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C: Energy Conversion and Storage; Energy and Charge Transport 2

Indium-Rich AgInS-ZnS Quantum Dots – Ag:Zn Dependent Photophysics and Photovoltaics Steven M. Kobosko, and Prashant V. Kamat J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03001 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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The Journal of Physical Chemistry

Indium-Rich AgInS 2-ZnS Quantum Dots – Ag:Zn Dependent Photophysics and Photovoltaics Steven M. Kobosko1,2 and Prashant V. Kamat1,2,3,* 1

Radiation Laboratory, 2Department of Chemical and Biomolecular Engineering, and 3

Department of Chemistry and Biochemistry

University of Notre Dame, Notre Dame, Indiana 46556, United States

_____________________________________________________________________________________ *Corresponding author: Prashant V. Kamat ([email protected])

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ABSTRACT AgInS 2 -ZnS solid solution quantum dots prepared with varying Ag:Zn ratio demonstrate composition-dependent photophysical properties. Absorption and emission processes are extremely complex in these compounds due to easily formed crystallographic defects which serve as intrabandgap states and provide additional excitation and relaxation pathways. In addition to valence to conduction band absorption, defect states located within the bandgap are responsible for tail absorption in these nanoparticles and are assigned to Ag In antisite defects. These AgInS 2 ZnS quantum dots display wavelength-dependent photoluminescence decays along with large stokes shifts and long photoluminescence lifetimes, strongly suggesting that donor-acceptor pair recombination is the dominant radiative pathway. The excited state interaction between AgInS 2 ZnS and TiO 2 is studied through the use of transient absorption spectroscopy and a fast photoinduced electron transfer rate constant of 5 × 1011 s-1 is determined. This interaction with TiO 2 is further probed by testing various compositions of AgInS 2 -ZnS in liquid-junction solar cells, with the optimum device power conversion efficiency reaching 1.83%.

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INTRODUCTION Metal chalcogenide semiconductor quantum dots such as PbS and CdSe have been extensively explored for photovoltaic applications.1–4 The ability to tune their optical and electronic properties through size quantization effects makes them suitable for light energy conversion devices.5,6 To date, photovoltaic cells utilizing PbS quantum dots as light harvesters have exceeded 11% certified power conversion efficiency (PCE).2 Since the heavy metal aspects of these semiconductors are of concern both from a human health and environmental standpoint, multinary semiconductor chalcogenides consisting of nontoxic metals are now being considered as alternate photovoltaic materials.7,8 Ternary chalcogenide quantum dots such as CuInS 2 and AgInS 2 are regarded as emerging materials for their use in thin film and nanocrystal photovoltaic devices. These ternary semiconductor compounds possess bulk bandgaps matched well to the solar spectrum along with high absorption coefficients across the visible spectrum.9,10 Efficiencies of photovoltaic devices have steadily risen for numerous multinary materials over the past few years. Solar cells using CuInS 2 nanocrystals have achieved certified PCEs as high as 6.6% while devices using AgInS 2 as light harvesting material have reached a peak efficiency of 2.9%.11,12 Another ternary chalcogenide material, AgBiS 2 , has demonstrated a certified PCE of 6.3% for a solid-state photovoltaic device.10 Solar cells containing zinc-diffused AgInSe 2 nanocrystals have attained a PCE of 3.5% whereas a study from Du et al. reported a PCE of 11.6% for a solar cell employing CuInSe 2 -ZnSe alloyed quantum dots.13,14 Although AgInS 2 is considered to be a potential contender for photovoltaic applications, its device efficiencies remain quite low (less than 3%). Because of favorable optical properties and tunability, AgInS 2 and its solid solution with zinc sulfide AgInS 2 -ZnS (AIZS) are being actively researched for a variety of applications such as photovoltaics,12,15–17 LEDs,18–20 medical imaging,21–25 and photocatalysis.26–32 Key to improving solar cell efficiency is a robust understanding of the photophysical mechanisms governing these multinary compounds. The photophysical properties of ternary and quaternary materials are very complex and differ greatly from heavy-metal-containing binary chalcogenide nanocrystals. CdSe quantum dots exhibit distinct excitonic transitions in the absorption spectra, small stokes shifts, narrow photoluminescence (PL) peak widths and short emission lifetimes.3,6,9,33–35 The excitonic peaks 3 ACS Paragon Plus Environment


