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Nanocrystal Size-Dependent Efficiency of Quantum Dot Sensitized Solar Cells in the Strongly-Coupled CdSe Nanocrystals/TiO System 2

Hyeong Jin Yun, Taejong Paik, Benjamin T. Diroll, Michael E. Edley, Jason B. Baxter, and Christopher B. Murray ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05552 • Publication Date (Web): 25 May 2016 Downloaded from http://pubs.acs.org on May 27, 2016

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ACS Applied Materials & Interfaces

Nanocrystal Size-Dependent Efficiency of Quantum Dot Sensitized Solar Cells in the Strongly-Coupled CdSe Nanocrystals/TiO2 System

Hyeong Jin Yun†, Taejong Paik†, §, Benjamin Diroll†, Michael E. Edley∥, Jason B. Baxter∥, and Christopher B. Murray†, ‡, *

†

Department of Chemistry, and ‡Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States. ∥

Department of Chemical and Biological Engineering, Drexel University, Philadelphia, Pennsylvania 19104, United States.

§

School of Integrative Engineering, Chung-Ang University, Seoul, 06974, South Korea

---------------------------------------------------*To whom correspondence should be addressed. E-mail: [email protected].

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Abstract

Light absorption and electron injection are important criteria determining solar energy conversion efficiency. In this research, monodisperse CdSe quantum dots (QDs) are synthesized with five different diameters, and the size-dependent solar energy conversion efficiency of CdSe quantum dot sensitized solar cell (QDSSCs) is investigated by employing the atomic inorganic ligand, S2-. Absorbance measurements and transmission electron microscopy show that the diameters of the uniform CdSe QDs are 2.5, 3.2, 4.2, 6.4 and 7.8 nm. Larger CdSe QDs generate a larger amount of charge under the irradiation of long wavelength photons, as verified by the absorbance results and the measurements of the external quantum efficiencies. However, the smaller QDs exhibit faster electron injection kinetics from CdSe QDs to TiO2 because of the high energy level of CBCdSe, as verified by time-resolved photoluminescence and internal quantum efficiency results. Importantly, the S2- ligand significantly enhances the electronic coupling between the CdSe QDs and TiO2, yielding an enhancement of the charge transfer rate at the interfacial region. As a result, the S2--ligand helps improve the new size-dependent solar energy conversion efficiency, showing best performance with 4.2 nm-sized CdSe QDs, whereas conventional ligand, mercaptopropionic acid, does not show any differences in efficiency according to the size of the CdSe QDs. The findings reported herein suggest that the atomic inorganic ligand reinforces the influence of quantum confinement on the solar energy conversion efficiency of QDSSCs.

KEYWORDS: CdSe QDs, quantum confinement effect, quantum dot sensitized solar cell, ligand exchange, charge transfer rate, size-dependency, photovoltaics 2


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INTRODUCTION

Quantum dot sensitized solar cells (QDSSC) have recently attracted considerable interest as the next generation solar energy harvesting using various semiconductor quantum dots (QDs), such as CdSe,1-4 CdS,1, 2, 5 CdTe,1, 2 and so on.6-10 QDSSC is a typical liquid junction-based photovoltaic cell that have similar configuration to a dye sensitized solar cell, except that QDs are used as light absorber to generate photo-excited carriers instead of organic dyes.11-13 The QDs are attached to the wide band gap semiconductor electrodes, such as TiO2,1, 2, 5, 8, 14, 15 ZnO,3, 4, 16 and SnO2,17 and electron and hole are then generated by the irradiation of solar light. The photogenerated electron is injected into the conduction band of the wide band gap semiconductor, while the hole is transported to the counter electrode via a polysulfide electrolyte. To improve solar energy conversion efficiency, both the photo-generation of charges and their transfer at the interfacial region should be improved. As a light absorber, QDs have the benefit that their light absorption can be managed easily by controlling their size14, 18-21 and shape.22 The band-gap of QDs is tunable because of the quantum confinement effect, which shows size-dependent properties of semiconductor nanocrystals. The gap between the conduction (CB) and valence band (VB) of nanocrystals increases with decreasing crystallite size. In particular, the energy level of the conduction band (ECB) is influenced by their sizes more than the energy level of the valence band (EVB) because the effective mass of an electron is smaller than that of a hole.14, 23, 24 P. Kamat, et al. recently showed that the ECB of CdSe tends to shift from -3.94 to -3.65 eV with decreasing size of the CdSe nanocrystals from 4.2 to 2.8 nm, while the EVB shifts from -6.13 to -6.16 eV.24 Charge transfer between QDs and TiO2 is driven mainly by the differences in the ECB of the QDs and 3



