Ag(I)-Thiolate Protected Silver Nanoclusters for Solar cells

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Ag(I)-Thiolate Protected Silver Nanoclusters for Solar cells: Electrochemical and Spectroscopic Look into Photoelectrode/Electrolyte Interface Muhammad A Abbas, Seog Joon Yoon, Hahkjoon Kim, Junghyun Lee, Prashant V. Kamat, and Jin Ho Bang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00049 • Publication Date (Web): 06 Mar 2019 Downloaded from http://pubs.acs.org on March 6, 2019

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ACS Applied Materials & Interfaces

Ag(I)-Thiolate Protected Silver Nanoclusters for Solar cells: Electrochemical and Spectroscopic Look into Photoelectrode/Electrolyte Interface Muhammad A. Abbas,† Seog Joon Yoon,‡ Hahkjoon Kim,ξ Junghyun Lee,§ Prashant V. Kamat,*,‡ and Jin Ho Bang*,†,§,ǁ †

Nanosensor Research Institute, Hanyang University, 55 Hanyangdaehak-ro, Sangnok-gu, Ansan, Gyeonggi-do 15588, Republic of Korea ‡ Notre Dame Radiation Laboratory and Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States ξ Department of Chemistry, Duksung Women’s University, Seoul 01369, Republic of Korea § Department of Bionano Technology, Hanyang University, Ansan, Republic of Korea ǁ Department of Chemical and Molecular Engineering, Hanyang University, Ansan, Republic of Korea ABSTRACT: Intrinsic low stability and short excited lifetimes associated with Ag nanoclusters (NCs) are major hurdles that have prevented the full utilization of the many advantages of Ag NCs over their longtime contender, Au NCs, in light energy conversion systems. In this report, we diagnosed the problems of conventional thiolated Ag NCs used for solar cell applications and developed a new synthesis route to form aggregation-induced emission (AIE)-type Ag NCs that can significantly overcome these limitations. A series of Ag(0)/Ag(I)-thiolate core/shell structured NCs with different core sizes were explored for photoelectrodes, and the nature of the two important interfacial events occurring in Ag NC-sensitized solar cells (photoinduced electron transfer and charge recombination) were unveiled by in-depth spectroscopic and electrochemical analyses. This work reveals that the subtle interplay between the light absorbing capability, charge separation dynamics, and charge recombination kinetics in the photoelectrode dictate the solar cell performance. In addition, we demonstrate significant improvement in the photocurrent stability and light conversion efficiency that have not been achieved previously. Our comprehensive understanding of the critical parameters that limit the light conversion efficiency lays a foundation on which new principles for designing Ag NCs for efficient light energy conversion can be built. KEYWORDS: nanoclusters, solar energy conversion, aggregation-induced emission, electron transfer, charge recombination INTRODUCTION In recent years, a few atom metal NCs featuring molecule-like discrete electronic structures have gained unprecedented attention for their use in various applications because of their unique optical,1-7 optoelectronic,8-16 and catalytic properties.17-23 Apart from these physical characteristics of NCs, a suitable HOMO– LUMO gap for visible light absorption and a long lifetime of excited states for efficient charge separation make them emerging candidates for solar energy harvesting applications.24 The ability of thiolated Au and Ag NCs to transfer excited electrons to TiO2 and generate photocurrent has been demonstrated in the literature.15-16,25-26 Since Au NCs are the most stable and versatile NCs, they were the first to be successfully implemented in photoelectrodes.15 The term “metal-cluster-sensitized solar cells (MCSSCs)” was later coined to describe this new type of solar cell.13 Given the short history of MCSSCs (the first conceptual demonstration was in 2010),24 the development of MCSSCs is still at an early stage compared to other types of solar cells. However, recent advances in synthesis methodologies and comprehensive knowledge gained from extensive experimental and computational efforts have resulted in striking developments in MCSSCs. As an example of these advancements, our group recently demonstrated a breakthrough in which the best power conversion efficiency (PCE) of 3.8% was achieved using Au18(SR)15 NCs.11 This success provided a new

impetus to explore Ag, the relatively inexpensive cousin of Au. Extending the utilization of NCs to Ag NCs poses a challenge because the physical properties of Ag NCs are quite different from Au NCs even though they share similar bulk properties. Bigioni and co-workers revealed that the most stable Au and Ag NCs have very different structures, hinting at significant differences in the excited state dynamics, which dictate charge separation in NC-based heterojunctions.27-28 They also showed that the optical density of Ag NCs is significantly greater than that of Au NCs, which has been attributed to the position of d-electrons in Ag.27,29 A stronger quantum confinement effect resulting from a lower density of states and no Landau damping is expected for Ag NCs as well.28 These superior optical properties suggest that Ag NCs hold great promise as a new class of solar energy harvester. However, the implementation of thiolated Ag NCs as a light harvester in MCSSCs has seen very limited progress to date. The first demonstration of photocurrent generation by Ag NCs was reported by the Tatsuma group in 2013.16 After this initial promising report, several reports on the utilization of Ag NCs have followed,8,30-31 but their performance fell short of expectations with the best PCE of 0.59%. Even worse, the photocurrent was highly unstable. Exposure to illumination for a few tens of seconds caused degradation of the photocurrent by more than 90%.31

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While the physical properties of Ag NCs have not yet been explored as much as those of Au NCs, it is commonly accepted that thiolated Ag NCs face two main challenges in light energy conversion applications: poor stability and short excited state lifetime. Only some well-defined NCs like Ag44(SR)30 are stable,32-33 most Ag NCs suffer from low stability. In addition, Ag NCs have much shorter excited state lifetimes compared to Au NCs, which could result in limitations in photoinduced electron transfer.34 Interestingly, Ashenfelter et al. have reported that the quantum yield (QY) of photoluminescence (PL) has an inverse relationship with the size of Ag NCs; that is, small Ag NCs tend to have better PL QY compared to the larger ones.29 They postulated that the small Ag NCs had fewer non-radiative recombination pathways, which led to the better QY. Small NCs are also considered to be more stable than larger NCs due to their larger HOMO–LUMO gap.27 However, this gives rise to a conundrum when choosing thiolated Ag NCs as a solar energy harvester. While small NCs have better excited state lifetimes, lower self-recombination, and better stability, they would have a larger HOMO–LUMO gap that limits light absorption. To harvest a broader range of the solar spectrum, large Ag NCs with a smaller HOMO–LUMO gap are preferred; however, they tend to have a much shorter excited state lifetime and poor stability. We speculated that these problems encountered in conventional Ag NCs could be circumvented to a large extent by employing AIE-type Ag NCs that feature a Ag(0)/Ag(I)-thiolate core/shell structure.35 The presence of the Ag(I)-thiolate complexes that are much less prone to oxidation can greatly enhance the stability of Ag NCs by protecting the inner Ag(0) core. In addition, they can promote phosphorescence from ligand-tometal charge transfer (LMCT), leading to dramatically enhanced PL (i.e., a long excited state lifetime).36 In this work, we devised a strategy to synthesize a series of Ag(0)/Ag(I)-thiolate NCs with varying Ag(0) core sizes to scrutinize their potency as light harvesters in MCSSCs. On the other hand, the greatly improved stability of the AIE-type Ag NCs allowed us to explore charge transfer and recombination dynamics in Ag NCsensitized photoelectrodes using in-depth spectroscopic and electrochemical analyses. Based on the size-dependent photoelectrochemical behavior of Au NCs that we revealed recently, larger Ag NCs can harvest a broader range of the solar spectrum. However, this might be accompanied by unfavorable changes in recombination kinetics and charge separation abilities of the NCs, which could eventually adversely affect the PCE.11 Therefore, it is very important to investigate the photovoltaic properties of a series of Ag NCs to truly highlight the potential of Ag NCs as a sensitizer in MCSSCs. Using the AIE-type Ag NCs, we were able to achieve a PCE of 1.40% and significantly improve solar cell stability, which marked an important advance in the development of Ag-based MCSSCs.31 Comprehensive physical insights into the working mechanism of Ag NC-based MCSSCs (including carrier relaxation dynamics, photoinduced electron-transfer kinetics, and interfacial charge recombination kinetics) are also provided to shed light on the implications of this important advance and identify the remaining challenges. EXPERIMENTAL Synthesis of Ag(0)/Ag(I)-thiolate core/shell NCs. The synthesis protocol is depicted schematically in Scheme S1. Briefly, 12.5 mL of 20 mM AgNO3 and 7.5 mL of 50 mM reduced-glutathione (GSH) were added to a 500 mL flask containing 200 mL of deionized water. White precipitates appeared instantly as