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and sharp absorption onsets in II-VI and IV-VI nanocrystals indicate that the absorption is dominated by strong band edge (valence to conduction band) transitions. The small stokes shifts and narrow PL peak widths are indicative of band edge radiative recombination, which is the dominant radiative channel for well-passivated samples.6 Conversely, the optical properties of multinary quantum dots such as AgInS 2 and AIZS are characterized by a lack of excitonic peaks and appearance of a long tail in the absorption spectra, large stokes shifts, broad PL, and long emission lifetimes.28,36–39 A recent investigation by our group showed that CuInS 2 quantum dots have two distinct absorption pathways which gives I-IIIVI2 quantum dots their characteristic broad absorption spectra. The first optical transition is band edge absorption and the second is a sub-bandgap transition arising from copper-related states lying above the valence band.40 The dominant PL mechanisms for both bulk and nanoscale I-III-VI 2 materials are generally attributed to radiative donor-acceptor pair (DAP) recombination centers arising from intrinsic defects.23,38,41–47 A study by Zhang et al. explored the nature of point defects and defect pairs in the CuInSe 2 system.48 The copper vacancy (V Cu ) which was formed near (0.03 eV) the valence band served as shallow acceptor state. The indium vacancy (V In (-/0)) had a deeper acceptor level located 0.17 eV above the valence band. Antisite defects (copper or indium substituting for the other, Cu In and In Cu ) were deeper traps. They also found that the defect pair (2V Cu - + In Cu 2+) had low formation energies and at least partially accounted for the large off-stoichiometric tolerance in I-III-VI 2 compounds. A study of bulk AgInS 2 by Hattori et al. identified sulfur vacancies and silver interstitial defect sites (V S and Ag int ) as donor states, while silver vacancies and sulfur interstitial defect sites (V Ag and S int ) served as acceptor states.49 Chevallier et al., identified the origin of radiative DAP (Donor Acceptor Pair) recombination in AIZS quantum dots.50 They characterized the defect complex (In Ag 2+ + 2V Ag -) through x-ray photoelectron spectroscopy (XPS) measurements and proposed this complex to be one of the prevalent radiative DAP centers in AIZS, especially for indium-rich samples. They also proposed the alloying mechanism of zinc into − the crystal lattice to be �2 Zn → Zn+ Ag + ZnIn �. Thus, one silver atom and one indium atom are

replaced by 2 zinc atoms. Since photoluminescence quantum yield increased with the inclusion of

zinc addition, it was concluded that incorporation of zinc provides increased radiative recombination centers in these multinary compounds.51 4 ACS Paragon Plus Environment

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In addition to interstitial defects surface states also play a major role in dictating the photodynamics of multinary chalcogenides. In I-III-VI 2 quantum dots, surface defects serve primarily as non-radiative recombination centers and exhibit faster charge recombination rates as compared to those originating from intrinsic defects.9,37,47,52 Passivation of these quantum dots with high bandgap semiconductors such as ZnS in a core-shell configuration, or with appropriate surface ligands reduces the surface defect contribution to the overall recombination and generally leads to higher PL quantum yields.12,38,53–55 Radiative recombination at surface defects often overlap with lower energy emission arising from DAP states in these multinary chalcogenides.56 Recently Eychmueller and coworkers have isolated size-selected series of water-soluble luminescent Ag−In−S (AIS) and core/shell AIS/ZnS quantum dots (QDs) to probe their luminescence properties.57 An increase of the structural imperfection/disorder of QD with the decrease of the mean AIS size was deduced in this study from the Urbach absorption “tail” below the fundamental absorption edge. Broadening of the distribution of sub-bandgap states was noted for smaller size particles. In continuation of this study these authors also presented an alternate exciton trap model and explained the broad PL bands of AIS QDs to the electron−phonon interaction and the vibrational relaxation.58 In the present work, we explore the photophysical properties of Indium-rich AIZS quantum dots with an average particle diameter of ~4 nm and their dependence on the Ag:Zn ratio using steady state and transient spectroscopic techniques. The excited state interaction between AIZS quantum dots and TiO 2 using transient absorption spectroscopy and photovoltaic performance of solar cell devices employing different Ag:Zn ratios are discussed.



