Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 2391−2399
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Cosensitization of Structurally Simple Porphyrin and AnthraceneBased Dye for Dye-Sensitized Solar Cells Kamani Sudhir K. Reddy,† Yen-Chiao Chen,‡ Chih-Chung Wu,‡ Chia-Wei Hsu,† Ya-Ching Chang,‡ Chih-Ming Chen,*,‡ and Chen-Yu Yeh*,† †
Department of Chemistry and Research Center for Sustainable Energy and Nanotechnology and ‡Department of Chemical Engineering, National Chung Hsing University, Taichung 402, Taiwan
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S Supporting Information *
ABSTRACT: Since their introduction, dye-sensitized solar cells (DSCs) have achieved huge success at a laboratory level. Recently, research is concentrated to visualize large DSC modules at the commercial platform. In that aspect, we have tested structurally simple porphyrin-based dye SK6 and anthracene-based dye CW10 for DSCs application under simulated 1 sun (AM 1.5G) and indoor light sources. These two dyes can be easily synthesized and yet are efficient with cell performances of ca. 5.42% and ca. 5.75% (without coadsorbent/ additive) for SK6 and CW10, respectively, under AM 1.5G illumination. The power conversion efficiency (PCE) of SK6 reported in this work is the highest ever reported; this is achieved by optimizing the adsorption of SK6 on TiO2 photoanode using the most suitable solvent and immersion period. Cosensitization of SK6 with CW10 on TiO2 surface has boosted cell performance further and achieved PCE of ca. 6.31% under AM 1.5G illumination. Charge-transfer properties of individual and cosensitized devices at TiO2/dye/electrolyte interface were examined via electrochemical impedance spectroscopy. To understand the cell performances under ambient light conditions, we soaked individual and cosensitized devices under T5 and light-emitting diode light sources in the range of 300−6000 lx. The PCE of ca. 22.91% under T5 light (6000 lx) with JSC = 0.883 mA cm−2, VOC = 0.646 V, and FF = 0.749 was noted for the cosensitized device, which equals a power output of 426 μW cm−2. These results reveal that DSCs made of structurally simple dyes performed efficiently under both 1 sun (AM 1.5G) and indoor light conditions, which is undoubtedly a significant achievement when it comes to a choice of commercial application. KEYWORDS: cosensitization, dim light, dye-sensitized solar cells, organic dye, porphyrin
1. INTRODUCTION Because of their ease of fabrication, low cost of manufacture, yet efficient performances under outdoor and indoor light sources, dye-sensitized solar cells (DSCs) have emerged rapidly as an alternative to conventional silicon solar cells. Extensive research was devoted to DSCs since the first report in 1991.1 Recently, the power conversion efficiency (PCE) of ca. 13% for porphyrin-based dye and ca. 14% for organic dye have been reported.2−4 On the other hand, perovskite solar cells (PSCs) with PCEs of ca. 20−22% were reported recently.5 The excellent performances of DSCs and PSCs demonstrate that they are in close proximity to surpass the silicon solar cells as cost of manufacture is concerned. Recent studies revealed that silicon solar cells are not suitable as portable power sources for indoor application because of their low performances under ambient light conditions.6 In general, simulated 1 sun illumination carries highly intense photon flux at 350−1200 nm,7 whereas the emission spectra of light sources in an indoor environment covers only 350−650 nm.8−10 The silicon solar cells can absorb photons between 350−1000 nm,11 with plenty of photons throughout this region, thus performing better than © 2017 American Chemical Society
DSCs under simulated 1 sun illumination. In contrast, under dim light conditions their capability of absorbing photons beyond the visible region is wasted due to lack of availability of photons beyond 650 nm. Above discussed drawback of silicon solar cells illustrates that DSCs and PSCs seem to be the frontrunners to surpass silicon solar cells. Though PSCs exhibit high PCEs (ca. 20−22%), the long-term stability must be improved before practical application appears. Therefore, at present DSCs seem to be a promising alternative to conventional siliconbased solar cells. To fabricate a large DSC module, production of dye molecule in large quantity is indispensable. The champion dyes such as YD2-o-C8, GY50, SM315, and ADEKA-1 are efficient with PCEs of >12%, yet synthesizing these dyes in large quantity becomes hectic.2−4,12,13 Therefore, there is still considerable room for improvement in the development of dyes, and the synthesis of structurally simple dyes with excellent PCEs are in urgent need. Received: August 27, 2017 Accepted: December 27, 2017 Published: December 27, 2017 2391