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a result of the formation of Ag–GSH complexes, and this mixture was stirred for 2 min (Figure S1A). The precipitates dissolved completely when the pH of the reaction mixture was changed to 11 by adding a 1 M NaOH solution, forming a transparent solution (Figure S1B). Heating this solution at 60, 70, or 90 °C for 1 h under stirring at 500 rpm yielded initial products of three different Ag NCs, whose differences were evident from the color change of the transparent solution from light yellow to dark yellow to light orange (Figure S1C). After the solution was naturally cooled to room temperature, the pH was changed again to 3.75 by adding 1 M HCl. This abrupt pH change resulted in an instantaneous darkening of the solution color that signified the aggregation of Ag(I)-thiolate complexes on the surface of the Au(0) core (Figure S1D). The color of the NC solution became darker over time. Hence, the NC solution was aged for 24 h at room temperature to complete this process (Figure S1E), and no further change in the color was observed after 24 h. Any precipitates formed during this period were removed via centrifugation, and the NC solution was then concentrated using a rotary evaporator to prepare a dipping bath for sensitization. Characterization. The absorption spectra of the Ag(0)/Ag(I)thiolate core/shell NCs in solutions and on TiO2 films were measured with a UV-vis spectrophotometer (SCINCO S-3100) equipped with a diffuse reflector. The separation of the Ag NCs was carried out on a 4-slab gel electrophoresis unit (Bio-Rad Mini-PROTEAN Tetra Cell). A 100×73 mm B-type slab was prepared with 30 wt% acrylamide monomers, and the eluted buffer solution contained 192 mM glycine and 25 mM tris(hydroxymethylamine). Concentrated Ag NC solution was dispersed in a 10% (v/v) glycerol/water solution. The polyacrylamide gel electrophoresis (PAGE) unit was run for 1 h at 160 V after 20 µL of the Ag NC solution was loaded onto the stacking gel. Dynamic light scattering (DLS) experiments were carried out using a particle analyzer (Litesizer™ 100, Anton Paar) to monitor the aggregation of the Ag(I)-thiolate complexes upon pH change. Transmission electron microscopy (TEM) images of the Ag NCs were acquired using a TITAN 80-300 (FEI) electron microscope. X-ray photoelectron spectroscopy (XPS) was carried out using a PHI Versa Probe system equipped with a 100 W Al K Alpha X-ray source. The emission and excitation spectra of Ag NCs were measured with a spectrofluorometer from Horiba Scientific (Nanolog). Time-resolved PL and transient absorption spectroscopy (TAS) were performed using Ag NCs adsorbed on semi-transparent ZrO2 and TiO2 films. A custombuilt inverted fluorescence microscope (Olympus, IX-70) was employed to measure PL lifetimes. The excitation beam (375 nm, 100 μW) was generated using a picosecond pulsed diode laser operating at 80 MHz (Picoquant, LDH-P-C-375B), which was focused into the Ag NC samples, and the resulting photons streaming from the detector (GaAs photomultiplier tube, Hamamatsu, H7422-40) were registered using a time-correlated single photon counting (TCSPC) board (Timeharp260, Picoquant, PICO model). TAS spectra were recorded by a transient absorption spectrometer (Ultrafast systems), where 130 fs pulses (1 mJ/pulse with 1 kHz repetition rate) were generated by a femtosecond laser system (Clark MXR CPA-2010). 95% of the total beam was used as the pump beam, and the remaining 5% was used to generate the probe beam. A CaF2 crystal was used to generate a white light continuum for the probe beam. A 387 nm laser was used as an excitation source, and the laser power was set to 15 μJ/cm2. All the measurements were performed in vacuum conditions to prevent degradation.


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Solar Cell Fabrication and Evaluation. Following a procedure reported previously, 10 µm-thick mesoporous TiO2 films followed by a 5 µm-thick scattering layer were deposited on TiCl4-treated fluorine-doped tin oxide (FTO) glass.11,37 The Ag(0)/Ag(I)-thiolate NCs were loaded onto the TiO2 films by dipping them into the prepared Ag NC solution at pH 3.75 for 48 h in the dark. After taking out the TiO2 films, they were rinsed with deionized water and ethanol sequentially to wash off unattached Ag NCs. [Co(phen)3(PF6)2]2+ and [Co(phen)3(PF6)3]3+ complexes with 1,10-phenanthroline (phen) ligands were synthesized as a redox couple in the electrolyte following the procedure reported previously.38 The electrolyte was composed of 0.2 M Co[(phen)3(PF6)2]2+, 0.03 M Co[(phen)3(PF6)3]3+, 0.1 M LiClO4, and 0.5 M 4-tert-butylpyridine in anhydrous acetonitrile. A hot-melt ionomer (Surlyn) was used as the separator, and it was melted at 90 °C on Ptsputtered FTO glass that was used as the counter electrode. A drop of electrolyte was placed on the counter electrode with the separator, which was assembled carefully with the photoanode, and held together with steel clips. The assembled solar cells were illuminated by a solar simulator (HAL-320, Asahi Spectra), and a Keithley 2400 source meter was used to record photocurrent density–photovoltage (J–V) curves. Prior to the measurements, the solar simulator was calibrated to 1 sun with a standard silicon diode (CS-20, Asahi Spectra). Custom-built equipment with a monochromator from Newport, a Xenon lamp, and a Keithley source meter was used to measure the incident photon-to-current efficiency (IPCE) spectra. An electrochemical workstation (REF600-25049) from Gamry Instruments was employed to perform the electrochemical impedance spectroscopy (EIS) of MCSSCs in the frequency range of 0.01 Hz to 100 kHz under dark conditions. Echem Analyst software from Gamry Instruments was used to fit the EIS spectra. RESULTS AND DISCUSSION Synthesis and Characterization of Ag(0)/Ag(I)-thiolate core/shell NCs. From a design point of view, three characteristics of NCs must be considered to assemble MCSSCs. The first is the stability of NCs in the solution. NCs should be able to remain in the solution for an extended period of time to allow the adsorption of NCs onto TiO2 films. The second issue is strong binding between NCs and TiO2. The NC ligand should have functional groups that can anchor to the TiO2 surface. Third, NCs must have the ability to undergo charge transfer with a redox couple and remain chemically stable. Keeping the above requirements in mind, our search for a stable Ag NC sensitizer started with thiolated Ag44(SR)30 NCs, which are known to be very stable Ag NCs.33 Although 5-mercapto-2-nitrobenzioc acid (MNBA)protected Ag44(MNBA)30 NCs were indeed very stable in the solution phase and could be adsorbed onto TiO2 films as well, they decomposed instantly when they came in contact with redox couples (Co2+/Co3+ or I-/I3-) (Figure S2A). GSH-protected Ag16(GSH)9 NCs were our next choice because they were reported to be reasonably stable in an aqueous solution.39 While Ag16(GSH)9 NCs were capable of being anchored to TiO2, a fully-assembled solar cell showed almost no photocurrent with the Co2+/Co3+ redox couple. A photocurrent density of over 600 μA/cm2 was observed with iodide-based electrolyte, but it degraded very rapidly within few seconds.8 From these attempts, we learned that GSH could be a good choice for a ligand, but

