CdS Quantum Dot Composite Sensitizer and Its Applications to

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CuS/CdS Quantum Dot Composite Sensitizer and Its Applications to Various TiO2 Mesoporous Film-Based Solar Cell Devices Myoung Kim, Uchirbant Altantuya, and Hyo Joong Lee Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b00324 • Publication Date (Web): 18 Jun 2015 Downloaded from http://pubs.acs.org on July 1, 2015

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CuS/CdS Quantum Dot Composite Sensitizer and Its Applications to Various TiO2 Mesoporous FilmBased Solar Cell Devices Myoung Kim,† Altantuya Ochirbat ‡, and Hyo Joong Lee †, ‡, *

Department of Bioactive Material Sciences† and Chemistry‡ Chonbuk National University, Jeonju, 561-756, South Korea (ROK)

*To whom correspondence should be addressed: [email protected] (H. J. Lee)

Tel.:+82-63-270-3353, Fax: +82-63-270-3408.

ABSTRACT

A nanoscale composite sensitizer composed of CuS and CdS quantum dots (QDs) was prepared by a simple but effective layer-by-layer reaction between a metal cation (Cu2+ or Cd2+) and a sulfide anion (S2–). The as-prepared composite CuS/CdS QD sensitizer displayed 1

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an enhanced photon-to-current conversion over the sensitizing range of the visible spectrum compared to the counterpart of the pure CdS sensitizer. At the optimized ratio of the deposited amounts of CuS and CdS, the best CuS/CdS-sensitized mesoporous TiO2 cell with a polysulfide electrolyte showed an overall power conversion efficiency of 3.60% with a short circuit current (Jsc) of 11.77 mA/cm2, an open circuit voltage (Voc) of 0.65 V, and a fill factor (FF) of 0.47. From the transmission electron microscopy images, the initially deposited CuS seemed to take a nucleation site to accumulate more CdS in the later deposition. The kinetic studies by impedance and Voc decay measurements also revealed the CuS/CdS and CdS QD sensitizers made a similar interface between TiO2 and the electrolyte but the former had a larger resistance of charge transfer with a longer lifetime of excitons after light absorption than the latter. To enhance further the sensitizing power, a multilayer QD sensitizer of CuS/CdS/CdSe was prepared by successive ionic layer adsorption and reaction (SILAR). This led to the best performance of 4.32% overall power conversion efficiency. Finally, a hybrid sensitizing system of inorganic QD (CuS/CdS) and organic dye (coded MK2) was tested with a [Co(bpy)3]2+/3+ redox mediator. The CuS/CdS/MK-2 dye-sensitized cell showed over 3.0% under the standard illumination condition (1 sun). KEYWORDS: Quantum dot-sensitizer, composite quantum dot, CuS/CdS, multilayer, hybrid sensitizer, quantum dot-sensitized solar cells.

1.

Introduction

In the past two decades, nanocrystalline semiconducting quantum dots (QDs) have attracted much attention in a variety of research fields for advanced optoelectronic applications.1,2 Most trials that utilized QDs as a building unit of a light emitter were based 2


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on an efficient charge recombination inside QDs for bright emission over a broad spectrum range.3 In photovoltaic applications, however, QDs have been used firstly over mesoporous metal oxide films as a photosensitizer working based on an effective charge separation out of QDs after incident light absorption,4-6 and later studied intensively in terms of charge transport in QD films for photovoltaics.7-9 After much work aimed at creating effective QD sensitizers and QD films, a moderate output of power conversion (1–8%) can now be expected from a few examples, such as cadmium, lead, or antimony chalcogenides.10-15 Although many studies have reported on QD-sensitized solar cells from pure QD compounds composed of binary elements at the early stage, a new breakthrough was needed to find more efficient QD sensitizers, and this has been pursued in several new directions: First, nontoxic ternary QDs, such as CuInS2(Se)2 and AgInS2, were prepared by a colloidal or in-situ chemical bath deposition (CBD) process and they appeared to be an ideal target QD sensitizer with a proper band gap of 1.0 to 1.5 eV.16-19 Second, a small amount of a foreign element, such as Mn and Pb, was added to the main component of the CdS QD for making composite or doped QDs during the preparation step. This strategy induced a moderate to large increase in power conversion efficiency depending on the experimental conditions.20,21 Third, two or more QDs were accumulated in a layer-by-layer fashion in the pursuit of both efficient light absorption and favorable charge separation between different QD layers. The most successful results were obtained from CdS/CdSe deposited sequentially over mesoporous TiO2 films by CBD methods.22,23 In addition, the pre-synthesized colloidal QD sensitizers were self-assembled successively to form a multilayer on mesoporous TiO2 or ZnO films using an electrostatic force between differently charged molecular or inorganic ligands surrounding the QDs.24,25 Fourth, a hybrid sensitizing system composed of inorganic QDs and molecular dyes was suggested to draw a synergistic contribution from each 3


