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.