Influence of electrolyte in transport and recombination in dye-sensitized solar cells studied by impedance spectroscopy

doi:10.1016/j.solmat.2004.07.017

Solar Energy Materials and Solar Cells

Volume 87, Issues 1–4, May 2005, Pages 117-131

https://doi.org/10.1016/j.solmat.2004.07.017 Get rights and content

Abstract

The main features of the characteristic impedance spectra of dye-sensitized solar cells are described in a wide range of potential conditions: from open to short circuit. An equivalent circuit model has been proposed to describe the parameters of electron transport, recombination, accumulation and other interfacial effects separately. These parameters were determined in the presence of three different electrolytes, both in the dark and under illumination. Shift in the conduction band edge due to the electrolyte composition was monitored in terms of the changes in transport resistance and charge accumulation in TiO₂. The interpretation of the current–potential curve characteristics, fill factor, open-circuit photopotential and efficiency in the different conditions, was correlated with this shift and the features of the recombination resistance.

Introduction

The optimization of dye-sensitized solar cells for their commercial use as an alternative power source of energy is the origin of research dedicated to the identification and description of the physical processes, such as injection, transport, accumulation and recombination of charge, that determine and limit the performance of the device [1].

Impedance spectroscopy is a well-known technique used for the study of electrochemical systems. The method is easy to measure but the correct interpretation of results needs the use of a suitable model. In the case of dye-sensitized solar cells the structure is a complex network of interconnected titanium dioxide (TiO₂) nanosized colloids deposited on a transparent conducting oxide (TCO) and permeated with a redox electrolyte. This is the reason why impedance models published so far, to describe the behavior of the dye-sensitized solar cell, are restricted to certain working conditions [2], [3] or apply only to separate parts of the cell [4], [5].

In this work we aim to describe the general features of impedance spectroscopy of complete dye-sensitized solar cells in a broad range of experimental conditions, both in the dark and under illumination, for the full range of operating conditions, i.e. from open circuit to short circuit passing through the different possible loads attainable by the solar cell under illumination. The results are interpreted using impedance models based on transmission lines [6] that describe the transport, accumulation and recombination of electrons in the semiconductor phase of the cells. Furthermore, the effect that electrolytes with different ion composition have on these properties is also analyzed in order to monitor the displacement of the conduction band of the semiconductor [7] and to describe differences found in parameters such as the open-circuit potential, fill factor, short-circuit current and efficiency of the different samples.

Section snippets

Experimental

Dye-sensitized solar cells were made from 12 nm diameter colloids deposited onto bare TCO (F:SnO₂, Hartford 8 Ω/□), sintered at 450 °C for half an hour and sensitized with cis-(NCS)₂(2,2′-bipyridyl-4,4′dicarboxylate)₂Ru(II) (N3 dye from Solaronix) overnight. Three different electrolytes were used in this study as shown in Table 1, where 3-MPN stands for 3-methoxypropionitrile, and MBI for 1-methylbenzimidazole, which is expected to have a similar effect as 4-tert-butylpyridine (4-TBP). Cells were

Results and modeling

Typical impedance spectra of a dye-sensitized solar cell at different applied potentials are shown in Fig. 1. Experimental data (○) have been fitted (—) to the model represented by the equivalent circuit shown in Fig. 2(a). In this figure, the representation of the network of colloids of TiO₂ has been simplified to a columnar model. The equivalent circuit elements have the following meaning [6]:

•
$C_{μ}$ (=c_μL) is the chemical capacitance that stands for the change of electron density as a function

Discussion

The observation of the impedance spectrum of Fig. 1(d) has two important implications on the description of the behavior of the dye-sensitized solar cell. The first one is related with the condition $R_{t} < R_{r}$ that yields to a value of the normalized diffusion length $L_{d} / L = \sqrt{R_{r} / R_{t}}$ [10] greater than 1. A diffusion length of electrons larger than the thickness of the film means that the transit time is shorter than the lifetime, and this is a necessary condition to efficiently collect the charge injected

Conclusions

A transmission line-based model was successfully used to describe the electrochemical behavior of dye-sensitized solar cells with different electrolytes in the dark and under illumination from the results of impedance spectroscopy. The characteristic impedance spectra have been discussed and the conditions needed to obtain an efficient dye-sensitized solar cell were commented on.

The parameters for transport, accumulation and recombination processes have been separated and determined using the

Acknowledgments

The authors want to acknowledge Eric Lewin for the preparation of the cells. This work was supported by projects BFM2001-3604 from MCyT, 02G014.31/1 from Fundació Bancaixa and JMB/JG/AP from Generalitat Valenciana, and by the Ångstrom Solar Center program of the Swedish Energy Agency (Swedish Foundation for Strategic Environmental Research).

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Influence of electrolyte in transport and recombination in dye-sensitized solar cells studied by impedance spectroscopy

Abstract

Introduction

Section snippets

Experimental

Results and modeling

Discussion

Conclusions

Acknowledgments

Electrochim. Acta

Electrochim. Acta

Sol. Energy Mater. Sol. Cells

Electrochim. Acta

J. Phys. Chem. B

Appl. Phys. Lett.

J. Phys. Chem. B

J. Phys. Chem. B

Phys. Chem. Chem. Phys.

J. Electrochem. Soc.

J. Phys. Chem. B