DOI: 10.1021/acsami.7b12960 ACS Appl. Mater. Interfaces 2018, 10, 2391−2399
Research Article
ACS Applied Materials & Interfaces Incident light of indoor and outdoor environments is distributed between the range of ∼200 and ∼100 000 lx (SI unit of illumination). At present time, most of the light sources in domestic or commercial spaces are revolutionized by fluorescent lamps and light-emitting diodes (300−10 000 lx).14 The emission spectra of indoor lights cover between 450 and 700 nm, and most of the dyes used in DSCs show excellent absorption in this region, which means they can harvest a major share of incident light to electricity. For instance, Y1A1 (19.3% at 300 lx), TY6 (28.6% at 6000 lx), and D35 (28.9% at 1000 lx) are in strong agreement with the above statement, and their performances reveal that DSCs are presently the best in the venue of indoor application.8−10 Above results illustrate that excellent performances under indoor light can be obtained when the absorption wavelengths of sensitizers match with the emission wavelengths of incident lights. The devices that come under Internet of Things (IoTs) and wireless network systems (WNSs) are expected to reach ∼20.8 billion units by 2020, which means they are ubiquitous either indoor or outdoor, or even in remote environment.15 Powering these small devices in a sustainable way is feasible via efficient photovoltaics such as DSCs. A minimum of tens to hundreds of microwatt power (μW) is sufficient to charge these devices;16 a DSC working under 200 lx with power input (Pin) of 61.3 μW cm−2 would generate a power output (Pout) that is ca. 13.7 μW cm−2, which translates to a PCE of 22.3%.10 Generally, the illuminance of light under indoor, rainy, and cloudy environment varies between 300 and 20 000 lx. Aforementioned Pout of DSC under indoor light indicates that it could power the devices for IoTs and WNSs.17,18 In DSCs, sensitizers and redox couples are the key components that share nearly 33.59% of total research conducted in DSCs.19 Sensitizers such as ruthenium complexes,20 Zn porphyrins,2 perylenes,21 and anthracenes9 have been shown to be excellent performers, thanks to their intriguing properties such as metal-to-ligand charge transfer (MLCT), high molar absorption coefficient (ε), and strong absorption at visible and near-infrared regions (NIR).2,12,22 Among aforementioned sensitizers, porphyrin and anthracenebased dyes were considered to be easily accessible, because of their ease of functionalization and moderate yields to obtain target dyes from commercially available starting materials. Because of this, a large number of device performances were documented from these two types of sensitizers.9,12,13,20,21,23−29 A typical zinc porphyrin dye exhibits a strong Soret band at 400−450 nm (S0 → S2 transitions) and moderate Q-bands at 600−650 nm (S0 → S1 transitions). On the other hand, an anthracene chromophore bearing a donor and an acceptor entity generally exhibits blue and red absorption peaks ascribed to π−π* transitions in visible region (350−650 nm).30 Thus, a combination of these two categories is a good choice to harvest the light in outdoor and indoor environments. Therefore, we have picked well-established, fundamental zinc porphyrin SK6, and a novel anthracene dye CW10 with a donor (N,N-bis(4(dodecyloxy)phenyl) and an acceptor (4-ethynylbenzoic acid) entity (Figure 1). These two dyes can be synthesized in four steps at gram scale. In general, zinc porphyrin dyes show a dip in absorption between Soret and Q-bands, which approximately fall at around 500−560 nm, whereas typical anthracene-based D-π-A dyes show strong absorption between 450 and 580 nm.9,30−32 Therefore, a cosensitization of SK6 with CW10 was expected to fill up the absorption dip of SK6.33−35 Three kinds of devices were fabricated using SK6, CW10, and SK6 + CW10
Figure 1. Molecular structures of SK6 and CW10.
and soaked under AM 1.5G and ambient light (T5 and LED light). The device fabricated using SK6 + CW10 (1:1 ratio) shows PCE of ca. 6.31% under AM 1.5G illumination and PCE of ca. 22.58% under T5 light source (6000 lx), which equals Pout of 426 μW cm−2, and are sufficient to charge devices for IoTs and WNSs.
