Surface engineered angstrom thick ZnO-sheathed TiO2 nanowires as photoanodes for performance enhanced dye-sensitized solar cells

Abstract

This paper presents a systematic study on the effects of angstrom-thick atomic layer deposited (ALD) ZnO sheaths on hydrothermally-grown TiO₂ nanowires (NWs) used as photoanodes in dye-sensitized solar cells (DSSCs). We designed, synthesized and characterized the samples prepared using different numbers of ZnO cycles and compared their photovoltaic (PV) performances. The device consisting of TiO₂ NWs coated with the optimum thickness (two cycles) of ZnO shell exhibits a three-fold increase in efficiency compared to a control reference device. This paper reports results and features that demonstrate the passivation of surface state traps upon deposition of ZnO shells. While this passivation of surface traps provides a reduction in the back-reactions of the surface state mediated electrons (K^trap_ET), it is speculated that ZnO-induced surface band bending (SBB) substantially reduces the recombination rate of the device by reducing the recombination rate of the conduction band (CB) electrons (K^CB_ET). Moreover, an enhancement in the amount of dye uptake for ZnO-coated TiO₂ samples is observed and explained with the isoelectric point (IEP) concept. In spite of the excellent PV power conversion efficiencies achieved by the first ZnO cycles, thicker layers impede the electron injection rate, reducing the efficiency of the device by capturing the photogenerated dye electrons in ZnO quantum wells. Here, we investigate the mechanisms contributing to this unprecedented change and correlate them with the enhancement in device efficiency.