211th ECS Meeting, Abstract #641, © The Electrochemical Society

Characterization and Modeling of Dye-sensitized Solar

Cells

LM Peter

Department of Chemistry, University of Bath

Bath BA2 7AY, United Kingdom

Recent progress towards understanding the  processes

taking place in dye-sensitized nanocrystalline solar cells(DSC) will be reviewed, and some areas characterized by

controversy or poor understanding will be highlighted.

Thermodynamic and kinetic criteria for successful celldesign will be summarized, and experimental results

obtained by several novel methods for characterizing the

stationary and dynamic properties of DCS will be

discussed. These methods include direct measurement of

the quasi Fermi level using an indicator electrode andcharge extraction measurements to determine the

energetic distribution of electron traps in the

nanocrystalline oxide (see below). The influence ofelectron trapping on dynamic measurements of electron

transfer and transport will be examined within the

framework of the quasi-static assumption, and a new

assessment of the electron diffusion length in the DSCwill be given, which suggests that collection of

 photoinjected electrons should be considerably more

efficient than previously assumed.

The charge extraction method involves allowing

the open circuit voltage of the dye cell to decay at opencircuit in the dark up to a pre-determined time, when the

cell is short circuited to extract the remaining charge. The

results obtained are illustrated by Figure 1, which showshow the experiment is repeated for different delay times

to obtain the trapped charge as a function of potential.

Figure 1. The top panel shows a series of five

experiments in which the open circuit decay was

interrupted by short circuiting the cell. The lower panel

shows the integrated current transients obtained, which

give the stored charge at the open circuit voltage valuechosen.

The open circuit decay also provides information about

shunting of the dye cell via the conducting glass substrate.

In the absence of a blocking layer, the decay is rapid as

electrons transfer via the fluorine-doped tin oxide layer to

the idodide/tri-iodide electrolyte. This process can be prevented by using a thin (50 nm) blocking layer of

compact TiO2 as illustrated in Figure 2

Figure 2. Open circuit photovoltage decay for dye cells

with and without a compact TiO2 blocking layer.

Analysis of the photovoltage decay for cells with a blocking layer provide information about the rate constant

for electron transfer from the nanocrystalline oxide to I3-

/I- as well as about the energetic distribution of electron

traps in the oxide [1]. Figure 3 shows an example of the

fit used to obtain these parameters (here Tc is atemperature that characterizes the energetic width of the

exponential trap distribution).

Figure 3. Photovoltage decay shown on a log-lin plot

together with the fit to theory used to obtain information

about the kinetics of electron transfer and the distributionof electron trapping states.

References.

Walker, A. B.; Peter, L. M.; Lobato, K.; Cameron, P. J.

 Journal of Physical Chemistry B 2006, 110, 25504.