2. RESULTS AND DISCUSSION 2.1. Synthesis. Synthetic procedures of SK6 are ubiquitous and the most suitable procedure for commercialization is condensation of commercially available pyrrole with benzaldehyde and methyl 4-formylbenzoate in refluxing propionic acid (Scheme 1).36 The obtained free base porphyrin was metalated using Zn(OAc)2, followed by hydrolysis under alkaline conditions yielding SK6 in gram scale. On the other hand, CW10 was synthesized in three steps by using commercially available 9-bromoanthracene (Scheme 2), which underwent a Buchwald-Hartwigg amination37 followed by a simple bromination using N-bromosuccinimide (NBS) in CH2Cl2. The brominated product was undertaken to employ a Sonogashira cross-coupling reaction38 with commercially available 4ethynylbenzoic acid to yield CW10 in gram scale.9,31 The synthetic routes are so simple that they are commercially viable to be synthesized in bulk quantity. The detailed experimental procedures of synthesis along with characterization were given in the Supporting Information. 2.2. Photophysical and Electrochemical Properties. The UV−visible absorption spectra of SK6 and CW10 were recorded in THF and shown in Figure 2 and the corresponding data were depicted in Table 1. Porphyrin SK6 shows a strong Soret band at 423 nm (ε = 473 × 103 M−1 cm−1) and two moderate Q-bands at 555 nm (19 × 103 M−1 cm−1) and 594 nm (8 × 103 M−1 cm−1). The organic dye CW10 shows broad absorption peaks at 401 nm (20 × 103 M−1 cm−1), 420 nm (18 × 103 M−1 cm−1), and 507 nm (13 × 103 M−1 cm−1). As seen in Figure 2, the broad absorption of CW10 between 450 and 550 nm is so important that a cosensitized solution of SK6 and CW10 would fill up the dip of absorption shown by SK6 at 450−550 nm. Therefore, a cosensitization of these two dyes on TiO2 surface would harvest the incident light to electrons in an efficient manner compared to that of individual dyes. As shown in Figure S1, the absorption wavelengths of these dyes are in close conjunction with the emission wavelengths of incident light in an indoor environment (i.e., either T5 or LED). To achieve high conversion efficiency, the driving forces for charge injection and dye regeneration must be sufficient. Thus, the energy levels of sensitizers play a crucial role in DSCs.27 To understand the electrochemical properties, cyclic voltammetry experiments were conducted for both the dyes in THF solution with ferrocene/ferrocenium (Fc/Fc+) as an internal reference and 0.1 M tetrabutylammonium hexafluorophosphate 2392
DOI: 10.1021/acsami.7b12960 ACS Appl. Mater. Interfaces 2018, 10, 2391−2399
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ACS Applied Materials & Interfaces Scheme 1. Synthetic Route of SK6a
a (i) Benzaldehyde, methyl 4-formlybenzoate, propionic acid (reflux), 9.9%. (ii) Zn(OAc)2, CH2Cl2, methanol, 99%. (iii) 2 M NaOH, THF/H2O, 5 h, 86%.
Scheme 2. Synthetic Route of CW10a
a
(i) Bis(4-(dodecyloxy)phenyl)amine, cat. Pd(OAc)2, P(t-Bu)3, NaOtBu, dry toluene, 72%. (ii) NBS, CH2Cl2, 81% (iii) 4-ethynylbenzoic acid, cat. Pd2(dba)3, AsPh3, THF/NEt3, 62%.
the dyes. As seen in Figure 3b, positions of HOMO energy levels (EHOMO) of both the dyes are positive compared to the potential of redox mediator (I−/I3−), and the driving forces are calculated as 0.63 and 0.34 eV for SK6 and CW10, respectively, ensuring a strong driving force for dye regeneration. The LUMO energy levels (ELUMO) of both the dyes are negative compared to Fermi level of TiO2 conduction band (TiO2 CB) and the calculated driving forces are 0.53 and 0.92 eV for SK6 and CW10, respectively, ensuring a strong driving force for charge injection. With enough driving forces for both charge injection and dye regeneration, these two dyes were expected to perform well as DSCs. 2.3. Photovoltaic Performance. In general, alcoholic or THF solutions of ZnP dyes were found to express better cell performances.39,40 Thus, both ethanol and THF solvents were tested (Table S1), and we found that the device made in ethanol solution showed better cell performance over that in THF under the same immersion period (4 h). The suitable immersion period was then next examined. Aggregation of ZnP on TiO2 photoanode is greatly related to the immersion period; a prolonged immersion period would enhance the tendency of aggregation on photoanode. As shown in Table S1, the PCEs of corresponding devices reveal an inverse relationship with
Figure 2. UV−visible absorption spectra of SK6 (purple) and CW10 (orange) recorded in THF at 25 °C. The extinction coefficient value of CW10 was magnified 10 times and the Q-band region of SK6 (dot) was magnified 5 times.
