Cosensitization of Structurally Simple Porphyrin and Anthracene

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Co-sensitization of Structurally Simple Porphyrin and Anthracene-based Dye for Dye-Sensitized Solar Cells Kamani Sudhir K Reddy, Yen-Chiao Chen, Chih-Chung Wu, ChiaWei Hsu, Ya-Ching Chang, Chih-Ming Chen, and Chen-Yu Yeh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12960 • Publication Date (Web): 27 Dec 2017 Downloaded from http://pubs.acs.org on December 28, 2017

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ACS Applied Materials & Interfaces

Co-sensitization of Structurally Simple Porphyrin and Anthracene-based Dye for Dye-Sensitized Solar Cells Kamani Sudhir K. Reddy,a Yen-Chiao Chen,b Chih-Chung Wu,b Chia-Wei Hsu,a Ya-Ching Chang,b Chih-Ming Chen,*b and Chen-Yu Yeh*a a

Department of Chemistry and Research Center for Sustainable Energy and Nanotechnology, National Chung Hsing University, Taichung 402, Taiwan

b

Department of Chemical Engineering, National Chung Hsing University, Taichung 402, Taiwan

KEYWORDS. co-sensitization, dim light, dye-sensitized solar cells, organic dye and porphyrin.

ABSTRACT

Since their introduction, dye-sensitized solar cells (DSCs) have achieved a huge success at 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 co-adsorbent / additive) for SK6 and CW10,

ACS Paragon Plus Environment

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respectively, under AM 1.5G illumination. The power conversion efficiency (PCE) of SK6 reported in this work is the highest than ever reported, this is achieved by optimizing the adsorption of SK6 on TiO2 photoanode using the most suitable solvent and immersion period. Co-sensitization 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 co-sensitized devices at TiO2/dye/electrolyte interface were examined via electrochemical impedance spectroscopy. In order to understand the cell performances under ambient light conditions, we soaked individual and co-sensitized devices under T5 and LED light sources in the range of 300 – 6000 lux. The PCE of ca. 22.91% under T5 light (6000 lux) with JSC = 0.883 mA cm-2, VOC = 0.646 V and FF = 0.749 was noted for the co-sensitized device, which equals to 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.

1. Introduction Owing to 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 ~ 20 to ~ 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


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cells are not suitable as portable power sources for indoor application, due to 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 at 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 DSCs under simulated 1 sun illumination. In contrast, under dim light conditions their capability of absorbing photons beyond 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 front-runners to surpass silicon solar cells. Though PSCs exhibit high PCEs (~ 20 to ~ 22%), the long-term stability must be improved before practical application appears. Therefore, at present DSCs seem to be 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, 1213

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. Incident light of indoor and outdoor environments is distributed between the range of ~ 200 to ~ 100,000 lux (SI unit of illumination). At present time, most of the light sources at domestic or commercial spaces are revolutionized by fluorescent lamps and light emitting diodes (300 – 10,000 lux).14 The emission spectra of indoor lights cover between 450 - 700 nm, and most of the dyes used in DSCs show excellent absorption at this region, which means they can harvest a major share of incident light-to-electricity. For instance, Y1A1 (19.3% at 300 lux), TY6 (28.6% at 6000 lux) and D35 (28.9% at 1000 lux) are in strong agreement with above


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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 (IoT) and wireless network systems (WNS) 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 micro-watt power (µW) is sufficient to charge these devices,16 a DSC working under 200 lux 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 – 20,000 lux. Aforementioned Pout of DSC under indoor light indicates that it could power the devices for IoTs and WNS.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 Perylenes21 and anthracenes9 have been shown to be the 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 infra-red regions (NIR).2, 12, 22 Among aforementioned sensitizers, porphyrin and anthracene-based dyes were considered to be easily accessible, owing to their ease of functionalization and moderate yields to get target dyes from commercially available starting materials. Due to this reason, 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


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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 at visible region (350 – 650 nm).

30

Thus, a combination of these two

categories is a good choice to harvest the light at 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 – 580 nm.9, 30-32 Therefore, a co-sensitization 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 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 lux), which equals to Pout of 426 µW cm-2, and are sufficient to charge devices for IoTs and WNS.

Figure 1. Molecular structures of SK6 and CW10.


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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 metallated using Zn(OAc)2, followed by hydrolysis under alkaline conditions yielded 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 4-ethynylbenzoic acid to yield CW10 in gram scale.9, 31 The synthetic routes are such simple that they are commercially viable to be synthesized in bulk quantity. The detailed experimental procedures of synthesis along with characterization were given in SI.

Scheme 1. Synthetic route of SK6. (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%.


