IMPROVING THE EFFICIENCY AND THE STABILITY OF DYE-SENSITIZED PHOTOCHEMICAL SOLAR CELLS AND WATER SPLITTING SYSTEM

Open Access
Author:
Lee, Seunghyun
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 08, 2010
Committee Members:
  • Thomas E Mallouk, Dissertation Advisor
  • Thomas E Mallouk, Committee Chair
  • Harry R Allcock, Committee Member
  • Nicholas Winograd, Committee Member
  • Michael Anthony Hickner, Committee Member
Keywords:
  • dye-sensitized solar cells
  • photonic crystals
  • non-volatile electrolytes
  • dye-sensitized photoelectrochemical water splittin
  • core-shell electrodes
Abstract:
Dye-sensitized solar cells (DSSCs) are promising solar energy conversion devices because of the low cost of their components. However, there are several challenges that must be overcome for their commecial development. The energy conversion efficiency of dye cells is still far below the level of silicon photovoltaic cells despite an enormous worldwide effort over the past two decades. The widely used N719 dye absorbs the light weakly in the 550 – 700 nm region. If red photons that are not absorbed by the dye can be utilized in DSSC, it will be possible to achieve a much higher efficiency. Several dyes with strong red absorbance have been reported, but because they have lower excited-state excess free energy, they typically have a lower quantum yield for charge injection. Instead of developing additional dyes, this dissertation introduces a novel way to enhance the device efficiency by designing a DSSC with a layer of titania inverse opal photonic crystals (PCs) that enhances light collection. The efficiency of DSSCs increased from 6.5 to 8.3% using titania PCs. This improvement derived from increases in both the short circuit photocurrent and open circuit photovoltage of the cell. Additionally, the enhancement is very sensitive to the degree of physical contact between the PC and nanocrystalline titania layers, and a few hundred nm gap between the layers is sufficient to decouple the two layers optically. Another issue that prevents DSSCs from commericialization is the use of volatile solvents in the electrolyte. This presents challenges for manufacturing and long term stability for outdoor applications. Hence, a novel solvent system, phosphazene-based nonvolatile electrolytes is examined to replace volatile solvents. Electrolytes based on hexa [methoxyethoxyethoxy cyclophosphazene] (MEE trimer) with dissolved LiI, NaI, NH4I, and 1-methyl-3-propylimidazolium (PMII) and I2 were examined. The anion conductivities were highest in the PMII system and decreased in the order PMII > NH4I > NaI > LiI. Photovoltaic measurements of DSSCs containing these electrolytes agree with the anion conductivity measurements. In addition, polyphosphazene-plasticizer blends with a dissolved PMII/I2 electrolyte gave better performance in DSSCs than equivalent poly(ethylene oxide)–plasticizer electrolytes. The efficiencies of these DSSCs are currently low (~1.9%) mainly due to a slow mass transport of I-/I3-. This dissertation addresses the primary loss mechanisms and proposes possible ways for improving polyphosphazene-based cells. The structure of the dye-sensitized solar cell can be modified to carry out the photoelectrolysis of water, producing H2 and O2. However, the quantum efficiency of the current dye-sensitized water splitting system is quite low, mainly due to fast charge recombination between photoinjected electrons in TiO2 and the oxidized dye at the surface. This study assesses the limitations of the current system, and investigates different insulating materials as a thin overlayer, for instance ZrO2 and Nb2O5 on the TiO2 electrode to improve the efficiency of the system. Having a thin layer of insulating materials increased the photocurrent and slowed charge recombination as confirmed by flash photolysis/transient absorption measurements.