Strategy for Improved Photoconversion Efficiency ... - ACS Publications

Article pubs.acs.org/JPCC

Strategy for Improved Photoconversion Efficiency in Thin Photoelectrode Films by Controlling π‑Spacer Dihedral Angle Deok-Ho Roh,†,§ Kwang Min Kim,†,§ Jung Seung Nam,† Un-Young Kim,† Byung-Man Kim,‡ Jeong Soo Kim,† and Tae-Hyuk Kwon*,†,‡ †

Department of Chemistry, School of Natural Science, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea ‡ School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea S Supporting Information *

ABSTRACT: Benzo[c][1,2,5]thiadiazole (BT) has been used in dye-sensitized solar cells (DSCs) for its light-harvesting abilities. However, as a strongly electron deficient unit, BT causes rapid back electron transfer (BET), which in turn lowers the photoconversion efficiency (PCE) of devices. Herein, we report a powerful strategy for retarding BET by controlling both the photoelectrode thickness and π-spacer dihedral angle. To achieve this, we introduced planar (BT-T) or twisted πspacers (BT-P, BT-MP, and BT-HT) between BT units and anchoring groups and used different photoelectrode thicknesses between 1.8 and 10 μm. Computational and experimental results show that twisted π-spacers were more efficient at retarding BET than the planar π-spacer. However, BET was found to be less important than expected, and light harvesting efficiency (LHE) played a critical role as the thickness of the photoelectrode decreased because charge collection efficiency was enhanced. The planar dye BT-T obtained the highest LHE, this value remained unusually high even in 1.8 μm photoelectrodes. As a result, BT-T gave a PCE of 6.5% (Jsc = 13.56 mA/ cm2, Voc = 0.67 V, and FF = 0.72) in thin 1.8 μm photoelectrodes with 3.5 μm scattering layers, which represented a roughly 40% enhancement compared to the PCE in 10 μm photoelectrodes (4.76%). Overall, these results provide a novel approach to achieving ultrathin and highly efficient flexible DSCs.

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INTRODUCTION Dye-sensitized solar cells (DSCs)1,2 have been receiving less attention than previously because of the greater interest in lead halide perovskite solar cells,3,4 which have attracted a great deal of attention due to their ease of processing and high efficiencies. However, DSCs are still fascinating photovoltaic systems because of their relatively environmentally friendly properties and device stability, as well as their transparency, variety of available colors, and progress toward flexible devices and building-integrated photovoltaics (BIPVs). Metal-free organic dyes have been focused on as alternatives to Ru complexes,5−7 owing to the ease of tailoring their chemical structures and energy levels via a process based on altering the donor−π−conjugation−acceptor (D−π−A) configuration.8−10 To enhance the absorption range, the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the sensitizer should be reduced by efficient intramolecular charge transfer (ICT). For this purpose, low band gap sensitizers have been reported that include the strongly electron deficient benzo[c][1,2,5]thiadiazole (BT) unit in the bridging framework of the D−π−A architecture.11−16 Although the BT unit extends the absorption © XXXX American Chemical Society

spectra of dyes, it also causes rapid back electron transfer (BET) of electrons injected into the conduction band of titanium dioxide (TiO2), causing them to return to the oxidized dyes, which in turn causes low photoconversion efficiency (PCE). In order to retard the BET and thereby increase the PCE, previous research has inserted π-spacers between BT units and anchoring groups.17,18 For example, a recently reported zinc porphyrin-based dye, SM315, recorded a PCE of 13% when π-spacers were introduced.19 However, the BT unit still caused fast BET, resulting in a low open circuit voltage (Voc), even though inserted π-spacer retards BET. However, it has been demonstrated that introducing twisted π-spacers that increase the dihedral angle between the BT units and the πspacers can reduced BET further.20 Although twisted π-spacers retard BET, these dyes show blue-shifted absorption spectra due to their poor ICT and low absorption abilities compared to planar dye systems. This means that these dyes require a thick TiO2 film (>10 μm) to improve their light harvesting efficiency Received: August 16, 2016 Revised: October 9, 2016 Published: October 11, 2016 A

DOI: 10.1021/acs.jpcc.6b08262 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. (a) Chemical structures of BT-series showing dihedral angles between BT and π-spacers. Schematic figures of (b) back electron transfer with different π-spacers and (c) enhancement of charge collection efficiency in a thin 1.8 μm photoelectrode compared with a thicker 10 μm photoelectrode.

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EXPERIMENTAL SECTION Materials Preparation. All chemicals, solvents, and reagents were purchased from commercial suppliers (SigmaAldrich, Alfa Aesar, Matrix Scientific) and used without further purification. All anhydrous solvents were prepared by molecular sieves prior to use. 4-(7-Bromobenzo[c][1,2,5]thiadiazol-4-yl)-N,N-diphenylaniline (2). To a 250 mL two-necked round-bottom flask were added compound 1 (590 mg, 2.0 mmol), N,N-diphenyl-4(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)aniline (500 mg, 1.3 mmol), and Pd(PPh3)4 (46 mg, 0.04 mmol) under N2 atmosphere, 30 mL of degassed THF, and 4.0 mL of degassed 2 M K2CO3 (aq), and the resulting solution was stirred and heated at reflux (55 °C) for overnight. When the reaction was completed, water was added to quench the reaction. The product was extracted with dichloromethane. The organic layer was collected, dried over anhydrous MgSO4, and evaporated under reduced pressure. The remaining crude product was purified by column chromatography using hexane/CH2Cl2 mixture (3:1) as eluent to give compound 2 (380 mg, 62%) as orange solid. 1H NMR (400 MHz, CDCl3) δ (ppm): 7.90 (d, J = 7.6 Hz, 1H), 7.78−7.82 (m, 2H), 7.54 (d, J = 7.6 Hz, 1H), 7.27−7.32 (m, 4H), 7.17−7.20 (m, 6H), 7.08 (t, J = 7.6 Hz, 2H). 4-(7-(4-(Diphenylamino)phenyl)-benzo[c][1,2,5]thiadiazol-4-yl)benzaldehyde (3). To a 250 mL two-necked round-bottom flask were added compound 2 (100 mg, 0.22 mmol), 4-formylphenylboronic acid (39 mg, 0.26 mmol), Pd2(dba)3 (6.7 mg, 0.06 mmol), and [(t-Bu)3PH]BF4 (3.8 mg, 0.013 mmol) under N2 atmosphere, 10 mL of degassed THF, and 0.5 mL of degassed 2 M K3PO4 (aq), and the resulting solution was stirred and heated at reflux (60 °C) for overnight.

