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Enhanced performance of CdTe quantum dot sensitized solar cell via anion exchanges
Accepted Manuscript Enhanced performance of CdTe quantum dot sensitized solar cell via anion exchanges Xuehua Shen , Jianguang Jia , Yuan Lin , Xiaowen Zhou PII:
S0378-7753(14)02039-4
DOI:
10.1016/j.jpowsour.2014.12.022
Reference:
POWER 20300
To appear in:
Journal of Power Sources
Received Date: 9 September 2014 Revised Date:
29 November 2014
Accepted Date: 6 December 2014
Please cite this article as: X. Shen, J. Jia, Y. Lin, X. Zhou, Enhanced performance of CdTe quantum dot sensitized solar cell via anion exchanges, Journal of Power Sources (2015), doi: 10.1016/ j.jpowsour.2014.12.022. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphic Enhanced performance of CdTe quantum dot sensitized solar cell via anion exchanges
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Xuehua Shen, Jianguang Jia, Yuan Lin, Xiaowen Zhou
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Enhanced performance of CdTe quantum dot sensitized solar cell via anion exchanges Xuehua Shen, Jianguang Jia, Yuan Lin, Xiaowen Zhou Highlights
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Eco-friendly synthesis of CdTe/CdS QDs is developed through an anion exchange. Light harvesting is increased due to the extension of light absorption range. Charge recombination is reduced while charge transport and collection is improved. A power conversion efficiency of 2.44% is achieved which is 15 times that of CdTe based QDSSC.
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Enhanced performance of CdTe quantum dot
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sensitized solar cell via anion exchanges Xuehua Shen†, ‡, Jianguang Jia†*, Yuan Lin§, Xiaowen Zhou§*
†Department of Physical Chemistry, School of Science, Beijing University of Chemical
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Technology, Beijing 100029, China
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§Key Laboratory of Photochemistry, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡College of Life Science, Tarim University, Alar 843300, China
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Corresponding authors: J. Jia ([email protected]), X. Zhou ([email protected])
Abstract: We report on an eco-friendly way to prepare CdTe/CdS quantum dots for quantum dot sensitized solar cell (QDSSC). CdTe/CdS quantum dots are synthesized through an anion
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exchange between CdTe quantum dots (QDs) and S2- in aqueous solution at low temperature
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under ambient condition. The resultant QDs are bonded onto TiO2 with the help of thioglycolic acid bifunctional molecule. The uniform distribution of QDs throughout the TiO2 mesoporous film depth is confirmed by the energy dispersive X-ray (EDX) elemental mapping. Absorption, dark current, impedance spectroscopy, and intensity-modulated photocurrent analyses prove that anion exchange can efficiently extend the absorption range, suppress the charge recombination, increase the electron injection as well as accelerate the electron transportation in the cell. In
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combination with CdS post-treatment, a solar-to-energy conversion efficiency of 2.44% is achieved for CdTe/CdS QDSSC, which is more than 15 times that of the CdTe based cell.
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Keywords: Anion exchange, Cadmium telluride, Quantum dot sensitized solar cell, Solar-toenergy conversion. 1. Introduction:
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Quantum dot sensitized solar cell (QDSSC), in which a semiconductor of narrow band-gap served as the photosensitizer, has attracted growing research interest as the next generation solar
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cells due to the superior intrinsic properties of quantum dots (QDs), including high molar extinction coefficients, easily tunable band-gaps, large intrinsic dipole moments as well as possible multiple carrier generation [1-3]. Although large progress have been achieved, which leads to a significant increase of the energy conversion efficiency from less than 1% to over 6%
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[4-7], the cell efficiency of QDSSC is still far behind either the efficiency of dye sensitized solar cell (DSSC) or the theoretical thermodynamic efficiency of QDSSC (∼ 66%). This is partly ascribed to the insufficient light absorption of QDs, the low electron injection efficiency and high
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charge recombination at the surface/interfaces of semiconductors. In QDSSC, the incident light is absorbed by QD sensitizer to produce electron-hole pair, which
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is followed by the injection of electrons into the large band-gap semiconductor (mostly TiO2) and the scavenging of holes by a redox couple in electrolyte. As the core part of QDSSC, the property of QD sensitizer plays a very important role in determining the performance of QDSSCs. Basically, an efficient sensitizer should have broad absorption range as well as a fast electron injection to TiO2. Therefore, the searching of panchromatic QD sensitizers with suitable band structure has been one of the core tasks for the enhancement of cell efficiency of QDSSCs.
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Various kinds of materials such as PbS, CdS, CdSe, CdTe, InAs, In2S3, PbSeS, CuInS2, AgBiS2, CdSexTe1-x, CdS/CdSe, CdTe/CdSe etc have been explored [8-20]. Of particular interest in this context are the heterostructured QDs, in which two kinds of semiconductors are combined to
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form a core/shell structure [6,15,19-26]. Due to their different band alignment of the constituent semconductors, core/shell structured QDs can efficiently promote charge separation by reducing the charge recombination. In addition, the formation of core/shell structure can also shift the
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absorption edges to longer wavelength related to its two constituents, originated from its decreased effective bandgap that determined by the lower conduction band and higher valence
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band of the two constituents, and thus leads to the extending of light absorption. These features of core/shell QDs render them as a good light harvesting material for QDSSC. For instance, Wang et al fabricated a QDSSC using type-Ⅱ CdTe/CdSe core/shell nanocrystals as the photosensitizer [20]. Cell performance investigations showed that cell efficiency of the resultant
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QDSSC (6.76%) is much higher than that of using plain CdSe QDs (4.49%). Beside the type-Ⅱ QDs, type-I QDs was also employed as the photosensitizer for QDSSC. By using an inverted type-I CdS/CdSe core/shell QD, a QDSSC with cell efficiency of 5.32% was fabricated by Pan et
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al [21]. Nevertheless, in almost all the cases, core/shell structured QDs were synthesized through an epitaxial growth route (mostly an organometallic route), in which a high reaction temperature
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or toxic organics are always necessary to obtain QDs of high qualities. In this paper, we report an alternative way to prepare core/shell structured QDs for QDSSC. We selected CdTe because of its nearly ideal band gap (1.5 eV) and high optical absorption coefficient (>104 cm-1). CdTe/CdS QDs were obtained through a Te2- for S2- anion exchange in aqueous solution. QDs were tethered onto mesoporous TiO2 film by a linker-assisted assembly route with the aid of thioglycolic acid. Photovoltaic performance investigations showed that the
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cell efficiency of the resultant CdTe/CdS sensitized QDSSC is more that 15 times that of the CdTe sensitized QDSSC, which is ascribed to the enhanced light harvesting and charge separation, as well as the decreased charge recombinations due to its superior properties of type-
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Ⅱ structure of CdTe/CdS core/shell QDs. Since all the processes were performed in aqueous solution at lower temperature (<100℃) without using any toxic organics, this method may provide a low-cost and environmentally friendly way for construction of efficient QDSSC.
