AgInS2âZnS Quantum Dots: Excited State Interactions with TiO2 and

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AgInS2−ZnS Quantum Dots: Excited State Interactions with TiO2 and Photovoltaic Performance Steven M. Kobosko,†,‡ Danilo H. Jara,†,§ and Prashant V. Kamat*,†,‡,§ †

Radiation Laboratory, ‡Department of Chemical and Biomolecular Engineering, and §Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: Multinary quantum dots such as AgInS2 and alloyed AgInS2−ZnS are an emerging class of semiconductor materials for applications in photovoltaic and display devices. The nanocrystals of (AgInS2)x−(ZnS)1−x (for x = 0.67) exhibit a broad emission with a maximum at 623 nm and interact strongly with TiO2 nanostructures by injecting electrons from the excited state. The electron transfer rate constant as determined from transient absorption spectroscopy was 1.8 × 1010 s−1. The photovoltaic performance was evaluated over a period of a few weeks to demonstrate the stability of AgInS2−ZnS when utilized as sensitizers in solar cells. We report a power conversion efficiency of 2.25% of our champion cell 1 month after its fabrication. The limitations of AgInS2−ZnS nanocrystals in achieving greater solar cell efficiency are discussed. KEYWORDS: AgInS2, quantum dots, electron transfer, emission quenching, solar cells

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respectively,15 and a high absorption coefficient, AIS quantum dots can serve as sensitizers in solar cells. Recent efforts have focused on synthetic procedures16−19 and photophysical properties of AIS quantum dots.6,20−23 The nanocrystals of ternary semiconductors can be alloyed with ZnS, thus enabling greater tunability of their optical properties. Alloying ZnS with AIS results in a blue-shift of the absorption and emission along with the enhancement in the photoluminescence (PL) quantum yield.24 The intricacies of optical properties of alloyed AgInS2−ZnS (AIZS) nanocrystals are discussed elsewhere.25−31 AIZS nanostructures have also been found useful in bioimaging,32,33 LED devices,29,34,35 and photocatalytic hydrogen generation.36−38 Although AIS has been employed in liquid-junction quantum dot solar cells (QDSCs), most studies report low power conversion efficiencies of ≤2%.39−43 A recent publication by Cai et al. reports a peak PCE of 2.91% for a AgInS2 QDSC.44 To boost the efficiency of QDSCs further, it is important to establish the excited state and charge transfer properties of AIZS quantum dots. We now report excited state dynamics of AIZS nanocrystals as well as their interaction with TiO2 nanostructures. Photoelectrochemical properties and stability of quantum dot solar cells employing AIZS quantum dots as sensitizers are also discussed.

INTRODUCTION Semiconductor quantum dots have gained popularity in recent years because of their potential applications in bioimaging, sensing, photovoltaic electricity generation, display devices, and photocatalysis.1 The ability to tune the absorption properties and modulate the bandgap of quantum dots with size and shape make them good candidates for use in photovoltaic applications. Metal chalcogenides (e.g., PbS and CdSe) are among the most studied quantum dots for elucidating excited state dynamics as well as solar cell applications.2−4 The heavy metal content in these types of materials has raised concerns for their utilization in practical systems. The recent emergence of ternary quantum dots offers an attractive alternative to the binary chalcogenide quantum dots.5 Compounds such as AgInS2 and CuInS2 possess many of the advantageous properties of binary quantum dots and avoid using heavy metal elements. These ternary semiconductors offer flexibility for tuning optical and electronic properties not only with particle size but also with composition.6 For example, we have recently shown that the absorption and emission of CuInS2 can be systematically varied by employing a different metal cation ratio.7 Among the ternary semiconductors, CuInS2 and CuInSe2 quantum dots have been reported to be effective sensitizers in solar cells.8−13 Although most of the studies report relatively low power conversion efficiency (PCE < 5%), a recent study reported 11.6% PCE for a solar cell employing CuInSe2−ZnSe alloyed quantum dots.14 Another promising ternary material for photovoltaic applications is AgInS2 (AIS). With a bulk bandgap of 1.87 and 1.98 eV for tetragonal and orthorhombic phases, © 2017 American Chemical Society

Special Issue: Hupp 60th Birthday Forum Received: November 14, 2016 Accepted: January 19, 2017 Published: February 3, 2017 33379

