Letter pubs.acs.org/OrgLett
Asymmetric 8H-Thieno[2′,3′:4,5]thieno[3,2‑b]thieno[2,3‑d]pyrroleBased Sensitizers: Synthesis and Application in Dye-Sensitized Solar Cells Zhihui Wang,†,‡ Mao Liang,*,‡ Huanhuan Dong,‡ Ping Gao,† Ya Su,† Peng Cai,† Shijie Ding,† Jing Chen,*,† and Song Xue‡ †
Jiangsu Provincial Key Laboratory of Palygorskite Science and Applied Technology, College of Chemical Engineering, Huaiyin Institute of Technology, Jiangsu Province, Huaian 223003, P. R. China ‡ Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, Department of Applied Chemistry, Tianjin University of Technology, Tianjin 300384, P.R.China S Supporting Information *
ABSTRACT: Asymmetric 8H-thieno[2′,3′:4,5]thieno[3,2-b]thieno[2,3-d]pyrrole (TTP) unit has been synthesized and used as a rigid and coplanar π-bridge to construct four new metal-free sensitizers W05−08. These dyes sensitized solar cells exhibit high photocurrent due to strong light harvesting capability. Among these dyes, the W05based cell in conjunction with cobalt(II/III) redox shuttle exhibits the best power conversion efficiency up to 9.22%.
In this contribution, we first conceive a new asymmetric 8Hthieno[2′,3′:4,5]thieno[3,2-b]thieno[2,3-d]pyrrole (TTP) unit, and employ it as the π-conjugated bridge to construct four novel organic dyes (W05−08, Figure 1). To the best of our
D
evelopment achieved by dye-sensitized solar cells (DSSCs) has been highly dependent on the photosensitizer, which plays a decisive role in sunlight capturing, charge generation/transfer, and photovoltaic performance. Tremendous research attempts have been devoted to designing and synthesizing new sensitizers, i.e., ruthenium polypyridyl complexes,1 zinc porphyrin complexes,2 and organic sensitizers.3 Recently, Kakiage et al. demonstrated a marvelous power conversion efficiency (PCE) of 14.5% based on the silylanchor organic dye ADEKA-1 in conjunction with a cobalt(II/ III) redox shuttle, highlighting the great potential of organic sensitizers in DSSCs application.4 To optimize the HOMO/LUMO energy levels as well as the photophysical properties of organic dyes, tuning π-bridges is an effective strategy. Considerable efforts have been devoted to fused-ring thiophenes (FTPs), such as dithieno[3,2-b:2′,3′d]silole,5 cyclopentadithiophene,6 dithieno[3,2-b:2′,3′-d]pyrrole,7 tetrathienothiophene,8 dithieno[2,3-d:2′,3′-d′]thieno[3,2-b:3′,2′-b′]dipyrrole (DTDP),9 N-annulated thienocyclopentaperylene,10 and dithienopicenocarbazole.11 Nevertheless, most of these works focus on symmetric FTPs, FTPs with an asymmetric structure are rarely exploited, mainly owing to their great challenge of synthesis.12 Moreover, in our recent work, we proposed that a twisted structure is indispensable for reducing the charge recombination in FTPs based organic dye-sensitized solar cells.13 Studies revealed that at least 1% PCE improvement can be achieved by introducing the hexyl group in the center of organic dyes without bulky donors. This is important for designing high efficient organic dyes. So, we believe the time and hard work involved in synthesis of fused-ring thiophenes with side chains are worthwhile. © 2017 American Chemical Society
Figure 1. Chemical structures of W05−08.
knowledge, no asymmetric FTPs (with three or above three rings) containing side chain have been reported in the DSSCs field.3c,d In order to improve the solubility, reduce the intermolecular aggregation, and suppress interfacial charge recombination, hexyl side chain and 4-hexyloxylphenyl were incorporated at the C3 and the N-position of the TTP core, respectively. To better understand the effect of conjugating orientation of asymmetric TTP on the performance of the dyes, W05 and W07 were synthesized. To improve the light harvest capacity of dyes and form a twisted structure, W06 and W08 Received: May 15, 2017 Published: July 6, 2017 3711
DOI: 10.1021/acs.orglett.7b01465 Org. Lett. 2017, 19, 3711−3714
Letter
Organic Letters were designed with 3-hexylthiophene ring inserted in the πconjugated spacer. The synthetic approaches to W05−08 are depicted in Scheme 1. Compound 4 was synthesized from ester 1 via three
acid) interaction. Among these sensitizers, W05 with hexyl side chain facing the triphenylamine (TPA) donor displays the shortest absorption spectra at 484 nm, with a maximum molar absorption coefficient (ε) of 68.3 × 103 M−1 cm−1. By contrast, a red-shift maximum absorption wavelength (λmax) along with an enhanced ε value of 71.4 × 103 M−1 cm−1 at 508 nm was presented for W07, primarily attributing to the smaller dihedral angle (25.6° vs 48.7°) between the TPA donor with the TTP segment (Figure S3). On the other hand, incorporation of 3hexylthiophene unit in the molecular skeleton of W05 brings forth a bathochromic shift of W06 (λmax = 511 nm) by 27 nm. This is due to the extended π-conjugated system, which effectively enhanced the extent of electron delocalization over the whole molecule. Nevertheless, only a very small red-shifted absorption peak (2 nm) was observed for W08 (λmax = 510 nm) with respect to W07, mainly due to its twist