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TiO2. Thus, the size of QDs is one of the most critical factors to determine the charge transfer

Scheme 1. Comparison of charge transfer between TiO2 and (A) smaller (d = 2.8 nm) and (B) larger sized-CdSe QDs (d = 4.2 nm). The ECB of each material was estimated by P. V. Kamat, et al.24

rate. E. Cánovas, et al. quantitatively studied on the relation between QDs size and charge transfer rate constant from PbSe QDs to SnO2 by monitoring with picosecond Terahertz spectroscopy. It is clearly shown that the charge transfer rate constant monotonically increases with the decrease in the PbSe QDs size (1.44 109 s-1 for PbSe QDs with 7.76 nm and 8.33 109 s-1 for PbSe QDs with 2.15 nm).25 As presented in Scheme 1, the energy difference in the conduction band of CdSe QDs (CBCdSe) and TiO2 (CBTiO ) is larger when smaller CdSe QDs are attached to TiO2. Therefore, photo2

generated electrons can be transferred more rapidly from smaller CdSe QDs to TiO2. Small CdSe QDs, however, cannot absorb the long wavelengths of light, which means less of the solar

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spectrum can be used. On the other hand, larger CdSe QDs absorb long wavelength light, but they are unfavorable for electron injection. This competitive relationship between light absorption and electron injection according to the size of the QDs might lead to the optimal QD size to provide maximum solar energy conversion efficiency. U. Banin, et al. synthesized CdSe QDs with diameters of 2.5, 3.4, 4.0, and 5.5 nm, and then deposited them on TiO2 substrate by electrophoresis to form the photoanode of QDSSCs.26 Interestingly, Jsc tends to increase with increasing size of CdSe QDs. P. Kamat, et al. tuned the band structure of ternary cadmium chalcogenides QDs by changing the composition of Se and S. Although CdSeS QDs with green emission (Se:S = 1:50) shows the highest charge transfer rate constant, CdSeS QDs having the excitonic peak at longer wavelength (Se:S = 1:5) exhibits better QDSSCs performance. X. H. Zhong, et al. have also shown this tendency that QDs absorbing long wavelength light provide a better solar energy conversion efficiency of QDSSCs,27 which is not consistent with our hypothesis about the competition between light absorption and electron injection. However, those studies used organic ligands on the surface of QDs. The presence of organic ligands on the QD surface, which are used to control growth and stabilize QDs as a colloidal solution, significantly inhibits charge transfer at the interfacial region leading to inefficient solar cells. Our recent report has shown that atomic sized inorganic ligand, S2-, significantly enhances the charge transfer rate constant at the interfacial region compared to conventional short organic ligand MPA and TGA.28 S2--ligand exchange is executed easily by dipping the CdSe QDs sensitized photoanode into a formamide solution of K2S, which is verified by FT-IR experiments.28-31 This treatment remarkably enhances the charge transfer kinetics at the interfacial region in both CdSe QDs/TiO2 and CdSe QDs/electrolyte. Internal charge transfer rate constant between CdSe QDs and TiO2 is increased from 1.03 107 (oleate-capped) to 1.32 5



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108 s-1 (S2--capped), and charge transfer resistance of photoanode is decreased from 193.5 (oleate-capped) to 36.13 Ωcm2 (S2--capped). The enhancement of the charge transfer kinetics leads to 10 times higher solar energy conversion efficiency than the conventional treated ones. In the present study, S2- ligand exchange is used to examine the new size-dependent efficiency of QDSSCs. The enhancement of charge transfer can change the conventional size dependence of the photovoltaic performance. This paper reports the results of a systematic study on the influence of the CdSe QDs size on the charge transfer rate constant and subsequently the photovoltaic performance of QDSSCs when QDs are capped with an atomic inorganic ligand, S2-. Five different CdSe QDs were prepared, each having different, but monodisperse, diameters. The monodisperse QDs were then loaded on the TiO2/FTO photoanode by electrophoretic deposition. S2--capped CdSe QDs are used to investigate size-dependent charge transfer under visible light irradiation, and the results are compared with those of conventional MPA-capped QDs. The interfacial electron transfer processes are analyzed by measuring the fluorescence decay and internal quantum efficiency (IQE). Their photovoltaic performance is examined by measuring the photocurrent density-voltage (J-V) characteristics and electrochemical impedance under irradiation with simulated solar light and examining the external quantum efficiency (EQE).