DOI: 10.1021/acsami.6b14604 ACS Appl. Mater. Interfaces 2017, 9, 33379−33388

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min. Following glass cleaning, the working electrodes were immersed in a 40 mM TiCl4 aqueous solution and heated to 70 °C for 30 min. The FTO was then rinsed with DI water and ethanol. Next, the active TiO2 layer (Solaronix Ti-Nanoxide T/SP paste) was applied by the doctor blade technique. The active layers were approximately 0.2 cm2 in size. The electrodes were then left to dry at room temperature for 1 h, dried at 80 °C for 1 h, and annealed at 500 °C for 1 h. On top of the active layer, a TiO2 scattering layer (Solaronix Ti-Nanoxide R/SP paste) was applied by doctor blade, followed by drying at room temperature for 1 h, drying at 80 °C for 1 h, and annealing at 500 °C for 1 h. The WEs were again immersed in a TiCl4 solution and heated to 70 °C for 30 min. Finally, the electrodes were calcined at 500 °C for 30 min. Solar Cell Preparation: EPD of Quantum Dots. Electrophoretic deposition (EPD) was used to load the quantum dots onto the TiO2 active layer. A working electrode and a blank piece of FTO were placed in a solution of colloidal quantum dots. For EPD, the AIZS quantum dots were transferred from toluene to chloroform. The two pieces of FTO were placed 0.4 cm from each other, and a voltage of 1250 V/cm was applied for 2 min. Solar Cell Preparation: ZnS SILAR Treatment. Following sensitization by EPD, successive ionic layer adsorption and reaction (SILAR) was performed on the working electrodes. First, the sensitized electrodes were immersed in a 0.1 M Zn(NO3)2 methanol solution for 1 min, rinsed with methanol, and dried; then, they were immersed in a 0.1 M Na2S mixture of methanol and water (1:1) for 1 min, rinsed, and dried. Two cycles of SILAR were applied to create a ZnS blocking/passivation layer. Solar Cell Preparation: Electrolyte. A polysulfide electrolyte was made by dissolving Na2S and sulfur powder in DI water. An aqueous polysulfide solution (2 M Na2S, 2 M sulfur) was prepared and sonicated until all powder was dissolved. Some samples were prepared with a modified electrolyte in which this stock solution was diluted with 1 part methanol, thus yielding 1 M polysulfide (1:1 water/ methanol) solution. Solar Cell Preparation: Counter Electrode. Counter electrodes (CEs) were prepared by first cleaning FTO with the glass cleaning procedure outlined above for the working electrode. Following cleaning, 100 nm of copper metal was evaporated onto the electrodes using an Oerlikon Leybold UNIVEX 250 Thermal Evaporator. Next, the CEs were sulfurized by immersion in a 1 M aqueous polysulfide solution for 15 min, which results in the formation of CuxS.45 Solar Cell Preparation: Completing the Cell. Parafilm was cut to dimensions just larger than the CuxS square on the counter electrode for use as a spacer between electrodes. The CE was then heated to ∼70 °C to melt the Parafilm slightly and create better adhesion to the FTO. Next, 1 or 2 drops of electrolyte solution was dropped into the basin created by the Parafilm, and a working electrode was placed on top. Binder clips were used to hold the two electrodes together. Finally, indium contacts were soldered to each end of FTO on their respective electrodes. Photovoltaic Measurements. The completed solar cells were tested using a 2-electrode setup on a Princeton Applied Research PARStat 2273 potentiostat under 1 sun conditions. A 300 W Xe lamp illuminated the solar cell through an AM 1.5 G filter at a power density of 100 mW/cm2. The voltage was swept from short circuit current to open circuit voltage (forward sweep) at a scan rate of 15 mV/s. The incident photon to current efficiency (IPCE) measurements were taken on a Newport Oriel Quantum Efficiency Measurement System. Preparation of TiO2 Colloid Solution. A 10 mM stock solution of titania nanoparticles was made by dissolving 0.1 mmol titanium isopropoxide in 9.5 mL ethanol and 0.2 mL acetic acid under vigorous stirring. Preparation of Quantum Dot Films for Transient Absorption. TiO2 films were made by first cleaning a glass (SiO2) substrate, then applying a TiO2 paste (Solaronix Ti-Nanoxide T/SP paste) by doctor blade, and then annealing at 500 °C for 1 h. Colloidal quantum dots in chloroform were adsorbed onto the TiO2 film by soaking for 1 h. The other substrate consisted of AIZS quantum dots drop cast onto

