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Novel rod-shaped organic sensitizers for liquid and quasi-solid-state dye-sensitized solar cells
Accepted Manuscript Novel rod-shaped organic sensitizers for liquid and quasi-solid-state dye-sensitized solar cells Hai Zhang, Zhen-E. Chen, Jiefang Hu, Yanping Hong PII:
S0013-4686(18)32615-X
DOI:
https://doi.org/10.1016/j.electacta.2018.11.132
Reference:
EA 33133
To appear in:
Electrochimica Acta
Received Date: 3 September 2018 Accepted Date: 19 November 2018
Please cite this article as: H. Zhang, Z.-E. Chen, J. Hu, Y. Hong, Novel rod-shaped organic sensitizers for liquid and quasi-solid-state dye-sensitized solar cells, Electrochimica Acta (2018), doi: https:// doi.org/10.1016/j.electacta.2018.11.132. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Better planarity and conjugation of long rod-shaped molecules, resulting in higher electron transport efficiency and thus higher photoelectric
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conversion efficiency.
ACCEPTED MANUSCRIPT Novel rod-shaped Organic Sensitizers for Liquid and Quasi-solid-state Dye-sensitized Solar Cells Hai Zhanga*, Zhen-E Chena, Jiefang Hub, Yanping Hongc* School of Chemistry and Chemical Engineering, Academician workstation, Zunyi Normal
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a
College, Guizhou Zunyi 563002, China
Jiangxi Vocational Technical College of Industry Trade, Nanchang 330038, China
c
Jiangxi Key Laboratory of Natural Product and Functional Food, College of Food Science and
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b
*
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Engineering, Jiangxi Agricultural University, Nanchang 330045, China
Corresponding author: Tel.: +86 0851 28927159. Fax: +86 0851 28927159; Tel.: +86 0791
83813863.
Email addresses: [email protected] (H. Zhang), [email protected] (Y. Hong).
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ABSTRACT:
Two novel D−π−A dyes (FD and SD) have been synthesized for dye-sensitized solar cells (DSSCs).
The
two
rod-shaped
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di(1-benzothieno)[3,2-b:2',3'-d]pyrrole
molecules
and
with
with a
an different
identical donor
π-spacer group
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5,11-dihydroindolo[3,2-b]carbazole for FD or 11,12-dihydroindolo[2,3-a]carbazole for SD have been investigated. Compared to FD, the sensitizer SD not only showed a broader absorption spectrum extended to infrared region which was conducive to enhancing the light-trapping capability and thus to increase the photocurrent of the device fabricated with SD, but also exhibited better inhibition of dye aggregation resulting in a higher photovoltage. With the co-adsorption of chenodeoxycholic acid (CDCA), the photovoltaic performance of DSSCs based on the two dyes has dropped significantly. The results indicate that the aggregation of the two
ACCEPTED MANUSCRIPT dyes FD and SD can be controlled by linking multiple branched alkyl chains in the structure. Finally, FD and SD-based cells without CDCA achieved power conversion efficiencies (PCE) of 7.20% and 7.47%, respectively, in liquid DSSCs. Correspondingly, FD and SD displayed PCE
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values of 5.77% and 6.67% in quasi-solid-state DSSCs, respectively, under standard global 1.5 solar conditions.
Keywords: Metal-free sensitizer; Rod-shaped; Quasi-solid-state; Power conversion efficiency.
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1. Introduction
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In the past 10 years, the third-generation photovoltaic technology including dye-sensitized solar cells (DSSCs), polymer solar cells, and perovskite solar cells has been rapidly developed. Among them, the low-cost, high-efficiency and easy-fabrication dye-sensitized solar cells (DSSCs) have been attracted tremendous research interest by the researchers. Since 1991[1], photosensitizers
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have been studied in depth as one of the key components of DSSCs. To date, the most effective photosensitizers are ruthenium (Ru) polypyridine complexes [2, 3] and some complex porphyrin dyes [4, 5]. However, the high cost of the zinc-porphyrin dyes and ruthenium-based complexes,
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the difficult synthetic route and the tedious purification process limit their further application in
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DSSCs. On the contrary, metal-free organic sensitizers may bring greater hope for future applications due to their wide range of raw materials, flexible molecular design, and simple synthesis [6, 7].
Most organic sensitizers follow the design strategy of the donor−π-bridge−acceptor (D−π−A) model structure, which contains an electron donor, a conjugated π-spacer and an electron acceptor. Compared to the electron acceptor, the donor and the π-bridge have received more attention due to their structural diversity. In a variety of donor groups, sensitizers based on triphenylamine as a
ACCEPTED MANUSCRIPT donor have been extensively explored due to the distorted structure and good electron-donating ability [8−20]. However, the non-planar structure of triphenylamine also reduces the conjugation of molecules and restricts the transmission capacity of electrons, which is not conducive to the
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production of wider absorption bands and higher photocurrent. Therefore, the development of flat donor units will undoubtedly help solve this problem.
Based on the above considerations, two rod-like molecules have been designed. Due to the
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planar structure similar to pentacene, the two isomers 5,11-dihydroindolo[3,2-b]carbazole and
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11,12-dihydroindolo[2,3-a]carbazole is used as a donor unit, respectively. The electron-rich carbazole and indole units are contained in the two plane donors, which helps to increase the electron-donating
ability
and
hole-transporting
ability.
The
application
of
5,11-dihydroindolo[3,2-b]carbazole in the fields of light-emitting diodes (OLEDs) [21, 22],
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organic field-effect transistors (OFETs) [23, 24], hole transporting materials (HTM) [25] and photovoltaic cells [26−28] has proved its excellent performances. Considering that an important factor for obtaining good photovoltaic properties is related to the
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π−π stacking of dye molecules, the planar structure will certainly increase this trend. Currently,
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most researchers suppress this phenomenon mainly by using additives such as chenodeoxycholic acid (CDCA) or by introducing bulk groups in the donor or π-bridge. The use of bulky substituents to modify the shape of the chromophores makes them more spherical and limits intermolecular interactions, where the introduction of branched alkyl chains in the molecule has proven to be a simple, effective strategy to inhibit molecular aggregation and increase the photovoltage [29]. To achieve the goal mentioning above, two 2-ethylhexyl groups have introduced into the donor skeleton, and the influence of the position of the alkyl chain on the
ACCEPTED MANUSCRIPT photovoltaic performance is also one of the contents of our research. As is generally known, the π-spacer is critical for slowing charge recombination, accelerating the electron injection process, and broadening the light absorption range. The planar porphyrin
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ring is a typical representative of the π-spacer. Chlorophyll containing several porphyrin molecules plays a crucial role in photosynthesis, therefore, metal porphyrin sensitizers have been receiving continuous attention. In early studies, porphyrin-based sensitizers usually exhibited very
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low conversion efficiencies. Until 2011, with the development of the porphyrin push-pull system
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and the introduction of bulky substituents into the porphyrin structure to avoid π−π stacking of the molecule, the efficiency of the DSSC based on the Zn (II) porphyrin dye had been significantly improved, reaching 12.3% [5]. However, the complex synthesis process of porphyrin ring limits its wide range of applications in DSSCs. In general, the rod-shaped conjugated structure is more
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likely to inhibit aggregation than the cyclic conjugated structure. Consequently, finding the appropriate polycyclic structure as π-bridge has become the focus of some researchers [30−33]. From design philosophy, we still used linear structural elements for the selection of π-bridge
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structure to minimize the space distortion caused by multiple link elements. In the previous work,
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we reported a series of dithieno[3,2-b:2',3'-d]pyrrole (DTP)-based organic dyes for DSSCs, dyes with completely coplanar DTP ring as π-bridge showed outstanding photoelectric properties [34, 35]. Undoubtedly, the influence of the larger linear plane structure obtained by other aromatic rings fused with DTP on the photovoltaic cell is worth researching. Here, analog di(1-benzothieno)[3,2-b:2',3'-d]pyrrole (DBTP) of DTP ring was selected as the bridging unit of the two rod-like molecules. Similarly, in order to inhibit charge recombination as well as increase solubility, 2-ethylhexyl was linked to the N-atom of the DBTP ring.
