(641c) Electron Transport And Recombination In ZnO Nanowire Dye-Sensitized Solar Cells

The
dye-sensitized solar cell (DSSC) is one of the most promising alternatives to
conventional inorganic p-n junction based solar cells. A typical DSSC consists
of a porous film made out of sintered TiO₂ (or ZnO) nanoparticles, a
monolayer of dye adsorbed on the TiO₂ surface and a liquid
electrolyte. The electrolyte fills the pores of the nanoparticle film forming a
semiconductor-dye-electrolyte interface with large surface area. During
illumination of the cell, the dye molecules inject electrons into the TiO₂
nanoparticles. The injected electrons diffuse through the nanoparticle network
by hopping from particle to particle until they are collected at a transparent
conductive oxide (TCO) anode. Meanwhile, the charged dye molecules are reduced
through an electrochemical reaction with a reductant in the electrolyte. The
oxidized ionic species diffuse to the counter electrode and are reduced by
electrons that have been collected at the anode and have traveled through the
load to complete the circuit.

Electrons
injected into the TiO₂ move through the particle network by random
walk and visit a large number of nanoparticles (~10³-10⁶). 
During this random walk through the nanoparticle network, a large fraction of
the photogenerated electrons can recombine with the oxidized species in the
electrolyte before reaching the anode. The ratio of the electron transport time
constant to the recombination time constant determines the charge collection
efficiency and affects the overall solar cell power conversion.  In order
to have high charge collection efficiencies, the transport rate should be much
faster that the recombination rate.  This requirement limits the dye
sensitized solar cells to essentially a single electrolyte and redox mediator
system, I₃^-/I^-. 
Recombination of electrons with I₃^- at the TiO₂
surface is remarkably slow so that even with many particle-to-particle hops,
the majority of electrons are still collected at the anode.  However,
solar cells assembled with other electrolytes or solid state hole transporting
materials, in lieu of I₃^-/I^-, suffer from low
charge collection efficiencies because, in such devices, the transport and
recombination rates become comparable. On the other hand, alternative
electrolytes and hole transporting materials have desired advantages and may
improve long term reliability as well as open circuit voltage if the ratio of
recombination time constant to the transport time constant can be improved.

Recently,
photoanodes made up of nanowires instead of nanoparticles have been proposed as
an alternative. Nanowires have the potential to improve the electron transport
rate through the photoanode since the electron percolation through the
nanoparticle network is replaced by direct electron transport from the point of
injection to the TCO anode. Understanding electron transport and recombination
in nanowire DSSCs is one of the key steps to overcoming the current DSSC
efficiency limit.  In this study, we report on electron transport and
recombination properties of DSSCs made using ZnO nanowires aligned
perpendicularly to the TCO anode. More specifically, we studied the dependence
of transport and recombination times on light intensity and nanowire length
using transient photocurrent and photovoltage spectroscopies. Electron
transport times were measured using intensity-modulated photocurrent
spectroscopy (IMPS) and a new technique based on photocurrent decay. Both in
IMPS and in photocurrent decay methods, the solar cell is illuminated with a
laser whose intensity is varied as a function of time in a prescribed way and
the ensuing transient response of the solar cell photocurrent is
recorded.  In IMPS, a small sinusoidal modulation is superimposed on
constant illumination intensity and the transient response of the solar cell is
measured as a function of the modulation frequency.  The electron
transport time is determined from the Nyquist or Bode representation of the
solar cell response.  In the photocurrent-decay method, the laser
illumination intensity is rapidly changed from one steady state level to
another using a square wave modulation and the ensuing photocurrent decay is
recorded. Electron recombination times were measured using open-circuit
photovoltage-decay where the photovoltage decay at open circuit is monitored as
a function of time after the illumination is turned off. 

We
found that the electron transport and recombination rates are very sensitive to
the solar cell preparation steps.  Measured electron transport time
constants varied in the range 0.1-10 ms, while the electron recombination rates
were between 0.1-100 s. The electron transport time constant increases with
nanowire length but the dependence of the transport time constant on the
nanowire length, ℓ, is weaker
than the expected ℓ² scaling, indicating that trap states
still play a significant role in electron transport even in single crystal
nanowires.