EXPERIMENTAL METHODS

Synthesis of AgInS2−ZnS Colloidal Quantum Dots. A hotinjection method was used to synthesize AIZS nanocrystals following a modified procedure of an earlier reported method.26 First, 0.1 mmol AgNO3, 0.1 mmol InCl3, 0.1 mmol zinc stearate, 0.5 mmol oleic acid, 4 mmol dodecanethiol, and 20 mL of octadecene are loaded into a 50 mL three-neck round-bottom flask. The mixture is degassed at room temperature for 15 min under stirring. Then, the solution is purged with an inert gas (N2 or Ar) and heated to 100 °C for 15 min under vacuum. The solution is again degassed and purged, and the solution is heated to the injection temperature of 150 °C under an inert gas atmosphere. When the temperature has stabilized and the precursors have formed a translucent solution, a sulfur precursor solution, previously prepared by 0.3 mmol sulfur powder dissolved (by sonication) in 1 mL of oleylamine, is injected into the reaction mixture. After injection, the solution immediately turns dark brown. The reaction is annealed for 5 min before being quenched in roomtemperature water. After the reaction has cooled to room temperature, 20 mL of toluene is added to the mixture, and the colloidal quantum dot solution is centrifuged. Keeping the supernatant, a 1:1 ethanol/ methanol mix is added to precipitate the quantum dots. This washing is repeated 3 times, and the resulting quantum dots are finally redispersed in toluene for storage. Characterization. An FEI Titan 80-300 electron microscope was used to obtain transmission electron microscope (TEM) images. For sample preparation, a dilute solution of AIZS quantum dots dissolved in toluene was pipetted onto a carbon-coated copper grid and dried under vacuum at room temperature overnight. X-ray diffraction (XRD) measurements were taken on a Bruker D8 Advance DaVinci Xray diffractometer with a Cu Kα source (λ = 1.5406 Å). The quantum dots were scanned from a 2θ value of 22−70°, a 0.04° step interval, and 150 s per step. X-ray photoelectron spectroscopy (XPS) measurements were carried out by a PHI VersaProbe II system, and the elemental analysis was carried out using the proprietary software MultiPak. A Varian Cary-50 Biospectrometer was used to obtain the ultraviolet−visible (UV−vis) absorption spectrum. The steady state photoluminescence (PL) spectrum was collected with a Horiba Fluorolog spectrometer. The emission lifetime and electron transfer rate constant in solution were measured using a Horiba timecorrelated single photon counting (TCSPC) setup with a 458 nm excitation source. A femtosecond transient absorption pump−probe spectroscopy setup was used to determine the electron transfer rate constant on films. The pump consisted of a Clark MXR-2010 laser (excitation wavelength of 387 nm, 150 fs pulse width, and energy density of 40 μJ/cm2 per pulse), which excited the sample, and a white light probe was delayed in time to determine the decay across the visible spectrum. Determination of Quantum Yield. The photoluminescence (PL) quantum yield (ϕ) was calculated using the equation

ϕsample = ϕreference ×

Absreference Abssample

×

Area of PLsample Area of PLreference

×

2 ηsample 2 ηreference

, where ϕ

stands for quantum yield, Abs stands for the absorbance value at the PL excitation wavelength, and η is the refractive index of the solvent for the sample and reference. The ratio of integrated areas of the PL spectra is the last component of the equation. The reference chosen was tris(2,2′-bipyridyl)ruthenium(II) chloride dissolved in water because it has a good emission overlap with the AIZS quantum dots. The absorbance values were matched at 450 nm. The PL spectrum was obtained by exciting both samples at 450 nm and integrating the areas from 480 to 850 nm. Solar Cell Preparation: Working Electrode. The working electrode (WE), consisting of numerous TiO2 layers, was prepared using the following procedure. First, fluorine-doped tin oxide (FTO) glass from Pilkington (TEC-7) was cleaned in a soapy aqueous solution (Fisherbrand Versa-Clean) in an ultrasonic bath for 30 min. Then, the glass was rinsed three times with deionized (DI) water and put in an ethanol bath under sonication for 30 min. The FTO was then rinsed with ethanol and dried with a compressed air stream. The FTO glass was then plasma cleaned (Harrick Plasma PDC-32G) for 15 33380


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ACS Applied Materials & Interfaces a clean piece of glass. The two films were put into vacuum cells when undergoing transient absorption measurements.

X-ray photoelectron spectroscopy (XPS) was performed on the particles to elucidate the elemental composition and percentage of AIS to ZnS. Figure 2B shows the XPS survey scan and relevant element peaks, which were analyzed to determine elemental/compound ratios. Additional XPS graphs and tables are included in the Supporting Information. The stoichiometry of the particles was determined to be (Ag1.2In0.8S2)0.67(ZnS)0.33. The XPS analysis revealed that the particles contained 1.5 times more silver than indium, 2.4 times more silver than zinc, and for every ZnS compound present, there were two AIS compounds (2:1 Ag1.2In0.8S2/ZnS). Optical Properties of AIZS Quantum Dots. The steadystate absorption and photoluminescence (PL) emission spectra are shown in Figure 3A. The absorption spectrum shows a broad band with a long tail starting around 700 nm. The absorption of AIZS lacks a distinct excitonic peak and likely gets buried in the tail absorption arising from sub-bandgap optical transitions arising from defect sites within the bandgap.7,20,23 The steady-state PL spectrum shows a rather broad emission with a maximum around 623 nm. The PL quantum yield was determined to be 14.8% using ruthenium tris(bipyridine) as a reference. PL quantum yield values of multinary nanocrystals vary widely in the literature and are affected by numerous factors such as synthesis parameters, cation ratios, postsynthesis ZnS shell, and types of ligands. It has been shown that surface traps act as nonradiative recombination centers.51 Surface passivation techniques such as a postsynthesis ZnS shell reduce these trap centers, allowing for enhanced PL quantum yield.52,53 The quantum yield reported here (14.8%) is lower than some of the best reported values found in the literature for AIS or AIZS nanocrystals. Hamanaka et al. achieved a PL quantum yield over 60% for AIS quantum dots.54 This high result was attributed to the prevalence of donor−acceptor pair emission over surface defect recombination. An even more recent study by Cai et al. reported a PL quantum yield of 74% for their AIS quantum dots.44 Further enhancement of PL emission is reported in a paper by Torimoto et al. in which a quantum yield of 80% is achieved for AIZS quantum dots following a postsynthesis ZnS treatment.24 The emission spectrum also shows a shoulder around 660 nm, signifying that multiple emission decay pathways are present. Park et al. asserted that two different pathways arising from the two AIS phases, tetragonal and orthorhombic, were responsible for this behavior.22 However, recent studies with CuInS2 have shown that inter-band transitions and trap sites offer different radiative recombination pathways.7,55 By recording the excitation spectra at different emission wavelengths and observing longer lifetimes seen at low energy transitions, it was possible to establish these charge recombination pathways. Given the similarity of absorption and emission features with CuInS2 nanocrystals, we expect similar transitions to prevail in the AgInS2−ZnS system. We further established the involvement of traps in the radiative recombination of charge carriers by monitoring the emission decay at different wavelengths. The decay profiles were analyzed using a biexponential fit, and the kinetic parameters are included in Table 1. The higher energy emission (e.g., at 550 nm) exhibits a greater pre-exponential factor (A1) for the short lifetime component (110 ns) indicating that this radiative excited state deactivation pathway initially contributes more to the overall emission than that of the longer lifetime component. The fractional intensities (F1

