Annealing effect on Sb2S3-TiO2 nanostructures for solar cell applications
© Li et al.; licensee Springer. 2013
Received: 31 October 2012
Accepted: 9 February 2013
Published: 19 February 2013
Nanostructures composited of vertical rutile TiO2 nanorod arrays and Sb2S3 nanoparticles were prepared on an F:SnO2 conductive glass by hydrothermal method and successive ionic layer adsorption and reaction method at low temperature. Sb2S3-sensitized TiO2 nanorod solar cells were assembled using the Sb2S3-TiO2 nanostructure as the photoanode and a polysulfide solution as an electrolyte. Annealing effects on the optical and photovoltaic properties of Sb2S3-TiO2 nanostructure were studied systematically. As the annealing temperatures increased, a regular red shift of the bandgap of Sb2S3 nanoparticles was observed, where the bandgap decreased from 2.25 to 1.73 eV. At the same time, the photovoltaic conversion efficiency for the nanostructured solar cells increased from 0.46% up to 1.47% as a consequence of the annealing effect. This improvement can be explained by considering the changes in the morphology, the crystalline quality, and the optical properties caused by the annealing treatment.
KeywordsTiO2 Sb2S3 Nanorod Solar cells Annealing effect
Dye-sensitized solar cells (DSSCs) pioneered by O'Regan and Grätzel have been intensively investigated as a promising photovoltaic cell all over the world [1–5]. Until now, photovoltaic conversion efficiency of up to 11% has been reported for DSSCs in the laboratory . Although the conversion efficiency is impressive, the expense of the dye required to sensitize the solar cell is still not feasible for practical applications. Therefore, it is critical to tailor the materials to be not only cost effective but also long lasting. Recently, the utilization of narrow-bandgap semiconductors as a light-absorbing material, in place of conventional dye molecules, has drawn much attention. Inorganic semiconductors have several advantages over conventional dyes: (1) The bandgap of semiconductor nanoparticles can be easily tuned by size over a wide range to match the solar spectrum. (2) Their large intrinsic dipole moments can lead to rapid charge separation and large extinction coefficient, which is known to reduce the dark current and increase the overall efficiency. (3) In addition, semiconductor sensitizers provide new chances to utilize hot electrons to generate multiple charge carriers with a single photon. These properties make such inorganic narrow-bandgap semiconductors extremely attractive as materials for photovoltaic applications.
Recently, a range of nano-sized semiconductors has been investigated in photovoltaic applications including CdS [7–9], CdSe [10–13], Ag2S , In2S3, PbS , Sb2S3, Cu2O , as well as III-VI quantum ring . Among these narrow-bandgap semiconductors, Sb2S3 has shown much promise as an impressive sensitizer due to its reasonable bandgap of about 1.7 eV, exhibiting a strong absorption of the solar spectrum. The use of Sb2S3 nanoparticles, which may produce more than one electron–hole pair per single absorbed photon (also known as multiple exciton generation), is a promising solution to enhance power conversion efficiency. Furthermore, the creation of a type-II heterojunction by growing Sb2S3 nanoparticles on the TiO2 surface greatly enhances charge separation. All of these effects are known to increase the exciton concentration, lifetime of hot electrons, and therefore, the performance of sensitized solar cells. Limited research has previously been carried out with Sb2S3-TiO2 nanostructure for solar cell applications [20–22]. A remarkable performance was obtained in both liquid cell configuration and solid configuration. These findings were based on the use of porous nanocrystalline TiO2 particles; however, very little research has been conducted using single-crystalline TiO2 nanorod arrays. Compared with conventional porous polycrystalline TiO2 films, single-crystalline TiO2 nanorods grown directly on transparent conductive oxide electrodes provide an ideal alternative solution by avoiding particle-to-particle hopping that occurs in polycrystalline films, thereby increasing the photocurrent efficiency. Further enhancements in solid Sb2S3-sensitized solar cells demand a deeper understanding of the main parameters determining photoelectric behavior while also requiring additional research and insight into the electrical transporting process in these nanostructures.
Growth of single-crystalline rutile TiO2 nanorod arrays by hydrothermal process
TiO2 nanorod arrays were grown directly on fluorine-doped tin oxide (FTO)-coated glass using the following hydrothermal methods: 50 mL of deionized water was mixed with 40 mL of concentrated hydrochloric acid. After stirring at ambient temperature for 5 min, 400 μL of titanium tetrachloride was added to the mixture. The feedstock, prepared as previously described, was injected into a stainless steel autoclave with a Teflon lining. The FTO substrates were ultrasonically cleaned for 10 min in a mixed solution of deionized water, acetone, and 2-propanol with volume ratios of 1:1:1 and were placed at an angle against the Teflon liner wall with the conducting side facing down. The hydrothermal synthesis was performed by placing the autoclave in an oven and keeping it at 180°C for 2 h. After synthesis, the autoclave was cooled to room temperature under flowing water, and the FTO substrates were taken out, washed extensively with deionized water, and dried in open air.
