Plasmonic Effects on Dye-Sensitized Solar Cells

Abstract

Dye-sensitized solar cells (DSSCs) have received much attention on account of their low cost-to-performance ratio, easy scale-up, light weight, ability to work at wide angles, low intensities of incident light, and modifiable aesthetic features such as color and transparency. However, the energy conversion efficiency of DSSCs needs to be further improved to become an economically profitable alternative. To improve energy conversion efficiency of the DSSCs, many efforts were performed in various aspects, such as increasing light harvesting by using panchromatic dye or enlarging amount of dye adsorbed on TiO2 nanoparticles (NPs), and reducing charge transfer resistance of I3 reduction by using alternative counter electrode. In this thesis, as a part of the ongoing study, two types of approaches have been made to improve energy conversion efficiency of the DSSCs

1) developing new photoanode based on silver (Ag) NPs which has localized surface plasmon resonances (LSPRs) and 2) developing alternatives counter electrode based on graphene with gold (Au) NPs which has high catalytic activity and low charge-transfer resistance.

In the first part, we fabricated plasmonic DSSCs based on composite films consisting of TiO2 NPs and Ag NPs. The energy conversion efficiency of the plasmonic DSSCs was affected by the degree of the spectral overlap between the extinction bands of Ag NPs and two visible absorption bands of N719 dye, centered at 393 and 533 nm. Also, it was affected by the weight percent of Ag NPs to TiO2 NPs. The energy conversion efficiency was enhanced as increasing to a certain weight percent of Ag NPs but then decreased when the weight percent was further increased, which was due to aggregation of metal NPs. As a result of aggregation of metal NPs, the LSPRs were red-shifted and mismatched to the two visible absorption bands of N719 dye. Accordingly, in this case, the energy conversion efficiency was decreased. Therefore, the aggregation of metal NPs should be avoided in the fabrication of the composite films of TiO2 NPs and metal NPs to achieve a high energy conversion efficiency of surface plasmon-enhanced DSSCs. Next, to prevent aggregation of Ag NPs, a quasi-monolayer film based on Ag NPs was developed. Three kinds of Ag NPs with different size and extinction maximum wavelength were prepared and immobilized on a photoactive layer coated with poly(4-vinyl pyridine)(P4VP). As a result, developed quasi-monolayer film based on Ag NPs showed panchromatic behavior which absorbed in all of the visible range. By constructing a panchromatic quasi-monolayer between the photoactive and scattering layers, the efficiency of the plasmonic DSSCs was enhanced from 8.9 ± 0.3% to 11.0 ± 0.4%, mainly by increase the photocurrent density. Absorption of dye molecules might be enhanced at around the surface of Ag NPs by the LSPRs, because the quasi-monolayer of Ag NPs scattered light strongly. Owing to the enhanced absorption by LSPRs effect, we could greatly reduce the thickness of the photoactive layer, about one-half the optimum length.

As a second approach to achieve enhanced energy conversion efficiency, we fabricated a nanostructure-based graphene flake counter electrode by immobilization of gold nanoparticles (NPs) on fluorine-doped tin oxide (FTO) glass and the deposition of a thin layer of graphene flakes. The graphene flakes, fabricated using a thermal plasma jet system, were very thin and pure and had good crystallinity. Although their average size was larger than 100 nm, they were well dispersed in some solvents by sonication. Relatively large size and good crystallinity lead good conductivity, and their good dispersibility allow to fabricate uniform films. The efficiency of the DSSC with a graphene flake/Au NP/FTO counter electrode was as much as 9.71%, which is higher than that with a conventional Pt/FTO (9.02%) or graphene flake/FTO (8.91%) counter electrode. By measuring the power conversion and incident photo-conversion efficiency, we discovered that the high efficiency was due to the LSPRs effect of the Au NPs included between the graphene flakes and FTO. We also proved that the catalytic activity of a graphene flake/Au NP/FTO counter electrode was improved and the charge-transfer resistance at the electrode/electrolyte interface was decreased using cyclic voltammograms and electrochemical impedance spectroscopy.