4623.
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Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Low-energy-gap organic photosensitizers with phenalenothiophene and benzoindenothiophene as primary electron-donors for durable dye-sensitized solar cells Heng Wu a, b, Xinrui Xie b, Jing Zhang b, Sining Li a, *, Zuochun Shen a, Yi Yuan b, ** a b
National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, Harbin, 150001, China Department of Chemistry, Zhejiang University, Hangzhou, 310028, China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Two low-energy-gap organic dyes with long excited-state lifetimes were prepared. � Both dyes with stabilized LUMO still ensure efficient electron-injection. � A organic dye achieves an impressive efficiency of 10.9% without any coadsorbate. � 7H-phenaleno[1,2-b]thiophene based dye solar cell presents excellent durability.
A R T I C L E I N F O
A B S T R A C T
Keywords: Solar cell Photosensitizer Low energy gap Charge carrier dynamics
Reducing the optical energy gap of a photosensitizer to broaden the spectral response is of paramount importance to improve the power conversion efficiency of dye-sensitized solar cells (DSSCs). Herein, we report two isomeric photosensitizers with planar aromatic polycycles 7H-phenaleno[1,2-b]thiophene (PT) and 7H-benzo[6,7]indeno [1,2-b]thiophene (BIT) as the kernel units of electron-donors and 4-(benzo[c][1,2,5]thiadiazol-4-ylethynyl) benzoic acid (BTEBA) as the electron-acceptor. The efficient electron-acceptor BTEBA can bring forth an obvious downward shift of the lowest unoccupied molecular orbital energy level of an organic photosensitizer and thus shrink the energy gap significantly. Femtosecond fluorescence decay and nanosecond transient absorption measurements suggest that the two low-energy-gap photosensitizers present efficient charge separation yields in DSSCs. These two photosensitizers are used to make coadsorbate-free, high-photovoltage-output DSSCs in combination with a tris(2,20 -bipyridine)cobalt based redox electrolyte. DSSCs with the BIT based photosensitizer achieves up to 10.9% power conversion efficiency at the 100 mW cm-2, simulated AM1.5G conditions. The cell using the PT based photosensitizer presents an excellent stability under full sunlight soaking at 60 � C for 1000 h, retaining 88% of its initial efficiency.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Li), [email protected] (Y. Yuan). https://doi.org/10.1016/j.jpowsour.2020.227748 Received 5 September 2019; Received in revised form 9 January 2020; Accepted 12 January 2020 Available online 30 January 2020 0378-7753/© 2020 Elsevier B.V. All rights reserved.
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Journal of Power Sources 451 (2020) 227748
Fig. 1. (a) Chemical structures of D-A organic dyes H3 and H4 with 4-(benzo[c] [1,2,5]thiadiazol-4-ylethynyl)benzoic acid (BTEBA, marked in red) as the electron-acceptor. (b) Theoretical LUMO energy levels (values above color bars), HOMO energy levels (values under color bars), and HOMO/LUMO energy gaps ðΔEB3LYP H=L Þ of H3 and H4 in THF. (c) Steady UV–Vis absorption spectra (solid lines) and fluorescence spectra (short dash) of dyes in PhMe and THF. (d) Scatter plots of averaged time constants ðτÞ of fluorescence decay traces for dyes in PhMe and THF. The solid lines are displayed only as a guide to the eyes. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Table 1 Electronic absorption and fluorescence properties of dyes in THF and PhMe.a. Dye
MPW1K λTD [nm] ABS;MAX
λMEAS ABS;MAX [nm]
εMEAS ABS;MAX [mM
H3/THF H4/THF H3/PhMe H4/PhMe
553 573 559 580
531 550 538 558
27.7 29.0 27.1 30.4
a
1
cm 1]
MPW1K [nm] λTD FL;MAX
λMEAS FL;MAX [nm]
ΔνTD
710 724 667 678
767 809 690 704
4.00 3.64 2.90 2.55
MPW1K
[103 cm
1
]
ΔνMEAS [103 cm
1
]
5.79 5.82 4.09 3.72
MPW1K MPW1K Theoretical absorption maxima ðλTD Þ and fluorescence maxima ðλTD Þ were calculated at the TD-MPW1K/6-311G(d,p) level for dye molecules in PhMe ABS;MAX FL;MAX
MEAS MEAS and THF. Experimental absorption maxima ðλMEAS Þ were ABS;MAX Þ and fluorescence maxima ðλFL;MAX Þ were acquired from the spectra in Fig. 1c, and stokes shifts ðΔν derived from the electronic absorption and fluorescence spectra of dye dissoved in PhMe and THF.