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RESULTS AND DISCUSSION

CdSe QDs with different sizes are synthesized by a modification of the method reported previously.32 Five different CdSe QDs with a range of diameters are prepared by controlling the synthetic variables during the growth of the CdSe QDs. Figure S1 presents the absorbance spectra of the prepared CdSe QDs. Their first excitonic peaks are observed at 511, 551, 590, 633, and 651 nm according to their sizes, which indicate the size dependent band-gap of the CdSe QDs. The change in the band gap energy with the diameter of CdSe QDs is associated with the quantum confinement effect. The empirical relationship between the size of the CdSe (D) and the wavelength, where the first excitonic peak is positioned (λ1), is expressed as33

D = (1.6122 × 10-9)λ14 - (2.6575 × 10-6)λ13 + (1.6242 × 10-3)λ12 - (0.4277)λ1 + (41.57).

(1)

This empirical equation is only available when λ1 is shorter than 700 nm. Because the largest CdSe QDs among the prepared samples shows the first excitonic peak at 651 nm, this equation is satisfied with all the CdSe QDs prepared in this study. From equation 1, their estimated sizes are approximately 2.5, 3.1, 4.2, 6.4, and 7.8 nm, respectively. Figure 1 shows the transmission electron microscopy (TEM) images of the CdSe QDs with different sizes. The QDs show a uniform distribution of shapes and sizes. All the CdSe QDs are close to spherical nanocrystals. Through an analytical investigation from the TEM images, it is observed that uniform CdSe QDs with D = 2.5 ± 0.04, 3.2 ± 0.07, 4.5 ± 0.07, 6.7 ± 0.07, and 7.9 nm ± 0.12 have been prepared successfully. These results concur with the estimation from the absorbance spectra. Various nanoparticles with different morphologies can be deposited uniformly by 7



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electrophoretic deposition (EPD).22, 26, 28, 34, 35 Hence, to fabricate a QD-based photoanode, CdSe QDs are deposited on the TiO2/fluorinated tin oxide (FTO) electrode by EPD. EPD enables the

Figure 1. TEM image of the prepared CdSe QDs. The mean diameter of CdSe QDs are (A) 2.5 ± 0.04, (B) 3.2 ± 0.07, (C) 4.5 ± 0.07, (D) 6.7 ± 0.07, and (E) 7.9 nm ± 0.12.

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Figure 2. (A) SEM image of a cross section of CdSe/TiO2/FTO electrode prepared by electrophoretic deposition. (B) EDS depth profile of Cd/Ti ratio in the CdSe/TiO2/FTO electrode. The size of the CdSe QDs in this photoanode is 7.9 nm.

deposition of CdSe QDs on mesoporous TiO2 with almost uniform coverage over the entire electrode. Before the deposition of CdSe QDs, mesoporous TiO2 on FTO is fabricated through a 9



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modification of a published method.26, 28, 34 The active layer with small-sized TiO2 nanoparticles (~ 10 nm) and a scattering layer with a large-sized TiO2 particles (40 ~ 500 nm) are deposited successively on FTO electrodes using the doctor blade method. Two types of electrophoretic bath are prepared according to the size of the CdSe QDs. To deposit the relatively small CdSe QDs (2.5 and 3.1 nm), the bath is prepared by mixing 40 ml of a CdSe QDs toluene stock solution (105

M) with 50 ml of acetonitrile. To deposit larger CdSe QDS (4.5, 6.7 and 8.4 nm), the bath is

prepared by dispersing CdSe QDs in toluene as a solvent (10-5 M). Figure 2A shows a cross sectional scanning electron microscope (SEM) image of the photoanodes, in which the largest CdSe QDs (D = 7.9 nm) are deposited. The image shows a double-layered structure, including the CdSe QDs/TiO2 active and the scattering layer. The thickness of the active and scattering layers are approximately 7.5 and 11 µm, respectively. The depth profile of Cd/Ti is measured using an energy dispersive X-ray spectrometer installed in the SEM. As shown in Figure 2B, the Cd/Ti ratio is approximately 0.034 through the entire cross section, which indicates that even the largest CdSe QDs are deposited uniformly in the mesoporous TiO2 film. All the tested photoanodes show similar depth profiles of the Cd/Ti ratio through the entire cross section. After depositing CdSe QDs on TiO2, the long organic ligand, oleate, is exchanged with either mercaptopropionic acid (MPA), which is used conventionally, or inorganic atomic ligand, S2-, to enhance the charge transfer kinetics. Here, these ligand exchanged CdSe QDs are labelled as MPA-, and S2--CdSe QDs. Figures 3A and 3B show the absorbance spectra of the MPA- and S2-CdSe/TiO2 photoanode containing different sized CdSe QDs. In both cases, the first excitonic peak tends to shift toward a higher wavelength with increasing CdSe QD size, which is a similar