ACCEPTED MANUSCRIPT Figure 1 In this work, two rod-like molecules (FD and SD, Fig. 1) were finally synthesized and incorporated for the first time as the sensitizers of DSSCs. To understand the influence of this
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novel structure on the photoelectric properties of DSSCs, we systematically studied the photophysical, electrochemical and photovoltaic properties of the two sensitizers in liquid electrolyte and quasi-solid electrolyte. As a result, in liquid electrolyte, the efficiencies of DSSCs
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based on FD and SD were 7.2% and 7.47%, respectively, while the power conversion (PCE) of
quasi-solid-state DSSC based on FD. 2. Results and discussion 2.1. Molecular orbital calculations
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the quasi-solid-state DSSC based on SD increases by 15% in comparison to that of the
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Prior to the synthesis of these dye molecules, density functional theory (DFT) calculations were performed using the Gaussian 09 program suite at the B3LYP/6-31 G(d) level in vacuum to investigate the spatial structure and energy levels of the molecules. As shown in the optimized
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conformation (Fig. 2), the dihedral angles between the donor unit and the DBTP ring in the dyes
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FD and SD were calculated to be 37.16o and 36.62o, respectively, while the dihedral angles between the DBTP unit and the neighboring thiophene ring were calculated to be 22.02o and 22.14o, respectively. Obviously, apart from the space distortion of the above two connection points, the other structural units of the two molecules are completely co-planar, which effectively reduces the transmission resistance of the electrons in the molecule. Figure 2 The frontier molecular orbitals and the energy levels were also calculated. As listed in Table 1,
ACCEPTED MANUSCRIPT the calculated electronic parameters (HOMO, LUMO, Eg) for FD and SD were (−4.82, −2.62, 2.20 eV) and (−4.98, −2.62, 2.36 eV), respectively. It is difficult to determine which one will present a better electron transfer to TiO2, because the dyes FD and SD have quite similar energy
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levels. However, it can be observed that the two molecules present an intramolecular electron transfer that goes from the donor part and the π-bridge up to the electron acceptor part, which is consistent with other high-performance dyes.
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2.2. Absorption properties in solutions
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The UV−Vis absorption spectra and emission spectra of the two dyes in diluted solution of CH2Cl2 (2 × 10-5 M) are displayed in Fig. 3a and 3b, and the corresponding data are summarized in Table 1. The two dyes exhibited two distinct absorption bands between 300−400 nm and 410−570 nm, respectively. The former corresponds to the π−π* transition of the conjugated
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system, while the later can be ascribed to an efficient charge-separated state produced by the intramolecular charge transfer (ICT) between the donor and acceptor moiety. The absorption spectra of FD and SD displayed maximal absorption peaks at 495 nm with molar extinction
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coefficient (ε) of 46200 M-1 cm-1, at 488 nm with ε of 47400 M-1 cm-1, respectively. Notably, the
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spectra of the two dyes are very similar, but the spectrum of FD is red-shifted compared to that of SD, which may be ascribed to the fact that 5,11-dihydroindolo[3,2-b]carbazole in FD has stronger electron-donating ability than 11,12-dihydroindolo[2,3-a]carbazole in SD, resulting in a smaller energy band gap and a broader absorption spectrum. Another interesting point is that the relative intensities of the two chromophores in the π−π* and ICT bands are significantly different. In the FD absorption spectra, the low-wavenumber π−π* band is significantly higher than the right-side ICT band, but the SD absorption spectra shows an opposite result. In general, ICT conversion is
ACCEPTED MANUSCRIPT the property of the HOMO level to the LUMO level. Although the LUMO surface is similarly delocalized in both dyes, the HOMO surface extends over the whole DBTP ring in SD and to a noticeable lesser extent over DBTP ring in FD (Fig. 2). Therefore, the larger overlap between the
Figure 3 Table 1
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2.3. Electrochemical properties
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molar extinction coefficient [36].
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two frontier orbitals in SD translates into a higher transition probability, while exhibiting a higher
To estimate the energy level alignment of dyes FD and SD versus TiO2 and I−/I3− redox mediator, CV measurements were performed in CH3CN solution with addition of ferrocene as the internal standard. As shown in Fig. 4, the two dyes exhibited almost reversible behavior,
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indicating that the oxidized dye molecules could easily accept electrons of the electron donor (I−) in the electrolyte solution and underwent an oxidation-reduction reaction to return to the ground state. The first oxidation potential vs. NHE (Eox) corresponding to the highest occupied molecular
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orbital (HOMO vs. NHE) was calibrated by addition of 0.63 V to potential vs Fc/Fc+ (vs. NHE).
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The LUMO levels could be calculated from HOMO – E0-0. The data are summarized in Table 1. The HOMO values of FD and SD were found to be 0.90 V and 1.01 V, respectively. Apparently, they are all sufficiently positive than the energy level of iodide/triiodide redox (0.4 V vs. NHE), indicating that oxidized dye generated by the electron injection process can effectively accept electrons from electrolyte and regenerated. In addition, the calculated LUMO values were –1.28 V and –1.19 V, respectively, which were more negative than the conduction band of TiO2 (–0.5 V vs. NHE), indicating that the electron injection process from the photo-excited dye molecule to the
ACCEPTED MANUSCRIPT conduction band of TiO2 was energetically permitted. Figure 4 2.4. Photovoltaic properties of DSSCs
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Fig. 5 shows the photocurrent-voltage (J−V) curves and incident photo-current conversion efficiency (IPCE) action spectra of liquid electrolyte-based DSSCs obtained with FD and SD. The effects of co-adsorbent (CDCA) on the photovoltaic performances were also investigated. Both
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the dye-based devices exhibited strong response in the range of 350–600 nm with the highest
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values of 67% and 72% at 500 nm for FD and SD, respectively. As compared to that of the SD-based device, FD showed slightly higher response in the long wavelengths 578–800 nm but lower IPCE values at other spectral regions, which was in good agreement with absorption spectra as shown in Fig. 2.
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Figure 5 Table 2
As seen in Fig. 5b, in pure solvent CH2Cl2 system without CDCA, the FD-based device gave a
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short circuit photocurrent density (Jsc) of 16.10 mA cm-2, an open circuit voltage (Voc) of 646 mV,
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and a fill factor (FF) of 0.693, corresponding to a power conversion efficiency (PCE) of 7.20%, while the SD-based device gave Jsc of 14.98 mA cm-2, Voc of 732 mV, FF of 0.682, and PCE of 7.47%. It is clear that FD-based device obtained a higher photocurrent due to its wider absorption in the visible region, while SD-based showed a higher photovoltage, indicating that the N-alkyl substituted 11,12-dihydroindolo[2,3-a]carbazole ring could suppress more effectively dye aggregation. With the addition of the CDCA in the dye solution, the performances of the FD- and SD-based devices reduced significantly. The DSSCs based on the dyes FD and SD with 0.5 mM
ACCEPTED MANUSCRIPT CDCA exhibited poorer Jsc, Voc and FF values (12.09 mA cm-2, 624 mV, 0.645 for FD) and (9.76 mA cm-2, 644 mV, 0.683 for SD), respectively, leading to inferior PCEs of 4.86% and 4.42%. This result can be attributed to the co-adsorbed CDCA molecules occupying the binding sites on the
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TiO2 film, inducing a decrease in the amount of organic dyes loaded on the TiO2 films (Table 2). It is worth noting that as the CDCA content increased from 0 to 0.5 mM and further to 2 mM, the photovoltaic performances of FD and SD showed opposite trend. When the dye molecules were
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co-adsorbed with 2 mM CDCA, there was no large change in the photovoltage and the amount of
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dye adsorption, but there was a significant increase in photocurrent, from 12.09 mA cm-2 rose to 13.31 mA cm-2 for FD and from 9.76 mA cm-2 rose to 13.16 mA cm-2 for SD, respectively. One plausible explanation for this phenomenon is that a broader and higher IPCE response of these devices with 2 mM CDCA (Fig. 5a) suggests that the aggregation is indeed repressed, thus
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resulting in more efficient charge transport and collection, the final result is the increase of Jsc, however further experimentation need to be performed to confirm this.