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2. Experimental section:
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2.1. Preparation of CdTe/CdS QDs: CdTe NCs were prepared by an aqueous solution method as previously reported [27]. Typically, CdCl2·5H2O was dissolved in distilled water and thioglycolic acid (TGA) was added under stirring. After TGA was complete dissolved, the pH of the solution was adjusted to ca. 11 by dropwise addition of 1 M NaOH solution. H2Te gas was generated by the reaction of 1 M HCl with 1M KHTe and passed through the solution carried by
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N2 gas under violent stirring. After stirring for ca. 30 min in an ice-water bath, the reaction mixture was refluxed at 90℃ to get TGA stabilized CdTe NCs. CdTe/CdS QDs were obtained by an anion exchange route under ambient condition. Typically, the as prepared CdTe QDs was
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precipitated and washed with ethanol at least three times to remove the unreacted reactants and
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TGA. After redissolved in distill water, 1 M NaOH solution was added to adjust the pH of the CdTe solution mixture to ca. 10. The resultant CdTe NCs solution was then mixed with Na2S solution under ambient condition at 90℃ to initiate the anion exchange for producing CdTe/CdS QDs.
2.2. Fabrication of QDSSC: Nanoporous TiO2 was deposited onto a cleaned FTO glass substrate (F-doped SnO2, resistance: 7 Ω sq-1) by doctor-blading a TiO2 paste, being prepared by mixing TiO2 power (Degussa P25), n-butyl alcohol and Triton [28]. The thickness of the film
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was ca. 12 µm determined by SEM measurements. After drying at room temperature, TiO2 films were sintered at 450℃ for 30 min in air. While cooled down to 100℃, the films were soaked in
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CdTe or CdTe/CdS suspended solution at room temperature under ambient atmosphere for 24 hours for the binding of QDs. Before that a small amount of TGA was added to CdTe/CdS QDs suspended solution to assistant the binding of QDs onto TiO2. After washed with water and dried with N2, the electrode films were subjected to further post-treatments. Two kinds of post-
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treatments were performed using a successive ionic layer adsorption and reaction (SILAR) method. For ZnS treatment, QD-anchored electrodes were dipped alternatively into 0.1 M
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Zn(NO3)2 and 0.1 M Na2S alternately for 1 min/dip at room temperature (one cycle), while the samples were thoroughly rinsed with distilled water between dips. Similar process was used for CdS treatment excepted that the use of 0.1 M CdCl2 instead of 0.1 M Zn(NO3)2. The thicknesses of ZnS and CdS coatings were adjusted by controlling the repeating cycles of the SILAR
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procedure.
QDSSCs were assembled into a sandwich structure using the QD-anchored electrode as working electrode, a Cu2S on brass as the counter electrode and a 0.5 M S, 2.0 M Na2S and 0.2
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M KCl in water/ethanol (7:3 in volume) mixed solution as the electrolyte. 2.3. Chracterizations: UV–vis absorption spectra were measured by using Lab Tech-UV2000
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UV–Vis spectrophotometer. Scanning electron microscope (SEM) was performed using a Hitachi-S4700. Energy dispersive X-ray (EDX) spectrometers fitted to electron microscopes were used for elemental analysis. Transmission electron microscope (TEM) was recorded on a JEOL JEM-2100F, operating at an acceleration voltage of 200 kV.
The crystallographic
structures of the CdTe NCs were analyzed by X-ray diffraction (XRD) (Shimadzu 2500VB2) at 40 KV, 200 mA with Ni filter and Cu α-radiation.
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Photocurrent–voltage measurements were performed with a computer-programmed Keithley 2611 Source Meter at room temperature under illumination of AM1.5 simulated sunlight (Oriel 91160-1000, 100 mW cm−2). The incident light was calibrated with a power meter (model 350)
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and a detector (model 262). The active area of the cell is 0.2 cm2. Incident photo-to-current conversion efficiency (IPCE) was measured using a 500W xenon lamp combined a monochromator, where the incident photons were determined by using a standard silicon cell.
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Intensity-modulated photocurrent spectroscopy (IMPS) were performed using a green light emitting diode (λmax= 520 nm) driven by a solartron 1255B frequency-response analyzer. The
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LED provided both DC and AC components of the illumination. Electron Impedance Spectroscopy (EIS) was carried out at open-circuit voltage under illumination, using a frequency response analyzer (Solartron 1255B) coupled with an electrochemical interface system (Solartron analytical SI, 1287). The frequency range was 10-1 to 105 Hz and the amplitude of the AC voltage
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was 10 mV. 3. Results and discussions:
CdTe/CdS core/shell QDs were prepared through an anion exchange between CdTe QDs with
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S2-. CdTe QDs were synthesized via an aqueous solution in the presence of TGA. After precipitated and washed with ethanol, CdTe QDs were redissolved in water. The resultant CdTe
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suspension was then mixed with Na2S solution and refluxed at 90℃, during which the Te2- for S2- anion exchanges occurred. The replacements of Te2- in the outer-shell of CdTe with S2produce a layer of CdS on CdTe core, forming a CdTe/CdS core/shell nanostructure. This is supported by the X-ray powder diffraction (XRD) and transmission electron microscope (TEM) investigations that the XRD diffraction peaks of CdTe QDs after anion exchange is slightly shifted to higher angle while both morphology and sizes are kept almost unchanged (Fig. S1).
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Fig. 1A shows the typical absorption spectra of CdTe NCs before and after anion exchanges at 90℃ for different period of time. The as prepared CdTe QDs exhibits obvious excitonic absorption, indicating its narrow size distribution. The size of CdTe QD was estimated to be ca. 3
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nm. When anion exchanges were carried out, continuous shift of the absorption to longer wavelength was observed. This is ascribed to the formation of type-ⅡCdTe/CdS QDs due to the Te2- for S2- exchanges, which leads to the decrease of effective bandgap that determined by the
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conduction band of CdS and valence band of CdTe as shown in Fig.1 B [29]. With the help of
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TGA, CdTe/CdS QDs were tethered on TiO2 porous films by immersing TiO2 film electrodes in the QD suspended solution for 24 hours. During this period, gradual coloration of the TiO2 films was observed (inset of Fig. 1C), which confirms the success deposition of QDs from solution onto TiO2 films. The corresponding absorption spectra of the QD decorated TiO2 films are shown in Fig. 1C. As expected, the absorption of the films showed similar red shifts as those
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shown in Fig. 1A, demonstrating that the absorption range of QD dot decorated film is expanded to the longer wavelength. Such an extension of light absorption is expected to be of great benefit for enhancing the light harvesting and thus the cell efficiency of quantum dot sensitized solar
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cell.