structure with larger dihedral angle (42.6° vs 25.6°, Figure S3), which resulted in the inefficient conjugated degree. Upon absorption on the surface of the TiO2 film (Figure 2b), W05−08 exhibits a hypsochromic shift of 22−37 nm in comparison with these in solutions, which can be ascribed to the deprotonation of carboxylic acid. Noted that, the absorption of W05 anchored on the TiO2 film exhibits the strongest absorption peak with respect to its congeners W06−08, mainly attributing to the larger dye-loading amount (Γ, Table S1), which is partially responsible for its higher photocurrent (vide infra). Cyclic voltammograms (CV) of dye-sensitized electrodes were further carried out to investigate the electrochemical properties of TTP-based dyes (Figure S2 and Table S1). The HOMO levels of W05−08 were determined to be 0.96, 1.05, 0.87, and 0.92 V versus normal hydrogen electrode (NHE), respectively. As presented in Figure 3, the HOMO levels of
Scheme 1. Synthetic Routes to TTP-Based Organic Sensitizers W05−08
steps, i.e. bromination, hydrolysis, and decarboxylation. This compound reacted with (3-bromothiophen-2-yl)zinc(II) bromide to afford the key intermediate 5, which further yielded the TTP 6 by Buchwald-Hartwig C−N coupling. With TTP 6 in hand, Vilsmeier−Haack reaction and bromination afforded isomeric compound 8a and 8b. 4-Hexylthiophene-2-carbaldehyde is attached to TTP by Still-coupling reaction. Bromination of 7c furnishes 8c, which in turn is readily converted to 10c. Compound 8d was prepared by a similar procedure. Finally, the target dyes W05−08 were converted in excellent yield via knoevenagel condensation of the aldehydes 10a−10d with cyanoacrylic acid. Experimental details and characteristic data for all compounds are provided in the Supporting Information (SI). UV−vis absorption spectra of W05−08 were recorded in dichloromethane solutions (Figure 2a, data are collected in Table S1). All the dyes exhibit an intense absorption band in the regions of 400−600 nm, which can be ascribed to the intramolecular donor (triarylamine) to acceptor (cyanoacrylic
Figure 3. Energy-level diagram of the four sensitizers.
TTP-series dyes can be fine-tuned through modifying the orientation of the asymmetric π-conjugated spacer. By comparison with W05 and W06, the HOMO of W07 and W08 are negative shifted by 0.09 and 0.13 V, respectively, indicative of an attenuated driving force for the regeneration of the oxidized dye. Therefore, we proposed that alkyl side chain of bridged FTPs adjoin to the donor part rather than being close to the acceptor unit is favorable to descend the HOMO levels and thus achieve efficient dye-regeneration. On the other hand, the LUMO levels of W05−08 are calculated to be −1.29, −1.06, −1.28, and −1.23 V, respectively, which are more negative than that of TiO2 (−0.5 V vs NHE), ensuring sufficient energetic barriers for the excited electron injection to the conduction band. Apparently, molecular geometrical configuration has an important impact on photophysical and electrochemical properties of dyes. Further insight into the
Figure 2. Absorption spectra of W05−08 in CH2Cl2 solutions (a) and anchored on TiO2 film (b). 3712
DOI: 10.1021/acs.orglett.7b01465 Org. Lett. 2017, 19, 3711−3714
Letter
Organic Letters
alteration of TTP bridge orientation, partially benefiting from its augmented dye-load amount. Furthermore, for Co-phen (EF,redox = 0.62 V) cells, the driving force for regeneration of W05 is 0.34 V, much higher than that of W07 (0.25 V), thus making it favorable for fast regeneration, which contributes to the higher Jsc value of W05. However, W06/W08 featuring additional 3-hexylthiophene ring as π-spacer show inferior IPCE performance with respect W05/W07, despite the broader IPCE action spectra in the region of 650−750 nm, mainly arising from the severe charge recombination (vide infra). To scrutinize the origins of the trends in Voc, we further carried out charge extraction and controlled intensity modulated photovoltage spectroscopy (IMVS) measurements. As is known to all, Voc is intimately determined by both of the CB position and the charge recombination rate.14 As depicted in Figure S4a, the cells made with W05−08 exhibit a similar potential bias at a fixed extracted electron density (dn), indicative of the same CB edge of titania in contrast to the electrolyte Fermi-level. Therefore, we ascribe the primary reason for the Voc difference to the charge recombination in the interface between TiO2/dye/electrolyte. Figure S4b displays the electron lifetime (τ) against the charge density for the W05−08 cells. The electron lifetime increases in the order W08 < W07 < W06 < W05, which is well in accordance with their Voc performances. Note that, W05 exhibits distinctly longer lifetime than that of W07, which is attributed to the twist structure as well as the larger dye loading amount15 (Table S1 and Figure S3), while the adverse impact of a reduced Γ on Voc is over compensated by effective suppression of interfacial charge recombination as the dye transformation from W08 to W06. Moreover, electrochemical