(TBAPF6) as a supporting electrolyte (Figure 3, Table 1). Figure 3a shows reversible waves for the first oxidation of both
Table 1. Photophysical and Electrochemical Properties of SK6 and CW10 dye
absorptiona (ε), nm (103 M−1 cm−1)
emission (nm)
EHOMO (V vs NHE)
ELUMOb (V vs NHE)
Eredb (V)
E0‑0c (eV)
SK6 CW10
423.56 (473.63), 555.21 (19.68), 594.74 (8.61) 401.47 (20.32), 420.21 (1.80), 506.76 (1.34)
601, 651 648
1.03 0.74
−1.03 −1.48
−1.33 −1.65
2.05 2.16
a Molar absorption coefficients were calculated from UV−visible absorption spectra recorded in THF at 25 °C. bOxidation and reduction potentials were obtained from cyclic voltammetry experiments recorded at 25 °C in dry THF as a solvent and 0.1 M TBAPF6 as an electrolyte. cOptical band gaps were obtained from the formula E0‑0 = 1240/λonset.
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Figure 3. Electrochemical characterization. (a) Cyclic voltammogram of SK6 and CW10 measured in THF as the solvent. (b) A schematic energy level diagram of corresponding dyes.
Figure 4. (a) J−V curves and (b) IPCE spectra of devices based on SK6, CW10, and SK6 + CW10 dyes under AM 1.5G illumination.
Table 2. Photovoltaic Parameters of SK6, CW10, and SK6 + CW10 Based Cells Measured under Air Mass 1.5G Illumination dyea,b SK6 CW10 SK6 + CW10 N719
JSC (mA cm−2) 11.30 10.76 12.04 14.79
± ± ± ±
0.134 0.051 0.011 0.247
VOC (V) 0.689 0.739 0.732 0.782
± ± ± ±
FF
0.001 0.000 0.002 0.009
0.695 0.722 0.715 0.717
± ± ± ±
0.163 0.025 0.000 0.010
η (%)
JSC (mA cm−2)c
± ± ± ±
9.67 9.47 10.91 13.17
5.42 5.75 6.31 8.29
0.060 0.216 0.019 0.095
a
PCEs were tested with 0.36 cm2. bThree cells were fabricated for each entry and the average parameters were tabulated along with standard deviations. Pin used for the tested cells was 100 mW cm−2. cCalculated from IPCE.
whereas the VOC of the cell fabricated from CW10 is higher than that of SK6. It is noteworthy that diarylamines with long alkyl/alkoxy chains play a vital role in DSC by impeding dye aggregation and charge recombination and enhancing charge separation and charge injection. Thus, the donor moiety bis(4(dodecyloxy)phenyl)amine attached to anthracene in CW10 would efficiently suppress the dye aggregation and also block the electrolyte contact into TiO2 surface, leading to enhancement of VOC value. The cosensitized device (SK6 + CW10 in a 1:1 ratio) shows PCE of 6.31% (JSC = 12.04 mA cm−2, VOC = 0.732 V, and FF = 0.715). As expected CW10 filled up the dip shown by SK6 between Soret and Q-bands, so with strong absorption at 350−600 nm the cosensitized device would harvest more photons in this region; in return, the corresponding device has obtained enhanced JSC value. The following equation explains the relation between IPCE response and JSC value:
respect to immersion period. Therefore, we found that soaking the cells for 4 h is the optimum choice to obtain the best PCE from SK6. As organic dye, CW10 with two long alkoxy chains (OC12H25) would not face as severe aggregation as SK6, so the procedure that was established by our group earlier (ethanol:CHCl3/4:1) was employed with slight modifications.31 To test the reproducibility of the highest PCEs of individual and cosensitized devices, as much as seven repetitions were performed with a slight deviation in PCEs for all three cells. The J−V curves and IPCE spectra of individual and cosensitized cells measured under AM 1.5 G illumination were shown in Figure 4 and the corresponding data were tabulated in Table 2. The cell fabricated from SK6 dye shows PCE of 5.42% (JSC = 11.30 mA cm−2, VOC = 0.689 V, and FF = 0.695), and the cell fabricated from CW10 dye shows PCE of 5.75% (JSC = 10.76 mA cm−2, VOC = 0.739 V, and FF = 0.722). The JSC value of SK6 is slightly higher than that of CW10, 2394
DOI: 10.1021/acsami.7b12960 ACS Appl. Mater. Interfaces 2018, 10, 2391−2399
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Figure 5. (a) J−V curves of SK6-, CW10-, and SK6 + CW10-based devices measured under dark conditions. (b) The charge-transfer resistance (Rct) on the dye-coated TiO2 photoanode obtained from the impedance spectra of SK6-, CW10-, and SK6 + CW10-based devices.
