Rapid microwave-assisted self-assembly of a carboxylic acid

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Article Cite This: ACS Appl. Energy Mater. 2018, 1, 202−210

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Rapid Microwave-Assisted Self-Assembly of a Carboxylic-AcidTerminated Dye on a TiO2 Photoanode V. Leandri,† W. Yang,†,§ J. M. Gardner,*,‡ G. Boschloo,† and S. Ott† †

Department of ChemistryÅngström Laboratory, Uppsala University, 751 20 Uppsala, Sweden Division of Applied Physical Chemistry, Department of Chemistry, KTH Royal Institute of Technology, SE-114 28 Stockholm, Sweden

‡

S Supporting Information *

ABSTRACT: Self-assembly of carboxylic-acid-functionalized dyes on mesoporous, anatase TiO2 is at the heart of dye-sensitized solar cells (DSSCs). However, the process often requires 6−20 h of electrode immersion at room temperature in the dye-bath solutions. Here, we introduce a new, rapid microwave-assisted sensitization technique (MWAS), which significantly accelerates the sensitization process and yields high-quality, self-assembled films of an organic dye within 5 min. Targeted experiments show that the effects of the microwave radiation cannot be explained purely on the basis of the thermal component. The interaction of the microwave radiation with the conductive fluorine-doped tin oxide (FTO) electrical contact is a key aspect to consider and a unique feature of MWAS that is the likely cause for producing rapid self-assembly of the dye on the surface. KEYWORDS: dye-sensitized solar cells, sensitization, dye absorption, microwave, self-assembly, TiO2 grafting shown that the application of an electric field above and below the photoelectrode soaked in a N719 solution was able to reduce the sensitization time from 12 to 5−6 h.24,25 More recently, B. Kim et al. reported that nitric acid treatment of the TiO2 electrodes was able to further reduce the sensitization time from a N719 solution to 20 min.26 Furthermore, the temperature and concentration of the dye solution significantly influences the kinetics of dye adsorption on TiO2 surfaces. Increasing the solution temperature has been shown to be an effective way to increase the speed of molecular adsorption on the semiconductor surface.27,28 However, optimal conditions for the photoanode sensitization depend on the specific sensitizer used. For instance, F. Sauvage et al. have shown that the self-assembly of ruthenium dye C101 on TiO2 at 4 °C gives better device performance than at 60 °C.29 To the best of our knowledge, such studies have never been carried out on organic dyes, which may exhibit higher thermal stability in solution. Over the past few years, organic dyes have been demonstrated to be promising alternatives to coordination complexes for sensitizers in DSSC.30,31 In particular the LEG4 dye, which is based on a 4H-cyclopenta[2,1-b:3,4-b′] dithiophene (CPDT) unit, has been used as a benchmark in numerous studies.32−36 Microwaves (MWs) have been used in many different scientific areas and can be an efficient method for indirectly heating substrates or molecules.37−40 Important innovations in MW-assisted chemistry have enabled researchers to prepare nanomaterials and molecules selectively, in almost

1. INTRODUCTION Self-assembled monolayers (SAMs) are ordered molecular assemblies that spontaneously form on a solid substrate upon its immersion in a solution containing properly functionalized molecules.1 Carboxylic-acid-based SAMs on metal oxide surfaces are among the oldest and most studied,2,3,1,4 and in the past decades they have been successfully employed in various photovoltaic applications.5−7 Self-assembly of mercaptopropionic-acid-capped water-soluble quantum dots (QDs) onto an oxide film electrode has been demonstrated to be a facile deposition route for high QD loading, increasing the performance of photovoltaic devices.8−10 ZnO-coated substrates with 3-aminopropanoic acid SAMs have been proven to increase the morphological control over perovskite crystal growth, which has led to higher power conversion efficiencies of the photovoltaic devices.11 Self-assembly of metal complexes and organic dyes on the surface of mesoporous, anatase TiO2 photoanodes is an essential step in building dye-sensitized solar cells (DSSCs).12−19 The standard method employed for the molecular adsorption of dyes onto anatase TiO2 normally consists of immersing mesoporous metal oxide electrodes into a diluted solution of the sensitizing agent for 6−20 h at room temperature. However, for future applications it may be important to reduce the time of the sensitization process. Early in 2003, Nazeeruddin et al. reported efficient devices based on electrodes immersed in 20 mM solutions of N719 dye for as short as 10 min.20 While this was shown to be a viable alternative for N719,21−23 the dye concentrations are 100 times higher than normally employed. Such high concentrations may not be possible for other dyes due to their limited solubility or may result in dye aggregation. Recently, H. Seo et al. have © 2017 American Chemical Society

