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 TiO 2 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 TiO 2 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 TiO 2 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.

Emerging photoluminescence from defective vanadium diselenide nanosheets

“…However, the shoulder positioned at a higher energy of 531.8 eV shows the existence of chemisorbed oxygen species on the sample. This has been proved in several previous studies [76][77][78][79]. Based on the calculation of free energy by the classical nucleation theory, most of the water-derived hydroxyl groups, such as OH radicals and H 2 O, have a high tendency to be chemisorbed near imperfections, such as defects (like vacancy levels) and grain boundaries.…”

~3-nm ZnO Nanoislands Deposition and Application in Charge Trapping Memory Grown by Single ALD Step

“…This conclusion has been already confirmed by XPS measurement in which the O1s spectra show the existence of V O ’s on the as prepared samples, while there is no sign related to Zn i in Zn2p spectra. These oxygen vacancies are of three types: Vo (neutral, filled with 2 electrons, usually found at 0.4 eV from the valence band) V O +1 (charged, filled with one electron or unoccupied and doubly degenerate, usually found at 1.04 and 2.56 eV from the valence band, respectively) and V O +2 (charged, unoccupied and doubly degenerate, usually found at around 3.1–3.2 eV from the valence band)343637. Figure 6 shows these estimated defect levels within the bandgap of our bulk ZnO3738.…”

“…Moreover, Zinc-Oxide (ZnO) has recently received considerable attention by industry and research due to its excellent chemical and physical properties such as large bandgap, high exciton binding energy (60 meV), high transparency, high electron mobility, and high mechanical, chemical, and thermal stability at room temperature. These unique properties turn ZnO into a very attractive material with potential in numerous applications in electronics, optoelectronics, photocatalysis and laser devices78910111213. ZnO is most commonly deposited by either Atomic Layer Deposition (ALD) or physical vapor deposition such as sputtering.…”

Engineered ZnO‐TiO2 Nanospheres for High Performing Membrane Assimilated Photocatalytic Water Remediation and Energy Harvesting

“…[19][20][21] In our previous work, the composite of ZnO and TiO 2 was demonstrated to exhibit effective surface charge trapping with increased charge isolation as they have matching band energy levels thereby creating a perfect heterojunction with photo induced electrons enrichment in ZnO and holes in TiO 2 . [22][23][24][25] This lead to decrease recombination of charge exciton pairs consequently increasing the lifetime and efficiency of dye sensitized solar cell. The surge of the available surface charge of ZnO-TiO 2 can also result in it being an efficient photocatalyst ultimately improving the redox activity.…”