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component, where QDs could play the same role as sensitizing dyes in transferring charges after light absorption into electron- and hole-transporting materials26-28 or they could play a special role in transferring energy into dyes for enhancing power or widening a range of light absorption.29,30 Besides, QDs could also be used as an active layer by forming a QD-polymer hybrid film between two electrodes for a new type of solar cell device.31 In this study, we found a small amount of CuS could be combined successfully with a representative QD sensitizer, CdS, by a typical SILAR process to make an efficient CuS/CdS composite sensitizer under atmospheric conditions at room temperature. Then, by using this newly created CuS/CdS QD sensitizer, a multilayer of CuS/CdS/CdSe and a hybrid of CuS/CdS QD and MK-dye, were constructed step-by-step to demonstrate its useful sensitizing power in mesoporous TiO2-based solar cell devices.

2.

Experimental

2.1.

Preparation of QD- and QD/dye-sensitized solar cells

For a thin TiO2 blocking layer, fluorine doped tin oxide (FTO) coated glass (Solaronix, 8 Ω/□) was immersed into 40 mM TiCl4 aqueous solution for 30 min at 70 oC. Then it was taken out and gradually heated up to 500 oC. Using a screen printing system and commercial TiO2 pastes, a double layer of TiO2 film was coated on the TiCl4-treated FTO glass. The first TiO2 film of ~5 µm thickness (Dyesol, 18NR-AO; TiO2 particle size of 20-450 nm) and then a second layer of ~4 µm thick opaque TiO2 film (Dyesol, WER2-O; TiO2 particle size of 150250 nm) were coated, and they were annealed gradually up to 500oC for 60min in atmospheric condition. After that, the typical post-treatment of TiCl4 was performed in the same manner as the pre-treatment of TiCl4. 4


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A 0.020 M aqueous solution of copper acetate monohydrate [Cu(CH3CO2)2·H2O, SigmaAldrich] or sodium sulfide nonahydrate (Na2S·9H2O, Sigma-Aldrich) was used for each cationic and anionic source for depositing CuS by the SILAR process. For the preparation of CdS QD, a 0.10 M solution of cadmium acetate dihydrate [Cd(CH3CO2)2 ·2H2O, SigmaAldrich] in ethanol/water (1:1, v/v) and a 0.1 M solution of Na2S·9H2O in methanol/water (1:1, v/v) were used, respectively. To deposit CdSe, 0.050 M Cd(NO3)2 and 0.050 M sodium selenide in ethanol were used for successive SILAR processes with a dipping time of 1 min each. Sodium selenide (Na2Se) was prepared in-situ by adding 0.10 M NaBH4 to 0.050 M SeO2 in ethanol while the container was purged with N2.32 A ZnS layer for surface passivation was applied after the growth of CdS, CuS/CdS or CdS/CdS/CdSe QDs onto TiO2 films by using aqueous solutions of 0.10 M zinc acetate [Zn(CH3CO2)2, Sigma-Aldrich) and 0.10 M Na2S·9H2O. For testing the as-prepared QDsensitized cells, a polysulfide electrolyte (S2-/ Sn2-) was used with a composition of 2.0 M of S, 2.0 M Na2S and 0.2 M KCl in H2O while a cobalt redox couple was used in the study of QD/dye-hybrid sensitizing system with 0.20 M Co(bpy)32+, 0.05 M Co(bpy)33+, and 0.1 M LiClO4 dissolved in acetonitrile. A CuxS counter electrode was prepared by following a reported procedure33; a 100 nm-thick Cu film was deposited onto FTO glass by a sputtering system. Then, the substrate with Cu film was immersed in a polysulfide solution composed of 1.0 M of Na2S and 1.0 M S for 10 min and subsequently washed with DI water. For a very thin Al2O3 overlayer, the QD/dye–modified FTO electrode was dipped for 10 min in an alcoholic solution of 15 mM precursor of aluminum isopropoxide (Sigma-Aldrich). Then, the electrode was taken out and dried in air for 1 min. To make the expected hydrolysis reaction of the precursor to become more complete and certain, the electrode was dipped in water for 1 min and then dried in air for 1 h before assembly.34,35 5


