Size-dependent photoluminescence dynamics of CuInS2 quantum dots and charge injection on titanium oxide film

Size-dependent photoluminescence dynamics of CuInS2 quantum dots and charge injection on titanium oxide film

Accepted Manuscript Size-dependent photoluminescence dynamics of CuInS2 quantum dots and charge injection on titanium oxide film Yueli Liu, Tao Chen, ...

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Accepted Manuscript Size-dependent photoluminescence dynamics of CuInS2 quantum dots and charge injection on titanium oxide film Yueli Liu, Tao Chen, Zhuoyin Peng, Lei Wu, Keqiang Chen, Peng Zhou, Linlin Wang, Wen Chen PII:

S0925-8388(15)31439-0

DOI:

10.1016/j.jallcom.2015.10.183

Reference:

JALCOM 35738

To appear in:

Journal of Alloys and Compounds

Received Date: 4 August 2015 Revised Date:

17 October 2015

Accepted Date: 20 October 2015

Please cite this article as: Y. Liu, T. Chen, Z. Peng, L. Wu, K. Chen, P. Zhou, L. Wang, W. Chen, Sizedependent photoluminescence dynamics of CuInS2 quantum dots and charge injection on titanium oxide film, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.10.183. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Revision Manuscript for Journal of Alloys and Compounds

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Ref. No.: JALCOM-D-15-06039R2

Size-dependent photoluminescence dynamics of CuInS2 quantum

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dots and charge injection on titanium oxide film

Yueli Liua, Tao Chena, Zhuoyin Penga, Lei Wub, Keqiang Chena, Peng Zhoua, Linlin

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Wanga, Wen Chena,*

* To whom correspondence should be addressed:

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[a] State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, School of Materials Science and Engineering, Wuhan University of Technology, Wuhan, 430070, P. R. China

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Tel.: +86-27-8765-1107 Fax: +86-27-8776-0129

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E-mail: [email protected] (Wen Chen)

[b] School of Electronic and Electrical, Wuhan Railway Vocational College of Technology, Wuhan, 430205, P. R. China

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Abstract

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CuInS2 quantum dots (QDs) with different sizes are synthesized by a solvethermal process, and then linked on the FTO conducting glass and TiO2 nanoparticle films. Photoluminescence (PL) lifetimes measured by time-resolved PL decay indicate that

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PL lifetimes firstly increase then decrease with the size increasing of CuInS2 quantum

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dots, which is due to the ratio between the radiative excitons traps and nonradiative excitons traps. Temperature-independent PL spectra of the 4.9 nm CuInS2 QDs prove that there is no emission peak shift from 5K to 200K, which is related with the emission from the trapped states, and the linear fitting character of the temperature

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dependence of the full width at half maximum comes from the coupling of the carriers to the acoustic phonons modes. With the CuInS2 quantum dots linking on TiO2 nanoparticle films, the PL lifetime is obviously reduced. The gradually larger

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conduction band energy offset between CuInS2 quantum dots and TiO2 induces the PL

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lifetime reducing with the size decreasing of the CuInS2 QDs, which can induce the faster electron injection rate constant for the CuInS2 quantum dots with small size.

Keywords: Size-dependent; Time-resolved photoluminescence; Low temperature photoluminescence; CuInS2 quantum dots; Charge injection rate

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1. Introduction Semiconductor quantum dots (QDs) have attracted many investigations for the applications in optical photoluminescence (PL) devices, biological probes, solar cells,

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and so on, which is due to the unique properties of perfect photostability, size-dependent optical and electric properties, large extinction coefficients and multiple exciton generation [1-4]. In last decade, many investigations about QDs have

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been focused on the solar cells, due to the high theoretical maximum efficiency of

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the quantum dots-sensitized solar cells (QDSSCs) [5-7]. In order to study the mechanism of the QDSSCs, the charge carrier generation and injection process of the QDs, such as CdS, CdSe and PbS, have also been reported through the PL decay [8-10]. The lifetime of the QDs varies with the QDs size, which exhibits

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hundreds of picoseconds [11]. Moreover, in the presence of titanium oxide the charge injection rate may be significantly increased, which is also affected by the

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size dependence of QDs.

