Charge transfer resistance of spray deposited and compressed counter electrodes for dye-sensitized nanoparticle solar cells on plastic substrates

https://doi.org/10.1016/j.solmat.2005.05.007Get rights and content

Abstract

Electrochemical impedance spectroscopy was used to determine the effective charge transfer resistances of porous dye-sensitized solar cell counter electrodes prepared by low-temperature spray deposition and compression of conductive carbon and platinized Sb-doped SnO2 powders on indium tin oxide-coated plastic substrates. The charge transfer resistances were 0.5–2 and 8–13 Ω cm2, respectively, when using 3-methoxypropionitrile as the electrolyte solvent. The manufacturing method used lends itself to produce mechanically stable and even-quality electrodes in an easy and fast manner.

Introduction

Over the past 10 years significant amount of research effort has been targeted worldwide to the development of dye-sensitized TiO2 nanoparticle solar cells. Dye-sensitized solar cells (DSSCs) have certain advantages over conventional silicon and thin film photovoltaic devices. The possibility to low-cost PV production due to the simplicity of the manufacturing process and cost-effectiveness of most of the cell materials makes this type of solar cells interesting for mass production. The dye solar cell technology could be used on a short-term perspective for low-current electronic consumer applications, such as wearable electronics, electronic paper, smart labels, etc.

While the basic dye solar cell is often built on a glass substrate, technological and practical advantage could be gained if the dye solar cell concept could be successfully utilized with light and flexible substrates such as plastics and metal foils. This is also the general trend in the development of other thin film solar cells such as a-Si and CIGS. Plastic substrates are also well suited for continuous and high throughput manufacturing process, for example the so-called roll-to-roll manufacturing, used in the paper and coating industry.

This study concentrates on the preparation methods of counter electrodes (CEs) of dye solar cells made on plastic substrates and on their characterization by electrochemical impedance spectroscopy (EIS). The task of a CE in the DSSC is to return the charge from the external load to the electrolyte reducing the oxidized form of the iodide/triiodide redox couple in it and thus keeping the operating cycle of the cell running. The charge transfer resistance between the electrode and the electrolyte appears as a voltage loss at the CE and it contributes directly to the series resistance of the whole cell. This means that a good CE material must possess low charge transfer resistance (i.e. good electrocatalytic activity), but also good mechanical and chemical stability.

The most commonly used CE in DSSCs is a fluorine-doped tin oxide (FTO)-coated glass, deposited with a small amount of platinum, for example, by sputtering [1]. Papageorgiou et al. [2] developed a catalyst based on thermal decomposition of a platinum chloride precursor at about 400 °C. Because of its high electrocatalytic activity towards iodine/triiodide reduction at low Pt loading, chemical and mechanical stability, and easiness of preparation, it is widely used in dye solar cells based on glass substrates, but the high temperatures involved makes it incompatible with plastic substrates.

Carbon has been used as an alternative CE catalyst material to platinum [3], [4], [5], [6], [7], [8]. The lower catalytic activity of carbon compared to platinum can be compensated by increasing the active surface area of the electrode by using porous electrode structure. Kay and Grätzel [3] achieved good solar cell performance in a monolithic cell construction on FTO glass with 60 μm thick porous electrodes made by depositing a powder mixture of graphite, carbon black and nanocrystalline TiO2 and sintering at 450 °C. Burnside et al. [4] prepared similar electrodes by screen printing and Papageorgiou et al. [5] studied mass transport in the electrolyte in these porous electrodes. Imoto et al. [6] showed that with CEs based on high specific surface area-activated carbon materials and heat treated at 180 °C, solar cell performance comparable to platinum catalyst CEs could be reached. Carbon nanotubes have also been successfully used [7], and carbon black has been used to improve electrical contact between the CE and a polypyrrole hole transport layer in a solid-state DSSC [8].

Recently, conducting polymer poly(3,4-ethlynedioxythiophene) (PEDOT) doped with either p-toluenesulfonate (PEDOT–TsO) [9], [10] or polystyrenesulfonate (PEDOT–PSS) [11] has been identified as an efficient CE catalyst material for DSSCs. These polymer electrodes are prepared at low temperature (110 °C or below) which is compatible with plastic substrates.

