Quenching of the Excitonic Emission of ZnO Quantum Dots Due to

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Quenching of the Excitonic Emission of ZnO Quantum Dots Due to Auger-Assisted Hole Transfer to CdS Quantum Dots Supriya Ghosh, Mihir Ghosh, Pushpendra Kumar, Abdus Salam Sarkar, and Suman Kalyan Pal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11011 • Publication Date (Web): 11 Nov 2016 Downloaded from http://pubs.acs.org on November 12, 2016

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The Journal of Physical Chemistry

Quenching of the excitonic emission of ZnO quantum dots due to Auger-assisted hole transfer to CdS quantum dots

Supriya Ghosh, Mihir Ghosh, Pushpendra Kumar, Abdus Salam Sarkar and Suman Kalyan Pal School of Basic Sciences and Advanced Material Research Center, Indian Institute of Technology Mandi, Kamand 175005, H.P, India. *

Corresponding Author: Tel.: +91 1905 267062; Fax: +91 1905 267009; E-mail: [email protected]

Abstract The charge transfer mechanism in quantum dot (QD) donor-acceptor systems is yet poorly understood. Here, we utilize steady state and time-resolved emission spectroscopy to study photoinduced hole transfer from ZnO to CdS QDs. The observed quenching of the excitonic emission (both intensity and lifetime of ZnO QDs) in the presence of CdS QDs is attributed to the hole transfer from excited ZnO to CdS QDs. We have demonstrated that the variation of the hole transfer rate with the driving force do not follow the conventional Marcus model, rather fits with a new Auger-assisted transfer mechanism, where the excess energy is used for electronic excitation. Moreover, we have evidenced the consequences of the hole transfer through the measurement of the enhanced photoconductivity of the film made of the blend of ZnO and CdS QDs.

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Keywords: ZnO QDs, CdS QDs, time-resolved emission spectroscopy, hole transfer, Augerassisted model. 1. Introduction Owing to their unique features of high absorption coefficient,1 band gap tunability,2 multiple exciton generation (MEG)3 and large intrinsic dipole moments,4 quantum dots (QDs) have shown great promise for photovoltaic5,6,7,8 and photocatalytic9,10,11 applications. Quantum dot sensitized solar cell (QDSSC) is considered to be one of the promising next generation solar cells, which has drawn ample consideration due to simple and economical solution processed fabrication compared to the customary silicon solar cells.12,13,14 According to a theoretical simulation, the power conversion efficiency (PCE) of QDSSCs can reach up to 40%,15,16 which is much higher than that of the Schockley-Queisser limit (31%)17 for single bandgap solar cells. Conversely, the experimental efficiency of QDSCs is still very low, which may be accredited to the severe surface charge recombination18 and unfavorable internal band-gap structure.6 Till now, maximum accomplished efficiency of QDSSC is 6% by implementing doped, alloyed, and core-shell based QDs.19,20,21 Charge collection, especially hole extraction is the limiting factor to the efficiencies of QD based solar cells. A deep insight into the interfacial hole transfer could be beneficial for the improvement of device efficiencies. The device architecture of QD based photovoltaic devices could greatly improve not only from mechanical understanding of hole transfer, but also from establishing a mathematical relationship between the rate of hole transfer and corresponding driving force for charge transfer. The quantum efficiency of charge generation that determines the short circuit current of the device increases with enhancement in the rate of charge (or hole) transfer. In contrast, the driving force for charge transfer is lost by reducing the device open 2 ACS Paragon Plus Environment