Recently, CuInS2 with a smaller direct band gap of 1.5 eV can effectively

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improve the photovoltaic performance of QDSSCs due to its high absorption coefficient, which is well matched with the full solar spectrum [12-15]. However, the low photovoltaic efficiency of the QDSSCs still limits the development of the CuInS2 QDs. It is found that the photo-induced absorption and PL quantum yield of the CuInS2 QDs correspond to the size-effect [16-18]. Although several studies have been focused on the PL decay of the pure CuInS2 QDs and its application on the solar cells, there still have no integral conclusion for the PL carrier dynamics of the CuInS2 QDs. 3

ACCEPTED MANUSCRIPT In order to effectively improve the photovoltaic efficiency, it is significantly necessary to study the PL carrier dynamics of CuInS2 QDs with different sizes. The lifetime of the CuInS2 QDs and TiO2/CuInS2 system from PL decay may provide

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great possible direction for the QDSSCs. Herein, to study the charge generation and injection processes of the CuInS2 QDs systems, the CuInS2 QDs with various sizes are synthesized by a thermolysis process

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and then sensitized on the TiO2 nanoparticle films by a traditional assembly linking

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process. Temperature-dependent PL spectra are measured from 5K to 200K. The PL dynamics and lifetime of different sized CuInS2 QDs and TiO2/CuInS2 system are measured by the time-resolved photoluminescence (TRPL) spectra, which show a significant size-dependent behavior of the CuInS2 QDs.

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2. Experimental procedures

Chemicals: The precursor employed to prepare CuInS2 QDs were copper (I)

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chloride (CuCl, Alfa, 99.9%), indium (III) acetate (In(ac)3, Alfa, 99%), oleylamine (OA, Alfa, 90%). 3-Mercaptopropionic acid (MPA), methanol (HPLC grade), toluene

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(HPLC grade), sulfur acid (ACS grade), acetonitrile (HPLC grade) and ethanol (99.5%) were obtained from Sinopharm Group Chemical Reagent Co., Ltd. in China. Titanium (IV) isopropoxide (Alfa), polyethylene glycol (PEG, 20000 in molecular weight, Sinopharm Co., Ltd.) and ethyl cellulose (Sinopharm Co., Ltd.) were used to prepare TiO2 films. All the materials were directly used without further purification. The electrode substrate was fluorine-doped tin oxide (FTO) conducting glass (thickness: 2.2 nm, sheet resistance: 14 Ω/square), which was washed with distilled 4

ACCEPTED MANUSCRIPT water, acetone and ethanol in ultrasonic bath before using. Preparation of various sized CuInS2 QDs: CuInS2 QDs were prepared by a modified thermolysis method [17]. Typically, CuI (0.192 g, 1mmol), In(acac)3 (0.292

vigorous stirring in a 25 mL three-neck flask at 120

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g, 1 mmol) and n-dodecanethiol (1 mL) were dissolved in 10 mL of 1-octadecene with under Ar condition. After 30

min, 1 mL of oleic acid was added into the solution and maintained at the same

for 10-210 min to obtain QDs with different sizes and then the

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heated to 200

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temperature for another 30 min under continuous Ar flowing. Then the solution was

heating source was removed. Excess methanol was added to precipitate and centrifuge the products when the solution was cooled to room temperature. After centrifugation, the liquid portion was discarded, and toluene was added to disperse them. This

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precipitation/dispersion cycle was repeated several times for purification. Finally, the oleic acid-capped CuInS2 QDs were re-dispersed in fresh toluene. OA-capped CuInS2 QDs prepared by the thermalysis method were dried in the oven at 60

, and the

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brown.

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powders were obtained with different colors gradually from red to brown to dark

Preparation of TiO2/CuInS2 films: The OA-capped CuInS2 QDs/toluene

suspension was added to the methanol solution of MPA and tetramethylammonium hydroxide, and the mixture was then stirred for 2 h to obtain a clear suspension under the toluene solution containing MPA-capped CuInS2 QDs. The MPA-capped CuInS2 QDs were precipitated with ethanol and re-dispersed in methanol. The TiO2 films were prepared as our previous report [18]. The TiO2 films were firstly immersed into 5

ACCEPTED MANUSCRIPT a MPA (1 M) and sulfur acid (0.1 M) acetonitrile solution for 12 h, and then immersed into the CuInS2 QDs toluene solution for 2 days after rinsing with acetonitrile and methanol. Finally, the TiO2/CuInS2 films were annealed at 300

for 5 min. For

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comparison, the MPA-capped CuInS2 QDs were also used to prepare the FTO/CuInS2 films by the same process. All the films were kept in the dark condition.