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RESULTS AND DISCUSSION Characterization of AgInS2−ZnS Quantum Dots. Figure 1 shows TEM images and the size distribution histogram of the quantum dots. The particles were roughly spherical with an average diameter of 5.7 ± 1.2 nm.

Figure 1. (A,B) TEM images of AIZS nanocrystals. (C) Scanning transmission electron microscopy (STEM) image. (D) Size distribution histogram showing an average particle diameter of 5.7 ± 1.2 nm.

The X-ray diffraction (XRD) pattern of the AIZS nanoparticles, pictured in Figure 2A, shows a combination of two phases: orthorhombic AIS and cubic ZnS. A detailed study by Torimoto et al. analyzed the XRD patterns of AIZS particles with different ratios and found that a solid solution of orthorhombic AIS and cubic ZnS is formed.38 Panels C−E in Figure 2 were made with Diamond software46 using crystallographic information files (.cif) for the three crystal structures downloaded from the Crystallography Open Database47 and show 3D representations of these three crystal structures for the two possible AIS phases and the cubic phase of ZnS. The ICDD Database (2015)48 was used to find the standard peak positions and intensities of the three phases (of the bulk material) and are shown in different colors in Figure 2A beneath the AIZS experimental data. PDF 00-025-1330 was used for tetragonal AIS; PDF 00-025-1328 was used for orthorhombic AIS, and PDF 00-005-0566 was used for cubic ZnS. In2S3, In2O3, Ag2S, and Ag2O peaks were not found to correspond with the XRD data. In the bulk material, the low temperature phase of AgInS2 is tetragonal, whereas at higher temperature, it exists in the orthorhombic phase (phase transition temperature of 620 °C).49,50 Though the reaction temperature in colloidal quantum dot syntheses is much lower than 620 °C, nanoparticles can still crystallize into these phases that are normally attained through high-temperature treatment.16 33381


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Figure 2. (A) XRD pattern of AIZS nanocrystals showing a mixture of three phases. The reference patterns are powder diffraction files from the ICDD database. (B) XPS survey scan showing the relevant element peaks. (C−E) Crystal structure diagrams of the two AIS crystal phases and the cubic ZnS phase present in the nanoparticles. The white and blue spheres are Ag+ and In3+ ions, respectively, and the yellow and red spheres are S2− and Zn2+, respectively.

Figure 3. (A) Absorption and emission spectra of AIZS quantum dots in toluene. (inset) Solution of nanoparticles dissolved in toluene under ambient light and under UV illumination. (B) Wavelength-dependent photoluminescence emission decay excited by a 458 nm laser pulse.

Excited State Interactions with TiO2 Colloids. One important parameter in a sensitized solar cell is the electron injection rate from absorber to the conduction band of TiO2. Faster rates of electron injection into TiO2 minimize the charge carrier losses due to recombination processes within AIZS nanocrystals. The emission of AIZS quantum dots serves as an excellent probe to monitor their interaction with TiO2 colloids. The preparation of the TiO2 colloids is detailed in the Experimental Methods. Fifty microliter aliquots of the stock solution were added at a time to a 3 mL solution of AIZS quantum dots suspended in toluene resulting in TiO 2

and F2) give the weights of the two components and signify the proportion of the excited-state population that radiatively decays through each pathway.56 The long-lived component contributes mostly to the overall emission and increases in contribution between 650 and 800 nm. The average lifetime increases from 286 to 984 ns with increasing emission wavelength over the whole monitoring region (550−800 nm). The longer lifetime of low-energy emission further establishes the involvement of traps that lie below the conduction band. 33382


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nanoparticles. For the possibility of solvent and dilution effects to be ruled out, a control experiment was carried out in which aliquots of solvent (ethanol with acetic acid) were added. Figure S4 shows that both the steady state emission spectra and emission lifetimes show little change resulting from the solvent variation. Note that the emission spectra shown in Figures 3A and 4A are from the same batch of nanoparticles, the only difference being that the spectrum in 3A was taken immediately following synthesis and the spectra in 4A was taken after 6 months of storage (ambient atmosphere in the dark). This unusual change in emission characteristics will be explored in a future study. For the quenching effect to be better quantified, an equilibrium between surface bound and unbound AIZS is considered with an apparent equilibrium constant Kapp (eq 1)