JSC = e
∫ Io(λ) × IPCE(λ) dλ
low intensity (300−6000 lx) to a relatively higher intensity (1 sun >100 000 lx). Therefore, a mismatch of the output current between the measured JSC and the calculated one is expectable. Figure 5a shows J−V curves of DSCs in dark conditions; with the increase in bias potential, the onset of the dark current was observed first for the device based on SK6 dye. This indicates that this cell has a higher rate of charge recombination than other cells, whereas CW10-based cell exhibits a lower rate of charge recombination; this is ascribed to the presence of a diarylamine entity (bis(4-(dodecyloxy)phenyl)amine) with two long alkoxy chains that would block the contact of redox couple with TiO2 surface. To gain better insight of electron-transport property inside the DSC, electrochemical impedance of DSCs based on three dyes were measured within a range of bias potential in dark conditions. The charge-transfer resistance (Rct) on the dye-coated TiO2 photoanode obtained from the impedance spectra by fitting to the equivalent circuit are plotted in Figure 5b. In dark conditions, the applied potential drives the electrons to move from the FTO substrate into the electrolyte. At a low bias potential range (−0.30 to −0.44 V), the TiO2 mesoporous film (TiO2MF) on the photoanode behaves as an electrical insulator. Therefore, the primary pathway of the electrons is route A, as shown in the equivalent circuit (inset in Figure 5b). The electrons mainly go through the interface between uncovered FTO and electrolyte and charge recombination takes place there. Therefore, the corresponding values of Rct are the charge-transfer resistance at the FTO/electrolyte interface. Because the surface condition of FTO is identical for all tested DSCs, the values of Rct are similar at the lower bias potential range. In contrast, at intermediate (−0.44 to −0.60 V) and high bias potentials (−0.60 to −0.80 V), the TiO2MF becomes more and more electrically conductive. In this case, the primary pathway of electrons is route B, where the electrons transport into the TiO2MF and then go into the electrolyte through the TiO2/electrolyte interface. The charge recombination takes place mainly on the TiO2 surface. Therefore, the measured values of Rct correspond to the charge-transfer resistance at the TiO2/electrolyte interface, or more precisely speaking the TiO2/dye/electrolyte interface. It is found that the Rct values of all tested DSCs at the intermediate and high bias potential range are in the following order: CW10 > SK6 + CW10 > SK6. The Rct order also represents the ability of suppressing the charge recombination. That is, the higher the Rct, the lower the charge recombination rate, which is in good agreement with the results of dark current (Figure 5a).
(1)
As shown in Figure 4b, SK6 shows an average of 74% IPCE at Soret and Q-band regions, whereas CW10 shows an average of 78% between 450 and 580 nm, giving similar JSC values for SK6 and CW10. On the other hand, an increase in IPCE response was observed in SK6 + CW10-based device over that of individual devices throughout the region 320−580 nm. This elevation in IPCE response was achieved by filling up the dip between Soret and Q-bands. The photovoltaic results reveal that a suitable immersion period and solvent could control the aggregation of SK6 on the surface of TiO2, and synergistic effect of a cosensitized device is pivotal to get enhanced PCEs. As listed in Table 2, the PCE of cosensitized device achieved around 77% that of device based on N719 dye under similar test conditions. Incident light was expected to diffuse and reflect upon contact with the surface of the DSC, which would significantly influence the cell performances for unmasked devices, and in such cases, the reliability of PCEs is a question mark. As shown in Figure S2 and Table S2, the elevated JSC values of unshielded/unmasked individual and cosensitized devices over that of shielded/masked devices are in conjunction with above-mentioned rationale.11 As expected, the cosensitized device achieved higher PCE than the individual dye-sensitized cells. We also measured the capacitances of the DSCs (Figure S4). The capacitance of cosensitized solar cell (SK6 + CW10) is similar to that of individual cells (SK6 and CW10), showing that the hysteresis effect due to capacitance is not an influential factor for the improved PCE of the cosensitized device.41 We further calculated the values of JSC from the IPCE data (Figure 4b) and tabulated them in Table 2 for comparison. A discrepancy was found when comparing the calculated values of JSC and the masked ones measured from the J−V curves. The measured JSC was about 10−15% higher than the values of JSC calculated from the IPCE data. This mismatch of 10−15% was below a threshold (20%) set for a comprehensively quantitative analysis of questionable published data, showing that the mismatch of JSC in our study is reasonable.42 The discrepancy can be explained by the difference in the illumination conditions such as light intensity.42 Because the light intensity of IPCE was much smaller than 1 sun illumination, the charge recombination under weak light became more severe and influenced the output current, resulting in a lower current density. Generally, the output current increases with increasing illumination intensity. However, a nonlinear relationship is often observed as the illumination intensity is boosted from a 2395
DOI: 10.1021/acsami.7b12960 ACS Appl. Mater. Interfaces 2018, 10, 2391−2399
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ACS Applied Materials & Interfaces
Figure 6. (a) Current density vs illuminance, (b) voltage vs illuminance, (c) PCE vs illuminance graphs of DSCs fabricated with mask (0.36 cm2) by SK6, CW10, and SK6 + CW10 dyes, and (d) Pin vs Pout graphs of corresponding cells under T5 light.