Received: October 31, 2017 Accepted: December 12, 2017 Published: December 12, 2017 202

DOI: 10.1021/acsaem.7b00088 ACS Appl. Energy Mater. 2018, 1, 202−210

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ACS Applied Energy Materials

substrates, with the active TiO2 layer screen-printed as reported in the previous section, were cut (H 1.8 cm, L 0.9−1.0 cm), and the edges on the conductive side of the glass were carefully smoothed by using a flat metal file to avoid any sharp regions that could accumulate charges during the microwave exposure and potentially cause a current discharge that damages the electrode. Two cut electrodes were combined leaving the screen-printed sides exposed and placed inside a microwave vial (Biotage, 2−5 mL size). After addition of 3 mL of the same dye-bath solution, the vial was sealed and placed into the microwave cavity. The desired temperature for the reaction was set, and the power was automatically modulated in order to maintain the constant temperature. Typical temperature, power, and pressure diagrams for the MWAS of photoanodes at 100 °C are reported in the Supporting Information (Figure S2). Dye-Sensitized Solar Cell Characterization. Current−voltage (I−V) measurements were carried out with a Keithley 2400 source/ meter and a Newport solar simulator (model 91160); the light intensity was calibrated using a certified reference solar cell (Fraunhofer ISE), to an intensity of 1000 W m−2. For current density−voltage (J−V) measurements a black mask with an aperture of 0.6 cm × 0.6 cm, slightly larger than the active area (0.5 cm × 0.5 cm), was used.44 The instruments for measuring incident photon-to-current conversion efficiency (IPCE), electron lifetime, and charge transport time have been previously described.45 The reported efficiencies are from a batch of 5 solar cells. ATR-IR Spectroscopic Analysis. ATR-IR spectra were recorded from 4000 to 650 cm−1 using a PerkinElmer (Spectrum One) instrument equipped with an MCT detector as the average of 128 scans at 2 cm−1 resolution. The ATR-IR spectrum of LEG4 was recorded from LEG4 powder. ATR-IR spectra of LEG4 on TiO2 were recorded sensitizing 16 electrodes (8 + 8) consisting of transparent TiO2 (Dyesol 18 NR-T paste, 3 layers, total thickness around 9 μm) either by MWAS in the reported conditions, or by standard room temperature immersion. The sensitized-TiO2 was scratched from the electrode, collected as a fine crystalline powder, and placed on a diamond ATR window for ATR-IR measurements.

quantitative yields, and with higher precision than using conventional heating.41 Here, we report the development of a microwave-assisted technique, which is able to reduce the sensitization time of electrodes immersed in a 0.2 mM solution of LEG4 dye, from 14 h to 5 min. The devices assembled with photoanodes sensitized via microwave-assisted sensitization (MWAS) technique exhibit efficiencies similar to those for devices assembled from photoanodes sensitized overnight from the same dye solution at room temperature. Further investigations suggest that the performance of the photoanodes prepared by the MWAS technique is not ascribable just to the higher temperature employed.

