Nanocomposites for Photovoltaic Energy Conversion

Abstract

Induction 1.1 Composite materialsA composite is a multiphase solid material.It is incorporated by two or more individual materials through physical or chemical methods.The performance of different materials to complement each other will produce synergistic effects (Ivanov et al., 2008;Lu et al., 2010).The overall property of composite materials is better than that of each original material to meet different requirements.These properties include mechanical, electrical, thermal, optical, electrochemical, catalytic behaviors etc.For example (Lu et al., 2010), investigating the synergistic effect of carbon nano-fiber (CNF) and carbon nano-paper on the shape recovery of shape memory polymer (SMP) composite shows that the combination of CNF and carbon nano-paper can improve the thermal and electrical conductivities of the SMP composite, which are much better than each of the components.The individual materials in a composite are referred to as two constituent materials.The two constituent materials are matrix and reinforcement.The matrix material is a frame to support the reinforcement material.So the reinforcement material can keep its relative position.The reinforcement is a functional material which can enhance the matrix properties.Composite metal matrix (substrates) includes aluminum, copper, titanium, magnesium and its alloys and nonmetallic matrix includes synthesized resin, rubber, ceramics, graphite and carbon and so on.Reinforcement mainly includes glass fiber, carbon fiber, boron fiber , aramid fiber, silicon carbide fiber , asbestos fiber, whisker, metal wire and fine grain etc.In order to make use of synergistic effect to improve composite properties, optimum combination of matrix and reinforcement should be chosen. Nanocomposites and their propertiesA nanocomposite is a special composite where one of the phases has one, two or three dimensions less than 100 nanometers (nm, 10 -9 m).A nanocomposite also includes the material where the structures between the different phases that make up the material are in nano-scale (Beecroft & Ober, 1997).In the broadest sense, nanocomposites can also include porous media, colloids, gels and polymers, because in these materials the particles or structures are in nano scale.One nanometer is equivalent to the length to tightly line up www.intechopen.comAdvances in Composite Materials for Medicine and Nanotechnology 212 10100 atoms.Nano-materials include nano-powders, nano-fibers, nano-particles and nanothin films.Their structures are between atom (molecular) size and macro size.Materials in nano-scale have special effects such as quantum size effect, surface effect, small-size effect and macroscopic quantum tunneling effect etc. Quantum size effectQuantum size effect is one of the fundamental physical properties of nano-particles.When the particle size decreases to a nano-scale, the electronic energy levels of the particle atom become discrete and the energy gap becomes wider.This phenomenon is called quantum size effect.This effect does not come into play when the particles are in micro dimensions.However, quantum effects become dominant when the particles sizes decrease to nanometer, especially when the dimensions are in 100 nanometers or less.So the nanometer size dimension is also called quantum realm.The quantum size effect leads the nanomaterials have very different magnetic, optical, acoustic, thermal, electrical and superconducting properties with conventional materials, such as very high non-linear optics, optical catalysis, high oxidability and reducibility etc. Due to quantum size effect, the galvanomagnetic and thermoelectric properties (electrical conductivity s, the Hall coefficient RH, charge carrier mobility m, and the Seebeck coefficient S) of nano-size PbSe thin film oscillate with the thin film thickness (3-200 nm) (Rogacheva et al., 2002).Silica glasses are doped with CdS and ZnS micro-crystals.Because of the quantum size effect, the optical absorption spectra and the photoluminescence spectra show that the optical absorption edges and the photoluminescence peaks shift to the higher energy side with decreasing the size of the micro-crystals (Zhao et al., 1994). Surface effectFor a spherical particle, the ratio of surface area and volume is inversely proportional to the particle diameter.When particle size decreases to nano-scale, more atoms appear on surface relatively.For example, when particle diameter is 10 nm, the surface atom is about 20% of the total atoms; the diameter of 1 nm, the percentage of surface atoms increases to 99%.Almost all of the atoms are in surface.The particle surface becomes large relative the particle volume and the surface atoms are easy to combine with each other.This structure shows a high chemical activity.So, for the nano-materials, surface energy and surface binding energy are rapidly increasing compared to bulk particles.This change is called surface effect.For example, Au particles with dimension about 2 nm have no stable states; they change from octahedron, decahedron to icosahedron multicrystals.They are not liquid, not solid and can be called quasi-solid. Small-size effectWhen the particle size is close to or smaller than the light wavelength, the particle's de Broglie wavelength, semiconductor coherence length and the particle's penetration depth, the particle has new properties such as optical, thermal, acoustic, electrical, magnetic and mechanical properties etc.This phenomenon is the small size effect.For example, when the metal particle size is small than the light wavelength, the metal's color will become black.This means nano metal particles have very good light absorption and can be optothermal, photoelectrical materials.The melt temperature of bulk gold is 1064 0 C, but 2 nm size gold particles melt temperature is only 327 0 C. Nano-scale metal particles can be insulator.www.intechopen.com