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Figure 6 Table 3
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The characteristic J−V and IPCE curves of FD and SD sensitized devices based on quasi-solid-state electrolyte are shown in Fig. 6 and the photovoltaic parameters are listed in Table 3. Compared to liquid electrolyte, the trend of the IPCE curves is the same. QSS-DSSC based on FD gave a relatively low IPCE action spectrum, and the maximum IPCE value was only 55% at 430 nm, while QSS-DSSC based on SD showed a higher IPCE values in the broad range of 340−610 nm, the maximum IPCE value appeared at 520 nm and reached 69%. As listed in Table 3, under the same conditions (AM 1.5), the SD based QSS-DSSC exhibited Jsc of 16.15 mA cm-2, Voc
ACCEPTED MANUSCRIPT of 649 mV and FF of 0.636, while the PCE reached 6.67%. QSS-DSSC based on FD gave a Jsc value of 17.11 mA cm-2, a Voc of 606 mV and a FF of 0.557, corresponding to the PCE value of 5.77%. Similarly, the higher PCE of the QSS-DSSC based on SD than that of the FD-sensitized
chains on the same side causes better inhibition. 2.5. Electrochemical impedance spectroscopy analysis
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cell, probably due to its higher fill factor, and the fact that the donor group substituted by alkyl
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Electrochemical impedance spectroscopy (EIS) was employed to investigate the electron
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recombination processes in DSSCs based on the dyes under darkness. EIS analysis of the liquid electrolyte-based DSSCs and quasi-solid-state electrolyte-based DSSCs were performed over the frequency range of 0.1 Hz to 106 Hz with an applied bias set at –0.67 V and –0.65 V, respectively. The Nyquist and Bode plots are shown in Fig. 7 and 8. Typically, the Nyquist plots show three which
are
assigned
to
the charge transfer resistance at
the counter
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semicircles,
electrode/electrolyte, the charge transfer resistance at the TiO2/dye/electrolyte interface, and the Warburg diffusion process of I−/I3− in the electrolyte. The radius of the second semicircle in the
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middle-frequency region corresponds to the value of the recombination resistance (Rrec) at the
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photoanode/dye/electrolyte interface. The calculated Rrec values are listed in Table 4. A smaller Rrec value means a larger charge recombination rate [37]. Evidently, the radius of the semicircle in the region of medium frequency increased in the order of FD (11.98 Ω) ﹤ SD (18.53 Ω) for the liquid electrolyte and FD (19.74 Ω) ﹤ SD (21.67 Ω) for the quasi-solid-state electrolyte. These observations are in good accordance with the trends of the experimental Voc. Figure 7 Figure 8
ACCEPTED MANUSCRIPT Table 4 The electron lifetimes were calculated according to the Bode plots using τe = 1/2πf, in which f represents the peak frequency of the lower frequency region [38]. With the quasi-solid-state
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electrolyte, the electron lifetimes for DSSCs based on FD and SD were 3.23, 4.18 ms, respectively. Meanwhile, the electron lifetimes for DSSCs based on FD and SD with liquid electrolyte were higher, reaching 3.39, 6.06 ms, respectively. The longer electron lifetime for the SD-based cell
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indicates the more effective suppression of the back reaction of the injected electrons with the I3−
that
the
combination
of
two
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in the electrolyte. Based on the results from EIS measurements and electron lifetimes, we believe adjacent
long-chain
alkyl
chains
and
bulky cyclic
11,12-dihydroindolo[2,3-a]carbazole electron donor in the dye SD brings about a better suppression of recombination between the injected electrons and the electrolyte, resulting in a rise
3. Conclusions
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of Voc.
In summary, two novel linear polycyclic organic sensitizers have been designed and
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synthesized. The results show that the introduction of linear planar units greatly reduce the space
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distortion in molecular and enhance the intramolecular charge transfer in the D−π−A system, yielding high molar absorptivity and a relatively narrow band-gap. In the liquid electrolyte, the sensitizer SD with two substituted alkyl groups on the same side of the donor unit gave more excellent photovoltaic performance than that of the sensitizer FD. Encouragingly, in quasi-solid-state electrolyte, the power conversion efficiency of SD-based DSSC reached 6.7%, compared to 5.6% for FD-based DSSC. This is mainly due to the relatively large charge transfer resistance and longer lifetime for the injected electrons, resulting in a higher voltage for DSSCs
ACCEPTED MANUSCRIPT based on SD. In addition, the PCE of DSSCs based on the two dyes was significantly reduced in the presence of the co-adsorbent CDCA. The result shows that in the absence of a co-adsorbent, multiple branched alkyl chains in these dye molecules can effectively inhibit dye molecule
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aggregation and improve photovoltaic performance. This work provides an alternative direction for rational molecular strategy to achieve high power conversion efficiency, especially to increase electron transfer efficiency and Jsc while avoiding reducing Voc.
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4. Experimental section
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4.1. Materials and instruments
Tetrahydrofuran and toluene were distilled in an argon atmosphere over sodium/benzophenone. All other reagents were purchased from Adamas, J&K, TCI chemicals, Sigma-Aldrich and used as such without any treatment. Liquids were transferred by using syringe. Thin layer chromatography
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(Merck silica gel 60 F254) was applied for monitoring the intermediates and final products and purification was carried out by column chromatography using silica gel (300–400 mesh). 4.2. Measurement and characterizations
H and 13C NMR spectra of the all intermediate and final products were measured on a Bruker
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1
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Avance 600 MHz spectrometer by using CDCl3 or THF-d8 and tetramethylsilane as an internal standard. The mass spectra of the prepared compounds were carried out at Finnigan MAT 95 XP analyzer spectrometer (Thermo Electron Corporation). For melting point XT4B microscopic melting point apparatus was used and uncorrected. UV3600 Plus spectrophotometer (Shimadzu Corporation) and Hitachi F–4600 spectrophotometers were used for the measurement of absorption and fluorescence spectra. The photocurrent-voltage (J−V) characteristics of the DSSCs were measured by Keithley 2400 source meter under simulated AM 1.5 G (100 mW/cm2)
ACCEPTED MANUSCRIPT illumination with a solar light simulator (Pecell-L15, Japan). A 1 KW Xenon arc lamp (Oriel) served as a light source and its incident light intensity was calibrated with an BS-520 standard silicon solar cell. The incident monochromatic photon-to-current conversion efficiency (IPCE)
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spectra of the DSSCs were conducted by illuminating monochromatic light from 300 to 800 nm on the basis of a PEC-S20 action spectrum measurement system. The electrochemical impedance spectroscopy (EIS) was conducted on the CS16X electrochemical workstation (Wuhan, China) in
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dark condition for analyzing the internal impedance characteristics of DSSCs by fitting the
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impedance spectra using Z-view software. Dye load amount on the films was measured by desorbing the with 0.1 M NaOH in THF/H2O (1:1) and measured by UV−Vis spectrum.