2. MATERIALS AND METHODS Materials. All chemicals were purchased from Sigma-Aldrich and used as received unless noted otherwise. The dye LEG4 (structural details in the Supporting Information, Figure S1) and the cobalt polypyridyl complexes used as the redox couple were supplied by Dyenamo AB (Sweden). FTO-coated glass substrates were purchased from Pilkington. TEC15 and TEC8 were used for working electrodes and counter electrodes, respectively. Dye-Sensitized Solar Cell Fabrication. FTO glass substrates (the working electrode) were cleaned in an ultrasonic bath with detergent solution (RBS 25 from Fluka analytical), ethanol (VWR DBH Prolabo purity of 99.9%), and deionized water. The glass substrates were pretreated in a 40 mM aqueous TiCl4 solution at 70 °C for 90 min and then rinsed with water and ethanol. After drying in air, the substrates were screen-printed (active area 0.25 cm2) with a diluted Dyesol 18 NR-T paste to form the active TiO2 layer. The diluted paste is composed of 60 wt % Dyesol 18 NR-T paste and of 40 wt % transparent paste (90 wt % terpineol and 10 wt % ethyl cellulose). The thickness of the active layer is around 6−7 μm after sintering. The substrates were dried at 125 °C for 10 min before being being screen-printed with a diluted TiO2 scattering paste. The diluted scattering paste is composed of 80 wt % Dyesol WER2-O and of 20 wt % transparent paste (90 wt % terpineol and 10 wt % ethyl cellulose). The total thickness of the electrodes is around 10 μm after sintering. The samples were heated gradually at 180 °C (10 min), 320 °C (10 min), 390 °C (10 min), and 500 °C (60 min) in an oven (Nabertherm Controller P320) with an air atmosphere. After sintering, the samples were once again treated with 40 mM aqueous TiCl4 at 70 °C, for 30 min. A final heating step (500 °C for 60 min) was performed. After sintering, the electrodes were cooled down to room temperature. In the case of the devices fabricated via the MWAS technique, the working electrodes were at room temperature prior to dye-bath immersion. For the realization of the standard devices, the electrodes were reheated at 90 °C prior to dye-bath immersion. The dye bath of LEG4 consisted of a 0.2 mM dye solution in acetonitrile/t-butanol (1:1 volume ratio). The photoanodes for standard devices were left in a dye bath overnight (14 h) or for a specified time, at room temperature and in the dark. After dye-bath immersion all the sensitized electrodes were rinsed with ethanol and assembled in a sandwich structure with the counter electrode. A 25 μm thick thermoplastic Surlyn frame was employed (Meltonix 1170−25 from Solaronix). The electrolyte was introduced in all the sealed devices through a predrilled hole by a vacuum backfilling technique, sealed with a thermoplastic Surlyn cover and a glass coverslip. The electrolyte composition is the following: 0.22 M Co(bpy)3(PF6)2, 0.04 M Co(bpy)3(PF6)3, 0.25 M 4-tert-butylpyridine, and 0.1 M LiClO4 in acetonitrile, where bpy is 2,2′-bipyridine. For counter electrode preparation, a predrilled one-hole conductive glass was cleaned following the same procedure reported for the working electrodes. The preparation procedure and the equipment used for the realization of electropolymerized PEDOT counter electrodes have been already described (electropolymerization time: 60 s).42,43 Microwave-Assisted Sensitization. Microwave-assisted sensitization (MWAS) was performed using a microwave reactor from Biotage (Initiator Classic) which operates at 2.45 GHz. FTO glass

3. RESULTS AND DISCUSSION The microwave apparatus provides a simple and safe tool to explore rapid MW-assisted sensitization (MWAS) of anatase TiO2 photoanodes at temperatures equal to and higher than the boiling point of the 1:1 v/v acetonitrile/t-butanol dye-bath solution (i.e., ∼80 °C). Scheme 1 shows the MWAS method. Scheme 1. Sensitization of DSSCs Electrodes via Microwave Sensitization Techniquea

a

Two electrodes with the appropriate dimensions (left) are inserted in a microwave vessel (center). Later, 3 mL of the dye-bath solution is introduced into the vessel. The sealed vessel is then placed in the microwave for sensitization. At the end of the process, the electrodes were rinsed with ethanol and dried (right).

The quality of the molecular layer assembled on the photoanodes is evaluated as a function of the performance of the solar cells constructed. Table 1 collects the photovoltaic performance under standard AM 1.5 sunlight of DSSCs based on LEG4 dye photoanodes sensitized via the MWAS technique, and standard room temperature immersion. It should be noted 203


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either delay or favor the recombination of electrons in TiO2 with the oxidized form of the redox couple.46,29 However, despite the lower Voc, the 100 °C MW-assisted sensitized electrodes systematically showed a slightly higher photocurrent density (Jsc) and fill-factor (FF), which finally result in comparable performances. The photoanodes prepared by the MWAS technique at 80 °C show Voc and FF values simliar to those prepared at 100 °C, but lower photocurrent density and overall lower efficiency, which reflect the importance of the operating temperature on the layer formation. No significant difference is observed between MWAS performed at 80 and 75 °C. Finally, solar cells assembled from TiO2 electrodes sensitized for 10 min at room temperature gave very poor performance due to the lower amount of dye absorbed. This result shows, once again, how dramatic the influence of the temperature is, and illustrates the potential of MWAS. We selected devices from photoanodes prepared from MWAS at 100 °C and standard method (14 h), and followed the evolution of their photovoltaic parameters over a period of 80 days (Figure 1). During this time the solar cells were stored in the dark, at room temperature, and measured on the reported days. Both devices exhibit long-term stability, and the overall conversion efficiencies significantly improve after 3−7 days, reaching a plateau after 20−40 days. The rapid efficiency growth occurring within the first week of observation is a phenomenon that has been previously reported in the literature, and is usually referred to as the “aging effect”.47,48,42 Bo Li et al. have suggested that the improved efficiency arises from the formation of blocking layers on the surface of nanocrystalline TiO2, resulting most likely from the intermolecular electrostatic interaction between 4-tert-butylpyridine