Fig. 2A shows the cross-sectional scanning electron microscopy (SEM) image after deposition
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of QDs. The film is composed of TiO2 nanoparticles, possessing a porous structure. The film thickness is measured to be ca. 12 µm. Fig 2 B-D shows the elements mapping of Cd, Te and S throughout the film. Clearly, it can be found that Cd, Te and S are well distributed throughout the whole film. The uniform distribution of Cd, Te and S suggests that CdTe/CdS QDs can penetrate throughout the mesoporous film depth leading to the uniform coverage of QDs around the porous TiO2 film.
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The QD/TiO2 films were employed to construct Sandwich-type QDSSCs using Cu2S on brass as the counter electrode and polysulfide composed of 0.5 M S, 2.0 M Na2S and 0.2 M KCl dissolved in water/ethanol as the electrolyte. Before that, a thin ZnS layer was deposited onto the
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QD/TiO2 films to improve the stability of QDSSC as well as to reduce the charge recombination at the TiO2/electrolyte and QD/electrolyte interfaces [30, 31]. Fig. 3 presents the typical photocurrent density - photovoltage (J-V) curves of QDSSCs under AM 1.5 irradiation (100 mW
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cm-2), while the corresponding open-circuit voltages (Voc), short-circuit current densities (Jsc), and overall solar-to-energy conversion efficiencies (η) were gathered in Fig. 4. The cell
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performance of the pristine CdTe based device is quite poor (Voc=255 mV, Jsc=0.65 mA/cm2 and =0.15%). This is in accordance with that of reported before [32]. When anion exchanges were performed, however, significant increases in Voc, Jsc as well as η were observed. Particularly, we noted that the cell performance is close related to the reaction time of the anion exchanges. As
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demonstrated in Fig. 4, Voc is rapidly increased from 255 mV for the as prepared CdTe QDs to ca. 455 mV for QDs after 3 h anion exchanges. After that, it keeps almost unchanged even further prolonging the exchange time. In comparison, Jsc is continuously increased from 0.65
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mA/cm2 for the as prepared CdTe QDs to 4.89 mA/cm2 for QDs after anion exchange for 24 h, although the increment is somewhat decreased after the first 1 h anion exchanges. As a result,
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cell efficiency was considerably improved from 0.15% to over 1.4%. In a controlled experiment, we prepared a CdTe/CdS cosensitized TiO2 photoanode by depositing CdS onto a CdTe QDs pre-decorated TiO2 film through a SILAR process. After ZnS deposition, Sandwich-type cell (denoted as CdTe-CdS) was constructed for J-V measurements. As shown in Fig. 5, the deposition of CdS on CdTe leads to the increase of VOC, JSC and cell efficiency compared to the CdTe cell (Fig. 5 a and b), owing to its passivation effect of CdS coating. Nevertheless, we noted that the cell efficiency of CdTe-CdS cell (0.30%) is much lower than those devices employing 8
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CdTe/CdS QDs prepared through anion exchanges (Fig. 5 b and c). This result indicates that ion exchange is superior to SILAR for the preparation of the core/shell QDs for QDSSC. This may partly ascribed to the excellent CdTe/CdS interface produced by Te2- for S2- anion exchange that
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replace the sublatice of Te2- in CdTe crystals by S2-, resulting in the decrease of charge recombination and increase of the electron injection from QDs to TiO2 as will be shown later. Furthermore, CdTe/CdS based QDSSC was post-treated by coating a thin CdS layer ahead of
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the deposition of ZnS passivation layer, which was performed by twice dipping the CdTe/CdS QD sensitized electrode film into Cd2+ and S2- solution alternatively. The plot d in Fig. 5 shows
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the typical J-V curve of the resultant QDSSC. As clearly demonstrated in the figure, we found that the deposition of CdS thin layer significantly improved the cell performance (Fig. 5 c and d). In particular, Voc is increased from 455 mV to 505 mV, while Jsc is increased from 4.89 mA/cm2 to 6.73mA/cm2. Consequently, an overall energy conversion efficiency reached to 2.44% was
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achieved for CdTe/CdS based QDSSC with CdS followed by ZnS post-treatments, showing near 70% increment compared to the QDSSC with the same CdTe/CdS QDs but only ZnS posttreatment. It needs to point out that the light absorption caused by the CdS post-treatment is
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neglected because of the very thin layer of CdS deposition, which was also supported by the fact that a very small photocurrent of ca. 0.1 mA cm-2 was measured when the same amount of CdS
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was deposited on TiO2. The benefit role of CdS post-treatment is in accordance with our previous report, ascribable to the increased charge injection and decreased charge recombination originated from the deposition of CdS [33]. Fig.6 shows the incident photon to current conversion efficiency (IPCE) of QDSSC with different configurations. Compared to CdTe based cell, the anion exchange leads to an obvious increase in IPCE throughout the entire spectrum, whereas the CdS post-treatment results in its further increase of IPCE. This result is consistence
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with the variation trend of JSC as shown in Fig. 5, verifying the benefit role of anion exchange and CdS post-treatment in enhancing the photocurrent of QDSSC. To unveil the superior photovaltaic properties of CdTe/CdS QD sensitized solar cell, dark
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current, electrochemical impedance spectroscopy (EIS) and intensity-modulated photocurrent spectroscopy (IMPS) investigations have been performed. The lower part of Fig. 5 compares the dark current of QDSSC with different configurations. One can find from this figure that the anion
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exchanges triggered a significant decrease in dark current, which was further diminished by the introduction of CdS post-treatment. This result suggests that anion exchange and CdS post-
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treatment can suppress the electron recombination in the cell. Fig. 7 shows the Nyquist plots (105 Hz ~ 10-1 Hz) of QDSSCs at open circuit under irradiation (AM1.5, 100 mW.cm-2). The charge transfer resistance is obtained by fitting the impedance data employing the equivalent circuit shown in the inset of Fig. 7, where Rs is the equivalent series resistance, Rct and CPE represent
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the charge transfer resistance and chemical capacitance at the TiO2/QDs/electrolyte interface. Since the same electrolyte and counter electrode were used, all the devices give out almost the same Rs (Table 1). However, there is an apparent difference in Rct. The Rct value of the
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CdTe/CdS based device is much lower than those of CdTe and CdTe-CdS based cells, while the CdS post-treatment leads to the further decrease in Rct. The decreased Rct indicates that the
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charge recombination is suppressed, while the electrons injection from QDs to the conduction band of TiO2 is increased. This is attributed to the decreased surface traps due to the formation of CdS on CdTe by Te2- for S2- anion exchange and the cascade band alignment of CdTe and CdS that favorable to increase electron injection from QDs to TiO2. Furthermore, the electron transport dynamics are characterized by IMPS measurement. Fig. 8 illustrates the typical IMPS spectra of cell devices with different cell configurations. The transit time (τd) of photogenerated electron is determined as before, using the equation of τd =1/2πfmin, where fmin is the minimum 10
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frequency in IMPS plot [34]. As displayed in Fig. 8, we found that τd for CdTe/CdS cell is much shorter than those of CdTe and CdTe-CdS cells, while the CdS post-treatment leads to further decrease of τd. This indicates that the formation of CdTe/CdS core/shell structure by Te2- for S2-
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anion exchange and the introduction of CdS deposition layer enhanced the electron transportation rate within the photoanode film. This observation is related to the larger charge generation in the CdTe/CdSe cells, which brings forward an enhanced electron concentration in the TiO2 substrate
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of the cells.
