Sustainable Design of Photovoltaics: Devices and Quantum Information

Introduction

In general terms, nanotechnology is described as a tool used in working with atoms and molecules in order to design and control the properties of nanomaterials and nanosystems, on a scale ranging to several tens billionths of a meter. In terms of energy domain, nanotechnology can be successfully applied to diminish the energy production, use and storage, e.g. in solar energy, hydrogen conversion and thermoelectric devices, according to the “Roadmap Report Concerning the Use of Nanomaterials in the Energy Sector” from the 6^th Framework Program (Serrano et al. 2009).

One of the widest spread available energy to be recycled is represented by the solar energy, and can be used in:

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  Artificial photosynthesis: since hydrogen or carbohydrates are produced by water splitting;

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  Thermal-solar systems: for solar collectors;

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  Passive solar technology: by increasing the solar lighting and heating;

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  Photovoltaic (PV) technology: by converting directly the light in electric current;

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  Biomass technology: the electricity from steam or biofuels at their turn produced from the complex carbohydrates resulted from the chemical transformation which occur in plants in presence of solar radiation.

Photovoltaic (PV) solar cells use the photoelectric effect from the sun radiation in order to produce electricity, meaning that the light photons are changed in electrical current. PV cells are divided in three categories: first generation of solar cells (silicon wafer-based solar cells), second generation (thin epitaxial layers of semiconductors deposited on the lattice-matched wafers), and third generation (cells based on quantum dots – Figure 1).

Figure 1.

Schematic representation of a solar water splitting system

Redrawn and adapted from Takabayashi et al. (2004).

Nano-crystal quantum dots (Ross & Nozik 1982) are nanoparticles with direct bandgap semiconductors used in thin film solar cells based on silicon or conductive transparent oxide (CTO), e.g. indium-tin-oxide (ITO), substrate with nano-crystals as a coat. The efficiency of such system is given by the fact that the quantum dots can emit multiple electrons per solar photon, the particle size determining the absorption and emission spectra. Another class of cells, the dyes-sensitized solar cells represents an alternative proposed nanotechnology to the conventional solar cells based on silicon. In this case, the dye cells absorb the light and the produced electrons are injected in the semiconductor conduction band. The light harvesting increasing is given by the high surface area of the nanoparticles, due to the fact the charge separation occurs at the interface between titanium and dye molecules. Composite photovoltaics is the newest developed technology and combine mesoporous metal oxides or conductive polymers with high surface areas to make a single multi-spectrum layer by increasing the internal reflection with nanoparticles (Serrano et al. 2009).

Recently, was discovered that the solar energy can be stored directly through hydrogen as a result of the photocatalytic water electrolysis which uses the PV energy to break water molecules in hydrogen and oxygen. This process is based on nanotechnology, precisely employing the semiconductor nanoparticle catalyst systems which are based on CdS, SiC, CuInSe₂ or TiO₂, and is still in the research phase because of the low reported cost per conversion efficiency (Jang et al. 2008; Sebastian et al. 2008; Silva et al. 2008; Ni et al. 2007).