Table 3. Photovoltaic Parameters of DSCs Fabricated with SK6, CW10, and SK6 + CW10 Dyes under T5 Light Source with a Mask of 0.36 cm2 (Illuminance = 6000 lux) dyea SK6 CW10 SK6+CW10 N719
JSC (mA cm−2) 0.827 0.771 0.883 0.912
± ± ± ±
0.008 0.004 0.002 0.025
VOC (V) 0.585 0.674 0.646 0.652
± ± ± ±
FF
0.002 0.002 0.004 0.011
0.760 0.762 0.749 0.733
± ± ± ±
0.001 0.000 0.002 0.013
η (%)
Poutb,c (μW cm−2)
± ± ± ±
366.57 395.46 426.10 435.86
19.46 20.95 22.58 23.43
0.282 0.013 0.015 0.118
a Three cells were fabricated each time and the average parameters were tabulated along with standard deviation. bPower input of 6000 lx is equal to 1.86 mW cm−2. cPout was calculated as per the formula η = Pout/Pin. Photovoltaic data from T5 light source (300−6000 lx) was incorporated in Table S3.
In the aspect of commercial application under indoor light, we tested the PCEs for individual and cosensitized devices under T5 and LED light sources (300−6000 lx); the corresponding results are shown in Figure 6 and Table 3 (parameters under LED were furnished in Figure S3 and Table S4 in the Supporting Information). All three cells have shown maximum PCEs (ηmax) under 6000 lx from T5/LED light source; J−V curves of DSCs under T5 light source (6000 lx) were furnished in Figure 7. Performances of all the devices under T5 light were found to be higher than that under LED light. SK6-based device has obtained ηmax of ca. 19.46% (JSC = 0.827 mA cm−2, VOC = 0.585 V, and FF = 0.760), which equals a Pout of ca. 366 μW cm−2. The device based on CW10 has obtained ηmax of ca. 20.95% (JSC = 0.771 mA cm−2, VOC = 0.674 V, and FF = 0.762), corresponding to a Pout of ca. 395 μW cm−2. The cosensitized device has obtained ηmax of ca. 22.58% (JSC = 0.883 mA cm−2, VOC = 0.646 V, and FF = 0.749), which is equal to a Pout of ca. 426 μW cm−2. As shown in Tables S2 and S4, the Pout of individual dyes, either SK6 or CW10, at 300 lx of T5 light is less than that of Y1A1 (ca. 18.2 μA cm−2) or
Figure 7. J−V curves of SK6-, CW10-, and SK6 + CW10-based devices under T5 light source (6000 lx).
