Fabrication of MgO-coated TiO2 nanotubes and application to dye-sensitized solar cells

Abstract

Dye-sensitized solar cells (DSCs) are more spotlighted than conventional photovoltaic devices due to their relatively low cost, easy fabrication and high efficiency. However, there are limitations to increase the conversion efficiency of DSCs. The limiting factors are the quantity of dye adsorption and charge recombination between TiO 2 electrode and electrolyte. Coating other materials such as high energy band gap insulators or semiconductors on the TiO 2 electrode enhances dye adsorption and reduces charge recombination. We fabricated DSCs based on bare TiO 2 nanotube arrays and 0.02 and 0.04 M MgO coated TiO 2 nanotube arrays. MgO layer increased the photovoltage and photocurrent. The overall conversion efficiency of DSCs using 0.02 M MgO coated TiO 2 nanotubes was 1.61%. MgO formed insulating layers between TiO 2 nanotube array electrode and electrolyte. Charge recombination was inhibited at the interfaces of TiO 2 nanotube array electrode and electrolyte by MgO insulating layers. MgO coating also improved dye adsorption because iso-electric point (IEP) of MgO was larger than TiO 2 . When the IEP of coating material is larger than TiO 2 , the chemical attraction between the electrode surface and Ru-based dye molecule is increased.

Organic and inorganic surface passivations of TiO2 nanotube arrays for dye-sensitized photoelectrodes

“…Increased photocurrents by PC 61 BM interlayer overcame V oc offset to negative potential and improved the power conversion efficiency of DSCs. It has been reported that blocking back-transfer of photoelectrons not only reduces charge recombination between TiO 2 films and liquid electrolyte but also improves V oc [8,19]. However, V oc was decreased by PC 61 BM interlayer despite of its blocking effect.…”

“…Firstly, surface-passivating layer reduces charge recombination between TiO 2 films and liquid electrolyte [5,8,15,19]. Electrons in TiO 2 films usually resides at surface trap states [30].…”

“…It was also reported that surface trap sites in TiO 2 films were decreased by surface-passivating layer [15]. Secondly, surface passivation of photoelectrodes is closely related to dye adsorption by introducing the concept of isoelectric point [8,9,16,18,19]. If surface-passivating materials have a higher isoelectric point (pI >7) than TiO 2 , the surface of photoelectrodes is positively charged during immersion process in N719 ethanol solution.…”

“…High band gap (E g ) oxides such as MgO [8,19], SrTiO 3 [16,18], ZnO [5], WO 3 [15], etc. were selected for surface-passivating materials, because the position of conduction band for surface-passivating materials should be higher than that of TiO 2 conduction band.…”

“…The thickness of surface-passivating layers was controlled to optimized value. When the thickness of surface-passivating layers is too thick, charge injection from dye molecules to TiO 2 film can be hindered and photocurrent is reduced [8]. The concept of isoelectric point [20] was used to select surface-passivating materials and to interpret the effect of interlayer between TiO 2 film and liquid electrolyte [9,17,19].…”