the GSH-protected Ag NCs were generally prone to oxidation and suffer from severe non-radiative recombination.8 Scheme 1. Schematic Illustration of pH-Induced Aggregation of Ag(I)-Thiolate Complexes on Ag(0) NC Surface.

To overcome these hurdles, we paid attention to the AIE-type Ag NCs that have recently been introduced to the NC research community.35-36 This new type of NC showed a significantly long lifetime of excited states by activating the radiative decay via the LMCT mechanism. In addition, they are much more stable than conventional NCs due to protection against oxidation offered by the Ag(I)-thiolate complexes. However, since the Ag(I)-thiolate complexes have low solubility in the acidic condition that is required to adsorb the NCs on the TiO2 surface (Figure S2B), the AIE-type Ag NCs suffered from very low adsorption on TiO2. Therefore, an elaborate synthesis strategy was needed, in which the focus would be placed solely on fulfilling the required characteristics of Ag NCs mentioned above. As a remedy to this challenge, we exploited the pH-driven aggregation of the Ag(I)-thiolate complexes on the surface of Ag NCs to synthesize the Ag(0)/Ag(I)-thiolate core/shell NCs (Scheme 1). In our new strategy, Ag NCs were initially formed by heating Ag(I)-thiolate complexes in a basic solution; then, a subsequent pH change made the solution acidic and promoted the aggregation of unreacted Ag(I)-thiolate complexes onto the preformed Ag(0) NC core. This was possible because lowering the pH resulted in the protonation of carboxylic groups of GSH that initiated the formation of Ag(I)-thiolate shells via hydrogen bonding.40-41 The thermal reduction was carried out at pH 11 at different temperatures (60, 70, and 90 °C) to control the core size. The as-obtained Ag NCs were very weakly emissive because there was no aggregation of Ag(I)-thiolate complexes on the Ag(0) NC core because of the negative charge that developed in the basic solution. However, changing the pH from 11 to 3.75 gave rise to a dramatic increase in PL (Figure 1A, S3), which was a signature of AIE. The aggregation of Ag(I)-thiolate complexes on the Ag(0) NC surface was also evidenced by the increase in the hydrodynamic diameter of Ag NCs upon pH change that was determined by DLS. All the NCs prepared at 60, 70, and 90 °C showed a gradual increase in the hydrodynamic diameter when the pH was changed from 11 to 3.75 (Figure 1B, S4). One might argue that the red-shift in the absorption spectrum after the pH change (Figure S5) was a result of the formation of larger NCs at the expense of smaller NCs during aging. However, when the pH of the NC solution was changed back to 11, the UV-vis spectrum shifted back to its original positions (Figure S6) and the hydrodynamic diameter of NCs also decreased again (Figure 1B, S4). This indicated the reversibility



of the aggregation of the Ag(I)-thiolate complexes on the Ag(0) NCs. This strategy significantly improved the stability of Ag(0)/Ag(I)-thiolate NCs at pH 4, where the sensitization was performed. It turned out later that this strategy could greatly promote the adsorption Ag NCs on TiO2. Three different Ag(0)/Ag(I)-thiolate NCs obtained at 60, 70, and 90 °C will hereafter be referred to as NC-1, NC-2, and NC-3, respectively. (A)

(B) Mean Hydrodynamic Diameter (nm)

NC-1 NC-2 NC-3

Integrated PL Intensity (a.u.)

(C)

NC-1 NC-2 NC-3

4.0 3.5 3.0 2.5 2.0

10.9 7.0 5.5 3.8 5.5 7.0 10.9

10.9 7.0 5.5 3.8 5.5 7.0 10.9

pH

pH

0.4 0.08

0.3

Absorbance

0 a

0.2 400

NC-1

b

NC-2

c

NC-3

600

0.1

0 300

a c

400

different in nature as they produced PLs of different colors under UV light. The PAGE results showed that the color of the NCs red-shifted from NC-1 to NC-3, affirming the increase in average NC size from NC-1 to NC-3. We tried in vain to identify the Ag(0)/Ag(I)-thiolate NCs using mass spectrometry. The identification of small Ag NCs via the mass analysis has been notoriously difficult because they are subject to fragmentation during electrospray-ionization,29,43-45 which is also the reason that our attempts at mass spectrometry failed. However, TEM analysis confirmed the formation of NCs whose average sizes fell in the 2-3 nm range and increased in size with increasing synthesis temperature (Figure S7). Additional information related to the Ag(0)/Ag(I)-thiolate NCs was obtained from the XPS analysis. The oxidation states of Ag in the three NC mixtures were resolved by XPS analysis, and the intensity ratio of Ag0 and Ag+ peaks was used to estimate the Ag0/Ag+ ratio in the NCs (Figure 2C).35,46 The Ag0/Ag+ ratio increased from 1.08 to 1.18 to 1.44 for NC-1, NC-2, and NC-3, respectively, which further substantiates the hypothesis that NC size grew with an increase in synthesis temperature.42,47 The high ratios of Ag+ species also confirmed the formation of Ag(0)/Ag(I)-thiolate core/shell NCs.35

500

600

700

(C)

Ag3d5/2 Raw Ag0 Ag+

Ag3d3/2 NC-3

800

Wavelength (nm)

Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 1. (A) Integrated PL intensity and (B) mean hydrodynamic diameters of Ag NCs measured at various pH conditions. Insets of (A) show the digital photographs of NC-3 under UV light during a pH change from 11 to 3.5 and to 11. (C) UV-vis absorption spectra of the Ag(0)/Ag(I)-thiolate NCs. The left inset shows zoomed-in spectra near the absorption edge, and the right insets are the digital photographs of Ag(0)/Ag(I)-thiolate NCs.

