In this thesis, the dynamics and quantum yields of electron injection occurring in
liquid and solid state dye sensitised solar cells (DSSCs) based on titanium dioxide
(TiO2) anodes sensitised with Ru – polypyridyl or organic dyes have been measured.
The electron injection process is investigated through both experimental and
modelling studies. A transient emission technique based on time correlated single
photon counting (TCSPC) has been developed to measure the kinetics and yields of
injection occurring in both films and devices. Other processes occurring in the device
are probed using a range of experimental techniques, including transient absorption
spectroscopy and transient photovoltage.
Initially the principles of the TCSPC measurement technique are introduced and the
procedure for measuring the injection in samples is outlined. Comparison of
appropriate control sample measurements, which show transient emission decay
dynamics in the absence of electron injection, with the TiO2 sample traces enables the
quantification of injection occurring in each experimental sample. TCSPC emission
decays associated with each sample are then fitted using stretch exponential functions
constrained by two degrees of freedom. This TCSPC technique for measuring electron
injection dynamics is validated by showing agreement with previously published
kinetics for an analogous system as measured by a well established ultrafast transient
absorption technique. The fits to the TCSPC decay dynamics are also shown to be
accurately replicated by Monte Carlo integrations based on a previously published
model of the active dye / TiO2 interface in the DSSCs. The technique is extended to
probing DSSCs employing a range of different sensitisers and measuring the kinetics
under different operating conditions occurring within the DSSCs where injection is
found to only depend strongly on the concentration of potential determining additives.
The first results chapter describes the TCSPC technique and gives examples of the
data analysis procedures associated with each transient emission decay measurement.
The agreement between injection kinetics measured using TCSPC with those
measured using ultrafast transient absorption technique is highlighted. The model of
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the active dye / TiO2 DSSC interface is introduced and Monte Carlo integrations
based on this physical model are shown to agree well with the experimental data.
The second results chapter extends the measurement of injection kinetics to different
Ru – polypyridyl based sensitisers. Injection kinetics are measured for a structure –
function dye series and the observed variations in the kinetics and yields are explained
with reference to the dye / TiO2 interface. The measurements are extended to
completely solid state DSSCs and successful fitting of the TCSPC data with
integrations based on the physical model show dispersive injection kinetics observed
in solid state DSSCs are controlled by the same parameters as the liquid cells.
The third chapter looks at a variety of factors which may affect injection in complete,
operating DSSCs. The factors addressed include presence of the commonly used
iodide / triiodide redox couple, residual effects of acid versus base film synthesis
procedures, effect of increasing the Fermi level in the DSSC and changing the
concentration of potential determining ions in the redox electrolyte. The major
controlling factor is found to be the concentration of the potential determining,
commonly used tert – butyl pyridine device additive and implications of this on
DSSC performance are discussed.
The last chapter compares device parameters for DSSCs based on successful organic
sensitiser with DSSCs based on the commonly used Ru – polypyridyl N719. Features
which control the performance of organic dyes in general are outlined and the reduced
performance of DSSCs employing these dyes is explained.