Function of Tetrabutylammonium on High-Efficiency Ruthenium

Jul 1, 2019 - We had reported previously that DSC sensitized with a heteroleptic ...... Wang, H.; Peter, L. M. Influence of electrolyte cations on ele...
0 downloads 0 Views 992KB Size
Download PDF
This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.

Article Cite This: ACS Omega 2019, 4, 11414−11423

http://pubs.acs.org/journal/acsodf

Function of Tetrabutylammonium on High-Efficiency Ruthenium Sensitizers for Both Outdoor and Indoor DSC Application The-Duy Nguyen, Chun-Han Lin, Chi-Lun Mai, and Chun-Guey Wu* Department of Chemistry and Research Center for New Generation Light Driven Photovoltaic Modules, National Central University, Jhong-Li, 32001 Taoyuan, Taiwan, ROC

Downloaded via 95.181.217.165 on July 29, 2019 at 09:28:26 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: The function of tetrabutyl ammonium ions (TBA+) in a sensitizer used in dye-sensitized solar cells (DSC) is contradictory. TBA+ can reduce unwanted charge-recombination by protecting the TiO2 surface and reduce dye aggregation, enhancing the photovoltaic performance. It will also compete with the dye-loading on the TiO2 film, decreasing the short-circuit current density of the cell. Three ruthenium sensitizers (DYE III, DUY11, and DUY12 containing two H+, one H+/one TBA+, and two TBA+, respectively) were prepared to systematically investigate the function of TBA+ in a dye for DSC under both standard sunlight and indoor illumination. The optical properties and frontier orbital energy level of the sensitizers are not influenced significantly by the number of TBA+. Under the standard 1 sun illumination, DSCs based on DUY11 (containing one H+ and one TBA+) achieved the highest power conversion efficiency (PCE) of 11.47%. Overall, optimized DSCs sensitized by the three ruthenium dyes all have the PCE over 10%, which is higher than that (9.95%) of N719-dyed cell fabricated at the same conditions. Under the illumination of a light emitting diode (LED), DSCs sensitized by DUY11 also have the highest efficiency of 19%. Furthermore, DUY12 with two TBA+ exhibits superior photovoltaic performance compared to a DYE III (containing two H+ in the anchoring ligands)-dyed cell; although these two dyes have similar photovoltaic performance under standard 1 sun lighting. The important function of TBA+ in reducing the charge recombination (by protecting TiO2 surface and avoiding dye aggregation) of a DSC under indoor lighting (when small number of electrons were excited by weak light) is also revealed.

■

replacing the H+ in the sensitizer with another less acidic ion such as tetrabutylammonium ion (TBA+ ) (called dye deprotonation) will result in higher VOC, but lower JSC (due to less efficient electron-injection from the photoexcited dye to TiO2) of the corresponding cell. Consequently, there is an optimal degree of deprotonation for a sensitizer to maximize the product of JSC and VOC of the cell to reach the highest PCE. TBA+ ion, similar to GuSCN (guanidinium thiocyanate), known as an additive in the electrolyte solution can cover the TiO2 surface, suppress the charge-recombination (electron on TiO2 reacts with the oxidant species (I3−) in the electrolyte) without changing significantly the TiO2 ECB level due to its dilute charge density.6,7 Therefore, the drawback of protonation on the TiO2 film can be reduced by replacing the acidic protons with TBA+ ions in the sensitizer. This strategy was proved successfully in two representative ruthenium-based dyes, N38 and N749 (black dye).9 In 2003, Nazeeruddin10 et al. investigated the photovoltaic properties of five N3-based

INTRODUCTION Almost three decades have passed since the first dye-sensitized solar cell (DSC) using the trimetric ruthenium complex as a sensitizer was announced by O’Regan and Grätzel in 1991.1 Most of the small-area lab-testing DSCs show a middling ( DUY11 > DUY12) of the absorption intensity when adsorbed on the TiO2 film is contrary to the trend (DYE III < DUY11 < DUY12) of their absorption coefficient (when dissolved in EtOH). This is due to the differences in the dye-loadings which will be discussed more in the later paragraphs. Furthermore, the absorption patterns of the three dye-loaded TiO2 although are nearly alike, the change in the intensity ratio of low-energy band (EL)/high energy band (EH) for DUY12 when adsorbed on TiO2 is not the same as that of DYE III or 11416

DOI: 10.1021/acsomega.9b00431 ACS Omega 2019, 4, 11414−11423

ACS Omega

Article

efficient pathway for the electron to inject from the excited sensitizer into the TiO2 conduction band via an anchoring ligand. These orbital distributions indicate that the ligand-toligand charge-transfer (LLCT) transitions may be also important for the light-harvesting ability of the sensitizer. The lowest-energy bands in the absorption spectra (see Figure 2) originated from the mixed metal/LLCT [(M/L)LCT] transitions, not solely from the metal-to-ligand charge-transfer (MLCT) transition as reported in several ruthenium sensitizers.38,43 The HOMO locates on the ruthenium and two thiocyanate ligands; therefore, when the sensitizer was oxidized after absorbing the photo and injecting the electron to TiO2, the dye cation with a β-LUSO (β-lowest unoccupied spin orbital, which is the regeneration site of the dye cation)44 was placed on the ruthenium and two thiocyanate ligands (see Figure S4, Supporting Information). The β-LUSO significantly distributes on the two soft sulfur atoms of the thiocyanate ligands along with the HOMO energy that is 0.11 V more positive than the redox potential of I2•−/I− couple guaranteeing efficient dye-regeneration. Combining the large dye absorption coefficient, the devices based on the three dyes were expected to exhibit high JSC values. The experimental and calculated absorption spectra along with the calculated oscillator strengths (f) and the electron density difference maps of DYE III dye are displayed in Figure S5, Supporting Information. The calculated transition assignments are summarized in Table S2, Supporting Information. The calculated absorption spectrum is quite close to the experimental absorption spectrum, implying that the TD-DFT calculated data can be used to explain the electron transitions in these three sensitizers. The S3, S4, S5, and S13 transitions composed of both metal-to-ligand and ligand-to-ligand charge transitions are the evidence to support that the lowest-energy band (∼540 nm) originates from the (M/L)LCT transitions but not from solely the MLCT transition. The origin of the S24, S27, and S38 transitions suggests that the middle energy band (∼365 nm) contains also the (M/L)LCT transition, even though the contribution of LLCT is superior to the (M/ L)LCT transition. On the other hand, the highest-energy band (∼307 nm) is assigned simply to the π → π* transition of the anchoring ligand.3,38 The transitions S3, S4, S5, S24, S27, and S38 all contribute to the photocurrent production when applied in DSC because these transitions are from the metal/ ligand to the anchoring ligand when the dye molecules are excited by photos. Transition S13 having 23% from the HOMO to the LUMO + 4 (which locates mainly on the anchoring ligand) also contributes to the photocurrent production. From the oscillator strengths of these positive transitions, high current density of the cell based on these dyes is expected. Photovoltaic Performance of the DSCs Sensitized by Three Dyes under Standard 1 Sun Illumination. To study the function of TBA+ in the anchoring ligand on the photovoltaic performance of the sensitizers, iodide electrolytes containing various amounts (0, 0.3, 0.5, and 0.7 M) of 4-tertbutylpyridine (TBP) were used for the first try because TBP has the function similar to TBA+ in the sense of TiO2 surface protection. The purpose of this study is to see if TBA+ can totally replace TBP in the electrolyte to improve the cell photovoltaic performance. The photovoltaic parameters of the resulting cells are summarized in Table 1. In general, the Jsc of the cells decreases when the concentration of TBP in the electrolyte increases for all three sensitizers. It is probably due

