Characteristics of gradient-interface-structured ZnCdSSe quantum dots with modified interface and its application to quantum-dot-sensitized solar cells

Accepted Manuscript Title: Characteristics of Gradient-Interface-Structured ZnCdSSe Quantum Dots with Modified Interface and Its Application to Quantum-Dot-Sensitized Solar Cells Authors: Da-Woon Jeong, Jae-Yup Kim, Han Wook Seo, Kyoung-Mook Lim, Min Jae Ko, Tae-Yeon Seong, Bum Sung Kim PII: DOI: Reference:

S0169-4332(17)32015-9 http://dx.doi.org/doi:10.1016/j.apsusc.2017.07.025 APSUSC 36559

To appear in:

APSUSC

Received date: Revised date: Accepted date:

30-3-2017 11-6-2017 4-7-2017

Please cite this article as: Da-Woon Jeong, Jae-Yup Kim, Han Wook Seo, KyoungMook Lim, Min Jae Ko, Tae-Yeon Seong, Bum Sung Kim, Characteristics of Gradient-Interface-Structured ZnCdSSe Quantum Dots with Modified Interface and Its Application to Quantum-Dot-Sensitized Solar Cells, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.07.025 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.

Characteristics

of

Gradient-Interface-Structured

ZnCdSSe

Quantum Dots with Modified Interface and Its Application to Quantum-Dot-Sensitized Solar Cells

Da-Woon Jeonga,b, Jae-Yup Kimc, Han Wook Seoa, Kyoung-Mook Lima, Min Jae Kod, TaeYeon Seongb, Bum Sung Kima,*

a

Korea Institute of Industrial Technology, Korea Institute for Rare Metals, Incheon 21999,

Republic of Korea b

Department of Materials Science and Engineering, Korea University, Seoul 02841, Republic

of Korea c

Department of Chemical Engineering, Hoseo University, Asan 31499, Republic of Korea

d

*

Department of Chemical Engineering, Hanyang University, Seoul 04763, Republic of Korea

Corresponding author: Bum Sung Kim, E-mail: [email protected], Tel: +82-32-850-

0286, Fax: +82-32-850-0304

Highlights 

The change of λmax and quantum yield according to the synthesis duration of ZnCdSSe QDs at 300 °C with S-to-Se ratios of 10:1 (S = 0.14 mM, Se = 0.014 mM) and 10:2 (S = 0.14 mM, Se = 0.028 mM).



X-ray diffraction patterns of ZnCdSSe QDs with S-to-Se ratios of 10:1 and 10:2.



Impedance spectra in the dark state at −0.45 V of ZnCdSSe QDs with S-to-Se ratios of 10:1 and 10:2.

1

Abstract Colloidal quantum dots (QDs) are attractive materials for application in photovoltaics, LEDs, displays, and bio devices owing to their unique properties. In this study, we synthesized gradient-interface-structured ZnCdSSe QDs and modified the interface based on a thermodynamic simulation to investigate its optical and physical properties. In addition, the interface was modified by increasing the molar concentration of Se. QDs at the modified interface were applied to QD-sensitized solar cells, which showed a 25.5% increase in photoelectric conversion efficiency owing to the reduced electron confinement effect. The increase seems to be caused by the excited electrons being relatively easily transferred to the level of TiO2 owing to the reduced electron confinement effect. Consequently, the electron confinement effect was observed to be reduced by increasing the ZnSe (or Zn1-xCdxSe)-rich phase at the interface. This means that, based on the thermodynamic simulation, the interface between the core QDs and the surface of the QDs can be controlled. The improvement of optical and electronic properties by controlling interfaces and surfaces during the synthesis of QDs, as reported in this work, can be useful for many applications beyond solar cells.

