Plasmonic Enhanced Dye-Sensitized Solar Cells with Redox-Controlled Electrolyte

Abstract

After the evolution of the third generation photovoltaic cells, dye-sensitized solar cell (DSSC) has been considered as one of the most promising candidates with merits of easy fabrication and low material cost. With the development of DSSCs, conventional solid-state photovoltaic technologies are challenged by devices functioning at molecular or nanometer scale. DSSCs have large flexibility in shape, color, and transparency, and its mechanism is well understood.

Plasmonic enhanced DSSCs with metallic nanostructures suffer from corrosion problems, especially with the presence of iodine/triiodide (I⁻/I₃⁻) in the electrolyte. In this research work, an alternative approach was introduced by compensation of iodine corrosion with a modified liquid electrolyte. In contrast to existing method of surface preservation for the plasmonic nanostructures, the redox-controlled electrolyte (RCE) contains iodoaurates intermediates, i.e. gold(I) diiodide (AuI₂^-) and gold(III) tetraiodide (AuI₄^-) with optimal concentrations, such that these intermediates are readily reduced to gold nanoparticles during the operation of the DSSCs. As corrosion and redeposition of gold occur simultaneously, it effectively provides corrosion compensation to the plasmonic gold nanostructures embedded in the photoanode. Cycling tests on DSSCs with various amount of gold content in the RCE of DSSCs show that the dissolution and deposition of gold in the RCE are reversible and repeatable. This gold deposition on the TiO₂ photoanode results in forming a Schottky barrier (SB) at the metal-semiconductor interface and effectively inhibits the recombination of the electron-hole pair. Therefore, the RCE increases the short-circuit current, amplifies the open-circuit voltage, and reduces the impedance of TiO₂/dye interface. The power conversion efficiency of DSSC was improved by 57 % after incorporating the RCE.

The I⁻/I₃⁻ redox couple has been widely used as the redox mediator. The main advantage of this redox couple is the suppressed recombination of injected electrons with triiodide providing high photocurrents. However, the I⁻/I₃⁻ electrolytes can cause large internal potential losses for oxidized dye regeneration. In addition, there may be problems of corrosive and competitive visible light absorption. The introduction of new copper complexes as promising redox mediators in DSSCs may enhance power conversion efficiency. Therefore, a copper bipyridyl complex, Cu(I/II)(dmp)₂TFSI_2/1 was studied as a new redox couple for DSSCs. Due to the small reorganization energy between Cu(I) and Cu(II) species, this copper complex can sufficiently regenerate the oxidized dye molecules with close to unity yield at driving force potentials as low as 0.1 V. However, the solar-to-electrical power conversion efficiencies for [Cu(dmp)₂]^2+/1+ based electrolytes was only about 1.3 %, when N719 dye was still used under the 1000 W m^-2 AM 1.5G illumination. The use of copper model complexes with a distorted tetragonal geometry, in which the structural change between the Cu(I) and Cu(II) complexes is minimized, provides a promising strategy to develop efficient and low cost photovoltaic cells, because nature has successfully utilized such a strategy to develop efficient electron-transport systems using blue copper protein as electron mediators.

To understand the essential factor governing the functions of materials, computational method has been extensively applied. An investigation was carried out on a number of chemical reactions of complete DSSCs that allows the performance of different device compositions and architectures to be predicted by density functional theory (DFT). The model also allows the complicated chemical reactions behavior of a DSSC during the many sophisticated operation to be interpreted in terms of more fundamental processes in the cell. Further analyses revealed that by using the DFT method to investigate a path when the initial and final states have been identified. An important class of algorithms that is capable of such analysis is the chain of state method that represents the path joining reactants and products as a sequence of structures. The nudged-elastic-band (NEB) method was used to find the minimum energy path (MEP) between a given initial and final state of the chemical reactions mentioned above. The results helped to explain the paths of ionic interactions in the DSSC. Our findings may pave a new way towards the development of plasmonic enhanced DSSCs with corrosion-free electrolyte for various applications.

Date of Award	18 Mar 2019
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Lawrence WU (Supervisor)