Quantum Dots In Solar Cells

AAKASH GUPTA UE 5501 E.C.E. 7TH SEMESTER U.I.E.T., PANJAB UNIVERSITY

CONTENTS • Nanoparticle • Quantum Dots – – – – –

Quantum Dot Degrees of freedom Structure and Formation How Quantum dots work Quantum Dot features

• Solar Cells – Working – Construction

• Need for Quantum dots in Solar cells

continued… • Quantum dots in Solar cells – – – – – –

Infrared photovoltaic cells Multiple exciton generation solar cells Quantum dot dye sensitized solar cells Rainbow Solar cells Intermediate band solar cells Luminescent concentrator cells

• References

NANOPARTICLE • In nanotechnology, a particle is defined as a small object that behaves as a whole unit in terms of its transport and properties • According to size: – fine particles cover a range between 100 and 2500 nm – ultrafine particles are sized between 1 and 100 nm

• Nanoparticles may or may not exhibit size-related intensive properties

QUANTUM DOTS • Non-traditional semiconductor • Crystals composed of periodic groups of II-VI, III-V, or IV-VI materials • Range from 2-10 nanometres (10-50 atoms) in diameter • An electromagnetic radiation emitter with an easily tunable band gap • 0 degrees of freedom

Quantum Dot

Quantum Dot Layer

Quantum Dot Layer

DEGREES OF FREEDOM Bulk Crystal (3D)  3 Degrees of Freedom (x-, y-, and z-axis)

Quantum Wire (1D)  1 Degree of Freedom (x-axis)

Quantum Well (2D)  2 Degrees of Freedom (x-, and y-axis)

Quantum Dot (0D)  0 Degrees of Freedom (electron is confined in all directions)

Structure vs. Energy

Quantum Dots are sometimes called “artificial atoms”

Quantum Dots (Structure and Formation) Self-Assembly (a.k.a StranskiKrastanow Method): Mismatched lattice constants cause surface tension which results in Qdot formation with surprisingly uniform characteristics. GaAs  5.6533 Å InAs  6.0584 Å

HOW QUANTUM DOTS WORK ? • Bands and band gaps – Electrons and Holes – Range of energies

• Quantum confinement – Exciton Bohr Radius – Discrete energy levels

• Tunable band gap – the size of the band gap is controlled simply by adjusting the size of the dot

continued... • Emission frequency depends on the bandgap, therefore it is possible to control the output wavelength of a dot with extreme precision • Small nanocrystals absorb shorter wavelengths or bluer light • Larger nanocrystals absorb longer wavelengths or redder light • The shape of the dot also changes the band gap energy level

QUANTUM DOT FEATURES • Tunable Absorption Pattern – bulk semiconductors display a uniform absorption spectrum, whereas absorption spectrum for quantum dots appears as a series of overlapping peaks that get larger at shorter wavelengths – the wavelength of the exciton peaks is a function of the composition and size of the quantum dot. Smaller quantum dots result in a first exciton peak at shorter wavelengths

• Tunable Emission Pattern – the peak emission wavelength is bell-shaped (Gaussian) – the Stoke's Shift – the peak emission wavelength is independent of the wavelength of the excitation light

continued... – bandwidth of the emission spectra, denoted as the Full Width at Half Maximum (FWHM) stems from : • the temperature, natural spectral line width of the quantum dots, and the size distribution of the population of quantum dots within a solution or matrix material

• Molecular Coupling – QDs can be attached to a variety of molecules via metal coordinating functional groups. For eg: thiol, amine, nitrile, phosphine, phosphine oxide – by bonding appropriate molecules to the surface, the quantum dots can be dispersed or dissolved in nearly any solvent or incorporated into a variety of inorganic and organic films – the surface chemistry can be used to effectively alter the properties of the quantum dot, including brightness and electronic lifetime

continued... • Quantum Yield – The percentage of absorbed photons that result in an emitted photon is called Quantum Yield (QY) – controlled by the existence of nonradiative transition of electrons and holes between energy levels – greatly influenced by the surface chemistry

• Adding Shells to Quantum Dots – Shell =several atomic layers of an inorganic wide band semiconductor • it should be of a different semiconductor material with a wider bandgap than the Core – reduces nonradiative recombination and results in brighter emission – also neutralizes the effects of many types of surface defects

SOLAR CELL • Photovoltaic cell is a device that converts solar energy into electricity by the photovoltaic effect • Represents the entire electromagnetic radiation (visible light, infrared, ultraviolet, x-rays, and radio waves) • Assemblies of cells are used to make solar modules

WORKING • When a photon is absorbed, the energy of the photon is transferred to an electron in the crystal lattice • This generates an electron-hole pair • The flow of electrons is a DC current (I) and electric field of the cell is a voltage (V) • Here power (P) is given by: P = VI • An inverter is used to convert DC into AC

CONSTRUCTION • Photovoltaic cells are made of two thin sheets of silicon that are separated from each other. • An antireflective coating is applied to the top of the cell to reduce reflection • A glass cover plate then protects the cell from weathering • PV Construction Technologies: – Single Crystalline – Polycrystalline or Multicrystalline – Amorphous or Thin-film

NEED FOR QUANTUM DOTS IN SOLAR CELLS • Reduction in cost: – of each kilowatt of electricity produced – of raw materials – of processes used to convert the raw materials into functional cells

• Inefficiencies in conventional single junction solar cells – inability to absorb all of the solar energy – inability to convert all of the photon energy that is absorbed to free electrons and holes