In general, the photocurrent of a solar cell can be estimated by J SC = qηlhηccηinj I 0 , where I0 is the
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incident photon flux, ηlh, ηcc and ηinj are the light harvesting, electron collection and injection efficiencies. As proved by UV-Vis, J-V, EIS and IMPS measurements, the Te2- for S2- anion exchange led to the extension of the light absorption range, the increases of the electron injections from QDs to TiO2 and the electron transportation rate within the film, as well as the
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suppression of the charge recombination in the cell. As a result, ηlh, ηcc and ηinj are increased, and thus leading to the enhancement of photocurrent. On the other hand, it is known that the
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photovoltage Voc of a cell is determined by the Jsc and J0:
Voc =
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where kB is the Boltzmann constant, T is the temperature,
is a parameter related with the
nonlinear recombination [35]. Therefore, it is expected that the enhanced Jsc and decreased J0 will increase the Voc, and thereby the cell efficiency. The benefit role of the introduction of CdS post-treatment is ascribed to its further passivation of QDs and the blockage of the TiO2 surface, resulting in the further inhibition of charge recombination within the cell and thus improves the cell efficiency [33].
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3. Conclusions: In summary, we have reported a novel method to prepare CdTe/CdS core/shell quantum dots for QDSSC. CdTe/CdS QDs were prepared through an anion exchange between CdTe QDs
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and S2- in aqueous solution under ambient condition at low temperature. QDs were bonded onto TiO2 with the help of TGA bifunctional molecule. EDX elemental mapping investigations demonstrate that the QDs are uniformly distributed throughout the mesoporous film depth.
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Photovoltaic investigations showed that the resultant QDSSC exhibits much higher cell efficiency compared to the CdTe cell as well as the device made from CdTe/CdS core/shell QDs
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prepared by SILAR route. The superior property of CdTe/CdS based cell is ascribed to its typeⅡ core/shell structures of CdTe/CdS QDs formed by T2- for S2- anion exchange, that can efficiently extend light absorption, suppress the charge recombination in the cell, and meanwhile increase electron injections from QDs to TiO2 and the electron transportation within the film as
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proved by the UV-vis, J-V, IS and IMPS investigations. In combination with CdS post-treatment, CdTe/CdS QD based QDSSC achieved an overall energy conversion efficiency of 2.44%, showing more than 15 times that of the CdTe cell. The cell efficiency might be further improved
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by optimizing the photoanode, such as the use of QD sensitizer with longer absorption onsets via the modification of the synthetic route etc. Nevertheless, the present work provides an alternative
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eco-friendly efficient way to construct QDSSC with enhanced solar to energy conversion efficiency.
Acknowledgments
This work was supported by the Fundamental Research Funds for the Central Universities (YS1406) References:
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[33] L. Mu, C. Liu, J. Jia, X. Zhou, Y. Lin, J. Mater. Chem. A 1 (2013) 8353-8357. [34] X. Yan, L. Feng, J. Jia, X. Zhou, Y. Lin, J. Mater. Chem. A 1 (2013) 5347-5352.
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[35] F. Fabregat-Santiago, G. Garcia-Belmonte, I. Mora-Sero, J. Bisquert, Phys. Chem. Chem.
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Phys. 13 (2011) 9083−9118.
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Figure captions: Fig.1 (A) Absorption spectra of a series of samples collected at different time points (0, 1, 3, 7,
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11, 15, and 24 h) during the anion exchange reactions at 90℃. (B) Schematic diagram of CdTe/CdS core/shell quantum dot indicating the relative band positions. (C) Absorption spectra
QD-sensitized TiO2 films.
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of QD-sensitized TiO2 films using the QDs shown in (A). Inset in (C) show the photographs of
Fig.2 (A) cross-sectional SEM image of CdTe/CdS QD sensitized TiO2 film, (B-D) elemental
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mapping of Cd, Te and S throughout the film depth.
Fig.3 J-V cureves of QDSSCs using CdTe QDs before (a) and after anion exchange for 1h (b), 3h (c), 7h (d), 11h (e), 15 (f), and 24h (g).
Fig.4 Dependent of open-circuit voltage (VOC), short-circuit current density (JSC) and solar-to-
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energy conversion efficiency (η) on the anion exchange time. The dash lines are guide for eye. Fig. 5 J-V cureves of QDSSCs with different photoanode under illumination (upper) and dark
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(lower): (a) CdTe QD sensitized TiO2 film post-treated by ZnS, (b) CdTe/CdS QD sensitized TiO2 film prepared by depositing CdS through a SILAR process onto a CdTe QDs pre-decorated
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TiO2 film and post-treated by ZnS, (c) CdTe/CdS QD sensitized TiO2 film using the anion exchanged CdTe/CdS QDs and post-treated by ZnS, (d) same CdTe/CdS QD sensitized TiO2 film of (c) post-treated by CdS and ZnS. Fig. 6 Incident photon-to-current spectra of QDSSCs with different photoanode: (a) CdTe QD sensitized TiO2 film post-treated by ZnS, (b) anion exchanged CdTe/CdS QD sensitized TiO2 post-treated by ZnS, (c) anion exchanged CdTe/CdS QD sensitized TiO2 film post-treated by
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CdS and ZnS. Fig. 7 Electrochemical impedance spectra of QDSSCs with different configurations as shown in
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Fig. 5. Fig. 8 Intensity-modulated photocurrent spectra of QDSSCs with different configurations as
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shown in Fig. 5.
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Fig. 1
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Fig. 2
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1.5
η
1.0 0.5
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Voc / mV
0.0 500 400
5 4 3 2 1 0
10
30
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Time/ h
20
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JSC / mA cm
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300
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Fig. 7
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Fig. 6
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Table 1. Fitted impedance parameters of different QDSSCs.
η (%)
Rct(Ω)
CdTe-ZnS
15.5
225.8
255
0.65
0.15
CdTe-CdS-ZnS
18.0
162.2
385
1.15
0.30
CdTe/CdS-ZnS
15.8
144.0
485
4.89
1.44
CdTe/CdS-CdS-ZnS
24.0
99.9
505
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Rs(Ω)
6.73
2.44
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Voc (mV)
JSC (mAcm2)
Photoanode
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Fig. S1 X-ray powder diffraction (XRD) patterns of CdTe NCs before and after anion exchanges at 90℃. Insets show their corresponding TEM imagines.
