This study investigates a one-step method for the fabrication of a TNWs-covered TNAs (TNWs/TNAs) hybrid structure, using a mixture of EG and water containing NH4F electrolyte with and without mechanical stirring. The morphology of the TNWs/TNAs structure was then examined by changing the anodizing voltage and processing time, to elucidate the detailed formation mechanism of TNWs/TNAs.
In specific, various TNAs and TNWs/TNAs structure can be fabricated by using anodizing voltage from 25 to 60 V for various processing time. In addition, we add a stirring bar to rotate the electrolyte during the anodizing experiment. This increases the TNA tube length but postpones the emergence of nanowire. Through the control of anodizing voltage, processing time, and mechanical stirring, the thickness of TNAs and TNWs/TNAs structure are kept the same at 12 mm, while the tube diameter and wall thickness are varied according to the processing conditions.
The DSSC performance of TNWs/TNAs and TNAs structures was then investigated and compared with the film made of TiO2 nanoparticles using a DSSC area size of 0.28cm2. For TNWs/ TNAs h ybrid structure ( 30V, 8 hours ), the conversi on effi ci ency and Js c are 1.85%, 5.27 mA/cm2, compared to 1.04% and 3.81 mA/cm2 in a TNA only film. The 44.3% improvement in conversion efficiency can be attributed to the enhanced dye adsorption (21%) and mass transport in TNWs/TNAs compared to TNAs only. Moreover, the conversion efficiencies in TNWs/TNAs and TNAs only films fabricated under 40 V are lower than those under 30 V due to lower surface area originated from larger inner diameter, as evidenced by dye adsorption amount. When the length up to 15 µm, the efficiency (η%) is further increased to 2.02%. Although the efficiency of TNWs/TNAs film is still lower than that
of TiO2 nanoparticle film, TNWs/TNAs films provide a simple and efficient method to be a flexible DSSC.
Future Work
There are plenty of opportunities to improve the performance in the DSSC based on TNWs/TNAs hybrid structure. Several approaches are recommended below for future work:
Methods for enhancing the dye adsorption amount to induce higher photocurrent
(a) To enhance the surface area through smaller tube diameter and larger surface roughness
(1) We can use different anodizing voltage to fabricate the different inner diameter nanotube array and expecting for a better dye adsorption.
(2) The surface area of TNWs/TNAs hybrid structure can be enhanced by increasing its surface roughness. This can be achieved by using a two-step process: (1) first, to carry out the anodizing process with a stirring bar to obtain a more homogeneous and collapsed TNWs. And then (2) After the formation of nanowire using stirring method, the stirring can to shut down to enhance the etch rate in the top section of TNAs and nanorwires. (3) to apply different rate of rotation and/or a periodic variation voltage to deliver various roughness inside the nanotubes. [80]
(b) To improve the adsorption of dye onto TNWs/TNAs hybrid structure.
Currently, vacancies are still observed after the dye has been immersed in to hybrid structure for 48 hours. Therefore, it may be useful to add a coadsorbent into DSSCs system which can serve as a buffer blocking water and triiodide from the surface of TiO2. Diphenylphosphinic acid (DPPA) is one of the coadsorbent candidate [105]
(c) To improve the electrolytes
One can change the source of I ions, or modify the concentration of electrolyte for providing electrons to redox the oxidized dye molecules. [106,107]
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