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Figure 3. Absorbance spectra of (A) MPA and (B) S2--capped CdSe QDs on TiO2 for different sized-CdSe QDs.

pattern to the absorbance spectra of the CdSe QDs in toluene. This means that larger CdSe QDs can absorb long wavelength photons in the visible light region, and can generate larger amounts of charges under solar light irradiation. Smaller CdSe QDs, however, only absorb relatively short wavelength photons up to 550 nm, which limits their use as a solar light absorber. The firstextinction peak of the MPA-capped CdSe TiO2 photoanodes is located at an identical wavelength to the CdSe QDs in toluene. This suggests that there are no discernible changes in the size and shape of the CdSe QDs during MPA-ligand exchange. After ligand exchange using the S2- ion, however, the first excitonic peak shifts slightly towards a longer wavelength. Previous research has shown that ligand exchange to S2- does not influence the size and shape of the QDs.28-31 This 11



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can be attributed to electronic coupling between the short S2--capped CdSe QDs and TiO2 substrate.36 The S2- ligand contacts the CdSe QDs and TiO2 directly without an insulating gap, resulting in enhanced electronic coupling between the QDs and TiO2. Interestingly, the differences in λ1 before and after S2--ligand exchange (△λ1) tends to decrease with increasing diameter of CdSe QDs. △λ1 of 2.5 nm-sized CdSe QDs is 31 nm, and then decreases gradually to 8 nm for the 7.9 nm-sized CdSe QDs. It is reported that the electronic wavefunction of QDs extends spatially well beyond the nanocrystals surface.36 In particular, small particle has more of the wavefunction outside the particle giving more electronic coupling. In addition, smaller CdSe QDs have a higher ECB in their band structure. That is, the energy gap between CBTiO and CBCdSe 2

becomes larger when smaller QDs are used. As a result, strong electronic coupling with an atomic S2- ligand makes an extinction peak shift toward a longer wavelength when smaller S2-capped CdSe QDs come in contact with TiO2. In other words, smaller CdSe QDs show stronger electronic coupling with TiO2 through S2- ligand exchange. This shows indirectly that photogenerated electrons can be transferred more rapidly from the smaller CdSe QDs to the conduction band of TiO2. The internal charge transfer rate constants (ket) between CdSe QDs and TiO2 are investigated by time-resolved photoluminescence (TRPL) experiments.28 ket is calculated from the TRPL results using a 405 nm monochromatic laser to excite the QDs. The emission decay is monitored at the emission maximum. ket can be estimated by comparing the lifetime of the electrons generated in CdSe QDs on TiO2 and glass. An electron generated in CdSe QDs is injected into the conduction band of TiO2 in the photoanode while no electron transfer occurs in the QDs coated on glass. Other deactivation processes of photo-excited electrons of ligand-exchanged QDs might be identical on TiO2 and glass. This is supported by the fact that each QD deposited 12


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Figure 4. Decay profile of the photoluminescence from (A) MPA and (B) S2--capped CdSe QDs on glass and TiO2 for different sized-CdSe QDs. The decays are monitored at the wavelength of the fluorescence peak maximum for each sample. The left photographs are the CdSe/TiO2/FTO electrodes fabricated with different sized-CdSe QDs.

on TiO2 and glass show similar decay patterns in the TRPL experiments. Figures 4A and 4B displays the emission decay profiles of each-sized MPA- and S2--CdSe QDs/TiO2 photoanode, respectively. The fluorescence decay can be fitted to a bi-exponential function using equation 2. 34, 37

It exp exp

(2)

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This generates two lifetime values, τ1 and τ2, with corresponding amplitudes of A1 and A2. The intensity-average lifetime, , is calculated using the parameters obtained from equation 2, as follows: 34, 37

(3)

Figure 5. Calculated ket values according to the size of the CdSe QDS capped by MPA and S2ligands.

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Finally, ket is estimated from equation 4,

!"