S0378-7753(14)02039-4
DOI:
10.1016/j.jpowsour.2014.12.022
Reference:
POWER 20300
To appear in:
Journal of Power Sources
Received Date: 9 September 2014 Revised Date:
29 November 2014
Accepted Date: 6 December 2014
Please cite this article as: X. Shen, J. Jia, Y. Lin, X. Zhou, Enhanced performance of CdTe quantum dot sensitized solar cell via anion exchanges, Journal of Power Sources (2015), doi: 10.1016/ j.jpowsour.2014.12.022. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphic Enhanced performance of CdTe quantum dot sensitized solar cell via anion exchanges
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Xuehua Shen, Jianguang Jia, Yuan Lin, Xiaowen Zhou
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Enhanced performance of CdTe quantum dot sensitized solar cell via anion exchanges Xuehua Shen, Jianguang Jia, Yuan Lin, Xiaowen Zhou Highlights
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Eco-friendly synthesis of CdTe/CdS QDs is developed through an anion exchange. Light harvesting is increased due to the extension of light absorption range. Charge recombination is reduced while charge transport and collection is improved. A power conversion efficiency of 2.44% is achieved which is 15 times that of CdTe based QDSSC.
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Enhanced performance of CdTe quantum dot
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sensitized solar cell via anion exchanges Xuehua Shen†, ‡, Jianguang Jia†*, Yuan Lin§, Xiaowen Zhou§*
†Department of Physical Chemistry, School of Science, Beijing University of Chemical
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Technology, Beijing 100029, China
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§Key Laboratory of Photochemistry, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡College of Life Science, Tarim University, Alar 843300, China
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Corresponding authors: J. Jia ([email protected]), X. Zhou ([email protected])
Abstract: We report on an eco-friendly way to prepare CdTe/CdS quantum dots for quantum dot sensitized solar cell (QDSSC). CdTe/CdS quantum dots are synthesized through an anion
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exchange between CdTe quantum dots (QDs) and S2- in aqueous solution at low temperature
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under ambient condition. The resultant QDs are bonded onto TiO2 with the help of thioglycolic acid bifunctional molecule. The uniform distribution of QDs throughout the TiO2 mesoporous film depth is confirmed by the energy dispersive X-ray (EDX) elemental mapping. Absorption, dark current, impedance spectroscopy, and intensity-modulated photocurrent analyses prove that anion exchange can efficiently extend the absorption range, suppress the charge recombination, increase the electron injection as well as accelerate the electron transportation in the cell. In
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combination with CdS post-treatment, a solar-to-energy conversion efficiency of 2.44% is achieved for CdTe/CdS QDSSC, which is more than 15 times that of the CdTe based cell.
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Keywords: Anion exchange, Cadmium telluride, Quantum dot sensitized solar cell, Solar-toenergy conversion. 1. Introduction:
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Quantum dot sensitized solar cell (QDSSC), in which a semiconductor of narrow band-gap served as the photosensitizer, has attracted growing research interest as the next generation solar
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cells due to the superior intrinsic properties of quantum dots (QDs), including high molar extinction coefficients, easily tunable band-gaps, large intrinsic dipole moments as well as possible multiple carrier generation [1-3]. Although large progress have been achieved, which leads to a significant increase of the energy conversion efficiency from less than 1% to over 6%
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[4-7], the cell efficiency of QDSSC is still far behind either the efficiency of dye sensitized solar cell (DSSC) or the theoretical thermodynamic efficiency of QDSSC (∼ 66%). This is partly ascribed to the insufficient light absorption of QDs, the low electron injection efficiency and high
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charge recombination at the surface/interfaces of semiconductors. In QDSSC, the incident light is absorbed by QD sensitizer to produce electron-hole pair, which
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is followed by the injection of electrons into the large band-gap semiconductor (mostly TiO2) and the scavenging of holes by a redox couple in electrolyte. As the core part of QDSSC, the property of QD sensitizer plays a very important role in determining the performance of QDSSCs. Basically, an efficient sensitizer should have broad absorption range as well as a fast electron injection to TiO2. Therefore, the searching of panchromatic QD sensitizers with suitable band structure has been one of the core tasks for the enhancement of cell efficiency of QDSSCs.
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Various kinds of materials such as PbS, CdS, CdSe, CdTe, InAs, In2S3, PbSeS, CuInS2, AgBiS2, CdSexTe1-x, CdS/CdSe, CdTe/CdSe etc have been explored [8-20]. Of particular interest in this context are the heterostructured QDs, in which two kinds of semiconductors are combined to
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form a core/shell structure [6,15,19-26]. Due to their different band alignment of the constituent semconductors, core/shell structured QDs can efficiently promote charge separation by reducing the charge recombination. In addition, the formation of core/shell structure can also shift the
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absorption edges to longer wavelength related to its two constituents, originated from its decreased effective bandgap that determined by the lower conduction band and higher valence
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band of the two constituents, and thus leads to the extending of light absorption. These features of core/shell QDs render them as a good light harvesting material for QDSSC. For instance, Wang et al fabricated a QDSSC using type-Ⅱ CdTe/CdSe core/shell nanocrystals as the photosensitizer [20]. Cell performance investigations showed that cell efficiency of the resultant
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QDSSC (6.76%) is much higher than that of using plain CdSe QDs (4.49%). Beside the type-Ⅱ QDs, type-I QDs was also employed as the photosensitizer for QDSSC. By using an inverted type-I CdS/CdSe core/shell QD, a QDSSC with cell efficiency of 5.32% was fabricated by Pan et
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al [21]. Nevertheless, in almost all the cases, core/shell structured QDs were synthesized through an epitaxial growth route (mostly an organometallic route), in which a high reaction temperature
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or toxic organics are always necessary to obtain QDs of high qualities. In this paper, we report an alternative way to prepare core/shell structured QDs for QDSSC. We selected CdTe because of its nearly ideal band gap (1.5 eV) and high optical absorption coefficient (>104 cm-1). CdTe/CdS QDs were obtained through a Te2- for S2- anion exchange in aqueous solution. QDs were tethered onto mesoporous TiO2 film by a linker-assisted assembly route with the aid of thioglycolic acid. Photovoltaic performance investigations showed that the
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cell efficiency of the resultant CdTe/CdS sensitized QDSSC is more that 15 times that of the CdTe sensitized QDSSC, which is ascribed to the enhanced light harvesting and charge separation, as well as the decreased charge recombinations due to its superior properties of type-
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Ⅱ structure of CdTe/CdS core/shell QDs. Since all the processes were performed in aqueous solution at lower temperature (<100℃) without using any toxic organics, this method may provide a low-cost and environmentally friendly way for construction of efficient QDSSC.