Table 1. Photovoltaic Parameters of DSCs Based on Three Ru-Sensitizers Using Different Concentrations of TBP in the Electrolytea dye

[TBP] (mol/l)

JSC (mA/cm2)

VOC (V)

FF

ηmax (%)

DYE III

0.7 0.5 0.3 0 0.7 0.5 0.3 0 0.7 0.5 0.3 0

17.41 18.41 19.12 20.06 17.30 18.16 19.09 19.82 16.90 17.68 18.75 19.59

0.78 0.77 0.77 0.65 0.78 0.78 0.78 0.66 0.77 0.77 0.80 0.68

0.75 0.74 0.69 0.64 0.75 0.75 0.70 0.66 0.75 0.74 0.71 0.67

9.97 10.51 10.13 8.30 10.12 10.61 10.47 8.70 9.76 10.16 10.56 8.96

DUY11

DUY12

a Cell fabrication parameters: TiO2 film thickness:14 (11 + 3) μm; dye solution: 0.2 mM corresponding dye and 20 mM chenodeoxycholic acid (CDCA) in a mixture of acetonitrile/isopropanol/DMSO (volume ratio 1:1:3); dye loading time: 14 h; electrolyte: 0.05 M LiI, 1.0 M butyl-3-methylimidazolium iodide (BMII), 0.03 M I2, 0.1 M guanidinium thiocyanate (GuSCN), various amount of TBP in a mixture of acetonitrile/valeronitrile (volume ratio 85:15).

to TBP hindering dye regeneration by blocking the electrolyte to contact the oxidized dyes or increasing the viscosity of the electrolyte solution, slowing down the I3− diffusion. Or TBP reduces the electron injection driving force because it raises the TiO2 ECB. The effect of TBP on the Voc of the cells for the three sensitizers is not the same. The DYE III (containing no TBA+ ions)-based cell has higher Voc when the electrolyte contains more TBP due to the better protection of TiO2 surface by TBP. As a result the highest efficiency cell was obtained when 0.5 M TBP was presented in the electrolyte solution. On the other hand, the highest efficiency of the DUY12 (containing two TBA+ ions) dyed cell was found when the electrolyte solution contains 0.3 M TBP. The data prove that TBA+ in a sensitizer can function as TBP additive in the electrolyte to reduce the charge recommendation by protecting the uncovered TiO2 surface. On the other hand, the side-effect of the adsorbed TBP, which raises the TiO2 conduction band and decreases the Jsc of the cell, will not occur in TBA+. This function of TBA+ is important especially when the dye molecule has high LUMO energy level. Furthermore, the absorption data illustrated in Figure 2A show that dyecontained TBA+ has higher absorption coefficient than the analogue-contained H+ due to the enhanced conjugation length (related to the deprotonation). This is also the advantage of replacing H+ by TBA+ in dye molecules. Nevertheless, TBP mended the uncovered TiO2 surface after the dye molecules were loaded; therefore, it will not affect the dye-loading but TBA+, which adsorbed on TiO2 with dye molecules simultaneously, will lower the dye-loading, and decrease the Jsc of the DSC. The photovoltaic data listed in Table 1 clearly reveal the dilemma of using sensitizers containing TBA+. As a result, the best photovoltaic performance dye with two anchoring groups was generally the one containing one TBA+ and one H+ (compared to those having two TBA+ or two H+). Data displayed in Table 1 also indicated that the function of TBA+ ion in the dye molecule is very close to TBP additive in the electrolyte in the sense of TiO2 protection Therefore, after 11417

