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High-Performance Dye-Sensitized Solar Cells with Conducting Polymer/ Carbon Nanotube Composites as counter electrode

Dye-sensitized solar cells (DSCs) have attracted strong attention due to the respectable energy conversion efficiency, the simple device fabrication process, and low cost. Though lots of effort has been made in the work electrode, the counter electrode also plays a key role by catalyzing the reduction of the redox species. A high-performance DSC requires the counter electrode to be highly catalytic and high conductive. Hence, platinum (Pt), which is a good catalyst for the reduction of the redox species, such as triiodide/iodide, is usually used as the counter electrode of the DSC. However, the noble Pt remarkably increases the cost of the DSC. In addition, the best Pt counter electrode of the DSC is produced by a high-temperature hydrolysis process. The high-temperature hydrolysis process brings a problem to deposition Pt on a flexible plastic substrate, which is needed for the flexible DSCs. Thus, other materials have also been investigated to replace Pt as the counter electrodes of the DSCs, such as carbon black, conductive polymers, and polymer/platinum composites. Carbon nanotube (CNT) shows strong potential as the counter electrode for the DSCs due to the fast electron transfer on CNTs. A crucial step for the CNT as the counter electrode is the formation of a high-quality CNT film on the substrate. We investigated to disperse multi-wall carbon nanotubes (MWCNTs) in aqueous solution of conducting poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, chemical structure shown in the inset of Figure 2). The PEDOT:PSS/MWCNTs composite films were fabricated by spin coating the composite solution on fluorine-doped tin oxide (FTO) glass substrates. Figure 1 shows the SEM image of a MWCNT/PEDOT:PSS composite film on FTO glass substrate. The MWCNTs were clearly observed, and they formed a network in both composite films (Figure 1). The composite films were used as the counter electrodes of the DSCs. The DSCs were fabricated with a porous TiO2 layer of 14 mm in thickness as the work electrode, cis- bis(isothiocyanato) bis(2,2'-bipyridyl-4,4'-dicarboxylato)- ruthenium(II)) as the dye and a PEDOT:PSS/MWCNT film as the counter electrode. The photovoltaic behavior of the DSCs was tested with AM 1.5 solar simulator (Oriel). The current (I)–voltage (V) curves of the DSCs with different counter electrodes are shown in Figure 2. The DSC with the MWCNT/PEDOT:PSS counter electrode exhibited high photovoltaic performance: Jsc = 15.5 mA cm-2, Voc = 0.66 V, FF = 0.63, and η = 6.5%. This performance is close to that of the DSC with the Pt counter electrode, and is significant higher than that with the MWCNT/PSSA counter electrode. In conclusion, dye-sensitize solar cells with a MWCNT/PEDOT:PSS film as the counter electrode were demonstrated. The counter electrodes were fabricated by a low-cost solution processing at room temperature. The devices exhibit high photovoltaic performance, which is close to the DSCs with Pt as the counter electrode.

Figure 1: SEM image of a PEDOT:PSS/MWCNT film

Figure 2: Current-voltage curve of a dye-sensitized solar cell with a MWCNT/PEDOT:PSS composite film as the counter electrodes. The inset is the chemical structure of PEDOT:PSS.

Contact Person : Dr. Ouyang Jianyong

E-mail: mseoj@nus.edu.sg

Tel : 6874 1472
Fax: 6776 3604