Nanomaterials for Improved Dye–Sensitized Solar Cells

With the growing interest in green energy, solar cells have become a potential solution to mitigate the problems of global warming. Scientists and engineers are seeking next-generation photovoltaic systems exhibiting high efficiencies, low costs, and the possibility of high-throughput manufacture. Since the pioneering studies of Grätzel et al., dye-sensitized solar cells (DSSCs) have blossomed to have great potential for commercialization.³⁷ DSSCs provide some advantages over conventional solid state solar cells: insensitivity to oblique light incidence and temperature effects;

very-low-cost, simple fabrication; transparency; and flexibility. DSSCs comprise four main building blocks: a photoanode of TiO2 nanoparticles (NPs), a light-absorbing dye, an iodide electrolyte, and a Pt counter-electrode.³⁸ Photo-excitation of the dye results in injection of electrons into the TiO2 NPs, while holes are released to the iodide/triiodide couples in the electrolyte. Carriers are transported in the conduction band of TiO2 and diffuse to the charge collector of the transparent conductive oxide (TCO). The iodide is reduced to triiodide at the counter-electrode. The circuit is completed through electron migration through the external load.³⁹

In the past few years, increases in DSSC efficiencies have been attributed to improvements in the syntheses of ruthenium complex dyes and in the nanoscale architectures of the TiO₂ networks.^40-41 Several ruthenium dyes have been synthesized that provide excellent IPCEs, some reaching greater than 11%.^42-43 Although the development of photoanodes has been relatively lagging, it remains one of the most promising avenues toward enhancing device performance. The ability to fabricate modern TiO₂ networks is expected to have a positive impact on solar cell performance by providing more-direct migration pathways with improved charge transport efficiencies.⁴⁴ Porous networks of TiO₂ NPs typically serve as the photoanode of DSSCs on the surface of transparent conducting glass. Unfortunately, the broad distribution of interconnections resulting from a randomly packed NP network significantly retards the transport dynamics.⁴⁵ The structured disorder associated with the contacts between the TiO₂ NPs enhances the scattering of free electrons, thereby reducing electron mobility.⁴⁶

Recently, TiO₂ nanotube (NT) arrays have been considered as alternative modern electrodes, replacing TiO2 NPs, for highly efficient DSSCs.^47-50 TiO2 NT arrays, which possess an ordered and strongly interconnected nanoscale photoanode architecture, provide superior electron lifetimes and more-direct migration pathways for electron percolation; they also result in markedly higher charge-collection efficiencies and light-harvesting efficiencies relative to those of traditional NP films.

TiO₂ NT arrays are prepared through anodization of thick-film Ti foils or Ti films on transparent conducting glasses. Intuitively, NT-based DSSCs should have higher efficiencies than their NP-based counterparts; practically, however, the overall conversion efficiencies that have been reported previously are similar for both types of film structures. The results are imputed to aggregation of the NTs, arising from the capillary forces, during the fabrication process. The individual NTs are transformed

into clusters of bundled NTs during the anodization process or electrolyte injection, and may be accompanied by cracking of the film. Similar phenomena have been observed for other one-dimensional nanostructures (e.g., Si nanowires and carbon NTs).⁵¹ The bundling of NTs, much like porous NP films, produces many unnecessary interconnections. These contacts substantially retard electron transport and the dye-loading capacity, thereby leading to poor efficiency.⁵²

Hupp and co-workers have developed one solution to this problem by separating the TiO2 NTs using commercial porous anodic alumina (PAA) membranes.^53-54 The reactive gas precursor was coated into PAA conformably using an atomic layer deposition (ALD) system to form TiO2 NTs along the pore wall. Kang et al. fabricated highly ordered TiO₂ NTs using the same PAA templating method.⁵⁵ A modified sol–gel route was used to infiltrate the commercial PAA with Ti(OiPr)4, which was subsequently converted into TiO₂ NTs. The photovoltaic performance of such TiO2/PAA cells was, however, barely acceptable. Theoretically, the separating of NTs, without interconnection, should improve electron transport, leading to higher photoefficiencies; in addition, the alumina layer should slow the recombination of photogenerated electrons on the TiO₂ conduction band and holes in the electrolyte or the oxidized dye. A problem inherent to these devices is poor contact between commercial PAA and the TCO layer, resulting in a loss of current transportation.

Besides, macro/micro-crack formation during the transfer of freestanding PAA films onto TCO substrates can also have a negative effect on photovoltaic performance.

Growing PAA directly on TCO substrates might overcome the aforementioned problems. A uniform PAA, with sufficient adhesion to the TCO layer to withstand thermal and surface treatment, is imperative for DSSC fabrication.^56-57 Although promising in theory, practical implementation of this approach has two major challenges. The first is poor anodization on the Al/TCO bilayer, mainly due to poor

adhesion between Al and TCO. When a noble metal or TCO serves as the underlying substrate, the evolution of gaseous O2 produces stress that damages the PAA/TCO adhesion. As many previous reports have described, the upper PAA eventually delaminates from the underlayer, weakening the performance of the resulting cells.^9,58 The second major challenge is forming a thick alumina barrier at the interface between the PAA and the TCO layer.^59-60 The formation of PAA, either from Al foil or from Al and a substrate, accompanies a barrier layer at the bottom of the pore channels. The isolated barrier layer would prohibit the connection of the pore channels and the substrate; in the DSSC application, it would block current transport between the TiO2 NTs and the TCO layer. In addition, because the current density is inversely proportional to the exponent of barrier thickness for a given anodization voltage,⁶¹ the high current density produced by a highly conductive TCO would result in a thick alumina barrier, which would be difficult to remove selectively using dilute phosphoric acid while also widening the pores.⁶²