DOI: 10.1021/acsomega.9b00431 ACS Omega 2019, 4, 11414−11423

ACS Omega

Article

the N719-based cell exhibits an efficiency of only 9.95%. The differences in the photovoltaic performance between them result from the content of TBA+ in the dye molecules. TBA+ also occupies the TiO2 surface, leading to the lower dyeloading (DYE III > DUY11 > DUY12) as seen in the data listed in Table 3. However, DUY12 has higher absorption coefficient (ε) than DUY11 and DYE III. As a result, the dye absorption (dye loading time absorption coefficient) on the practical photoanode is DUY11 > DUY12 > DYE III. Overall, the lower dye-loading of DUY11 was compensated by its higher absorption coefficient, consequently, DUY11-sensitized DSC has the highest efficiency. DUY12-dyed TiO2 has higher dye absorption compared to DYE III-adsorbed TiO2; however, the corresponding cell has lower Jsc. DUY12 with both H+ ions substituted by TBA+ ions and no acidic H+ to lower the ECB of TiO2 when adsorbed, the electron injection of the resulting DSC has less efficiency, resulting in lower Jsc.46 TBA+ in a sensitizer can reduce the Voc loss by protecting the TiO2 surface, and DSC based on DUY12 containing two TBA+ shows the highest VOC. As a result, the overall conversion efficiency (10.88%) of the DUY12 dyed cell is lower than that of the DUY11-sensitized cell but close to that of the cell based on DYE III. The IPCE curves of the DSCs sensitized with three dyes illustrated in Figure 3B revealed that the IPCE values of the DUY12-based cell is obviously lower than the cells based on the other two dyes at almost the whole wavelength, consistent with the order of the Jsc values obtained from the I−V measurements. Overall, considering the solubility and photovoltaic performance, the optimal structure of the ruthenium dye with two anchoring groups is the one containing one H+ and one TBA+ ions. This study reveals that just like the thiocyanate ruthenium sensitizer containing three (e.g., N749)10,11 or four (e.g., N3)9 carboxylate anchoring groups, the best photovoltaic performance dye with two anchoring groups (e.g., DYE III series) is also probably the one having one H+ and one TBA+ ions, although the difference in the efficiency is not significant. Charge-Transfer Kinetics of the Devices. Nyquist plots (under illumination and in dark) of electrochemical impedance spectra (EIS) for the cells sensitized by the three ruthenium dyes are displayed in Figure 4. The TiO2/dye/electrolyte interfacial (R2) and series (Rs) resistances extracted from EIS are also summarized in Table 3. Under illumination, the trend of the R2 value is in the order of DYE III (18.6 Ω) < DUY11 (18.9 Ω) < DUY12 (19.5 Ω), even though the energies of the frontier orbitals for these three dyes are very close. The smaller interfacial resistance, R2, indicates more facile electron

optimizing the TBP concentration in the electrolyte, the highest efficiency of cells based on three sensitizers are very close under the standard light illumination (see Table 1). Replacing the acidic H+ in the sensitizer with TBA+ provides an alternative method to optimize cell performance with less amount of TBP used. DUY11 containing one H+ and one TBA+ was used as a sensitizer to optimize the thickness of the TiO2 films because the previous reports3,12−22 indicate that a ruthenium sensitizer containing one TBA+ ion generally has the highest efficiency among other H+/TBA+ analogues. The photovoltaic parameters of the devices using the various thicknesses of TiO2 photoanodes are summarized in Table 2. Table 2. Photovoltaic Parameters of DUY11-Sensitized Cells Using Various TiO2 Thicknessesa TiO2 thickness (μm)

JSC (mA/cm2)

VOC (V)

FF

ηmax (%)

9 12 18 22

16.82 18.10 23.28 18.16

0.80 0.79 0.76 0.78

0.72 0.72 0.65 0.75

9.69 10.28 11.47 10.61

a

Cell fabrication parameters: dye solution: 0.2 mM DUY11 and 20 mM CDCA in a mixture of acetonitrile/isopropanol/DMSO (volume ratio 1:1:3); dye loading time: 14 h; electrolyte: 0.05 M LiI, 1.0 M butyl-3-methylimidazolium iodide (BMII), 0.03 M I2, 0.1 M guanidinium thiocyanate (GuSCN), 0.5 M TBP in a mixture of acetonitrile/valeronitrile (volume ratio 85:15).

The data showed that JSC increases with increasing (from 12 to 18 μm) TiO2 film thickness because a thicker TiO2 film loads more dye molecules and thus, leads to the enhancement in the photocurrent of the cell. Nevertheless, the increasing of TiO2 film thickness (TiO2 is an insulator) results in the increase in the series resistance of the cell,45 therefore, both VOC and FF of the cell decrease (see Table 2). Moreover, further increasing the TiO2 thickness (up to 22 μm) leads to a decrease in the efficiency of the cell because of the poor quality of thick TiO2 film. Overall, the optimal TiO2 thickness is 18 μm, and the highest efficiency of DUY11-sensitized DSC is up to 11.47%. The same fabrication condition except the concentration of TBP in the electrolyte (0.5 M for DYE III, 0.3 M for DUY12, the optimal concentration used in thin TiO2 film, see Table 1) were used for preparing DSCs based on DYE III and DUY12 dyes, and the J−V and IPCE curves of devices sensitized by the three dyes under AM 1.5G simulated sunlight are displayed in Figure 3. The photovoltaic parameters, dye-loading, and dye absorption (dye-loading times dye absorption coefficient) are collected in Table 3. DSCs based on DYE III and DUY12 achieve the PCE of 10.76 and 10.88%, respectively, whereas

Figure 3. (A) J−V and (B) IPCE curves of DYE III, DUY11, and DUY12 devices under the illumination of AM 1.5G simulated sunlight (100 mW/ cm2). 11418

DOI: 10.1021/acsomega.9b00431 ACS Omega 2019, 4, 11414−11423

ACS Omega

Article

Table 3. Photovoltaic Parameters of DSCs Based on DYE III, DUY11, DUY12, and N719 Measured under the Illumination of AM 1.5G Simulated Sunlight, Dye-Loading, and TiO2/Dye/Electrolyte Interface Resistances (R2) in Dark and under Lightinga b

dye

JSC (mA/cm2)

VOC (V)

FF

ηmax (%),bηavg (%)

DYE III DUY11 DUY12 N719

23.15 23.28 20.10 18.04

0.72 0.76 0.78 0.78

0.64 0.65 0.69 0.71

10.76, 9.34 ± 0.82 11.47, 10.12 ± 0.94 10.88, 9.29 ± 0.85 9.95

avg dye loading (10−7 mol/cm2) 2.14 2.11 1.95

c

dye absorption (10−3 cm−3)

R2 (Ω), in dark

R2 (Ω), under light

4.90 5.02 4.97

256 294 304

18.6 18.9 19.5

a Cell fabrication parameters: TiO2 film thickness: 18 μm; dye solution: 0.2 mM corresponding dye and 20 mM CDCA in a mixture of acetonitrile/ isopropanol/DMSO (volume ratio 1:1:3); dye loading time: 14 h; electrolyte: 0.05 M LiI, 1.0 M butyl-3-methylimidazolium iodide (BMII), 0.03 M I2, 0.1 M guanidinium thiocyanate (GuSCN), TBP (0.3 M for DUY12, the others are 0.5 M) in a mixture of acetonitrile/valeronitrile (volume ratio 85:15). bAverage of six cells. cDye absorption = dye-loading × dye absorption coefficient.