(LHE). However, this approach is some way from being an economic process and is unsuitable for flexible devices that require high performances and thin photoelectrodes.21,22 In addition, it has previously been reported that the PCE values of porphyrin-based dyes in thin photoelectrode devices (2.4 μm, 5.6%) were still low compared to those of thick photoelectrode devices (11.5 μm, 8.8%).23 Therefore, it is important to develop new sensitizers and methods for overcoming BET in thin photoelectrode DSCs to obtain high PCE values for practical applications. To overcome these issues, we designed and synthesized the following simple D−π−A dyes: BT-0 without a π-spacer, BT-P with a phenyl spacer, BT-MP with a 3-methylphenyl spacer, BT-T with a thiophene spacer, and BT-HT with a 4hexylthiophene spacer. The different π-spacers had different dihedral angles between the BT unit and the π-spacer (Figure 1a). Furthermore, we investigated the effect of photoelectrode thickness on BET in devices with photoelectrodes between 1.8 and 10 μm with a 3.5 μm scattering layer. This systematic investigation indicated that BET in sensitizers was gradually reduced as the dihedral angle between the π-spacer and the BT unit increased (Figure 1b), while their light harvesting abilities were reduced. However, the effect of BET dramatically reduced in thin photoelectrode devices (1.8 μm) compared with thick photoelectrode devices (10 μm) because of the enhanced charge collection efficiency of the sensitizers in thin photoelectrodes, as shown in Figure 1c. As a result, the best photovoltaic performance (PCE = 6.5%, short circuit current density (Jsc) = 13.56 mA/cm2, Voc = 0.67 V, and fill factor (FF) = 0.72) was given by a dye with a planar π-spacer, BT-T, using a thin 1.8 μm photoelectrode. B


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(aq). The reaction mixture heated at reflux (90 °C) for overnight. When the reaction was completed, water was added to quench the reaction. The product was extracted with dichloromethane. The organic layer was collected, dried over anhydrous MgSO4, and evaporated under reduced pressure. The remaining crude product was purified by column chromatography using hexane/CH2Cl2 mixture (5:1) as eluent to give compound 6 (120 mg, 46%) as yellow powder. 1H NMR (400 MHz, CDCl3) δ (ppm): 7.95 (d, J = 1.2 Hz, 1H), 7.83 (d, J = 7.6 Hz, 1H), 7.67 (d, J = 7.6 Hz, 1H), 7.06 (q, J1 = 2.0 Hz, J2 = 0.8 Hz, 1H), 2.67−2.71 (m, 2H), 1.65−1.73 (m, 2H), 1.30−1.41 (m, 6H), 0.88−0.91 (m, 3H). 13C NMR (100 MHz, CDCl3) δ (ppm): 153.78, 151.79, 144.50, 138.06, 132.23, 129.59, 127.36, 125.56, 122.04, 112.03, 31.67, 30.44, 29.00, 22.61, 14.09. 5-(7-Bromobenzo[c][1,2,5]thiadiazol-4-yl)-4-hexylthiophene-2-carbaldehyde (7). To a 250 mL two-necked roundbottom flask were added compound 6 (50 mg, 0.13 mmol) under N2 atmosphere and 5 mL of 1,2-dichloroethane, and then POCl3 (24 μL, 0.26 mmol) and 1-formylpiperidine was added at ice bath. The reaction mixture heated at reflux (90 °C) for overnight. When the reaction was completed, water and 1 M NaOAc were added slowly to deal with excess POCl3. The product was extracted with dichloromethane. The organic layer was collected, dried over anhydrous MgSO4, and evaporated under reduced pressure. The remaining crude product was purified by column chromatography using hexane/CH2Cl2 mixture (1:2) as eluent to give compound 7 (48 mg, 90%) as yellow powder. 1H NMR (400 MHz, CDCl3) δ (ppm): 10.10 (s, 1H), 8.03 (s, 1H), 7.90 (d, J = 8.0 Hz, 1H), 7.82 (d, J = 7.6 Hz, 1H), 3.01−3.05 (m, 2H), 1.72−1.80 (m, 2H), 1.31−1.43 (m, 6H), 0.88−0.91 (m, 3H). 13C NMR (100 MHz, CDCl3) δ (ppm): 206.94, 182.19, 153.37, 146.39, 137.93, 132.10, 131.13, 127.26, 125.82, 31.55, 31.45, 30.91, 29.0, 28.65, 22.54, 14.04. 5-(7-(4-(Diphenylamino)phenyl)benzo[c][1,2,5]thiadiazol4-yl)-4-hexylthiophene-2-carbaldehyde (8). To a 250 mL two-necked round-bottom flask were added compound 7 (48 mg, 0.12 mmol), N,N-diphenyl-4-(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)aniline (52 mg, 0.14 mmol), Pd2(dba)3 (2.4 mg, 0.0024 mmol), and [(t-Bu)3PH]BF4 (1.4 mg, 0.005 mmol) under N2 atmosphere, 5 mL of degassed THF, and 0.2 mL of degassed 2 M K3PO4 (aq), and the resulting solution was stirred and heated at reflux (60 °C) for overnight. When the reaction was completed, water was added to quench the reaction. The product was extracted with dichloromethane. The organic layer was collected, dried over anhydrous MgSO4, and evaporated under reduced pressure. The remaining crude product was purified by column chromatography using hexane/ CH2Cl2 mixture (1:2) as eluent to give compound 8 (43 mg, 65%) as red solid. 1H NMR (400 MHz, CDCl3) δ (ppm): 10.10 (s, 1H), 8.07 (s, 1H), 8.03 (d, J = 7.6 Hz, 1H), 7.89 (d, J = 8.8 Hz, 2H), 7.32 (d, J = 7.6 Hz, 1H), 7.28−7.32 (m, 4H), 7.18−7.21 (m, 6H), 7.06−7.10 (m, 2H), 3.02−3.06 (m, 2H), 1.74−1.81 (m, 2H), 1.33−1.44 (m, 6H), 0.88−0.92 (m, 3H). 13 C NMR (100 MHz, CDCl3) δ (ppm): 182.14, 153.85, 153.46, 148.51, 148.46, 147.76, 147.26, 137.28, 130.46, 134.43, 130.01, 129.39, 127.53, 126.75, 125.09, 124.28, 123.56, 122.47, 31.58, 31.47, 29.03, 28.70, 22.56, 14.05. LC/Q-TOF MS: Found m/z 596.1795, calc. for C35H31N3NaOS2 596.1801. 2-Cyano-3-(7-(4-(diphenylamino)phenyl)benzo[c][1,2,5]thiadiazol-4-yl)acrylic acid (BT-0). To a 250 mL two-necked round-bottom flask were added compound 9 (50 mg, 0.12 mmol) and cyanoacetic acid (313 mg, 3.68 mmol) under N2