4.3. Fabrication of DSSCs
The working electrode composed of mesoporous TiO2 layer (8 µm nanocrystalline transparent
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layer and 4 µm scattering layer) on FTO glass was prepared following the reported procedure [34]. The active area of the TiO2 film was controlled to be 0.4 × 0.4 cm-2 for photovoltaic performance
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measurements. The TiO2 films were immersed into a solution of the dyes (0.2 mM dye in CH2Cl2) for 16 h, namely sensitization. The liquid electrolyte used in this work was composed of 0.6 M
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1-methyl-3-propylimidazolium iodide (PMII), 0.1 M guanidinium thiocyanate, 0.07 M I2, 0.05 M LiI, and 0.5 M tert-butyl-pyridine. Sodium lauryl sulfate, ammonium persulfate, sodium hydrogencarbonate and deionized water were heated and stirred in a constant temperature water bath, and nitrogen was passed for 20 minutes. After the resultant mixture was copolymerized with methyl methacrylate and vinyl acetate for 1 h, added aluminum sulfate to break emulsion and then filtered, washed with water and alcohol, respectively, and vacuum dried to prepare the polymer. The quasi-solid-state gel electrolyte was prepared as follows: 12% (wt%) of poly(methyl
ACCEPTED MANUSCRIPT methacrylate-co-vinyl acetate) and the liquid electrolyte were mixed in 3-methoxypropionitrile, heated at 60−70 °C for 1 h, then cooled and repeated several times, until all solids dissolved and a clear gel appeared.
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Acknowledgements We are grateful to the National Natural Science Foundation of China (21462054 and 61368006), the Scientific Research Fund of Guizhou Provincial Science and Technology Department, China
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(LH[2015]7036, [2015]4003 and [2016]1419), the Natural Science Foundation of Guizhou
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Provincial Education Department, China ([2014]42), and the Science and Technology Cooperation talent Project of ZunYi City, China ([2016]14) for the financial support. References
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Figure Captions
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Fig. 1. Chemical structure of the dyes FD and SD.
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Fig. 2. Optimized structures and electron distributions in HOMO and LUMO levels of FD and SD.
Fig. 3. (a) UV−Vis spectra of FD and SD in CH2Cl2 solution and (b) Normalized absorption and
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emission spectra.
Fig. 4. Cyclic voltammograms curves of FD and SD and ferrocene/ferrocenium internal reference.
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Fig. 5. (a) IPCE action spectra and (b) J− −V characteristics of DSSCs based on FD and SD with liquid
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electrolyte (L: liquid DSSCs).
Fig. 6. (a) IPCE action spectra and (b) J− −V characteristics of DSSCs based on FD and SD with quasi-solid-state electrolyte (QSS: quasi-solid-state DSSCs).
Fig. 7. (a) EIS Nyquist and (b) EIS Bode plots for liquid DSSCs based on the two sensitizers measured in the dark under −0.67 V bias (L: liquid DSSCs).
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measured in the dark under −0.65 V bias (QSS: quasi-solid-state DSSCs).
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Better planarity and conjugation of long rod-shaped molecules, resulting in higher electron transport efficiency and thus higher photoelectric
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conversion efficiency.
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Table 1 Absorption and Emission Data for organic dyes in CH2Cl2. λmaxa/nm
εa/M-1 cm-1
λemb/nm
λintc/nm
HOMOd/V (vs. NHE)
E0-0e/eV
FD SD
495 488
46200 47400
642 644
570 563
0.90 1.01
2.18 2.20
EHOMOg/eV
–1.28 –1.19
–4.82 –4.98
ELUMOg/eV –2.62 –2.62
Absorption maximum of the dyes measured in CH2Cl2 with a concentration of 2 × 10-5 M. ε: Molar extinction coefficient at λmax. b Emission maxima of dye in CH2Cl2. c λint (λintersection) obtained from the cross point of normalized absorption and emission spectra in CH2Cl2 solution. d HOMO levels were measured in CH3CN with 0.1 M TBAPF6 as supporting electrolyte, Fc+/Fc as an internal standard, scan rate 50 mV/s. e E0-0 = 1240/λint. f LUMO was calculated from HOMO – E0-0. g EHOMO and ELUMO by DFT calculation (B3LYP, 6-31 G (d) level).
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a
LUMOf/V (vs. NHE)
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dye
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Table 2 Photovoltaic Parameters for DSSCs of Organic Dyes. Voca(Vocb)/mV
Jsca(Jscb)/mA·cm-2
FFa(FFb)/%
PCEa(PCEb)/%
FD SD FD + 0.5 mM CDCA SD + 0.5 mM CDCA FD + 2 mM CDCA SD + 2 mM CDCA N719
646 (648 ± 5) 732 (734 ± 9) 624 (624 ± 2) 664 (666 ± 8) 629 (630 ± 3) 667 (669 ± 4) 705 (705 ± 6)
16.10 (16.05 ± 0.19) 14.98 (15.16 ± 0.18) 12.09 (11.96 ± 0.13) 9.76 (9.81 ± 0.10) 13.31 (13.35 ± 0.17) 13.16 (12.96 ± 0.22) 17.96 (17.97 ± 0.15)
69.30 (69.02 ± 0.39) 68.17 (66.73 ± 1.44) 64.47 (64.61 ± 0.59) 68.26 (67.52 ± 0.74) 67.41 (66.54 ± 0.87) 62.06 (62.00 ± 0.35) 67.00 (66.33 ± 0.67)
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7.20 (7.18 ± 0.02) 7.47 (7.42 ± 0.05) 4.86 (4.83 ± 0.05) 4.42 (4.41 ± 0.02) 5.65 (5.59 ± 0.06) 5.45 (5.38 ± 0.07) 8.50 (8.41 ± 0.09)
Dye loading (mol cm-2) 2.39 × 10-7 2.13 × 10-7 2.05 × 10-7 1.75 × 10-7 2.06 × 10-7 1.57 × 10-7
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TiO2 thickness 8 + 4 µm and 0.16 cm2, [dye] = 0.2 mM, dipping solvent CH2Cl2, dipping time 16 h, under AM 1.5 illumination (100 mW/cm2). a The best device parameters. b The average device parameters (obtained from five devices).
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Table 3 Photovoltaic Parameters for DSSCs of Organic Dyes. electrolyte
Voca(Vocb)/mV
Jsca(Jscb)/mA·cm-2
FD SD
quasi-solid quasi-solid
606 (607 ± 1) 649 (650 ± 4)
17.11 (16.92 ± 0.19) 16.15 (16.14 ± 0.24)
FFa(FFb)/%
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55.69 (55.99 ± 0.3) 63.57 (63.34 ± 1.3)
PCEa(PCEb)/% 5.77 (5.75 ± 0.03) 6.67 (6.64 ± 0.04)
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TiO2 thickness 8 + 4 µm and 0.16 cm2, [dye] = 0.2 mM, dipping solvent CH2Cl2, dipping time 16 h, under AM 1.5 illumination (100 mW/cm2). a The best device parameters. b The average device parameters (obtained from five devices).
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Table 4 Electrochemical parameters of the DSSCs with various dyes. electrolyte
Rrec (ohm)
fmax (Hz)
FD SD FD SD
liquid liquid quasi-solid quasi-solid
11.98 18.53 19.74 21.67
46.96 26.27 49.25 38.08
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Dyes
τe (ms) 3.39 6.06 3.23 4.18
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1、 Two novel rod-shaped dyes were successfully synthesized and utilized for dye-sensitized solar cells. 2、 The di(1-benzothieno)[3,2-b:2',3'-d]pyrrole structure was first introduced as a π-bridge unit for sensitizers.