Keywords: Quantum Dots, Solar Cell, QDSCs, gradient interface, modified interface

1. Introduction Over the last few decades, the synthesis of colloidal quantum dots (QDs) has attracted much attention owing to their unique properties, including a high absorption coefficient and widely tunable emission characteristics [1-3]. Furthermore, colloidal QDs can be easily processed using solution-based methods [1-6]. Previous studies have investigated the application of colloidal QDs in light-emitting diodes (LEDs) [7], solar cells [8], displays [9], and biological labels [10]. In particular, various QDs based on metal chalcogenides, such as PbS, CdSe, PbSe, Cu-In-S, and Cu-In-Se, have been extensively studied for application in low-cost photovoltaic devices [8, 11-24]. Compared to other next-generation solar cells, QD 2

solar cells have unique advantages owing to the potential of multiple exciton generation [2528]. In recent years, the synthesis of II-VI quaternary (or ternary) alloyed QDs and their characterization and application have been attracting attention [29-33]. Quaternary (or ternary) alloyed QDs fabricated using the one-pot synthesis method without any additional shell process are characterized by their internal composition and their optical and physical properties [34, 35]. In the present study, we synthesized gradient-interface-structured ZnCdSSe QDs by the one-pot synthesis method and elucidated the internal structure and synthesis mechanism of QDs by thermodynamic simulation. In addition, we applied modified-interface QDs to QDsensitized solar cells (QDSCs) to investigate the photoelectrical characteristics according to the internal interface. The photoelectric properties of QDSCs depend strongly on the interface of the QDs, and the interface modification of the QDs is an important factor for improving the performance. We expect that the improvement of optical and electronic properties by controlling the interface and surface during the synthesis of QDs will be useful for many applications beyond solar cells.

2. Experimental Procedure 2.1 Preparation of precursor solutions Cadmium oxide(Sigma-Aldrich, 99.99%), zinc acetate(Sigma-Aldrich, 99.99%), selenium powder(Sigma-Aldrich, 99.99%), sulfur powder(Sigma-Aldrich, 99.98%), 1-octadecene (ODE, Sigma-Aldrich, 90%), oleic acid (Sigma-Aldrich, 90%), trioctylphosphine (TOP, Sigma-Aldrich, 97%) were commercially available and used without further purification. The Zn-Cd precursor solution was prepared by dissolving 0.14 mM of Zinc acetate and 0.014 mM of cadmium oxide in a solution mixture of 16.63 g of oleic acid and 47.40 g of ODE. The mixture was heated to 160 C for 30 min, filled with Ar gas, and further heated to 310 C to form a clear mixture solution of Zn-Cd precursor. TOP-S-Se precursor solution was prepared by dissolving 0.014 or 0.028 mM of Se powder and 0.14 mM of S powder in 4.986 g of TOP at room temperature for 1 h.

2.2 One-pot synthesis of ZnCdSSe colloidal QDs

3

For the synthesis of ZnCdSSe QDs, Zn-Cd precursor was then maintained at 310 C with continuous stirring in 500 mL three neck flask. At this temperature, TOP-S-Se precursor were rapidly injected into the reaction flask. After the injection, QDs were annealed at 300 C for various duration times with continuous stirring [36]. A number of aliquots were collected in vials containing 5 mL cold hexane to quench further QDs growth at various intervals such as S, 1, 2, 4, 6, 8, 10, and CoolingR.T. (S = Immediately after injection, Cooling R.T. = After Cooling to the R.T.). The samples were purified by solution of toluene and an excess amount of ethanol (3 time, 4500 rpm for 15 min at 10 C); they were then redispersed in hexane or dichloromethane for the characterization or application in solar cells, respectively.