Table 1. Biexponential Fit Parameters and Average Emission Lifetimes for the Monitored Wavelengths A1 A2 τ1 (ns) τ2 (ns) F1 a F2 ⟨τ⟩ (ns)b a

F1 =

550 nm

600 nm

650 nm

700 nm

750 nm

800 nm

0.54 0.46 100.3 348.7 0.25 0.75 285.5

0.63 0.37 204.4 558.2 0.38 0.62 423.0

0.64 0.36 294.1 801.9 0.39 0.61 602.3

0.54 0.46 309.2 928.1 0.28 0.72 755.8

0.45 0.55 316.8 1028.7 0.20 0.80 883.9

0.39 0.61 259.4 1091.9 0.13 0.87 983.7

A1τ12 + A2τ22 A1τ1 + A2τ2

= F1τ1 + F2τ2

A1τ1 b ⟨τ ⟩ A1τ1 + A2τ2

=

concentrations between 1.6 and 7.7 mM. Figure 4A shows that PL intensity decreases with increasing addition of TiO2

K app

AIZS + TiO2 ⇐ ⇒ [AIZS⋯TiO2 ]

(1)

Eq 2 was used to find the association constant ϕf0 ϕf0 − ϕfobsd

=

ϕf0 ϕf0 − ϕf′

+

57

ϕf0 K app(ϕf0 − ϕf′)[TiO2 ]

(2)

where ϕf0 is the quantum yield of unadsorbed quantum dots, ϕfobsd is the observed quantum yield at different TiO2 concentrations, and ϕf′ is the quantum yield of unbound quantum dots. The plot of

ϕf0 ϕf0

− ϕfobsd

versus the inverse of TiO2

concentration is shown in Figure S5. Obtaining a linear fit with an R2 value of 0.99, the analysis using this model is deemed valid. Kapp was determined to be 4200 M−1, which signifies that the quantum dots bind strongly to the TiO2 particles in solution. By employing the experimentally determined value for ϕf0 of 14.8% for AIZS quantum dots, we obtain a value for ϕf′ of 1.3% from the intercept. The net quenching efficiency (ϕ0f − ϕf′/ϕ0f ) = 91% under these experimental conditions. We also monitored the emission decay to probe the excited state interaction with TiO2 (Figure 4B). The AIZS suspensions in toluene with and without TiO2 were excited with a 458 nm laser, and the emission was monitored at 650 nm. With the addition of TiO2 colloids, an additional decay component with a shorter lifetime appeared. We attribute this component to reflect electron transfer from AIZS to TiO2 (described with eq 3). The analysis of the emission decay is given in Table S4.

Figure 4. (A) Emission spectra of quantum dots before and after TiO2 colloid addition. (B) Emission decay (monitored at 650 nm) of quantum dots without (a) and with (b) quencher present.

ket

[AIZS*⋯TiO2 ] → AIZS(h) + TiO2 (e)

(3)

Figure 5. (A) Transient absorption spectra recorded following laser pulse excitation (387 nm) of AIZS quantum dots on SiO2. (B) Transient absorption spectra for AIZS quantum dots adsorbed onto a TiO2 film. (C) The bleaching recovery monitored at 550 nm for AIZS quantum dots deposited on (a) TiO2 film and (b) SiO2. The black traces are biexponential fits. 33383


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ACS Applied Materials & Interfaces Because the excitation pulse width is ∼1 ns, we could not resolve the fast processes that occur in the subnanosecond time scale. Further insight into the fast electron transfer process was obtained using femtosecond transient absorption spectroscopy. Electron Injection Rates of AIZS Quantum Dots on TiO2 Films. Pump−probe femtosecond transient absorption spectroscopy was employed to investigate the interaction of quantum dots with TiO2 on an ultrafast time scale. AIZS quantum dots were deposited onto two different substrates: a mesoporous TiO2 film and a glass slide (SiO2). These films were transferred to a spectroscopic cell, which had a provision for evacuation. The films were excited with 387 nm laser pulse, and transient spectra were recorded at different delay times. Representative difference spectra showing a small bleaching recovery of the AIZS quantum dots on SiO2 are shown in Figure 5A. Much faster bleach recovery is seen in Figure 5B, which shows the transient absorption spectra of AIZS on a TiO2 film. Charge separation due to laser excitation of AIZS quantum dots is represented by the bleaching of absorption in the 450 to 650 nm region. Figure 5C shows the bleaching recovery of AIZS on TiO2 and on SiO2 over the 1.5 ns time window of the experiment. The data was normalized and fit to biexponential decays using eq 4 y = A1e−t / τ1 + A 2 e−t / τ2

1.1 ns. The long lifetime component was attributed to radiative recombination and back electron transfer from TiO2 to quantum dots. The short component for AIZS on SiO2 was attributed to fast, nonradiative recombination (possibly arising from surface defect states). The short component for AIZS on TiO2 was taken to be a combination of nonradiative recombination and electron transfer to TiO2. If we assume that these are the two pathways for the short components, we can obtain the electron transfer rate constant using eq 5 ket =

and the fitting parameters are given in Table 2. Table 2. Fitting Parameters and Electron Transfer Rate Constant of AIZS Quantum Dots on SiO2 and TiO2 Films τ1 (ps) amp. (%) τ2 (ps) amp. (%) ket (s−1)