1. Introduction
DSCs is the main factor influencing the charge collection efficiency. Meanwhile, continuous explorations on electron-rich donors and electron-deficient acceptors have been performed to prolong the excited-state lifetime of D-A organic photosensitizers by constructing exotic polycyclic heteroaromatics (PHAs) [7,8]. It is also necessary to attach the flexible side chains onto the photoactive backbone to alleviate the adverse intermolecular π-π stacking of dye molecules grafted on the surface of titania, which is normally considered to engender dissipative exciton annihilation. In this context, some organic dyes have been prepared and achieved impressive power conversion efficiencies (PCEs) of over 12% in conjunction with a cobalt-based redox electrolyte, such as perylene dye C275 [9], C281 [10], anthracene dye R6 [7], silyl-anchor dye AKDEKA-1 [11], and triazatruxene dye ZL003 [12]. In spite of the gradual improvement in efficiency, the commercial application of DSSCs based on a cobalt electrolyte is heavily hindered by the long-term sta bility issue [13,14]. Recently, we prepared two naphthalene based D-A organic dyes H1
Solar energy conversion is one of the most promising approaches to solve the problems of fossil fuel depletion and environmental pollution. In this regard, dye-sensitized solar cells (DSSCs) have been proposed as one of the next generation photovoltaic technologies due to the low-cost and facile manufacture procedure [1]. The exploration of photosensi tizers including ruthenium-polypyridine [2,3] and zinc-porphyrin complexes [4] as well as metal-free organic dyes [5,6] has been inten sively carried out in the past decades. Donor–acceptor (D-A) organic sensitizers are characteristic of abundant raw materials, elastic molec ular designs, and gorgeous colors. In general, the structure of a sensitizer as the kernel component of DSSCs plays a pivotal role in dominating light absorption and regulating the compactness of self-assembling molecular layer on the surface of wide-band-gap titania. To improve the harvesting yield of near-infrared (NIR) solar photons, the common strategy is to design a low optical energy gap sensitizer. Interface recombination relative to charge transport through the photoanodes in 2
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Journal of Power Sources 451 (2020) 227748
Fig. 2. Reduced density gradient (RDG) isosurfaces (s ¼ 0.5 au.) for the S0 and Seq 1 states of H3 and H4 in PhMe. The surfaces are colored on a blue-green-red scale, according to the values of the electron density multiplied by the sign of the second Hessian eigenvalue ðsignðλ2 ÞρÞ, ranging from 0.04 to 0.02 au. Blue represents strong attractive interactions, and red represents strong nonbonded overlap. The S0 state: (a) H3; (c) H4. The Seq 1 state: (e) H3; (g) H4. Plots of the RDG as a function of signðλ2 Þρ. The S0 state: (b) H3; (d) H4. The Seq 1 state: (f) H3; (h) H4. The hexyl chain was substituted with ethyl for the sake of computational efficiency. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 3. (a, b) Wavelength-dependent average time constant ðτÞ of fluorescence decay traces for H3 and H4 grafted oxide films in conjunction with a cobalt electrolyte: (a) alumina; (b) titania. Pump wavelength: 530 nm. (c) Plots of fluorescence quenching yield ðQYÞ as a function of fluorescence wavelength ðλÞ for H3 and H4. (d) Plots of time constants of electron injection as a function of λ. (e,f) Normalized transient absorption traces at 1300 nm of dye-grafted titania films with (e) H3 and (f) H4, which are also in contact with an inert electrolyte composed of 0.5 M 4-tert-butypyridine and 0.1 M lithium bis(trifluoromethanesulfonyl)imide in acetonitrile or a cobalt electrolyte. Pulse fluence: 25 μJ cm 2. The excitation wavelength was selected at a 0.5 optical density of dye-grafted titania films to yield a similar dis tribution profile of vertically excited states in the testing samples. Excitation wavelength: 618 nm for H3; 640 nm for H4.
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Journal of Power Sources 451 (2020) 227748
Fig. 4. (a) EQE curves of DSSCs with a cobalt electrolyte. (b) LHE curves of dye-sensitized transparent titania films (7.0 μm-thick). (c) Plots of electron diffusion length (Ln ) as a function of density of states (DOS). The short dash line represents thickness of the mesoporous titania layer of DSSCs.