The size distribution of the Ag(0)/Ag(I)-thiolate NCs depended directly on the temperature, where a higher temperature yielded larger Ag NCs on average. This deduction was confirmed by various characterization techniques. Typically, the HOMO–LUMO gap of NCs depends on the size, where smaller NCs have a larger HOMO–LUMO gap and vice versa. In certain cases, however, larger NCs can have a larger HOMO– LUMO gap than smaller NCs because the gap also depends on the structure of the NCs.4,42 In our case, all the NCs were prepared under similar conditions. Therefore, NCs were expected to have similar structures, as implied by the featureless absorption spectra (Figure 1C). Nonetheless, the absorption edge of the NCs shifted towards the red region with an increase in size. The absorption edge shifted from 520 to 600 nm when the temperature increased from 60 to 90 °C (Figure 1C), indicating that larger NCs were formed at elevated temperatures, which was also evident from the different colors of each NC solution. Upon separation by PAGE, NC-1 showed almost a single band with a light yellow color (Figure 2A). However, NC-2 resolved into three bands with light yellow (band 1) and dark yellow (band 2 and 3) colors with all three bands showing similar intensities (Figure 2A). NC-3 was resolved into four bands, while bands 4 and 5 seemed to merge as one with similar deep red colors. However, it was evident from Figure 2B that bands 4 and 5 were

NC-2

NC-1

375 372 369 366 Binding Energy (eV)

Figure 2. Digital photographs of PAGE gel after separation taken under (A) visible and (B) UV light. (C) XPS Ag 3d spectra of NC1, NC-2, and NC-3.

Investigation of Excited States and Electron Transfer Dynamics. Examining PL can provide a glimpse into the excited state behavior of the Ag(0)/Ag(I)-thiolate NCs. Since PL from NCs is intimately associated with the characteristics of the ligandmetal interface,29,48 a subtle change at the interface often leads to a dramatic difference in the PL properties. The Ag(0)/Ag(I)thiolate NCs explored in this work were no exception. While the Ag NCs formed at pH 11 showed a barely visible PL, the Ag(0)/Ag(I)-thiolate NCs generated by the pH change were much more luminescent (Figure 3). All the NCs exhibited broad emission spectra and a Stokes shift of more than 200 nm, which was consistent with the general characteristics of the AIE-type NCs.35 The illumination of the NC bands separated by PAGE under UV light showed that bands 4 and 5 were predominantly fluorescent, and their abundance in the NCs solution decreased


PL (a.u.)

Normalized Intensity

300 400 500 600 700 800 Wavelength (nm)

1.0

NC-1 NC-2 NC-3

0.8 0.6 0.4 0.2 0 0

60 120 Time (ns)

180

QY

(1)

𝜏

(2)

As given in Table 1, the non-radiative recombination was the slowest in NC-3 and the fastest in NC-1. These results were consistent with the QYs and suggest that NC-1 was more prone to the non-radiative recombination than its larger counterparts. Given the size-dependency of the PL in conventional Ag NCs,29 this observation was a very important finding in our study because the absorption capability and the lifetime of the excited state showed the same trend in the Ag(0)/Ag(I)-thiolate NCs, thus offering a way to mitigate the issues associated with conventional Ag NCs mentioned above. (A)

Figure 3. Excitation and PL spectra (excitation at 410 nm) of (A) NC-1, (B) NC-2, and (C) NC-3 and their absorbance traces (for clarity the vertical axes for absorbance traces are not shown). (D) PL lifetime decay curves of all the Ag NCs. The insets of (A-C) show digital photographs of the respective NCs under UV light.

Table 1. Average PL Lifetime (τavg), Quantum Yield (QY), and Radiative (kr) and Non-Radiative (knr) Rate Constants of NC-1, NC-2, and NC-3. a

Sample

τavg (ns)

QY (%)

kr (s-1)

knr (s-1)

NC-1-ZrO2

74.9

0.6

8.0 × 104

1.33 × 107

NC-2-ZrO2

84.1

1.5

1.8 × 105

1.17 × 107

NC-3-ZrO2

125.04

5.9

4.7 × 105

7.53 × 106

a The PL decay curves in Figure 3D were analyzed by the tri𝐴 𝑒 𝐴 𝑒 ) and τavg was calexponential fit (A 𝑒 ∑ 𝐴𝜏 / ∑ 𝐴 𝜏 . culated using the following equation: 𝜏

NC-1 and NC-2 spectra (Figure 3A, B). This may have originated from the radiative decay of band 1. The excitation spectra of all the NCs matched quite well with their absorption spectra. This confirmed that the PL originated from the Ag NCs rather than the impurities present in the solution. On a relative basis, NC-3 showed the brightest PL, and NC-1 exhibited the faintest PL (insets in Figure 3). The relative PL intensities of the Ag(0)/Ag(I)-thiolate NCs are provided in Figure S8 for a quick comparison, and the QYs of NC-1, NC-2 and NC-3 measured against standard R6G were determined to be 0.6 1.5, and 5.9%, respectively. This trend was also reflected in the PL lifetimes of all the Ag NCs (Figure 3D). PL decay curves were fitted with the tri-exponential decay function, and the fitting results are summarized in the Table 1. While the overall PL lifetimes of NC-1, NC-2 and NC-3 were shorter than those of Au NCs,3-4,35-

(B) 1.0

a b

0.8

NC-1-ZrO2 NC-1-TiO2

0.6 0.4 a

0.2 0

b

0

50

100

Time (ns)

150

(C)

1.0

a b

0.8

NC-2-ZrO2 NC-2-TiO2

0.6 0.4 a

0.2 0

b

0

50

100

Time (ns)

150


300 400 500 600 700 800 Wavelength (nm)

(D)

(C)

they were substantially longer than those of typical Ag NCs, whose PL lifetimes were in the range of few ns or ps. The average PL lifetimes (τave) of NC-1, NC-2, and NC-3 were determined to be 74.9, 84.1, and 125.04 ns, respectively. To gain more insight into the excited state dynamics of NC-1, NC-2 and NC-3, these PL lifetimes and the QYs were used to calculate the rate constants of the radiative (kr) and non-radiative (knr) recombination occurring in the Ag NCs based on the following relationships:50


Excitation (a.u.)

300 400 500 600 700 800 Wavelength (nm)

36,49

PL (a.u.)

(B)

PL (a.u.)

(A) Excitation (a.u.)

with a reduction in the synthesis temperature. NC-3 had the most prominent bands 4 and 5, while they were barely visible in NC-1. Apart from bands 4 and 5, band 1 also showed a faint PL under UV light. The PL spectra of all the NCs showed two dominant peaks at 633 and 675 nm, which might be attributed to bands 4 and 5. Another peak at 614 nm was also visible in

Excitation (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



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1.0

a b

0.8

NC-3-ZrO2 NC-3-TiO2

0.6 0.4

a b

0.2 0

0

50

100

150

Time (ns)

Figure 4. PL decay curves of (A) NC-1, (B) NC-2, and (C) NC-3 adsorbed on ZrO2 (red) and TiO2 (olive) films. The solid lines represent tri-exponential kinetic fits.