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Scheme 2. Synthetic route of CW10. (i) Bis(4-(dodecyloxy)phenyl)amine, cat. Pd(OAc)2, P(tBu)3, NaOtBu, dry toluene, 72%. (ii) NBS, CH2Cl2, 81% (iii) 4-ethynylbenzoic acid, cat. Pd2(dba)3, AsPh3, THF/NEt3, 62%. 2.2. Photophysical and electrochemical properties

Figure 2. UV-Visible absorption spectra of SK6 (purple) and CW10 (orange) recorded in THF at 25 0C. The extinction coefficient value of CW10 was magnified 10 times and the Q-band region of SK6 (dot) was magnified 5 times. 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 x 103 M-1 cm-1) and two moderate Q bands at 555 nm (19 x 103 M1

cm-1) and 594 nm (8 x 103 M-1 cm-1). The organic dye CW10 shows broad absorption peaks at


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401 nm (20 x 103 M-1 cm-1), 420 nm (18 x 103 M-1 cm-1) and 507 nm (13 x 103 M-1 cm-1). As seen in Figure 2, the broad absorption of CW10 between 450 – 550 nm is such important that, a co-sensitized solution of SK6 and CW10 would fill up the dip of absorption shown by SK6 at 450 – 550 nm. Therefore, a co-sensitization of these two dyes on TiO2 surface would harvest the incident light-to-electrons in an efficient manner than 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 at indoor environment (i.e. either T5 or LED). Table 1. Photophysical and electrochemical properties of SK6 and CW10. Absorption (ε), a

Emission

EHOMO

ELUMO

Ered

E0-0

nm (103 M-1 cm-1)

(nm)

(V vs NHE)

(V vs NHE) b

(V) b

(eV) c

SK6

423.56 (473.63), 555.21 (19.68), 594.74 (8.61)

601, 651

1.03

-1.03

-1.33

2.05

CW10

401.47 (20.32), 420.21 (1.80), 506.76 (1.34)

648

0.74

-1.48

-1.65

2.16

Dye

a

Molar absorption coefficients were calculated from UV-Visible absorption spectra recorded in THF at 25 0C. b Oxidation and reduction potentials were obtained from cyclic voltammetry experiments recorded at 25 0C in dry THF as a solvent and 0.1 M TBAPF6 as an electrolyte. c Optical band gaps were obtained from the formula E0-0 = 1240 / λonset.

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.


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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 (TBAPF6) as a supporting electrolyte (Figure 3, Table 1). Figure 3a shows reversible waves for the first oxidation of both the dyes. As seen in Figure 3b, positions of HOMO energy levels (EHOMO) of both the dyes are positive than the potential of redox mediator (I-/I3-), and the driving forces are calculated as 0.63 eV 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 than fermi level of TiO2 conduction band (TiO2 CB) and the calculated driving forces are 0.53 eV 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 well perform 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 respective to immersion period. Therefore, we found that soaking the cells for 4 hours is the optimum choice


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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 In order to test the reproducibility of the highest PCEs of individual and co-sensitized devices, as much as seven repetitions were performed with a slight deviation in PCEs for all three cells.

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. Dye a,b

JSC (mA cm-2)

VOC (V)

FF

η (%)

JSC (mA cm-2) c

SK6

11.30 ± 0.134

0.689 ± 0.001

0.695 ± 0.163

5.42 ± 0.060

9.67

CW10

10.76 ± 0.051

0.739 ± 0.000

0.722 ± 0.025

5.75 ± 0.216

9.47

SK6 + CW10

12.04 ± 0.011

0.732 ± 0.002

0.715 ± 0.000

6.31 ± 0.019

10.91

N719

14.79 ± 0.247

0.782 ± 0.009

0.717 ± 0.010

8.29 ± 0.095

13.17

PCEs were tested with (0.36 cm2). b Three 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. c Calculated from IPCE. a


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The J-V curves and IPCE spectra of individual and co-sensitized 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, 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, 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 blocks the electrolyte contact into TiO2 surface, leading to enhancement of VOC value. The co-sensitized 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 co-sensitized device would harvest more photons in this region, in return the corresponding device has obtained enhanced JSC value. Following equation explains the relation between IPCE response and JSC value: = ×

(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


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aggregation of SK6 on the surface of TiO2, and synergistic effect of a co-sensitized device is pivotal to get enhanced PCEs. As listed in Table 2, the PCE of co-sensitized 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

co-sensitized

devices

over

that

of

shielded/masked devices are in conjunction with above mentioned rationale.11 As expected, the co-sensitized devices achieved higher PCE than the individual dye-sensitized cells. We also measured the capacitances of the DSCs (Fig. S4). The capacitance of co-sensitized 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 one sun illumination, the charge recombination under weak light became more severe and influenced the output current, resulting to a lower current density. Generally, the output current increases with increasing illumination


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intensity. However, a nonlinear relationship is often observed as the illumination intensity is boosted from a low intensity (300 - 6000 lux) to a relatively higher intensity (one sun > 100,000 lux). Therefore, a mismatch of the output current between the measured JSC and the calculated one is expectable.