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EXPERIMENTAL METHODS Synthesis of AgInS 2 -ZnS Nanoparticles. Colloidal quantum dots were synthesized using a hot-injection method. Precursor salts consisting of 0.1, 0.2 or 0.3 mmol silver acetate [Ag(OAc)], 0.6 mmol In(OAc) 3 , and 0.1, 0.2, or 0.3 mmol Zn(OAc) 2 were put together in a round bottom flask, depending on desired cation ratio. 5 mL of octadecene, 5 mL of oleylamine, and 2 mL of octanethiol were then added to the reaction flask. The mixture was degassed under vacuum for 20 min at room temperature. A nitrogen atmosphere was then introduced, and the solution was heated to 110 °C. Immediately upon reaching this temperature, a sulfur precursor (consisting of 1 mmol sulfur powder dissolved in 2.5 mL oleylamine) was injected into the reaction vessel. The temperature was then raised to 150 °C and the solution was held at this temperature for 10 min. The reaction flask was then allowed to cool to room temperature and ~10 mL of toluene was added. Ethanol was then used to precipitate the nanoparticles out of solution under centrifugation. The resulting precipitate was then dispersed in toluene. Instrumentation. Transmission electron microscopy (TEM) images were obtained using an FEI Titan 80-300 system. A PHI VersaProbe II system was used for XPS measurements. Steady state absorption measurements were obtained from a Varian Cary-50 Bio spectrometer. A Horiba Fluorolog spectrometer was used to measure the steady state photolumenescence and excitation spectra. A time-correlated single photon counting (TCSPC) setup from Horiba was used to measure the PL lifetime of the colloidal quantum dots in toluene. Pump-probe transient absorption spectroscopy was used to monitor excited state processes in the picosecond time regime. The system consisted of a Clark MXR-2010 laser with the pump having an excitation wavelength of 387 nm, 150 fs pulse width, and an energy density of 50 µJ/cm2 per pulse. Part of the beam was split off and passed through a CaF 2 crystal to create the white light continuum which constituted the probe beam. For solar cell measurements, a 2-electrode potentiostat setup was used to record the current density vs voltage curves (scan rate of 15 mV/s) as well as current density vs time plots. The solar simulator consisted of a 300 W Xe lamp generating the beam of light which was passed through an AM 1.5G filter while the power density incident the solar cell was kept at a value of 100 mW/cm2 (1 sun illumination). A Princeton Applied Research PARStat 2273 potentiostat was used for recording J-V characteristics of the solar cell. Incident photon to current efficiency (IPCE) measurements were carried out with a Newport Oriel Quantum Efficiency Measurement System. 6 ACS Paragon Plus Environment

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Quantum Yield Calculation. The PL quantum yield (ϕ) was determined using the equation: 𝜙𝜙𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 = 𝜙𝜙𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 ×

𝐴𝐴𝐴𝐴𝐴𝐴𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 𝐴𝐴𝐴𝐴𝐴𝐴𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠

×

𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 𝑜𝑜𝑜𝑜 𝑃𝑃𝑃𝑃𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠

𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 𝑜𝑜𝑜𝑜 𝑃𝑃𝑃𝑃𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟

×

2 𝜂𝜂𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠

2 𝜂𝜂𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟

.

where Abs is the absorbance f the sample and reference at excitation wavelength, the Area of PL refers to the integrated area under the photoluminescence spectra, and η is the refractive index of the solvents. The quantum yield standard used was Tris(2,2′-bipyridyl)ruthenium(II) chloride in water. The absorbance values were matched at the excitation wavelength of 450 nm and the photoluminescence spectra were monitored between 480 and 850 nm. Transient Absorption Sample Preparation. Two different substrates were used for transient absorption measurements: The first sample was prepared by depositing AIZS quantum dots in toluene onto glass substrates spin coat and then allowed to dry. The other sample was deposited on the mesoscopic TiO 2 film. The TiO 2 films were first prepared by applying Solaronix Ti-Nanoxide T/SP paste via doctor blade onto glass substrates. The films were annealed at 500 °C for 1 h. Colloidal quantum dots were applied to the films via electrophoretic deposition (EPD). The samples were placed in vacuum cells which were degassed prior to measurement. Solar Cell Preparation. Working electrodes (photoanodes) were made by applying numerous TiO 2 layers to fluorine-doped tin oxide (FTO) glass and described in detail in our previous publication.17 Quantum dots were sensitized to working electrodes via EPD. The quantum dots were dissolved in a 1:1 mixture of toluene and chloroform and 100 V was applied between a photoanode and blank piece of FTO for 6 min. Following EPD procedure, a Successive Ionic Layer Adsorption and Reaction (SILAR) method was used to deposit a ZnS passivation layer on the sensitized electrodes. The electrodes were first dipped in a 0.05 M Zn(NO 3 ) 2 ethanol solution for 1 min then rinsed with methanol. Second, the electrodes were dipped in a 0.05 M Na 2 S mixture of methanol and water (1:1) for 1 min followed by rinsing with methanol. Four of these SILAR cycles were applied to each f the working electrodes. An aqueous polysulfide electrolyte was employed as a redox couple for the solar cells. The electrolyte consisted of 2 M Na 2 S and 2 M sulfur powder dissolved in deionized water. Counter electrodes were prepared by thermally evaporating copper onto FTO glass substrates. The resulting 100 nm films were then sulfurized by dipping into polysulfide electrolyte for 10 min. 7 ACS Paragon Plus Environment