2.3. Preparation of QD-sensitized working electrodes Fluorine-doped tin oxide (FTO) glass (Pilkington, TEC-8, 8 Ω/sq) was cleaned with ethanol using an ultrasonic cleaner for 15 min followed by UV/Ozone cleaning for 15 min to remove organic contaminants. A dense blocking layer was coated onto the cleaned FTO glass by spin-casting a 0.1 M Ti(IV) bis (ethyl acetoacetato) diisopropoxide (Aldrich, 98%) in 1butanol (Aldrich, 99.8%) solution. A transparent paste of 20 nm-sized TiO2 particles was prepared as previously described [37]. The weight ratio of the constituents in this paste was as follows: TiO2/ethyl cellulose/lauric acid/terpineol = 0.18/0.05/0.02/0.75. A scattering paste was prepared using 500 nm-sized TiO2 particles (G2, Showa Denko, Japan) with the same constituents as the transparent paste. The prepared transparent TiO2 paste was deposited on a dense blocking layer-coated FTO glass using the doctor blade technique followed by an annealing at 500 °C for 30 min in air. For direct adsorption of QDs, the annealed TiO2/FTO glass electrode was dipped into the QDs dispersed in dichloromethane at room temperature [38]. After adsorption of the QDs, the QD-sensitized TiO2 electrode was washed with dichloromethane and dried using a nitrogen gun. Finally, the electrode was passivated with a ZnS layer three times by alternate dipping into 0.1 M Zn(CH3COO)2 and 0.1 M Na2S aqueous solutions for 1 min [39].

2.4. Electrode assembly The Cu2S counter electrode was prepared by chemical etching of a brass foil (Alfa Aesar, 0.25 mm thick) in HCl solution (DAEJUNG, 35–37 wt%) at 80 °C for 25 min, followed by 4

washing with deionized (DI) water and drying with a nitrogen gun. An aqueous polysulfide solution composed of 1.0 M Na2S and 1.0 M S (99.998%, Aldrich) was deposited onto the etched brass foil, leading to a porous Cu2S film on the surface, and subsequently rinsed with DI water [40]. The QD-sensitized TiO2 working electrode was assembled with the Cu2S counter electrode using a thermal adhesive film (Surlyn, thickness: 60 μm, Dupont 1702) followed by injection of aqueous polysulfide electrolyte (1.0 M Na2S and 1.0 M S) through pre-drilled holes at the working electrode. The active area for each cell was in the range of 0.25±0.03 cm2, which was measured using a CCD camera (Moticam 1000) with an image analysis program.

2.5 Characterization of ZnCdSSe QDs For the Characterization of ZnCdSSe QDs, Diluted solutions of the QDs in hexane were placed in 1-cm3 quartz cuvettes, and their absorption and corresponding fluorescence were measured. UV–vis absorption spectra were acquired on a double-beam UV–vis spectrometer (Optizen 3220UV, MECASYS, Daejeon, KR). PL quantum yield (QY) measurements, the absorbance of the QDs and the reference rhodamine 6 G was adjusted to be comparable (A < 0.08). The crystal phases of the QDs were confirmed by X-ray diffraction (XRD) (X’Pert-pro MRD, PANalytical, Almelo, NLD) with Cu Kα radiation (λ = 1.5406 Å). The photoluminescence (PL) spectra were taken on a Maya2000 pro spectrometer (Ocean Optics, Dunedin, FL). A Spherical Aberration Corrected Scanning Transmission Electron Microscope (Cs corrected-STEM) (JEM-ARM 200F, JEOL, Tokyo, JP) was used at 200 kV to obtain high-resolution images of QDs. Photocurrent density–voltage (J–V) measurements were performed at one sun light intensity (100 mW/cm2) using a 1600 W Xenon lamp (Yamashita Denso YSS-200A solar simulator) equipped with an AM 1.5 G filter. Each cell was covered with a black mask having an aperture to avoid the overestimation caused by the additional illumination through the lateral space [41]. The electrochemical impedance spectra were obtained in the dark state using a Solartron 1287 potentiostat and a Solartron 1260 frequency response detector at −0.45 V. The applied sinusoidal perturbations were 10mV, and the frequencies were ranged from 1 Hz to 100 kHz.