Fig. 5. (a) J V curves of H3 and H4 based DSSCs measured at the 100 mW cm 2, AM 1.5G condition. (b) Charge extracted from the dye-grafted titania films ðQCE Þ as a function of VOC : (C) Plots of half-lifetime ðt TPD 1=2 Þ of electrons in the titania films versus.QCE :
and H2 featuring the coplanar aromatic units 7H-phenaleno[1,2-b] thiophene (PT) and 7H-benzo [6,7]indeno[1,2-b]thiophene (BIT) [15]. The H1-based device exhibits an excellent stability during full-sunlight soaking at 60 � C for 1000 h, retaining 90% of its initial PCE. In this work, we employ an efficient electron-acceptor 4-(benzo[c] [1,2,5] thiadiazol-4-ylethynyl)benzoic acid (BTEBA) [10] to replace electron-acceptor 4-(benzo[c] [1,2,5]thiadiazol-4-yl)benzoic acid (BTBA) [16], constructing two simple low-energy-gap organic dyes H3 and H4 (Fig. 1a). Preliminary theoretical calculations show that the introduction of more electron-deficient acceptor BTEBA does not impact the highest occupied molecular orbital (HOMO) energy levels, but re sults in an obviously downward displacement of the lowest unoccupied molecular orbital (LUMO) energy levels. The influence of electron-acceptor BTEBA on light-harvesting, excited state dynamics, and photovoltaic characteristics of these two dyes will be further sys tematically investigated.
Table 2 Photovoltaic parameters of cells with H3, H4, and C268 measured at the 100 mW cm 2, AM1.5G condition. Cell
2 JEQE SC [mA cm ]
JSC [mA cm
H3 H4 C268
15.16 16.53 17.95
15.21 16.58 16.33
2
]
VOC [mV]
FF [%]
PCE [%]
926 902 826
73.1 72.6 74.4
10.3 10.9 10.0
a EQE JSC
was calculated by wavelength integation of the product of the standard AM 1.5G emission spectrum (ASTM G173-03) and the EQE curve measured at the short-circuit.
before use. The synthetic details on H3 and H4 and structure charac terization with 1H NMR, 13C NMR, MALDI-TOF, and elemental analysis are elaborated in the Electronic supplementary information.
2. Experimental
2.2. Fabrication and characterization of DSSCs
2.1. Materials
A bilayer titania film was screen-printed on fluorine-doped tin oxide (FTO) glass (1.6 mm, white glass, NSG) and employed as the negative electrode (photoanode) of a DSSC. The bilayer film is composed of a 4.0μm-thick mesoporous layer formed with small anatase particles (Dyesol, 30NR-D), and a 3.0-μm-thick macroporous layer formed with large anatase particles (paticle size, 350–450 nm, Dyesol, WER4-0). The de tails on film and cell manufacture were elaborated in a former paper [17]. Dye-loading was carried out by dipping a titania film in a dye solution overnight. The dyes (150 μM) were dissolved in a solvent
Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), 1-ethyl-3methylimidazolium bis(trifluoromethanesulfonyl)imide (EMITFSI), decamethylferrocene (DMFc), ferrocene (Fc), 4-tert-butylpyridine (TBP), bis(triphenylphosphine)palladium(II) chloride (Pd(PPh3)2Cl2), n-butyl lithium (n-BuLi), trimethyltin chloride (Me3SnCl), potassium hydroxide (KOH), and phosphoric acid were purchased from Energy Chemical. Toluene (PhMe) and tetrahydrofuran (THF) were dried and distilled 4
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2.4. Theoretical calculations Quantum calculations were performed by using the 6-311G(d,p) basis set in the Gaussian 09 software package. The conductor-like polarized continuum model (C-PCM) [20] was applied to simulate the solvent (PhMe and THF) effect. The universal B3LYP exchange-correlation functional [21] was selected to optimize the ground-state geometries. The TD-MPW1K hybrid functional, which features 42% Hartree-Fock exchange, was employed to compute the vertical electron transitions and excited-state geometries [22,23]. The analysis of reduced density gradient non-covalent interaction (RDG-NCI) was conducted by a wavefunction software Multiwfn [24]. 2.5. Photophysical measurements and data analysis The steady UV–Vis spectra were measured by the G1103A spec trometer (Agilent). The static fluorescence spectra were tested with a QEPro spectrometer (Ocean Optics). Femtosecond fluorescence upconversion traces were implemented with a time-resolved fluorescence spectrometer (Halcyone Fire, Ultrafast). The fundamental pulses were generated with a Ti: Sapphire laser system (Astrella, 800 nm, 35 fs, 7 mJ per pulse, 1 kHz repetition rate, Coherent Inc.). The output of femto second pulses at 800 nm was divided into two parts: the large portion was sent to one optical parametric amplifier (OPA) to generate the pump pulse (530 nm), and the small portion was sent to another OPA to generate the gate pulse (1200 nm). Both the wavelength-dependent fluorescence emitted from the randomly moving sample and the 1200 nm gate pulse were focused onto a BBO crystal to generate a sum fre quency light, which then passed through a monochromator and detected by a photomultiplier tube (PMT). This setup is very useful to monitor the broad fluorescence emission of DSSC dyes in the infrared region. Subnanosecond transient absorption (TA) traces were measured using a pump-probe transient absorption spectrometer (EOS, Ultrafast System). The time-dependent fluorescence intensities ðIFL Þ at all detected wavelengths of each dye were globally fitted with several time constants according to Eq. (1)
Fig. 6. Evolution of photovoltaic parameters measured at the 100 mW cm 2, AM1.5G condition of cells with H3 and H4, which were aged under full sunlight at 60 � C.