TiO2

55.2 ± 8.2

28.0 ± 1.7 60 1120.9 ± 73.9 40 1.8 × 1010

τ1(TiO2)

−

1 τ1(SiO2)

(5)

where τ1(TiO2) is the short lifetime component of AIZS on the TiO2 film and τ1(SiO2) is the short lifetime component of AIZS on SiO2. The electron transfer rate constant determined from eq 5 was 1.8 × 1010 s−1. Du et al. reported a record QDSCcertified efficiency of 11.6% using CuInSe2−ZnSe quantum dots.14 In this study, they also reported electron transfer rate constants to TiO2. They determined that the electron transfer rate constant for CuInSe2 was 2.4 × 1010 s−1 and for CuInSe2− ZnSe was 9.1 × 1010 s−1, whereas the corresponding solar cell efficiencies were 9.75 and 11.91%, respectively (for the champion devices). Because our value of 1.8 × 1010 s−1 is of the same order of magnitude as those calculated in Du et al.’s study, we can deduce that electron injection from sensitizer into TiO2 is not a limiting factor in a photovoltaic device. Because alloying AIS with zinc changes the bandgap, and thus the conduction band edge, a future study will look into varying the silver/zinc ratio to optimize the electron transfer rate/band alignment to TiO2. Quantum Dot Solar Cells. AIZS quantum dots were employed as sensitizers in liquid-junction solar cells to test their viability in photovoltaics. The devices were fabricated as detailed in the Experimental Methods. For this study, two electrolytes were utilized: a 2 M Na2S/2 M S aqueous electrolyte and a 1 M Na2S/1 M S mixed solvent electrolyte consisting of 1:1 water/methanol. Three solar cells utilizing each electrolyte were fabricated (for a total of six cells). The six solar cells were tested over a period of 7 weeks to evaluate their device stability over time. Current density vs voltage (J−V) curves for all six devices are shown in Figure S1, and

(4)

SiO2

1

AIZS quantum dots on the SiO2 substrate had a short lifetime component (τ1) of 55.2 ps. The long lifetime component was much longer than the 1.5 ns time window of the experiment; thus, an accurate time constant could not be extracted for τ2. The short component for AIZS on a TiO2 film was calculated to be 28 ps, whereas the long component was

Figure 6. Current density vs voltage (J−V) curves for two select solar cells: one employing a purely aqueous electrolyte and the other a mixed methanol/water electrolyte on (A) the day they were made and (B) day 31. (inset) Photo of one of the solar cells. 33384


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increase in the first 31 days has not been identified and will be explored in a subsequent study. We speculate that it might be an equilibration process (such as cumulative light-soaking effect) taking place that improves both current and voltage of these devices for the first month after fabrication. All solar cells were stored in the dark under ambient atmosphere, being exposed to light only when being tested. The AIZS quantum dots appear to be very stable, as the solar cells still operate 50 days after fabrication. Figure S6 shows the incident photon to current efficiency (IPCE) measurement of two solar cells, one containing a purely aqueous electrolyte and one with mixed electrolyte on day 1. The two devices absorb a large portion of the visible spectrum (between 400 nm and ∼600 nm) and convert ∼15 and ∼20% to electricity in the range of 400 to 500 nm for water and mixed electrolyte, respectively. Though the range of the visible spectrum is sufficient, this low IPCE gives insight into why the photocurrent and PCE of these quantum dot solar cells remain low. The IPCE needs to greatly improve in these visible wavelengths to see an improvement in PCE. The methanolcontaining electrolyte had a slightly better IPCE than did the pure aqueous one, which is a reason for the marginally higher PCE on day 1. Though methanol is known to act as a sacrificial donor, it may improve performance for a variety of reasons. For instance, improved “wetting” of methanol over water can allow for better percolation of electrolyte into the mesoporous TiO2 layer and thus provide more intimate contact between the quantum dot and electrolyte.58 Additionally, McDaniel et al. claimed that the lower viscosity of methanol relative to water leads to a decrease in series resistance.45 Also in that paper, the group tested their QDSCs with electrolyte-containing methanol over a period of 71 days and found that the performance did not worsen but rather improved over time. It is important to note, however, that methanol acts as a hole scavenger and undergoes an irreversible reaction when employed in an electrolyte.59 Although methanol can offer short-term stability, it will eventually degrade and cease providing enhanced photocurrent. Therefore, methanol cannot be viewed as a viable option for use in practical liquid-junction solar cells. The solar cells made in this study demonstrate lower PCE than the best reported efficiency of 2.91% for an AIS QDSC. One reason for the relatively lower efficiency is that the AIZS quantum dots in this study show a PL quantum yield of only