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ACS Applied Materials & Interfaces TY6 (ca. 15.8 μW cm−2), but the Pout of SK6 + CW10 at 600 lx is higher than that of Y1A1 or TY6 at 300 lx. This superior performance of cosensitized device over that of individual devices of SK6 or CW10 is attributed to the excellent spectral match with the emission wavelength of incident light (Figure S1 and Figure 4b). As seen in Figure 6, the photovoltaic data of cosensitized device are comparable to those of dye N719 under T5 light source. The large Pout expressed by device based on SK6 + CW10 would allow DSC modules to be used in an indoor environment. The photovoltaic data under LED light source were given in Table S3, where the trend is like that of device results obtained under T5 light source. The corresponding J−V curves and PCEs were roughly proportional to Pin of incident light. An aging test of cosensitized device based on SK6 + CW10 was performed in a programmable oven to evaluate the stability of the DSC under working conditions. To overcome the volatile problem of the AN/VN-based electrolyte, a low-volatile MPN-based electrolyte was used in the stability test. As shown in Figure 8, the PCE of cosensitized cell approximately
that of cells fabricated from individual dyes. This contributes enhanced JSC and PCE values for SK6 + CW10-based cell. The PCE of SK6 reported in this work under 1 sun illumination is highest as compared to those reported using the same dye. All three kinds of cells SK6, CW10, and SK6 + CW10 showed excellent cell performances under dim light (T5 and LED). The SK6 + CW10-based device with most suitable spectral response to the indoor lights throughout the visible region performed in an excellent manner. The overall cell performance of 22.58% was achieved under T5 light source (6000 lx), which is undoubtedly a remarkable achievement for DSC based on facile dyes. The photostability experiment results reveal that the cosensitized cell has remarkable photostability for a period of 600 h under 1 sun illumination (retained >90% of its initial PCE). This achievement would attract attention from the researchers looking for facile dyes toward commercialization of DSCs. We are now developing simple dyes for indoor applications.
4. EXPERIMENTAL SECTION 4.1. General Information. Reagents and chemicals were purchased from available commercial sources and used without any further purification unless otherwise noted. CH2Cl2 and trimethylamine were dried using CaH2, THF was dried using sodium metal, and a water/oxygen free environment was detected by a benzophenone ketyl study. 1H and 13C NMR spectroscopy for synthesized dyes were performed on a Varian spectrometer at 400 and 100 MHz, respectively. Mass spectra were recorded on a Bruker APEX II spectrometer operating in the positive ion detection mode. UV−visible absorption measurements were performed on a Varian Cary 50 spectrophotometer and emission spectra were recorded on a JASCOFP 6000 fluorescent spectrophotometer. Electrochemical studies were carried out on CH Instruments Model 750A. Homemade threeelectrode cells were used to analyze redox potentials of sensitizers, equipped with a BAS glassy carbon (0.07 cm2) disk as the working electrode, a platinum wire as the auxiliary electrode, and a homemade Ag/AgCl (supersaturated H2O solution) as the reference electrode. The reference electrode Ag/AgCl was separated from the bulk solution by a double junction filled with an electrolyte solution (0.1 M TBAPF6). Potentials are reported against Ag/AgCl and calibrated the accuracy with respective to standard ferrocene/ferrocenium (Fc/Fc+) couple, which occurs at E1/2 = +0.63 V vs NHE. Glassy carbon working electrode was polished with 0.03 μm alumina on Buehler felt pad and washed with deionized water and dry THF prior to each experiment; reproducibility of individual potential values was within ±5 mV. 4.2. Device Fabrication. Fluorine-doped tin oxide (FTO) glass (13 Ω/□, 3.1 mm thick, 8% haze, Nippon Sheet Glass Co., Ltd., Japan) was cleaned sequentially in two ultrasonic baths containing 4% glass cleaner (PK-LCG545, Parker corporation Co., Ltd., Japan) and deionized water respectively for 30 min. After cleaning, the FTO glass was used as the substrate for TiO2 mesoporous film, and dye adsorption. The FTO glass substrate was first immersed in a 40 mM TiCl4 aqueous solution at 70 °C for 60 min, followed by rinsing with deionized water and ethanol and sintering in an oven at 450 °C for 30 min to form a compact TiO2 underlayer on the FTO surface. A TiO2 nanocrystalline mesoporous film was screen-printed on FTO glass using a commercial TiO2 paste (Ti-Nanoxide T/SP, Solaronix SA, Switzerland) with a thickness and area of 12 μm and 0.16 cm2, respectively. A TiO2 scattering film was then screen-printed on the mesoporous film using a commercial TiO2 paste (PST-400C, JGC Catalysts and Chemicals Ltd., Japan) with a thickness of 2 μm. After screen printing, the TiO2-coated FTO glass was sintered in an oven sequentially at 325 °C for 5 min, 375 °C for 5 min, 450 °C for 15 min, and 500 °C for 15 min to form a TiO2 nanocrystalline network with an anatase structure. Subsequently, the TiO2-coated FTO glass was again immersed in a 40 mM TiCl4 aqueous solution at 70 °C for 60 min,
Figure 8. Photovoltaic properties of SK6 + CW10-based device over a period of 600 h.
remained at a stable value during the 600 h aging test consisting of heating at 35 °C for 250 h, 45 °C for 250 h, and 35 °C for 100 h, showing that the device based on SK6 + CW10 was stable in the simulated aging environment. It is found that the PCE of DSC fabricated with SK6 + CW10 retained more than 90% of its initial PCE (from 3.90% to 3.54% during the period of 600 h). A fluctuant behavior is found in the FF and VOC during the aging test. Detailed stability assessment of the SK6 + CW10 cosensitizer requires more investigations and will be discussed in the near future.