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RESULTS AND DISCUSSION Characterization and Photophysical Properties of the AgInS 2 -ZnS Nanoparticles. A hot-injection method was used to synthesize octanethiol/oleylamine-capped AIZS quantum dots (See Experimental Methods for the procedure). Three compositions of AIZS nanoparticles were prepared with cation precursor ratios Ag:In:Zn of 3:6:1, 2:6:2, and 1:6:3, referred to henceforth as AIZS 361, AIZS 262, and AIZS 163, respectively. The quantum dots were dispersed in toluene

Figure 1. TEM images of (A) AIZS 361 (B) AIZS 262 and (C) AIZS 163 nanoparticles with with cation precursor ratios Ag:In:Zn of 3:6:1, 2:6:2, and 1:6:3 respectively. and were stable in air for many months. Transmission Electron Microscopy (TEM) was used to discern the size and shape of the AIZS nanoparticles. Figure 1 shows TEM images of AIZS 361, 262, and 163 quantum dots with cation precursor ratios, Ag:In:Zn of 3:6:1, 2:6:2, and 1:6:3. AIZS 361 nanoparticles had an average diameter of 4.3 ± 0.8 nm, AIZS 262 nanoparticles had an average diameter of 3.7 ± 0.7 nm, and AIZS 163 nanoparticles had an average diameter of 4.0 ± 0.8 nm. Size distribution histograms are given in Figure S1. The elemental compositions of the AIZS nanoparticles were determined by XPS. Table 1 shows the atomic percentages of the three cations Ag, In, and Zn present in these three quantum dot samples. In each case, the silver content was slightly greater while zinc content was lower than the expected composition from the precursor concentrations. The indium percentage was quite close to the expected value. XPS data


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Table 1. Elemental composition of the nanoparticles as determined by XPS.

and elemental percentages including sulfur are provided in the Supporting Information (Figure S2S4).

The absorption spectra (Figure 2A) show monotonically increasing absorption with increasing photon energy without exhibiting distinct excitonic peaks. We observe a blueshift in the absorption edge as more zinc is incorporated into the nanoparticles. The solid solution between AgInS 2 and ZnS has been shown to form an alloy across a wide range of compositions with the resulting bandgap being tunable between 1.87 eV and 3.6 eV.28 The absorption onset for AIZS 361 quantum dots is around 650 nm, for AIZS 262 around 580 nm, and for AIZS 163 around 550 nm. The tail absorption is attributed to the intrabandgap transitions which arise from the defect states that overshadows the excitonic peak. A recent study on water-soluble AIZS quantum dots compared the size of the nanoparticles offered an alternate explaination based on the Urbach tail absorption.57 It was found that the Urbach energy increased as nanoparticle size decreased, indicating that smaller quantum dots possess more imperfections, disorder, and/or defects. It was shown in an earlier study that there are two distinct absorption pathways in CuInS 2 quantum dots which account for the long absorption tail and lack of distinct excitonic peak.40 One absorption pathway is band edge absorption while the other is excitation from copper defect states lying above the valence band to the conduction band. This second pathway accounts for the long absorption tail observed in ternary nanocrystals. Similarly, we can attribute the tail seen in the absorption spectra of Figure 2A to the corresponding silver defect states lying within the bandgap in our AIZS quantum dots. Based on the experimental work done by Jara et al. and theoretical work on defect energy levels by Zhang et al., we surmise that the antisite defect of silver on an indium site (Ag In ) is a probable defect responsible for this lower energy (tail) absorption.40,48 Thus 9 ACS Paragon Plus Environment