photovoltaic parameters along with average efficiency are given in Table S1. For brevity, Figure 6 shows the J−V curves for two select solar cells (one of each electrolyte) on the day of fabrication (day 1) and 31 days later. For testing, the voltage was swept from short circuit current to open circuit voltage (forward scan) at a rate of 15 mV/s using a solar simulator at 1 sun conditions (AM 1.5 G at 100 mW/cm2). A summary of the photovoltaic parameters for the two devices is given in Table 3. On day 1, Table 3. Photovoltaic Parameters of the Two Solar Cells on Days 1 and 31 Jsc (mA/cm2) Voc (V) FF efficiency (%)

water day 1

mixed day 1

water day 31

mixed day 31

2.23 0.335 0.65 0.48

2.85 0.323 0.63 0.58

3.31 0.386 0.52 0.67

8.34 0.490 0.55 2.25

the solar cell with the methanol-containing electrolyte had a slightly higher power conversion efficiency (PCE) than the cell containing the purely aqueous electrolyte. One month later, however, the cells with the mixed electrolyte had an average PCE of over 2% with the best cell yielding 2.25%, whereas the aqueous solar cells only slightly improved from their day 1 values. Both short circuit current and open circuit voltage improved with increasing time, whereas the fill factor decreased. The stability of the photocurrent was probed using a light chopper and applying a 0.2 V bias to the solar cells. For both the purely aqueous and methanol-containing electrolytes, the photoresponse was reproducible and the photocurrent remained stable during illumination (Figure 7A). Next, to assess the device functionality over time, solar cell performance was regularly evaluated over a period of 7 weeks. Figure 7B shows device performance over time with average efficiencies and error bars calculated from statistical analysis of the six photovoltaic cells. It can be seen that after approximately 2 weeks the cells containing methanol start to drastically improve, whereas the aqueous cells have more or less plateaued from day 4. We see a peak in efficiency around day 31 and then a subsequent decline. We attribute this loss in efficiency to electrolyte evaporation due to inadequate sealing between the two electrodes, degradation of the CuxS counter electrode, and methanol acting as a sacrificial donor. The reason for the

Figure 7. (A) Current density vs time of two solar cells with different electrolytes on the day of fabrication under 0.2 V bias. (B) Photovoltaic performance of the solar cells (average of 3 devices for each electrolyte) over a period of 7 weeks, stored in the dark under ambient atmosphere. 33385


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Arthur J. Schmitt Leadership Fellowship for supporting his graduate study. D.H.J. would like to thank Comisión Nacional ́ y Tecnológica (CONICYT) for the de Investigación Cientifica Becas Chile Scholarship, code 72110038.

14.8%. Because surface defects have been shown to be nonradiative recombination centers,51 quantum dots possessing a large amount of surface defects will undergo a large amount of undesirable recombination when employed in a solar cell, lowering efficiency. The PL quantum yield for the best performing QDSC in Cai et al.’s study was 74%, which translated to a solar cell PCE of 2.91%.44 Our future work on AIZS quantum dots will take this into account by altering synthesis parameters and ligands to minimize surface defects. Another beneficial change might be to change the ligand used in this study to mercaptopropionic acid (MPA). This bifunctional linker can anchor AIZS quantum dots to the TiO2 by attaching to AIZS with the thiol and to TiO2 with the carboxyl group. Compared with dodecanethiol ligand used in this study, MPA is a shorter chain ligand that might provide faster electron injection rates to TiO2 and potentially better surface coverage of quantum dots on TiO2, enhancing photovoltaic performance. The highest reported PCEs for AIS and CuInSe2−ZnSe QDSCs (2.9144 and 11.91%,14 respectively) have utilized MPA as ligand.

