CuInS2 quantum dot-sensitized TiO2 nanorod array photoelectrodes: synthesis and performance optimization
© Zhou et al.; licensee Springer. 2012
Received: 30 September 2012
Accepted: 18 November 2012
Published: 27 November 2012
CuInS2 quantum dots (QDs) were deposited onto TiO2 nanorod arrays for different cycles by using successive ionic layer adsorption and reaction (SILAR) method. The effect of SILAR cycles on the light absorption and photoelectrochemical properties of the sensitized photoelectrodes was studied. With optimization of CuInS2 SILAR cycles and introduction of In2S3 buffer layer, quantum dot-sensitized solar cells assembled with 3-μm thick TiO2 nanorod film exhibited a short-circuit current density (Isc) of 4.51 mA cm−2, an open-circuit voltage (Voc) of 0.56 V, a fill factor (FF) of 0.41, and a power conversion efficiency (η) of 1.06%, respectively. This study indicates that SILAR process is a very promising strategy for preparing directly anchored semiconductor QDs on TiO2 nanorod surface in a straightforward but controllable way without any complicated fabrication procedures and introduction of a linker molecule.
KeywordsCuInS2 Quantum dots TiO2 Nanorod arrays Photoelectrochemical properties
Since the introduction of an important advancement of using a nanostructured dye-sensitized photo-active electrode in a solar cell by O'Regan and Grätzel in 1991 , the dye-sensitized solar cells (DSSCs) have attracted a lot of attention in the past two decades and been considered as a potential low-cost alternative to conventional silica-based solar cells [2–5]. The latest energy conversion efficiency of DSSCs was reported to exceed 12% . Further improvement of the efficiency of DSSCs is impeded by the design of new dyes which could absorb all photons above a threshold energy of 1.3 to 1.4 eV (roughly 940 to 890 nm) without affecting the injection efficiency and regeneration rate [7, 8]. Another attractive strategy is to use semiconductor quantum dot (QD) as a substitute for organic dye [9–12]. For enhancement of the conversion efficiency, it is still necessary to select a semiconducting material with the proper band gap that absorbs strongly for photon energies above 1.3 eV. Ternary chalcopyrite CuInS2, which is a direct band gap semiconductor with Eg = 1.55 eV (bulk) has many favorable features including high absorption coefficient (105 cm−1) and proper band gap well matched to the solar spectrum [13, 14], as well as non-toxicity and good stability. It has been demonstrated as a promising photosensitizer successfully used in quantum dot-sensitized solar cells (QDSSCs) [15, 16].
Up to now, the reports on CuInS2-based QDSSCs are almost exploited a presynthesis method, in which the CuInS2 colloidal QDs are presynthesized and anchored to the electrodes by means of bifunctional linker molecules or direct adsorption [16, 17]. This process suffers from rather low QD loading and relatively weaker electronic coupling between QDs and TiO2. Another approach for QD sensitization is direct growth of QDs on TiO2 by successive ionic layer adsorption and reaction (SILAR), in which the ions in the precursor solution are adsorbed directly onto the bare surface of TiO2 to form a very thin conformal covering film [19, 20]. The SILAR process has recently emerged as the best method for adsorbing QDs onto TiO2 electrodes, owing to its facile and reproducible preparation, high QD loading together with well controllable in size and density of the target semiconductor QDs, and efficient electron transfer to TiO2[18, 20]. Very recently, Chang et al. have reported CuInS2 QD-sensitized TiO2 nanoparticle film by SILAR process . For assembly of QDSSCs, one dimensional (1D) TiO2 nanostructure arrays possess the superiority over other nanomaterials due to its more open structure which was preferable for both sensitizer and electrolyte filling . Moreover, 1D nanostructure can provide a direct and efficient pathway for electrons from sensitizer to conductive substrate compared to the disordered electron pathway in nanoparticles [23–26]. Therefore, TiO2 has been fabricated into various 1D nanostructure arrays such as nanowires (NWs), nanorods (NRs), and nanotubes for photovoltaic devices. Single-crystalline TiO2 NW or NR array is preferable over polycrystalline one in electron transfer because of electron scattering or trapping at grain boundaries of polycrystal . However, the exploitation of CuInS2 QD-sensitized single-crystalline TiO2 NRs for QDSSCs has not been systematically investigated.