Figure 5. a) J-V curves of SK6, CW10 and SK6 + CW10 based devices measured under dark condition. 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. Figure 5a shows J-V curves of DSCs in dark condition, 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 a 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 condition. The charge transfer resistance (Rct) on the dye-coated TiO2 photoanode obtained from the impedance spectra by fitting to the


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equivalent circuit are plotted in Figure 5b. In dark condition, the applied potential drives the electrons to move from the FTO substrate into the electrolyte. At a low bias potential range (0.30 V 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, i.e. the higher the Rct the lower the charge recombination rate, which is in good agreement with the results of dark current (Figure 5a).


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Figure 6. a) Current density vs-, b) Voltage vs-, 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. In the aspect of commercial application under indoor light, we tested the PCEs for individual and co-sensitized devices under T5 and LED light sources (300 – 6000 lux), the corresponding results are shown in Figure 6 and Table 3 (parameters under LED were furnished in Figure S3, and Table S4 in SI). All the three cells have shown maximum PCEs (ηmax) under 6000 lux from T5 / LED light source, J-V curves of DSCs under T5 light source (6000 lux) were


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furnished in Figure 7. Performances of all the devices under T5 light were found to be higher than that of 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 to 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 co-sensitized 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 Table S2 and Table S4, the Pout of individual dyes, either SK6 or CW10, at 300 lux of T5 light is less than that of Y1A1 (ca. 18.2 µA cm-2) or TY6 (ca. 15.8 µW cm-2), but the Pout of SK6 + CW10 at 600 lux is higher than that of Y1A1 or TY6 at 300 lux. This superior performance of co-sensitized 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 co-sensitized 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 at indoor environment. The photovoltaic data under LED light source were given in Table S3, where the trend is as that of device results obtained under T5 light source. The corresponding J-V curves and PCEs were roughly proportional to Pin of incident light.


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Figure 7. J-V curves of SK6, CW10 and SK6 + CW10 based devices under T5 light source (6000 lux). 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). Dye a

JSC (mA cm-2)

VOC (V)

FF

η (%)

Pout (µW cm-2) b, c

SK6

0.827 ± 0.008

0.585 ± 0.002

0.760 ± 0.001

19.46 ± 0.282

366.57

CW10

0.771 ± 0.004

0.674 ± 0.002

0.762 ± 0.000

20.95 ± 0.013

395.46

SK6+CW10

0.883 ± 0.002

0.646 ± 0.004

0.749 ± 0.002

22.58 ±0.015

426.10

N719

0.912 ± 0.025

0.652 ± 0.011

0.733 ± 0.013

23.43 ± 0.118

435.86

a

Three cells were fabricated each time and the average parameters were tabulated along with standard deviation. b Power input of 6000 lux is equal to 1.86 mW cm-2. c Pout was calculated as per the formula η = Pout/Pin. Photovoltaic data from T5 light source (300 – 6000 lux) was incorporated in Table S3.

An aging test of co-sensitized 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 co-sensitized cell approximately


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remained at a stable value during the 600 h aging test consisting of heating at 35 οC for 250 h, 45 ο

C for 250 h, and 35 οC 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 co-sensitizer requires more investigations and will be discussed in the near future.

Figure 8. Photovoltaic properties of SK6 + CW10 based device over a period of 600 h. 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 blamed 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 Q band regions shown by SK6 was successfully compensated by utilizing the suitable dye, i.e. CW10, as


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a result, the co-sensitized cell has shown better IPCE response over 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 the 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 excellent. The overall cell performance of 22.58% was achieved under T5 light source (6000 lux), which is undoubtedly a remarkable achievement for DSC based on facile dyes. The photostability experiment results revel that the co-sensitized 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 towards 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 water/oxygen free environment was detected by benzophenone ketyl study. 1H and 13C NMR spectroscopy for synthesized dyes were performed on a Varian spectrometer at 400 MHz and 100 MHz respectively. Mass spectra were recorded on a Bruker APEX II spectrometer operating in the positive ion detection mode. UVVisible absorption measurements were performed on a Varian Cary 50 spectrophotometer and emission spectra were recorded on a JASCO-FP 6000 fluorescent spectrophotometer. Electrochemical studies were carried out at CH Instruments Model 750A. Homemade three-