(4)

#$%&&

where TiO and glass are the intensity-averaged lifetime of the photo-generated electrons in 2

the CdSe QDs on TiO2 and glass, respectively. ket is presented in Figure 5 and all fitting results are summarized in Table S1 in the supporting information. In both cases, the fluorescence decay curves of CdSe/TiO2 show larger differences from CdSe/glass with decreasing CdSe QD size, which indicates that ket is increasing with decreasing CdSe QD size. In the smaller QDs, the energy level of CB shifts in the upper direction because of the quantum confinement effect. This leads to a larger potential gap between the CBTiO and CBCdSe. Because electron transfer from the 2

CdSe QDs to TiO2 is driven by this energy gap, smaller CdSe QDs exhibit a fast electron transfer rate. Nevertheless, when QDs are capped with MPA, the value of ket is comparatively low for all the CdSe QDs tested. The ket of 2.5 nm-sized MPA-CdSe/TiO2 photoanode is 4.13 × 107 s-1, and ket then decreases with increasing CdSe QD size. On the other hand, the S2- ligand enhances the charge transfer kinetics significantly, yielding a high value of ket. In particular, ket of the 2.5 nm-sized S2--CdSe/TiO2 photoanode is 2.68 × 108 s-1, which is approximately 6.5 times higher than the value for the 2.5 nm-sized MPA-CdSe/TiO2. Although this value also tends to decrease with increasing size of CdSe QDs, ket is still high, even on the large CdSe QDs. ket of the 8.4 nm-sized S2--CdSe/TiO2 photoanode is 8.57 × 107 s-1, which is approximately 7.2 times higher than that of the idendically sized MPA-CdSe/TiO2. The size dependent photovoltaic performance of the CdSe QDSSCs is examined by measuring the external (EQE) and internal quantum efficiencies (IQE). For the EQE and IQE measurements, 15



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each photoanode is assembled with an Au sputtered FTO counter electrode using a Surlyn spacer. A polysulfide water/methanol (3:7) solution is used as the Red/Ox coupling electrolyte. EQE is calculated by measuring the photocurrent of each cell at different monochromatic excitations. Figures 6A and 6B shows the EQE curves of MPA- and S2--CdSe QDSSCs, according to the size of the CdSe QDs, respectively. The photocurrent action spectra match well with the absorbance spectra of the corresponding photoanodes. The S2--CdSe QDs provide a higher photocurrent than conventional MPA-capped QDs because of the enhancement of the charge transfer kinetics at the interfacial region. The value of the EQE peak at the position corresponding to λ1 (EQEpeak) tends to decrease with increasing CdSe QD size for both MPA- and S2--capped samples. The EQEpeak of the 2.5 nm-sized MPA-capped CdSe QDSSCs is 19.5 %, which decreases gradually decrease with increasing CdSe QD size to 4.3 % for the 7.9 nm-sized QDs. On the other hand, 2.5 nmsized S2--CdSe QDSSCs provide a particularly high EQEpeak of 37.8 %, which is almost double that of the MPA-capped ones. The influence of the CdSe QDs size on the electron injection rate is investigated by measuring the IQE of all the QDSSCs. The IQE is strongly related to the electron injection efficiency, independent of the absorbed light. The IQE is calculated by dividing the EQE by the absorbance. Figures 6C and 6D presents the IQE results of the MPAand S2--CdSe QDSSCs, respectively. The IQE tends to increase with decreasing CdSe QD size for both MPA- and S2--CdSe QDSSCs. The IQE of the 2.5 nm-sized MPA-CdSe QDSSC is approximately 45 % in the range between 460 to 520 nm. The IQE for MPA-CdSe QDSSCs decrease gradually to approximately 10 % for 7.9 nm-sized CdSe QDs. The 2.5 nm-sized S2-CdSe QDSSCs show the highest IQE (~ 90 %) among the samples tested. Although the IQE decreases drastically with increasing QDs size, the 7.9 nm-sized S2--CdSe QDSSC still shows a high IQE of approximately 40 %. The IQE results show good agreement with the TRPL results. 16


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Figure 6. EQE for each QDSSC, whose CdSe QDs are capped by (A) MPA and (B) S2-. IQE for each QDSSC, whose CdSe QDs are capped by (C) MPA and (D) S2-.