Figure 4. EIS (Nyquist plots) of DYE III-, DUY11-, and DUY12-sensitized cells (A) under standard 1 Sunlight and (B) in dark.

Figure 5. (A) Electron transport time (τtr) and (B) electron lifetime (τn) as a function of light intensity for DYE III-, DUY11-, and DUY12sensitized devices.

injection and dye regeneration in the cells.47 The more facile electron injection and dye regeneration of the DYE III-based cell is because DYE III contains higher number of acidic H+ that lowers the TiO2 ECB upon adsorbing on it, thus increasing the driving force for the electron injection. On the other hand, in dark, the interfacial resistance (R2) represents the coverage of TiO2 surface: the higher the R2 value, the better the TiO2 coverage. Interestingly, the R2 order (DYE III (256 Ω) < DUY11 (294 Ω) < DUY12 (304 Ω), see Table 3) measured in dark is the same as that under illumination. The sensitizer containing more TBA+ ions exhibits larger interfacial resistance, supporting the fact that TBA+ adsorbed on TiO2 surface can block the electron transfer from TiO2 to I3− in the electrolyte, reduce the charge recombination, resulting in high Voc of the cell. The light intensity-related electron transport time (τtr, the time taken by the electrons on TiO2 transiting across the photoanode) and electron lifetime (τn, also called charge recombination time, the time taken by the electrons on TiO2 to recombine with the oxidant in the electrolyte) extracted

from the intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS), respectively, are displayed in Figure 5. The electron transport times of all cells are more than one order of magnitude shorter than the electron lifetime, allowing the electron to transport to the photoanode before recombining with the oxidant in the electrolyte.25 That is why the cells sensitized by these three dyes all have PCE over 10%. The electron transport time (τtr) provides the information regarding the number of injected electrons in the TiO2 conduction band and the quality (resistance) of the TiO2 film. The τtr values for the cells based on DYE III are very close to that of DUY11 cell at all light intensities measured. Supposing the quality of the TiO2 films is pretty much the same, the similar τtr value suggests that the number of injected electrons in the TiO2 conduction band for DYE III- and DUY11-dyed cells is also very close. The τtr value of DUY12based cell is slightly lower than the other two dyes sensitized cells, resulting in lower Jsc. The trend of the electron lifetime (τn) for the three cells is in ascending order of DYE III < 11419

DOI: 10.1021/acsomega.9b00431 ACS Omega 2019, 4, 11414−11423

ACS Omega

Article

DUY11 < DUY12 (see Figure 5B). Longer τn value suggests the cell with less charge recombination (or better TiO2 surface coverage). The dye-loading is in descending order of DYE III > DUY11 > DUY12 (see Table 3). DUY12 has the lowest dyeloading, but it covers the TiO2 surface more completely (by the assistance of TBA+); the corresponding cell has the longest τn value, and therefore has the highest Voc (see Table 3). The electron lifetime data reaffirmed the importance of TBA+ in dye molecules to reduce the charge recombination by adsorbing on TiO2 surface. Nevertheless, TBA+ cannot lower the Fermi level of TiO2 anode as efficiently as H+ does and decrease the dye loading by occupying TiO2 surface, resulting in lower Jsc. If the LUMO level of the ruthenium dyes can be raised without sacrificing too much light-harvesting ability, the benefit of TBA+ will be more manifest. Photovoltaic Performance of the DSCs Sensitized by Three Dyes under Room-Light Illumination. Indoor lighting conditions are very different compared with the solar irradiance outdoors both in the light intensity and spectrum response. In general, indoor illumination has an intensity between 200 and 2000 lux with major visible light. On the other hand, the intensity of a full solar irradiation (having ca. 45% NIR and IR photos48) is 100 mW cm−2 (AM 1.5G), corresponding to about 100 000−110 000 lux. Therefore, the sensitizer designed for outdoor and indoor application may not be totally the same. Nevertheless, only few reports (including iodide-based electrolytes in combination with a rutheniumbased dye,49−51 or copper electrolyte coupled with organic dyes52) exist that focus on designing the sensitizers to be used for indoor-light harvesting. It is valuable to know the function of TBA+ in the sensitizers when indoor DSC application is addressed. The absorption spectra of DYE III, DUY11, and DUY12 immobilized on TiO2 and the emission spectra of the indoor lighting (T5 fluorescent and LED lights) overlap in a broad wavelength range from 300 to 800 nm (see Figure S6, Supporting Information).32 It is expected that the cells under room-lighting will have higher PCE compared to those under the irradiation of standard AM 1.5G sunlight. To investigate the function of TBA+ ion in dye molecules for indoor DSC application, we test the photovoltaic performance of the DSCs under LED lighting. The PCEs of the DYE III-, DUY11-, DUY12-, and N719-sensitized cells under LED light illumination with various intensities are illustrated in Figure 6. The detailed photovoltaic parameters and the original I−V curves are displayed in Tables S3−S6 (Supporting Informa-