n X
IFL ∝
t = τi Þ � IRF;
Ai expð
(1)
i¼1
mixture of chloroform and ethanol at a volume ratio of 1/19 for both H3 and H4. A Surlyn O-ring (thickness, 25 μm) was used to stick a dyed titania electrode and a platinized FTO electrode under a proper me chanical pressure at 130 � C. Electrical measurements were performed as reported in previous papers [18,19]. The composition of a cobalt based redox electrolyte employed in the device manufacture is listed as fol lows: 0.25 M tris(2,20 -bipyridine)cobalt(II) di[bis(tri fluoromethanesulfonyl)imide, 0.10 M tris(2,20 -bipyridine)cobalt(III) tris [bis(trifluoromethanesulfonyl)imide], 0.5 M TBP, and 0.1 M LiTFSI in acetonitrile.
where Ai is the amplitude, t and τi represent the delay time and the time constant, respectively, and IRF is the instrumental response function. The amplitude-weighted average lifetimes ðτÞ of fluorescence decays can be derived from Eq. (2) , ! n n X X τ¼ Ai τ i Ai Ai > 0 : (2)
2.3. Electrochemical measurements
where τðT; λÞ and τðA; λÞ represent the average lifetimes of fluorescence decay traces for dyes on titania and alumina, respectively. The calcu lation of time constant of electron injection ðτei ðλÞÞ is based on Eq. (4)
i¼1
i¼1
The calculation of quenching yield ðQYðλÞÞ is based on Eq. (3) � � � � τðT; λÞ QY λ ¼ 1 � 100%; τðA; λÞ
A CHI660C electrochemical workstation was used to measure cyclic voltammograms in combination with a three-electrode electrochemical cell equipped with a glass carbon as working electrode, a platinum gauze as counter electrode, and a silver wire as auxillary electrode. Impedance spectroscopy measurements of DSSCs were performed under illumina tion of a white light-emitting diode on an IM6ex electrochemical workstation, with a frequency range from 50 mHz to 1 MHz and a po tential modulation of 20 mV. A potential bias (V) was applied to equal the open-circuit photovoltage ðVOC Þ at each irradiation intensity, meeting the requirement of zero current. The obtained impedance spectra were fitted with the Z-view software (v2.80, Scribner Associates Inc.).
τei ðλÞ ¼
1 1=τðT; λÞ
: 1=τðA; λÞ
(3)
(4)
The transient absorption traces at 1300 nm (Fig. 3c and d) were fitted with a multi-exponential function for easy determination of the halfreaction time constants. The calculation of hole injection yield ðφhi Þ is based on Eq. (5) ! tCo 1=2 φhi ¼ 1 inert � 100%; (5) t1=2 5
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Fig. 7. (a, b) EQE curves of fresh (solid symbols) and aged (open symbols) cells: (a) H3; (b) H4. The cells were aged under full sunlight at 60 � C for 1000 h. (c, d) LHE curves of DSSCs with a 4 μm-thick TiO2 film: (c) H3; (d) H4. (e, f) Normalized kinetic traces of transparent DSSCs with a 4 μmthick TiO2 film at the fluorescence wave lengths of 800 nm and 880 nm: (e) H3 and (f) H4. (g, h) Normalized transient absorp tion traces upon femtosecond laser pulse excitation of transparent DSSCs with a 4 μmthick TiO2 film: (g) H3; (h) H4.
bear a similar HOMO/LUMO energy gap ðECV H=L Þ: Our experiments have
where tCo 1=2 represents the half-reaction time constant of transient ab
further proved the benefit of using BTEBA instead of BTBA for a lowenergy-gap DSSC dye [8,10]. Density functional theory (DFT) calcula tions at the B3LYP/6-311G(d,p) level shown in Fig. 1b could well simulate the relative tendencies of electrochemical HOMO and LUMO energy levels of H3 and H4.