As verified by the EQE results, larger CdSe QDs generate an eletron hole pair over a wide range of visible light irradiation. In particular, the 7.9 nm-sized CdSe QDs produce electricity under the irradiation of long wavelengths (λ ≤ 700 nm). On the other hand, the 2.5 nm-sized CdSe QDs can just cover relatively short wavelength visible light (λ ≤ 600 nm). Small CdSe QDs, however, show a high electron injection efficiency by exhibiting a high IQE. In particular, 17



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the S2- ligand enhances the charge transfer kinetics significantly at the interfacial region. Owing to competition between the light absorbance and charge injection efficiency according to the size of CdSe QDs, this remarkable enhancement of electron transfer kinetics will lead to a different size dependency of the solar energy conversion efficiency compared to the conventional results reported previously. 26, 27 For a further study of their photovoltaic performance, the current density-voltage (J-V) characteristics are examined for each sized MPA- and S2--CdSe QDSSCs under the illumination of AM 1.5 G solar simulated light (Figure 7). Table 1 lists the short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF), and the overall power conversion efficiency (η). For all cases, the Jsc corresponds to the integration of the photocurrent curves versus the monochromatic excitation wavelength, which is used to obtain the EQE results. With the MPA ligand, the solar energy conversion performance is not affected by the size of the CdSe QDs,

Figure 7. J-V characteristics of each QDSSC whose CdSe QDs are capped by (A) MPA and (B) S2- under AM 1.5 irradiation with an incident power of 100 mW/cm2.

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Table 1. Photoelectrochemical parameters of each QDSSCs using MPA and S2--capped CdSe/TiO2 as the photoanode. Ligand

MPA

S2-

QDs Size (nm)

JSC (mA/cm2)

VOC (V)

FF

η (%)

2.5

5.18

0.30

0.49

0.7

3.1

5.25

0.29

0.49

0.8

4.2

5.73

0.28

0.49

0.8

6.4

5.78

0.28

0.45

0.7

7.8

5.82

0.27

0.44

0.7

2.5

7.97

0.46

0.50

1.8

3.1

9.43

0.45

0.48

2.0

4.2

13.4

0.44

0.44

2.6

6.4

11.7

0.39

0.44

2.0

7.8

11.4

0.38

0.41

1.8

which is a similar result to that reported elsewhere.26 η does not change significantly with the size of CdSe QDs. This is likely due to the balance between light absorbance and electron injection. In the case of S2--CdSe QDs, however, η and Jsc are optimized using the 4.2 nm-sized CdSe QDs. Voc tends to increase with decreasing CdSe QD size. As verified by the TRPL and IQE experiments, the S2--ligand enhances the electron injection efficiency. In particular, smaller CdSe QDs show very high electron injection performance, and can provide a high Voc. Moreover, S2- ligand leads even larger CdSe QDs to inject a photo-induced electron into the CB of TiO2 rapidly. In particular, 4.2 nm-sized S2--CdSe QDs exhibit a high electron transfer rate constant (1.29 108 s-1) and absorbance of wide-band visible light up to 650 nm. The 4.2 nm-sized S2-19



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CdSe QDSSCs have an overall solar power conversion efficiency of 2.6 %, which is 3.7 times higher than that of the MPA-CdSe QDSSCs. The charge transfer resistance at the interface between photoanode and electrolyte is estimated by electrochemical impedance spectroscopy (EIS). This technique is useful for characterizing the interfacial electrochemical charge transfer of QDSSC,15, 38 DSSCs,39, 40 and photoelectrochemical cells.41 Figure 8 presents representative Nyquist plots of the photovoltaic cells using MPA and S2-capped CdSe/TiO2 as the photoanode. Each marker in Figure 8 indicates the experimental impedance, while the corresponding solid lines are the theoretical spectra fitted from the equivalent circuit shown in Figure S2. The ohmic serial resistance (RS) corresponds to the electrolyte and FTO resistance, and the resistances, Rct, C and Rct A, correspond to the charge transfer process occurring at the Au counter electrode and polysulfide electrolyte/CdSe/TiO2 interface, respectively. The curves of the theoretical simulation agree with the experimental impedance

results,

thereby

validating

the

proposed

equivalent

circuit

of

this

photoelectrochemical cell. Table S2 list all the parameters extracted for each photovoltaic cell. Two semicircles are found for each Nyquist plot; interfacial electrochemical charge transfer occurred on the cathode (at higher frequency) and anode (at lower frequency), respectively. Their radius is strongly related to the Faradaic reduction/oxidation charge transfer resistance of each electrode. No significant change in the radius of the semicircle is observed at higher frequencies in the Nyquist plot for each photovoltaic cell because the Au-coated cathode is used for all experiments. For MPA-CdSe/TiO2 photoanode, the value of Rct, A does not change significantly with CdSe QD size. For the S2--capped QDs, however, an obvious change in the radius of the semicircle is observed at lower frequencies. The Rct, A of QDSSCs is reduced to 47.3 Ωcm2 when 4.2 nm-sized S2--CdSe QDs are used as the light absorber. For all sizes of CdSe QDs, the S2-20


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capped photoanodes show a much lower Rct, A due to