Characterization and measurements: The crystal structure and microstructures

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of the CuInS2 QDs was characterized by X-ray diffraction (XRD, PertPro,

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PANalytical, Netherlands) patterns and transmission electron microscope (TEM, JEM-2100F, JEOL, Japan). UV-vis absorption spectra (UV-2550, Shimadzu, Japan) and photoluminescence spectra (FS-2400, Shimadzu, Japan) were employed to characterize the absorption and excitation properties of samples. Time-resolved

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photoluminescence spectra (HORIBA Fluoromax-4, France) were employed to measure the photoluminescence dynamics and photoluminescence decay of the various sized CuInS2 QDs and TiO2/CuInS2 films. Temperature-dependent

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photoluminescence spectra of 4.9 nm CuInS2 QDs were conducted from 5K to 200K.

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3. Results and discussion TEM images of the various samples with the increasing of the reaction time are

shown in Fig. 1, and it shows that the as-prepared CuInS2 QDs are monodisperse. The insets in each image is the size distribution histograms of the samples, which reveals that the CuInS2 QDs synthesized by different reaction time have the diameters of 2.1 ±0.2 nm, 2.8±0.3 nm, 3.4±0.1 nm, 4.2±0.2 nm, 4.9±0.2 nm, 5.6±0.3 nm, 6.3± 0.3 nm and 7.1±0.3 nm, respectively. It reveals that the QDs have narrow size 6

ACCEPTED MANUSCRIPT distributions with a relative standard deviation around 10%. The insert image in Fig. 1(c) is the HRTEM image of a 3.4 nm QDs, the well-resolved lattice image indicates that the CuInS2 QDs are single crystal, and the interplanar spacing of 0.196 nm is

which further proves the existence of CuInS2 QDs.

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corresponding to the crystal lattice distance of {204} crystal planes of CuInS2 phase,

The crystal structure of the samples is confirmed by the XRD patterns in Fig.

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2(a), which shows that there is pure tetragonal CuInS2 phase existing, and all of the

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peaks reflected from (112), (200), (204), (301), (116) and (224) crystal planes match well with the standard pattern of tetragonal CuInS2 phase (JCPDS NO: 00-085-1575) [19], which could indicate the formation of the CuInS2 phase by the thermalysis process. Moreover, it can be seen that the full width at half maximum (FWHM) of the

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various CuInS2 QDs samples is gradually reduced with the increasing of thermalysis time, which indicates that the diameter of the sample increases with the increasing of reaction time. According to the Sherrer formula [20-21], the sizes of the CuInS2 QDs

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can be calculated in Fig. 2(a), which proves that the sizes of CuInS2 QDs are from 2.1

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nm to 7.1 nm, and it is accord with the results from the HRTEM images in Fig. 1. It is interesting to see that there is a broad shoulder at around 27º (2-Theta), which may be due to the effect of the surface state of QDs. In order to point out the influence of DDT on the surface state of QDs, the as-prepared QDs are annealed at 300

for 1h.

The XRD pattern in Fig. 2(b) indicates that the phase structure of the QDs is still of CuInS2 with a sharp peak at about 27º, which is similar with the ones of Cu12Sb4S13 nanocrystals in our previous work [22]. 7

ACCEPTED MANUSCRIPT Various sized CuInS2 QDs are dispersed in toluene solution with the same concentration for the optical properties measurement. The optical absorption and PL spectra of various sized CuInS2 QDs are shown in Fig. 3. In Fig. 3(a), all the CuInS2

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QDs have the absorption edges with long wavelengths from 600 nm to 850 nm. The absorption edge gradually shifts to the higher wavelength with the size increasing of CuInS2 QDs. Meanwhile, it can be seen in Fig. 3(b) that the PL emission peaks of

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various sized CuInS2 QDs also shift to the visible wavelength from 620 nm to 830

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nm with the size increasing. The red-shift phenomenon of the absorption edges and PL emission peaks can represent the quantum confinement effect of the CuInS2 QDs. Moreover, the optical band gaps of the various sized CuInS2 QDs can be calculated by the following Tauc equation (Eq. 1) [23-24]:

(1)

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αhν=A(hν-Eg)1/2

Where α is the absorption coefficient, hν is the photon energy, A is the proportionally coefficient, Eg is the band gap energy.