When the reaction was completed, water was added to quench the reaction. The product was extracted with dichloromethane. The organic layer was collected, dried over anhydrous MgSO4, and evaporated under reduced pressure. The remaining crude product was purified by column chromatography using hexane/ CH2Cl2 mixture (2:3) as eluent to give compound 3 (63 mg, 60%) as red solid. 1H NMR (400 MHz, CDCl3) δ (ppm): 10.12 (s, 1H), 8.17 (d, J = 8.0 Hz, 2H), 8.06 (d, J = 8.0 Hz, 2H), 7.90 (d, J = 8.8 Hz, 2H), 7.86 (d, J = 7.2 Hz, 1H), 7.79 (d, J = 7.6 Hz, 1H), 7.29−7.33 (m, 4H), 7.19−7.23 (m, 6H), 7.07−7.10 (m, 2H). 4-(7-(4-(Diphenylamino)phenyl)-benzo[c][1,2,5]thiadiazol-4-yl)-3-methylbenzaldehyde (4). To a 250 mL two-necked round-bottom flask were added compound 2 (100 mg, 0.22 mmol), 4-formyl-2-methylphenylboronic acid (43 mg, 0.26 mmol), Pd2(dba)3 (11 mg, 0.01 mmol), and [(tBu)3PH]BF4 (6 mg, 0.02 mmol) under N2 atmosphere, 10 mL of degassed THF, and 2.0 mL of degassed 2 M K3PO4 (aq), and the resulting solution was stirred and heated at reflux (60 °C) for overnight. When the reaction was completed, water was added to quench the reaction. The product was extracted with dichloromethane. The organic layer was collected, dried over anhydrous MgSO4, and evaporated under reduced pressure. The remaining crude product was purified by column chromatography using hexane/CH2Cl2 mixture (2:3) as eluent to give compound 4 (70 mg, 65%) as red solid. 1H NMR (400 MHz, CDCl3) δ (ppm): 10.1 (s, 1H), 7.91 (m, 1H), 7.89 (m, 2H), 7.85 (d, J = 8.0 Hz, 1H), 7.77 (d, J = 7.2 Hz, 1H), 7.59 (d, J = 7.2 Hz, 1H), 7.56 (s, 1H), 7.28−7.33 (m, 4H), 7.19−7.25 (m, 6H), 7.06−7.11 (m, 2H), 2.29 (s, 3H). 13C NMR (100 MHz, CDCl3) δ (ppm): 192.16, 154.18, 153.43, 148.30, 147.37, 143.78, 137.89, 136.14, 133.76, 132.09, 131.58, 131.12, 130.38, 129.98, 129.61, 129.37, 127.19, 126.70, 125.00, 123.44, 122.68, 20.50. LC/Q-TOF MS: Found m/z 520.1461, calc. for C32H23N3NaOS 520.1454. 5-(7-(4-Diphenylamino-phenyl)-benzo[c][1,2,5]thiadiazol4-yl)-thiophene-2-carbaldehyde (5). To a 250 mL two-necked round-bottom flask were added compound 2 (154 mg, 0.34 mmol), 5-formyl-2-thienylboronic acid (63 mg, 0.40 mmol), Pd2(dba)3 (7 mg, 0.007 mmol), and [(t-Bu)3PH]BF4 (4 mg, 0.01 mmol) under N2 atmosphere, 5 mL of degassed THF, and 0.7 mL of degassed 2 M K3PO4 (aq), and the resulting solution was stirred and heated at reflux (60 °C) for overnight. When the reaction was completed, water was added to quench the reaction. The product was extracted with dichloromethane. The organic layer was collected, dried over anhydrous MgSO4, and evaporated under reduced pressure. The remaining crude product was purified by column chromatography using hexane/ CH2Cl2 mixture (2:3) as eluent to give compound 5 (112 mg, 68%) as red solid. 1H NMR (400 MHz, CDCl3) δ (ppm): 9.98 (s, 1H), 8.23 (d, J = 4.0 Hz, 1H), 8.06 (d, J = 7.6 Hz, 1H), 7.90 (d, J = 8.4 Hz, 2H), 7.86 (d, J = 4.0 Hz, 1H), 7.25 (d, J = 7.6 Hz, 1H), 7.29−7.33 (m, 4H), 7.19−7.22 (m, 6H), 7.07−7.11 (m, 2H). 13C NMR (100 MHz, CDCl3) δ (ppm): 182.98, 153.82, 152.67, 148.94, 148.57, 147.25, 143.26, 136.79, 134.60, 130.02, 129.92, 129.40, 127.80, 127.62, 126.72, 125.11, 124.17, 123.59, 122.42. 4-Bromo-7-(3-hexylthiophen-2-yl)-benzo[c][1,2,5]thiadiazole (6). To a 25 mL two-necked round-bottom flask were added 3-hexylthiophene-2-boronic acid pinacol ester (200 mg, 0.68 mmol), compound 1 (300 mg, 1.0 mmol), Pd(PPh3)4 (32 mg, 0.03 mmol) under N2 atmosphere, 10 mL of degassed toluene, 5 mL of degassed EtOH, and degassed 2 M Na2CO3 C


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added to mixture. The reaction mixture heated at reflux (90 °C) for overnight. When the reaction was completed, water and 1 M HCl solution were added to deal with piperidine. The product was extracted with dichloromethane. The organic layer was collected, dried over anhydrous MgSO4, and evaporated under reduced pressure. The remaining crude product was purified by column chromatography using CH2Cl2/MeOH/ AcOH mixture (10:1:0.1) as eluent to give compound BT-T (40 mg, 58%) as dark red solid. 1H NMR (400 MHz, DMSO) δ (ppm): 8.18−8.19 (m, 2H), 8.12 (d, J = 7.6 Hz, 1H), 7.91 (d, J = 8.8 Hz, 2H), 7.81 (d, J = 7.6 Hz, 1H), 7.76 (d, J = 4 Hz, 1H), 7.27−7.30 (m, 4H), 7.03−7.11 (m, 8H). 2-Cyano-3-(5-(7-(4-(diphenylamino)phenyl)benzo[c][1,2,5]thiadiazol-4-yl)-4-hexylthiophen-2-yl)acrylic acid (BTHT). To a 250 mL two-necked round-bottom flask were added compound 8 (50 mg, 0.09 mmol) and cyanoacetic acid (221 mg, 2.6 mmol) under N2 atmosphere and 20 mL of anhydrous CHCl3/CH3CN (3:1), and then piperidine (1 mL, 10.4 mmol) was added to mixture. The reaction mixture heated at reflux (90 °C) for overnight. When the reaction was completed, water and 1 M HCl solution were added to deal with piperidine. The product was extracted with dichloromethane. The organic layer was collected, dried over anhydrous MgSO4, and evaporated under reduced pressure. The remaining crude product was purified by column chromatography using CH2Cl2/MeOH/ AcOH mixture (10:1:0.1) as eluent to give compound BT-HT (41 mg, 82%) as dark red solid. 1H NMR (400 MHz, DMSO) δ (ppm): 8.21 (s, 1H), 8.08 (s, 2H), 7.91 (d, J = 7.2 Hz, 2H), 7.82 (s, 1H), 7.33−7.36 (m, 4H), 7.05−7.12 (m, 8H), 2.78− 2.73 (m, 2H), 1.62 (s, 2H), 1.25−1.29 (m, 6H), 0.84 (s, 3H). 13 C NMR (125 MHz, DMSO) δ (ppm): 163.85, 152.91, 151.78, 149.74, 147.53, 146.63, 137.72, 132.14, 131.89, 129.91, 129.75, 129.41, 129.21, 126.94, 126.81, 124.42, 123.67, 123.45, 121.82, 118.91, 30.78, 30.33, 28.15, 28.04, 21.75, 13.60. LC/QTOF MS: Found m/z 663.1853, calc. for C38H32N4NaO2S2 663.1859. Characterization Methods and Instrumentation. 1H and 13C nuclear magnetic resonance (NMR) spectra were obtained at FT-NMR 400 and 600 MHz Agilent Spectrometer as solution in CDCl3 and DMSO. Chemical shifts are reported in parts per million (ppm, δ) and referenced from tetramethylsilane. High resolution mass spectra (HRMS) were measured on JMS-700 (JEOL, Japan) mass spectrometer. UV−visible spectra were acquired on Cary 5000 (Agilent Technologies, Inc.) spectrophotometer. The photoluminescence spectra were measured on a fluorometer (Cary Eclipse, Varian Inc.). Cyclic voltammetry (CV) measurements were carried out using a Model 1287A potentiostat (Solartron Analytical, AMETEK, Inc.) equipped with a standard threeelectrode configuration, comprising indium tin oxide (ITO) working electrode, a Ag/AgCl (saturated KCl solution) reference electrode, and a Pt wire counter electrode. The measurement was done in anhydrous dichloromethane with tetrabutylammonium hexafluorophosphate (0.1 M) as the supporting electrolyte under a nitrogen atmosphere at a rate of 50 mV/s. The potential of Ag/AgCl reference electrode was calibrated by using the ferrocene/ferrocenium redox couple (Fe/Fe+). DSCs Fabrication. The fluorine-doped tin oxide (FTO, Nippon Sheet Glass Co., Ltd.) glass, which is used as current collector, was cleaned sequentially with D.I. water, ethanol, acetone, and ethanol for 15 min. After cleaning sequence, the glass was treated with UV-ozone system to make FTO’s surface