CEs on plastic substrates have usually been prepared by sputtering a small amount of platinum, e.g. [12], [13], [14], [15], [16]. Lindström et al. [17] developed an alternative method where the CE is prepared by compressing powder materials onto indium-doped tin oxide (ITO) coated polyethyleneterephtalate (PET) substrate at room temperature. Two types of materials were used in their study: either a graphite–carbon black–nanocrystalline TiO2 mixture similar to that used for monolithic cells on FTO glass [3], [4], but without heat treatment, or thermally platinized Sb-doped SnO2 particles. Compared to vacuum sputtering of Pt, these compressed CEs offer an interesting alternative where the manufacturing of the CE sheets could be made in a high throughput continuous process at room temperature. These compressible CE materials were selected as the materials for the present study.

The goal of this work was to study how well the compressed powder CEs meet the demands for electrocatalytic activity and durability, and what are their prospects from the point of view of reliable and fast manufacturing process, the underlying motive being to eventually upscale DSSC production, including CE, from laboratory into industrial scale.

The powder materials were spray deposited onto flexible ITO–PET substrates and compacted at room temperature to yield porous CE films. The electrodes were employed in a thin layer cell configuration, and their overall charge transfer resistances were determined with EIS. Thermal platinum electrodes on FTO-coated glass were prepared and characterized for comparison.

The results show that the charge transfer resistances obtained with these materials are well competitive to those obtained with platinum on glass. The manufacturing process is easy and fast, yielding electrodes with even quality. The adherence to the substrates is adequate and tolerance to mechanical stress good during the cell manufacturing.

Section snippets

Substrates

FTO glass (TEC 15 from Pilkington) with sheet resistance of about 15 Ω/sq. supplied by Hartford Glass Co. was used as the conductive substrate for thermal platinum CEs. ITO–PET (NV-CT-CHO1S-M-7) with sheet resistance of about 60 Ω/sq. and total film thickness of 190 μm supplied by Bekaert Specialty Films was used as the plastic substrate. The size of all substrates was 20×20 mm.

Before use, the substrates were washed first with warm water and mild detergent, then rinsed with distilled water, and

Interpretation of the measured electrochemical impedance spectra

Fig. 2, Fig. 3 show typical impedance spectra of compressed carbon powder and Sb:SnO2+Pt electrodes measured using the cell configuration depicted in Fig. 1. The measured impedance spectra were dominated by two characteristic frequencies resulting in two impedance arcs in the Nyquist plot (Fig. 2) and correspondingly two peaks in the imaginary part of the impedance (−ZIm) vs. frequency plot (Fig. 3).

As justified by theoretical reasoning and experimental evidence discussed in detail in the

Conclusions

The results obtained here show that low temperature spray deposition followed by compacting by compression is a promising manufacturing technique for DSSC CEs made of alternative materials, i.e. materials other than platinum on glass substrate.

Porous, conductive carbon powder or platinized Sb-doped SnO2 CEs sprayed and compressed on conductive plastic substrates showed satisfactory mechanical stability. Their charge transfer resistances were close to those of thermal platinum on FTO glass.

Acknowledgements

Financial support from the National Technology Agency of Finland (Tekes) is gratefully acknowledged. J. H. is grateful for the scholarship from the Nordic Energy Research (NEFP). The authors thank Dr. Jani Kallioinen for supplying the ITO–PET substrates for the study, Degussa for the carbon black and TiO2 samples, Milliken Chemicals for the Sb:SnO2 sample, Dupont for the Surlyn sheets and Mr. Christian Orassaari and Mr. Harri Jaronen for performing compaction of the powder electrodes.

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      When the dye is excited by sunlight, the CE transfers electrons from the external circuit to the redox couple. Generally, Pt is used in the CEs of DSCs since it has excellent catalytic activity for the effective reduction of I−/I3− and good electrical conductivity (Hauch and Georg, 2001; Gratzel, 2004; Nazeeruddin et al., 2005; Ahn et al., 2013; Halme et al., 2006; Murakami and Grätzel, 2008; Papageorgiou, 2004; Lee et al., 2010). However, it has the disadvantages of being relatively expensive and easily corroded by the liquid iodide electrolyte (Chiba et al., 2006; Olsen et al., 2000; Luo et al., 2009).

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