Electron transfer from NCs to TiO2 is one of the basic requirements for the operation of MCSSCs. Therefore, the investigation of charge injection kinetics can identify any limitations in the MCSSCs. The first evidence of the electron transfer was obtained by the steady-state PL measurements, where the relative PL intensities from Ag NCs adsorbed on ZrO2 and TiO2 films were compared (Figure S9). There was no PL quenching observed in the ZrO2 films because the bandgap of ZrO2 (~5 eV) was too wide to accept excited electrons from the Ag NCs (Scheme 2A). In contrast, almost complete PL quenching was observed in the TiO2 films, which was ascribed to the electron transfer from the Ag NCs’ LUMO level to the conduction band of TiO2 (Scheme 2B). This observation was further validated by measuring the PL lifetime of each NC (Figure 4, Table 2). The PL lifetime significantly decreased when the Ag(0)/Ag(I)-thiolate NCs were coupled with TiO2 due to the presence of an additional relaxation pathway (i.e., electron transfer). The apparent rate constants (kc) of the electron transfer determined in this experiment showed similar values for NC-1 and NC-3 (Table 2). However, NC-2 exhibited a faster rate constant than NC-3 by approximately an order of magnitude. On the basis of the obtained kinetic parameters, we were able to conclude that the rate constants for the electron transfer and the internal non-radiative recombination were of the same order (~107 s-1), and hence they competed against each other upon photoexcitation (Scheme 2C).



Table 2. PL Lifetime Componentsa and Apparent Rate Constants (kc)b Calculated from Time-Resolved PL Measurements. Sample

A1

τ1 (ns)

A2

τ2 (ns)

A3

τ3 (ns)

τavg (ns)

NC-1-ZrO2

0.51

0.48

0.23

6.56

0.26

80.64

74.86

NC-1-TiO2

0.63

0.98

0.27

5.54

0.10

58.30

43.80

NC-2-ZrO2

0.35

0.7

0.28

9.79

0.37

90.73

84.06

a

𝜏

6

7

6.8×10

0.43

0.58

0.39

5.97

0.18

43.24

33.84

NC-3-ZrO2

0.14

1.75

0.24

16.37

0.62

130.68

125.04

NC-3-TiO2

0.33

2.1

0.37

12.06

0.31

72.91

kc

9.5×10

NC-2-TiO2

6

8.3×10

61.36

The PL decay curves in Figure 4 were analyzed by the tri-exponential fit (A 𝑒 𝐴 𝑒 ∑ 𝐴𝜏 / ∑ 𝐴 𝜏 . b kc was calculated using the following equation: 𝑘 1/𝜏 1/𝜏

𝐴 𝑒 .

), and τavg was calculated using

Scheme 2. Schematic Illustration of Charge Transfer Occurring in the Ag(0)/Ag(I)-Thiolate NCs Coupled with (A) TiO2 and (B) ZrO2 upon Photoexcitation and (C) Various Rate Constants Involved in the Ag NC–TiO2 Couple.

(A)

(B)

(C) S2

Ec

ICT

LUMO

Ec Eg=3.2 eV

LUMO

Eg=5.0 eV

HOMO

Ev

S1

HOMO

TiO2

Ag NCs

a b c d

0.5

1 ps 10 ps 100 ps 1000 ps

500 600 Wavelength (nm)

3

1.0 a b c d

0.5 0 400

700

(E)

a

d 1 ps 10 ps 100 ps 1000 ps


4

∆A (× 10-3)

∆A (× 10-3)

∆A (× 10-3)

d

0 400

a

2.0

1.5

1.0

(D)

(C) 2.5 a

1.5

C.B. of TiO2 k nr ≈ 10 7

ICT= Intramolecular Charge Transfer

(B) 2.0

a

k r ≈ 10 5

S0

ZrO2 (A) 2.0

kc ≈ 10 7

hv

Ev

1.5

a b c d

0.5 0 400

700

(F)

a

d

1.0

1 ps 10 ps 100 ps 1000 ps


3

700

a

∆A (× 10-3)

d

1

a b c d

0 400

(G)

1 ps 10 ps 100 ps 1000 ps


2

d a b c d

1 0 400

700


(H) 1.0

1.0

0.6

Normalized ∆A

a

0.8

b

0.4 0.2

a b

0 0

NC-1-ZrO2 NC-1-TiO2

400 800 1200 Time (ps)

0.4 a b

0

a b c d

1 ps 10 ps 100 ps 1000 ps


700

1.0

b

0.2

d

1

(I)

a

0.6

2

0 400

700

0.8

0 1600

1 ps 10 ps 100 ps 1000 ps

Normalized ∆A

∆A (× 10-3)

2

∆A (× 10-3)

3

Normalized ∆A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 13

NC-2-ZrO2 NC-2-TiO2

400 800 1200 Time (ps)

0.8

a

0.6 b

0.4 0.2

a b

0 1600

0

400

NC-3-ZrO2 NC-3-TiO2

800 1200 Time (ps)

1600

Figure 5. Time-resolved difference absorption spectra of Ag(0)/Ag(I)-thiolate NCs: (A) NC-1-ZrO2, (B) NC-2-ZrO2, (C) NC-3-ZrO2, (D) NC-1-TiO2, (E) NC-2-TiO2, and (F) NC-3-TiO2. Transient absorption–time profiles recorded at 660 nm following 387 nm laser excitation: (G) NC-1, (H) NC-2, and (I) NC-3 on ZrO2 (open symbols) and TiO2 (solid symbols) films.


Page 7 of 13 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3. Fitted Kinetic Parametersa and the Rate Constants of Electron Transfer (ket)b Calculated from Transient Decays of NC-1, NC-2, and NC-3 at 660 nm. Sample ZrO NC-1

2

TiO

2

ZrO NC-2

2

TiO

2

ZrO NC-3

2

TiO

2

y0

A1

τ1 (ps)

A2

τ2 (ps)

R2 c

τavg (ps)

0.65

-

-

0.36

694

0.98

694

0.57

0.07

4.17

0.35

661

0.99

547

0.50

-

-

0.50

2810

0.91

2810

0.45

0.07

7.17

0.46

989

0.99

865

0.55

0.07

12.79

0.35

622

0.98

523

0.35

0.17

1.41

0.45

650

0.99

470

ket (s-1) 3.88×10

8.00×10

2.16×10

8

8

8

Excited state traces in Figure 5 were fitted using these equations: y y 𝐴 𝑒 / 𝐴 𝑒 / or y y 𝐴 𝑒 / . b ket values were c 2 calculated using the following relationship: 𝑘 1/𝜏 1/𝜏 . R represents the goodness of fit, and a value of 1 corresponds to a perfect fit a

