Aggregation effect of silver nanoparticles on the energy conversion efficiency of the surface plasmon-enhanced dye-sensitized solar cells

We have fabricated Ag nanoplates that have two broad extinction bands in the visible region, which are found in similar spectral regions of two visible absorption bands of N719 dye. The efficiency of the dye-sensitized solar cells (DSSCs) based on composite films consisting of TiO₂ and Ag nanoplates was affected by the degree of the spectral overlap between these bands and the weight percent of Ag nanoplates to TiO₂ nanoparticles (NPs). By optimizing the size and geometry of Ag nanoplates and the weight percent of Ag nanoplates to TiO₂ NPs, the energy conversion efficiency was improved from 8.7% to 10.3%. The energy conversion efficiency was significantly enhanced by including Ag nanoplates instead of Ag nanospheres. However, the efficiency increased up to 0.35wt% of Ag nanoplates but then decreased when the weight percent was further increased. The cause of the efficiency decrease for a further increase of Ag weight percent was studied. N719 dye has two strong absorption bands centered at 393 and 533nm, while black dye centered at 410 and 610 nm. The in-plane mode of the localized surface plasmon of Ag nanoplates near 530 nm is red-shifted when they are aggregated. Therefore, an enhanced absorption is expected on or near the surface of the isolated Ag nanoplates for N719 dye, while aggregated ones for black dye. The extinction of the TiO₂ NP/Ag nanoplate composite films adsorbed N719 dye was the highest when the weight percent of Ag nanoplates was 0.35 wt%, while that adsorbed black dye increased with increasing the weight percent up to 0.7 wt%. This means that aggregation of Ag nanoplates took place significantly when the percent was higher than 0.35 wt%. When the weight percent of Ag nanoplates was higher than 0.35 wt%, the efficiency of the plasmon enhanced absorption of N719 dye might be decreased by aggregation and consequently the energy conversion efficiency was decreased. Therefore, it is suggested that to get a high efficiency of surface plasmon-enhanced DSSCs, the aggregation of metal NPs should be controlled in the fabrication of the composite films.