A

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B

Figure 2. (A) Absorption and (B) normalized PL spectra of (a) AIZS 361, (b) AIZS 262, and (c) AIZS 163 in toluene. Emission spectra were recorded using an excitation wavelength of 450 nm.

we can attribute two excitation pathways responsible for separation of charge carriers in AIZS (reactions 1 & 2). + AIZS + hν → AIZS (e− CB + hVB )

(1)

0 − AIZS (Ag − In ) + hν (lower energy) → AIZS (Ag In + eCB )

(2)

PL spectra of the three AIZS compositions presented in Figure 2B shows the large stokes shift and broad emission, characteristic of I-III-VI 2 nanomaterials. As with the absorption spectra, a blueshift in the PL is observed with increasing zinc content. The PL quantum yield calculated

A

B

C

Figure 3. PL decay of (A) AIZS 361 (B) AIZS 262 and (C) AIZS 163 measured by TCSPC for monitoring wavelengths of (a) 500 nm, (b) 550 nm, (c) 600 nm, (d) 650 nm and (e) 700 nm. The laser excitation wavelength used was 458 nm. 10 ACS Paragon Plus Environment

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using a ruthenium tris(bipyridyl) complex as reference was found to be 17.3% for AIZS 361, 24.9% for AIZS 262, and 26.5% for AIZS 163. Thus, the quantum yield increases as more zinc is incorporated into the nanoparticles. The fluorescence lifetime was recorded for each sample using TCSPC (Figure 3). The colloidal quantum dots in toluene were excited with a 458 nm pulsed laser and the PL was monitored between 500 and 700 nm over a 2 µs time window. For all compositions, the PL lifetime increases with increasing emission wavelength, and had average lifetimes extending into the hundreds of nanoseconds. This behavior strongly suggests that DAP recombination is the dominant radiative mechanism in these alloyed AIZS nanoparticles. Higher energy photons originate from donors in close proximity to acceptors, while lower energy photons originate from DAPs spatially farther apart. Since the high energy pairs are located close to each other, the probability for recombination is higher due to larger wavefunction overlap.59 The PL decay displayed a multiexponential character with emission at longer wavelengths exhibiting longer lifetimes for all components. Tables S7 – S9 show the biexponential fitting 𝐴𝐴 𝜏𝜏2 +𝐴𝐴 𝜏𝜏2

parameters. Average lifetimes which were calculated using the formula: 〈τ〉 = 𝐴𝐴1 𝜏𝜏1 +𝐴𝐴2 𝜏𝜏2 . When 1 1

2 2

monitoring the emission near the bandgap (i.e., 500 nm) we see fast decay components (τ 1 ) of 6.4,

12.1, and 16.4 ns for AIZS 361, 262, and 163, respectively. The slow decay component (τ 2 ) grows from around 100 ns for shorter wavelengths (500 nm) to over 700 ns at longer wavelengths (700 nm). The fastest radiative processes in semiconductors are generally attributed to band edge (conduction to valence band) emission. We see evidence of this process from the fast decay lifetimes when monitoring near the bandgap. At longer wavelengths, the longer lifetimes suggest that DAP recombination is primarily responsible for emission of the less energetic photons. PL decay lifetimes intermediary between these two processes are free-to-bound transitions, which consists of an electron in the conduction band recombining with a hole in an acceptor defect level.59 These three radiative pathways are summarized in reactions 3 – 5. + AIZS(e− CB + hVB ) → AIZS + hν

+ ′ AIZS(e− CB ) + A𝑑𝑑𝑑𝑑𝑑𝑑 (h ) → AIZS + A𝑑𝑑𝑑𝑑𝑑𝑑 + hν

D𝑑𝑑𝑑𝑑𝑑𝑑 (e− ) + A𝑑𝑑𝑑𝑑𝑑𝑑 (h+ ) → D𝑑𝑑𝑑𝑑𝑑𝑑 + A𝑑𝑑𝑑𝑑𝑑𝑑 + hν′′

(3)

(4)

(5)

D def and A def stand for defect donor level and defect acceptor level, respectively. Reaction 3 represents band edge emission and plays a minor role in the overall fluorescence for these 11 ACS Paragon Plus Environment