sorption trace of a transparent DSSCs in combination with a cobaltbased electrolyte, and t inert 1=2 represents the half-reaction time constant of transient absorption trace of a transparent dummy cell using an inert electrolyte without the cobalt based redox couple. 3. Results and discussion
3.2. Photophysical properties
3.1. Energy levels
Fig. 1c shows the steady-state UV–Vis absorption and fluorescence spectra of dyes H3 and H4 dissolved in PhMe and THF. Key spectro scopic data are collected in Table 1. The maximum absorption wave lengths ðλMEAS ABS;MAX Þ of both dyes in PhMe display red-shifted absorption
To evaluate the influence of electron acceptors (BTEBA versus BTBA) on the HOMO and LUMO energy levels of PT and BIT based dyes, cyclic voltammograms (CVs) of H3 and H4 dissolved in tetrahydrofuran (THF) were measured under nitrogen and also compared with those of H1 and H2. In our experiments, EMITFSI was used as the supporting electrolyte and DMFc was added to the electrolyte as an internal reference. The potential was further calibrated with respect to the standard redox couple ferrocene/ferrocenium (Fc/Fcþ). The measured redox potential of Fc is 488 mV more positive than that of DMFc. The HOMO and LUMO energy levels can be obtained from CV curves shown in Fig. S3 by use of equation E ¼ 4:88 eEonset ; where Eonset represents the redox potential of a ground state dye molecule in THF. In contrast to dyes H1 and H2 with electron acceptor BTBA, H3 and H4 with BTEBA feature ~100 meV stablized LUMO energy levels and comparable HOMO levels. H3 and H4
peaks compared with those in THF. In contrast to dye H3, the λMEAS ABS;MAX values of dye H4 are red-shifted, by 20 nm in PhMe and 19 nm in THF. The maximum molar absorption coefficients ðεMEAS MAX Þ in PhMe are 27.7 � 103 M-1 cm-1 for dye H3 and 30.4 � 103 M-1 cm-1 for dye H4, similar to those in THF (27.1 � 103 M-1 cm-1 for H3 and 29.0 � 103 M-1 cm-1 for H4). Time-dependent DFT (TDDFT) calculations show that the relative MPW1K tendency of theoretical λTD values is in good agreement with that ABS;MAX of experimental ones. The S1←S0 vertical electronic excitations of dyes H3 and H4 are mainly due to the transition from HOMO and HOMO-1 to LUMO (Table S1). An apparent intramolecular charge-transfer feature 6
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Fig. 8. (a) Charge extracted from the dye-grafted titania films ðQCE Þ as a function of VOC for DSSCs before and after aging under full sunlight at 60 � C for 1000 h. (b) CE Plots of half-lifetime ðt TPD of fresh and aged cells. (c, d) Pictorial illustration of the self-assembling films of H3 and H4 1=2 Þ of electrons in the titania films versus Q
grafted on the surface of titania to mitigate the adverse contact between titania and electrolyte, and to control the diffusion and permeation of lithium ions: (c) H3; (d) H4.
can be perceived from the contour plots of HOMO and LUMO in Figs. S5 and S7. The fluorescence maximum ðλMEAS FL;MAX Þ of either H3 or H4 in THF is red-shifted compared with that in PhMe, owing to the more stabilized excited state in the polar solvent. Moreover, dye H4 possesses a redshifted fluorescence peak in contrast to that of H3. As tabulated in Table 1, H3 and H4 in either THF or PhMe show large Stokes shift ðΔvMEAS Þ. The large energy losses upon vertical excitation have indicated the presence of vibration, torsional, and solvation relaxation. A larger ΔvMEAS in THF could be detected against that in PhMe for each dye. In addition, the tendencies of measured fluorescence maxima and Stokes shifts are in good accord with those based on DFT and TD-DFT calculations. Noncovalent interactions (NCI) analysis [25] is calculated for dyes H3 and H4 at the ground state ðS0 Þ and the first equilibrium excited state ðSeq 1 Þ. The H⋯S, N⋯H, H⋯S, and H … π intramolecular interactions of both dyes are identified by the reduced density gradient (RDG) iso surfaces shown in Fig. 2. Note that the value (ρsignλ2 ) of the electron density multiplied by the sign of the second Hessian eigenvalue is considered to be an indicator of interaction strength, and the sign of ρsignλ2 represents the interaction type in which the negative shown by blue or green color in RDG isosurfaces is indicative of attractive inter action, and the positive shown by red color is indicative of nonbonding. The spikes in the negative ρsignλ2 region (Fig. 2) for H3 and H4 at the S0 state are observed to move to larger negative values at the Seq 1 state, demonstrating that the noncovalent interactions between the benzo[c] [1,2,5]thiadiazole (BT) unit and the adjacent thiophene unit are enhanced after conformation relaxation of the vertical excited states. This is consistent with the improved planarity of the conjugated back bones, as revealed by the significantly reduced dihedral angle between thiophene (green) and BT (red) from the S0 state to the Seq 1 state (Figs. S6 and S8). To integrally acquire the excited-state properties of H3 and H4 in