atmosphere, and 20 mL of anhydrous CHCl3/CH3CN (3:1), and then piperidine (1.45 mL, 14.7 mmol) was added to mixture. The reaction mixture heated at reflux (90 °C) for overnight. When the reaction was completed, water and 1 M HCl solution were added to deal with piperidine. The product was extracted with dichloromethane. The organic layer was collected, dried over anhydrous MgSO4, and evaporated under reduced pressure. The remaining crude product was purified by column chromatography using CH2Cl2/MeOH/AcOH mixture (10:1:0.1) as eluent to give compound BT-0 (50 mg, 86%) as dark red solid. 1H NMR (400 MHz, CDCl3) δ (ppm): 9.35 (s, 1H), 8.94 (d, J = 7.6 Hz, 1H), 7.97 (d, J = 8.8 Hz, 2H), 7.87 (d, J = 8.0 Hz, 1H), 7.31−7.35 (m, 4H), 7.19−7.22 (m, 6H), 7.10−7.14 (m, 4H). 2-Cyano-3-(4-(7-(4-(diphenylamino)phenyl)benzo[c][1,2,5]thiadiazol-4-yl)phenyl)acrylic acid (BT-P). To a 250 mL two-necked round-bottom flask were added compound 3 (120 mg, 0.25 mmol) and cyanoacetic acid (633 mg, 7.4 mmol) under N2 atmosphere and 16 mL of anhydrous CHCl3/ CH3CN (3:1), and then piperidine (3.0 mL, 30 mmol) was added to mixture. The reaction mixture heated at reflux (110 °C) for overnight. When the reaction was completed, water and 1 M HCl solution were added to deal with piperidine. The product was extracted with dichloromethane. The organic layer was collected, dried over anhydrous MgSO4, and evaporated under reduced pressure. The remaining crude product was purified by column chromatography using CH2Cl2/MeOH/ AcOH mixture (10:1:0.1) as eluent to give compound BT-P (110 mg, 81%) as dark orange solid. 1H NMR (400 MHz, DMSO) δ (ppm): 8.37 (s, 1H), 8.26 (d, J = 8.4 Hz, 2H), 8.20 (d, J = 8.4 Hz, 2H), 8.09 (d, J = 7.2 Hz, 1H), 8.00 (d, J = 8.8 Hz, 2H), 7.96 (d, J = 7.6 Hz, 1H), 7.35−7.39 (m, 4H), 7.11− 7.13 (m, 6H), 7.09−7.10 (m, 2H). 2-Cyano-3-(4-(7-(4-(diphenylamino)phenyl)benzo[c][1,2,5]thiadiazol-4-yl)-3-methylphenyl)acrylic acid (BT-MP). To a 250 mL two-necked round-bottom flask were added compound 4 (60 mg, 0.12 mmol) and cyanoacetic acid (306 mg, 3.6 mmol) under N2 atmosphere and 20 mL of anhydrous CHCl3/CH3CN (3:1), and then piperidine (1.42 mL, 14.4 mmol) was added to mixture. The reaction mixture heated at reflux (90 °C) for overnight. When the reaction was completed, water and 1 M HCl solution were added to deal with piperidine. The product was extracted with dichloromethane. The organic layer was collected, dried over anhydrous MgSO4, and evaporated under reduced pressure. The remaining crude product was purified by column chromatography using CH2Cl2/MeOH/AcOH mixture (10:1:0.1) as eluent to give compound BT-MP (45 mg, 66%) as dark red solid. 1H NMR (400 MHz, CDCl3) δ (ppm): 8.37 (s, 1H), 8.05 (d, J = 8 Hz, 1H), 8.01 (s, 1H), 7.90 (d, J = 8.4 Hz, 2H), 7.78 (d, J = 7.2 Hz, 1H), 7.60 (d, J = 7.2 Hz, 1H), 7.57 (d, J = 8.0 Hz, 1H), 7.29− 7.33 (m, 4H), 7.19−7.24 (m, 6H), 7.06−7.11 (m, 2H). 13C NMR (100 MHz, CDCl3) δ (ppm): 154.11, 153.44, 148.34, 147.36, 143.21. 138.18, 133.90, 133.55, 131.76, 131.45, 130.32, 129.99, 129.76, 129.38, 128.54, 126.71, 125.02, 123.46, 122.64, 20.60. LC/Q-TOF MS: Found m/z 587.1511, calc. for C35H24N4NaO2S 587.1512. 2-Cyano-3-(5-(7-(4-(diphenylamino)phenyl)benzo[c][1,2,5]thiadiazol-4-yl)thiophen-2-yl)acrylic acid (BT-T). To a 250 mL two-necked round-bottom flask were added compound 5 (60 mg, 0.12 mmol) and cyanoacetic acid (313 mg, 3.7 mmol) under N2 atmosphere and 20 mL of anhydrous CHCl3/ CH3CN (3:1), and then piperidine (1.5 mL, 14.7 mmol) was D


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The Journal of Physical Chemistry C Scheme 1. Synthesis of BT-Series

dipped in dye solution (0.2 mM in CHCl3/EtOH = 7:3 mixed solution). Each cell is dipped in this solution for several hours in dark condition. After dipping process, the cells were cleaned with anhydrous ethanol for remove unabsorbed dye molecules. The FTO glass was drilled by hand drill for making hole for inject electrolyte and washed with D.I. water, ethanol, and acetone sequentially. After removing residual contaminants, the Pt catalyst was deposited on the FTO surface by brushing with H2PtCl6 solution (10 mM H2PtCl6·6H2O in ethanol). After deposition the glass was on heat treatment at 400 °C for 15 min. The photoanode and Pt-counter electrode were assembled into a sandwich type and sealed with a 25 μm thickness thermoplastic sealing film (Meltonix 1170−25). The redox electrolyte was introduced into the interspace between the working and the counter electrode through the hole by a vacuum backfilling force, and then the hole was sealed up by same 25 μm film and cover glass (Marlenfeld GmbH and Co.) at 110 °C.