tion) and Figures S7−S10 (Supporting Information), respectively. The PCE of all cells is higher at stronger (LED) light intensity (between 400 and 6000 lux studied in this article). The DUY11-sensitized cell (which has the best photovoltaic performance at standard 1 sun illumination) also has the highest PCE under all studied light intensities. The DUY11-based cell exhibits the PCE up to 19% at ca. 1000 lux of LED lighting. This promises the applications of DSCs under room lighting conditions. It is interesting to note here that under LED light, DSC sensitized with DUY12 (containing two TBA+ in the anchoring groups) has higher PCE than the DYE III (contains no TBA+ ion)-based cell, although these two cells have similar PCE value under standard 1 sun illumination. Under weak light illumination (when the number of the excited electrons is small), protection of TiO2 surface to avoid the charge recombination and keeping the high TiO2 Fermilevel (by adsorbed less protons) to achieve high Voc are more important than under strong light illumination. TBA+ performs its functions (protects TiO2 surface without changing its fermilevel) nicely along this line. This could be the reason that under LED illumination, the PCE of DUY12 cell is obviously higher than that of DYE III cell. Furthermore, the TBA+ ion in the sensitizer and sensitizer anion may undergo synergistic interactions in the adsorbed state as the optical data shown in Figure 2B and the corresponding discussion in the previous paragraph. Thus, TBA+ may suppress unfavorable dye aggregation after adsorbing on TiO2 to reduce the charge recombination.52 In conclusion, three thiocyanate ruthenium complexes (named as DYE III, DUY11, and DUY12) with two H+, one H+, one TBA+, and two TBA+ in the bidentated anchoring groups were synthesized by an efficient one-pot reaction with microwave assistance. At standard AM 1.5G, 100 mW/cm2 condition, DSCs sensitized by DYE III, DUY11, and DUY12 exhibited excellent conversion efficiencies of 10.76, 11.47, and 10.88%, respectively; all are higher than that (9.95%) of the N719-based cell. Replacing the acidic H+ with TBA+ in dye molecules can reduce the unwanted charge recombination by avoiding dye aggregation and protecting TiO2 surface without significantly affecting (compared to proton) the ECB of TiO2 when used in DSCs. Nevertheless, TBA+ also occupies the TiO2 surface to lower the dye loading, decreasing the Jsc. As a result, the highest efficiency device is the one sensitized by the dye containing one H+ and one TBA+ (such as DUY11 in this study). The DUY11-based device also exhibits the highest overall efficiency (PCE of 19% at ca. 1000 lux) under LED illumination at various intensities. Furthermore, under LED lighting, the DSC based on DUY12 (with two TBA+) has higher PCE compared to that dyed by DYE III (contains two H+); even these two dyes have similar photovoltaic performance under standard 1 sun light. The result reveals the important role of TBA+ in TiO2 surface protection under room lighting condition (when small number of electrons were excited by the weak light). This study provided another point (replacing TBA+ with H+ in dye molecule) in molecular engineering of ruthenium-based sensitizers for dye-sensitized solar cells, especially in indoor environments. We expect that our study will have a practical impact in designing new highefficient sensitizers to harvest the ambient light energy to power electronic devices or to extend their battery lifetime for the autonomously operated electric devices.

Figure 6. PCE of the DSCs sensitized by DYE III, DUY11, DUY12, and N719 dyes under the illumination of LED light at various intensities. 11420

DOI: 10.1021/acsomega.9b00431 ACS Omega 2019, 4, 11414−11423

ACS Omega

■

Article

EXPERIMENTAL SECTION Materials and Measurements. All reagents were obtained from the commercial resources and used as received unless specified. Solvents were dried over 4 Å molecular sieves before use. Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker 500 MHz NMR spectrometer. Highresolution mass spectra (HRMS) were obtained using a JMS700 MStation Mass Spectrometer. Elemental analyses were carried out with a PerkinElmer 2400 CHNS/O analyzer. UV− vis spectra were measured using a Cary 300 Bio spectrometer. Voltammetric measurements were performed in a singlecompartment, three-electrode cell with a Pt wire counter electrode and a platinum disk working electrode. The reference electrode was Ag+/Ag, and the supporting electrolyte was 0.1 M tetrabutylammonium perchlorate (TBAClO4) in DMF. The square-wave voltammograms (potential step increment: 5 mV; frequency: 25 Hz) of sensitizers dissolved in EtOH were recorded using a Potentiostat/Galvanostat (PGSTAT 30, Autolab, Eco-Chemie, the Netherlands), and ferrocene was used as an internal calibration standard. Electrochemical impedance spectroscopy (EIS) measurements were carried out using an Eco Chemie Autolab PGSTAT30 Potentiostat/ Galvanostat in 50 mV voltage steps with a sinusoidal potential perturbation of 10 mV in dark or under illumination with simulated sunlight. IMPS and IMVS were recorded with a Zahner Zennium controlled intensity modulated photospectroscopy CIMPS-1 potentiostat installed with a light intensity modulated function. The measurements of EIS, IMPS, and IMVS as well as the relative calculations can be found in our previous articles.30,34 DFT Calculation. All computational calculations were performed using the Gaussian 09 program similar to our previous report.35 Device Fabrication and Photovoltaic Performance Characterization. A 19 μm (or 15 μm or 9 or 5.5 μm) TiO2 layer (purchased from Taiwan DSC PV (TDP), Taiwan) and a 3 μm scattered layer of anatase TiO2 (400 nm in diameter, prepared in our laboratory) were deposited sequentially on a fluorine-doped tin oxide (FTO) conducting glass using screen printing to make the photoanode. After sintering at 500 °C for 4 h, the TiO2 film was immersed into a solution of 0.2 mM sensitizer containing 20 mM CDCA as a co-adsorbent in a mixture of acetonitrile/isopropanol/DMSO (volume ratio 1:1:3) for 14 h at room temperature. The platinum counter electrode was made through a typical thermal decomposition of H2PtCl6 onto a FTO glass. The two electrodes were assembled and sealed with a 25 μm thick Surlyn spacer. A solution of 0.05 M LiI, 1.0 M butyl-3-methylimidazolium iodide (BMII), 0.03 M I2, 0.1 M guanidinium thiocyanate (GuSCN), and TBP (0, 0.3, 0.5, or 0.7 M) in a mixture of acetonitrile/valeronitrile (volume ratio = 85:15) was used as the electrolyte for the devices. The cell active area is 0.16 cm2 obtained by using a dark mask. Other detailed processes for TiO2 film preparation and device fabrication can be found in our previous article.3 The dye loading was estimated by desorbing the dye molecules from the TiO2 practical photoanode with a solution of 0.05 M TBAOH in EtOH and then the corresponding UV−vis spectra were used for quantitative analysis. For photovoltaic characterization under indoor lighting, a LED (Innotec Inc. Taiwan, LUX-T14401AL/NT8) light source was used. Irradiation from the artificial light sources

