Plasmonic-enhanced Dye-Sensitized Solar Cells with Ag and Au Nanoparticles

Abstract

Dye-sensitized solar cell (DSSC) is one of the efficient devices for generating electrons from solar light energy. Their advantages are low-cost processing, and various colors and transparent design for building integrated photovoltaics (BIPVs). Although DSSCs have numerous advantages, their power conversion efficiency (PCE) is lower than that of other solar cells. There are several ways to develop highly efficient DSSCs, such as improving light harvesting and electron transport. Applying plasmonic metal nanoparticles (NPs), exhibiting localized surface plasmon resonances (LSPRs), to the DSSCs seems to be very effective for highly efficient DSSCs. Because their strong plasmonic near-fields increase the photon scattering cross-section enormously, thereby increasing the overall dye absorption and efficiency improvement by many different route such as light trapping. Therefore, when plasmonic metal NPs are applied to the DSSCs, light harvesting or carrier collection can be improved. Many attempts have been made to apply plasmonic metal NPs to increase the efficiency in DSSCs. Among the various plasmonic metal NPs, Ag and Au NPs are the most widely used metallic materials for triggering plasmonic enhancement in solar cells, because of their remarkable optical properties. The maximum absorption wavelengths of spherical Ag and Au NPs are approximately 400 nm and 530 nm, respectively. The absorption bands of these NPs are well matched to those of N719 dye which is the most commonly used Ru-based dye molecules for DSSCs. Therefore, in this thesis, Ag and Au NPs were incorporated together in DSSCs to enhancing two absorption bands of N719 dye at the same time for highly efficient DSSCs. First, we fabricated Ag and Au NPs and used them to fabricate plasmonic DSSCs based on double-layered composite films. Compared to DSSCs without metal NPs, the PCE of the double-layered plasmonic DSSC enhanced from 8.42% to 10.03%, corresponding to 19% enhancement due to LSPR effect of Ag and Au NPs. The high efficiency of double-layered plasmonic DSSC might be due to a well optical spectra matching between the LSPRs of Ag and Au NPs and two strong absorption bands of N719 dye. For double-layered plasmonic DSSCs, the plasmonic metal NPs were dispersed into the TiO2 photoactive layer and play a role as light harvesting and charge separation sites. By LSPR effect of Ag and Au NPs, the electric field around the metal NPs enhances and the light absorption cross section of the dye and the number of generated photoelectrons increase which in turn increases the efficiency of DSSCs. Next, to fabricate highly efficient DSSCs, it is required to prevent the aggregation of metal NPs in the fabrication process of the composite film with TiO2 and metal NPs. Therefore, we fabricated plasmonic layer consisting of Ag and Au NPs and incorporated on the TiO2 photoactive layer of DSSCs. In this experiment, the plasmonic layer was fabricated by immobilizing plasmonic metal NPs on the surface of the TiO2 film coated with poly(4-vinylpyridine) (P4VP) to prevent the aggregation of metal NPs. The optimal conditions for metal NPs, such as immobilizing time and order, were examined. When both Au and Ag NPs were employed together at optimum conditions as the plasmonic layer, the PCE further improved from 8.39% to 10.17%, corresponding to 21.16% enhancement compared to DSSCs without metal NPs. The significant improvement of the PCE could be attributed to the LSPRs of plasmonic layer consisting of Au and Ag NPs. The plasmonic layer, which is located between the photoactive and scattering layers, functions as light scattering site and results in increase optical path length of the incident light, the light absorption and the electron transfer yields. Lastly, multi-shaped Ag NPs were prepared and applied to DSSCs to enhance their PCE by broad absorption in visible region. Prepared multi-shaped Ag NPs were composed of various shapes such as spherical, rod, and triangle structures, which exhibited broader absorption than that of the spherical Ag NPs. The absorption of the plasmonic layer based on multi-shaped Ag and Au NPs could cover the absorption range of N719 dye. To study the plasmonic effect of the multi-shaped Ag NPs, we have compared the effect of spherical Ag NPs and multi-shaped Ag NPs on the photovoltaic properties of DSSCs based on a layer-by-layer structure and a composite film structure with Ag and Au NPs. The maximum absorption wavelength (λmax) of the multi-shaped Ag NPs is 420 nm, including the shoulder with a full width at half maximum (FWHM) of 121 nm. This is a broad absorbance wavelength compared to spherical Ag NPs, whose λmax is 400 nm, without the shoulder of 61 nm FWHM. For DSSCs based on layer-by-layer structure with multi-shaped Ag and Au NPs, the PCE increased from 9.90% to 10.22%, a 3.2% enhancement, compared to DSSCs with spherical Ag and Au NPs. The PCE of the DSSCs based on layer-by-layer structure with multi-shaped Ag and Au NPs enhanced by 21.09%, compared to DSSCs without metal NPs. Similar to the layer-by-layer structure, the PCE of DSSCs based on the composite film structure with multi-shaped Ag and Au NPs increased from 9.99% to 10.34%, a 3.5% enhancement, compared to DSSCs with spherical Ag and Au NPs. The PCE of the DSSCs based on composite film structure with multi-shaped Ag and Au NPs enhanced by 20.51%, compared to DSSCs without metal NPs. It is concluded that the DSSCs with spherical Ag or multi-shaped Ag NPs was improved by the plasmonic effect, and the DSSCs with multi-shaped Ag NPs, which have broader absorption wavelengths range in the absorption of N719 dye at 393nm, exhibited better PCE than the DSSCs with spherical Ag NPs.