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3、 The effects of two isomers 5,11-dihydroindole[3,2-b]carbazole and 11,12-dihydroindole[2,3-a] carbazole as donor units on photovoltaic properties were investigated.
4、 In liquid electrolyte, the power conversion efficiencies of DSSCs based on FD and SD reached
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7.2% and 7.47%, respectively.
S0013-4686(18)32615-X
DOI:
https://doi.org/10.1016/j.electacta.2018.11.132
Reference:
EA 33133
To appear in:
Electrochimica Acta
Received Date: 3 September 2018 Accepted Date: 19 November 2018
Please cite this article as: H. Zhang, Z.-E. Chen, J. Hu, Y. Hong, Novel rod-shaped organic sensitizers for liquid and quasi-solid-state dye-sensitized solar cells, Electrochimica Acta (2018), doi: https:// doi.org/10.1016/j.electacta.2018.11.132. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Better planarity and conjugation of long rod-shaped molecules, resulting in higher electron transport efficiency and thus higher photoelectric
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conversion efficiency.
ACCEPTED MANUSCRIPT Novel rod-shaped Organic Sensitizers for Liquid and Quasi-solid-state Dye-sensitized Solar Cells Hai Zhanga*, Zhen-E Chena, Jiefang Hub, Yanping Hongc* School of Chemistry and Chemical Engineering, Academician workstation, Zunyi Normal
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a
College, Guizhou Zunyi 563002, China
Jiangxi Vocational Technical College of Industry Trade, Nanchang 330038, China
c
Jiangxi Key Laboratory of Natural Product and Functional Food, College of Food Science and
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b
*
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Engineering, Jiangxi Agricultural University, Nanchang 330045, China
Corresponding author: Tel.: +86 0851 28927159. Fax: +86 0851 28927159; Tel.: +86 0791
83813863.
Email addresses: [email protected] (H. Zhang), [email protected] (Y. Hong).
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ABSTRACT:
Two novel D−π−A dyes (FD and SD) have been synthesized for dye-sensitized solar cells (DSSCs).
The
two
rod-shaped
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di(1-benzothieno)[3,2-b:2',3'-d]pyrrole
molecules
and
with
with a
an different
identical donor
π-spacer group
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5,11-dihydroindolo[3,2-b]carbazole for FD or 11,12-dihydroindolo[2,3-a]carbazole for SD have been investigated. Compared to FD, the sensitizer SD not only showed a broader absorption spectrum extended to infrared region which was conducive to enhancing the light-trapping capability and thus to increase the photocurrent of the device fabricated with SD, but also exhibited better inhibition of dye aggregation resulting in a higher photovoltage. With the co-adsorption of chenodeoxycholic acid (CDCA), the photovoltaic performance of DSSCs based on the two dyes has dropped significantly. The results indicate that the aggregation of the two
ACCEPTED MANUSCRIPT dyes FD and SD can be controlled by linking multiple branched alkyl chains in the structure. Finally, FD and SD-based cells without CDCA achieved power conversion efficiencies (PCE) of 7.20% and 7.47%, respectively, in liquid DSSCs. Correspondingly, FD and SD displayed PCE
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values of 5.77% and 6.67% in quasi-solid-state DSSCs, respectively, under standard global 1.5 solar conditions.
Keywords: Metal-free sensitizer; Rod-shaped; Quasi-solid-state; Power conversion efficiency.
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1. Introduction
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In the past 10 years, the third-generation photovoltaic technology including dye-sensitized solar cells (DSSCs), polymer solar cells, and perovskite solar cells has been rapidly developed. Among them, the low-cost, high-efficiency and easy-fabrication dye-sensitized solar cells (DSSCs) have been attracted tremendous research interest by the researchers. Since 1991[1], photosensitizers
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have been studied in depth as one of the key components of DSSCs. To date, the most effective photosensitizers are ruthenium (Ru) polypyridine complexes [2, 3] and some complex porphyrin dyes [4, 5]. However, the high cost of the zinc-porphyrin dyes and ruthenium-based complexes,
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the difficult synthetic route and the tedious purification process limit their further application in
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DSSCs. On the contrary, metal-free organic sensitizers may bring greater hope for future applications due to their wide range of raw materials, flexible molecular design, and simple synthesis [6, 7].
Most organic sensitizers follow the design strategy of the donor−π-bridge−acceptor (D−π−A) model structure, which contains an electron donor, a conjugated π-spacer and an electron acceptor. Compared to the electron acceptor, the donor and the π-bridge have received more attention due to their structural diversity. In a variety of donor groups, sensitizers based on triphenylamine as a
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production of wider absorption bands and higher photocurrent. Therefore, the development of flat donor units will undoubtedly help solve this problem.
Based on the above considerations, two rod-like molecules have been designed. Due to the
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planar structure similar to pentacene, the two isomers 5,11-dihydroindolo[3,2-b]carbazole and
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11,12-dihydroindolo[2,3-a]carbazole is used as a donor unit, respectively. The electron-rich carbazole and indole units are contained in the two plane donors, which helps to increase the electron-donating
ability
and
hole-transporting
ability.
The
application
of
5,11-dihydroindolo[3,2-b]carbazole in the fields of light-emitting diodes (OLEDs) [21, 22],
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organic field-effect transistors (OFETs) [23, 24], hole transporting materials (HTM) [25] and photovoltaic cells [26−28] has proved its excellent performances. Considering that an important factor for obtaining good photovoltaic properties is related to the
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π−π stacking of dye molecules, the planar structure will certainly increase this trend. Currently,
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most researchers suppress this phenomenon mainly by using additives such as chenodeoxycholic acid (CDCA) or by introducing bulk groups in the donor or π-bridge. The use of bulky substituents to modify the shape of the chromophores makes them more spherical and limits intermolecular interactions, where the introduction of branched alkyl chains in the molecule has proven to be a simple, effective strategy to inhibit molecular aggregation and increase the photovoltage [29]. To achieve the goal mentioning above, two 2-ethylhexyl groups have introduced into the donor skeleton, and the influence of the position of the alkyl chain on the
ACCEPTED MANUSCRIPT photovoltaic performance is also one of the contents of our research. As is generally known, the π-spacer is critical for slowing charge recombination, accelerating the electron injection process, and broadening the light absorption range. The planar porphyrin
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ring is a typical representative of the π-spacer. Chlorophyll containing several porphyrin molecules plays a crucial role in photosynthesis, therefore, metal porphyrin sensitizers have been receiving continuous attention. In early studies, porphyrin-based sensitizers usually exhibited very
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low conversion efficiencies. Until 2011, with the development of the porphyrin push-pull system
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and the introduction of bulky substituents into the porphyrin structure to avoid π−π stacking of the molecule, the efficiency of the DSSC based on the Zn (II) porphyrin dye had been significantly improved, reaching 12.3% [5]. However, the complex synthesis process of porphyrin ring limits its wide range of applications in DSSCs. In general, the rod-shaped conjugated structure is more
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likely to inhibit aggregation than the cyclic conjugated structure. Consequently, finding the appropriate polycyclic structure as π-bridge has become the focus of some researchers [30−33]. From design philosophy, we still used linear structural elements for the selection of π-bridge
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structure to minimize the space distortion caused by multiple link elements. In the previous work,
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we reported a series of dithieno[3,2-b:2',3'-d]pyrrole (DTP)-based organic dyes for DSSCs, dyes with completely coplanar DTP ring as π-bridge showed outstanding photoelectric properties [34, 35]. Undoubtedly, the influence of the larger linear plane structure obtained by other aromatic rings fused with DTP on the photovoltaic cell is worth researching. Here, analog di(1-benzothieno)[3,2-b:2',3'-d]pyrrole (DBTP) of DTP ring was selected as the bridging unit of the two rod-like molecules. Similarly, in order to inhibit charge recombination as well as increase solubility, 2-ethylhexyl was linked to the N-atom of the DBTP ring.