PhMe and THF, we measured femtosecond fluorescence up-conversion traces [26,27] in a broad wavelength band range with a 1200 nm gate pulse. Note that very few studies [15] have been performed on the excited state properties of DSSCs dyes with this technique. All kinetic traces can be globally fitted by five time constants and the processed data are provided in Fig. S15 S18 and Table S2 S5, with analytical details listed in experimental section. As Fig. 1d presents, the average time constant (τ) of both dyes are raised by 1–3 orders of magnitude with the red-shifting of fluorescence wavelength. Eventually, the τ reaches a steady value at long fluorescence wavelengths interpreted as the lifetime eq of Seq 1 for each dye molecule. The S1 lifetime in PhMe is 868 ps for H3 and 1055 ps for H4, which are slightly reduced compared to those of H1 and H2 (Fig. S4), owing to the HOMO/LUMO energy gap shrinkage [28]. In THF, H4 features a Seq 1 lifetime of 224 ps, which is about twice longer than H3 (111 ps). The shorter Seq 1 lifetime of each dye in THF compared with that in PhMe is attributed to the acceleration of non-radiative recombination. Although dye H4 bears a smaller optical energy gap, its equilibrium excited state lasts longer, especially in a polar solvent, which is important for sufficient charge separation in the high dielectric environment of DSSCs. 3.3. Photoinduced charge transfer and photocurrent action spectra of solar cells As presented in Fig. S9a, there is a blue-shifted fluorescence maximum of either H3 or H4 on titania in contrast to that on alumina, which could be associated with the occurrence of electron injection from the nonequilibrium hot excited state to titania and thereby reduced emission from the quilibrium excited state. Note that in our experiments the dyed oxide films are in contact with a cobalt-based electrolyte for device fabrication. To further ascertain the yield ðφei Þ and kinetics ðτei Þ of electron injection, we also executed femtosecond fluorescence up7
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mA cm-2, an open-circuit photovoltage ðVOC Þ of 926 mV, and a fill factor (FF) of 73.1%, yielding a PCE of 10.3%. Meanwhile, the H4 cell has an improved PCE of 10.9%, generated from the obviously enhanced JSC of 16.58 mA cm-2, the VOC of 902 mV, and the FF of 72.6%. The higher JSC value of H4 is attributed to the broader photocurrent response compared with that of H3. Note that the PCE of the device based on the highperformance C268 dye [8] under the same condition is 10.0% ðJSC ¼ 16.33 mA cm 2, VOC ¼ 826 mV, FF ¼ 74.4%). Furthermore, the PCEs of DSSC with the well-known dyes N719 and Z907 under the same device condition are 2.1% and 6.9% [15]. The high photovoltage output of low-energy-gap organic dyes H3 and H4 contribute to the impressive power conversion efficiencies. We then carried out charge extraction (CE) and transient photo voltage decay (TPD) measurements [32,33] to gain a deep insight into the inherent interfacial energetics and kinetics behind the VOC variation. As Fig. 5b shows, the charges ðQCE Þ stored in titania of the H3 and H4 cells were similar at the same potential bias. However, the H3 cell CE than that of the H4 presents a longer half-lifetime ðtTPD 1=2 Þ at a fixed Q
conversion measurements (Fig. S19 S22) on dyed films. All fluores cence decay traces can be globally fitted by four time constants, and the fitting parameters are listed in Table S6 S9. The three fluorescence decays at 660 nm, 760 nm, and 860 nm (Fig. S9b) for a H3 dyed titania film are obviously different. It is apparent that the τ values of decay traces for the dyed films (Fig. 3a and b) increase by 1–3 orders of magnitude along with the red-shifting of fluorescence wavelength. The τ at longer fluorescence wavelength is regarded as the Seq 1 lifetime, being about 300 ps for each dye on alumina, which is much larger than that (~40 ps) of the dyed titania samples. The Seq 1 lifetimes of H3 and H4 on alumina are shorter than those in THF, which can be rationalized by the joint contribution of dye aggregation and high dielectric environment of electrolyte. Fluorescence quenching yields ðQYÞ and time constants ðτei Þ of electron injection were depicted in Fig. 3c and d. On account of vibrational, torsional, and solvent relaxtions of the nonequilibrium excited states, the lower fluorescence QY values at short