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The sandwich-type cells were fabricated from the QD- or QD/dye-sensitized photoanodes and the CuxS or Pt counter electrodes, respectively, with a 60-µm thick polymer film (Surlyn, Dupont 1702). Finally, the polysulfide or cobalt (II/III) electrolyte was introduced through the previously drilled holes at the counter electrode. 2. 2. Measurements The current-voltage and open-circuit voltage decay characteristics were analyzed under a standard illuminating condition (100 mW cm-2) using a solar simulator (Peccell, PEC-L01) and a potentiostat (IVIUM, Compactstat). The scan rate and voltage step of the J-V measurement were 50 mV/s and 2 mV, respectively. Under dark condition, electrochemical impedance measurement was done at a forward bias of 0.70 V superimposed with an amplitude of 10 mV ac from 106 to 10-1 Hz. The incident photon-to-current efficiency (IPCE) data were collected from 380 nm to 800 nm by a light source (ABET 150W Xenon lamp, 13014) with a monochromator (DONGWOO OPTORN Co., Ltd., MonoRa-500i) and the potentiostat (IVIUM, Compactstat) based on DC method without chopper and light bias. A black tape with a slightly larger aperture (3.5 x 8.5 mm) than the photoactive region (3 x 8 mm) was used as a mask to avoid the diffuse light which can induce any inflated photocurrents by blocking the passage of incident light over the glass electrode except the sensitized area. Optical absorbance was measured with a UV-VIS-NIR spectrophotometer (Cary 100). Transmission electron microscopy (TEM) imaging was carried out using a JEOL (JEM-ARM-200 F) microscope operated at 200kV.

6


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3.

Results and discussion

The chemical bath-deposited CdS has been used most frequently as an inorganic semiconducting photosensitizer in the research of solar cells36-38 and photo-catalysts39 because of its easy preparation under atmospheric conditions and its favorable band positions for an efficient charge transfer into each electron- and hole-accepting material after light absorption. However, the relatively large band gap (~2.5 eV) of CdS has always limited its sensitizing power up to ~550 nm where only a fraction of the visible spectrum is absorbed. To solve this intrinsic limitation of CdS utilization, a small amount of a foreign element, such as Pb or Mn, was introduced during the initial or whole stage of CdS deposition. This could induce an effect of narrowing the band gap of CdS and thus increasing the generated photocurrents.20,21,40 Although it was difficult to get an exact description of the formation of the nanocomposite or doped state by the major CdS and the minor PbS or MnS, the asprepared PbS/CdS or MnS:CdS QD sensitizer has shown an enhanced photon-to-current conversion efficiency over a broader spectrum range compared to the corresponding pure CdS sensitizer. In this study, we introduced a small amount of CuS before the growth of the main CdS QD to create a nanocomposite QD sensitizer of CuS/CdS by applying the same SILAR-deposition process to all preparation steps. After checking the optimal amount of Cu relative to Cd under the current experimental conditions, we found that the best photovoltaic result was obtained by a combination of deposition by 1 X CuS and 7 X CdS SILAR cycles. As seen in Fig. 1(a) and summarized in Table 1, the deposition of a small amount of CuS before CdS increased the overall power conversion efficiency from 2.82% to 3.60% due to a large increase in Jsc followed by a small increment of Voc with almost the same FF. If the cell was measured without a black mask having an aperture area slightly larger than the active 7