is generally expressed in photometric units (lux, illuminance) rather than radiometric ones (W/m2, irradiance) to represent the light intensity. By measuring the radiant flux and the luminous flux of the lamp, the luminous efficacy conversion factor (incident illuminance/incident irradiance) was calculated, being 302 ± 3 lm/W for our measuring system. This value was then used to evaluate the efficiency of the PV devices. The illuminance was appraised by a luxmeter (Optimum optoelectronics Corp., Taiwan, SRI2000F). Current−voltage (I−V) measurements under LED illumination were performed in a customized designed iron box (0.65 m x 0.7 m x 2.0 m) composed of height-tunable lift with black walls inside and a 4 lamps holder incorporated at the top and four supporting plates underneath along the vertical axis. The incident power density was adjusted by the distance between the top lamps and the supporting (sample) plate. The spectroradiometer was placed on the supporting plate for measuring light intensity, then the DSC was placed at the same place for taking I−V curves via a computer-controlled digital source meter (Keithley 2400, USA). After I−V measurements, the spectroradiometer was placed back to the supporting plate for checking the variation in the light intensity. The door was kept closed during I−V measurements to avoid the ingress of light from outside. It is worth mentioning that 1 sun radiation, that is 1000 W/m2 AM1.5G, corresponds to an illuminance of 100 000−110 000 lux.36,37 The light bulb employed in our system, namely LED, presents irradiance which is two orders of magnitude smaller than that at STC and narrower emission spectra with minor components in the IR range. Thus, the source of heat which affect the PV device efficiency is very small during indoor-lighting.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b00431.

■

Details of synthesis, optical data of DYE III, DUY11, and DUY12 in EtOH and adsorbed on TiO2 films, selective transition states for DYE III in EtOH, photovoltaic parameters of DYE III-, DUY11-, DUY12-, and N719sensitized DSCs measured under LED light at various illuminances, 1H NMR spectra of DYE III, DUY11, and DUY12, square-wave voltammograms of DYE III, DUY11, and DUY12, first ten frontier orbitals of DYE III, calculated β-LUSO distribution of [DYE III], UV− vis spectrum in EtOH and calculated absorption spectrum with the oscillator strengths of DYE III sensitizer, absorption spectra of DYE III, DUY11, and DUY12 immobilized on TiO2 films and emission spectra of T5 and LED fluorescent lights, J−V curves of DYE III-, DUY11-, DUY12-, and N719-cell under LED light at various illuminances (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Chun-Guey Wu: 0000-0001-8540-5602 Notes

The authors declare no competing financial interest. 11421

DOI: 10.1021/acsomega.9b00431 ACS Omega 2019, 4, 11414−11423

ACS Omega

■

Article

complexes to the anatase(101) surface. J. Phys. Chem. C 2010, 114, 8398−8404. (16) Liu, S.-H.; Fu, H.; Cheng, Y.-M.; Wu, K.-L.; Ho, S.-T.; Chi, Y.; Chou, P.-T. Theoretical study of N749 dyes anchoring on the (TiO2)28 surface in DSSCs and their electronic absorption properties. J. Phys. Chem. C 2012, 116, 16338−16345. (17) Wang, Z.-S.; Yamaguchi, T.; Sugihara, H.; Arakawa, H. Significant efficiency improvement of the black dye-sensitized solar cell through protonation of TiO2 films. Langmuir 2005, 21, 4272− 4276. (18) Sodeyama, K.; Sumita, M.; O’Rourke, C.; Terranova, U.; Islam, A.; Han, L.; Bowler, D. R.; Tateyama, Y. Protonated carboxyl anchor for stable adsorption of Ru N749 dye (black dye) on a TiO2 anatase (101) surface. J. Phys. Chem. Lett. 2012, 3, 472−477. (19) Chen, J.; Wang, J.; Bai, F.-Q.; Pan, Q.-J.; Zhang, H.-X. Theoretical studies on structural and spectroscopic properties of photoelectrochemical cell ruthenium sensitizers, derivatives of AR20. Int. J. Quantum Chem. 2013, 113, 891−901. (20) Song, L.-Q.; Xie, P. H.; Wang, X. S.; Hou, Y. J.; Zhang, B. W.; Cao, Y.; Li, W. Y.; Zhang, J. B.; Xiao, X. R.; Lin, Y. Synthesis of new ruthenium (II) bipyridyl complexes and studies on their photophysical and photoelectrochemical properties. Chin. J. Chem. 2010, 21, 644−649. (21) Fan, S.-H.; Zhang, A.-G.; Ju, C.-C.; Gao, L.-H.; Wang, K.-Z. A Triphenylamine-grafted imidazo[4,5-f][1,10]phenanthroline ruthenium(II) complex: acid − base and photoelectric properties. Inorg. Chem. 2010, 49, 3752−3763. (22) Li, C.; Wu, S.-J.; Wu, C.-G. Structural design of ruthenium sensitizer compatible with cobalt electrolyte for a dye-sensitized solar cell. J. Mater. Chem. A 2014, 2, 17551−17560. (23) DiMarco, B. N.; O’Donnell, R. M.; Meyer, G. J. Cationdependent charge recombination to organic mediators in dyesensitized solar cells. J. Phys. Chem. C 2015, 119, 21599−21604. (24) Bandara, T. M. W. J.; Fernando, H. D. N. S.; Furlani, M.; Albinsson, I.; Dissanayake, M. A. K. L.; Ratnasekera, J. L.; Mellander, B.-E. Effect of the alkaline cation size on the conductivity in gel polymer electrolytes and their influence on photo electrochemical solar cells. Phys. Chem. Chem. Phys. 2016, 18, 10873−10881. (25) Katoh, R.; Kasuya, M.; Kodate, S.; Furube, A.; Fuke, N.; Koide, N. Effects of 4-tert-Butylpyridine and Li ions on photoinduced electron injection efficiency in black-dye-sensitized nanocrystalline TiO2 films. J. Phys. Chem. C 2009, 113, 20738−20744. (26) Ozawa, H.; Okuyama, Y.; Arakawa, H. Effects of cation composition in the electrolyte on the efficiency improvement of black dye-based desensitizeddyesensitized solar cells. RSC Adv. 2013, 3, 9175−9177. (27) Wyss, P.; Moehl, T.; Zakeeruddin, S. M.; Grätzel, M. Influence of cations of the electrolyte on the performance and stability of dye sensitized solar cells. J. Mater. Chem. 2012, 22, 24424−24429. (28) Lee, S.-H. A.; Jackson, A.-M. S.; Hess, A.; Fei, S.-T.; Pursel, S. M.; Basham, J.; Grimes, C. A.; Horn, M. W.; Allcock, H. R.; Mallouk, T. E. Influence of different iodide salts on the performance of dyesensitized solar cells containing phosphagensphosphazene-based nonvolatile electrolytes. J. Phys. Chem. C 2010, 114, 15234−15242. (29) Andrade, L.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Aguilar Ribeiro, H.; Mendes, A.; Grätzel, M. Influence of sodium cations of N3 dye on the photovoltaic performance and stability of dyesensitized solar cells. ChemPhysChem 2009, 10, 1117−1124. (30) Watson, D. F.; Meyer, G. J. Cation effects in nanocrystalline solar cells. Coord. Chem. Rev. 2004, 248, 1391−1406. (31) Wang, P.; Gao, F.; Liu, S.; Cai, N.; Zhang, J.. Dipyridyl ruthenium dye containing S, Se and Te electron-repellent groups used in dye-sensitized solar battery. Patent CN 101412855 B, Sep 28, 2011. (32) Barber, G. D.; et al. Utilization of direct and diffuse sunlight in a dye-sensitized solar cell-silicon photovoltaic hybrid concentrator system. J. Phys. Chem. Lett. 2011, 2, 581−585. (33) Chen, C.-Y.; Jian, Z.-H.; Huang, S.-H.; Lee, K.-M.; Kao, M.-H.; Shen, C.-H.; Shieh, J.-M.; Wang, C.-L.; Chang, C.-W.; Lin, B.-Z.; Lin, C.-Y.; Chang, T.-K.; Chi, Y.; Chi, C.-Y.; Wang, W.-T.; Tai, Y.; Lu, M.-