In this study, using the SILAR procedures, CuInS2 QDs were successfully assembled onto vertically oriented single-crystalline TiO2 nanorod array (NRA), which was grown directly onto transparent conductive fluorine-doped tin oxide (FTO) substrates. A detailed structural characterization and photoelectrochemical investigation of the CuInS2-sensitized TiO2 nanorod array photoelectrodes were discussed in this article. Furthermore, by introduction of a cadmium-free In2S3 buffer layer to adjust the interfacial properties of CuInS2 and TiO2, the photoelectrical properties of QDSSCs were remarkably improved.
Copper (II) sulfate (CuSO4, 99%), indium (III) sulfate (In2(SO4)3, 98.0%), indium(III) nitrate (99.9%), sodium sulfide (Na2S, 98%), and titanium butoxide (97%) were purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd. Potassium phosphate monobasic (KH2PO4, 99.99%), sodium hydroxide (NaOH, 98%), sodium sulfite (Na2SO3, 97%), and concentrated hydrochloric acid (HCl, 37% by weight) were obtained from Tianjin Chemical Reagents Company (Tianjin, China). All the materials were used directly without further purification. Triply deionized water (resistivity of 18.2 MΩ cm−1) was obtained from a Milli-Q ultrapure water system (EMD Millipore Corporation, MA, USA). FTO-coated glass slides (F: SnO2, 14 Ω/square, Nippon Sheet Glass Group, Tokyo, Japan) were thoroughly washed with a mixed solution of deionized water, acetone, and 2-propanol (volume ratios of 1:1:1) under sonication for 60 min.
Synthesis of TiO2 NRAs
The TiO2 NRAs were grown directly on transparent FTO substrates by a hydrothermal method; details of the synthesis procedure can be found in Liu and Aydil . In a typical synthesis, 30 mL of concentrated HCl was added to 30 mL of deionized water with stirring. After 5 min of stirring, 1 mL of titanium butoxide was added dropwise to the solution and stirred continuously for another 5 min to obtain a clear transparent solution. The resulting solution was then transferred into a 120-mL Teflon-lined stainless-steel autoclave. Then, one piece of cleaned FTO glass was placed into the autoclave at an angle of about 45° against the wall of the Teflon lining with the conducting side facing down. Subsequently, the autoclave was sealed and placed inside an electronic oven. The hydrothermal synthesis was conducted at 150°C for 20 h, and the obtained TiO2 NRAs on FTO glass substrates were taken out of the cooled autoclave, rinsed extensively with distilled water, and finally dried in air.
Fabrication of CuInS2 QD-sensitized TiO2 NRA electrodes
CuInS2 QDs were attached to TiO2 NRAs by the SILAR process, which was similar to that described by Wu et al. . Briefly, the TiO2 nanorod array substrate was dipped sequentially in aqueous solutions of 0.1 M In2(SO4)3 for 60 s, and S ion precursor solution (0.075 M Na2S, with pH equal to 11.3 adjusted by a buffer composed of 0.1 M KH2PO4 and 0.1 M NaOH) for 240 s, following in 0.01 M CuSO4 aqueous solutions for 20 s, and S ion precursor solution for 240 s. Between each dip, the films were rinsed with deionized water for 30 s to remove excess precursors and dried in air before the next dipping. Such an immersion procedure is termed as one cycle for copper indium sulfide deposition, and this immersion cycle was repeated several times until the desired amount of CuInS2 QDs was incorporated. To increase the crystallinity and the concentration of sulfur in the SILAR-deposited CuInS2, the samples were annealed in furnace under sulfur ambiance (using S powder as the S source) at 500°C for 30 min after SILAR deposition.
A In2S3 buffer layer was introduced between TiO2 and CuInS2 layer also by SILAR. For In2S3 deposition from their precursor solutions, 0.1 M indium nitrate in ethanol was used as cation source, and 0.1 M sodium sulfide in 1:1 methanol and water as anion source.
The as-prepared CuInS2 QD-sensitized TiO2 NRA electrodes were characterized by various analytical and spectroscopic techniques. The morphology of the sample was studied by a field-emission scanning electron microscopy (FESEM, JSM-7001 F, JEOL Co., Ltd., Beijing, China). Transmission electron microscopy (TEM), and high-resolution TEM (HRTEM) investigations were carried out by a JEOL JEM-2100(UHR) microscope operating at 200 kV. The samples were detached from the FTO substrate, then dispersed in ethanol by sonication, and dropped onto a carbon film supported on a copper grid. Structure characterizations of the CuInS2-sensitized TiO2 NRA films were conducted using X-ray diffraction (XRD). The XRD patterns were recorded using a Philips X'Pert PRO X-ray diffractometer (Royal Philips Electronics, Amsterdam, The Netherlands) with Cu Kα1 radiation (λ = 1.5406 Å) from 20 to 70° at a scan rate of 2.4° min−1. X-ray tube voltage and current were set at 40 kV and 40 mA, respectively. The absorption spectra for CuInS2 QD-sensitized TiO2 NRA electrodes were recorded on a CARY5000 UV-visible NIR spectrometer (Agilent Technologies Inc., CA, USA).