Figure 8. Nyquist plots for each QDSSC whose CdSe QDs are capped by (A) MPA and (B) S2under AM 1.5 irradiation with an incident power of 100 mW/cm2.

the enhanced electronic coupling. These results agree with the Jsc measurements. Here, the use of a S2- ligand on the CdSe QDs is responsible for the new observation of size dependent photovoltaic performance of QDSSCs. The band gap of QDs is tunable because of the quantum confinement effect, which can control light absorption in solar cells. To date, there has been no confirmation of the unique properties of QDs in QDSSCs yet, because the organic ligands inhibit charge transfer at the interfacial region. The solar energy conversion efficiency has been dependent mainly on the QDs absorbance, which means that the larger QDs tend to 21



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show better solar cell performance. Previous studies have claimed that the effective QDSSCs can be fabricated using CdSe QDs that absorb longer wavelengths because they can absorb a broad spectrum of solar light with a high extinction coefficient. The S2- ligand, however, significantly enhances the charge transfer kinetics at the interfacial region. This enhancement complements the charge transfer kinetics, which has not been properly represented in previous studies due to the long organic ligands. In particular, 4.2 nm-sized S2--CdSe QDSSCs, exhibiting moderate light absorbance and charge injection, show the highest photovoltaic performance of QDSSCs because of the smallest interfacial charge transfer resistance on the photoanode.

22


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CONCLUSIONS

The size dependent solar energy conversion efficiency of QDSSCs with an atomic inorganic ligand (S2-) is investigated using uniform CdSe QDs with diameters of 2.5, 3.2, 4.2, 6.4, and 7.8 nm. The absorption and EQE measurements show that larger sized CdSe QDs generate charges under the irradiation of long wavelength photons, which is advantageous to utilizing the solar spectrum. Electron injection from the CdSe QDs to TiO2 is enhanced when smaller sized CdSe QDs are used because of the high energy level of CBCdSe, which is verified by TRPL and IQE. The competition between the generation and injection of electrons with QDs size results in a new size dependent photovoltaic performance of CdSe QDSSCs that is not observed in the earlier MPA-CdSe QDs systems. On the other hand, the S2- ligand significantly enhances the charge transfer rate at the interfacial region by eliminating the barrier to charge transfer. As a result, 4.2 nm-sized S2--CdSe QDSSCs show photovoltaic performance of 2.6 %, which is 3.7 times higher than conventional MPA-CdSe QDSSCs. This study clearly shows that an atomic inorganic ligand, S2-, maximizes the influence of quantum confinement on the solar energy conversion efficiency of QDSSCs.

23



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EXPERIMENTAL DETAILS

Synthesis of CdSe quantum dots. The CdSe seeds are prepared by modifying the synthesis method previously reported.28, 32 After nucleation, the temperature is raised to 280 oC for the growth step. A mixture of 0.1 M of selenium in octadecene (x ml) and 0.5 M of cadmium oxide in oleic acid (x/5 ml) is injected into the reaction flask at a rate of 1 mL/min. x is 5, 20, 30 and 40 for synthesizing the 3.2, 4.2, 6.8 and 7.8 nm-sized CdSe QDs, respectively. They are purified by precipitation with the addition of isopropyl alcohol, and re-dispersed in toluene to form stable colloidal dispersions. 2.5 nm-sized CdSe QDs are prepared by the similar method to synthesis of CdSe seeds,28 except that the nucleation temperature is 190 oC. Once the reaction flask has reached 190 oC, it is cooled rapidly to room temperature.

Characterization. The shapes and sizes of synthesized CdSe QDs are examined by transmission electron microscopy (TEM, JEOL-2100) equipped with a Gatan Peltier cooled CCD camera. The high-resolution cross-sectional scanning electron microscopy (SEM) images of QDsensitized TiO2 anodes are recored using a JEOL 7500F. The optical absorbacne are obtained using

a

UV/vis/NIR

spectrophotometer

(Cary

5000,

Agilent).

The

time-resolved

photoluminescence (TRPL) curves are acquired through time-correlated single photon counting (TCSPC) methods using a Fluorolog-3 spectrofluorometer. A 405 nm pulsed diode laser (Picoquant) with a repetition rate of 2.5 MHz is used as the excitation source.