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Scheme 1. Absorption and deactivation mechanisms in AIZS quantum dots. Process 1 is absorption and recombination from band edge transitions. Process 2 is absorption and recombination between defect levels, located above the valence band, and the conduction band. Process 3 is DAP recombination arising from defects. Solid lines represent absorption or emission of a photon, while dashed lines represent nonradiative processes.

quantum dots, as we observe a large stokes shift and little overlap between the absorption and PL spectra (Figure 2B). Reaction 4 represents free-to-bound transitions which emit less energetic photons relative to band edge emission (hν’ < hν). The acceptor defects responsible for emission from this type of pathway has been identified as surface defects in AIZS nanocrystals.56 Reaction 5 represents radiative DAP recombination. These radiative pathways, as well as absorption pathways described in reactions 1 and 2, are summarized in Scheme 1. The types of defects responsible for emission in these multinary chalcogenides can be put into two main categories: intrinsic crystallographic defects and surface defects.37 Numerous studies on AgInS 2 and AIZS have suggested that surface defects provide faster recombination pathways while intrinsic defects are responsible for the longer radiative lifetimes.37,47,52,56,60 Additionally, surface defects provide mainly non-radiative recombination pathways while intrinsic defects are much more fluorescent.9,38,53,55 Surface defects have been shown to have both free-tobound and DAP character.56 Thus, the relaxations shown in processes 2 and 3 from the scheme are possible recombination pathways due to surface defects. A recent work by Eychmueller and coworkers presents a self-trapped exciton model to explain the broad nature of the emission band.58 Strong electron-phonon interactions along with a 12 ACS Paragon Plus Environment

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vibrational relaxations were shown to influence the position and shape of the photoluminescence spectra in ZnS capped AIS quantum dots. In this report, the authors argue that the broad PL is an inherent property which does not depend on defectiveness of the lattice. While this alternate explanation may have some grounds, the majority of work by others attribute long wavelength absorption and associated emission to the inherent defects.7,48,50 The photoluminescence response in the long wavelength region as confirmed from the excitation spectra (Figure S5) confirms the availability of midgap states to direct excitation. The dominant radiative mechanism in these I-III-VI2 chalcogenide quantum dots has been shown to be radiative DAP recombination (process 3) arising from intrinsic crystallographic defects.23,38,44–47 The specific intrinsic defects which participate in emission have not yet been identified with certainty due to the complexity of nanoparticle compounds containing three to four elements, along with the fact that many intrinsic point defects and defect pairs form easily in these I-III-VI 2 compounds.45,48 However, numerous theoretical and experimental studies have identified possible donors and acceptors, as well as some of their energy levels within the bandgap. These donor-acceptor energy levels are typically assigned to crystallographic defects such as vacancies, interstitial atoms, and antisite defects.23,48–50 From these reports and the spectroscopic data obtained on our indium-rich AIZS nanoparticles, we propose (In Ag 2+ + 2V Ag -) defect complexes, silver vacancies, and zinc-related emissive states as probable intrinsic defects responsible for the fluorescence seen in these materials.



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Transient Absorption Measurements. To elucidate photophysical processes of AIZS quantum dots in the picosecond time regime, pump-probe transient absorption experiments were performed on quantum dot films. Colloidal solutions of each composition were dropcast onto glass slides and allowed to dry at room temperature under ambient conditions. These samples were transferred to a quartz optical cell with the provision to evacuate and seal prior to measurement. The time-resolved difference absorption spectra were recorded at different probe delays following 387 nm laser (pulse width 150 fs) excitation. Representative difference absorption spectra of AIZS 361, 262, and 163 films on glass substrates are presented in Figure 4A-C. AIZS 361 possesses a very broad bleach which spans from about 420 to 650 nm. We clearly see two bleach maxima

A

B

C

D

E

F

G

H

I

Figure 4. Transient absorption spectra of (A) AIZS 361 (B) AIZS 262 and (C) AIZS 163 quantum dots on glass slides. Transient absorption spectra of (D) AIZS 361 (E) AIZS 262 and (F) AIZS 163 quantum dots sensitized onto TiO2 films. Bleaching recovery kinetics of (G) AIZS 361 (H) AIZS 262 and (I) AIZS 163 at 500 nm, 482 nm, and 440 nm, respectively. The kinetic traces were recorded (a) on glass and (b) on TiO2 substrates. 14 ACS Paragon Plus Environment