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region (0.24 cm2), our best cell approached an overall efficiency value of over 4.0% due to the well-known scattering effect from the surrounding region (data not shown).41 Table 1. A summarized data set of short circuit currents, open circuit voltages, fill factors and overall conversion efficiencies from a various combination of QD sensitizers prepared by the SILAR process (the number after designated QDs indicates the repeated times of each SILAR process). Jsc (mA/cm2)

Voc (V)

FF

η (%)

CdS(7)

9.15

0.63

0.49

2.82

CuS(1)/CdS(5)

9.98

0.62

0.48

2.97

CuS(1)/CdS(7)

11.77

0.65

0.47

3.60

CuS(1)/CdS(9)

11.98

0.65

0.43

3.35

PbS(1)/CdS(7)

16.50

0.57

0.37

3.48

Fig. 1 (a) J-V curves and (b) IPCE data from the best CdS(7)- and CuS(1)/CdS(7)-sensitized cells (schematic diagram of charge transfer at the interfaces and absorbance of two sensitizers in each inset of a and b).

When a series of SILAR depositions of CdS was extended up to the ninth cycle and their photovoltaic performances were tested with a polysulfide electrolyte, the optimum amount 8


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and ratio between Cu and Cd were observed at around the seventh cycle of CdS deposition. If the added amount of Cu was increased by a multiple deposition of CuS or by using a higher concentration of the Cu (II) precursor than the current of 20 mM, the overall efficiency always substantially decreased (data not shown). Only a small amount of CuS by one SILAR cycle was helpful in inducing more currents because the magnitude of band gap and the band positions move quickly to that of bulk CuS (~1.2 eV) as the amount of SILAR-deposited CuS increases, thus leading to an unfavorable band position for charge transfer although absorbing more incident light over a range of visible spectrum. These results are in agreement with the reported results from PbS/CdS where only a small amount of PbS was necessary to reach a maximum efficiency of about 4.0%.21,42 As for device stability, we have observed a normal behavior reported so far about QD-sensitized solar cells using a polysulfide electrolyte; the overall efficiency are gradually decreased mainly due to a slow leakage of water-based electrolyte and intrinsic instability of semiconducting materials in contact with an alkaline polysulfide solution. About 90% of the initial value was maintained after 1 week if there was no serious problem in sealing the cell. All data were obtained and compared from fresh cells to minimize the changes depending on the time after cell assembly. From the measurements of IPCE (Fig. 1b) and absorbance (inset, Fig. 1b), the additional presence of a small amount of CuS seems to strengthen the sensitizing power more effectively in the visible region and thus generated more photocurrents than without CuS. One possible reason could be adopted from the reported result in which Cu-doped metal chalcogenide was known to have a much longer PL lifetime, presumably due to a localized hole state.43 In this study, the hole localized in CuS for a relatively long time could be stabilized and then transfered effectively to electrolyte through CdS as depicted in schematic 9


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diagram in the inset of Fig. 1a. When the overall power conversion efficiency was compared (Table 1), the current CuS/CdS sensitizer was more efficient than the reported PbS/CdS under our experimental conditions due to a higher Voc and FF despite a smaller Jsc. The latter showed a longer tail up to the near-infrared region,21 while the former had an absorption-to-current feature until about 700 nm in the visible region (Fig. 1b). These different IPCE features by PbS and CuS are likely to arise from their different band gaps, which could influence the band gap of a composite QD with CdS. This finding provides a good guidance in designing a new composite or alloyed QD sensitizer by combining a small band gap QD with a larger one. More studies are underway to elucidate the detailed mechanism of mixing two different cations to construct a new QD sensitizer in which the band gap can be controlled by selecting an appropriate ratio of two or more cations.