Solar cell fabrication and characterization. 2 mm-thick fluorine-doped tin oxide (FTO) glass substrates (TCO22-7, Solaronix) are cleaned sequentially with a detergent solution, DI24


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water and ethanol using an ultrasonic bath for 15 min during each step. Cleaned FTO glass is treated in a UV-Ozone system for 30 min. Mesoporous TiO2 films are fabricated by layering a TiO2 paste using the doctor-blade technique. A TiO2 paste (T/SP, Solaronix) for the active layer is coated on a pre-cleaned glass substrate using the doctor-blade method, and sintered at 500 °C for 30 min. TiO2 paste for the scattering layer is prepared, as reported previously.28, 42 It is deposited onto the transparent TiO2 film, followed by annealing at 500 °C for 30 minutes. Two kinds of electrophoretic baths are prepared depending on the size of the CdSe QDs to be deposited. For the deposition of relatively small CdSe QDs (2.5 and 3.1 nm), the bath is prepared by mixing 40 ml of a CdSe QDs toluene stock solution (10-5 M) with 50 ml of acetonitrile. The addition of acetonitrile provides the appropriate polarity to the bath, which is required to deposit small nanocrystals. To deposit larger CdSe QDS (4.5, 6.7 and 8.4 nm), the bath is prepared by dispersing CdSe QDs in only a toluene solvent (10-5 M). The TiO2/FTO and FTO electrodes connected to the positive and negative terminals of the power supply (EV215, Consort), respectively, are immersed in the prepared QD solution bath while maintaining a separation of 1 cm. To deposit the smaller CdSe QDs with a diameter of 2.5 and 3.1 nm onto the TiO2 surface, a DC voltage (200 V for 2.5 nm QDs and 50 V for 3.1 nm QDs) is applied for 10 minutes, respectively, and the electrodes are then rinsed with toluene to remove the over-deposited CdSe QDs. This cycle is repeated 6 times. The over-deposited CdSe QDs are rinsed with toluene. To deposit the larger CdSe QDs with diameters of 4.5, 6.7 and 8.4 nm, a 1200 V DC voltage is applied for 120, 30 and 10 minutes, respectively, and the electrodes are then rinsed with toluene. These experimental variables for fabricating consistent photoanode with the same Cd/Ti ratio in the TiO2 photoanode layer are determined to investigate the influence of quantum confinement on the photovoltaic performance of the QDSSCs more precisely. Ligand exchange is performed 25



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Page 26 of 34

by dipping the CdSe/TiO2/FTO electrodes into each bath (MPA and K2S solution in formamide (10 mg/mL) for 1 hour. After liand exchange, the electrodes are cleaned with methanol. Three photoanodes are fabricated under each condition and charge transfer rate constants are obtained from their average. Sputtered Au (30 nm) on the cleaned FTO glass is used as cathode for QDSSCs. The photoanode and cathode are assembled in a sandwich type using a 25 µm thick sealing film (Surlyn®, Meltonix 1170-25, Solaronix) as a spacer. The electrolyte is prepared by dissolving 0.5 M of Na2S, 2 M of S, and 0.1 M of NaCl in water and methanol (3:7 vol.). The EQE is calculated by passing light through a monochromator (Oriel Cornerstone 130) and measuring the photocurrent and light irradiance. The J-V characteristics of the QDSSCs are obtained using a Keithley 2400 source-meter under the illumination of AM 1.5G solar simulated light (1 sun, 100 mW/cm2, Oriel instruments model 96000, Newport Co.). The EIS curves are recorded using a potentiostat (Series G™ 750, Gamry), over the frequency range, 100 kHz and 100 MHz, with an AC voltage amplitude of 10 mV at 0 V vs. VOC. The impedance spectra are interpreted by a nonlinear least-square fitting procedure using commercial software (ZVIEW 2TM).

26


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ACKNOWLEDGMENTS

This received primary support for development of the QDSSC’s and measurement of the charge transfer dynamics from a collaborative NSF grant (NSF CBET-1335821 for C.B.M. and NSF CBET-1333649 for J.B.B. Secondary support for QD synthesis and characterization was provided by the U.S. Department of Energy Office of Basic Energy Sciences, Division of Materials Science and Engineering (Award No. DE-SC0002158). H.J.Y is grateful for the support of a postdoctoral fellowship from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF2012R1A6A3A03039474). C.B.M. is grateful for the support of Richard Perry University Professorship. J.B.B. acknowledges support from an NSF CAREER Award (CBET-0846464).

27



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Supporting Information Available: Absorbance spectra of the various as-synthesized CdSe QDs in toluene, equivalent electronic circuit of the electrochemical system for the quantum dot sensitized solar cells, kinetic analyses of emission decay for each photoanode, and parameters extracted from fitted results of EIS spectra for each QDSSC. This material is available free of charge via the Internet at http://pubs.acs.org.

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