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The optical band gaps of the various sized CuInS2 QDs can be determined by the

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curves of hν-(αhν)2 in Fig. 3(c). The optical band gap gradually decreases from 2.35 eV to 1.6 eV, which is much approached to the band gap (1.5 eV) of the bulk CuInS2. Furthermore, the band gap energies (E(R)) of the various sized CuInS2 QDs can also be calculated by Brus model as following (Eq. 2) [25]: E ( R) = Egbulk +

h 2π 2 1 1 1.8e 0.124e3 1 1 ( + ) − − ( + ) −1 2 2 2 2eR me mh 4πε r h (4πε ) me mh

(2)

Where R is the size of CuInS2 QDs, E gbulk is the band gap (1.5 eV) of the bulk CuInS2, h is the Planck constant, e is the electronic charge, me is the effective mass 8

ACCEPTED MANUSCRIPT of the electron, mh is the effective mass of the hole, ε is the dielectric constant. In the CuInS2 QDs system, E gbulk =1.5 eV, me = 0.16 mo, mh = 1.3 mo, ε = 11, where the mo is the mass of electron. It can be seen the comparison for the different calculation

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band gaps of the CuInS2 QDs in Fig. 3(d), which shows that the experimental calculation band gap are well matched with the calculation from the Brus model. These results further prove that the sizes of CuInS2 QDs are from 2.1 nm to 7.1 nm.

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It is observed that the PL emission has the red-shift phenomenon with the size

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increasing of CuInS2 QDs. However, the influence of the size of CuInS2 QDs on the PL dynamics and lifetime still needs to be demonstrated. The TRPL spectra can be employed not only in the investigation of surface electron trapped states but also in the analysis of PL dynamics in the semiconductor nanocrystals, which can provide the

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significant information about the PL lifetime of various sized CuInS2 QDs to control the optical performance.

Time-resolved laser techniques are considered to be a powerful tool for the

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measurement of the lifetime of PL emission. Fig. 4(a) shows the TRPL decays of

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various sized CuInS2 QDs under their emission peak. Due to the fact that the distribution in the electron-holes recombination rate constants may influence the decay kinetics, the PL decays are multiexponential [26]. It can be seen that the PL decay rates of various sized CuInS2 QDs are obviously different. In order to clarify the PL decay rate, the multiexponential function model is used to calculate the average PL lifetime as following (Eq. 3) [27]:

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α1τ 12 + α 2τ 2 2 + α 3τ 32 α1τ 1 + α 2τ 2 + α 3τ 3

(3)

Where τ is the average lifetime, αnτn is the each decay parameters of component in the multiexponential function model.

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The average PL lifetimes of various sized CuInS2 QDs are shown in Fig. 4(b). It can be found that the lifetimes of the CuInS2 QDs firstly increase then decrease with

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the size increasing. It is well known that the intrinsic defects, size-dependent band gap and surface defects are all involved in the PL emission. The radiative lifetime of PL

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emission is one important optical property of semiconductor QDs, as different radiative lifetimes may correspond to different electron-hole recombination mechanisms [17]. In our case, the PL lifetime of 185 ns is the longest one when the size of CuInS2 QDs is 4.2 nm. These size-dependent PL dynamics of CuInS2 QDs can

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be understood by the surface trapped states and the intrinsic electron-hole exchange reaction. The multiexponential PL decay will induce more radiative excitons

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recombination. The excitons from the CuInS2 QDs may be trapped to recombine either in the internal states or on the surface, which will induce either the fast or the