When a nanoscale formation of CuS/CdS QDs over TiO2 was observed by high-resolution TEM after each SILAR deposition step, the initially deposited CuS nanoparticles look to be well-scattered over a TiO2 particle-connected film with a density of about a few particles per one TiO2 particle (Fig. 2a). As the deposition cycle for CdS progressed, more CdS appeared to be spreading and accumulating over the TiO2/CuS, finally leading to a thin film-like aggregate (Fig. 2b-e and the lower part of Supplement Fig. 2). Without the initial deposition of CuS, less CdS were deposited over the TiO2 (Fig. 2f and the upper part of Supplement Fig. 2). This is also consistent with what was observed from absorbance after each SILAR deposition step (Fig.1b inset and Supplement Fig.1). Thus, a small amount of CuS deposited firstly appears to play a role as the nucleation site for more deposition of CdS later.

10


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Fig. 2 TEM images of small pieces scraped off from (a) TiO2/CuS(1), (b-e) TiO2/CuS(1)/CdS(1) ~ CdS(7), and (f) TiO2/CdS(7)

If more QDs are present onto the TiO2 film, more incident light will be absorbed but not guaranteed to be converted into current in a proportional way. The photocurrent increased gradually up to the 7th cycle of CdS deposition and then almost leveled off with a deteriorated fill factor (Table 1), which means the internal quantum efficiency (IQE) is also increased up to the stage of optimum size of QD sensitizer, but not proportionally. This could arise probably from two facts; first, the size of QD sensitizer is also critical in charge separation and internal recombination after light absorption.44-46 Second, the presence of two much QDs inside the mesopore of TiO2 film could hinder the electrolyte to flow effectively for hole-transporting which could increase the solution resistance leading to a low fill factor. 11


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Therefore, there is always an optimum size of QD sensitizer in solar cells based on mesopore TiO2 films. In current experimental conditions, an optimum size of QD sensitizer was around 7th cycle of CdS deposition. To check the interfacial electrochemical properties of the as-prepared CuS(1)/CdS(7) and CdS(7) QD-sensitized cells, Impedance and Voc decay were measured and the obtained results were compared (Fig. 3). Under dark at a forward bias, the shape of the Nyquist plots of the two cells shown in Fig. 3a was very similar except for the magnitude of the second semicircle, which reflects the interfacial charge-transfer resistance (Rct) between TiO2 and the electrolyte in the transmission line model applied to the mesoporous electrode-based QD cells.46,47 The Rct obtained from the semicircle at higher frequencies is larger for the CuS(1)/CdS(7)-sensitized TiO2 electrode as compared to the CdS(7)-deposited one because of a higher surface coverage of CuS/CdS QDs over TiO2 particles which was confirmed clearly by TEM images (Fig. 2). These densely packed QD sensitizers by initial deposition of CuS could hinder electron flow from TiO2 to electrolyte, thus reducing the recombination at TiO2/QD/electrolyte. The same trend was observed by applying a typical passivation layer of ZnS over main QD sensitizers in QD-sensitized solar cells based on mesoporous metal oxide films.46 From the measurement of Voc decay,48 we clearly saw the CuS(1)/CdS(7) sensitizer had a slower decay of open-circuit voltage than the pure CdS(7), which indicates a longer lifetime of the excitons generated after light absorption in the CuS(1)/CdS(7)-sensitized cell compared to the CdS(7) cell. This situation of slow relaxation of exciton could be helpful in generating a higher short-circuit current in the former than in the latter. From these impedance and Voc decay analyses along with absorbance and TEM measurements, more QDs present at the interface of TiO2/electrolyte should be beneficial in 12


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increasing 1) absorption of incident light, 2) resistance of charge transfer (Rct), and 3) exciton life time after excitation, which all could contribute the increase of power conversion efficiency. But, after reaching an optimal size or structure of QD sensitizer layer, additional growth of QDs is not always better due to a compromise with an internal recombination in the QD absorber which was also pointed out by Hodes.49

Fig. 3 Impedance (a) and Voc decay (b) data from CdS(7)- and CuS(1)CdS(7)-sensitized cells.