While the kinetic information obtained in our PL analysis was beneficial for understanding the charge transfer from NCs to TiO2, the information is rather limited and cannot be used to obtain a comprehensive understanding of the photoinduced charge transfer process because it usually occurs in the sub-picosecond to picosecond time window. We therefore performed time-resolved TAS experiments to gain more insight into the excited state behavior of the Ag NCs and the electron transfer dynamics in the Ag NC–TiO2 system. Figure 5 displays the TAS spectra of the Ag(0)/Ag(I)-thiolate NCs on ZrO2 and TiO2 films and transient absorption-time profiles recorded at 660 nm. Regardless of the types of substrate, all the Ag NCs showed a broad excited state absorption extending over the whole visible light spectrum. Unlike larger clusters (e.g., Ag32(SG)19, Ag44(MNBA)30) where strong bleaching along with a distinct excited state absorption feature are observed,28,34,40,51-53 the TAS spectra of the Ag NCs were rather featureless with no bleaching, which resembled the TAS spectrum of small Ag NCs (e.g., Ag15(SG)11).28 This result implied that the Ag(0)/Ag(I)-thiolate NCs were primarily composed of small NCs that were no larger than Ag15(SG)11. Another noticeable feature was that the transient signal (ΔA) from the Ag(0)/Ag(I)-thiolate NCs (even on ZrO2) decayed gradually with time (Figure 5A-C), unlike the Au NCs that showed very little change in difference absorption spectra after 1000 ps10. This indicated that the excited state lifetime of the Ag(0)/Ag(I)-thiolate NCs was shorter than that of typical Au NCs, implying the presence of a faster relaxation pathway in these NCs.10,25 On the other hand, the decay of ΔA became more noticeable when the Ag(0)/Ag(I)-thiolate NCs were coupled with TiO2 (Figure 5D-F), affirming that TiO2 was capable of capturing the excited electrons from the Ag(0)/Ag(I)-thiolate NCs. The ΔA-time profiles could represent internal relaxation and electron transfer kinetics; therefore, more insight into this interfacial electron transfer process was gained by comparing the decay kinetics (Figure 5G-I). When the Ag(0)/Ag(I)-thiolate NCs were coupled with ZrO2, the profile of ΔA decay with time could be attributed solely to the internal charge recombination because of the inaccessibility of the conduction band for electron transfer. Interestingly, the decay traces of NC-1 and NC-2 were fitted using mono-exponential fits with a single long lifetime component of 694 and 2810 ps

for NC-1 and NC-2, but that of NC-3 was fitted to a bi-exponential function with a short lifetime component of 12.58 ps and a long lifetime component of 622 ps (Table 3). Most NCs having a distinct core/shell structure exhibited two lifetime components in their excited states: a short lifetime component associated with the metallic core, and a long lifetime component linked to the outer shell.10,25,54 Therefore, while the structural information of the Ag(0)/Ag(I)-thiolate NCs was largely unknown, this suggested that the larger Ag(0) core in NC-3 than those in NC-1 and NC-2 might play a significant role in charge relaxation dynamics. The ΔA decays of the Ag(0)/Ag(I)-thiolate NCs were noticeably different when they were coupled with TiO2 because additional charge relaxation via electron transfer became available (Scheme 2). Therefore, a comparison between the decay traces of the Ag NCs on ZrO2 and those of TiO2 can provide information related to electron injection kinetics. The rate constants of the electron transfer (ket) were estimated by calculating the difference between the reciprocals of the average lifetimes of each Ag(0)/Ag(I)-thiolate NC coupled with TiO2 and ZrO2 (Table 3). While NC-2 showed a slightly larger ket than others, the ket values of all the Ag NCs were of the same order (108 s-1), revealing that the electron injection kinetics were similar regardless of the size. Given that the order of ket was significantly smaller than those found in quantum dot-based photoelectrodes (109-1010 s-1),55-59 we inferred that the presence of a short lifetime component in NC-3 would be detrimental for efficient light harvesting because it would be unlikely to participate in the electron transfer event, thus limiting the photon-to-current conversion efficiency of NC-3. This speculation will be discussed in detail in the following section. Evaluation of Solar Cell Performance and Investigation of Events at the Electrode/Electrolyte Interface. The stability of the Ag(0)/Ag(I)-thiolate NCs was high enough to fabricate Ag NC-sensitized MCSSCs. This allowed us to perform an in-depth investigation of their photoelectrochemical behavior. As anticipated from the average size, the light absorbing capability of the NC-sensitized TiO2 photoanode increased from NC-1 to NC-3 (Figure 6A). The red-shift in the absorption edge was consistent with the trend found in the solution of all



three NCs. Compared to NC-1, the overall absorbance of NC-2 and NC-3 in the range of 450-600 nm increased by 20% and 50%, respectively. Fully working solar cell devices were assembled with the Co2+/Co3+ redox couple as the electrolyte, Pt as the counter electrode, and the Ag NC-sensitized TiO2 as the photoanode. Solar cells sensitized with NC-1, NC-2 and NC-3 will be referred to as NC-1-SC, NC-2-SC, and NC-3-SC, respectively. Figure 6B displays the J–V curves of these three MCSSCs, and the solar cell parameters are summarized in Table 4. While the highest PCE was obtained with NC-2-SC, the highest short-circuit current (JSC) was observed with NC-3-SC, and the largest open-circuit voltage (VOC) was with NC-1-SC. The JSC of the solar cells increased with an increase in average NC size, which could be ascribed to the increase in absorption capability of the NCs at first glance. However, the extent of increase in JSC of NC-3-SC did not match its absorbance capability, implying the photon-to-photocurrent conversion efficiency of NC-3-SC was lower than that of NC-1-SC and NC-2-SC. This aspect will be discussed further in detail. In contrast, the VOC showed an opposite trend to the size of the NCs. Therefore, the highest PCE of 1.39% was achieved by NC-2-SC, which resulted from a compromise between VOC and JSC. This compromise was also partly attributed to the fill factor (FF), which showed a negative trend with an increase in size (Table 4). It is worth mentioning that, apart from the enhanced PCE, significantly improved stability was obtained in the current work (Figure 6C). The photocurrent of NC-2-SC degraded to 70% of its initial value after 1000 s of light exposure. Decreases in photocurrent of 75% for NC-1-SC and 60% for NC-2-SC from the initial photocurrent were observed under the same conditions. The photocurrent stability seemed to depend on the NC size. (A)

0.5

NC-1

0.4

0

NC-2

NC-3

a b

0.3 0.1

(C)

a b c

375

c

NC-1 NC-2 NC-3

450 525 600 675 Wavelength (nm)

3.0

c b

2.5 2.0

Normalized Current

Current Density (mA/cm2)

0.6

0.2

NC-3 had a higher ratio of Ag(0) atoms than NC-1 and NC-2, which may have caused a larger loss in the photocurrent in NC3-SC compared to NC-1-SC. As discussed by Kumar et al., the stability of Ag NCs also depends on the HOMO–LUMO gap; a smaller HOMO–LUMO gap results in lower stability in the NCs.27 This assertion was consistent with our results. While this improved stability was still no match for the Au NC-sensitized MCSSCs (95% retention after 1000 s illumination),11 this achievement was not trivial given the instantaneous degradation of Ag NC-based MCSSCs upon illumination. This boost in stability resulted from the presence of Ag(I)-thiolate complexes, which resisted oxidation. The Ag(I)-thiolate complexes were wrapped around the Ag(0) NC core, which protected the core against direct exposure to the redox couple, and hence slowed the photocurrent degradation. We investigated the internal working mechanism of these MCSSCs via in-depth EIS analysis to determine the various performance-limiting parameters and to elucidate charge recombination kinetics at the photoelectrode/electrolyte interface. The Nyquist plots in Figure S10 consisted of two half-circles; the high frequency region corresponds to the electrolyte/counter electrode interface, and the low frequency half-circle corresponds to the photoelectrode/electrolyte interface. Both halfcircles were connected by a linear region, corresponding to the transmission line that was used to determine the charge transport properties of the photoelectrodes. The transmission line model developed by Bisquert and co-workers11,60 was adopted to extract various physical parameters that helped elucidate the solar cell performance (Figure S11).