Table 1. Photovoltaic Details of Average DSSCs Assembled from Photoanodes Sensitized by MWAS and Standard Technique, under Simulated 1 Sun Intensity (1000 W m−2) technique (sensitization time) μw 100 °C (5 min)a,c,d μw 80 °C (5 min)a,c,d μw 75 °C (5 min)a,c,d standard (14 h)b,c,d standard (10 min)b,c,d

Voc (mV)

Jsc (mA cm−2)

FF (%)

η (%)

790(±8)

12.2(±0.1)

67(±0.1)

6.5(±0.2)

790(±9)

11.3(±0.1)

67(±0.1)

6.0(±0.2)

795(±8)

11.3(±0.1)

66(±0.1)

6.0(±0.2)

830(±8) 400(±10)

12.0(±0.1) 1.17(±0.1)

66(±0.1) 49(±0.1)

6.6(±0.2) 0.2(±0.2)

Microwave settings: temperature set at 100, 80, or 75 °C. The power is allowed to vary in order to keep the temperature constant. Conditions: equilibration time, 40 s; stirring, no; cooling, no. Additional details are reported in the Supporting Information (Figures S2 and S3). bElectrodes (preheated at 90 °C) immersion in a 3.0 mL solution of LEG4 at room temperature, stored in the dark (14 h or 10 min). cParameters of DSSCs measured within 1 h after sealing. dThe values in the parentheses report the sample standard deviation. Histograms reporting the efficiency of each solar cell in all the used batches are available in the Supporting Information (Figure S4). a

that complete dye coverage in DSSCs with cobalt-based mediators is critical for optimal device performance.46 The photovoltaic devices built from photoanodes sensitized via 14 h room temperature immersion and MWAS techniques at 100 °C perform in a similar fashion. A marked difference in the Voc, higher for the standard devices, may be derived from different dye coverage or assembly. As previously discussed, the way in which the sensitizer self-assembles on the surface depends on the adsorption temperature employed. This may

Figure 1. Photovoltaic parameters of DSSCs with photoanodes sensitized via 100 °C MWAS technique (red ■) and by standard room temperature immersion for 14 h (black ●), over a period of 80 days. The day in which the devices were sealed is considered day 1. Error bars represent the statistical averaging based on 5 solar cells. I−V curves and IPCE of these cells after 14 and 80 days are reported in the Supporting Information (Figure S5). 204


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ACS Applied Energy Materials and the 1,2-dimethyl-3-propylimidazolium ions.49 The blocking layers reduce the interfacial reaction of the TiO2 electrons with I3− ions, resulting in higher Voc, FF, Jsc, and overall conversion efficiency. However, the data reported in Figure 1 clearly show a similar trend for cobalt-based electrolyte. This observation suggests, for the first time, that the aging effect is mainly related to the additional components of the electrolyte (i.e., lithium salt and 4-tert-butylpyridine), rather than the specific redox mediator or solvent employed. As a consequence of the aging effect, the fill-factor (FF) is subjected to a significant improvement over the observed period of time. The values are initially equal to 66−67%, reaching 75% after 80 days for both MWAS and standard photoanodes. On the other hand, the photocurrent density (Jsc) is subjected to a significant variation. Initially, due to the aging effect, it significantly improves for both devices causing the notable increase in the overall efficiency. However, a light but continuous decrease can be seen after 14 days. The reduction is more pronounced for the standard devices, which exhibit appreciably lower photocurrent after 80 days. Nevertheless, the inferior Jsc of the standard devices is compensated by their relatively higher opencircuit voltage (Voc), which finally results in similar power conversion efficiency. Finally, a lower Voc associated with higher sensitization temperature has already been described, and our data agree with those reported in the literature.27,29 The performance of devices assembled from photoelectrodes immersed at room temperature in the dye-bath solution for 10 min has been monitored for a total period of 14 days. Figure 2 shows how their properties changed during this time.

charge extraction, and transport time measurements, on the three types of sensitized photoanodes (Figure 3).