hydrophilic and clean for 20 min. A thin compact layer of TiO2 was prepared on the FTO by dipping the glass into the 40 mM TiCl4 aqueous solution in 70 °C oven for 30 min and rinsed with D.I. water and ethanol. The transparent TiO2 active layer were coated by screen printing of transparent TiO2 paste (average 18 nm, DONGJIN SEMICHEM CO., Ltd.) and scattering TiO2 paste (Ti-Nanoxide R/SP) on the treated FTO. The film thickness is controlled by repeating screen printing process with one mask (MESH S/T250, Emulsion 12 μm), which corresponds to 1.8 μm after one time, 3.5 μm after two times, and 5.1 μm after three times printing. For a thicker active layer, another mask (MESH S/T150, Emulsion 50 μm) was used. It provided 5.0 μm film after printing, 10 μm film for two times printing. The printed photo anode (0.16 cm2, 0.4 × 0.4 cm2) was gradually heated at 150 °C for 10 min, 325 °C for 5 min, 327 °C for 5 min, 450 °C for 15 min, and 500 °C for 30 min. Sintered FTO glass was immersed in TiCl4 aqueous solution again for making another thin TiO2 layer and heated at 500 °C for 30 min. After these processes, the electrode was E


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The Journal of Physical Chemistry C Photoelectrochemical Characterization of BT-series. The current−voltage (J−V) characteristics were recorded with a digital source meter (Series 2600A, Keithley Instruments, Inc.) under standard AM 1.5G conditions (1000 W/m2) simulated by a photovoltaic efficiency measurement system (IQE-200, Newport Corporation). The photovoltaic measurements were conducted with a black mask (0.5 × 0.5 cm2) for preventing overestimation. Intensity modulated photocurrent/ intensity modulated photovoltage (IMPS/IMVS) were acquired from the electrochemical workstation (ZENNIUM XPOT, ZAHZER-elektrik GmbH and Co. KG) with a frequency response analyzer under an intensity-controlled monochromatic LED (463 nm). Electrochemical impedance spectroscopy (EIS) was carried out using a Model 1287A potentiostat and 1260A Impedance/Gain-phase analyzer combined (Solartron Analytical, AMETEK, Inc.). Computation Detail. Density functional theory (DFT)24 and time-dependent density functional theory (TD-DFT)25 calculation were performed with the Gaussian 09 package.26 The ground state and oxidized state geometries of BT-series were optimized in vacuum state by using the hybrid functional B3LYP27 and the 6-311G** basis set. TD-DFT calculation were carried out on the B3LYP optimized ground state geometries in vacuum state by using the hybrid functional CAM-B3LYP28 and the 6-311G** basis set.

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RESULTS AND DISCUSSION Design and Synthesis. We introduced a 3-methylphenyl spacer (BT-MP) and a 4-hexylthiophene spacer (BT-HT) to control the dihedral angle between the π-spacer and the BT unit, and compared the materials with BT-P and BT-T, respectively. BT-0 was prepared according to a literature procedure.29 Scheme 1 shows a detailed synthetic scheme for the BT-containing materials. The Suzuki−Miyaura coupling of 4,7-dibromobenzo[c][1,2,5]thiadiazole with N,N-diphenyl-4(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-aniline afforded compound 2 in a 62% yield. Compound 2 is an important intermediate for the synthesis of BT-P, BT-MP, and BT-T. Compounds 3, 4, and 5 were all obtained through Pd-catalyzed coupling of compound 2 with π-spacers (4-formylphenylboronic acid, 4-formyl-2-methylphenylboronic acid, and 5-formyl2-thienylboronic acid). The synthesis of BT-HT was slightly different to those of other dyes. 3-hexylthiophene-2-boronic acid pinacol ester was coupled with compound 1 under Suzuki conditions to afford compound 6 in a 46% yield. The Vilsmeier−Haack reaction gave the aldehyde, compound 7, which then underwent a Suzuki reaction with 4(diphenylamino)phenylboronic acid pinacol ester to yield compound 8 in a 65% yield. Finally, the BT-series were obtained by the Knoevenagel condensation reaction Computational Study. To identify the properties of the πspacers, we performed density functional theory (DFT)24 calculations with the hybrid functional B3LYP27 and the 6311G** basis set. Figure 2 shows the DFT calculated optimized ground state geometries of the BT-series, with dihedral angles of 1.0° for BT-0, 33.3° for BT-P, 57.3° for BT-MP, 0.3° for BT-T, and 53.5° for BT-HT. We also calculated the spin densities of the oxidized states of the dyes (Figure S2), when the arrangement and orientation of the organic dyes on the TiO2 surface were similar to each other. Spin density represents the probability that the electrons injected from the dyes into the conduction band of TiO2 will recombine with the oxidized dyes. This recombination process is also known as BET.30

Figure 2. DFT calculated optimized ground state geometries of BTseries, with dihedral angles between BT units and π-spacers indicated.

Analysis of the spin density for BT-0 shows that the hole is highly delocalized along the whole backbone of the dye because there was no π-spacer to retard BET, which results in increased BET. The planar dye BT-T has a similar spin density to BT-0. In contrast, the twisted dyes, BT-P, BT-MP, and BT-HP, have a less spin density over the entire backbone, especially in the acceptor units. Based on this information, it was expected that BT-0 and the planar dye BT-T would have severe BET, while the other twisted dyes would exhibit less BET. In Figure S4, it can be seen that the HOMO of the BT-series were delocalized from the triphenylamine donor group to the BT units, while the LUMO were delocalized from the BT units to the electron-accepting anchoring groups, including across the π-spacer. Hence, both the HOMO and LUMO showed sufficient overlap between the donor and acceptor groups, making it possible for charge transfer to occur from the exited dyes to the TiO2 conduction band. Optical Properties. The absorption spectra of the BTseries measured in CHCl3/EtOH (7:3) solutions and on thin photoelectrodes (1.8 μm) with chenodeoxycholic acid (CDCA) are shown in Figure S7a and Figure 3a, respectively. Their detailed optical parameters are also summarized in Table 1. The BT-series absorb over a range of 300−600 nm. The shorter wavelength bands are ascribed to the localized aromatic π−π* electronic transitions of the sensitizers, while the longer wavelength bands correspond to ICT. The ICT peak of BTMP in both solution and films exhibited the largest blue shift compared with that of the reference BT-0 because it possessed the largest dihedral angle (57.3°), followed by BT-P (33.3°). In contrast, the ICT peaks of BT-T and BT-HT presented similar red shifts compared with that of BT-0 because of additional F


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absorbance of the photoelectrode. In the BT-series, A at ICT peak increased in the order BT-MP (439 nm) < BT-HT (491 nm) < BT-0 (477 nm) < BT-P (454 nm) < BT-T (482 nm) (Figure 3a). In Figure 3b, it can be seen that BT-T exhibited the highest LHE as well as the broadest absorption. However, BT-MP showed the poorest LHE. Electrochemical Properties. Cyclic voltammograms (CV) were measured to evaluate the oxidation potentials (Eox vs normal hydrogen electrode (NHE)) of the BT-series (Figure S10). The reduction potentials (Ered vs NHE) were estimated from the optical band gap (E0−0) measured from the onset of the UV−visible absorption spectra. The electrochemical parameters of the BT-series are summarized in Table 1. Overall, the dyes had similar values of Eox, while Ered varied according to the π-spacer used (Figure 3c). The E0−0 increased in the order of BT-T ≈ BT-HT < BT-0 < BT-P < BT-MP, which agreed with the results of the DFT calculations (Figure S3). As depicted in Figure 3c, the Eox of the BT-series were sufficiently more positive than the I−/I3− redox couple (0.35 V vs NHE), and the Ered lay above the edge of the conduction band of TiO2 (−0.5 V vs NHE), indicating that the electron injection (K1) and regeneration (K4) would be energetically permitted. Characterization of Photovoltaic Performance. We evaluated the photovoltaic performances of the BT-series using DSCs with different photoelectrode thicknesses ranging from 1.8 to 10 μm under standard AM 1.5G conditions (1000 W/ m2). The PCE (η) can be determined from the current−voltage (J−V) curves of DSCs fabricated using the BT-series using η=