5

3. Results and Discussion In our previous work, we reported a mechanism for the one-pot synthesis of gradientinterface quaternary ZnCdSSe quantum dots through thermodynamic simulations [34]. Fig. 1 (S-to-Se ratio of 10:1; S = 0.14 mM; Se = 0.014 mM; sample name: “S_Se_10-1”) and Table 1 summarize the following: a core composed of a Cd-rich phase was formed within 1 min, following which a shell composed of a Zn-rich phase was formed, resulting in gradientinterface QDs. 𝐸𝑔(𝑞𝑑) = 𝐸𝑏𝑢𝑙𝑘 +

ℎ2

1

1

1.786𝑒 2

8𝑅

𝑚𝑒

𝑚ℎ

4𝜋𝜀0 𝜀𝑟 𝑅 2

2(

∗ +

∗) −

,

(1)

where 𝐸𝑔(𝑞𝑑) is the band gap of a quantum dot, 𝐸𝑏𝑢𝑙𝑘 is the band gap of a quantum dot, 𝑅 is the radius of a quantum dot, 𝑚𝑒∗ is the effective mass of an excited electron, 𝑚ℎ∗ is the effective mass of an excited hole, 𝜀𝑟 is the dielectric constant, 𝜀0 is the permittivity of vacuum, and ℎ is the Planck’s constant. Here, we consider the type-1 QD model and assume that the recombination occurs only in the core and emits light. Consequently, the core size is calculated using the Brus formula in Eq. (1) to be approximately 4.9 nm [42]. The total size of the gradient-interface quaternary ZnCdSSe QDs (S_Se_10-1) was measured using the Cscorrected STEM as approximately 8.7 nm. Considering that the size of the core is approximately 4.9 nm in the quantum dots with a total size of approximately 8.7 nm, the thickness of the gradient-interface shell is approximately 1.9 nm. A QDSC device was fabricated using such quantum dots. Its J-V characteristics were measured and incident photon-to-electron conversion efficiency (IPCE) analysis was conducted as shown in Fig. 2. From the J-V characteristics, a photoelectric conversion efficiency of approximately 0.59% was obtained with a Voc of 0.47 V, Jsc of 2.55 mA (1.74 mA from IPCE), and fill factor (FF) of 49.56%. The low Jsc appears to be due to the thick gradient-interface shell, which has a quantum-well structure. Based on the thermodynamic simulation, we increased the concentration of Se to induce the formation of a ZnSe-rich layer at the interface without all of the Se being consumed in the formation of CdSe, and we synthesized the QDs as shown in Fig. 3. Fig. 3 and Table 2 show the absorption and photoluminescence (PL) spectra of ZnCdSSe QDs with a S-to-Se ratio of 10:2 (S = 0.14 mM; Se = 0.028 mM; sample name: “S_Se_10-2”). The emission peak appeared at 541.33 nm (± 6

0.05; full width at half maximum (FWHM) = 23.81 nm) immediately after the injection of the S-Se precursor solution. After reaction for 1 min, the emission peak was observed at 559.85 nm (± 0.16; FWHM = 25.30 nm) with a red-shift of approximately 15 nm, and no further peak shift was observed thereafter. The behavior is similar to that of the gradient-interface quaternary ZnCdSSe QDs (S_Se_10-1) in Fig. 1 and Table 1 described above. As with the S_Se_10-1 QDs, the synthesis of the core part of the S_Se_10-2 QDs is completed in 1 min. However, immediately after the injection of the TOP-S-Se precursor in S_Se_10-1 and S_Se_10-2, λmax is 516.81 (± 2.53; FWHM = 22.45 nm) and 541.33 nm (± 0.05; FWHM = 23.81 nm), respectively, which is a difference of 26.52 nm. Considering that nucleation occurs immediately after the injection of the TOP-S-Se precursor, this indicates that the synthesis of the core part of the S_Se_10-2 QDs immediately after nucleation is significantly faster than that of the S_Se_10-1 QDs, which can be attributed to the increase in Se concentration. The detailed reasons will be described below. After all the reactions were completed, the PL spectra of both types of QDs were analyzed at room temperature, and the λmax of S_Se_10-1 and S_Se_10-2 were 550.67 (± 0.17; FWHM = 33.07 nm) and 559.11 nm (± 0.55; FWHM = 38.40 nm), respectively, which constitutes a red-shift of 8.44 nm with increasing Se concentration. The detailed reasons will be described below. Fig. 4 shows the change of λmax and quantum yield according to the synthesis duration of ZnCdSSe QDs at 300 °C with S_Se_10-1 and S_Se_10-2. The two graphs in Fig. 4 show three major differences, including the ones described above. The first is the difference in λmax immediately after injection of the TOP-S-Se precursor. The second is the difference in saturation λmax after reaction for 1 min. The third is the decrease in the overall quantum yield (QY) of S_Se_10-2 QDs with increasing Se concentration. The reason for the first and third major differences is that, in the synthesis of the sample S_Se_10-2, a TOP-S-Se precursor with a relatively high concentration of Se was injected, resulting in more rapid nucleation; it can be assumed that the defects of the core portion are increased along with the rapid growth. The reason for the second major difference is that, because of the increased concentration of Se, a CdSe-rich core was formed, but not all Se was consumed, which seems to be due to the formation of the ZnSe (or Zn1-xCdxSe)-rich phase interface, which leads to a reduction in electron confinement. This reduction in electron confinement is observed by red-shifting the emission peak [43]. 7