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circuit voltage. In order to obtain maximum efficiency, one should achieve a higher charge transfer rate by losing minimum potential energy in the form of driving force. The charge transfer rate in molecular systems is governed by Marcus theory,22,23,24,25,26 which suggests that the electron transfer rate initially increases with driving force up to a point where driving force is equal to the reorganization energy (normal region) and then decreases with the further increase of the driving force (inverted region). A considerable number of work has been devoted to develop theoretical model for both electron27,28,29 and hole transfer30,31,32 in QD-molecular systems. Unlike molecules, QDs possess inherent heterogeneity with respect to the radius, the total number of surface ions per QD, the ratio of surface cations to surface anions, and the number of bound surface ligands per QD. Nonetheless, there could be a number of different possible configurations for the binding of a surface ligand to the QD surface. In spite of various heterogeneities and distributions in QDs, many people were able to describe the relation between the electron transfer rate from QDs to molecular acceptor and driving force on the lights of Marcus theory through controlled and systematic experiments.33,34,28 Zhu et al.28 did not observe Marcus inverted region for the photoinduced electron transfer from QDs (CdS, CdSe, and CdTe) to molecular acceptors within a wide span of driving force. They rationalized the observed deviation from the Marcus picture by introducing a new model called Auger-assisted electron transfer model. According to this model the potential energy lost in the electron transfer to the molecule is coupled to the excitation of the residual hole in the valence band (like Auger recombination in QDs) avoiding the unfavorable Franck–Condon overlaps in the Marcus inverted region. Because of the coupling of electron transfer with hole excitation and many potential energy states of hole in QDs, some transition could be barrier less leading to saturation of the electron transfer rate at the highest driving forces. Recently, Alivisatos and coworkers35



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investigated hole transfer from CdSe/CdS core-shell QDs to six different molecules and reported lack of Marcus inverted regime in the relationship between the hole transfer rate and driving force, which was interpreted using an Auger-assisted transfer mechanism. In contrary, Zheng et al.36 were successfully explained the relation between the hole transfer rate from QDs to metal oxide using Marcus theory. Thus, we can see that there is no general consensus regarding the model for charge transfer in QD-based systems and requires further study. In this paper, we examine the quenching of the excitonic emission of ZnO QDs in the presence of different size CdS QDs using time-resolved emission spectroscopy (TRES). Observed emission quenching was attributed to the photoinduced hole transfer from ZnO QDs to CdS QDs. The hole transfer rate varies with the driving force and the variation is explained, for the first time, by Auger-assisted transfer mechanism. The photoconductivity of ZnO-CdS QDs composite film was found to enhance as a consequence of the photoexcited hole transfer from ZnO QDs to CdS QDs.

2. Experimental Section 2.1 Materials The chemicals used in the experiments are zinc acetate dihydrate (Zn(OAc)2.2H2O, Merck, Germany), tetramethyl ammonium hydroxide (TMAOH; 25 wt%) in methanol (Loba Chemie Private Limited), cadmium chloride (CdCl2. 2.5H2O) (99%), 3-mercaptopropionic acid (MPA, >99%), thiourea (99%), methanol (>99%, HPLC grade) from Sigma Aldrich and absolute ethanol (>99% AR, S D Fine-Chem Limited) without further purification. Deionized water was used for the preparation of solutions.


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2.2. Synthesis of ZnO QDs Nanometer-size ZnO particles were prepared following the procedure adopted by Wood et al.37 with some modification. For a typical preparation, 0.995 g of Zn(OAC)2. 2H2O was dissolved in 100 mL of absolute ethanol and heated up for proper dissolution by forming a clear solution. Then 2.8 ml of 25% (w/w) TMAOH in methanol was added with constant injection rate into it under vigorous stirring condition in a water bath at room temperature. After that 1 ml of QD solution was stirred for 30 min with MPA (0.012 mmol, 1.28 mg) and 2 ml of HPLC grade methanol. The resulting reactants were centrifuged at 6000 rpm for 5 minutes and the supernatants were discarded. The resulting precepitates were redesolved in HPLC grade methanol and bi-functional MPA capped ZnO QDs were obtained. 2.3. Synthesis of CdS QDs CdS QDs were synthesized following the procedure introduced by Schneider et al.38 Both, 0.175 mmol CdCl2. 2.5H2O (40 mg) and 0.289 mmol of thiourea (22 mg) were dissolved in 7 ml of ultrapure water in a 100 ml conical flask. An aqueous solution (10 ml) of MPA (42 mg, 0.395 mmol) was then added to it. The pH was adjusted to 10.0 with a 1.0 M NaOH solution. The typical molar ratio of 1:1.7:2.3 for Cd2+, thiourea and MPA was maintained in our experiments. The solution was purged with pure nitrogen gas for 30 min and transferred into a teflon-lined stainless steel autoclave. The autoclave was maintained at 105°C for 1, 1.5, 2, 2.5 and 3 hour (s) for synthesis of CdS-1, CdS-2, CdS-3, CdS-4 and CdS-5 QDs, respectively. Finally, the solution was cooled down to room temperature.