To examine the photovoltaic properties of CuInS2 QD-sensitized TiO2 NRA electrodes, the electrodes were assembled into cells using a Pt-coated counter-electrode facing it, which had been prepared by sputtering with 100 nm of Pt on cleaned FTO glass using radio frequency sputtering at a power of 150 W and a working pressure of 3 × 10−3 Torr with argon gas for 60 s. The sandwich-type solar cells were then sealed with 60-μm thick hot-melt film (Surlyn 1702, Dupont, DE, USA) by hot pressing. Polysulfide electrolyte consisting of 0.24 M Na2S and 0.35 M Na2SO3 in aqueous solution was injected into the interelectrode space by capillary force. A mask with an aperture of 0.16 cm2 (0.4 cm × 0.4 cm) was used to define the active area of the cell and prevent stray light from producing photocurrents. The photocurrent-voltage (I-V) curves were measured under an illumination of a solar simulator (Oriel class A, SP91160A, Newport Corporation, CA, USA) at one sun (AM1.5, 100 mW cm−2) irradiation calibrated with a Si-based reference. A Keithley model 2400 digital source meter (Keithley Instruments, Inc., OH, USA) was used to record the I-V characteristics by applying an external bias potential to the cell and measuring the photocurrent.
Results and discussion
Characterization of CuInS2 QD-sensitized TiO2 nanorod array
Photoelectric properties of CuInS2 QD-sensitized TiO2 nanorod arrays
Photovoltaic performance of the CuInS 2 -based QDSSC devices with different SILAR cycles
Fill factor (%)
Photovoltaic performance for CuInS 2 -sensitized TiO 2 NRA photoelectrodes with and without In 2 S 3 buffer layer
J sc I(mA/cm2)
Fill factor (%)
It should be mentioned that the efficiency of the CuInS2-based QDSSCs present in our study is still limited, which may be attributed to the limitation of TiO2 NR length. The typical thickness of TiO2 nanoparticle films is about 13 μm, but for TiO2 NR array films used in our experiment, the length was just about 3 μm. As a result, although there are many advantages of 1D TiO2 NRs, the insufficient length resulted in poor QD loadings and light harvesting, which constrained the efficiency of TiO2 NR cells to relatively lower levels than that of nanoparticle-based ones. To further improve photovoltaic performances of 1D nanostructure-based QDSSCs, it is necessary to pay more attention to the internal surface area of TiO2 NRAs and interfacial properties that are very critical to determine the fate of excitons generated inside the semiconductor QDs.
In this study, for the first time, we have employed a facile SILAR process to deposit CuInS2 QD onto TiO2 NRAs, which was prepared by a simple hydrothermal method. The CuInS2 QD-sensitized TiO2 NRAs were used as photoanodes to assemble sandwiched QDSSCs. The effect of SILAR cycles on the photoelectrochemical performance of the CuInS2-sensitized solar cells was investigated. With optimal CuInS2 SILAR cycles and introduction of In2S3 buffer layer to modify the interface, the best photovoltaic performance with an energy conversion efficiency of 1.06% under AM 1.5 G illuminations, an open-circuit photovoltage of 0.56 V, a short circuit current density of 4.51 mA cm−2, and a FF of 0.41 were achieved. The present CuInS2-based QDSSC fabrication approach combined the advantages of 1D TiO2 NRAs and in situ growing of the target semiconductor sensitized layers and buffer layer by SILAR, which can be used for construction of other useful optoelectronic devices and composite catalysts.
ZZ is a Ph.D. candidate in the Key Laboratory for Special Functional Materials of Ministry of Education, Henan University. SY, JF, and ZH are all masters degree students on Inorganic Material Chemistry. WZ is a Ph.D. degree holder on Analytical Chemistry. ZD is the distinguished professor and research director in the Key Laboratory for Special Functional Materials of Ministry of Education. SW is a full professor on Material Chemistry and Physics.
This work was supported by the National Natural Science Foundation of China (20871041 and 20903033) and the New Century Excellent Talents in University (NCET-08-0659).
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