We fabricated QDSCs using S_Se_10-2 QDs with reduced electronic confinement. The JV characteristics under illumination and IPCE spectra of QDSCs with both types of QDs are shown in Fig. 5, and the results are summarized in Table 3. In particular, the Jsc was increased by the ZnSe (or Zn1-xCdxSe)-rich phase in the shell interface, resulting in an increase of the photoelectric conversion efficiency by approximately 25.5%. This seems to be caused by the excited electrons being relatively easily transferred to the level of TiO2 due to the reduced electron confinement effect. It also means that, based on the thermodynamic simulation, the interface between the core QDs and the surface of QDs can be controlled. Fig. 6 shows the XRD patterns of ZnCdSSe QDs with S_Se_10-1 and S_Se_10-2. Both types of QDs show a clear zincblende structure, and the (111) planes of S_Se_10-1 QDs and S_Se_10-2 QDs are observed at 27.89° and 27.40°, respectively. The shift by approximately 0.5° is not a small value when considering the d(111) of CdSe (d(111) of pure CdSe = 3.5100; 2θ = 25.3545), Zn0.7Cd0.3Se (d(111) of Zn0.7Cd0.3Se = 3.3319; 2θ = 26.7342), ZnSe (d(111) of pure ZnSe = 3.3279; 2θ = 27.2253), ZnS (d(111) of pure ZnS = 3.1231; 2θ = 28.5582). Fig. 7 shows the transmission electron microscopy (TEM) images of ZnCdSSe QDs with S_Se_10-1 and S_Se_10-2. Overall, the two types of synthesized QDs do not show a large difference in size. As a result of TEM-energy dispersive spectroscopy (EDS) analysis, the composition of the QDs of S_Se_10-2 was Zn87Cd13S60Se40. (In the case of S_Se_10-1 condition, the composition of Zn0.69Cd0.31S0.83Se0.17 was shown [34].) As the concentration of Se precursor was increased, Se but also Zn were increased in the synthesized QDs. To shift the peak to a lower angle, it is possible that 1) the size of the CdSe-rich core increases, or 2) the ZnSe (or Zn1-xCdxSe)-rich phase increases in the Zn-based shell interface. From the information on S_Se_10-1 QDs calculated and measured above, the size of the CdSe-rich core and the total size of QDs were 4.90 nm and 8.72 nm, respectively. Then, assuming that the QDs are spherical, the volume of CdSe-rich core and Zn-based shell can be calculated as 61.60 nm3 and 285.57 nm3, respectively. The volume ratio between the CdSe-rich core and Zn-based shell is 17.74%:82.26%. In other words, even if the size of the CdSe-rich core increases by as much as the red-shift of the PL λmax (8.44 nm), the volume occupied by the CdSe-rich core in the overall volume is not large. Therefore, considering the PL analysis results, it can be reasonably assumed that the ZnSe (or Zn1-xCdxSe)-rich phase, rather than the CdSe-rich core, increased in the Zn-based shell interface. Fig. 8 compares the energy and kinetic 8

