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.

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.

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 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.

The morphology of the rutile TiO₂ nanorod arrays is shown in Figure 2a. The SEM images clearly show that the entire surface of the FTO glass substrate was uniformly covered with ordered TiO₂ nanorods, and the nanorods were tetragonal in shape with square top facets. This nanorod array presented an easily accessed open structure for Sb₂S₃ deposition and a higher hole transferring speed for the whole solar cell. No significant changes in nanorod array morphology were observed after annealing at 400°C. As-synthesized Sb₂S₃-TiO₂ nanostructure is shown in Figure2b, indicating a combination of the Sb₂S₃ nanoparticles and TiO₂ nanorods. The Sb₂S₃-TiO₂ nanostructure after annealing at 300°C for 30 min is shown in Figure 2c. Compared to the CdS-TiO₂ nanostructure, in which 5-to 10-nm CdS nanoparticles distributed uniformly on the TiO₂ nanorod 9 , the as-deposited Sb₂S₃ particles differed with a larger diameter of approximately 50 nm and often covered several TiO₂ nanorods. This structural phenomenon was observed much more so in the annealed sample, where at least some melting of the low melting point (550°C) Sb₂S₃ clearly occurred. After the annealing treatment, the size of Sb₂S₃ particles increased, which enabled the Sb₂S₃ particles to closely contact the TiO₂ nanorod surface. This solid connection between Sb₂S₃ nanoparticles and the TiO₂ nanorods was beneficial to the charge separation and improved the overall properties of the sensitized solar cells.

X-ray diffraction (XRD) patterns of the bare TiO₂ nanorod array, the as-synthesized Sb₂S₃-TiO₂ nanostructure, and the annealed nanostructure are shown in Figure 3. Note in Figure 3a that the TiO₂ nanorod arrays grown on the FTO-coated glass substrates had a tetragonal rutile structure (JCPDS no. 02–0494), which may be attributed to the small lattice mismatch between FTO and rutile. The as-synthesized Sb₂S₃-TiO₂ nanostructure exhibited a weak diffraction peak (Figure 3b) at 2θ = 28.7°, corresponding to the (230) plane of orthorhombic Sb₂S₃. As the annealing temperature increased, more diffraction peaks were observed, and the peaks became more distinct at the same time. Figure 3c shows the XRD pattern of the nanostructure annealed at less than 300°C. All of the reflections were indexed to an orthorhombic phase of Sb₂S₃ (JCPDS no. c-74-1046) 23 . The shape of the diffraction peaks indicates that the product was well crystallized.

The UV-visible absorption spectra of Sb₂S₃-TiO₂ nanostructure samples are shown in Figure 4. An optical bandgap of 2.25 eV is estimated for the as-synthesized Sb₂S₃ nanoparticles from the absorption spectra, which exhibits obvious blueshift compared with the value of bulk Sb₂S₃. After being annealed at 100°C, 200°C, and 300°C for 30 min, the bandgap of Sb₂S₃ nanoparticles was red shifted to 2.19 eV (565 nm), 2.13 eV (583 nm), and 1.73 eV (716 nm), respectively. When annealed at 400°C, the absorption spectra deteriorated, which may be attributed to the oxidation as well as the evaporation of the Sb₂S₃ nanoparticles. The Sb₂S₃-TiO₂ nanostructure annealed at 300°C shows an enhanced absorption in the visible range, which is of great importance for solar cell applications and will result in higher power conversion efficiency. As shown by the XRD patterns and SEM images, this red shift in the annealed samples may be explained by the annealing-induced increase in particle size at the elevated temperatures. The annealing effect on the optical absorption spectra of bare TiO₂ nanorod arrays was also studied (not included here). No obvious difference was found between the samples with and without annealing treatment. This result suggests that although annealing changes the morphology and crystallinity of Sb₂S₃ nanoparticles, it does not significantly affect the optical property of TiO₂ nanorod arrays.

The photocurrent-voltage (I-V) performances of the solar cells assembled using Sb₂S₃-TiO₂ nanostructures annealed under different temperatures are shown in Figure 5. The I-V curves of the samples were measured under one sun illumination (AM1.5, 100 mW/cm²). Compared with the solar cell based on as-grown Sb₂S₃-TiO₂ nanostructure, the solar cell performances correspondingly improved as the annealing temperatures increased from 100°C to 300°C. The open-circuit voltage (V _oc) improved from 0.3 up to 0.39 V, and the short-circuit current density (J _sc) improved from 6.2 up to 12.1 mA/cm². A power conversion efficiency of 1.47% for the sample with annealing treatment was obtained, indicating an increase of 219% (as compared to the 0.46% for the as-grown sample) as a consequence of the annealing treatment. The photovoltaic performance of annealed Sb₂S₃-TiO₂ nanostructured solar cell under 400°C deteriorated, which coincides with the absorption spectrum. Detailed parameters of the solar cells extracted from the I-V characteristics are listed in Table 1.

V _oc, open-circuit voltage; J _sc, integral photocurrent density; FF, fill factor; η, power conversion efficiency.

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 24 . 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 25 .