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(1) Bera, D.; Qian, L.; Tseng, T.-K.; Holloway, P. H. Quantum Dots and Their Multimodal Applications: A Review. Materials 2010, 3 (4), 2260−2345. (2) Chuang, C.-H. M.; Brown, P. R.; Bulović, V.; Bawendi, M. G. Improved Performance and Stability in Quantum Dot Solar Cells through Band Alignment Engineering. Nat. Mater. 2014, 13 (8), 796− 801. (3) Kim, G.-H.; García de Arquer, F. P.; Yoon, Y. J.; Lan, X.; Liu, M.; Voznyy, O.; Yang, Z.; Fan, F.; Ip, A. H.; Kanjanaboos, P.; Hoogland, S.; Kim, J. Y.; Sargent, E. H. High-Efficiency Colloidal Quantum Dot Photovoltaics via Robust Self-Assembled Monolayers. Nano Lett. 2015, 15 (11), 7691−7696. (4) Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V. Quantum Dot Solar Cells. Harvesting Light Energy with CdSe Nanocrystals Molecularly Linked to Mesoscopic TiO2 Films. J. Am. Chem. Soc. 2006, 128 (7), 2385−2393. (5) Zhong, H.; Bai, Z.; Zou, B. Tuning the Luminescence Properties of Colloidal I−III−VI Semiconductor Nanocrystals for Optoelectronics and Biotechnology Applications. J. Phys. Chem. Lett. 2012, 3 (21), 3167−3175. (6) Dai, M.; Ogawa, S.; Kameyama, T.; Okazaki, K.; Kudo, A.; Kuwabata, S.; Tsuboi, Y.; Torimoto, T. Tunable Photoluminescence from the Visible to near-Infrared Wavelength Region of NonStoichiometric AgInS2 Nanoparticles. J. Mater. Chem. 2012, 22 (25), 12851−12858. (7) Jara, D. H.; Stamplecoskie, K. G.; Kamat, P. V. Two Distinct Transitions in CuxInS2 Quantum Dots. Bandgap versus Sub-Bandgap Excitations in Copper-Deficient Structures. J. Phys. Chem. Lett. 2016, 7 (8), 1452−1459. (8) Jara, D. H.; Yoon, S. J.; Stamplecoskie, K. G.; Kamat, P. V. SizeDependent Photovoltaic Performance of CuInS2 Quantum DotSensitized Solar Cells. Chem. Mater. 2014, 26 (24), 7221−7228. (9) Santra, P. K.; Nair, P. V.; George Thomas, K.; Kamat, P. V. CuInS2-Sensitized Quantum Dot Solar Cell. Electrophoretic Deposition, Excited-State Dynamics, and Photovoltaic Performance. J. Phys. Chem. Lett. 2013, 4 (5), 722−729. (10) Pan, Z.; Mora-Seró, I.; Shen, Q.; Zhang, H.; Li, Y.; Zhao, K.; Wang, J.; Zhong, X.; Bisquert, J. High-Efficiency “Green” Quantum Dot Solar Cells. J. Am. Chem. Soc. 2014, 136 (25), 9203−9210. (11) Li, W.; Pan, Z.; Zhong, X. CuInSe2 and CuInSe2−ZnS Based High Efficiency “green” Quantum Dot Sensitized Solar Cells. J. Mater. Chem. A 2015, 3 (4), 1649−1655. (12) Kim, J.-Y.; Yang, J.; Yu, J. H.; Baek, W.; Lee, C.-H.; Son, H. J.; Hyeon, T.; Ko, M. J. Highly Efficient Copper−Indium−Selenide Quantum Dot Solar Cells: Suppression of Carrier Recombination by Controlled ZnS Overlayers. ACS Nano 2015, 9 (11), 11286−11295. (13) Pernik, D. R.; Gutierrez, M.; Thomas, C.; Voggu, V. R.; Yu, Y.; van Embden, J.; Topping, A. J.; Jasieniak, J. J.; Vanden Bout, D. A.; Lewandowski, R.; Korgel, B. A. Plastic Microgroove Solar Cells Using CuInSe2 Nanocrystals. ACS Energy Lett. 2016, 1 (5), 1021−1027. (14) Du, J.; Du, Z.; Hu, J.-S.; Pan, Z.; Shen, Q.; Sun, J.; Long, D.; Dong, H.; Sun, L.; Zhong, X.; Wan, L.-J. Zn−Cu−In−Se Quantum Dot Solar Cells with a Certified Power Conversion Efficiency of 11.6%. J. Am. Chem. Soc. 2016, 138 (12), 4201−4209. (15) Shay, J. L.; Tell, B.; Schiavone, L. M.; Kasper, H. M.; Thiel, F. Energy Bands of AgInS2 in the Chalcopyrite and Orthorhombic Structures. Phys. Rev. B 1974, 9 (4), 1719−1723. (16) Wang, D.; Zheng, W.; Hao, C.; Peng, Q.; Li, Y. General Synthesis of I−III−VI2 Ternary Semiconductor Nanocrystals. Chem. Commun. 2008, No. 22, 2556−2558. (17) Liu, M.; Zeng, H. C. General Synthetic Approach to Heterostructured Nanocrystals Based on Noble Metals and I−VI,

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CONCLUSIONS AgInS2−ZnS alloyed quantum dots were shown to interact with TiO2 nanoparticles when excited by photon irradiation. TiO2 colloids had an emission quenching effect on the AIZS nanocrystals. Using ultrafast pump−probe transient absorption spectroscopy, an electron transfer rate constant was determined to be 1.8 × 1010 s−1 for quantum dots sensitized onto a TiO2 film. Quantum dot-sensitized solar cells made using these quantum dots demonstrated a peak power conversion efficiency of 2.25%, and the material remained stable over a period of 7 weeks being employed in this application.

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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/acsami.6b14604. Solar cell performance (J−V) curves and photovoltaic parameters for all devices, transient absorption spectra of quantum dots in solution, fitting of XPS elemental peaks, and solvent effect of steady-state and transient PL (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Prashant V. Kamat: 0000-0002-2465-6819 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research described herein was supported by the Division of Chemical Sciences, Geosciences and Biosciences, Basic Energy Sciences, Office of Science, U.S. Department of Energy through Grant DE-FC02-04ER15533. This is document no. NDRL 5155 from Notre Dame Radiation Laboratory. The authors would like to thank Seog Joon Yoon for collecting the XRD data. We thank the ND Energy Materials Characterization Facility (MCF) for the use of the XPS and Thermal Evaporator. We also thank Notre Dame Integrated Imaging Facility (NDIIF) for electron microscopy facilities. S.M.K thanks the 33386


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