characteristics of photoanodes with these two types of QDs, and we obtained the electrochemical impedance spectrum at -0.45 V under dark conditions (Measured at -0.45 V similar to Voc of both devices). As shown in the inset of Fig. 8, the equivalent circuit model consists of series resistance (RS), the impedance at the electrolyte/counter electrode (RCE and CPE1), and at the electrolyte/QD-sensitized TiO2 electrode (Rct and CPE2). The CPE stands for a “constant phase element”, which is used for the interfacial capacitance of porous and rough solid electrodes [44, 45]. The chemical capacitance (Cμ) of the photoanode can be obtained from CPE2. The impedance parameters fitted by ZView software are listed in Table 3. The Cμ values were very similar for both QDs, indicating that the band edge position of the QD-sensitized TiO2 electrode was not affected by the type of QDs [44, 46]. On the other hand, Rct of S_Se_10-2 QDs was clearly lower than that of S_Se_10-1 QDs by about one order of magnitude. This trend implies that the charge transfer between the QD-sensitized TiO2 electrode and the electrolyte can occur more easily for S_Se_10-2 QDs [47, 48]. It is believed that the ZnSe (or Zn1-xCdxSe)-rich phase is formed at the Zn-based shell interface and the electron confinement is reduced, resulting in red shift and reduced band gap. We expect that this result will be useful for many applications beyond solar cells by improving the optical and electronic properties through control of the interface and surface during the synthesis of QDs.

5. Conclusions In this study, we synthesized gradient-interface-structured ZnCdSSe QDs and modified the interface based on thermodynamic simulation to investigate optical and physical properties. The photovoltaic performance was analyzed by applying the modified-interface QDs to QDSCs, which resulted in a 25.5% increase in photoelectric conversion efficiency due to the reduced electron confinement effect. When the molar concentration of Se was increased, the ZnSe (or Zn1-xCdxSe)-rich phase increased at the interface and the electron confinement effect was reduced. This implies that, based on the thermodynamic simulation, the interface between the core QDs and the surface can be controlled. In addition, the photoelectric conversion efficiency was enhanced by 25.5% with the increased ZnSe (or Zn1xCdxSe)-rich

phase at the interface. We expect that the improvement of optical and electronic 9

properties by controlling the interface and surface during the synthesis of the QDs will be useful for many applications such as optical sensor and optical materials beyond solar cells.

Acknowledgement The authors acknowledge financial support from the Regional Industry Nurturing Program, “Development of high sensitive and functional organic/inorganic hybrid plastic composite for LED (Project No. A010400068),” funded by the Ministry of Trade Industry & Energy (MOTIE).

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16

Figures

Figure 1. (a) Absorption spectrum and (b) photoluminescence spectrum of ZnCdSSe QDs with a S-to-Se ratio of 10:1 (S = 0.14 mM; Se = 0.014 mM) (reproduced from D.-W. Jeong et al. Appl. Surf. Sci. 2017; in press, with permission from Elsevier) [34].

17

Figure 2. (a) J-V Characteristics under illumination (1 sun light intensity with AM 1.5G filter) and (b) IPCE spectra of the ZnCdSSe QDSCs for a S-to-Se ratio of 10:1 (0.14 mM:0.014 mM). (c) Extrapolated plot of [ln(1-EQE)hv]2 vs hv from the IPCE spectra, used to obtain the optical band gap of the ZnCdSSe QDs (S:Se = 10:1).

18

Figure 3. (a) Absorption spectrum and (b) photoluminescence spectrum of ZnCdSSe QDs with a S-to-Se ratio of 10:2 (0.14 mM:0.028 mM).