ACKNOWLEDGMENTS Financial support from the Ministry of Science and Technology (MOST), Taiwan, ROC (grand number: NSC104-2113-M-008-002-MY3) is greatly acknowledged. The device fabrication and photovoltaic characterizations were carried out in the Advanced Laboratory of Accommodation and Research for Organic Photovoltaics (AROPV), MOST, Taiwan, ROC.

■

REFERENCES

(1) O’Regan, B.; Grätzel, M. A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature 1991, 353, 737− 740. (2) Shalini, S.; Balasundaraprabhu, R.; Kumar, T. S.; Prabavathy, N.; Senthilarasu, S.; Prasanna, S. Status and outlook of sensitizers/dyes used in dye sensitized solar cells (DSSC): a review. Int. J. Energy Res. 2016, 40, 1303−1320. (3) 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.; Zakeeruddin, S. M.; Grätzel, M. Highly efficient light-harvesting ruthenium sensitizer for thin-film dye-sensitized solar cells. ACS Nano 2009, 3, 3103−3109. (4) Zhang, L.; Cole, J. M. Anchoring groups for dye-sensitized solar cells. ACS Appl. Mater. Interfaces 2015, 7, 3427−3455. (5) Yan, S. G.; Hupp, J. T. Energetics of electron transfer at the nanocrystalline titanium dioxide semiconductor/aqueous solution interface: pH invariance of the metal-based formal potential of a representative surface-attached dye couple. J. Phys. Chem. B 1997, 101, 1493−1495. (6) Wang, H.; Peter, L. M. Influence of electrolyte cations on electron transport and electron transfer in dye-sensitized solar cells. J. Phys. Chem. C 2012, 116, 10468−10475. (7) Ozawa, H.; Okuyama, Y.; Arakawa, H. Effects of cation composition in the electrolyte on the efficiency improvement of black dye-based dye sensitized solar cells. RSC Adv. 2013, 3, 9175−9177. (8) Nazeeruddin, M. K.; Zakeeruddin, S. M.; Humphry-Baker, R.; Jirousek, M.; Liska, P.; Vlachopoulos, N.; Shklover, V.; Fischer, C.-H.; Grätzel, M. Acid − base equilibria of (2,2’-bipyridyl-4,4’-dicarboxylic acid)ruthenium(II) complexes and the effect of protonation on charge-transfer sensitization of nanocrystalline titania. Inorg. Chem. 1999, 38, 6298−6305. (9) Nazeeruddin, M. K.; Péchy, P.; Renouard, T.; Zakeeruddin, S. M.; Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Shklover, V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Grätzel, M. Engineering of efficient panchromatic sensitizers for nanocrystalline TiO2-based solar cells. J. Am. Chem. Soc. 2001, 123, 1613−1624. (10) Nazeeruddin, M. K.; Humphry-Baker, R.; Liska, P.; Grätzel, M. Investigation of sensitizer adsorption and the influence of protons on current and voltage of a dye-sensitized nanocrystalline TiO2 solar cell. J. Phys. Chem. B 2003, 107, 8981−8987. (11) Nazeeruddin, M. K.; De Angelis, F.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Grätzel, M. Combined experimental and DFT-TDDFT computational study of photoelectrochemical cell ruthenium sensitizers. J. Am. Chem. Soc. 2005, 127, 16835−16847. (12) Pashaei, B.; Shahroosvand, H.; Graetzel, M.; Nazeeruddin, M. K. Influence of ancillary ligands in dye-sensitized solar cells. Chem. Rev. 2016, 116, 9485−9564. (13) Xie, M.; Chen, J.; Bai, F.-Q.; Wei, W.; Zhang, H.-X. Theoretical studies on the interaction of ruthenium sensitizers and redox couple in different deprotonation situations. J. Phys. Chem. A 2014, 118, 2244−2252. (14) Pizzoli, G.; Lobello, M. G.; Carlotti, B.; Elisei, F.; Nazeeruddin, M. K.; Vitillaro, G.; De Angelis, F. Acid−base properties of the N3 ruthenium(II) solar cell sensitizer: a combined experimental and computational analysis. Dalton Trans. 2012, 41, 11841−11848. (15) Schiffmann, F.; VandeVondele, J.; Hutter, J.; Wirz, R.; Urakawa, A.; Baiker, A. Protonation-dependent binding of ruthenium bipyridyl 11422