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slow PL decay lifetime. The trapped energies from the excitons will be transformed by the radiative and nonradiative recombination [28-29]. Due to the high quality of CuInS2 QDs, the possibility of the excitons trapped on the surface of CuInS2 QDs may be increased. With the size decreasing of CuInS2 QDs, the energy enhancement will induce the radiative excitons trapped to increase the PL decay lifetime. However, the nonradiative excitons trapped are related to the size of CuInS2 QDs. With the size decreasing, the surface to volume ratio will become much higher, which may induce 10

ACCEPTED MANUSCRIPT more nonradiative excitons trapped [30]. The enhancement of nonradiative excitons trapped will reduce the PL decay lifetime. Therefore, the ratio between the radiative excitons trapped and nonradiative excitons trapped will achieve the best value at a

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desirable size of CuInS2 QDs, which may induce to obtain the longest PL decay lifetime, and it is found the longest PL decay lifetime at 4.2 nm.

Normalized PL spectra of 4.9 nm CuInS2 QDs exhibit that the PL peak positions

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locate at 751 nm (0.61 eV) within the whole temperature range from 5K to 200K, as

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shown in Fig. 5(a). It is shown that no obvious and pronounced temperature dependence is observed, which may suggest that the special character originates from the trapped state emission. Normally, the trapped state in the exciton relaxation is very important, as the photo-induced carriers could be captured by the trapped states from

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the 1S-1S excition state, as well as from the higher lying states to those states promoted by the optical excitation [31]. The similar phenomenon has been pointed out by the results from the resonant femtosecond pump-probe spectroscopy of CdTe

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[32], CdSSe [33] and CdS [34], or from TRPL spectroscopy of colloidal CdSe [35]

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and PbS QDs [31].

Fig. 5(b) shows the FWHM values of the intrinsic CuInS2 QDs exciton emission

as a function of temperature. The temperature dependence of the FWHM is due to the exciton-phonon interactions, the total linewidth can be described as the sum of three terms: An inhomogeneous broadening term and two other terms representing homogeneous broadening due to acoustic and optical phonon–exciton interactions, respectively [36-38], which can be described by the following equation: 11

ACCEPTED MANUSCRIPT FWHM(T) = Γinh +σT +γNLO(T)

(4)

where Γinh is originated from the temperature-independent inhomogeneous contribution, which illustrates the QDs size, shape, and environment variations 0. σ is an

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distribution of linewidth of a single QD in the ensemble at T

exciton-acoustic-phonon coupling coefficient, while γ is a temperature-independent linewidth parameter, which is used to character the total linewidth due to interactions.

NLO(T)=[exp(ELO/kBT)1]-1,

and

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exciton-LO-phonon

it

is

the

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Bose-Einstein distribution of LO phonons. In our case, the narrow FWHM range is observed from 83 nm (67 meV) to 86.2 nm (69.5 meV) within the whole temperature range from 5K to 200K, which also improves the narrow size distribution of CuInS2 QDs. Moreover, the linear fitting character implies that the dominant contribution to

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the line broadening in CuInS2 QDs originates from the coupling of the carriers to the acoustic phonons modes.

In the FTO/CuInS2 and TiO2/CuInS2 systems, the different sized CuInS2 QDs

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are linked on the surface of FTO conducting glass and TiO2 nanoparticle films by

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employing the MPA ligand regent. PL decay dynamics of different FTO/CuInS2 and TiO2/CuInS2 electrodes are illustrated from TRPL spectra in Fig. 6. From the multiexponential function model, the average PL lifetime can be calculated, as shown in Fig. 6(a). Firstly, although there is the same phenomenon for the PL lifetime of various sized CuInS2 QDs comparing with that of the CuInS2 QDs solution, the PL lifetime value is obviously reduced after the linking on the surface of FTO conducting glass. With the transient PL excited, in CuInS2 QDs solution system the 12

ACCEPTED MANUSCRIPT photo-induced electron-hole recombination reaction from CuInS2 QDs can be gradually weaken by the size-dependent CuInS2 QDs, achieving the long lifetime of nearly 200 ns. However, after linking on the surface of FTO conducting glass, several

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excited electrons may inject into FTO conducting glass, which will induce the electron-hole separation to quickly weaken the electron-hole recombination reaction. This result may reduce the PL lifetime of FTO/CuInS2 comparing with that of CuInS2

