Theoretical study on the interaction of iodide electrolyte/organic dye with the TiO2 surface in dye-sensitized solar cells

The iodide/triiodide interaction with the dye on a semiconductor surface plays a significant role in understanding the dye-sensitized solar cells (DSSCs) mechanism and improving its efficiency. In the present study, density functional theory (DFT) calculations were used to determine the interaction between the complexed iodide redox couple with dye/TiO2 for the relevance of DSSCs. Three new metal-free organic dyes noted as D1Y, D2Y and D3Y, featured with D-π-A configuration were designed by varying functional groups on the donor moiety. We analyzed the structural and electronic properties of these dyes when standing alone and being adsorbed on the oxide surface with the iodide electrolyte. Of the designed dyes, the incorporation of a strong donor unit in D1Y and D2Y sensitizers in conjunction with iodide electrolytes on the TiO2 surface provides better adsorption and electronic properties in comparison to those from the dye alone on the TiO2 surface. Analysis of density of states (DOS) indicates that the introduction of a strong electron-donating group into the organic dye, mainly D1Y and D2Y with an iodide electrolyte on the surface remarkably upshifts the Fermi energy, thereby improving the efficiency of the DSSCs by an increase of the open-circuit voltage (Voc). The present finding constitutes the basis for achieving a deeper understanding of the intrinsic interaction taking place at the electrolyte/dye/TiO2 interface and provides us with directions for the design of efficient dyes and redox electrolytes for improving DSSCs.

Flexible energy generation and storage devices: focus on key role of heterocyclic solid-state organic ionic conductors

This review addresses the vital role of solid-state electrolytes to develop highly efficient, customizable flexible energy generation and storage devices.

Effects of the terminal donor unit in dyes with D-D-π-A architecture on the regeneration mechanism in DSSCs: a computational study

This theoretical study on dye-sensitized solar cells (DSSCs) includes design strategies for dye donor units to improve the efficiency of DSSCs, and further illuminates the organic dye regeneration mechanism. We have designed a series of new organic sensitizers based on a D-D-π-A architecture to exhibit easy electron transfer and to have remarkable light harvesting properties in the visible region by density functional theory (DFT) and time-dependent (TD)-DFT calculations. Furthermore, the interaction of the organic sensitizers with the conventional redox electrolyte using the triiodide/iodide couple (I3-/I-) is investigated. Our calculations indicate that incorporation of strong electron-donating groups remarkably improves the charge transfer characteristics, optoelectronic properties and rapid dye regeneration as compared to less electron donating substituents. In addition, our study demonstrates the possibility of second electron injection from the oxidized dye complex to the semiconductor surface, which further confirms our recently proposed dye regeneration mechanism.

Microscopic observation of dye molecules for solar cells on a titania surface

The lateral distribution and coverage of Ru-based dye molecules, which are used for dye-sensitized solar cells (DSCs), were directly examined on a titania surface using high-resolution scanning transmission electron microscopy (STEM). The clean surface of a free-standing titania nanosheet was first confirmed with atomic resolution, and then, the nanosheet was used as a substrate. A single dye molecule on the titania nanosheet was visualized for the first time. The quantitative STEM images revealed an inhomogeneous dye-molecule distribution at the early stage of its absorption, i.e., the aggregation of the dye molecules. The majority of the titania surface was not covered by dye molecules, suggesting that optimization of the dye molecule distribution could yield further improvement of the DSC conversion efficiencies.

Effects of triphenyl phosphate as an inexpensive additive on the photovoltaic performance of dye-sensitized nanocrystalline TiO2 solar cells

In this study, triphenyl phosphate (TPP) is applied as an effective and inexpensive additive in the dye sensitized solar cells (DSSCs) and an increase in the photoelectric conversion efficiency is obtained of almost 24%.

Rapid Dye Regeneration Mechanism of Dye-Sensitized Solar Cells

During the light-harvesting process of dye-sensitized solar cells (DSSCs), the hole localized on the dye after the charge separation yields an oxidized dye, D(+). The fast regeneration of D(+) using the redox pair (typically the I(-)/I3(-) couple) is critical for the efficient DSSCs. However, the kinetic processes of dye regeneration remain uncertain, still promoting vigorous debates. Here, we use molecular dynamics simulations to determine that the inner-sphere electron-transfer pathway provides a rapid dye regeneration route of ∼4 ps, where penetration of I(-) next to D(+) enables an immediate electron transfer, forming a kinetic barrier. This explains the recently reported ultrafast dye regeneration rate of a few picoseconds determined experimentally. We expect that our MD based comprehensive understanding of the dye regeneration mechanism will provide a helpful guideline in designing TiO2-dye-electrolyte interfacial systems for better performing DSSCs.

DFT investigation on organic dyes with cross-conjugated cyano groups

Organic dye molecules with the acceptor moieties, cyano groups, cross-conjugated to the donor moieties, have been investigated theoretically. Density functional theory (DFT) calculations on such cross-conjugated molecules reveal the effects of cross-conjugation on the geometric and electronic structures of the molecules. The cross-conjugated cyano groups in the dye molecules are found effective to alter the charge population and the frontier orbital energy levels of the dyes. The effects of cross-conjugation of cyano group on the molecular conformation, the charge transfer, and polarity of the dyes are discussed.

Redox Reaction Mechanisms with Non-triiodide Mediators in Dye-Sensitized Solar Cells by Redox Potential Calculations

We investigate reaction mechanisms of the redox mediators in dye-sensitized solar cells through systematic calculations of redox potentials of possible cobalt complexes and iodides in acetonitrile solution by use of the thermodynamic cycle method with continuum solvent model. The calculated redox potentials were in good agreement with the experimental values, although the experimentalists used different reference electrodes. The maximum open circuit voltage (VOC) of the mediators calculated in this work indicate that the I2(•-)/2I(-) and I2/I2(•-) as well as the net I2/2I(-) redox reactions can dominate at both photoanode and counter-electrode.