## 1 Introduction  
Since the initial development of dye-sensitized solar cells (DSSCs) in 1991 by the Gratzel group [1], global research has continued due to their low cost, ease of fabrication, and high power conversion efficiency [2; 3]. Titanium dioxide nanoparticles (TiO\({}_{2}\) NPs) are typically used as the photoanode in DSSCs because they have a desirable direct band gap (3.2 eV) and a large surface area for adsorbing dyes; both help to generate electrons [4; 5; 6; 7]. However, TiO\({}_{2}\) NPs are randomly networked, and the countless grain boundaries within them lead to material defects and charge recombinations that inhibit smooth electron transport [8; 9; 10]. In recent years, TiO\({}_{2}\) nanotube arrays (NTAs) have been explored as an alternative to TiO\({}_{2}\) NPs [11; 12; 13]. The TiO\({}_{2}\) NTAs can be fabricated using an electrochemical method (i.e., anodization) [11; 14], and their well-ordered, vertically aligned tubular structures serve as direct electron pathways; this enhances not only electron transport, but also charge separation [15; 16]. However, despite their merits, the barrier layer on the bottom of the TiO\({}_{2}\) NTAs may impede charge transfer and electrolyte diffusion. To overcome this problem, we recently removed the bottom layer of TiO\({}_{2}\) NTAs using argon ion (Ar\({}^{+}\)) milling, which resulted in improved electron transport and improved electrolyte diffusion [17]. There have been an increasing number of studies that add carbon to TiO\({}_{2}\) NTAs in order to improve the charge separation and transfer of electrons; this is due to carbon's superior electrical properties by \(\pi\)-\(\pi\) conjugation [18; 19; 20]. Many researchers have explored the application of carbon materials in solar cell technologies. Carbon 60 (C\({}_{60}\) or \"fullerene\") and carbon nanotubes (CNTs) are well known for their roles as electron acceptors and charge separators in organic solar cells [21; 22]. When incorporated in organic solar cells, CNTs act as exciton dissociation sites and hopping centers for hole transport [23], and in DSSCs, graphene mixed with TiO\({}_{2}\) NPs plays a role in promoting charge separation and movement [24]. As mentioned, TiO\({}_{2}\) NTAs were developed as alternatives to TiO\({}_{2}\) NPs. While it is not simple to blend carbon materials with TiO\({}_{2}\) NTAs, we recently reported a method for enriching freestanding TiO\({}_{2}\) NTAs with carbon for use in DSSCs. When a small amount of carbon was deposited on TiO\({}_{2}\) NTAs, compared to those without carbon enrichment, the energy conversion efficiency increased by approximately 22.4% [25]. We tentatively attributed this to an improved efficiency of electron transport by the \(\pi\)-\(\pi\) conjugation introduced through carbon enrichment. A plasmonic effect triggered by metal NPs (such as silver and gold) can be used to enhance photoabsorption in solar cells [26; 27; 28]. When incident photons pass by Ag NPs, they cause electron vibration and photo scattering of the nanoparticles, which facilitates photon control more efficiently [29]. The metal NPs were incorporated through mixing with TiO\({}_{2}\) sol in the DSSCs, or with precursors of the active layer in organic solar cells. However, it is difficult to insert metal NPs into the channels of TiO\({}_{2}\) NTAs, as the fabrication of DSSCs based on TiO\({}_{2}\) NTAs requires. We recently devised a simple method for the complete formation of Ag NPs in the channels of TiO\({}_{2}\) NTAs using ultraviolet (UV) irradiation [13; 30]. These NPs helped the dyes to generate electrons, as was demonstrated by a high current density in the DSSCs. To date, we have confirmed the separate effects of both carbon enrichment and the incorporation of Ag NPs in previous studies. However, the effects of adding both carbon and Ag NPs remain unknown. Here, we report such effects on the performance of DSSCs--in terms of enhanced electron transport and plasmonic effects--when enriching freestanding TiO\({}_{2}\) NTAs with one or both materials.