ACCEPTED MANUSCRIPT Figure 1 In this work, two rod-like molecules (FD and SD, Fig. 1) were finally synthesized and incorporated for the first time as the sensitizers of DSSCs. To understand the influence of this
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novel structure on the photoelectric properties of DSSCs, we systematically studied the photophysical, electrochemical and photovoltaic properties of the two sensitizers in liquid electrolyte and quasi-solid electrolyte. As a result, in liquid electrolyte, the efficiencies of DSSCs
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based on FD and SD were 7.2% and 7.47%, respectively, while the power conversion (PCE) of
quasi-solid-state DSSC based on FD. 2. Results and discussion 2.1. Molecular orbital calculations
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the quasi-solid-state DSSC based on SD increases by 15% in comparison to that of the
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Prior to the synthesis of these dye molecules, density functional theory (DFT) calculations were performed using the Gaussian 09 program suite at the B3LYP/6-31 G(d) level in vacuum to investigate the spatial structure and energy levels of the molecules. As shown in the optimized
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conformation (Fig. 2), the dihedral angles between the donor unit and the DBTP ring in the dyes
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FD and SD were calculated to be 37.16o and 36.62o, respectively, while the dihedral angles between the DBTP unit and the neighboring thiophene ring were calculated to be 22.02o and 22.14o, respectively. Obviously, apart from the space distortion of the above two connection points, the other structural units of the two molecules are completely co-planar, which effectively reduces the transmission resistance of the electrons in the molecule. Figure 2 The frontier molecular orbitals and the energy levels were also calculated. As listed in Table 1,
ACCEPTED MANUSCRIPT the calculated electronic parameters (HOMO, LUMO, Eg) for FD and SD were (−4.82, −2.62, 2.20 eV) and (−4.98, −2.62, 2.36 eV), respectively. It is difficult to determine which one will present a better electron transfer to TiO2, because the dyes FD and SD have quite similar energy
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levels. However, it can be observed that the two molecules present an intramolecular electron transfer that goes from the donor part and the π-bridge up to the electron acceptor part, which is consistent with other high-performance dyes.
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2.2. Absorption properties in solutions
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The UV−Vis absorption spectra and emission spectra of the two dyes in diluted solution of CH2Cl2 (2 × 10-5 M) are displayed in Fig. 3a and 3b, and the corresponding data are summarized in Table 1. The two dyes exhibited two distinct absorption bands between 300−400 nm and 410−570 nm, respectively. The former corresponds to the π−π* transition of the conjugated
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system, while the later can be ascribed to an efficient charge-separated state produced by the intramolecular charge transfer (ICT) between the donor and acceptor moiety. The absorption spectra of FD and SD displayed maximal absorption peaks at 495 nm with molar extinction
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coefficient (ε) of 46200 M-1 cm-1, at 488 nm with ε of 47400 M-1 cm-1, respectively. Notably, the
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spectra of the two dyes are very similar, but the spectrum of FD is red-shifted compared to that of SD, which may be ascribed to the fact that 5,11-dihydroindolo[3,2-b]carbazole in FD has stronger electron-donating ability than 11,12-dihydroindolo[2,3-a]carbazole in SD, resulting in a smaller energy band gap and a broader absorption spectrum. Another interesting point is that the relative intensities of the two chromophores in the π−π* and ICT bands are significantly different. In the FD absorption spectra, the low-wavenumber π−π* band is significantly higher than the right-side ICT band, but the SD absorption spectra shows an opposite result. In general, ICT conversion is
ACCEPTED MANUSCRIPT the property of the HOMO level to the LUMO level. Although the LUMO surface is similarly delocalized in both dyes, the HOMO surface extends over the whole DBTP ring in SD and to a noticeable lesser extent over DBTP ring in FD (Fig. 2). Therefore, the larger overlap between the
Figure 3 Table 1
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2.3. Electrochemical properties
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molar extinction coefficient [36].
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two frontier orbitals in SD translates into a higher transition probability, while exhibiting a higher
To estimate the energy level alignment of dyes FD and SD versus TiO2 and I−/I3− redox mediator, CV measurements were performed in CH3CN solution with addition of ferrocene as the internal standard. As shown in Fig. 4, the two dyes exhibited almost reversible behavior,
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indicating that the oxidized dye molecules could easily accept electrons of the electron donor (I−) in the electrolyte solution and underwent an oxidation-reduction reaction to return to the ground state. The first oxidation potential vs. NHE (Eox) corresponding to the highest occupied molecular
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orbital (HOMO vs. NHE) was calibrated by addition of 0.63 V to potential vs Fc/Fc+ (vs. NHE).
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The LUMO levels could be calculated from HOMO – E0-0. The data are summarized in Table 1. The HOMO values of FD and SD were found to be 0.90 V and 1.01 V, respectively. Apparently, they are all sufficiently positive than the energy level of iodide/triiodide redox (0.4 V vs. NHE), indicating that oxidized dye generated by the electron injection process can effectively accept electrons from electrolyte and regenerated. In addition, the calculated LUMO values were –1.28 V and –1.19 V, respectively, which were more negative than the conduction band of TiO2 (–0.5 V vs. NHE), indicating that the electron injection process from the photo-excited dye molecule to the
ACCEPTED MANUSCRIPT conduction band of TiO2 was energetically permitted. Figure 4 2.4. Photovoltaic properties of DSSCs
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Fig. 5 shows the photocurrent-voltage (J−V) curves and incident photo-current conversion efficiency (IPCE) action spectra of liquid electrolyte-based DSSCs obtained with FD and SD. The effects of co-adsorbent (CDCA) on the photovoltaic performances were also investigated. Both
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the dye-based devices exhibited strong response in the range of 350–600 nm with the highest
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values of 67% and 72% at 500 nm for FD and SD, respectively. As compared to that of the SD-based device, FD showed slightly higher response in the long wavelengths 578–800 nm but lower IPCE values at other spectral regions, which was in good agreement with absorption spectra as shown in Fig. 2.
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Figure 5 Table 2
As seen in Fig. 5b, in pure solvent CH2Cl2 system without CDCA, the FD-based device gave a
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short circuit photocurrent density (Jsc) of 16.10 mA cm-2, an open circuit voltage (Voc) of 646 mV,
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and a fill factor (FF) of 0.693, corresponding to a power conversion efficiency (PCE) of 7.20%, while the SD-based device gave Jsc of 14.98 mA cm-2, Voc of 732 mV, FF of 0.682, and PCE of 7.47%. It is clear that FD-based device obtained a higher photocurrent due to its wider absorption in the visible region, while SD-based showed a higher photovoltage, indicating that the N-alkyl substituted 11,12-dihydroindolo[2,3-a]carbazole ring could suppress more effectively dye aggregation. With the addition of the CDCA in the dye solution, the performances of the FD- and SD-based devices reduced significantly. The DSSCs based on the dyes FD and SD with 0.5 mM
ACCEPTED MANUSCRIPT CDCA exhibited poorer Jsc, Voc and FF values (12.09 mA cm-2, 624 mV, 0.645 for FD) and (9.76 mA cm-2, 644 mV, 0.683 for SD), respectively, leading to inferior PCEs of 4.86% and 4.42%. This result can be attributed to the co-adsorbed CDCA molecules occupying the binding sites on the
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TiO2 film, inducing a decrease in the amount of organic dyes loaded on the TiO2 films (Table 2). It is worth noting that as the CDCA content increased from 0 to 0.5 mM and further to 2 mM, the photovoltaic performances of FD and SD showed opposite trend. When the dye molecules were
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co-adsorbed with 2 mM CDCA, there was no large change in the photovoltage and the amount of
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dye adsorption, but there was a significant increase in photocurrent, from 12.09 mA cm-2 rose to 13.31 mA cm-2 for FD and from 9.76 mA cm-2 rose to 13.16 mA cm-2 for SD, respectively. One plausible explanation for this phenomenon is that a broader and higher IPCE response of these devices with 2 mM CDCA (Fig. 5a) suggests that the aggregation is indeed repressed, thus
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resulting in more efficient charge transport and collection, the final result is the increase of Jsc, however further experimentation need to be performed to confirm this.
