The Role of Hole Transport between Dyes in Solid-State Dye-Sensitized Solar Cells

Abstract

In dye-sensitized solar cells (DSSCs) photo-generated positive charges are normally considered to be carried away from the dyes by a separate phase of hole transporting material (HTM). We show that there can also be significant transport within the dye monolayer itself before the hole reaches the HTM. We quantify the fraction of dye regeneration in solid state DSSCs that can be attributed to this process. By using cyclic voltammetry and transient anisotropy spectroscopy we demonstrate that the rate of inter-dye hole transport is prevented both on micrometer and nanometer length scales by reducing the dye loading on the TiO2 surface. The dye regeneration yield is quantified for films with high and low dye loadings (with and without hole percolation in the dye monolayer) infiltrated with varying levels of HTM. Inter-dye hole transport can account for >50% of the overall dye regeneration with low HTM pore filling. This is reduced to about 5% when the infiltration of the HTM in the pores is optimized in 2μm thick films. Finally, we use hole transport in the dye monolayer to characterize the spatial distribution of the HTM phase in the pores of the dyed mesoporous TiO2.

Donor-acceptor polymer for the design of All-Solid-State dye-sensitized solar cells

“…The inter-molecular electrostatic energy of this interaction is -8.72 kcal/mol, which further confirms the evidence of charge transferring between HTM and N3. It is well-known from the literature that a good amount of this type of dye work as donor-π-electrons -acceptor where a large orbital density of the LUMO should cover the carboxylic anchoring groups and a small electronic cloud density cover their HOMO [11][12][13][14][15][16]53]. This similar type of phenomenon is observed in the molecular orbitals of N3 (Fig.S5) which is consistent with the already reported work [11][12][13][14][15][16]53].…”

“…It is well-known from the literature that a good amount of this type of dye work as donor-π-electrons -acceptor where a large orbital density of the LUMO should cover the carboxylic anchoring groups and a small electronic cloud density cover their HOMO [11][12][13][14][15][16]53]. This similar type of phenomenon is observed in the molecular orbitals of N3 (Fig.S5) which is consistent with the already reported work [11][12][13][14][15][16]53]. The higher electronic cloud density at LUMO compared to that of HOMO shows that the charge injection is favored from HTM to dye and prevents charge recombination in the dye molecule.…”

“…The dye molecule (photosensitizer) absorb light and injects its excited electrons into the conduction band of TiO2 and become oxidized. The redox electrolyte act as an intermediate to transfer hole from dye to counter electrode for regeneration of dye [11][12][13]. Although, highly efficient DSSCs have been reported but they are facing longterm performance and durability problems which are stem to liquid electrolytes; the I 1-/I 3-redox couple, leads to serious problems such as electrode corrosion and electrolyte leakage [14][15][16].…”

Enhancement of hole mobility in hybrid titanium dioxide/poly(3-hexylthiophene) nanocomposites by employing an oligothiophene dye as an interface modifier

“…More recently, it has been shown that overall device performance of TiO2 / polymer solar cells can be enhanced by more than a factor of two using a self-assembled monolayer 23 on TiO2 nanoparticles with permanent dipole pointing towards the TiO2 surface or pointing towards polymer when compared to a control device with no interface modifiers. It has also been demonstrated that inter dye hole-transport 24,25,26 may contribute to current collection in solid-state dye-sensitized solar cells.…”

How mobile are dye adsorbates and acetonitrile molecules on the surface of TiO2 nanoparticles? A quasi-elastic neutron scattering study

“…However, in addition to these functions, lateral hole diffusion can occur in the dye monolayer by charge transfer between molecules anchored to adjacent sites on the TiO 2 surface11121314151617181920. These interesting phenomena have been shown to improve the performance of solid state DSSCs2122, and may play a role in recombination within liquid DSSCs and devices for water splitting23. Indeed, DSSCs can work without a separate hole transporting phase, with holes being collected solely via the dye monolayer18.…”