impedance spectra (EIS) measurements (Figure S5) were also carried out on the DSSCs in the dark under forward bias (−0.65 V). The interfacial charge transfer resistance (Rct) decreases in the order W05 (2018 Ω) > W06 (1482 Ω) > W07 (388 Ω) > W08 (318 Ω), which is in good agreement with the above-mentioned results. The J−V characteristics of all-solid-state DSSCs (ssDSSCs) based on W05−08 using Spiro-OMeTAD as the holetransporting materials have been carried out under the same light irradiation (Figure S6 and Table S2). The ssDSSC fabricated with W05 shows the best PCE of 3.27% (Jsc of 9.01 mA cm−2, a Voc of 780 mV, and a FF of 0.47) among the four dyes. The ssDSSCs sensitized by W05/W06 featuring an aryl chain adjoin to donor display higher photovoltaic performance in comparison with these of W07/W08, which is well in accord with the trend of W05−08 based liquid-state DSSCs in conjunction with Co-electrolyte. So we proposed that it is better to place the alkyl group close to donor for asymmetric fused-ring thiophenes based organic dyes. In conclusion, four new organic sensitizers W05−08 featuring asymmetric 8H-thieno[2′,3′:4,5]thieno[3,2-b]thieno[2,3-d] pyrrole (TTP) segment as the central linkage have been successfully designed and synthesized. All sensitizers exhibit high molar extinction coefficients and broad photospectra response, contributing to good solar cells performance. When the introduction of hexyl side chain in the TTP unit facing the TPA donor, a twisted structure with larger dihedral angle can be achieved, which is not only beneficial to improve the driving force for dye regeneration, but also enhance the suppression of interfacial charge recombination. As a result, the cell made with the resulted W05 displays the highest PCE of 9.22%, indicating that TTP-bridged sensitizers could be a promising candidate in
optimized geometrical configuration and electron distribution of TTP-based dyes, density functional theory (DFT) calculations were conducted at B3LYP/6-31G level, as shown in Figure S3. The photovoltaic characteristics of DSSCs based on W05− 08 in conjunction with Co-phen electrolyte were evaluated under AM 1.5G irradiation at 100 mW cm−2 (see details in the SI), and the performance parameters were summarized in Table 1. As Figure 4 presents, the DSSC based on W05 exhibits the Table 1. Photovoltaic Parameters for DSSCs Employing the Cobalt Electrolyte under the Optimized Conditions dye a
W05 W06a W07a W08a
Jsc/ mA cm−2
Voc/mV
FF
PCE/%
15.2 14.8 14.3 14.5
885 852 821 818
0.68 0.66 0.70 0.69
9.22 8.37 8.22 8.18
a [Co(phen)3]2+/3+ electrolye; and the detailed components of cobalt electrolyte are provided in the Supporting Information.
Figure 4. J−V plots and IPCE action spectra (the inset curves) for DSSCs based on W05−08.
best power conversion efficiency (PCE) of 9.22% among these dyes, with a short-circuit photocurrent density (Jsc) of 15.2 mA cm−2, an open-circuit photovoltage (Voc) of 885 mV, and a fill factor (FF) of 0.68, respectively. When the orientation of the conjugated bridge TTP is turned over, the Jsc and Voc of the resulting W07 (Jsc: 14.3 mA cm−2; Voc: 821 mV) are both distinctly depleted, leading to a lower PCE of 8.22%. Meanwhile, the DSSC based on W06 (incorporating additional 3-hexylthiophene ring into π-spacer; Jsc: 14.8 mA cm−2, Voc: 852 mV, FF: 0.66) shows an attenuated efficiency of 8.37% as compared to that of W05, which is ascribed to severe interfacial charge recombination (see Figures S4b and S5). Similar trend can also be observed between W07 and W08 based cells. The difference between the observed Jsc values for W05−08 cells can be evaluated by estimating the monochromic incident photon-to-current conversion efficiency (IPCE) measurements, as shown in Figure 4 (the inset curves). All these dyes exhibit a stronger and broader response in the entire visible region with a high plateau of >80% indicating that all TTP-based dyes can efficiently covert the light to photocurrent. It is valuable to note that W05 exhibits a higher IPCE plateau with a maximum of 89.8% at 510 nm relative to W07 (84.2% at 520 nm) as the 3713
DOI: 10.1021/acs.orglett.7b01465 Org. Lett. 2017, 19, 3711−3714
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Organic Letters
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highly efficient DSSCs. Further studies for new asymmetric fused-ring thiophenes through molecular modifications are underway in our laboratory.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b01465. Experimental details, characterization date for all new compounds, details of DSSCs fabrication, cyclic voltammogram, emission spectra, molecular orbitals, and EIS plots (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] *E-mail:
[email protected] ORCID
Mao Liang: 0000-0003-0808-3851 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (No. 21602074, 21174046, 21373007, and 21376179) and Natural Science Foundation of Jiangsu Province (BK20150416).
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REFERENCES
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