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arising from band edge absorption (reaction 1) and shallow defect states (reaction 2) in these quantum dot films. The 387 nm photons are capable of exciting both valence band electrons and electrons residing in intrinsic defect sites to the conduction band. This is evident from the bleaching of the absorption in the band edge as well as tail absorption region (see panel A in Figure 4). Such a bleaching response in the transient absorption spectra is similar to the one observed in the case of CuInS 2 quantum dots.40 As we decrease the Ag content we see narrowing down of the bleach and minimization of the low energy band. This is reflective of decreased tail absorption seen in the absorption spectra. AIZS 262 exhibits bleach between 400 and 580 nm and AIZS 163 shows bleach at wavelengths less than 550 nm. In order to probe the photosensitization properties of AIZS quantum dots we adsorbed these nanoparticles onto mesoporous TiO 2 films by EPD. The favorable energetics is expected to induce charge injection from excited AIZS into TiO 2 similar to those observed in other quantum dot solar cells (QDSCs).61-64 We probed the excited state interaction between these two semiconductors in the picosecond time regime by exciting AIZS quantum dots with a 387 nm pump laser and recording time-resolved transient absorption spectra. Figure 4D-F show the transient absorption spectra for the three compositions of AIZS quantum dots adsorbed on TiO 2 films. The overall bleach seen following the excitation of AIZS quantum dots on TiO 2 exhibit bleaching similar to the corresponding ones observed on glass slides, but with a faster recovery. However, a major difference is the observation of additional induced absorption feature, seen at longer wavelengths. This induced absorption is attributed to trapped electrons in TiO 2 after the electrons are injected from excited AIZS quantum dots. Similar broad absorption in the red-infrared region has been observed in directly excited colloidal TiO 2 nanoparticles, and photoinduced electron transfer from CdSe quantum dots to TiO 2 films.62,63 We ruled out the possibility of direct excitation of TiO 2 by exciting a TiO 2 film with same transient absorption spectroscopic conditions. Figure S6 shows the

transient absorption spectra of this blank TiO 2 sample excited with the 387 nm laser. No detectable signal was observed over the entire 1500 ps time window. Based on these observations we can conclude that electrons that are transferred from AIZS quantum dots to the TiO 2 are responsible for the induced absorption in the transient spectra. This signal becomes fully developed 2 ps after laser pulse excitation, and this photoinduced electron transfer mechanism is given by reaction 6. 𝜏𝜏 < 2 ps

AIZS(e− + h+ ) + TiO2 �⎯⎯⎯⎯� AIZS(h+ ) + TiO2 (e− ) ACS Paragon Plus Environment

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15


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If we assume that the electron transfer occurs with a lifetime of 2 ps, we obtain an electron 1

transfer rate constant �k et = τ� of 5 × 1011 s-1. This electron transfer rate constant is greater than the one we reported earlier (k et = 1.8 × 1010 s-1) for silver-rich AIZS quantum dots.17 The increased indium and zinc content is likely to shift the conduction band level to higher energies thus providing a greater driving force for electron transfer.64 The bleach recovery of these quantum dot films on glass are presented in Figure 4G-I as the red data points (a). The bleached absorption is long-lived and decayed little within the time window of our measurements. The long bleaching recovery is in agreement with the long excited lifetime observed through emission decay experiments. Table 3.3 provides the biexponential fit

parameters for the bleach recoveries for all AIZS quantum dot compositions both on glass substrates and TiO 2 films. For each composition, we see a faster bleach recovery with TiO 2 than without. The additional deactivation pathway for photoexcited charge carriers is the cause of this faster decay. The bleaching recovery and the decay of induced absorption were followed over the 1.5 ns time window. Interestingly the bleach recovery monitored at 482 and 440 nm for AIZS 262 and AIZS 163 respectively, as well as the induced absorption decay monitored at 650 nm shown in Figure S7 (Supporting Information) show similar kinetics, thus supporting recombination of electrons residing in TiO 2 and the residual hole in the AIZS quantum dots (reaction 7). AIZS(h+ ) + TiO2 (e− ) → AIZS + TiO2

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AIZS Quantum Dot Solar Cells. The charge injection from excited AIZS into TiO 2 nanostructures was further evaluated by constructing liquid-junction QDSCs. The specific details of device fabrication can be found in the Experimental Methods. In brief, phonoanodes consisting of numerous TiO 2 layers on FTO glass were made using the doctor blade technique to apply pastes followed by firing at 500 °C. Quantum dots were then adsorbed onto the TiO 2 films by EPD. A 100 nm thin film of Cu x S on FTO was used as counter electrode and an aqueous solution containing 2 M sulfur / 2 M Na 2 S was used as a regenerative redox electrolyte. Three devices of each composition were fabricated to evaluate reproducibility of photoelectrochemical performance. Individual current density vs voltage (J-V) curves and photovoltaic parameters for these solar cells are included in the Supporting Information (Figure S8 and Table S12,