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QDs solution, as shows in Eqs. 5 and 6. To deeply understand of the behavior of the

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nonradiative excitons trapped of the CuInS2 QDs in the FTO/CuInS2 and TiO2/CuInS2 systems, the temperature dependent PL spectra are considered to be an effective route, and further work will be undertaken in the future work. CuInS2 + hυ → CuInS2 (e- + h+)

(6)

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CuInS2 (e-) → FTO

(5)

Secondly, in Fig. 7(a), the PL lifetimes of CuInS2 QDs are obviously reduced comparing with that of the FTO/CuInS2 after CuInS2 QDs linked on TiO2 nanoparticle

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films. This is attributed to the excited electrons injection from CuInS2 QDs to TiO2

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nanoparticle films. It is well known that many factors can affect the electron transfer, such as the size of the quantum dots, the surface state, the ligand exchange treatments, the interfacial combination between the quantum dots and other substrates (FTO glass, TiO2 film, etc.), and so on [39]. It is reported that there are two possible pathways for the electron transfer when the quantum dots are linked with FTO glass or TiO2 nanoparticles films [40], which may lead to drastically different results in measurements of electron transfer depending on relative values of recombination and 13

ACCEPTED MANUSCRIPT photon absorption rates. In both scenarios, photon absorption by a neutral QD results in the generation of a single exciton, which then undergoes charge separation as the electron is transferred to the TiO2 film. If the recombination of separated charges

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(electron in the FTO or TiO2 film and hole in the QD) is relatively slow, then the hole still remains in the QD when the next photon is absorbed. The result is a positively charged exciton (i.e., a positive trion) that has a very short lifetime determined by the

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nonradiative Auger recombination rate.

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Therefore, with the PL excited, CuInS2 QDs can effectively generate the photo-induced electrons from valence band (VB) to conduction band (CB), which also maintains the photo-induced holes in their VB to separate the electron-hole pairs, and then the photo-induced electrons flow towards the FTO electrodes, as shown in Eqs. 5,

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7 and 8.

CuInS2 (e- + h+) + TiO2 → CuInS2 (h+) + TiO2 (e-)

(7)

TiO2 (e-) → FTO

(8)

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This will induce the sharply reduction of electron-hole recombination reaction to

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obtain the shorter PL lifetime. Moreover, the PL lifetimes of TiO2/CuInS2 electrodes are gradually increased with the size increasing of CuInS2 QDs. The band gap of TiO2 nanoparticles is about 3.2 eV, while the band gaps of these various sized CuInS2 QDs are from 1.6 eV to 2.4 eV. It can be seen in Fig. 7(b) that the CB energies of these CuInS2 QDs are higher than that of TiO2. CB energies of these CuInS2 QDs are gradually increased with the size decreasing, which induces that the CB energy offset between CuInS2 and TiO2 becomes larger. This larger CB energy offset will increase 14

ACCEPTED MANUSCRIPT the electron injection rate from CuInS2 to TiO2, which may extremely reduce the PL lifetime of TiO2/CuInS2 electrodes, and our results are well accord with some similar works [40]. The electron injection rate constants of all the electrodes can be calculated

ket =

1

τ (CuInS



2 + TiO2 )

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by the PL lifetimes described as Eq. 9, and the detailed values are shown in Table 1. 1

τ ( CuInS

(9)

2)

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It shows that the 2.1 nm CuInS2 QDs have the shorter average PL lifetime of 0.5 ns, which indicates the faster electron injection in the TiO2/CuInS2 system. For

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calculation, these 2.1 nm TiO2/CuInS2 electrodes have the faster electron injection rate constant of 1.941×109 s-1. Although CuInS2 QDs with the small size they have faster electron injection rate constant, the ultraviolet and visible light absorption properties of the CuInS2 QDs will be limited to influence the more electrons

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generation under the sunlight, which is further improved by the photovoltaic conversion efficiency of the fabricated QDSSCs in our previous work [18].

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4. Conclusions

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In summary, various sized CuInS2 QDs are synthesized and linked on the FTO conducting

glass

and

TiO2

nanoparticle

films.