(B)

0.7 Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 13

a

1.5 a b c

1.0 0.5 0

750

0

NC-1-SC NC-2-SC NC-3-SC

0.15

0.30 0.45 0.60 Voltage (V)

0.75

0.90

1.2 1.0 0.8

a

0.6

c

b

0.4

a b c

0.2 0

0

250

500 Time (s)


750

1000

Figure 6. (A) UV-vis absorption spectra of NC-1, NC-2, and NC-3 adsorbed on TiO2 films. (B) J–V curves and (C) normalized photocurrent stability of NC-1-SC, NC-2-SC, and NC-3-SC. The inset of (A) shows the digital photographs of the corresponding photoelectrodes.

Table 4. J–V Characteristics of Ag(0)/Ag(I)-Thiolate NC-Sensitized MCSSCs. Sample

JSC (mAꞏcm-2)

VOC (mV)

FF

PCE (%)

NC-1-SC

1.53 ± 0.09

844 ± 14

0.780 ± 0.013

1.00 ± 0.06

NC-2-SC

2.31 ± 0.07

811 ± 11

0.743 ± 0.010

1.39 ± 0.06

NC-3-SC

2.50 ± 0.07

741 ± 20

0.712 ± 0.014

1.32 ± 0.05


10-2

0.9

102

100 0.6

0.7 0.8 Vecb (V)

0.9

10-4

10-1 0.6

(F) 10-3

10-3

100

101

(E)



10-4

0.7 0.8 Vecb (V)

0.9

10-5 10-6 0.6

0.7 0.8 Vecb (V)

0.9

102


10-5 0.6


0.7 0.8 Vecb (V)

0.9

101 0.6

0.7 0.8 Vecb (V)

0.9

Figure 7. (A) Chemical capacitance (Cμ) vs. Fermi voltage (VF). (B) Recombination resistance (Rr), (C) electron lifetime (τn), (D) electron conductivity (σn), (E) electron diffusion coefficient (Dn), and (F) small-perturbation diffusion length (λn) vs. Vecb of Ag(0)/Ag(I)-thiolate NC-sensitized MCSSCs.

External bias voltages (Vappl) were applied during the EIS measurements to set the Fermi level of TiO2, but there was a voltage drop (Vs + VCE) across the cells due to the internal resistance (RS) and the charge transfer resistance at the counter electrode (RCE). This voltage drop was subtracted to obtain the actual voltage (VF) that contributed to the change in the Fermi 𝑉 𝑉 𝑉 ). VF is defined as the Fermi voltlevel (𝑉 age, and it is equivalent to the shift in equilibrium Fermi level (EF0) against the quasi-Fermi level (EFn), which is expressed by 𝐸 𝐸 /𝑞, where q is the the following equation: 𝑉 positive elementary charge. Figure 7A exhibits the plot of the chemical capacitance (Cμ) vs. VF, where a slight difference in Cμ that resulted from the difference in the size of Ag NCs was found. Cµ represents the density of states in TiO261 and is related to the difference between EFn and the conduction band position (ECB) of TiO2 as expressed below:60 𝐶 ∝ exp

(3)

where kB and T are the Boltzmann constant and the absolute temperature, respectively. Cµ decreased with an increase in NC size (Figure 7A). According to Equation 3, a lower Cµ at the same VF would suggest an upward shift in the ECB of the TiO2 because EFn was fixed at the same level by the applied bias voltage. Given the definition of VOC (the difference between the ECB of TiO2 and the redox potential of the electrolyte), we inferred from the trend in Figure 7A that VOC should increase with an increase in the size of the Ag NCs. However, this was not the case in the MCSSCs. This discrepancy suggests that Cµ was not the only determining factor for the VOC trend and the VOC also depended on other factors, among which interfacial recombination kinetics are the most important. Before going on, an important prerequisite for the EIS analysis had to be taken into account.60 Equation 3 shows that a higher Cµ (i.e., a smaller difference between ECB and EFn) would also lead to a higher electron density in the TiO2; therefore, it would be pointless to compare the recombination resistance (Rr) and other physical parameters that vary with the electron density in the conduction band of TiO2. Therefore, to compare the different MCSSCs on the basis of the same number of electrons in TiO2, this difference in the electron density should be calibrated by inducing an

equivalent conduction band (Vecb) condition that was obtained by shifting the voltage axis in Cµ and making all the Cµ values overlap (Figure S12).11,37,60,62-64 The most important parameter from the EIS analysis was the recombination resistance (Rr), that could shed light on the trends of VOC and PCE in the J–V curves. As shown in Figure 7B, NC-1-SC showed the highest Rr, and the trend in Rr followed the same pattern as the VOC in the J–V curves (NC-1SC>NC-2-SC>NC-3-SC). The same inverse proportionality to the size of the NCs was observed in the electron lifetime (τn), which was calculated by τn = RrCμ (Figure 7C). Charge recombination and electron injection kinetics are two primary factors that determine VOC. Given the similar electron injection kinetics (Table 3), we concluded that the difference in Rr was the primary reason for the VOC trend. The electron conductivity (σn) and electron diffusion coefficient (Dn) also decreased with an increase in the NC size (Figure 7D-E). This might hinder the charge collection performance of MCSSC sensitized with larger NCs. Our understanding of the charge collection performance of NC-1-SC, NC-2-SC, and NC-3-SC became clearer when we examined the small-perturbation diffusion length (λn). Figure 7F shows that the λn value of the NCs decreased with an increase in the NC size. The λn of NC-1-SC in the high voltage range was more than 100 µm. Electrons had to move through a network of interconnected TiO2 particles before they were collected at the FTO, as predicted by the diffusion equation.65-66 Thus, the charge collection efficiency of NC-1-SC should be near 100% because only a 15 µm-thick TiO2 film was employed to fabricate the photoanode.67 On the other hand, the λn values of NC-3-SC fell in the range of 18–50 µm, and hence this shortened diffusion length likely hindered the charge collection ability of NC-3-SC. We believed that this is the reason for the minute improvement in JSC from NC-2-SC to NC-3-SC. While NC-3 showed a large increase in absorption compared to NC-2, this dramatic increase in the absorption was not realized as JSC in NC-3-SC because of this limitation. (A)

0.7

a b c

0.6

4 a b

0.3

3

c

2

0.2

1

0.1 0 300

400

500

600

6

5

0.5 0.4

(B)

6

NC-1 NC-2 NC-3

700

0 800

5 J (mA/cm2)

(D)

0.7 0.8 VF (V)

101


τn (s)

Rr (Ω . cm2)

10-2

10-3 0.6

(C) 103

Dn (cm2 . s-1)

Cμ (F . cm-2)


Solar Flux (1018 photons/s)

(B)

10-1

λn (μm)

(A)

σn (Ω-1 . cm-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Absorbance

Page 9 of 13

a b c

a

4 3 2

a

a

Jmax Jmax-corrected Actual JSC

b b

b c

c

c

1 0

NC-1

NC-2 Sample

NC-3

Wavelength (nm)

Figure 8. (A) UV-vis absorption spectra of Ag(0)/Ag(I)-thiolate NC-sensitized TiO2 photoelectrodes and AM 1.5G solar spectrum. (B) Jmax, corrected Jmax, and an actual JSC for NC-1-SC, NC-2-SC, and NC-3-SC.