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Figure 6 Table 3
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The characteristic J−V and IPCE curves of FD and SD sensitized devices based on quasi-solid-state electrolyte are shown in Fig. 6 and the photovoltaic parameters are listed in Table 3. Compared to liquid electrolyte, the trend of the IPCE curves is the same. QSS-DSSC based on FD gave a relatively low IPCE action spectrum, and the maximum IPCE value was only 55% at 430 nm, while QSS-DSSC based on SD showed a higher IPCE values in the broad range of 340−610 nm, the maximum IPCE value appeared at 520 nm and reached 69%. As listed in Table 3, under the same conditions (AM 1.5), the SD based QSS-DSSC exhibited Jsc of 16.15 mA cm-2, Voc
ACCEPTED MANUSCRIPT of 649 mV and FF of 0.636, while the PCE reached 6.67%. QSS-DSSC based on FD gave a Jsc value of 17.11 mA cm-2, a Voc of 606 mV and a FF of 0.557, corresponding to the PCE value of 5.77%. Similarly, the higher PCE of the QSS-DSSC based on SD than that of the FD-sensitized
chains on the same side causes better inhibition. 2.5. Electrochemical impedance spectroscopy analysis
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cell, probably due to its higher fill factor, and the fact that the donor group substituted by alkyl
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Electrochemical impedance spectroscopy (EIS) was employed to investigate the electron
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recombination processes in DSSCs based on the dyes under darkness. EIS analysis of the liquid electrolyte-based DSSCs and quasi-solid-state electrolyte-based DSSCs were performed over the frequency range of 0.1 Hz to 106 Hz with an applied bias set at –0.67 V and –0.65 V, respectively. The Nyquist and Bode plots are shown in Fig. 7 and 8. Typically, the Nyquist plots show three which
are
assigned
to
the charge transfer resistance at
the counter
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semicircles,
electrode/electrolyte, the charge transfer resistance at the TiO2/dye/electrolyte interface, and the Warburg diffusion process of I−/I3− in the electrolyte. The radius of the second semicircle in the
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middle-frequency region corresponds to the value of the recombination resistance (Rrec) at the
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photoanode/dye/electrolyte interface. The calculated Rrec values are listed in Table 4. A smaller Rrec value means a larger charge recombination rate [37]. Evidently, the radius of the semicircle in the region of medium frequency increased in the order of FD (11.98 Ω) ﹤ SD (18.53 Ω) for the liquid electrolyte and FD (19.74 Ω) ﹤ SD (21.67 Ω) for the quasi-solid-state electrolyte. These observations are in good accordance with the trends of the experimental Voc. Figure 7 Figure 8
ACCEPTED MANUSCRIPT Table 4 The electron lifetimes were calculated according to the Bode plots using τe = 1/2πf, in which f represents the peak frequency of the lower frequency region [38]. With the quasi-solid-state
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electrolyte, the electron lifetimes for DSSCs based on FD and SD were 3.23, 4.18 ms, respectively. Meanwhile, the electron lifetimes for DSSCs based on FD and SD with liquid electrolyte were higher, reaching 3.39, 6.06 ms, respectively. The longer electron lifetime for the SD-based cell
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indicates the more effective suppression of the back reaction of the injected electrons with the I3−
that
the
combination
of
two
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in the electrolyte. Based on the results from EIS measurements and electron lifetimes, we believe adjacent
long-chain
alkyl
chains
and
bulky cyclic
11,12-dihydroindolo[2,3-a]carbazole electron donor in the dye SD brings about a better suppression of recombination between the injected electrons and the electrolyte, resulting in a rise
3. Conclusions
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of Voc.
In summary, two novel linear polycyclic organic sensitizers have been designed and
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synthesized. The results show that the introduction of linear planar units greatly reduce the space
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distortion in molecular and enhance the intramolecular charge transfer in the D−π−A system, yielding high molar absorptivity and a relatively narrow band-gap. In the liquid electrolyte, the sensitizer SD with two substituted alkyl groups on the same side of the donor unit gave more excellent photovoltaic performance than that of the sensitizer FD. Encouragingly, in quasi-solid-state electrolyte, the power conversion efficiency of SD-based DSSC reached 6.7%, compared to 5.6% for FD-based DSSC. This is mainly due to the relatively large charge transfer resistance and longer lifetime for the injected electrons, resulting in a higher voltage for DSSCs
ACCEPTED MANUSCRIPT based on SD. In addition, the PCE of DSSCs based on the two dyes was significantly reduced in the presence of the co-adsorbent CDCA. The result shows that in the absence of a co-adsorbent, multiple branched alkyl chains in these dye molecules can effectively inhibit dye molecule
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aggregation and improve photovoltaic performance. This work provides an alternative direction for rational molecular strategy to achieve high power conversion efficiency, especially to increase electron transfer efficiency and Jsc while avoiding reducing Voc.
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4. Experimental section
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4.1. Materials and instruments
Tetrahydrofuran and toluene were distilled in an argon atmosphere over sodium/benzophenone. All other reagents were purchased from Adamas, J&K, TCI chemicals, Sigma-Aldrich and used as such without any treatment. Liquids were transferred by using syringe. Thin layer chromatography
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(Merck silica gel 60 F254) was applied for monitoring the intermediates and final products and purification was carried out by column chromatography using silica gel (300–400 mesh). 4.2. Measurement and characterizations
H and 13C NMR spectra of the all intermediate and final products were measured on a Bruker
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1
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Avance 600 MHz spectrometer by using CDCl3 or THF-d8 and tetramethylsilane as an internal standard. The mass spectra of the prepared compounds were carried out at Finnigan MAT 95 XP analyzer spectrometer (Thermo Electron Corporation). For melting point XT4B microscopic melting point apparatus was used and uncorrected. UV3600 Plus spectrophotometer (Shimadzu Corporation) and Hitachi F–4600 spectrophotometers were used for the measurement of absorption and fluorescence spectra. The photocurrent-voltage (J−V) characteristics of the DSSCs were measured by Keithley 2400 source meter under simulated AM 1.5 G (100 mW/cm2)
ACCEPTED MANUSCRIPT illumination with a solar light simulator (Pecell-L15, Japan). A 1 KW Xenon arc lamp (Oriel) served as a light source and its incident light intensity was calibrated with an BS-520 standard silicon solar cell. The incident monochromatic photon-to-current conversion efficiency (IPCE)
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spectra of the DSSCs were conducted by illuminating monochromatic light from 300 to 800 nm on the basis of a PEC-S20 action spectrum measurement system. The electrochemical impedance spectroscopy (EIS) was conducted on the CS16X electrochemical workstation (Wuhan, China) in
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dark condition for analyzing the internal impedance characteristics of DSSCs by fitting the
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impedance spectra using Z-view software. Dye load amount on the films was measured by desorbing the with 0.1 M NaOH in THF/H2O (1:1) and measured by UV−Vis spectrum.
