Nano structures for solar cell applications. Oğuz Gülseren

Transcription

1 Nano structures for solar cell applications Oğuz Gülseren Bilkent University Department of Physics Ankara, Turkey Bilkent University

2 Outline General design considerations. Method of calculations. Metallic nano particles in a homogenous medium. Metallic nano particles in an inhomogeneous medium. Interaction ti between the nanoparticles. Conclusion. Bilkent University

3 What is Solar Energy? Energy produced by the sun Clean, renewable source of energy Harnessed by solar collection methods such as solar cells Converted into usable energy such as electricity Photovoltaic (solar) panel Sources: Sun and Set of solar panels electrical power lines

4 Solar Cells are Converters of Energy Light energy Solar cell - converts light energy to electricity Solar cells are devices that take light energy as input and convert it into electrical energy Electrical energy (carried through wires) Sources: 4http://

5 The Solar Cell Light with energy e greater e than the band gap energy e of semiconductor is absorbed Energy is given to an electron in the crystal lattice The energy excites the electron; it is free to move hν > E g A positive hole is left in the electron s place This separation of electrons and holes creates a voltage and a current p pn-junction n -holes - electrons

6 Photovoltaic Solar Cells Generate electricity it directly from sunlight 2 Main types: Silicon-based solar cell Single-crystal silicon (traditional) Widespread Expensive to manufacture Dye-sensitized ( nano ) Newer, less proven Inexpensive to manufacture Flexible Dye- sensitized solar cell Sources:

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9 Tandem Cells Intrinsic efficiency limit for a solar cell using a single semiconducting material is 31%. E g1 > E g2 > E g3 Light with energy below the band gap Cell 1 (E g1 ) of the semiconductor will not be absorbed The excess photon energy above the Cell 2 (E band gap is lost in the form of heat. g2 ) Single crystal GaAs cell: 25.1% AM1.5, 1x Multi-junction (MJ) tandem cell Cell 3 (E g3 ) Maximum thermodynamically achievable efficiencies are increased to 50%, 56%, and 72% for stacks of 2, 3, and 36 junctions with appropriately optimized energy gaps GaInP/GaAs/Ge tandem cells (efficiency 40%)

10 Multi-junction vs. Multi-band junction3 I junction2 junction1 Multi-junction Single gap (two bands) each junction N junctions N absorptions Effi i 30 40% Multi-band Single junction (no lattice-mismatch) N bands N (N-1)/2 gaps N (N 1)/2 absorptions Add one band add N absorptions Efficiency~30-40% N (N-1)/2 absorptions

11 Introduction Current technology 90% of current solarcells are based on crystalline Silicon wafers thickness micron too expensive to produce electricity 40% of the cost is due to material used. P Plasmonic Solarcell Thin film solar cells with metallic nanoparticles. thickness 1-2micron deposited on cheap surfaces like glass. The main problem is absorbance b of flight in this thin film structure. N Solar cell is a diode. Light is absorbed in junction (depletion layer). Charges created in depletion layer separates and forms a potential difference.

12 Plasmonics for improved photovoltaic devices Harry A. Atwater and Albert Polman A limitation in all thin-film solar cell applications is that absorbance of near band gap light is small For high efficiency solar cells, minority carrier diffusion lengths should be several times the materials thickness for all photocarriers to be collected.

13 Light trapping Scattering from metal nanoparticles at the surface of the solar cell The excitation of localized surface plasmons in metal nanoparticles embedded in the semiconductor The excitation of surface plasmon polaritons at the metal/semiconductor interface. A corrugated metal back surface couples light to SPPs

14 Plasmonic Solar cell design Metal Nano particles(mnp) prefer forward scattering of light if placed on surface of dielectric. 10% coverage is enough to scatter all incoming light due to large cross section. If MNP placed at junction it behaves like an antenna absorbs light (create plasmon) and dissipates to creates charge pairs. A metallic back gate allows excitation of surface plasmon polariton waves which interact with incoming light.

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17 Finite difference time domain (FDTD) and boundary element methods (BEM) for plasmonic calculations FDTD Time dependent solution of Maxwell equations. Space part discretizated t d on a dual lattice shifted by half a lattice constant so that both B and E field can be propagated. (Yee lattice) A time Fourier transformation is necessary to find fluxes on surfaces and as a result cross sections for steady state. Due to high values of effective refractive index in metals it needs big mesh sizes and therefore it is slow. No restriction on geometry. BEM Surface charge redistribution is calculated l by matching green functions inside and outside the boundary. Cross section data and field lines can be obtained from surface charges. No time propagations, all result for steady state. Fast calculations due to only discretization necessary on the boundary. Not suitable when two object with very different sizes due to large matrices to accurately define both surfaces.