fluorescence wavelengths are detected for both dyes. The fluorescence QY values of the equilibrium excited states are 89% for H3 and 87% for H4. As pre sented in Fig. 3d, electron injection time constants of both dyes gradu ally increase by several times along with the red-shift of fluorescence wavelength, which can be understood by the driving force dependent electron injection kinetics. The τei values of electron injection for H3 and H4 at the Seq 1 states are almost the same owing to their nearly identical LUMO energy levels. Next, pump-probe transient absorption experiments were performed to measure the traces at 1300 nm, in order to evaluate nano second millisecond kinetics of interfacial charge-transfer reactions of oxidized dye molecules only with electrons in titania, or with both electrons in titania and cobalt(II) ions in the redox electrolyte [29,30]. We employed a multi-exponential function to fit transient decay traces for the accurate assessment of half-reaction time constants ðt1=2 Þ. When dyed titania films are in contact with an inert electrolyte composed of 0.5 M TBP and 0.1 LiTFSI, slow kinetic traces (Fig. 3e and f) which are intrinsically connected with the interfacial charge recombination of photo-oxidized dye molecules with electron in titania occur in the millisecond time region, with time constants ðt inert 1=2 Þ being 2.1 ms for the
cell (Fig. 5c). Considering the same titania conduction band (CB) edge, the H3 cell stored more charge and possessed higher quasi-Fermi-level than the H4 cell, nicely interpreting the VOC distinction. The dye load amounts ðcm Þ on the surface of titania were further measured to be 1.47 � 10 8 mol cm 2 μm 1 for dye H3 and 1.26 � 10 8 mol cm 2 μm 1 for dye H4. Accordingly, the larger VOC value for the H3 cell may originate from a more compact and well-organized molecular layer on the surface of tiania. 3.5. Long-term light and thermal stability of DSSCs
To test the long-term stability of DSSCs under heat stress and continuous one-sun illumination, the cobalt-based DSSCs covered with a 50 μm-thick polyester film (Preservation Equipment Ltd, UK) as a UV cutoff filter were irradiated at 60 � C under a Suntest CPS plus lamp (ATLAS GmbH, 100 mW cm 2) for 1000 h. The evolution of photovoltaic parameters (VOC ; JSC ; FF, and PCE) is presented in Fig. 6. The H3 cell shows an excellent stability, retaining 88% of its original PCE after full sunlight soaking at 60 � C for 1000 h, much better than that of 79% for the H4 cell. The open-circuit voltage drop after accelerated thermal aging is a general plight in DSSCs. The VOC drops are 107 mV for the H3 cell and 144 mV for the H4 cell. Simultaneously, the JSC value of the H3 cell is increased by 0.39 mA cm 2, while that of the H4 cell is reduced by 0.44 mA cm 2. The reduced ratios of FF for the H3 and H4 cells are about ~3%. In order to uncover the cause of JSC variation observed for the aged cells, we first measured external quantum efficiencies of the aged H3 and H4 cells (Fig. 7a and b). The EQE maxima of both aged cells are increased from 85% to 90%. As depicted in Fig. 7c and d, the LHEs of the aged cells with H3 and H4 were nearly not altered by full-sunlight soaking and thermal stress, suggesting the strong binding of dye mole cule on the surface of mesoporous titania and the photo-stability of dye molecules. As shown in Fig. 7g and h, the fresh and aged cells with H3 and H4 keep the similar transient absorption decay traces, implying that hole injection kinetics of the H3 and H4 cells after aging were not changed. Nevertheless, the femtosecond fluorescence kinetic traces (Fig. 7e and f) of the aged H3 and H4 cells have suggested faster electron injection kinetics due to the negative shift of the titania CB edge, which is related to the increased EQE maxima. Obviously, the electron injec tion acceleration was the main reason for the JSC augmentation for the H3 cell. However, the aged H4 cell presents a sublinear response in photocurrent under one sun illumination. We deduce that the thermal and light stress may induce insufficient mass transport of Co(III) species to the platinum electrode [34]. In consideration of DSSCs with the same redox electrolyte, the VOC drop after 1000 h aging under dual stress should stem from the shift of the quasi-Fermi-level ðEF;n Þ of the mesoporous titania film, which is
H3 sample and 1.8 ms for H4. Moreover, in the presence of a cobalt electrolyte for cell fabrication, accelerated kinetic traces due to hole injection from photo-oxidized dye molecules to electrolyte occur in the microsecond time domain, with time constants ðtCo 1=2 Þ being 28 μs for the