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2. Experimental section:
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2.1. Preparation of CdTe/CdS QDs: CdTe NCs were prepared by an aqueous solution method as previously reported [27]. Typically, CdCl2·5H2O was dissolved in distilled water and thioglycolic acid (TGA) was added under stirring. After TGA was complete dissolved, the pH of the solution was adjusted to ca. 11 by dropwise addition of 1 M NaOH solution. H2Te gas was generated by the reaction of 1 M HCl with 1M KHTe and passed through the solution carried by
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N2 gas under violent stirring. After stirring for ca. 30 min in an ice-water bath, the reaction mixture was refluxed at 90℃ to get TGA stabilized CdTe NCs. CdTe/CdS QDs were obtained by an anion exchange route under ambient condition. Typically, the as prepared CdTe QDs was
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precipitated and washed with ethanol at least three times to remove the unreacted reactants and
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TGA. After redissolved in distill water, 1 M NaOH solution was added to adjust the pH of the CdTe solution mixture to ca. 10. The resultant CdTe NCs solution was then mixed with Na2S solution under ambient condition at 90℃ to initiate the anion exchange for producing CdTe/CdS QDs.
2.2. Fabrication of QDSSC: Nanoporous TiO2 was deposited onto a cleaned FTO glass substrate (F-doped SnO2, resistance: 7 Ω sq-1) by doctor-blading a TiO2 paste, being prepared by mixing TiO2 power (Degussa P25), n-butyl alcohol and Triton [28]. The thickness of the film
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was ca. 12 µm determined by SEM measurements. After drying at room temperature, TiO2 films were sintered at 450℃ for 30 min in air. While cooled down to 100℃, the films were soaked in
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CdTe or CdTe/CdS suspended solution at room temperature under ambient atmosphere for 24 hours for the binding of QDs. Before that a small amount of TGA was added to CdTe/CdS QDs suspended solution to assistant the binding of QDs onto TiO2. After washed with water and dried with N2, the electrode films were subjected to further post-treatments. Two kinds of post-
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treatments were performed using a successive ionic layer adsorption and reaction (SILAR) method. For ZnS treatment, QD-anchored electrodes were dipped alternatively into 0.1 M
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Zn(NO3)2 and 0.1 M Na2S alternately for 1 min/dip at room temperature (one cycle), while the samples were thoroughly rinsed with distilled water between dips. Similar process was used for CdS treatment excepted that the use of 0.1 M CdCl2 instead of 0.1 M Zn(NO3)2. The thicknesses of ZnS and CdS coatings were adjusted by controlling the repeating cycles of the SILAR
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procedure.
QDSSCs were assembled into a sandwich structure using the QD-anchored electrode as working electrode, a Cu2S on brass as the counter electrode and a 0.5 M S, 2.0 M Na2S and 0.2
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M KCl in water/ethanol (7:3 in volume) mixed solution as the electrolyte. 2.3. Chracterizations: UV–vis absorption spectra were measured by using Lab Tech-UV2000
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UV–Vis spectrophotometer. Scanning electron microscope (SEM) was performed using a Hitachi-S4700. Energy dispersive X-ray (EDX) spectrometers fitted to electron microscopes were used for elemental analysis. Transmission electron microscope (TEM) was recorded on a JEOL JEM-2100F, operating at an acceleration voltage of 200 kV.
The crystallographic
structures of the CdTe NCs were analyzed by X-ray diffraction (XRD) (Shimadzu 2500VB2) at 40 KV, 200 mA with Ni filter and Cu α-radiation.
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Photocurrent–voltage measurements were performed with a computer-programmed Keithley 2611 Source Meter at room temperature under illumination of AM1.5 simulated sunlight (Oriel 91160-1000, 100 mW cm−2). The incident light was calibrated with a power meter (model 350)
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and a detector (model 262). The active area of the cell is 0.2 cm2. Incident photo-to-current conversion efficiency (IPCE) was measured using a 500W xenon lamp combined a monochromator, where the incident photons were determined by using a standard silicon cell.
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Intensity-modulated photocurrent spectroscopy (IMPS) were performed using a green light emitting diode (λmax= 520 nm) driven by a solartron 1255B frequency-response analyzer. The
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LED provided both DC and AC components of the illumination. Electron Impedance Spectroscopy (EIS) was carried out at open-circuit voltage under illumination, using a frequency response analyzer (Solartron 1255B) coupled with an electrochemical interface system (Solartron analytical SI, 1287). The frequency range was 10-1 to 105 Hz and the amplitude of the AC voltage
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was 10 mV. 3. Results and discussions:
CdTe/CdS core/shell QDs were prepared through an anion exchange between CdTe QDs with
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S2-. CdTe QDs were synthesized via an aqueous solution in the presence of TGA. After precipitated and washed with ethanol, CdTe QDs were redissolved in water. The resultant CdTe
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suspension was then mixed with Na2S solution and refluxed at 90℃, during which the Te2- for S2- anion exchanges occurred. The replacements of Te2- in the outer-shell of CdTe with S2produce a layer of CdS on CdTe core, forming a CdTe/CdS core/shell nanostructure. This is supported by the X-ray powder diffraction (XRD) and transmission electron microscope (TEM) investigations that the XRD diffraction peaks of CdTe QDs after anion exchange is slightly shifted to higher angle while both morphology and sizes are kept almost unchanged (Fig. S1).
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Fig. 1A shows the typical absorption spectra of CdTe NCs before and after anion exchanges at 90℃ for different period of time. The as prepared CdTe QDs exhibits obvious excitonic absorption, indicating its narrow size distribution. The size of CdTe QD was estimated to be ca. 3
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nm. When anion exchanges were carried out, continuous shift of the absorption to longer wavelength was observed. This is ascribed to the formation of type-ⅡCdTe/CdS QDs due to the Te2- for S2- exchanges, which leads to the decrease of effective bandgap that determined by the
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conduction band of CdS and valence band of CdTe as shown in Fig.1 B [29]. With the help of
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TGA, CdTe/CdS QDs were tethered on TiO2 porous films by immersing TiO2 film electrodes in the QD suspended solution for 24 hours. During this period, gradual coloration of the TiO2 films was observed (inset of Fig. 1C), which confirms the success deposition of QDs from solution onto TiO2 films. The corresponding absorption spectra of the QD decorated TiO2 films are shown in Fig. 1C. As expected, the absorption of the films showed similar red shifts as those
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shown in Fig. 1A, demonstrating that the absorption range of QD dot decorated film is expanded to the longer wavelength. Such an extension of light absorption is expected to be of great benefit for enhancing the light harvesting and thus the cell efficiency of quantum dot sensitized solar
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cell.