Along with the limited charge collection efficiency, the difference in charge relaxation dynamics observed in NC-3 could be another reason for the limited photocurrent boost. The TAS analysis revealed that NC-1 and NC-2 showed a single long lifetime component that was longer than ~600 ps. Therefore, we deduced that all the charge carriers in NC-1 and NC-2 could participate in the charge injection. Conversely, NC-3 exhibited two excited state lifetime components. Given the order of ket and the short component of 12.8 ps (which contributed 17% to the overall excited state lifetime), it is unlikely that this contributed to the photocurrent. This could put constraints on the amount of absorbed light NC-3 can convert to photocurrent as demonstrated in Au25(SR)18 NC-sensitized MCSSC.14 To visualize this hypothesis, we determined the maximum theoretical

9 ACS Paragon Plus Environment


current density (Jmax) that can be produced by each NC if only absorption and the solar spectrum were taken into account (Figure 8). Jmax was then calculated using the equation below: 𝐽

𝐴𝑏𝑠 𝜆 Φ 𝜆 𝑑𝜆

(4)

where Abs(λ) and Φ(λ) are the film absorbance and the AM 1.5G solar flux, respectively. The Jmax was corrected by eliminating the short lifetime component that may not contribute to photocurrent. As seen in Figure 8B, the corrected Jmax of NC-3SC was significantly lower than the Jmax. Therefore, the different excited state behavior of the Ag(0)/Ag(I)-thiolate NCs may also play a role in suppressing the increase in the JSC from NC2-SC to NC-3-SC despite the significant increase in light absorption. The origin of the JSC was further understood by analyzing the IPCE spectra (Figure S13A). NC-1-SC showed a maximum IPCE of 19.3% at 420 nm, and it declined to zero at ~600 nm. With NC-2-SC, the maximum IPCE value of 19.4% was also observed at 420 nm, but the higher spectral region was covered for the photocurrent generation, which eventually led to an increase in JSC in NC-2-SC. In NC-3-SC, the IPCE values were lower than those of NC-2 in the 400–510 nm range, which we attributed to the faster recombination kinetics. However, the lower energy region (520–750 nm) was exploited more efficiently for photocurrent generation due to the red-shifted absorption edge of NC-3. IPCE is the product of various physical phenomena, which can be expressed as follows:65,68 (5) 𝐼𝑃𝐶𝐸 𝜂 𝜂 𝜂 where 𝜂 is the light harvesting efficiency, 𝜂 is the charge separation efficiency, and 𝜂 is the charge collection efficiency. If we ignore the absorption by the counter electrode and as:65,68 the electrolyte in the TiO2 pores, we can calculate 𝜂 𝜂

𝜆

𝑇

𝜆 1

𝑒

(6)

where 𝑇 𝜆 is the transmittance of the FTO glass substrate, d is the thickness of the TiO2 film, and 𝛼 𝜆 is the absorbance calculated using coefficient of the photoelectrodes. The 𝜂 Equation (6) is given in Figure S13B. As the NC size increased, 𝜂 increased over the broader spectral range, which coincided with their absorbance profiles. If the JSC was governed solely by the 𝜂 , NC-3-SC would have produced a higher JSC than NC-2-SC, but that was not the case. This implied that JSC or both. 𝜂 in NC-3-SC was limited by either 𝜂 or 𝜂 and 𝜂 are collectively called the internal quantum efficiency (IQE), which can be determined by normalizing the IPCE by (Figure S13C). The IQE can serve as a measure of the 𝜂 ability of solar cells to convert absorbed photons in the photocurrent. Compared to NC-2-SC, NC-3-SC exhibited lower IQE values over the whole measured range (400–700 nm). This result was in good agreement with the insight obtained by TAS and EIS analyses, highlighting the importance of suppressing undesirable interfacial events to enhance light conversion efficiency.

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oxidation-resistive Ag(I)-thiolate complexes acted as a protective shell and enhanced PL by promoting the LMCT mechanism. We devised a pH-driven aggregation strategy to prepare Ag(0)/Ag(I)-thiolate NCs, where Ag(I)-thiolate complexes reversibly aggregated on the surface of Ag(0) cores upon pH change. The solar cells employing these Ag(0)/Ag(I)-thiolate NCs outperformed any other Ag NC-based solar cells that have been reported in terms of conversion efficiency and stability. In addition, in-depth spectroscopic and electrochemical analyses of a series of core/shell Ag(0)/Ag(I)-thiolate NCs with different core sizes revealed that there was a compromise between light absorbing capability, charge separation dynamics, and charge recombination kinetics depending on the size. This comprehensive understanding of the Ag NC-sensitized solar cells lays a foundation on which a new design principle for Ag NCs for efficient light energy conversion can be built. Despite the big leap achieved in this work, Ag NCs are inferior to Au NCs for light energy conversion applications. However, according to the new insights provided in our investigation, it would be feasible to exploit their full potential if an elaborate control strategy can be developed that can ensure the stability of Ag NCs and enhance their carrier lifetimes.

ASSOCIATED CONTENT Supporting Information. Synthesis scheme, digital photographs, and UV-Vis spectra of Ag NCs at various stages of synthesis. DLS and PL spectra, TEM images, and difference absorption spectra at various time scales. Nyquist plots, an equivalent circuit, and IPCE analysis of Ag NC-sensitized based solar cells. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected] (JHB) * [email protected] (PVK)

Author Contributions MAA and SJY contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by grants from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF2016R1A1A1A05005038, NRF-2018R1E1A2A02086254, and NRF-2018M3D1A1089380) and by the Ministry of Education (NRF-2018R1A6A1A03024231). This is contribution number NDRL No. 5232 from the Notre Dame Radiation Laboratory, which is supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through award DE-FC02-04ER15533.

REFERENCES CONCLUSIONS The many advantages of Ag over its longtime rival Au make Ag NCs promising in light energy conversion applications. However, the intrinsic low chemical stability of Ag NCs along with their short excited state lifetimes have prevented their widespread use as photosensitizers. In this report, these challenges were overcome by the use of AIE-type Ag NCs, where

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Ag(I)-Thiolate Protected Silver Nanoclusters for Solar cells

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