H3 sample and 21 μs for H4. Overall, the hole injection yields ðφhi Þ for H3 and H4 are 99%. We further measured photocurrent action spectra of dyed titania films in combination with a tris(2,20 -bipyridine)cobalt(II)/(III) based redox couple (for details see Fabrication and characterization of DSSCs), and the photos of DSSCs with H3 and H4 are shown in Fig. S13. As depicted in Fig. 4a, the H3 and H4 cells display a comparable external quantum efficiency (EQE) peak value of 85%. In addition, the H4 cell possesses higher EQE values in the wavelength range from 610 nm to 800 nm. In comparison to H3, H4 grafted on titania presents augmented light-harvesting efficiencies (LHEs) at longer wavelengths (Fig. 4b), which nicely interprets the broadened photocurrent response of the H4 cell. As shown in Fig. 4c, the electron diffusion length ðLn Þ plotted as a function of density of states (DOS) in DSSCs are comparable for the H3 and H4 cells, implying that both cells should have similar charge collection yields [31]. The Ln values of both cells are larger than the thickness of nanocrystalline TiO2 layer, suggesting close-to-unity charge collection yield. 3.4. Photovoltaic performance of DSSCs Typical photocurrent density-voltage (J-V) curves (Fig. 5a) of the H3 and H4 cells were measured under the illumination of 100 mW cm-2, simulated AM1.5G sunlight. Cell parameters are compiled in Table 2. The H3 cell affords a short-circuit photocurrent density ðJSC Þ of 15.21 8
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inherently related to the conduction band edge ðEc Þ and free electron density ðnc Þ of titania. As Fig. 8a presents, the aged H4 cell stores more charges than that of the aged H3 cell at a fixed voltage, suggesting that the downward Ec movements for the aged H3 and H4 cells should be different. As shown in Fig. 8b, the aged H3 cell displays an evidently longer half-lifetime ðt TPD 1=2 Þ of titania electrons compared to the aged H4
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cell, which interprets the VOC trend. It is known that lithium ions could downward shift the CB edge of metal oxide semiconductors [35]. Taking account of the optimized molecular geometry, the dye loading amount, and the electrical measurements, we assume that the H3 dye layer could possess less permeation channel with respect to the H4 dye layer as demonstrated in pictorial scheme (Fig. 8c and d). The self-assembling molecular layer on the surface of titania with H3 is more compact and better organized than that with H4, retarding the andante penetration of lithium ions into titania and mitigating the interfacial charge recombination. 4. Conclusions In summary, we have used 4-(benzo[c] [1,2,5]thiadiazol-4-yle thynyl)benzoic acid as the electron-acceptor to prepare two narrow-energy-gap isomeric photosensitizers, in combination with coplanar aromatic units of phenalenothiophene and benzoindenothio phene for electron-donors. In contrast to the phenalenothiophene based photosensitizer, the benzoindenothiophene counterpart has achieved a higher power conversion efficiency owing to the broadened spectral response. However, the phenalenothiophene based dye has presented a better retention ratio of power conversion efficiency under full-light soaking at 60 � C for 1000 h, due to the smaller drop of photovoltage and the improved output of photocurrent. Our studies have suggested that a joint effort on wise design of organic photosensitizers and inter face control should be taken into consideration to further enhance both efficiency and stability of colorful dye-sensitized solar cells. Notes The authors declare no competing financial interest. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was supported by the National Science Foundation of China (Grants 51673165 and 91733302), the National 973 Program (Grant 2015CB932204), and H. Glass. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2020.227748. References [1] B. O’Regan, M. Gr€ atzel, A low-cost, high-efficiency solar cell based on dyesensitized colloidal TiO2 films, Nature 353 (1991) 737–740. [2] C.-C. Chou, K.-L. Wu, Y. Chi, W.-P. Hu, S.J. Yu, G.-H. Lee, C.-L. Lin, P.-T. Chou, Ruthenium(II) sensitizers with heteroleptic tridentate chelates for dye-sensitized solar cells, Angew. Chem. Int. Ed. 50 (2011) 2054–2058. [3] Y. Huang, W.-C. Chen, R. Ghadari, X.-P. Liu, X.-Q. Fang, T. Yu, F.-T. Kong, Highly efficient ruthenium complexes with acetyl electron-acceptor unit for dye sensitized solar cells, J. Power Sources 396 (2018) 559–565. [4] M. Urbani, M. Gr€ atzel, M.K. Nazeeruddin, T. Torres, Meso-substituted porphyrins for dye-sensitized solar cells, Chem. Rev. 114 (2014) 12330–12396.
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