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2.4. Spectroscopic and microscopic techniques The steady state electronic absorption spectra of all the synthesized QDs were measured by using a quartz cuvette of 1 cm path-length by means of Shimadzu UV-Vis 2450 spectrophotometer at the ambient temperature (300 K). Emission spectra of the samples were recorded by using Cary Eclipse fluorescence spectrophotometer (Agilent Technologies). The emission was detected at right angles to the direction of excitation light in order to avoid stray light. The time resolved emission measurements were carried out using a time correlated single photon counting (TCSPC) spectrometer, Fluorolog Modular Spectrofluorometer (HORIBA Scientific). The samples were excited with 284 nm light from a nanoLED. A PMT based detector was used for detection of the emitted photons through a monochromator. The instrument response of the TCSPC set-up was measured by collecting the scattered light from a ludox suspension. Reduced , Durbin–Watson (DW) parameter and residuals were used to judge the goodness of the fit. Transmission electron microscopy (TEM) images of the QD samples were recorded by an instrument (Tecnai G2 F20, FEI Co., USA) operated at an accelerating voltage of 200 kV. The energy dispersive spectroscopy (EDS) was performed from the same TEM instrument with an energy dispersive X-ray detector (Bruker, Co., Germany). 2.5. Fabrication and characterization of devices Bulk heterojunction devices were fabricated from ZnO and CdS QD blends, which were prepared by dissolving both the QDs in cosolvent (deionized water and ethanol) and stirring for several hours at room temperature. Chemically etched, patterned indium tin oxide (ITO) coated glass substrates (Sigma Aldrich, USA) having sheet resistance 8-12 Ω/□ were used for the device fabrication. The substrates were cleaned by ultrasonic treatment in detergent, deionized water, 6 ACS Paragon Plus Environment

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acetone and isopropyl alcohol and subsequently dried in a vacuum oven. A layer of QD mixture was then prepared by drop casting 100µl of the mixed solution onto the ITO coated glass substrates, which was kept at 75oC. Aluminum cathode (100 nm thick) on the QD layer was deposited by thermal evaporation in a vacuum chamber at a pressure of 10-6 mbar.39 Electrical measurements were performed using Keithley-2400 source meter.40 The photoconducting behavior of the fabricated devices was characterized under the illumination of 100 mW/cm2 with AM1.5G irradiation by an OAI Trisol solar simulator calibrated with a standard Si solar cell.

3. Results and Discussion 3.1. Optical properties of QDs A major endeavor of the present study is to comprehend the hole-transfer behavior from ZnO QDs to CdS QDs. Nonetheless, before exploring the charge-transfer in the excited state, it is very significant to monitor the optical properties of QDs. The steady-state absorption spectrum of ZnO QDs having the first excitonic peak around 328 nm is shown in figure 1a. The sharp peak of the excitonic band of ZnO QDs suggests a narrow size distribution for the QDs. QD size was estimated from the excitonic band-gap41 and found to be 3.2 nm. Figure 1b depicts the absorption spectrum of CdS QDs of five different sizes (CdS-1, CdS-2, CdS-3, CdS-4 and CdS-5) with excitonic peaks around 353, 362, 376, 389 and 398 nm, respectively. Diameter (d) of CdS QDs was estimated using the following equation 42 () = (−6.6521 × 10 ) + (1.9557 × 10 ) − (9.2352 × 10 ) + 13.29 (1) Thus calculated particle sizes were CdS-1: 2.01 nm, CdS-2: 2.23 nm, CdS-3: 2.44 nm, CdS-4: 2.59 nm and CdS-5: 2.81 nm. These values are very close to that obtained from Brus equation.43,44,45,46 TEM images and EDS spectrum ( SI figure S1 and S2) also provide evidence 7 ACS Paragon Plus Environment


for the formation of tiny CdS particles. The excitonic peaks for ZnO and CdS particles get blueshifted compared to the bulk (band gap of ∼3.30 eV47 and 2.40 eV48 for ZnO and CdS, respectively). Since the radii of particles are less than the Bohr exciton radii for both ZnO and CdS (2.87 nm49 and 5.8 nm,50 respectively), the blue-shift in the absorption spectra may be

1.2

(a)

1.0 0.8 0.6 0.4 0.2 0.0 300 350 400 450 500 550 Wavelength (nm)

Intensity

400

Absorbance (norm.)

regarded as confinement-induced shift.