Figure 3. Semilogarithmic plot of the electron lifetime at open circuit as a function of the Voc (top), charge-extracted plot (middle), and log−log plot of electron transport time at short-circuit condition as a function of the photocurrent (bottom) for DSSCs assembled from differently sensitized photoanodes: MWAS at 100 °C (red ●), standard room temperature immersion for 14 h (black ■) and 10 min (blue ▲). Error bars represent the statistical averaging based on 2 solar cells, and they have been calculated for all the measurements (due to the high precision of the measurements, some error bars may not be visible).

Figure 2. Photovoltaic details of DSSCs with photoanodes sensitized by standard room temperature immersion for 10 min, over a period of 14 days.

After 10 min immersion, the amount of dye absorbed on the surface of the TiO2 electrodes is visibly low. As a consequence, the number of electrons injected into the conduction band of TiO2 is poor, resulting in a low-lying quasi-Fermi level, which causes the low Voc observed in the photovoltaic devices.50,51 Poor distribution of dyes on the surface of TiO2 may lead to greater charge recombination, which lowers both the Voc and fill-factor.46,52 However, despite the relatively scarce amount of sensitizer, the devices clearly show enhanced performances after a 14 days aging period. With consideration of the performances of days 1 (Table 1) and 14 (Voc = 405 mV; Jsc = 1.95 mA cm−2; FF = 50%; η = 0.4%), the devices exhibit a 170% increased efficiency after aging. As previously observed, the higher photocurrent is the main reason for the improved efficiency. To account for the differences in the above-mentioned photovoltaic performance, we conducted electron lifetime,

The electron lifetime (Figure 3, top) is determined under open-circuit conditions and only depends on the equilibrium between injected electrons and recombination of the injected electrons with the oxidized form of the redox couple. DSSCs bearing MWAS photoanodes exhibit a slightly lower electron lifetime (3.2 ms at 795 mV) compared to those of the 14 h standard devices (3.7 ms at 825 mV). However, both of them showed a longer electron lifetime, more than 2 orders of amplitude, than those based on the standard 10 min. Similar trends were also observed in the case of the charge extraction measurement (Figure 3, middle), which measures the accumulated charges within the mesoporous films at a different voltage at open-circuit conditions and is widely used in the literature to conclude the band edge movement of the TiO2. 205


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ACS Applied Energy Materials Herein, we have found that the charge extraction curves of the films sensitized from MWAS methods are similar to that of the standard 14 h within experimental errors, but both largely shifted to higher voltages compared to the films sensitized only 10 min in the standard methods. Finally, in the photocurrent response measurement (Figure 3, bottom), determined from the transient photocurrent decay of DSSCs at short-circuit under small-light perturbation, we observe, again, a similar photocurrent response from the standard 14 h sensitized films and the films fabricated via MWAS, whereas the photocurrent response of the films sensitized from the standard 10 min are found to be dramatically accelerated. The above measurements indicated that the films sensitized from the standard 10 min method have insufficient dye coverage, as previous studies have clearly shown by observation of reduced electron lifetime, charge extraction shifted to lower voltages, and faster photocurrent response (which is mostly contributed from the very fast charge recombination loss instead of a fast transport).46 On the other hand, the MWAS method is sufficient to guarantee high dye coverage, evident from the electronic properties that are rather similar to those of DSSCs fabricated from the 14 h standard sensitization method. The slight photovoltage decrease (∼30 mV) of the DSSCs by the MWAS methods is primarily due to the decrease of its electron lifetime. The reason for this reduced electron lifetime is not clear yet, but it is suspected to be related to the formation of dye aggregates, due to its short-time self-assembly of dye molecules in a very large amount (for a saturated surface, the coverage for LEG4 was reported in the order of 10−7 mol cm−2),53 which was previously shown to have caused a higher recombination.26,27,29 The suggested presence of aggregates can also explain the relatively small loss of Jsc of DSSCs fabricated from MWAS in the stability studies (Figure 1). Assuming that changes in the DSSCs performance are primarily related to dye desorption, an excess of sensitizer on the TiO2 surface due to dye aggregation would prevent desorption of the surface-bound dye molecules, as the desorption of initial loosely bound dyes could saturate the surrounding electrolyte shifting the thermodynamic equilibrium for desorption. ATR-IR spectra of LEG4 powder, and LEG4 absorbed on TiO2 by standard room temperature (14 h) electrode immersion, and by MWAS technique (100 °C, 5 min), were recorded to investigate the binding mode of the dye on the surface. The results are reported in Figure 4. The general tendency of carboxylic acids is to form dimers both in solution and in the solid state. Accordingly, we can assign the two characteristic infrared stretching bands at 1718 and 1681 cm−1, visible in the spectra of LEG4 not absorbed on TiO2, to the νC=O of the monomer H-bonded and the dimer Hbonded, respectively.54−56 Interestingly, the IR spectrum of LEG4 absorbed on TiO2 by the standard method shows a weak band at 1718 cm−1 as well, suggesting the presence of a small amount of sensitizer anchored in a monodentate fashion.57,58 MWAS samples do not display any vibrational frequency associated with the carbonyl group stretching; therefore, we can rule out the presence of binding modes that differ from a bidentate chelating of a Ti atom, or a bidentate bridging between two different Ti atoms.57,59 This observation is further supported by the presence of an intense signal at 1582 cm−1 for the spectra of LEG4 on TiO2, which is assigned to OCO asymmetric stretching of the carboxylate (−COO−).60,57,6 The asymmetric stretching of the carboxylate unit is absent in the IR response of LEG4 in powder form, which shows intense signals