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electrode 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 (super saturated H2O solution) 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 firstly 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


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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, followed by a post sintering 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 co-sensitized 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 two-step dip-coating process.43 The post sintering condition for the Pt nanocluster layer was 500 °C for 30 min. The dyeadsorbed TiO2 photoanode and the PVP-capped Pt counter electrodes were assembled face-toface 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 pre-drilled 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 DSSCs 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 computer-controlled 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


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spectroscopy (EIS) analysis was performed with various bias potentials in dark condition using a potentiostat instrument (Autolab PGSTAT302N, Metrohm Autolab B.V., Netherlands). The EIS setting contained an AC amplitude of 10 mV with a frequency range of 100 kHz to 0.1 Hz, a two-electrode configuration for the DSSC, 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 measurement were carried out under customized design composed of heighttunable lift loaded with standard office lightning of T5 fluorescent lamp (FH14D-EX/T, China Electric Mfg Corporation, Taiwan) and a calibrated spectroradiometer embed at 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 spectroradiometer until it reached firmly stable condition. 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 V to 0.85 V, and a sampling time of 0.1 s. ASSOCIATED CONTENT Supporting Information. A Microsoft word document that consists of experimental procedures, Figures and Tables was given as Supporting information. AUTHOR INFORMATION


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Corresponding Author E-mail: [email protected]; [email protected] Author Contributions The manuscript was written through the contributions of all authors. All the authors have given approval to the final version of this manuscript. ACKNOWLEDGMENT The authors thank the Ministry of Science and Technology (MOST) and Ministry of Education in Taiwan for financial support. REFERENCES 1. O'Regan, B.; Graetzel, M., A Low-Cost, High-Efficiency Solar Cell Based on DyeSensitized Colloidal TiO2 Films. Nature 1991, 353, 737-740. 2. Mathew, S.; Yella, A.; Gao, P.; Humphry-Baker, R.; Curchod Basile, F. E.; AshariAstani, N.; Tavernelli, I.; Rothlisberger, U.; NazeeruddinMd, K.; Grätzel, M., Dye-Sensitized Solar Cells With 13% Efficiency Achieved through the Molecular Engineering of Porphyrin Sensitizers. Nat. Chem. 2014, 6, 242-247. 3. Kakiage, K.; Aoyama, Y.; Yano, T.; Oya, K.; Fujisawa, J.-I.; Hanaya, M., HighlyEfficient Dye-Sensitized Solar Cells With Collaborative Sensitization by Silyl-Anchor and Carboxy-Anchor Dyes. Chem. Commun. 2015, 51, 15894-15897. 4. Kakiage, K.; Aoyama, Y.; Yano, T.; Oya, K.; Kyomen, T.; Hanaya, M., Fabrication of A High-Performance Dye-Sensitized Solar Cell With 12.8% Conversion Efficiency Using Organic Silyl-Anchor Dyes. Chem. Commun. 2015, 5, 6315-6317. 5. Shin, S. S.; Yeom, E. J.; Yang, W. S.; Hur, S.; Kim, M. G.; Im, J.; Seo, J.; Noh, J. H.; Seok, S. I., Colloidally Prepared La-Doped BaSnO3 Electrodes for Efficient, Photostable Perovskite Solar Cells. Science 2017, 356, 167-171. 6. Apostolou, G.; Reinders, A.; Verwaal, M., Comparison of the Indoor Performance of 12 Commercial PV Products by A Simple Model. Energy Sci. Eng. 2016, 4, 69-85. 7. Liu, T.; Li, Y., Photocatalysis: Plasmonic Solar Desalination. Nat. Photon. 2016, 10, 361362. 8. Liu, Y.-C.; Chou, H.-H.; Ho, F.-Y.; Wei, H.-J.; Wei, T.-C.; Yeh, C.-Y., A Feasible Scalable Porphyrin Dye for Dye-Sensitized Solar Cells under One Sun and Dim Light Environments. J. Mater. Chem. A. 2016, 4, 11878-11887. 9. Tingare, Y. S.; Vinh, N. S.; Chou, H.-H.; Liu, Y.-C.; Long, Y.-S.; Wu, T.-C.; Wei, T.-C.; Yeh, C.-Y., New Acetylene-Bridged 9,10-Conjugated Anthracene Sensitizers: Application in Outdoor and Indoor Dye-Sensitized Solar Cells. Adv. Energy. Mater. 2017, 7, 1700032.


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Table of contents / Graphical abstract


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Cosensitization of Structurally Simple Porphyrin and Anthracene

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