3. CONCLUSION In conclusion, facile dyes for commercial application were synthesized in gram scale and tested for DSCs under both outdoor and indoor light sources. The structurally simple dye SK6 was determined so far to have severe aggregation problems on TiO2 surface, but with suitable solvent and immersion period, the DSC of dye SK6 can perform well. The organic dye CW10 was designed to block the electrolyte contact with TiO2 surface and successfully achieved the enhanced VOC and PCE over that of SK6. The dip in absorption between Soret and Qband regions shown by SK6 was successfully compensated by utilizing the suitable dye; that is, CW10, as a result, the cosensitized cell has shown better IPCE response compared to 2397
DOI: 10.1021/acsami.7b12960 ACS Appl. Mater. Interfaces 2018, 10, 2391−2399
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ACS Applied Materials & Interfaces followed by a postsintering treatment at 450 °C for 30 min to carry out post-treatment of the photoanode to enhance interconnectivity in the mesoporous TiO2 film. The dye adsorption was performed by immersing the TiO2-coated FTO glass in a dye solution for 4 h. Four dye solutions were prepared and the composition of cosensitized dyes was listed in Table 2. Another cleaned FTO glass was used as the substrate of the counter electrode and was coated with a catalytic polyvinylpyrrolidone (PVP)-capped Pt nanocluster layer using a twostep dip-coating process.43 The postsintering condition for the Pt nanocluster layer was 500 °C for 30 min. The dye-adsorbed TiO2 photoanode and the PVP-capped Pt counter electrodes were assembled in a face-to-face manner and sealed with a 25 μm thick Surlyn film having a 0.36 cm2 open window by hot-pressing at 115 °C for 10 s. Finally, a proper amount of liquid electrolyte (0.2 M LiI, 0.05 M I2, 0.3 M PMII, 0.5 M TBP, and 0.2 M TBAI in acetonitrile/ valeronitrile, 85:15, v/v) was injected into the sealed cell via a predrilled hole on the counter electrode side to fill the gap between the two electrodes. A mask with the size of 0.36 cm2 was attached to the illumination side of the cell prior to each measurement. 4.3. Photovoltaic Measurement. The assembled DSCs were evaluated under AM 1.5G (1 sun, 100 mW cm−2) illumination with a solar simulator (YSS-E40, Yamashita Denso Corp, Japan) and their PV outputs (photocurrent and voltage) were measured using a computercontrolled digital source meter (Model 2400, Keith-ley Instrument Inc., USA). The spectra of the incident photon-to-electron conversion efficiency (IPCE) were measured using a solar cell QE/IPCE measurement system (QE-3000, Titan Electro-Optics Co., Ltd., Taiwan). Electrochemical impedance spectroscopy (EIS) analysis was performed with various bias potentials in dark conditions using a potentiostat instrument (Autolab PGSTAT302N, Metrohm Autolab B.V., Netherlands). The EIS setting contained an alternating current amplitude of 10 mV with a frequency range of 100 kHz to 0.1 Hz, a two-electrode configuration for the DSC, and the photoanode as a working electrode. 4.4. Measurements under Dim Light. Dim light measurements were carried out as per the procedures of our documented report;43 the J−V measurements were carried out under customized design composed of height-tunable lift loaded with standard office lightning of T5 fluorescent lamp (FH14D-EX/T, China Electric Mfg Corporation, Taiwan) and a calibrated spectroradiometer embedded in an underlying system (ISM-Lux, Isuzu Optics, Japan). The acquired illuminance value was attained by altering T5 lamp-lifting platform to the moderate position and incessantly confirmed by a spectroradiometer until it reached firmly stable conditions. After that, we put the DSC device at the upper site of the spectroradiameter and measured the J−V curves via a computer-controlled digital source meter (Keithley 2400C, USA) under various dim light illumination. The J−V curve measurements were performed from short circuit to open circuit (normal scan) and vice versa (reverse scan) with a scan rate of 55 mV/s, a normal scan direction from −0.05 to 0.85 V, and a sampling time of 0.1 s.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the Ministry of Science and Technology (MOST) and Ministry of Education in Taiwan for financial support.
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b12960.
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REFERENCES
Experimental procedures, figures, and tables (PDF)
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