Behzad Rezaei, Ismaeil Mohammadi, Ali Asghar Ensafi, Mohammad Mohsen Momeni

A B S T R A C T
In this study, two types of open-ended and closed-ended freestanding TiO2 nanotube membrane (OE-TNT and CE-TNT, respectively) are fabricated, transferred onto a fluorine-doped tin oxide (FTO) substrate and applied in the dye-sensitized solar cell (DSSC). The open-ended TiO2 nanotube array (OE-TNT) is fabricated by applying a large-voltage pulse continued by chemical etching. To increase dye loading, an efficient hybrid photoanode architecture consisting of freestanding TNT membranes and TiO2 nanoparticles (TNPs) is fabricated by highfrequency AC-electrophoresis deposition (OE-EPD and CE-EPD). The photovoltaic performance, transient properties and charge transport resistances of four types of photoanodes are analyzed by photocurrent densityvoltage (I-V) curve, open circuit voltage decay (OCVD) measurements and electrochemical impedance spectra (EIS). The OE-TNT based DSSC exhibits a power conversion efficiency (PCE) of 3.11%, which is dramatically larger than the value for CE-TNT based DSSC (1.85%), which mainly is due to the enhancement of the fill factor by the opening end of TNT membrane; FF=61.1% vs. FF=38.6%, respectively. The slope of the I-V curve at V=VOC for OE-TNT based DSSC is higher than that for CE-TNT-DSSC leading to lower RS in OE-TNT-DSSC, in
agreement with EIS results, give rise to the good electrical connection of FTO with TNT membrane. Moreover, the PCE of the OE-EPD based DSSC has improved significantly to 96.8% due to the decoration of the OE-TNT by the TNTPs through AC-EPD technique. This PCE is higher than that of the PCE of CE-EPD based DSSC by 69.7%.

Keywords:
Dye-sensitized solar cell, Open-ended TiO2 nanotube, Electrophoretic deposition, Free-standing TiO2 nanotube membrane