18 Plasmon, light on metal Plasmons are collective motion of electrons under electric field. Electrons that are pulled away from their positive background result in oscillatory motion due to restoring Coulomb force. ε ( w) = ε 2 wp w ( w + i / τ ) w Dielectric function of Drude form. p = = ne ( ) 16 1 mε Where we estimate plasmon frequency for a 16 1 metal. Ultraviolet light has frequency 10 s so it passes through metal for instance. Relaxation time can be derived from Drude formula for conduction, for silver, s When Light wavelength is less than plasmon wavelength than thin film transmits else reflects. Almost all visible light is reflected back from silver that is why we see silver as white color. Here, determines the response of metal to light Absorbing b Reflecting transparent

19 Some theoretical considerations There is something about spherical symmetry when it comes to wave scattering. Optical theorem Gustav Mie Incident wave scattered wave Transmitted wave For a sphere solution inside and incoming light outside are written in terms of spherical harmonic. Than boundary match gives scattering states. If size parameter is too small than unity than one get only dipole excitation, that is 1 single term in spherical expansion enough to describe scattering. When size parameter increases you get quadruple or higher modes inside the sphere and scattering states has contributions from those.

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21 Manipulating Plasmon frequency of Silver nano particles Size Dielectric Medium Electric field Distribution. Dipole excitation. Shape

22 Plasmonics Clausius- Mosatti eq n. Since light wavelength is much bigger than radius of metallic nano particles, electric field on sphere can be assumed to be constant but with a time dependent phase. Scattering is dipolar since electric field alternatively polarizes sphere. When size increase higher mode excites. Polarization P = ε mαe 0 3 ε ε m α = 4ππ R ε 0 ε + 2ε m size geometry ε m σ sca = 4 k 2 6 π α σ abs = ( 2π λ )Im(α) σ = k Im(α) ext = ε = 2ε m ε 0 ε m ε Permittivity of vacuum Dielectric of host medium dielectric of metal (complex) Resonance condition determines plasmon resonance frequency.

23 Analyzing experimental data 1. No direct control on shapes since de-wetting technique for production Ag nano particle is relied on self organized process. Generally spheroid formation observed. 2. Plasmon resonance is neither at the spheroid in air nor in substrate only environment. 3. Higher order plasmon resonances are not observed. 4. Low wavelength peak not observable by the experiment. 5. Oblique view hinders information about immersion of Ag nano particles into substrate as well as their exact shape 6. Different substrate determines packing and size of Ag nanoparticles.

24 A sample calculation is done as below where we registered dipole peak position to compare with the experiment. Note that silver nano particle in air peak is not observable in experiment. In general dipped sphere and half sphere show a resonance akin to the substrate. ITO substrate of thickness 200nm is shown here. A sphere of radius r=30nm is used in this set of calculations.

25 Comparison of plasmon resonances Green triangles denote experimental e data. Sphere e on the surface (blue, plus), dipped sphere (red, star) and hemisphere(cyan, circle) for radi r=30, r=60, r=90nm simulations are shown.

26 Immersion of nano particle to substrate ¼ immersed D=120nm Ag nano particle into ITO is compared to experimental data here. We believe the experimental wide shoulder is due to inter-particle interactions.

27 Coupling to substrate A sphere of radius r=30nm is chosen. Principal cp axis ratio show prolate spheroid deformed in the direction of light incident. ITO substrate of t=200nm thickness is used.

28 Film thickness and distance to surface effects. t ITO t ITO

29 Inter-Particle interaction L is separation in x and y directions Sphere with radius r=60nm is L is separation in x and y directions. Sphere with radius r=60nm is used. Silver nano particles are in air.

30 Size effect on averaging D1 D2 D1 D3

31 Shape effect on averaging D D

32 Acknowledgments Calculations: Mehmet Can Günendi Dr. Gürsoy Akgüç Synthesis and experiments: İrem Tanyeli Prof. Alpan Bek Prof. RaşitTuran Supported by TÜBİTAK (Grant No: TBAG-109R037) Bilkent University