Pin

(2)

where Pin is the incident irradiance of sunlight on the cell. The photovoltaic performances of the DSCs fabricated using the BT-series are summarized in Figure 4 and Table 2. It is noteworthy that BT-T, which had the most planar structure and therefore the most BET, showed a completely different dependence of PCE on the photoelectrode thickness than the twisted structures, BT-P, BT-MP, and BT-HT (Figure 4a). The Jsc and PCE of twisted sensitizers (BT-P, BT-MP, and BTHT) were gradually decreased as the thickness of the photoelectrode decreased (Table 2). The reason is their poor LHE on thin photoelectrode (1.8 μm) due to their twisted structures (Figure S9). However, the PCE of BT-T gradually increased from 4.76% to 6.49% as the thickness of the photoelectrode decreased. The Jsc of BT-T was particularly enhanced by the decrease in the photoelectrode thickness (Figure 4b). This unusual result for BT-T was related to the LHE and the effect of BET in thin 1.8 μm photoelectrodes. This phenomenon is discussed in more detail in the Charge Collection Efficiency section. Figure 4c shows the J−V curves of the BT-series in thin photoelectrode (1.8 μm) devices. BT-0 exhibited poor PCE (2.39−2.76%) with low Jsc and Voc values at all photoelectrode thicknesses because of rapid BET.17,18 However, the PCE (4.81%) of BT-P showed improved PCE compared to BT-0 (2.76%) because the twisted phenyl spacer (33.3°) retarded the BET. The more twisted BT-MP (57.3°) exhibited a worse PCE (1.87%) than that of BT-P, which was attributed to its poor LHE (Figure 3b). Similarly, BT-HT (53.5°) showed a lower PCE (4.45%) than that of BT-T (0.3°, 6.49%). These results are in good agreement with the incident photon-to-electron

Figure 3. (a) Absorption spectra and (b) LHE of BT-series on thin photoelectrodes (1.8 μm). (c) Energy level diagrams of conduction band of TiO2, BT-series, and I−/I3− with electron pathway in DCSs (K1, electron injection; K2, back electron transfer; K3, recombination to electrolyte; K4, regeneration).

thiophene-based π-spacers. However, the ICT peak of BT-HT showed no blue shift compared with that of BT-T, although BT-HT (53.5°) had a larger dihedral angle than BT-T (0.3°). This could be related to aggregation effects caused by the long alkyl chain in BT-HT.31 In addition, the π-spacer affected not only shift in the ICT peak but also the LHE (Figure 3b). The LHE (λ) is described by32,33 LHE(λ) = 1 − 10−ε Γ = 1 − 10−A

Jsc Voc FF

(1)

where ε is the molar extinction coefficient of the sensitizer at a given wavelength (λ), Γ is the molar concentration of the sensitizer per projected surface area of the film, and A is the G


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The Journal of Physical Chemistry C Table 1. Optical and Electrochemical Properties of BT-Series dye BT-0 BT-P BT-MP BT-T

BT-HT

λmax, absa [nm]

εmax [L mol−1 cm−1]

λmax, absb [nm]

E0ox [V]

E0redc [V]

Eoptical0−0 [eV]

HOMOd [eV]

LUMOe [eV]

323 481 314 454 311 439 309 388 491 311 391 493

30,000 17,000 45,000 18,000 50,000 14,000 24,000 15,000 20,000 29,000 13,000 19,000

477

1.22

−0.99

2.21

−5.39

−3.18

454

1.18

−1.19

2.32

−5.35

−3.03

439

1.20

−1.23

2.43

−5.37

−2.91

482

1.15

−1.01

2.16

−5.32

−3.16

491

1.14

−1.02

2.16

−5.31

−3.15

Chloroform/EtOH = 7:3/20 μM solution. b1.8 μm TiO2 films. cE0red = E0ox − Eoptical0−0. dHOMO = −(Eox vs Fc+/Fc) − 4.8 eV. eLUMO = HOMO + Eoptical0−0. a

Figure 4. (a) Average PCE of BT-series and (b) J−V curves of BT-T with different photoelectrode thicknesses. (c) J−V curves and (d) IPCE spectra of BT-series with 1.8 μm photoelectrodes.

conversion efficiency (IPCE) values, as shown in Figure 4d. The sensitizers with π-spacers (BT-P, BT-MP, BT-T, and BTHT) generally exhibited higher IPCE values than BT-0 because the π-spacers efficiently reduced BET. Of these dyes, the planar BT-T showed a broader IPCE spectrum than the twisted dyes (BT-P and BT-MP) and higher IPCE values than the twisted BT-HT at all wavelengths owing to its improved LHE. This is

the same reason that the IPCE of BT-P was broader and higher than that of BT-MP. Furthermore, BT-T showed remarkably enhanced IPCE in the 400−600 nm region in thinner photoelectrodes compared with that in thick photoelectrodes (Figures S11 and S12). Consequently, in a 1.8 μm photoelectrode, BT-T provided the highest Jsc (13.6 mA/cm2), followed by BT-HT (10.4 mA/cm2), BT-P (9.1 mA/cm2), and H


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The Journal of Physical Chemistry C Table 2. Photovoltaic Parameters of BT-Series with Different Photoelectrode Thicknesses dye

La [μm]

Jsc [mA/cm2]

Voc [V]

FF

η (η)b [%]

BT-0

1.8 + 3.5 3.5 + 3.5 5.1 + 3.5 10 + 3.5 1.8 + 3.5 3.5 + 3.5 5.1 + 3.5 10 + 3.5 1.8 + 3.5 3.5 + 3.5 5.1 + 3.5 10 + 3.5 1.8 + 3.5 3.5 + 3.5 5.1 + 3.5 10 + 3.5 1.8 + 3.5 3.5 + 3.5 5.1 + 3.5 10 + 3.5

7.63 7.31 6.31 6.63 9.06 10.06 10.44 12.25 4.07 5.38 7.19 7.44 13.56 10.50 10.31 10.06 10.43 10.88 11.31 12.69

0.55 0.52 0.54 0.55 0.71 0.67 0.72 0.65 0.62 0.66 0.63 0.66 0.67 0.66 0.64 0.63 0.64 0.63 0.65 0.63

0.66 0.72 0.70 0.73 0.75 0.74 0.73 0.71 0.74 0.74 0.72 0.75 0.72 0.76 0.75 0.75 0.70 0.75 0.72 0.73

2.76 2.74 2.39 2.64 4.81 4.99 5.44 5.61 1.87 2.62 3.26 3.66 6.49 5.20 5.01 4.76 4.67 5.14 5.30 5.83

BT-P

BT-MP

BT-T

BT-HT

a

(2.36) (2.39) (2.12) (2.44) (4.66) (4.83) (5.24) (5.35) (1.82) (2.54) (3.12) (3.50) (6.12) (5.14) (4.77) (4.28) (4.31) (4.73) (5.00) (5.82)

Γc 0.83 1.28 1.53 4.02 0.75 1.22 2.13 3.73 0.19 0.60 1.89 2.20 0.75 1.39 2.28 4.21 0.19 0.85 1.34 1.95

Photoelectrode thickness with scattering layer. bThe average PCE values. cDye loading capacity (×10−7 mol/cm2).

