Dye-sensitized solar cells using ZnO nanotips and Ga-doped ZnO films

Abstract

Dye sensitized solar cell (DSSC) is a promising and low-cost photovoltaic device. In the DSSC, a monolayer of dye is anchored on the surface of a semiconductor oxide.Photoexcitation takes place inside the dye, and photogenerated charges are then separated at the dye/oxide interface. Nanostructured oxide films are particularly attractive for DSSCs as they provide large surface area for dye anchoring. The crystalline quality of oxide films has significant impact on the charge transport. It is important to reduce the charge traps in the oxide to speed up the charge transport. In the DSSC, the transparent conducting oxide serves as an optical window, which determines the amount of light entering the device, and as the electrode, which extracts photocurrent. In the cell design, the selection of semiconductor oxide and corresponding transparent electrode is critical to achieve efficient light harvesting, charge separation and extraction.ii In this dissertation, a novel photoelectrode, consisting of well-aligned ZnO nanotips and a Ga-doped ZnO (GZO) TCO film, is developed for DSSC applications. The n-type semiconductor ZnO nanotips provide large surface area for dye anchoring in conjunction with direct conduction pathways for charge transport, while the GZO film acts as the transparent electrode. ZnO nanotips and GZO films are grown by metalorganic chemical vapor deposition (MOCVD). GZO films (~400 nm) with sheet resistance of ~ 25 Ω/sq and transmittance over ~ 85% are achieved. ZnO nanotips have single crystalline quality and show free exciton emissions at room temperature. Photoelectrochemical cells are fabricated using liquid redox electrolyte and semi-solid gel electrolyte, respectively. UV light harvesting, which is directly generated by ZnO photoelectrode, is observed. The DSSC using liquid electrolyte exhibits a quantum efficiency at ~530nm of 65% and power conversion efficiency of 0.77%. By replacing the liquid electrolyte with gel electrolyte in the DSSC with the same structure, the open circuit voltage is increased from 610 mV to 726 mV and the overall power efficiency is increased to 0.89%. The aging testing shows that the DSSC using gel electrolyte has better stability than its liquid electrolyte counterpart.iii

ZnO Nanostructures for Dye‐Sensitized Solar Cells

“…The DSC performance of these nanotips has been investigated in previous studies. [126,127] The results confirmed that the energy-conversion efficiency of the cells increased with the length of the ZnO nanotips due to the increase in surface area of the photoelectrode film. An overall conversion efficiency of 0.55% was obtained for 3.2-mm-long ZnO nanotips.…”

“…It has also been demonstrated that the overall conversion efficiency could be increased to 0.77% by combining the ZnO nanotips with a Ga-doped ZnO film as a transparent conducting layer. [127] 3.4. ZnO ''Nanoflowers''…”

Efficient Inverted ITO-Free Organic Solar Cells Based on Transparent Silver Electrode with Aqueous Solution-Processed ZnO Interlayer

“…What is more, indium is a scarce and toxic material, and thus, the highest share in material cost [13,14] and energy input [15,16] of ITO is inevitable in the fabrication of ITO-based OSCs. Therefore, more and more efforts are dedicated to the ITO-free or indium-free transparent electrodes, such as poly (3,4-ethylenedioxythiophene):-poly(styrenesulfonate) (PEDOT:PSS) [17,18], various transparent conducting oxides (Al-doped ZnO (AZO) or Ga-doped ZnO (GZO)) [19][20][21], Ag nanowires [22,23], carbon nanotubes [24], graphene [25], and so on. However, although the PEDOT:PSS electrode can be prepared by simple solution process, its conductivity needs to be further improved [26,27].…”

Photovoltaic Characterization of Dye-sensitized Solar Cells with Natural Dyes using Synthesized ZnO nanostructures

“…Furthermore, since ZnO is transparent to the whole visible spectrum, it is a good TCO material. Chen et al have recently carried out this concept by growing ZnO nanorods on a ZnO film using a two-step method 18 . In this work we grew ZnO nanorods on a ZnO film using sol-gel technique.…”