(6hv) Developing Noble Metal/TiO2 and Swnt/TiO2 Composites to Improve Light Harvesting and Carrier Collection of Solar Cells

The performance of photovoltaic devices could be improved by using rationally-designed nanocomposites with high electron mobility to efficiently collect photo-generated electrons. Single-walled carbon nanotubes (SWNT) exhibit very high electron mobility, but the incorporation of such nanotubes into nanocomposites to create efficient photovoltaic devices is challenging. Here we report the synthesis of SWNT-TiO₂ nanocrystal core-shell nanocomposites using a genetically engineered M13 virus as a template. Using this method, SWNTs are stabilized without surfactants and surface modifications, and their electronic properties are preserved. In addition, close contact is achieved between SWNTs and TiO₂ nanocrystals. With the developed biological template approach, we demonstrate that well-dispersed semiconducting SWNTs can improve the power conversion efficiency of DSSCs. Even small fractions of nanotubes (typically 0.1 wt%) improve the power conversion efficiency by increasing the electron collection efficiency. We also show that both the electronic type and degree of bundling of the nanotubes in the SWNT/TiO₂ complex are critical factors in device performance. Semiconducting SWNTs improve the electron collection and metallic SWNTs decrease the electron collection in DSSCs. Moreover, debundled SWNTs affect the device performance more than bundled SWNTs. By optimizing the electronic type and aggregation state of SWNTs, we achieve a power conversion efficiency in the dye-sensitized solar cells of 10.6%.

Because SWNTs have good thermal conductivity in addition to high electron mobility, this approach might improve the stability of large DSSC modules. Moreover, biological engineering of multiple genes of the virus can extend this approach to creation of more complex structures. Though the route to DSSC improvement lies in the development of dyes with absorption extending into the infrared and better redox couples for higher voltages, we believe that our approach will enable the utilization of SWNTs in many practical photovoltaic devices that require efficient electron diffusion and reduced electron recombination, for instance, quantum dot solar cells, organic solar cells, and photoelectrochemical cells.

In photovoltaic devices, light harvesting (LH) and carrier collection have opposite relations with the thickness of the photoactive layer, which imposes a fundamental compromise for the power conversion efficiency (PCE). In addition, unbalanced LH at different wavelengths further reduces the achievable PCE. Localized surface plasmons (LSPs) are the elementary excitation states in noble metal nanoparticles (NPs), and can improve LH of photo-absorbers. Here, we report a novel approach to broadband balanced LH and panchromatic solar energy conversion using multiple-core-shell structured oxide-metal-oxide plasmonic nanoparticles (core-shell-shell titania-gold-titania nanoparticles). These nanoparticles feature tunable localized surface plasmon resonance frequencies (visible to infrared) and the required thermal stability during device fabrication. By simply blending the plasmonic nanoparticles with available photoactive materials, the broadband LH of practical photovoltaic devices can be significantly enhanced. We demonstrate a panchromatic dye-sensitized solar cell with an increased PCE from 8.3% to 10.8%, mainly through plasmon-enhanced photo-absorption in the otherwise less harvested region of solar spectrum. This general and simple strategy also highlights easy fabrication, and may benefit solar cells using other photo-absorbers or other types of solar-harvesting devices.

The general and simple strategy for broadband LH enhancement and panchromatic DSSCs is realized by matching LSP resonance (LSPR) wavelength (λ_LSPR) of plasmonic NPs with λ_Lo (wavelengths with low extinction coefficient) of existing photo-absorbers. Demonstrated by both finite-difference time-domain simulations and experiments, matching λ_LSPR with λ_Hi (wavelengths with high extinction coefficient) or λ_Lo affects LH differently. Although matching λ_LSPR with λ_Hi readily improves LH and PCE for optically-thin photo-absorbing layers, matching λ_LSPR with λ_Lo maximally increases LH and PCE for practical photovoltaic devices with optically-thick layers, through enhancing photo-absorption in the weakly-absorbing region. The multiple-core-shell oxide-metal-oxide plasmonic NPs developed and utilized here are advantageous over other geometries, by featuring: adjustable λ_LSPR of 600-1,000 nm, substantially enhanced near-field electromagnetic (EM) intensity, and preservable plasmonic properties during device fabrication. By matching λ_LSPR with λ_Lo of a common dye (N719), a panchromatic DSSC with broadband balanced LH and a PCE of 10.8% is achieved (30% increase comparing to DSSCs without plasmonic NPs).