Dye-sensitized solar cell with plasmonic gold nanoparticles modified photoanode

Abstract

The depletion of fossil fuel and environmental anxieties has led to a greater interest in renewable forms of energy. To a large extent, the most prevalent form of renewable energy presently is solar energy. The Dye-sensitized solar cell (DSSC) is a promising substitute for all too familiar silicon solar cells. DSSC uses dyes as light-harvesting pigments in the conversion of solar energy to electric energy. This study is about the fabrication of DSSC using a photoanode made up of TiO ₂ fused with gold nanoparticles. Gold nanoparticles enhance the performance of DSSC due to the plasmonic effect. The gold nanoparticles were made by the citrate method and characterized using UV–visible spectroscopy and dynamic light scattering. The performance of the solar cells was accessed via photocurrent and photovoltage measurements. The solar-to-electric power efficiency of the solar cells with gold nanoparticles was found to be about 50% higher than those without gold nanoparticles.

Introduction

With fossil fuels serving as a major energy source throughout much of the world, its use and overconsumption pose a severe detriment to the environment. As they are limited and depleting at high rates, the depletion of such nonrenewable energy sources is inevitable. Thus, the appeal for sources of renewable energy is growing. The International Energy Agency reports that the electricity demand will have mounted to approximately 70% by the year 2040 — this is largely due to the expected surge of economic growth in Africa, Middle East, China, South-East Asia and India in the future [1]. Energy sources that could serve as potential substitutes for the widely used coal, oil, and gas must be provided to avoid any complications that may intervene in the development of the previously mentioned economies. Recently, there have been exposed to several renewable energy options in the modern industrial world such as nuclear, hydroelectric, geothermal energy, and solar energy [2]. Solar energy has gained popularity and led to the implementation of various photovoltaic devices, more specifically solar cells. At the forefront of recent solar cell technologies lies the dye-sensitized solar cells (DSSCs).

DSSCs pioneered by Michael Grätzel and his colleagues in 1991 have become a very popular photovoltaic device [3]. DSSC function by absorbing incident sunlight and using the energy to induce an electron transfer reaction that culminates in the production of electricity [4], [5], [6]. DSSCs have distinct advantages over silicon solar cells and they include being low-cost, having easy fabrication processes, and being environmentally friendly. The main components of the cells include a photo-sensitized anode, a cathode, and a redox couple electrolyte. The photo-anode is made up of the dye-sensitized titanium dioxide film on a fluorine-doped tin oxide (FTO) glass slide. The titanium dioxide (TiO₂) has a big surface area which enables it to retain an enormous amount of dye molecules needed to capture photons.

The research on DSSCs has mainly focused on optimizing components of the devices to increases the efficiency of DSSC. The photoanode, composed of the dye-sensitized TiO₂ on an FTO glass has received the most attention since it is responsible for the capture of solar energy. Different types of sensitizing dyes including natural dyes [7], [8], quantum-dot sensitizer [9], [10], [11], metal-free organic dyes [12], [13], perovskite-based sensitizer [14], and inorganic ruthenium (Ru) -based dyes [15] and other metal-based organic dyes [16], [17].

Incorporating certain materials in the photoanode tends to enhance the ability of the dye to capture photons. For example, the absorption process can be enhanced when metal nanoparticles such as gold or silver nanoparticles are incorporated into the TiO₂ structure of the photoanode. The addition of noble material nanoparticles exhibits surface plasmonic resonance (SPR) within the device, which is the effect of electron oscillation in a structure stimulated by incident light. The solar cell can have Au or Ag nanoparticles incorporated into it to induce the plasmonic effect, which causes enhanced light absorption and scattering, and ultimately the potential improvement of the solar cell’s performance [18], [19], [20], [21]. The excitation associated with the surface plasmonic resonance is the result of incident light in the environs of the visible region of the spectrum — the particular resonance frequency, as well as absorption intensity of the surface plasmonic resonance, are dependent on such factors of the noble material nanoparticles themselves, more specific to this paper, the size of the nanoparticles [22], [23], [24], [25], [26], [27], [28].

In this study, gold nanoparticles of diameters ranging between 10 and 30 nanometers, synthesized by the Frens/Turkevich method, were used to modify the photoanode of the DSSC structure with synthetic dye, N719. Gold nanoparticles are easily made and are very economical. They have a large surface to size ratio and their surface chemistry allow for easy attachment to other materials. The performance of the modified DSSCs was investigated as a means of comparison for a traditional DSSC with N719 dye and an unaltered photoanode.

The principle of operation of the DSSC is displayed in Fig. 1. The photoanode made up of the TiO₂ sensitized film with the gold nanoparticles absorb incident light. The plasmonic effect of the gold nanoparticles enhances the ability of the dye to absorb radiant energy. Once the light is incident on the device, the dye molecules absorb energy and move the excited state where the electron is ejected and injected into the conduction band of the TiO₂. The oxidized dye picks an electron from the redox electrolyte resulting in the restoration of the ground state of the dye. The electron ejected from the dye is moved through the TiO₂ nanoparticles to the conductive part of the FTO glass and then through an external circuit, the electron reach the counter electrode. The electron at the counter electrode reduces triiodide ion (${\mathrm{I}}_3^{-}$) to iodide ion (I${}^{-}$); thus, the restoration of the ground state of the dye occurs owing to the receipt of the electron from iodide ion redox mediator, and I${}^{-}$ oxidizes to ${\mathrm{I}}_3^{-}$. Again, the oxidized mediator (${\mathrm{I}}_3^{-}$) moves towards the cathode and is reduced to iodide ion (I${}^{-}$).