DOI: 10.1021/acsomega.9b00431 ACS Omega 2019, 4, 11414−11423

ACS Omega

Article

D.; Tung, Y.-L.; Chou, P.-T.; Wu, W.-T.; Chow, T. J.; Chen, P.; Luo, X.-H.; Lee, Y.-L.; Wu, C.-C.; Chen, C.-M.; Yeh, C.-Y.; Fan, M.-S.; Peng, J.-D.; Ho, K.-C.; Liu, Y.-N.; Lee, H.-Y.; Chen, C.-Y.; Lin, H.-W.; Yen, C.-T.; Huang, Y.-C.; Tsao, C.-S.; Ting, Y.-C.; Wei, T.-C.; Wu, C.-G. Performance characterization of dye-sensitized photovoltaics under indoor lighting. J. Phys. Chem. Lett. 2017, 8, 1824−1830. (34) Li, J.-Y.; Lee, C.; Chen, C.-Y.; Lee, W.-L.; Ma, R.; Wu, C.-G. Diastereoisomers of ruthenium dyes with unsymmetric ligands for DSC: Fundamental chemistry and photovoltaic performance. Inorg. Chem. 2015, 54, 10483−10489. (35) Nguyen, T.-D.; Lin, C.-H.; Wu, C.-G. Effect of the CF3 substituents on the charge-transfer kinetics of high-efficiency cyclometalated ruthenium sensitizers. Inorg. Chem. 2017, 56, 252− 260. (36) Kandilli, C.; Ulgen, K. Solar illumination and estimating daylight availability of global solar irradiance. Energy Sources, Part A 2008, 30, 1127−1140. (37) Castaner, L.; Silvestre, S. Modelling photovoltaic systems using PSpice; John Wiley and Sons, 2002. (38) Yu, Q.; Liu, S.; Zhang, M.; Cai, N.; Wang, Y.; Wang, P. An extremely high molar extinction coefficient ruthenium sensitizer in dye-sensitized solar cells: The effects of π-conjugation extension. J. Phys. Chem. C 2009, 113, 14559−14566. (39) Gao, F.; Wang, Y.; Shi, D.; Zhang, J.; Wang, M.; Jing, X.; Humphry-Baker, R.; Wang, P.; Zakeeruddin, S. M.; Grätzel, M. Enhance the optical absorptivity of nanocrystalline TiO2 film with high molar extinction coefficient ruthenium sensitizers for high performance dye-sensitized solar cells. J. Am. Chem. Soc. 2008, 130, 10720−10728. (40) Boschloo, G.; Gibson, E. A.; Hagfeldt, A. voltammetry of iodide/triiodide redox electrolytes and its relevance to dye-sensitized solar cells. J. Phys. Chem. Lett. 2011, 2, 3016−3020. (41) Chen, C.-Y.; Wu, S.-J.; Li, J.-Y.; Wu, C.-G.; Chen, J.-G.; Ho, K.C. A new route to enhance the light-harvesting capability of ruthenium complexes for dye-sensitized solar cells. Adv. Mater. 2007, 19, 3888−3891. (42) Chen, C.-Y.; Pootrakulchote, N.; Chen, M.-Y.; Moehl, T.; Tsai, H.-H.; Zakeeruddin, S. M.; Wu, C.-G.; Grätzel, M. A new heteroleptic ruthenium sensitizer for transparent dye-sensitized solar cells. Adv. Energy Mater. 2012, 2, 1503−1509. (43) Kuang, D.; Klein, C.; Ito, S.; Moser, J.-E.; Humphry-Baker, R.; Zakeeruddin, S. M.; Grätzel, M. High molar extinction coefficient ioncoordinating ruthenium sensitizer for efficient and stable mesoscopic dye-sensitized solar cells. Adv. Funct. Mater. 2007, 17, 154−160. (44) Robson, K. C. D.; Hu, K.; Meyer, G. J.; Berlinguette, C. P. Atomic level resolution of dye regeneration in the dye-sensitized solar cell. J. Am. Chem. Soc. 2013, 135, 1961−1971. (45) Chen, J.; Chen, C.; Wu, S.; Li, J.; Wu, C.; Ho, K. On the photophysical and electrochemical studies of dye-sensitized solar cells with the new dye CYC-B1. Sol. Energy Mater. Sol. Cells 2008, 92, 1723−1727. (46) Yan, Y.; Shi, W.; Yuan, Z.; He, S.; Li, D.; Meng, Q.; Ji, H.; Chen, C.; Ma, W.; Zhao, J. The formation of Ti−H species at interface is lethal to the efficiency of TiO2-based dye-sensitized devices. J. Am. Chem. Soc. 2017, 139, 2083−2089. (47) Nguyen, T.-D.; Lin, C.-H.; Wu, C.-G. Effect of the CF3 substituents on the charge-transfer kinetics of high-efficiency cyclometalated ruthenium sensitizers. Inorg. Chem. 2017, 56, 252− 260. (48) Reich, N. H.; van Sark, W. G. J. H. M.; Turkenburg, W. C. Charge yield potential of indoor-operated solar cells incorporated into product integrated photovoltaic (PIPV). Renewable Energy 2011, 36, 642−647. (49) Lan, J.-L.; Wei, T.-C.; Feng, S.-P.; Wan, C.-C.; Cao, G. Effects of iodine content in the electrolyte on the charge transfer and power conversion efficiency of dye sensitized solar cells under low light intensities. J. Phys. Chem. C 2012, 116, 25727−25733.

(50) Kroon, J. M.; et al. Nanocrystalline dye-sensitized solar cells having maximum performance. Prog. Photovolt. Res. Appl. 2007, 15, 1−18. (51) Kontos, A. G.; Stergiopoulos, T.; Likodimos, V.; Milliken, D.; Desilvesto, H.; Tulloch, G.; Falaras, P. Long-term thermal stability of liquid dye solar cells. J. Phys. Chem. C 2013, 117, 8636−8646. (52) Freitag, M.; Teuscher, J.; Saygili, Y.; Zhang, X.; Giordano, F.; Liska, P.; Hua, J.; Zakeeruddin, S. M.; Moser, J.-E.; Grätzel, M.; Hagfeldt, A. Dye-sensitized solar cells for efficient power generation under ambient lighting. Nat. Photonics 2017, 11, 372−378.

11423

DOI: 10.1021/acsomega.9b00431 ACS Omega 2019, 4, 11414−11423