Intensity (norm.)

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1.2

(b)

0.9 0.6 0.3 0.0

300

350 400 450 Wavelength (nm)

500

(c)

300 200 100 0 350


550

Figure 1. (a) Normalized absorption (black) and photoluminescence spectra (blue) of ZnO QDs. (b) Normalized electronic absorption spectra of CdS-1 (purple), CdS-2 (green), CdS-3 (dark


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gray), CdS-4 (orange) and CdS-5 (red) QDs. (c) Steady state emission spectra of ZnO QDs (~ 0.9 µM) in absence (blue) and presence (red) of CdS-5 QDs (~ 0.13 µM). ZnO QDs show an intense emission band having maximum at ~ 370 nm (Figure 1a). This band is very close to the absorption onset of the QDs and hence can be safely assigned to the excitonic emission of ZnO QDs. It should be noted that ZnO QDs do not show any trap emission, which has a mximum around 510 nm.51 The excitonic emission of ZnO QDs is greatly quenched by CdS QDs (Figure 1c). 3.2. Time resolved emission studies and hole transfer Time resolve emission spectroscopy (TRES) is a very informative tool for investigating the excited state processes, e.g., energy and charge transfer. Time resolved emission spectra of ZnO QDs in the absence and presence of CdS QDs were measured following excitation by a

3000 2500

(a)

2000

1 ns

1500 1000 500 0

(b)

(a)

2 ns

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Counts

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350


1000 500 0

550

350 400 450 500 Wavelength (nm)

550

Figure 2. (a) and (b) TRES of ZnO QDs (~ 0.9 µM) in the absence (blue) and presence (red) of CdS-5 QDs (~ 0.13 µM) at 1 ns and 2 ns, respectively.



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a 284 nm light. Figure 2a and b show the emission spectra of ZnO and ZnO-CdS-5 QDs at different monitoring times (1 and 2 ns, respectively). It is clear from figure 2a and b that ZnO QDs exhibit only excitonic emission band around 370 nm, but no trap emission peak, which is supposed to appear at 510 nm. The excitonic emission from ZnO QDs is quenched in the presence of CdS-5 QDs (Figure 2a and b). The emission quenching is also observed for four other CdS QDs (CdS-1, CdS-2, CdS-3 and CdS-4) of different sizes (SI figure SI3 and SI4). CdS QD solution of same concentration that was used for the quenching experiment exhibits no emission and hence do not contaminate the emission from ZnO QDs. It is to be mentioned that in order to selectively excite ZnO QDs to eliminate the primary inner filter effect, all the emission measurements were carried out with dilute solutions (∼0.03 µM ) of CdS QDs, which have negligible absorption at the excitation wavelength (SI figure S5). Alignment of energy levels of ZnO and CdS QDs is shown in figure 3a. The conduction band (CB) of ZnO QD lies below the CB of CdS QD making photoinduced electron transfer from ZnO to CdS unfavorable. On the other hand, the less positive energetic position of the VB of CdS QD compared to the VB of ZnO QD infers possibility of hole transfer, which is a thermodynamically downhill process from photoexcited ZnO to CdS QD. Therefore the quenching of the emission of ZnO QDs could be attributed to the photoinduced hole transfer from the VB of a ZnO QD to the VB of a CdS QD (figure 3a). The hole transfer mechanism can be expressed as + ℎ → (" + ℎ# ) (2) (" + ℎ# ) + → (" ) + (ℎ# ) (3)


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3.3. Emission kinetics and hole transfer rates To corroborate the hole-transfer process, we have measured emission decay kinetics with low enough excitation fluence ensuring negligible contributions from multiexcitonic processes. Figure 3b shows the emission decay traces of only ZnO QDs and ZnO-CdS QDs mixed samples following excitation at 284 nm and monitoring the emission at 360 nm. It is apparent from figure 3b that the fluorescence lifetime of ZnO QDs is reduced in the presence of CdS QDs. The emission decay kinetics for pure ZnO QDs can be fitted biexponentially with time constants $% = 2.37 ns ( 98%) and $ = 10.39 ns ( 2 %) with average lifetime ($&' ) of 2.53 ns. Whereas,