Deposition of Sb2S3 nanoparticles with successive ionic layer adsorption and reaction method and annealing treatment
Successive ionic layer adsorption and reaction (SILAR) method was used to prepare Sb2S3 semiconductor nanoparticles. In a typical SILAR cycle, the F:SnO2 conductive glass, pre-grown with TiO2 nanorod arrays, was dipped into the 0.1 M antimonic chloride ethanol solution for 5 min at 50°C. Next, the F:SnO2 conductive glass was rinsed with ethanol and then dipped in 0.2 M sodium thiosulfate solution for 5 min at 80°C and finally rinsed in water. This entire SILAR process was repeated for 10 cycles. After the SILAR process, samples were annealed in N2 flow at varied temperatures from 100°C to 400°C for 30 min. After annealing, a color change was noted in the Sb2S3-TiO2 nanostructured samples, which were orange before annealing and gradually turned blackish as the annealing temperature increased.
Characterization of the Sb2S3-TiO2 nanostructures
The crystal structure of the Sb2S3-TiO2 samples were examined by X-ray diffraction (XD-3, PG Instruments Ltd., Beijing, China) with CuKα radiation (λ = 0.154 nm) at a scan rate of 2°/min. X-ray tube voltage and current were set at 40 kV and 30 mA, respectively. The surface morphology of the Sb2S3-TiO2 nanostructures was examined by scanning electron microscopy (SEM; FEI Sirion, FEI Company, Hillsboro, OR, USA). The optical absorption spectra were obtained using a dual beam UV-visible spectrometer (TU-1900, PG Instruments, Ltd.).
Solar cell assembly and performance measurement
Solar cells were assembled using a Sb2S3-TiO2 nanostructure as the photoanode. Pt counter electrodes were prepared by depositing an approximately 20-nm Pt film on FTO glass using magnetron sputtering. A 60-μm-thick sealing material (SX-1170-60, Solaronix SA, Aubonne, Switzerland) with a 3 × 3 mm aperture was pasted onto the Pt counter electrodes. The Pt counter electrode and the Sb2S3-TiO2 sample were sandwiched and sealed with the conductive sides facing inward. A polysulfide electrolyte was injected into the space between the two electrodes. The polysulfide electrolyte was composed of 0.1 M sulfur, 1 M Na2S, and 0.1 M NaOH which were dissolved in distilled water and stirred at 80°C for 2 h.
A solar simulator (Model 94022A, Newport, OH, USA) with an AM1.5 filter was used to illuminate the working solar cell at light intensity of one sun illumination (100 mW/cm2). A source meter (2400, Keithley Instruments Inc., Cleveland, OH, USA) was used for electrical characterization during the measurements. The measurements were carried out using a calibrated OSI standard silicon solar photodiode.
Results and discussion
Morphology and crystal structure of Sb2S3-TiO2 nanostructure
Optical property of the Sb2S3-TiO2 nanostructures
Photovoltaic performance of the solar cell based on Sb2S3-TiO2 nanostructure
Parameters of Sb 2 S 3 -TiO 2 nanostructured solar cells annealed at different temperatures
Sb2S3-TiO2 under 100°C
Sb2S3-TiO2 under 200°C
Sb2S3-TiO2 under 300°C
Sb2S3-TiO2 under 400°C
This significant improvement of the photovoltaic performance obtained for annealed Sb2S3-TiO2 nanostructured solar cells is explained by the following reasons: (1) An enhanced absorption of sunlight caused by the red shift of the bandgap will result in an enhanced current density. (2) Increase of Sb2S3 grain size by annealing will reduce the particle-to-particle hopping of the photo-induced carrier. This hopping may occur in an as-grown nanostructure with Sb2S3 nanoparticles. (3) Improvement of crystal quality of the Sb2S3 nanoparticles by annealing treatment will decrease the internal defects, which can reduce the recombination of photoexcited carriers and result in higher power conversion efficiency. (4) Good contact between the Sb2S3 nanoparticles and the TiO2 nanorod is formed as a result of high-temperature annealing. Such a superior interface between TiO2 and nanoparticles can inhibit the interfacial recombination of the injected electrons from TiO2 to the electrolyte, which is also responsible for its higher efficiency.
Our findings suggest the possible use of 3D nanostructure material grown by a facile hydrothermal method for sensitized solar cell studies. The drawback of this type of solar cell is a rather poor fill factor, which limits the energy conversion efficiency. This low fill factor may be ascribed to the lower hole recovery rate of the polysulfide electrolyte, which leads to a higher probability for charge recombination . To further improve the efficiency of these nanorod array solar cells, we advise that a new hole transport medium with suitable redox potential and low electron recombination at the semiconductor and electrolyte interface should be developed. Moreover, as reported by Soel et al., other contributions such as the counter electrode material may also influence the fill factor .
With a facile hydrothermal method, the single-crystalline TiO2 nanorod arrays were successfully grown on fluorine-doped tin oxide glass. Next, Sb2S3 nanoparticles were deposited by successive ionic layer adsorption and reaction method to form a Sb2S3-TiO2 nanostructure for solar cell applications. Annealing treatment was conducted under varied temperatures, and the optimal annealing temperature of 300°C was obtained. Obvious enhancement in visible light absorption was observed for the annealed samples. The photovoltaic performance for solar cells based on annealed Sb2S3-TiO2 nanostructure shows an increase of up to 219% in power conversion efficiency.
This work was supported by the National Key Basic Research Program of China (2013CB922303, 2010CB833103), the National Natural Science Foundation of China (60976073, 11274201, 51231007), the 111 Project (B13029), the National Found for Fostering Talents of Basic Science (J1103212), and the Foundation for Outstanding Young Scientist in Shandong Province (BS2010CL036).
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