Open-circuit voltage (V)

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(b)

0.7

CdS (7) CuS (1)/CdS(7)

0.6

0.5

0.4

0.3

0

5

10

15

Time (s)

After establishing a well-defined and reproducible condition for preparing an efficient CuS/CdS QD sensitizer, we made a multilayer QD sensitizer by combining CuS/CdS and CdSe, which were all prepared from one common SILAR process. Based on extensive published results,20,22,23,50 the CdS/CdSe has proven itself as an ideal model QD sensitizer when working with polysulfide electrolytes. When a small amount of CuS-incorporated CdS was used as the first sensitizing layer instead of pure CdS, the addition of CdSe as the second sensitizer was the most effective by a 2 X SILAR process for CdSe deposition, and then the overall conversion efficiency decreased gradually (Table 2). Without CuS, CdS needs to be 13


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combined with more CdSe for best results.20,22,23,50 This observation may result from the reduced band gap induced by the inclusion of CuS into the main CdS. After checking the optimum ratio for the best photovoltaic performance, CuS(1)/CdS(7)/CdSe(2) was found to give the highest conversion efficiency of 4.32% with 16.01 mA/cm2, 0.60 V, and 0.45 FF. After a few overlayers of CdSe, the short circuit current increased greatly from 11.77 to 16.01 mA/cm2 with a slight decrease in both Voc and FF. Therefore, only two cycles for depositing CdSe were sufficient to make the best performance cell.

Table 2. A summarized data set of photovoltaic performances from a various combination of designated QD multilayer sensitizers prepared by the all SILAR process Jsc (mA/cm2)

Voc (V)

FF

η (%)

CuS(1)/CdS(7)

11.77

0.65

0.47

3.60

CuS(1)/CdS(7)/CdSe(1)

14.32

0.60

0.43

3.69


16.01

0.60

0.45

4.32


15.96

0.49

0.41

3.21


13.55

0.39

0.38

2.01

Fig. 4 J-V curves obtained from a multilayer of designated QD sensitizers prepared by the all SILAR process 20 CuS(1)/CdS(7) CuS(1)/CdS(7)/CdSe(1) CuS(1)/CdS(7)/CdSe(2) CuS(1)/CdS(7)/CdSe(3) CuS(1)/CdS(7)/CdSe(5)

2

Current density (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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16

12

8

4

0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Voltage (V)

14


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Another proof-of-concept trial for enhancing the sensitizing power of QD is to make a hybrid sensitizer by adding organic molecular dyes over TiO2/inorganic QDs. So far, CdS was used in most cases for making a hybrid sensitizer due to its easy preparation under atmospheric conditions while CdSe looks more ideal in terms of the sensitizing range, but the latter is more difficult to be prepared than the former. From the current study, we have developed a new CuS/CdS composite sensitizer which can be prepared easily in air with a better sensitizing power over a broader range than pure CdS. Therefore, it would be interesting to combine CuS/CdS with a typical organic dye coded as MK-2 and then test the performance of a hybrid sensitizer. When testing a hybrid sensitizer of QD and dye in one cell, only cobalt (II/III) redox couples are currently known to be compatible with both sensitizers simultaneously.27,28,35,51,52 The typical I–/I3– redox relay is common to almost all dyes, but very corrosive to QDs, while the polysulfide is good with some QDs but detaches most dyes. Therefore, Co(II/III)(bpy)3 was selected to evaluate the as-prepared hybrid sensitizer from CuS(1)/CdS(7) and the organic dye coded as MK-2. As summarized in Table 3 and shown in the IPCE data of Fig. 5, the composite sensitizer of CuS(1)/CdS(7) had a better power generation over a broader range of the visible spectrum compared to CdS(7) when recycled with the Co(II/III)(bpy)3 redox couple under the standard illumination conditions. To further enhance the sensitizing power, the typical organic dye, MK-2, was selfassembled over the CuS(1)/CdS(7)-deposited mesoporous surface of TiO2 film. The overall power conversion efficiency was then elevated to almost 3.0% under a standard 1 sun condition, and the photon-to-current conversion was enhanced up to 800 nm, as shown in the IPCE measurements (Fig. 5). In the QD- and/or dye-sensitizer tested with the cobalt(II/III) redox mediator, a very thin metal oxide is known to work well as a kind of passivating layer to protect the inside sensitizer and retard the electron flow toward the electrolyte.34,35,49 15