Fig. 2A shows the cross-sectional scanning electron microscopy (SEM) image after deposition
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of QDs. The film is composed of TiO2 nanoparticles, possessing a porous structure. The film thickness is measured to be ca. 12 µm. Fig 2 B-D shows the elements mapping of Cd, Te and S throughout the film. Clearly, it can be found that Cd, Te and S are well distributed throughout the whole film. The uniform distribution of Cd, Te and S suggests that CdTe/CdS QDs can penetrate throughout the mesoporous film depth leading to the uniform coverage of QDs around the porous TiO2 film.
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The QD/TiO2 films were employed to construct Sandwich-type QDSSCs using Cu2S on brass as the counter electrode and polysulfide composed of 0.5 M S, 2.0 M Na2S and 0.2 M KCl dissolved in water/ethanol as the electrolyte. Before that, a thin ZnS layer was deposited onto the
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QD/TiO2 films to improve the stability of QDSSC as well as to reduce the charge recombination at the TiO2/electrolyte and QD/electrolyte interfaces [30, 31]. Fig. 3 presents the typical photocurrent density - photovoltage (J-V) curves of QDSSCs under AM 1.5 irradiation (100 mW
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cm-2), while the corresponding open-circuit voltages (Voc), short-circuit current densities (Jsc), and overall solar-to-energy conversion efficiencies (η) were gathered in Fig. 4. The cell
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performance of the pristine CdTe based device is quite poor (Voc=255 mV, Jsc=0.65 mA/cm2 and =0.15%). This is in accordance with that of reported before [32]. When anion exchanges were performed, however, significant increases in Voc, Jsc as well as η were observed. Particularly, we noted that the cell performance is close related to the reaction time of the anion exchanges. As
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demonstrated in Fig. 4, Voc is rapidly increased from 255 mV for the as prepared CdTe QDs to ca. 455 mV for QDs after 3 h anion exchanges. After that, it keeps almost unchanged even further prolonging the exchange time. In comparison, Jsc is continuously increased from 0.65
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mA/cm2 for the as prepared CdTe QDs to 4.89 mA/cm2 for QDs after anion exchange for 24 h, although the increment is somewhat decreased after the first 1 h anion exchanges. As a result,
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cell efficiency was considerably improved from 0.15% to over 1.4%. In a controlled experiment, we prepared a CdTe/CdS cosensitized TiO2 photoanode by depositing CdS onto a CdTe QDs pre-decorated TiO2 film through a SILAR process. After ZnS deposition, Sandwich-type cell (denoted as CdTe-CdS) was constructed for J-V measurements. As shown in Fig. 5, the deposition of CdS on CdTe leads to the increase of VOC, JSC and cell efficiency compared to the CdTe cell (Fig. 5 a and b), owing to its passivation effect of CdS coating. Nevertheless, we noted that the cell efficiency of CdTe-CdS cell (0.30%) is much lower than those devices employing 8
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CdTe/CdS QDs prepared through anion exchanges (Fig. 5 b and c). This result indicates that ion exchange is superior to SILAR for the preparation of the core/shell QDs for QDSSC. This may partly ascribed to the excellent CdTe/CdS interface produced by Te2- for S2- anion exchange that
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replace the sublatice of Te2- in CdTe crystals by S2-, resulting in the decrease of charge recombination and increase of the electron injection from QDs to TiO2 as will be shown later. Furthermore, CdTe/CdS based QDSSC was post-treated by coating a thin CdS layer ahead of
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the deposition of ZnS passivation layer, which was performed by twice dipping the CdTe/CdS QD sensitized electrode film into Cd2+ and S2- solution alternatively. The plot d in Fig. 5 shows
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the typical J-V curve of the resultant QDSSC. As clearly demonstrated in the figure, we found that the deposition of CdS thin layer significantly improved the cell performance (Fig. 5 c and d). In particular, Voc is increased from 455 mV to 505 mV, while Jsc is increased from 4.89 mA/cm2 to 6.73mA/cm2. Consequently, an overall energy conversion efficiency reached to 2.44% was
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achieved for CdTe/CdS based QDSSC with CdS followed by ZnS post-treatments, showing near 70% increment compared to the QDSSC with the same CdTe/CdS QDs but only ZnS posttreatment. It needs to point out that the light absorption caused by the CdS post-treatment is
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neglected because of the very thin layer of CdS deposition, which was also supported by the fact that a very small photocurrent of ca. 0.1 mA cm-2 was measured when the same amount of CdS
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was deposited on TiO2. The benefit role of CdS post-treatment is in accordance with our previous report, ascribable to the increased charge injection and decreased charge recombination originated from the deposition of CdS [33]. Fig.6 shows the incident photon to current conversion efficiency (IPCE) of QDSSC with different configurations. Compared to CdTe based cell, the anion exchange leads to an obvious increase in IPCE throughout the entire spectrum, whereas the CdS post-treatment results in its further increase of IPCE. This result is consistence
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with the variation trend of JSC as shown in Fig. 5, verifying the benefit role of anion exchange and CdS post-treatment in enhancing the photocurrent of QDSSC. To unveil the superior photovaltaic properties of CdTe/CdS QD sensitized solar cell, dark
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current, electrochemical impedance spectroscopy (EIS) and intensity-modulated photocurrent spectroscopy (IMPS) investigations have been performed. The lower part of Fig. 5 compares the dark current of QDSSC with different configurations. One can find from this figure that the anion
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exchanges triggered a significant decrease in dark current, which was further diminished by the introduction of CdS post-treatment. This result suggests that anion exchange and CdS post-
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treatment can suppress the electron recombination in the cell. Fig. 7 shows the Nyquist plots (105 Hz ~ 10-1 Hz) of QDSSCs at open circuit under irradiation (AM1.5, 100 mW.cm-2). The charge transfer resistance is obtained by fitting the impedance data employing the equivalent circuit shown in the inset of Fig. 7, where Rs is the equivalent series resistance, Rct and CPE represent
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the charge transfer resistance and chemical capacitance at the TiO2/QDs/electrolyte interface. Since the same electrolyte and counter electrode were used, all the devices give out almost the same Rs (Table 1). However, there is an apparent difference in Rct. The Rct value of the
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CdTe/CdS based device is much lower than those of CdTe and CdTe-CdS based cells, while the CdS post-treatment leads to the further decrease in Rct. The decreased Rct indicates that the
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charge recombination is suppressed, while the electrons injection from QDs to the conduction band of TiO2 is increased. This is attributed to the decreased surface traps due to the formation of CdS on CdTe by Te2- for S2- anion exchange and the cascade band alignment of CdTe and CdS that favorable to increase electron injection from QDs to TiO2. Furthermore, the electron transport dynamics are characterized by IMPS measurement. Fig. 8 illustrates the typical IMPS spectra of cell devices with different cell configurations. The transit time (τd) of photogenerated electron is determined as before, using the equation of τd =1/2πfmin, where fmin is the minimum 10
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frequency in IMPS plot [34]. As displayed in Fig. 8, we found that τd for CdTe/CdS cell is much shorter than those of CdTe and CdTe-CdS cells, while the CdS post-treatment leads to further decrease of τd. This indicates that the formation of CdTe/CdS core/shell structure by Te2- for S2-
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anion exchange and the introduction of CdS deposition layer enhanced the electron transportation rate within the photoanode film. This observation is related to the larger charge generation in the CdTe/CdSe cells, which brings forward an enhanced electron concentration in the TiO2 substrate
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of the cells.
