Nanostructured materials improve efficiency in excitonic solar cells

Third-generation solar cells promise many advantages over their traditional counterparts, such as low cost, nontoxic materials, and improved efficiency, while also maintaining acceptable long-term stability.¹ Excitonic solar cells (either dye- or quantum-dot-sensitized) are strong candidates for further developments in this field.²

While dye-sensitized cells have a 20-year history of development and are now competitive with their poly- and amorphous-silicon counterparts in overall cell efficiency and stability, quantum-dot approaches are at the very beginning of their functional exploitation and have thus far performed poorly. However, intense development efforts are aiming to enhance the overall photoconversion efficiency single-crystal nanowires of transparent conducting oxides into photoanodes. Pioneering work suggested the possibility of obtaining a photoelectrochemical system in which electronic transport takes place along the single-crystalline backbone of 1D transparent nanostructures (see Figure 1).^3–6 Thanks to the high electron mobility in single-crystal nanowires (approximately 100 times higher than in a polycrystalline network), this solution eliminates the drawback of polycrystalline photoanodes, where a single electron must pass thousands of grain boundaries before reaching the anode (with high recombination probability). In principle, this benefit could result in unprecedented cell efficiency, but to date only limited results have been obtained for nanowire-based cells.

Figure 1. Four different structures can be used as photoanodes in photoelectrochemical cells. (A) Polycrystalline network (traditional anode). (B) Polycrystalline nanotube. (C) Single-crystal nanowire array. (D) Network of single-crystalline nanowires and dispersed nanoparticles.

One of the most critical issues is the very limited specific surface of the nanowire bundle, which affects the optical density of the active layer. Engineered networks of mixed polycrystal powders and single-crystalline nanowires can merge the beneficial properties of both systems. These networks allow high optical density of the active layer, which results in nearly complete light absorption while maintaining a direct electron path (which minimizes recombination processes).⁷ Such systems can be profitably applied in both dye- and quantum-dot-based solar cells.

Figure 2. Scanning-electron-microscope images of three transparent conducting oxide materials integrated into photoanodes. (a) Traditional polycrystalline titanium dioxide (TiO₂). (b) Zinc oxide (ZnO) single-crystal nanowire bundle. (c) Composite network of ZnO single-crystal nanowires and TiO₂ polycrystals.

We have fabricated different networks of transparent conducting oxides with different morphologies for use as photoanodes. We considered three different systems (see Figure 2), including polycrystalline (traditional) titanium dioxide (TiO₂), single-crystal zinc oxide (ZnO) nanowires (1.5μm thick), and single-crystal ZnO nanowires mixed with polycrystalline TiO₂ (1.5μm thick). The almost similar electronic band structure of ZnO and TiO₂ guarantees perfect compatibility from the point of view of electron transport, limiting the formation of detrimental electric fields which could affect electron mobility. We sensitized photoanodes using the commercial ruthenium-based dye molecule N719 (Solaronix), and the triiodide/iodide (I₃⁻/I⁻) redox couple. Comparison of current-voltage curves of cells composed of ZnO nanowires versus the composite network (see Figure 3) indicates that the latter enhances the short-circuit current and cell efficiency. Nanonetworks reduced open-circuit voltage, likely due to higher recombination in the TiO₂ nanoparticles than in single-crystalline wires.

Figure 3. Current (j)-voltage curves of the dye-sensitized cells under 1 sun irradiation (airmass 1.5 global, 100mW/cm²). Solid line: ZnO nanowires. Dashed line: Network of ZnO nanowires and TiO₂ nanoparticles. η: Efficiency.

Our work demonstrates the effectiveness of composite nanonetworks in enhancing excitonic solar-cell efficiency. Optimizing the material improved network efficiency compared to a bare nanowire bundle. We hope to fabricate dye- and/or quantum-dot-sensitized cells with high efficiency by simply enhancing the thickness of the active layer, which is our next step.

Cariplo Foundation, Program of Relevant National Interest (PRIN) 2007, National Institute for the Physics of Matter-National Research Council (INFM-CNR) seed project, and Greenvision Ambiente are acknowledged for partial funding.