19

Figure 4. Change of λmax and quantum yield according to the synthesis duration of ZnCdSSe QDs at 300 °C with S-to-Se ratios of (a) 10:1 (0.14 mM:0.014 mM) and (b) 10:2 (0.14 mM:0.028 mM).

20

Figure 5. (a) J-V characteristics under illumination (1 sun light intensity with AM 1.5G filter) and (b) IPCE spectra of the ZnCdSSe QDSCs for S-to-Se ratios of 10:1 (0.14 mM:0.014 mM) and 10:2 (0.14 mM:0.028 mM). (c) Extrapolated plot of [ln(1-EQE)hv]2 vs hv from the IPCE spectra, used to obtain the optical band gap of the ZnCdSSe QDs (S-to-Se ratios of 10:1 and 10:2).

21

Figure 6. X-ray diffraction patterns of ZnCdSSe QDs with S-to-Se ratios of 10:1 (0.14 mM:0.014 mM) and 10:2 (0.14 mM:0.028 mM).

22

Figure 7. Transmission electron microscopy (TEM) images of ZnCdSSe QDs with S-to-Se ratios of (a) 10:1 (0.14 mM:0.014 mM) and (b) 10:2 (0.14 mM:0.028 mM).

23

Figure 8. Impedance spectra in the dark state at −0.45 V of ZnCdSSe QDs with S-to-Se ratios of (a) 10:1 (0.14 mM:0.014 mM) and (b) 10:2 (0.14 mM:0.028 mM).

24

Tables

Table 1. λmax, full width at half maximum (FWHM), and quantum yield (QY) of ZnCdSSe quantum dots after various synthesis reaction times at 300 °C with a S-to-Se ratio of 10:1 (S = 0.14 mM; Se

= 0.014 mM). (S = immediately after injection; Cooling R.T. = after cooling to room temperature) S :

Se

Duration Time at 300 oC

Ratio = 10 : 1 S

1 min

2 min

4 min

6 min

8 min

10 min

Cooling R.T.

λmax

516.81

549.27

550.90

552.26

551.70

550.90

550.42

550.67

(nm)

± 2.53

± 0.25

± 0.16

± 0.14

±0.18

± 0.18

± 0.20

± 0.17

22.45

24.43

25.63

28.20

30.24

31.44

32.33

33.07

10.45

50.53

73.37

88.96

80.02

82.70

81.63

95.85

FWHM (nm)

QY (%)

* (Reproduced from D.-W. Jeong et al. Appl. Surf. Sci. 2017; in press, with permission from Elsevier.)[34]

25

Table 2. λmax, full width at half maximum (FWHM), and quantum yield (QY) of ZnCdSSe quantum dots after various synthesis reaction times at 300 °C with a S-to-Se ratio of 10:2 (S = 0.14 mM; Se

= 0.028 mM). (S = immediately after injection; Cooling R.T. = after cooling to room temperature) S :

Se

Duration Time at 300 oC

Ratio = 10 : 2 S

1 min

2 min

4 min

6 min

8 min

10 min

Cooling R.T.

λmax

541.33

559.85

560.70

560.74

560.06

559.62

559.26

559.11

(nm)

± 0.05

± 0.16

± 0.64

± 0.67

±0.64

± 0.62

± 0.60

± 0.55

23.81

25.30

29.25

33.94

35.79

37.03

37.95

38.40

9.84

18.43

52.60

60.31

58.58

56.30

52.76

50.31

FWHM (nm)

QY (%)

26

Table 3. Summary of J-V characteristics and impedance parameters of ZnCdSSe QDSCs for S-to-Se ratios of 10:1 (0.14 mM:0.014 mM) and 10:2 (0.14 mM:0.028 mM).

QD

JSC (mA/cm2)

VOC (V)

FF (%)

η (%)

Cμ (μF/cm2)

Rct (Ω cm2)

S_Se_10-1

2.55

0.47

49.56

0.59

2864

82.89

S_Se_10-2

3.13

0.47

50.08

0.74

2996

8.57

27