In general, the photocurrent of a solar cell can be estimated by J SC = qηlhηccηinj I 0 , where I0 is the
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incident photon flux, ηlh, ηcc and ηinj are the light harvesting, electron collection and injection efficiencies. As proved by UV-Vis, J-V, EIS and IMPS measurements, the Te2- for S2- anion exchange led to the extension of the light absorption range, the increases of the electron injections from QDs to TiO2 and the electron transportation rate within the film, as well as the
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suppression of the charge recombination in the cell. As a result, ηlh, ηcc and ηinj are increased, and thus leading to the enhancement of photocurrent. On the other hand, it is known that the
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photovoltage Voc of a cell is determined by the Jsc and J0:
Voc =
k BT qβ
J sc + 1 ln J0
where kB is the Boltzmann constant, T is the temperature,
is a parameter related with the
nonlinear recombination [35]. Therefore, it is expected that the enhanced Jsc and decreased J0 will increase the Voc, and thereby the cell efficiency. The benefit role of the introduction of CdS post-treatment is ascribed to its further passivation of QDs and the blockage of the TiO2 surface, resulting in the further inhibition of charge recombination within the cell and thus improves the cell efficiency [33].
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3. Conclusions: In summary, we have reported a novel method to prepare CdTe/CdS core/shell quantum dots for QDSSC. CdTe/CdS QDs were prepared through an anion exchange between CdTe QDs
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and S2- in aqueous solution under ambient condition at low temperature. QDs were bonded onto TiO2 with the help of TGA bifunctional molecule. EDX elemental mapping investigations demonstrate that the QDs are uniformly distributed throughout the mesoporous film depth.
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Photovoltaic investigations showed that the resultant QDSSC exhibits much higher cell efficiency compared to the CdTe cell as well as the device made from CdTe/CdS core/shell QDs
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prepared by SILAR route. The superior property of CdTe/CdS based cell is ascribed to its typeⅡ core/shell structures of CdTe/CdS QDs formed by T2- for S2- anion exchange, that can efficiently extend light absorption, suppress the charge recombination in the cell, and meanwhile increase electron injections from QDs to TiO2 and the electron transportation within the film as
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proved by the UV-vis, J-V, IS and IMPS investigations. In combination with CdS post-treatment, CdTe/CdS QD based QDSSC achieved an overall energy conversion efficiency of 2.44%, showing more than 15 times that of the CdTe cell. The cell efficiency might be further improved
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by optimizing the photoanode, such as the use of QD sensitizer with longer absorption onsets via the modification of the synthetic route etc. Nevertheless, the present work provides an alternative
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eco-friendly efficient way to construct QDSSC with enhanced solar to energy conversion efficiency.
Acknowledgments
This work was supported by the Fundamental Research Funds for the Central Universities (YS1406) References:
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[1] R. S. Selinsky, Q. Ding, M. S. Faber, J. C. Wright, S. Jin, Chem. Soc. Rev. 42 (2013) 29632985.
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[2] P. V. Kamat, Acc. Chem. Res. 45 ( 2012) 1906-1915. [3] M. C. Beard, J. M. Luther, O. E. Semonnin, A. J. Nozik, Acc. Chem. Res. 46 (2013) 12521260.
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Figure captions: Fig.1 (A) Absorption spectra of a series of samples collected at different time points (0, 1, 3, 7,
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11, 15, and 24 h) during the anion exchange reactions at 90℃. (B) Schematic diagram of CdTe/CdS core/shell quantum dot indicating the relative band positions. (C) Absorption spectra
QD-sensitized TiO2 films.
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of QD-sensitized TiO2 films using the QDs shown in (A). Inset in (C) show the photographs of
Fig.2 (A) cross-sectional SEM image of CdTe/CdS QD sensitized TiO2 film, (B-D) elemental
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mapping of Cd, Te and S throughout the film depth.
Fig.3 J-V cureves of QDSSCs using CdTe QDs before (a) and after anion exchange for 1h (b), 3h (c), 7h (d), 11h (e), 15 (f), and 24h (g).
Fig.4 Dependent of open-circuit voltage (VOC), short-circuit current density (JSC) and solar-to-
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energy conversion efficiency (η) on the anion exchange time. The dash lines are guide for eye. Fig. 5 J-V cureves of QDSSCs with different photoanode under illumination (upper) and dark
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(lower): (a) CdTe QD sensitized TiO2 film post-treated by ZnS, (b) CdTe/CdS QD sensitized TiO2 film prepared by depositing CdS through a SILAR process onto a CdTe QDs pre-decorated
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TiO2 film and post-treated by ZnS, (c) CdTe/CdS QD sensitized TiO2 film using the anion exchanged CdTe/CdS QDs and post-treated by ZnS, (d) same CdTe/CdS QD sensitized TiO2 film of (c) post-treated by CdS and ZnS. Fig. 6 Incident photon-to-current spectra of QDSSCs with different photoanode: (a) CdTe QD sensitized TiO2 film post-treated by ZnS, (b) anion exchanged CdTe/CdS QD sensitized TiO2 post-treated by ZnS, (c) anion exchanged CdTe/CdS QD sensitized TiO2 film post-treated by
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CdS and ZnS. Fig. 7 Electrochemical impedance spectra of QDSSCs with different configurations as shown in
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Fig. 5. Fig. 8 Intensity-modulated photocurrent spectra of QDSSCs with different configurations as
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1.5
η
1.0 0.5
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Voc / mV
0.0 500 400
5 4 3 2 1 0
10
30
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Time/ h
20
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JSC / mA cm
2
300
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Fig. 5
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Fig. 7
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Table 1. Fitted impedance parameters of different QDSSCs.
η (%)
Rct(Ω)
CdTe-ZnS
15.5
225.8
255
0.65
0.15
CdTe-CdS-ZnS
18.0
162.2
385
1.15
0.30
CdTe/CdS-ZnS
15.8
144.0
485
4.89
1.44
CdTe/CdS-CdS-ZnS
24.0
99.9
505
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Rs(Ω)
6.73
2.44
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Voc (mV)
JSC (mAcm2)
Photoanode
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Fig. S1 X-ray powder diffraction (XRD) patterns of CdTe NCs before and after anion exchanges at 90℃. Insets show their corresponding TEM imagines.