Light Trapping on Plasmonic-Photonic Nanostructured Fluorine- Doped Tin Oxide

Plasmonic Au nanoparticles of ∼50−200 nm in diameter were generated via thermally assisted self-assembly from Au films evaporated on fluorine-doped tin oxide (FTO). A comparative study has been made on the light trapping effects of the plasmonic Au nanoparticles on original FTO and FTO with photonic nanopatterns fabricated using nanoimprint lithography. While strong localized surface plasmon resonance (LSPR) in the visible spectrum has been confirmed in both cases, quantitative differences exist and may be attributed to the Au nanoparticle morphology and their interface with FTO. In particular, the LSPR frequency depends on the Au nanoparticle structure and size, while the LSPR peak width is affected by FTO surface morphology (original or nanopatterned). It has been found that the combined plasmonic-photonic nanostructured FTO has the best light trapping, which agrees well with the finite difference time domain simulations and provides a promising transparent electrode for high-efficiency thin film solar cells and other optoelectronic devices. ■ INTRODUCTION Light management has been an important approach in the development of high-performance and low-cost thin-film photovoltaics for clean and sustainable solar energy that plays a major role in meeting world energy need. One approach is to incorporate photonic nanostructures for improved light scattering and thus light absorption in photovoltaics. Such structures have been successfully applied to photovoltaics in photoactive layers and back reflectors. Our recent work in generating photonic nanostructures directly on the transparent electrodes, such as fluorine-doped tin oxide (FTO), has led to enhanced light scattering and improved photocurrent in thin-film dye-sensitized solar cells. The unique advantage of this approach is in minimal interruptions to the photovoltaic devices grown atop. Plasmonic light trapping is regarded as an attractive approach to improve light management in solar cells and has prompted extensive studies recently in design consideration as well as performance characterization of plasmonic photovoltaics that generally involve incorporation of metal nanoparticles. Plasmon is light-induced collective oscillations of electrons on the metal surfaces, which becomes localized on metal nanoparticles due to the dimension restriction. When the light frequency is resonant with collective oscillation of electrons, both strong absorption and strong scattering of incident light may occur, which is known as localized surface plasmon resonance (LSPR). The frequency of the LSPR is determined primarily by the physical parameters of metal nanoparticle (complex dielectric constant, dimensions, and shapes), as well as the refractive index of the surrounding material. LSPR is expected to enhance light-harvesting in photovoltaics through scattering and local (typically in the range of tens to hundreds of nanometers) enhancement in electromagnetic fields. This work explores the combination of plasmonic and photonic nanostructures on FTO transparent electrodes, and the goal is to understand the benefits of this combination that could be effectively integrated for photovoltaic applications. With conducting both experiments and computer simulation and modeling, we evaluate merits and shortcomings of the combined plasmonic-photonic nanostructures as front electrode and back reflectors for improved light management in solar cells. A unique advantage of implementing plasmonicphotonic nanostructures on electrodes instead of inside the semiconductors is to minimize generating additional defects as charge traps that reduce charge mobility and increase charge recombination. Specifically, plasmonic Au particles were generated on photonic nanostructures as well as original FTO electrodes using a thermally self-assembly process. Measurements of light scattering and plasmonic effects on these samples in comparison with that on the photonic nanostructured FTO allow comparison of the combined effects of plasmonic-photonic nanostructures on FTO with separated effects of plasmonic and photonic counterparts. ■ EXPERIMENTAL SECTION Commercial FTO glass TEC 8 (Hartford Glass) with thickness of approximately 550−600 nm FTO layer coated on ∼3 mm thick glass was utilized in this experiment. Before processing, FTO glass was cleaned in an ultrasonic bath with detergent, DI water, acetone, and isopropanol in sequence. Au films were deposited using e-beam evaporation. The Au thickness of 6 nm Au was found to be optimal for obtaining Au nanoparticles of Received: March 8, 2013 Revised: April 18, 2013 Article