Optical

absorption

and

photoluminescence properties are depended on the size of CuInS2 QDs. TRPL measurements indicate that the PL lifetimes firstly increase then decrease with the size increasing of CuInS2 QDs. Low temperature PL spectra show that there is a temperature-independent PL peak position from 5K to 100K with a narrow FWHM distribution. Due to the electron injection, the PL lifetimes are obviously reduced by 15

ACCEPTED MANUSCRIPT the linking between the CuInS2 QDs and TiO2 nanoparticle films. PL lifetime gradually reduces with the size decreasing of the CuInS2 QDs due to the larger conduction band energy offset between CuInS2 QDs and TiO2, which can obtain the

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faster electron injection rate constant at small size. These results can further improve the application on optical and electronic devices of the CuInS2 QDs.

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Acknowledgements

This work is supported by International Science & Technology Cooperation

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Program of China (No. 2013DFR50710), Equipment pre-research project (No. 625010402), the National Nature Science Foundation of China (No. 51572205), Science and Technology Support Program of Hubei Province (No. 2014BAA096), the National Nature Science Foundation of Hubei Province (No. 2014CFB165), and the

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Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and

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Guangdong Province (No. GD201402).

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ACCEPTED MANUSCRIPT Figure Captions Table 1 Kinetic parameters of the various sized CuInS2 PL decay analysis Fig. 1 HRTEM images of the as-prepared CuInS2 quntum dots by differenr reaction

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times (Insets: the size distribution histograms of the samples): (a) 10 min; (b) 30min; (c) 60min; (d) 90min; (e) 120min; (f) 150min; (g) 180min; (h) 210 min

Fig. 2 XRD patterns of the samples: (a) various sized CuInS2 quantum dots; (b)

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CuInS2 quantum dots with the size of 2.1 nm by heat treatment at 300

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Fig. 3 Optical properties of various sized CuInS2 quantum dots: (a) UV-vis spectra; (b) PL spectra; (c) band gap of CuInS2 calculated by plots for (αhυ)2 versus hυ and (d) size-dependent optical bandgaps of CuInS2 QDs

Fig. 4 (a) Time-resolved photoluminescence spectra of various sized CuInS2 QDs; (b)

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Fig. 5 (a) Photoluminescence spectra of 4.9 nm CuInS2 QDs at different temperatures from 5K to 200K; (b) temperature dependence of FWHM

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Fig. 6 Time-resolved photoluminescence spectra of FTO/CuInS2 and TiO2/CuInS2

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Fig. 7 (a) Lifetimes of FTO/CuInS2 and TiO2/CuInS2; (b) schematic diagram of the band gaps of the various sized TiO2/CuInS2 electrodes

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Table 1 Kinetic parameters of the various sized CuInS2 PL decay analysis 2.8

3.4

4.2

nm

nm

nm

nm

4.9

5.6

6.3

7.1

nm

nm

nm

16ns

14.6ns

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Size

FTO/CuInS2 17.1ns 19.4ns 20.7ns 22.3ns 21.2ns 18.5ns 0.5ns

2.6ns

5.3ns

7.9ns

9.7ns

11.9ns 12.5ns 13.2ns

ket(×109s-1)

1.941

0.331

0.141

0.079

0.056

0.03

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0.018

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(a)

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Intensity (a. u.)

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(301)

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2θ (º)

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(200) (116)

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Fig. 2 XRD patterns of the samples: (a) various sized CuInS2 quantum dots; (b)

CuInS2 quantum dots with the size of 2.1 nm by heat treatment at 300

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Fig. 3 Optical properties of various sized CuInS2 quantum dots: (a) UV-vis spectra; (b)

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Fig. 4 (a) Time-resolved photoluminescence spectra of various sized CuInS2 QDs; (b) lifetimes of various sized CuInS2 QDs

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Fig. 5 (a) photoluminescence spectra of 4.9 nm CuInS2 QDs at different temperatures from 5K to 200K; (b) temperature dependence of FWHM

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Fig. 6 Time-resolved photoluminescence spectra of FTO/CuInS2 and TiO2/CuInS2

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1) CuInS2 quantum dots with different sizes are synthesized by a thermolysis process

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2) PL lifetimes firstly increase then decrease with the size increasing of CuInS2 QDs 3) There is no emission peak shift from 5K to 200K for the 4.9 nm CuInS2 QDs

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