Figure 5. IMVS and IMPS parameters of BT-series: (a) recombination lifetime, (b) transport time in thin photoelectrodes (1.8 μm), and (c) charge collection efficiencies in photoelectrodes of different thicknesses. (d) LHE of BT-T and BT-MP in photoelectrodes of different thicknesses.

I


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The Journal of Physical Chemistry C BT-MP (4.1 mA/cm2), which corresponded well with the trend in PCE. As a result, BT-T showed a ca. 40% enhancement in its PCE in the 1.8 μm photoelectrode compared to that seen in the 10 μm photoelectrode, reaching 6.5% (Jsc = 13.6 mA/cm2, Voc = 0.67 V, and FF = 0.72) compared to 4.76%. In contrast, the other twisted dyes (BT-P, BT-MP, and BT-HT) that allowed efficient retardation of BET showed optimal performances in thicker photoelectrodes (5.1−10 μm). In other words, thicker photoelectrodes enhanced the LHE of twisted dyes, so PCE values gradually increased from 4.81% to 5.61% for BT-P, from 1.87% to 3.66% for BT-MP, and from 4.67% to 5.83% for BTHT as the thickness of the photoelectrode increased. Based on these results, we can assume that BET in BT-T became less influential as the thickness of the photoelectrode decreased, as the charge collection efficiency was enhanced. Therefore, LHE played a more critical role than BET in thin photoelectrodes; this is discussed further in Charge Collection Efficiency section. BT-T exhibited remarkably enhanced Jsc values in thin photoelectrodes compared with those in thick photoelectrodes. Therefore, to improve our understanding of the factors influencing the Jsc of BT-T-based DSCs, we analyzed the effect of different photoelectrode thicknesses on the BET. The Jsc in DSCs can be determined by34 Jsc =

∫λ LHE(λ)Φinjectηcollect dλ

As was discussed earlier, BT-T had the highest LHE in thin photoelectrodes, thereby producing the greatest amount of charge. Taken together, these factors mean that large amounts of extracted charges reached the electrode in BT-T devices before BET could occur due to the rapid charge transport time. Thus, we can draw the conclusion that the LHE plays a more critical role than retarding BET in thin photoelectrodes, thereby enabling BT-T to exhibit the highest current density and PCE. To explain the variation in the photovoltaic performances of these dyes according to the thicknesses of the photoelectrodes, the charge collection efficiencies were measured for all thicknesses. Figure 5c shows the overall ηcollect of BT-series improved as the photoelectrode thickness decreased and that BT-T had the highest ηcollect (≥97%) of the series in thin photoelectrodes (1.8 μm), both of which are in good agreement with the device results. However, the other twisted dyes (BT-P, BT-MP, and BT-HT) did not show improved photovoltaic performance, although they also exhibited enhanced ηcollect in thin photoelectrodes. This is because these twisted dyes showed dramatically reduced LHE as the thickness of the photoelectrodes decreased, while the LHE of BT-T remained much more similar (Figure 5d and Figure S9). This also indicates that LHE is a more critical factor than reducing BET in thin photoelectrodes. Electrochemical Impedance Spectroscopy. EIS was measured to study the charge transfer resistances and electron lifetimes in DSCs fabricated with BT-series (1.8 μm thin photoelectrodes) under one sun illumination and open circuit conditions, applying a 10 mV AC perturbation amplitude within a frequency range of 0.1 Hz to 100 kHz.36 Table S5 shows the EIS fitting parameters for the BT-series. The plot of BT-0 could not be fitted as the shape of plot was close to the Gerischer pattern.37 In Figure 6, the first small semicircle in the

(3)

where the LHE (λ) is the light harvesting efficiency, Φinject is the charge injection efficiency, and ηcollect is the charge collection efficiency. When the same sensitizer was used and the only difference was the photoelectrode thickness, it is reasonable to assume that Φinject was a constant and that the LHE (λ) was generally reduced as photoelectrode thickness decreased (Figure S9). Based on this equation and the device results, we can expect that ηcollect would be the most important factor for understanding the Jsc of BT-T in thin photoelectrodes. A more in-depth study regarding the charge collection efficiency follows in the next section. Charge Collection Efficiency. The charge collection efficiency (ηcollect) is determined as35 ηcollect = 1 − τd /τrec

(4)

where τd is the charge transport time and τrec is the charge recombination time. Therefore, intensity modulated photovoltage spectroscopy (IMVS) and intensity modulated photocurrent spectroscopy (IMPS) were performed to evaluate τd, τrec, and ηcollect in BT-series. The plots of τrec and τd obtained at different light intensities are shown in Figure 5a,b, and the values of τrec, τd, and ηcollect are summarized in Table S4. In Figure 5a, it can be seen that the twisted dyes BT-P (6.14 ms), BT-MP (6.86 ms), and BT-HT (7.71 ms) showed longer τrec values than the planar dye BT-T (5.10 ms), as was expected. This indicated that the twisted π-spacers reduced the BET more efficiently than the planar π-spacer. Moreover, BT-T (5.10 ms) showed a similar τrec to that of BT-0 (4.51 ms). This indicated that the planar π-spacer was inefficient at retarding BET. However, as shown in Figure 5b, BT-T measured the fastest τd (0.14 ms) of all the BT-series, with the rest of the series increasing as follows: BT-P (0.29 ms) < BT-MP (0.43 ms) < BT-HT (0.49 ms) ≈ BT-0 (0.49 ms). Using these results and eq 4, it can be seen that BT-T reached the highest value of ηcollect (97%) in thin photoelectrodes (1.8 μm), followed by BT-P (95%), BT-HT (93%), BT-MP (93%), and BT-0 (89%).

Figure 6. Electrochemical impedance spectra for DSCs with BT-series: Nyquist plots measured under one sun illumination.