4.3. Fabrication of DSSCs
The working electrode composed of mesoporous TiO2 layer (8 µm nanocrystalline transparent
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layer and 4 µm scattering layer) on FTO glass was prepared following the reported procedure [34]. The active area of the TiO2 film was controlled to be 0.4 × 0.4 cm-2 for photovoltaic performance
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measurements. The TiO2 films were immersed into a solution of the dyes (0.2 mM dye in CH2Cl2) for 16 h, namely sensitization. The liquid electrolyte used in this work was composed of 0.6 M
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1-methyl-3-propylimidazolium iodide (PMII), 0.1 M guanidinium thiocyanate, 0.07 M I2, 0.05 M LiI, and 0.5 M tert-butyl-pyridine. Sodium lauryl sulfate, ammonium persulfate, sodium hydrogencarbonate and deionized water were heated and stirred in a constant temperature water bath, and nitrogen was passed for 20 minutes. After the resultant mixture was copolymerized with methyl methacrylate and vinyl acetate for 1 h, added aluminum sulfate to break emulsion and then filtered, washed with water and alcohol, respectively, and vacuum dried to prepare the polymer. The quasi-solid-state gel electrolyte was prepared as follows: 12% (wt%) of poly(methyl
ACCEPTED MANUSCRIPT methacrylate-co-vinyl acetate) and the liquid electrolyte were mixed in 3-methoxypropionitrile, heated at 60−70 °C for 1 h, then cooled and repeated several times, until all solids dissolved and a clear gel appeared.
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Acknowledgements We are grateful to the National Natural Science Foundation of China (21462054 and 61368006), the Scientific Research Fund of Guizhou Provincial Science and Technology Department, China
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(LH[2015]7036, [2015]4003 and [2016]1419), the Natural Science Foundation of Guizhou
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Provincial Education Department, China ([2014]42), and the Science and Technology Cooperation talent Project of ZunYi City, China ([2016]14) for the financial support. References
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Figure Captions
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Fig. 1. Chemical structure of the dyes FD and SD.
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Fig. 2. Optimized structures and electron distributions in HOMO and LUMO levels of FD and SD.
Fig. 3. (a) UV−Vis spectra of FD and SD in CH2Cl2 solution and (b) Normalized absorption and
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Fig. 4. Cyclic voltammograms curves of FD and SD and ferrocene/ferrocenium internal reference.
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Fig. 5. (a) IPCE action spectra and (b) J− −V characteristics of DSSCs based on FD and SD with liquid
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electrolyte (L: liquid DSSCs).
Fig. 6. (a) IPCE action spectra and (b) J− −V characteristics of DSSCs based on FD and SD with quasi-solid-state electrolyte (QSS: quasi-solid-state DSSCs).
Fig. 7. (a) EIS Nyquist and (b) EIS Bode plots for liquid DSSCs based on the two sensitizers measured in the dark under −0.67 V bias (L: liquid DSSCs).
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measured in the dark under −0.65 V bias (QSS: quasi-solid-state DSSCs).
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Better planarity and conjugation of long rod-shaped molecules, resulting in higher electron transport efficiency and thus higher photoelectric
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conversion efficiency.
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Table 1 Absorption and Emission Data for organic dyes in CH2Cl2. λmaxa/nm
εa/M-1 cm-1
λemb/nm
λintc/nm
HOMOd/V (vs. NHE)
E0-0e/eV
FD SD
495 488
46200 47400
642 644
570 563
0.90 1.01
2.18 2.20
EHOMOg/eV
–1.28 –1.19
–4.82 –4.98
ELUMOg/eV –2.62 –2.62
Absorption maximum of the dyes measured in CH2Cl2 with a concentration of 2 × 10-5 M. ε: Molar extinction coefficient at λmax. b Emission maxima of dye in CH2Cl2. c λint (λintersection) obtained from the cross point of normalized absorption and emission spectra in CH2Cl2 solution. d HOMO levels were measured in CH3CN with 0.1 M TBAPF6 as supporting electrolyte, Fc+/Fc as an internal standard, scan rate 50 mV/s. e E0-0 = 1240/λint. f LUMO was calculated from HOMO – E0-0. g EHOMO and ELUMO by DFT calculation (B3LYP, 6-31 G (d) level).
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Table 2 Photovoltaic Parameters for DSSCs of Organic Dyes. Voca(Vocb)/mV
Jsca(Jscb)/mA·cm-2
FFa(FFb)/%
PCEa(PCEb)/%
FD SD FD + 0.5 mM CDCA SD + 0.5 mM CDCA FD + 2 mM CDCA SD + 2 mM CDCA N719
646 (648 ± 5) 732 (734 ± 9) 624 (624 ± 2) 664 (666 ± 8) 629 (630 ± 3) 667 (669 ± 4) 705 (705 ± 6)
16.10 (16.05 ± 0.19) 14.98 (15.16 ± 0.18) 12.09 (11.96 ± 0.13) 9.76 (9.81 ± 0.10) 13.31 (13.35 ± 0.17) 13.16 (12.96 ± 0.22) 17.96 (17.97 ± 0.15)
69.30 (69.02 ± 0.39) 68.17 (66.73 ± 1.44) 64.47 (64.61 ± 0.59) 68.26 (67.52 ± 0.74) 67.41 (66.54 ± 0.87) 62.06 (62.00 ± 0.35) 67.00 (66.33 ± 0.67)
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7.20 (7.18 ± 0.02) 7.47 (7.42 ± 0.05) 4.86 (4.83 ± 0.05) 4.42 (4.41 ± 0.02) 5.65 (5.59 ± 0.06) 5.45 (5.38 ± 0.07) 8.50 (8.41 ± 0.09)
Dye loading (mol cm-2) 2.39 × 10-7 2.13 × 10-7 2.05 × 10-7 1.75 × 10-7 2.06 × 10-7 1.57 × 10-7
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TiO2 thickness 8 + 4 µm and 0.16 cm2, [dye] = 0.2 mM, dipping solvent CH2Cl2, dipping time 16 h, under AM 1.5 illumination (100 mW/cm2). a The best device parameters. b The average device parameters (obtained from five devices).
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Table 3 Photovoltaic Parameters for DSSCs of Organic Dyes. electrolyte
Voca(Vocb)/mV
Jsca(Jscb)/mA·cm-2
FD SD
quasi-solid quasi-solid
606 (607 ± 1) 649 (650 ± 4)
17.11 (16.92 ± 0.19) 16.15 (16.14 ± 0.24)
FFa(FFb)/%
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55.69 (55.99 ± 0.3) 63.57 (63.34 ± 1.3)
PCEa(PCEb)/% 5.77 (5.75 ± 0.03) 6.67 (6.64 ± 0.04)
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TiO2 thickness 8 + 4 µm and 0.16 cm2, [dye] = 0.2 mM, dipping solvent CH2Cl2, dipping time 16 h, under AM 1.5 illumination (100 mW/cm2). a The best device parameters. b The average device parameters (obtained from five devices).
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Table 4 Electrochemical parameters of the DSSCs with various dyes. electrolyte
Rrec (ohm)
fmax (Hz)
FD SD FD SD
liquid liquid quasi-solid quasi-solid
11.98 18.53 19.74 21.67
46.96 26.27 49.25 38.08
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τe (ms) 3.39 6.06 3.23 4.18
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1、 Two novel rod-shaped dyes were successfully synthesized and utilized for dye-sensitized solar cells. 2、 The di(1-benzothieno)[3,2-b:2',3'-d]pyrrole structure was first introduced as a π-bridge unit for sensitizers.
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3、 The effects of two isomers 5,11-dihydroindole[3,2-b]carbazole and 11,12-dihydroindole[2,3-a] carbazole as donor units on photovoltaic properties were investigated.
4、 In liquid electrolyte, the power conversion efficiencies of DSSCs based on FD and SD reached
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7.2% and 7.47%, respectively.