High efficiency front-illuminated nanotube-based dye-sensitized solar cells
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High efficiency front-illuminated nanotube-based dye-sensitized solar cells Kangle Li1, Stefan Adams1 1 Department of Materials Science and Engineering, National University of Singapore, 5 Engineering Drive 2, Singapore 117576 ABSTRACT A highly reproducible two-step anodization method is reported to fabricate anatase TiO2 nanotube layers. The nanotube membrane fabricated by this method is highly uniform and crackfree. Large area nanotube membranes can be transferred completely onto transparent FTO electrodes without the need for damaging ultrasonic agitation or acid treatment for application in front-illuminated nanotube-based dye-sensitized solar cells. A 16 m thin front-illuminated nanotube-based dye-sensitized solar cell produced using this method reaches an efficiency of 6.3% under 1 sun illumination AM1.5. INTRODUCTION Dye-sensitized solar cells (DSC) are the most cost-effective photovoltaic device without requiring elaborate apparatus to manufacture [1]. As highly ordered, vertically oriented anatase TiO2 nanotube arrays possess outstanding charge transport and carrier lifetime properties [2,3], their use as photo electrode for dye-sensitized solar cells [4,5] can help to boost the efficiency by improving the transport of the harvested charge carriers. Using nanotube arrays grown on Ti foils however requires a “back-illumination” design [6], where efficiency is limited by the need to illuminate the TiO2 nanotube layer through the iodide-containing and thus absorbing electrolytes. Therefore, front-illumination DSC can greatly utilize the illumination without suffering the light reflection, scattering and absorption by the electrolyte. There are currently two established ways to fabricate front-illuminated nanotube-based dye-sensitized solar cell [7-9]. Sputtering of Ti metal on fluorine-doped tin oxide (FTO) followed by complete anodization of the metal layer increases the cost of fabrication and only limited thickness of Ti metal can be sputtered, typically less than 3 μm leading to TiO2 nanotube membranes of suboptimal thickness < 5 μm [10]. The alternative method is to anodize a Ti metal foil and thereafter transfer the resulting titania nanotube membrane from the Ti foil onto FTO [11-13]. Previously, titania nanotube membranes were formed by anodizing thin Ti metal foils for several days until the metal foil is completely transformed [14]. For this method the nanotube membrane thickness is predetermined by the available Ti metal foils, so that the membranes are typically much thicker than what is expected to be suitable for front-illuminated DSC. When transferring nanotube membranes from a partially anodized metal foil, the main problem is to detach the membrane without damaging it. Of the two common techniques to detach nanotube membranes (ultrasonic agitation and acid treatment), ultrasonic agitation limits the reproducibility, as it typically results in cracking of the membranes rather than a detachment of an integer membrane. Cracking of the membrane was found to limit cell performance and a thick nanop
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