In this work, we study the influence of the distance between electrodes on the performance of dye-sensitized solar cells based on TiO using the organic dye LEG4 and a Cu(dmp) redox couple (dmp = dimethyl phenantroline). The solar cells are characterized by a large open circuit voltage of up to 1.03 V, and an efficiency of 8.2% has been achieved for a 5.3 μm thick TiO film using an epoxy resin-based sealed cell configuration with a minimal separation between electrodes. Transient short-circuit photocurrent measurements up to an intensity of 3 Suns show a significant decay in photocurrent after an initial peak current upon switching on the light for larger distance, resulting in a lower steady state photocurrent. For the smaller distance cells, the steady state photocurrent is linear with light intensity up to 2 Suns. Charge extraction measurements under short-circuit conditions show that reducing the distance between electrodes increases the electron collection efficiency and thus, the attainable photocurrent. Recombination losses increase with larger electrode separation distance and higher light intensity due to mass transport limitation of the redox mediator. Electrochemical impedance measurements confirm the effect of electrode distance on the redox couple transport, showing an additional loop with increasing distance. For the configuration where the TiO film is in very close proximity to the PEDOT-covered counter electrode, inductive behavior is observed at low frequencies. The inductive behavior disappears with the incorporation of an insulating porous ZrO layer. The equivalent circuit for the solar cell has been expanded to include this effect.
Any
optimization of dye-sensitized solar cells (DSSCs) must consider
the energetics and charge transfer kinetics of the dye, substrate,
and redox couple. Here, we use surface photovoltage spectroscopy to
probe the energetics and photochemical charge transfer efficiency
in fluorenyl-thiophene dye (OD-8)-sensitized ZnO films. Discrete photochemical
charge transfer events at the dye–ZnO interface and at the
dye– I–/I3
– or
[Co(2,2′-bipyridyl)3]3+/2+ interfaces
can be observed as negative photovoltage under dye excitation at 1.7
eV (460 nm). Without a redox couple, charge separation at the ZnO/dye
interface is only 4% effective, likely due to the short electron hole
separation distance. In the presence of the redox couples, charge
separation approaches 26–54% of the theoretical limit, emphasizing
the importance of the dye regeneration reaction via the redox couple.
On the basis of the open circuit voltage, charge separation in fully
assembled DSSCs is 100% efficient with iodide, but only 61% efficient
with the cobalt redox couple. This suggests that device improvements
are possible by optimizing the dye regeneration reaction with the
cobalt redox couple.
The relation between the morphological and textural properties and the performance of ZnO-based dye-sensitized solar cells is explored using ZnO materials prepared by a sonochemical method as a function of pH.
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