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Accordingly, when a very thin Al2O3 layer was applied to the TiO2/CuS/CdS/MK-2 dye, an increase in Jsc was also observed with an overall power conversion efficiency of 3.21%, as expected. This step-by-step modification of the mesoporous TiO2 electrode is considered effective in constructing versatile component-modulated nanoscale-multilayers for creating a new photosensitizer in solar cell devices as well as in composite photocatalysts depending on purpose-specific plans.

Table 3. A summarized data set of photovoltaic performances from a various combination of designated QD sensitizers and/or organic dye (MK-2) tested with Co(bpy)32+/3+ redox mediator. Jsc (mA/cm2)

Voc (V)

FF

η (%)

CdS(7)

2.85

0.54

0.63

0.97

CuS(1)/CdS(7)

4.46

0.62

0.59

1.63

CuS(1)/CdS(7)/MK-2

5.21

0.70

0.81

2.95

CuS(1)/CdS(7)/MK-2/Al2O3

5.88

0.70

0.78

3.21

MK-2

8.86

0.52

0.47

2.17

Fig. 5 IPCE data from designated QDs- and QDs/dye-sensitized cells

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4.

Conclusions

In this study, a new nanocomposite QD sensitizer of CuS/CdS was developed with an enhanced power conversion efficiency of 3.60% over its counterpart of pure CdS. This nanocomposite QD sensitizer can be prepared easily and reproducibly under normal atmospheric conditions and then utilized as a model QD sensitizer with improved activity as compared to CdS which has been adopted so far in most cases. When combining a narrow band gap of QD (CuS) with a wider one (CdS), control of the relative ratio of the amount between the two QDs is important to obtain an efficient nanocomposite sensitizer, which was also the case in the reported result for the PbS/CdS QD sensitizer. In overall, a small amount of CuS at the interface between TiO2 and main CdS QD seems to play a critical role of 1) inducing more deposition of CdS in the less cycle of SILAR process for optimum size QD and 2) stabilizing hole in the localized state of CuS for effective charge separation, finally leading to an enhancement of overall conversion efficiency. However, detailed investigations are needed to clarify the interface between the two different QDs. In addition, using this new QD composite as the first sensitizer, it was possible to make an efficient multilayer and hybrid sensitizer system through the step-by-step adsorption of cations/anions or molecular dye. This CuS/CdS composite sensitizer is also expected to fit well in the research of photocatalysts in which better photocatalytic activity is needed than what is provided by CdS. We believe that our findings will be helpful in designing and preparing new complex QD sensitizers, enabling them to compete in future photovoltaic and photocatalytic research.

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Acknowledgement H. J. Lee acknowledges the financial support by the National Research Foundation (no. 2012R1A1A1015528) and by the “Human Resource Development (project name: Advanced track for Si-based solar cell materials and devices, project number: 20124010100660)” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy.

Supporting Information Available Absorption spectra from CdS and CuS/CdS QDs after each SILAR cycle up to 3rd, 5th, and 7th CdS deposition over transparent TiO2 film are given. Magnified TEM images of CdS(7) and CuS(1)/CdS(7) QDs deposited by SILAR process onto the surface of TiO2 films are shown. This information is available free of charge via the Internet at http://pubs.acs.org/.

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CdS Quantum Dot Composite Sensitizer and Its Applications to

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