Nyquist plots represents the charge transfer resistance (Rpt) at the counter electrode/electrolyte interface. The radius of the second semicircle in the Nyquist plots reflects the charge transfer resistance (Rct) at the TiO2/dye/electrolyte interface, with a smaller radius indicating a lower Rct under one sun illumination condition (Figure 6). BT-T exhibited the lowest Rct (10.0 Ω), with the remaining dyes following the sequence: BT-HT (14.9 Ω) ≤ BT-P (17.1 Ω) < BT-MP (22.2 Ω) < BTJ


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The Journal of Physical Chemistry C Notes

0. The reason for this was that the planar dye, having the best LHE, had the largest amount of photoinjected charge, resulting in a reduction in Rct due to an increased amount of charge collection compared to charge recombination. These results agreed well with the PCE trend and help to explain why BT-T gave the highest PCE (6.5%) in thin photoelectrodes (1.8 μm). Photostability. The photostability of sensitizers is a critical factor for practical applications and long lifetimes for DSCs. Therefore, we evaluated the photostabilities of the BT-series using a simple and efficient method developed by Katoh and co-workers38 that uses light irradiation on dye-absorbed photoelectrodes without redox electrolyte. Figure S15 shows the absorption spectra of the BT-series absorbed on 1.8 μm photoelectrodes while aging for 0−60 min under one sun of illumination. It can clearly be seen that the photostability of the planar dye BT-T was much higher than those of the twisted dyes (BT-P, BT-MP, and BT-HT). This could be due to the efficient ICT in BT-T, which could stabilize the sensitizer in its oxidized state. Moreover, BT-T showed higher photostability than BT-0. These results indicate that the planar dye BT-T is suitable for use in thin photoelectrode DSCs due to its robust photostability and high performance.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support provided by the Ulsan National Institute of Science and Technology (Research Fund 1.140094.01 and 1.160092.01), the National Research Foundation of Korea (Grant 2016R1A2B4009239), and the Korea Institute of Energy Technology Evaluation and Planning (No. 20143030011560).

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(1) O’Regan, B.; Gratzel, M. A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature 1991, 353, 737− 740. (2) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. DyeSensitized Solar Cells. Chem. Rev. 2010, 110, 6595−6663. (3) Park, N.-G. Organometal Perovskite Light Absorbers Toward a 20% Efficiency Low-Cost Solid-State Mesoscopic Solar Cell. J. Phys. Chem. Lett. 2013, 4, 2423−2429. (4) Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The emergence of perovskite solar cells. Nat. Photonics 2014, 8, 506−514. (5) Grätzel, M. Recent Advances in Sensitized Mesoscopic Solar Cells. Acc. Chem. Res. 2009, 42, 1788−1798. (6) Yu, Q.; Wang, Y.; Yi, Z.; Zu, N.; Zhang, J.; Zhang, M.; Wang, P. High-Efficiency Dye-Sensitized Solar Cells: The Influence of Lithium Ions on Exciton Dissociation, Charge Recombination, and Surface States. ACS Nano 2010, 4, 6032−6038. (7) Chen, C.-Y.; Wang, M.; Li, J.-Y.; Pootrakulchote, N.; Alibabaei, L.; Ngoc-le, C.-h.; Decoppet, J.-D.; Tsai, J.-H.; Grätzel, C.; Wu, C.-G.; et al. Highly Efficient Light-Harvesting Ruthenium Sensitizer for ThinFilm Dye-Sensitized Solar Cells. ACS Nano 2009, 3, 3103−3109. (8) Mishra, A.; Fischer, M. K.; Bauerle, P. Metal-free organic dyes for dye-sensitized solar cells: from structure: property relationships to design rules. Angew. Chem., Int. Ed. 2009, 48, 2474−2499. (9) Imahori, H.; Umeyama, T.; Ito, S. Large π-Aromatic Molecules as Potential Sensitizers for Highly Efficient Dye-Sensitized Solar Cells. Acc. Chem. Res. 2009, 42, 1809−1818. (10) Wang, X.-F.; Tamiaki, H. Cyclic tetrapyrrole based molecules for dye-sensitized solar cells. Energy Environ. Sci. 2010, 3, 94−106. (11) Kim, J.-J.; Choi, H.; Lee, J.-W.; Kang, M.-S.; Song, K.; Kang, S. O.; Ko, J. A polymer gel electrolyte to achieve ≥ 6% power conversion efficiency with a novel organic dye incorporating a low-band-gap chromophore. J. Mater. Chem. 2008, 18, 5223. (12) Chen, L.; Li, X.; Ying, W.; Zhang, X.; Guo, F.; Li, J.; Hua, J. 5,6Bis(octyloxy)benzo[c][1,2,5]thiadiazole-Bridged Dyes for Dye-Sensitized Solar Cells with High Open-Circuit Voltage Performance. Eur. J. Org. Chem. 2013, 2013, 1770−1780. (13) Tang, Z. M.; Lei, T.; Jiang, K. J.; Song, Y. L.; Pei, J. Benzothiadiazole containing D-pi-A conjugated compounds for dyesensitized solar cells: synthesis, properties, and photovoltaic performances. Chem. - Asian J. 2010, 5, 1911−1917. (14) Velusamy, M.; Justin Thomas, K. R.; Lin, J. T.; Hsu, Y.-C.; Ho, K.-C. Organic Dyes Incorporating Low-Band-Gap Chromophores for Dye-Sensitized Solar Cells. Org. Lett. 2005, 7, 1899−1902. (15) Zhu, W.; Wu, Y.; Wang, S.; Li, W.; Li, X.; Chen, J.; Wang, Z.-s.; Tian, H. Organic D-A-π-A Solar Cell Sensitizers with Improved Stability and Spectral Response. Adv. Funct. Mater. 2011, 21, 756−763. (16) Cabau, L.; Vijay Kumar, C.; Moncho, A.; Clifford, J. N.; Lopez, N.; Palomares, E. A single atom change ″switches-on″ the solar-toenergy conversion efficiency of Zn-porphyrin based dye sensitized solar cells to 10.5%. Energy Environ. Sci. 2015, 8, 1368−1375. (17) Haid, S.; Marszalek, M.; Mishra, A.; Wielopolski, M.; Teuscher, J.; Moser, J.-E.; Humphry-Baker, R.; Zakeeruddin, S. M.; Grätzel, M.; Bäuerle, P. Significant Improvement of Dye-Sensitized Solar Cell Performance by Small Structural Modification in π-Conjugated DonorAcceptor Dyes. Adv. Funct. Mater. 2012, 22, 1291−1302.

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CONCLUSIONS In summary, we designed and synthesized five simple organic dyes with various π-spacers that provided different dihedral angles between the BT units and the anchoring group. DFT calculations, UV−vis spectroscopy, CV, IPCE, IMVS/IMPS, and EIS were performed to investigate the photovoltaic performances of the BT-series. From the DFT-calculated spin densities and IMVS results, it can be seen that the twisted πspacers reduced BET better than the planar π-spacer. However, twisted π-spacers caused blue shifts in the ICT bands and poor light harvesting abilities in thin photoelectrodes. As a result, the photovoltaic performances of the twisted dyes were poor in thin photoelectrodes (1.8 μm). In contrast, the planar dye BTT achieved the best PCE of 6.5% (Jsc = 13.6 mA/cm2, Voc = 0.67 V, and FF = 0.72) in devices with 1.8 μm photoelectrodes and a 3.5 μm scattering layer. This was because BT-T retained a high LHE and the effects of BET could be dramatically reduced in thin photoelectrodes (1.8 μm) due to the high charge collection efficiency (≥ 97%). Moreover, BT-T showed the best photostability of the BT-series. This result indicates that the planar π-spacer (thiophene) is a suitable unit for high performing BT-based sensitizers in thin photoelectrodes, regardless of the effect of BET. Therefore, this molecular engineering and device strategy enables fabrication of highly efficient, ultrathin, and flexible DSCs.

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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.jpcc.6b08262. Characterization data and additional optical, electrochemical, and DFT results for the BT-series (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +82-52-217-2947. Author Contributions §

These authors contributed equally to this work. K


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