Counts

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C o u n ts

10000

1000

1000 0

2 4 6 Time (ns)

8

100 IRF 10

(c) 1E9 kHT (S-1)

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1E8 400

600 800 1000 ∆GHole (meV) 11

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Figure 3. (a) Schematic diagram representing the Auger-assisted hole transfer process from electronically excited ZnO QDs to CdS QDs. Electronic excitation in the conduction band occurs during hole transfer. (b) Emission decay profiles of ZnO QDs (concentration ~ 0.9 µM) in the absence (blue) and in the presence of CdS-1 (concentration 0.18 µM, purple), CdS-2 (concentration 0.17 µM, green), CdS-3 (concentration 0.17 µM, dark gray), CdS-4 (concentration 0.16 µM, orange), CdS-5 (concentration 0.13 µM, red). Solid black lines are the fitted curves using biexponential functions. Excitation and emission wavelengths are 284 nm and 360 nm, respectively. Inset shows the emission decay profiles at early times. (c) Driving force vs rate constant plot. The dashed line shows behavior expected from two state non-adiabatic Marcus model and solid line shows behavior expected from Auger-assisted model. Filled square data points (black) are experimental data points corresponding to different sized CdS QDs. Reorganization energies are 750, 800 and 850 meV for red, green and bule curves, respectively.

fluorescence decay kinetics for ZnO-CdS-1, ZnO-CdS-2, ZnO-CdS-3, ZnO-CdS-4 and ZnOCdS-5 QD systems can be biexponentially fitted with average lifetimes of 1.78, 1.53, 1.26, 1.13 and 1.02 ns, respectively. Observed decrease of emission lifetime could be attributed to the hole transfer from ZnO QDs to CdS QDs. Associated rate constant can be calculated from the following expression, ()* =

1 $+,-./0

−

1 $+,-

(4)

Using equation 4, the hole-transfer rate constants are estimated to be 1.66×108, 2.58×108, 3.98×108, 4.90×108 and 5.85×108 s-1 for ZnO-CdS-1, ZnO-CdS-2, ZnO-CdS-3, ZnO-CdS-4 and ZnO-CdS-5 QD systems, respectively. 12 ACS Paragon Plus Environment

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3.4. Thermodynamics of hole transfer To ascertain the quantitative driving force for hole transfer, we have estimated VB and CB energies of ZnO and CdS QDs.51,43 ,44 The driving force for the hole transfer (∆3)456 ) can be calculated by subtracting the ground state ionization energy (IE) of the electron donor CdS QD

Table 1. Fluorescence lifetimes of ZnO QDs in the absence and presence of CdS QDs and corresponding hole transfer rates

QDs

τ1

τ2

τav

(Diameter)

(ns)

(ns)

(ns)

ZnO

2.37

10.4

2.53

1.16

-

(3.2 nm)

(98%)

(2%)

ZnO-CdS-1

1.20

3.55

1.78

1.13

1.66×108

(2.01 nm)

(75%)

(25%)

ZnO-CdS-2

1.01

3.38

1.53

1.15

2.58×108

(2.23 nm)

(78%)

(22%)

ZnO-CdS-3

0.89

3.36

1.26

1.21

3.98×108

(2.44 nm)

(85%)

ZnO-CdS-4

0.84

3.27

1.13

1.18

4.90×108

(2.59 nm)

(88%)

(12%)

ZnO-CdS-5

0.79

3.67

1.02

1.12

5.85×108

(2.81 nm)

(92%)

(8%)

78

kHT ( s-1)

(15%)



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from excited state electron affinity (EA) of the electron acceptor ZnO QD.52 ∆3)456 = 9:(+,-∗⁄+,-9(./0 ?⁄./0)

(5)

In terms of the singlet exciton energy 9@-AB of ZnO QDs (0.06 eV), we can express the EA of the excited state, 9:(+,-∗⁄+,-

Quenching of the Excitonic Emission of ZnO Quantum Dots Due to

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