• Traditional semiconductor materials are crystalline or rigid

QUANTUM DOTS IN SOLAR CELLS Quantum dots are being used in following photovoltaic cells: • Infrared photovoltaic cells • Multiple exciton generation solar cells • Quantum dot dye sensitized solar cells • Rainbow Solar cells • Intermediate band solar cells • Luminescent concentrator cells

INFRARED PHOTOVOLTAIC CELL • Transform infrared light into electricity • Nearly half of the approximately 1000 W/m3 of the intensity of sunlight is within the invisible infrared region • ‘Thermovoltaics’ - can even capture radiation from a fuel-fire emitter – co-generation of electricity and heat is quiet, reliable, clean and efficient – A 1 cm2 silicon cell in direct sunlight generates about 0.01W, but an efficient infrared photovoltaic cell of equal size can produce theoretically 1W in a fuel-fired system

• Infrared PVs make use of light-sensitive conjugated polymers polymers with alternating single and double carbon-carbon bonds

continued...

• Polymers are wrapped around lead sulphide quantum dots tuned (by size) to respond to infrared • The polymer poly(2-methoxy-5-(2’-ethylhexyloxy-pphenylenevinylene)] (MEH-PPV) on its own absorbs between ~400 and ~600 nm • Quantum dots of lead sulphide (PbS) have absorption peaks that can be tuned from ~800 to ~2000 nm • Wrapping MEH-PPV around the quantum dots shifted the polymer’s absorption into the infrared

MULTIPLE EXCITON GENERATION CELLS • Multiple excitons per absorbed photon happens when the energy of the photon absorbed is far greater than the semiconductor band gap • In bulk semiconductors the excess energy simply dissipates away as heat • In quantum dots, the rate of energy dissipation is significantly reduced

continued… • The charge carriers are confined within a minute volume, thereby increasing their interactions and enhancing the probability for multiple excitons to form • Quantum yield of 300 percent for 2.9nm diameter PbSe (lead selenide) quantum dots when the energy of the photon absorbed is four times that of the band gap

Quantum dot dye sensitized solar cells • Nanoparticles of titanium dioxide are used in these cells – Its band gap is too wide to absorb much sunlight

• Working: – TiO2 particles are coated with a metal organic ruthenium-based dye, the dye absorbs light, becomes oxidized (loses electrons), and injects these electrons into the TiO2

continued… –

These diffuse to the electrode while the holes pass to the LiI electrolyte

–

The electrons pass through an external load, doing work, then flow to the counter electrode

–

Here the electrons are carried by iodine ions to regenerate the dye through reduction (gain of electrons)

RAINBOW SOLAR CELLS • ‘Rainbow’ design involves arranging quantum dots according to size, so that each one acts on a specific wavelength of the electromagnetic spectrum, thus harvesting most of the power light carries • Size quantization effect - a quantum dot is able to absorb and convert only a specific wavelength of the EM spectrum, which is in direct relation to the size of the dot

INTERMEDIATE BAND SOLAR CELLS • In conventional single p-n junction solar cells a smaller bandgap would result in a larger photo-generated current, but the voltage delivered by the solar cell is always smaller than the bandgap • In intermediate band solar cells the photo-generated current increases without reducing the bandgap and thus the voltage • Theoretical efficiency: – No intermediate band: 40.7% – 1 intermediate band: 63.2% – 2 intermediate bands: 71.7%

continued… • Ideal intermediate band solar cell: 1) 2) 3) 4) 5)

Only radiative recombination One electron-hole pair per photon Absorption of all photons Eg>E No high energy photons in low energy processes Maximum concentration of solar radiation

• The quantum dots are simply placed in the pn-junction of the conventional solar cell, to form a pin-junction. Quantum dot layer

p

n

LUMINESCENT CONCENTRATOR CELLS Light

• Spectrum of the incoming light is converted such that it has a better match with the absorption spectrum of the solar cell • Can reduce spectral losses, especially in the case of a small absorption band, such as for dye sensitized solar cells and polymer solar cells

1 2

a

2 1 QD Solar Cell Transparent Matrix

Schematic 3D view of a luminescent concentrator. AM Light is incident from the top. The light is absorbed by a luminescent particle. The luminescence from the particle is randomly emitted. Part of the emission falls within the escape cone (determined by the angel (a)) and is leaving the luminescent concentrator at the side (1). The other part of the luminescence is guided to the Si cell by total internal reflection (2).

continued... • The LC consists of a transparent matrix material, usually a flat plate, with solar cells connected to one or more sides • The transparent matrix contains luminescent particles such as, e.g., organic dyes or quantum dots that absorb part of the spectrum • Part of the light emitted by the luminescent particles is guided towards the solar cells by total internal reflection • Both direct and diffuse sunlight is collected, making solar tracking unnecessary • Ideally, the luminescence spectrum of the luminescent particles matches the spectral response of the solar cell.This can be done by: – combining two infrared photons to get one photon in the visible (upconversion), or by splitting one ultraviolet photon into two visible photons (downconversion) – using different dyes which cover different parts of the spectrum

REFERENCES • • • • • •

http://www.evidenttech.com/ http://en.wikipedia.org/wiki/Solar_cell http://en.wikipedia.org/wiki/Quantum_dot http://softpedia.com/ http://www.sciencedaily.com/articles/ http://www.i-sis.org.uk/index.php/

• White Papers: – – – –

Brad Gussin, John Romankiewicz Peter Green L.H. Slooff, R. Kinderman, A. R. Burgers, J.A.M. van Roosmalen UC-Davis Physics,CM Journal Club