Au-nanoparticle-embedded open-ended freestanding tio₂ nanotube arrays in dye-sensitized solar cells for better electron generation and electron transport

Freestanding TiO₂ nanotube arrays (TNTAs) were prepared by an electrochemical method, and dye-sensitized solar cells (DSSCs) were fabricated with the open-ended freestanding TNTAs incorporated with Au nanoparticles (NPs). Open-ended freestanding TNTAs were prepared by etching the barrier layer of closed-ended freestanding TNTAs using an ion milling method, and Au NPs were incorporated into the channel of the open-ended freestanding TNTAs by an electrodeposition method. The Au-NP-embedded open-ended freestanding TNTAs were applied to DSSCs to improve the energy conversion efficiency (ECE) by better electron generation and electron transport. The ECE of DSSCs based on the closed-ended freestanding TNTAs with Au NPs increased to 6.116% from 5.502% for DSSCs based on the closed-ended freestanding TNTAs without Au NPs, an enhancement of 11.16% because of better electron generation by the plasmonic and charging effects of the Au NPs. However, the ECE of DSSCs based on the closed-ended freestanding TNTAs incorporated with Au NPs for 40 s decreased from 6.116 to 5.336% because aggregation of the Au NPs led to a decrease in the open-circuit voltage (V_oc) and fill factor. For enhanced ECE of DSSCs, the barrier layer of closed-ended freestanding TNTAs was etched by an ion milling method for 0, 30, 60, or 90 min to provide "open-ended freestanding TNTAs". Then, Au NPs were incorporated into the open-ended freestanding TNTAs. After the barrier layer was completely removed by the ion milling method for 90 min, the ECE of the DSSCs reached 7.120% because the electron transport and electrolyte diffusion were improved by the elimination of the barrier layer of the freestanding TNTAs.