Figure 4. ATR-IR transmittance spectra obtained from LEG4 dye powder (blue line, bottom), LEG4 on TiO2 absorbed by standard room temperature immersion for 14 h (black line, middle), and LEG4 on TiO2 absorbed by MWAS at 100 °C for 5 min (red line, top).

at 1602, 1561, 1519, 1493, and 1408 cm−1 deriving from the aromatic CC stretching of the donor and spacer moieties. In the spectra of the dye absorbed on TiO2, a prominent band located at 1377 cm−1, related to the OC O symmetric stretching of the carboxylate (COO−), is also present. Finally, the intense bands at 1289 cm−1 (LEG4 on TiO2), and at 1251 cm−1 (LEG4 powder), as well as the signals between 1118 and 1070 cm−1, correspond to the stretching vibrations of the CO and CN bonds on the donor moiety. Therefore, ATR-IR analysis reported in Figure 4 suggests that the binding mode of LEG4 dye on TiO2 is nearly equivalent for both techniques. In order to get a deeper understanding of the sensitization mechanism and separate the thermal and the microwave contribution, the TiO2 photoanodes were immersed in the refluxing dye-bath solution for 5 min. DSSCs assembled with these electrodes exhibited significantly lower performance than those constructed from MWAS and standard (14 h) electrodes. Figure 5 shows a comparison between the I−V characteristics of DSSCs constructed from photoanodes sensitized with different techniques. As previously mentioned, the temperature of the substrate during deposition can have a significant effect on the structure of the adsorbed species.5,29 Fast absorption of dye molecules on mesoporous metal oxide surfaces at room temperature is diffusion-limited, and the diffusion rate of the adsorbate molecules across the void space between the pores of the photoanode increases at a higher temperature.26,27 Therefore, DSSCs including TiO2 electrodes sensitized at reflux for 5 min have adsorbed a greater amount of sensitizer and are characterized by significantly higher Voc, Jsc, and overall conversion efficiency than those assembled from 10 min room temperature dye-bath immersion. However, despite similar sensitization time and temperature employed, MWAS at 80 °C for 5 min displays greatly improved performances (Figure 5 and Table 1). This evidently suggests that the fast sensitization time associated with the good performances deriving from MWAS must have an additional component. 206


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Figure 5. Current−voltage (I−V) curves of DSSC assembled from standard room temperature photoanode immersion for 14 h (black, ●), MWAS photoanodes at 100 °C (red, ■), MWAS photoanodes at 80 °C (green, ◆), photoanodes immersed for 5 min in the refluxing dye-bath solution (magenta, ★), and standard room temperature photoanodes immersion for 10 min (blue, ▲). The curves are related to the performances of the devices during the first day. Detailed photovoltaic parameters for the photoanodes refluxed for 5 min are reported.