A

B

Figure 5. (A) Current density vs voltage curves of three photovoltaic devices, one of each AIZS composition. (B) IPCE of the three devices. (inset) photographs of devices sensitized with AIZS 361 quantum dots (leftmost image), AIZS 262 quantum dots (middle image), and AIZS 163 quantum dots (rightmost image).

respectively). The solar cells were tested using a solar simulator under 1 sun illumination (AM 1.5) and the voltage was swept from 0V to just above open circuit voltage (V oc ). Figure 5A depicts the J-V curves for three select solar cells of different AIZS composition. Table 3 presents the photovoltaic parameters averaged from three identical devices for each composition. AIZS 262 performed best overall with the highest PCE of 1.83% and highest short circuit current density (J sc ), while AIZS 17 ACS Paragon Plus Environment


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163 had the highest V oc . AIZS 361 obtained a PCE of 1.56% and AIZS 163 had the lowest average PCE of 0.86%. AIZS 262 also had both higher V oc and J sc than did AIZS 361. It was not expected that AIZS 262 would have a higher J sc than AIZS 361 due to the fact that the more silver-rich sample had more coverage (~70 nm) of the visible spectrum (650 nm absorption onset vs 580 nm). The IPCE plot shown in Figure 5B gives insight to the wavelength-dependent photoconversion efficiency of the three different quantum dot compositions. Consistent with the steady state absorption measurements (Figure 2A), the IPCE follows the trend in which the onset of AIZS 361 occurs at the longest wavelength (~750 nm) whilst the onset of AIZS 262 occurs around 725 nm and the onset of AIZS 163 occurs around 650 nm. AIZS 361 is more efficient at turning the incident light into electrical energy between approximately 550 and 750 nm, but AIZS 262 shows much more efficient use of photons with wavelengths between 400 and 550 nm. The photocurrent was also monitored over time to evaluate the short-term stability of the solar cells. Figure 6 shows photocurrent density over a period of 5 min for each AIZS composition. A voltage bias of 0.2 V was applied and a light chopper was used to turn on and off the light beam (on for 20 s, off for 10 s). The results show that photocurrent was stable over this time window. AIZS 262 showed slightly higher current density than AIZS 361 while the current density was significantly lower for AIZS 163.


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Figure 6. Current density over a 300 second time window under a constant 0.2 V bias. A light chopper was employed to turn on and off the light (20 seconds on, 10 seconds off).

CONCLUSIONS Colloidal AIZS quantum dots approximately 4 nm in diameter and with similar size distributions synthesized with varying Ag:Zn ratio exhibit composition-dependent photophysical properties. From analysis of PL experiments, DAP radiative recombination arising from defects was established as the dominant radiative mechanism in these quaternary quantum dots. Two absorption pathways were established, one being valence band to conduction band absorption, and the other due to electrons in Ag In antisite defects with energy levels lying above the valence band edge being excited to the conduction band. A fast photoinduced electron transfer from excited AIZS quantum dots to nanostructured TiO 2 films was observed. The electron transfer rate constant was determined to be 5 × 1011 s-1 for all compositions. Evaluation of photosensitization properties of AIZS as sensitizers in liquid-junction QDSCs revealed AIZS 262 to be the champion light harvesting materials for photovoltaics among the three AIZS compositions tested with an average PCE of 1.83%.



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SUPPORTING INFORMATION Size distribution histograms, XPS data and compositional analysis, excitation spectra, TCSPC fit parameters, additional transient absorption plots and kinetics, and individual solar cell performance for all measured devices.

ACKNOWLEDGMENTS The research described herein was supported by t acknowledges the support of 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. This is document no. NDRL 5208 from Notre Dame Radiation Laboratory. The authors would like to thank Dr. Sergei Rouvimov for help in collecting TEM images and Dr. Subila Balakrishnan for help in TEM sample preparation. 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 would like to thank the Arthur J. Schmitt Leadership Fellowship and CEST-Bayer fellowship to continue graduate study at University of Notre Dame.

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