It is common knowledge that microwave energy is strongly absorbed by conductive films, which, as a consequence, are heated. The heating is primarily due to the generation of an electromagnetic field and resulting induced eddy current on the film surface that is responsible for the Joule effect.61,62 This phenomenon is well-known and has been widely used to promote annealing, sintering, synthesis, and deposition of different materials on conductive substrates such as fluorinedoped tin oxide (FTO) and indium-doped tin oxide (ITO).63−67 Since the glass substrates employed for the photoanodes preparation are coated with a conductive FTO layer, we expect its interaction with the microwave radiation. Indeed, we noticed that a small number of the TiO2 electrodes subjected to MWAS were damaged. SEM pictures of sensitized electrodes are shown in Figure 6. Additional images can be found in the Supporting Information (SI). Nevertheless, by carefully smoothing out the edges of the substrate and properly modulating the microwave radiation power (Table S1, Supporting Information), the damage can be easily prevented. However, the study of damaged electrodes can provide significant insights into the MWAS mechanism. Figure 6c shows a damaged TiO2 section of a photoanode sensitized via MWAS technique. Both the scattering and the transparent TiO2 layers have been removed, and the exposed glass substrate appears visibly damaged as well, bearing holes of different size. The damage not only is limited to the TiO2 area but also crosses the entire electrode (Supporting Information, Figures S6 and S7). This latter aspect suggests that the damage is most likely due to charge accumulation at the sides of the electrode, that eventually result in an electrostatic discharge which damages the substrates. This hypothesis is additionally supported by the observation that the energy and the heat produced by the electrostatic discharge are so intense as to melt and vaporize the TiO2, FTO layer, and even part of the glass substrate encountered in its pathway. Energy-dispersive X-ray

Figure 6. Scanning electron microscopy (SEM) images of TiO2 electrodes sensitized by standard room temperature immersion (a) and MWAS (b). Damaged electrode during MWAS (c).

(EDX) elemental mapping has been performed on the damaged electrode in order to confirm the identity of each exposed section (Supporting Information, Figure S8). It is reasonable to conclude that, in suitable conditions, the mechanism promoting the fast and efficient MWAS electrode is characterized by an intense electromagnetic field and current, which locally heats the sample. Electric field, current, and heat are therefore responsible for the faster and more efficient sensitization observed in comparison to the conventional heating.

4. CONCLUSION In conclusion, we have shown that the microwave-assisted sensitization (MWAS) technique can be a new, fast, and efficient method for a high-quality dye-layer formation on TiO2 photoanodes. The light-to-electricity conversion efficiency of DSSCs assembled with photoanodes sensitized via MWAS technique and 14 h standard room temperature dye-bath immersion is comparable. The performance of the devices has 207


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been monitored over a period of 80 days, showing comparable stability. However, the sensitization time required for MWAS is dramatically shorter compared to the standard method: 5 min and 14 h, respectively. ATR-IR measurements of TiO 2 nanoparticles sensitized via MWAS and standard techniques exhibit similar profiles, indicating an equivalent anchoring mode of the dye to the semiconductor surface. Equivalent heating temperature and immersion time were employed for the sensitization of photoanodes via a conventional heating and microwave technique, revealing that the latter results in markedly superior performance. We investigated the role of the microwave radiation deducing that its interaction with the conductive FTO layer results in a local electromagnetic field which induces an eddy current, producing local heating (Joule effect). Therefore, we concluded that these three aspects are responsible for the fast and efficient dye-layer formation observed employing microwave-assisted technique. We strongly believe that MWAS can be successfully extended to a wide variety of semiconductors and sensitizers/molecules even bearing different anchoring groups and for different purposes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.7b00088. Molecular structure of LEG4; pressure, temperature, and power graphs obtained from the microwave instrument used; histograms showing the device efficiency of the solar cells used in this study; I−V curves and IPCE spectra; and SEM and EDX of damaged electrodes (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

J. M. Gardner: 0000-0002-4782-4969 G. Boschloo: 0000-0002-8249-1469 S. Ott: 0000-0002-1691-729X Present Address

§ Department of Chemistry, Imperial College London, London SW7 2AZ, UK.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the Carl Trygger Foundation, the Swedish Research Council, the Swedish Energy Agency, and the Knut & Alice Wallenberg Foundation for financial support. J.M.G. would like to kindly acknowledge the support of the Swedish Government through the research initiative “STandUP for ENERGY”.

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REFERENCES

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