Nanotechnology Application for Solar Cells: Using Quantum Dots to Modify Absorption Properties QUANTUM NANOS INC.

Introduction How Classical Solar Cells Operate Absorption Coefficient (α) Definition and Relevance of α Physical Techniques for Measuring α Light Absorption of Quantum Dot Layers Reasons for Interest Into Quantum Dot Light Absorption Definition of a Quantum Dot Formula for Light Absorption of a Quantum Dot Comparison of α versus Energy for Bulk Material and Quantum Dot Researchers working on Light Absorption of Quantum Dots Dr. Sheila Baily Dr. Ryne Raffaelle Problem Statement – Determining the most optically absorbent semiconductor material Problem Solution Explanation of Theory Results

How Classical Solar Cells Operate1,2

How Classical Solar Cells Operate1,2

How Classical Solar Cells Operate1,2

How Classical Solar Cells Operate1,2

How Classical Solar Cells Operate1,2

How Classical Solar Cells Operate1,2

How Classical Solar Cells Operate1,2

How Classical Solar Cells Operate1,2

Absorption Coefficient α – Definition and Relevance of α3 Definition of Absorption Coefficient α A measure of the rate in decrease of electromagnetic radiation (as light) as it passes through a given substance; the fraction of incident radiant energy absorbed per unit mass or thickness of an absorber.

Absorption Coefficient α – Definition and Relevance of α3 Unit of Absorption Coefficient α The units of α are per length (cm-1)

Absorption Coefficient α – Definition and Relevance of α3 Unit of Absorption Coefficient α The units of α are per length (cm-1)

Absorption Coefficient α – Definition and Relevance of α4 Absorption Versus Transmission Transmission (t): a measure of conduction of radiant energy through a medium, often expressed as a percentage of energy passing through an element or system relative to the amount that entered.

Absorption Coefficient α – Definition and Relevance of α4 Absorption Versus Transmission Transmission (t): a measure of conduction of radiant energy through a medium, often expressed as a percentage of energy passing through an element or system relative to the amount that entered.

Absorption Coefficient α – Definition and Relevance of α4 Absorption Versus Transmission Transmission (t): a measure of conduction of radiant energy through a medium, often expressed as a percentage of energy passing through an element or system relative to the amount that entered.

Absorption Coefficient α – Physical Techniques for Measuring α5,6 Optical Transmission Measurement t – Measured transmission l – Sample thickness R - Reflectance

Light Absorption of Quantum Dots – Why We Are Interested7,8,13 These structures have great potential for optoelectronic applications, one of which may be solar cells Standard solar cells have a theoretical upper conversion rate of 33%, the theoretical limit on the conversion of sunlight to electricity is 67%

Light Absorption of Quantum Dots – Definition of a Quantum Dot9

Light Absorption of Quantum Dots – Definition of a Quantum Dot9 Quantum Dot Layer

Light Absorption of Quantum Dots – Definition of a Quantum Dot9 Quantum Dot Layer

Light Absorption of Quantum Dots – Formula7 _ Vav = Average Dot Volume pfi = 2d momentum matrix element a = polarization of light N(ћω) = density of states

Light Absorption of Quantum Dots – Formula12 Transmission for Quantum dots. For transmission through n planes of dots, each having the same dot density N and each dot experiencing the same optical field amplitude, the transmission fraction is: Tn=(1-σN)n ≈ (1-nσN) ; (σN << 1) σ represents a cross section of the layer

Light Absorption of Quantum Dots – Comparison of α versus Energy for Bulk Material and Quantum Dot9

Light Absorption of Quantum Dots – Comparison of α versus Energy for Bulk Material and Quantum Dot

Light Absorption of Quantum Dots – Comparison of α versus Energy for Bulk Material and Quantum Dot

Light Absorption of Quantum Dots – Comparison of α versus Energy for Bulk Material and Quantum Dot7

Light Absorption of Quantum Dots – Comparison of α versus Energy for Bulk Material and Quantum Dot7

Researchers Working on Light Absorption of Quantum Dot Layers Dr. S. Bailey Using quantum dots in a solar cell to create an intermediate band IEEE Photovoltaic Specialist Conference (PVSC) Executive Committee since 1987

Researchers Working on Light Absorption of Quantum Dot Layers11 Dr. Ryne R RIT NanoPower Laboratories Organic and Plastic Solar Cells Combined with Quantum Dot Layers

Problem Solution – Explanation of theory Photon Absorption z = propagation direction nr = refractive index omega = frequency alpha = absorption coefficient Laws of Conservation Energy Momentum Photon Emission Figures based on Singh textbook

Problem Statement – Determining the most optically absorbent semiconductor bulk Consider InP and GaAs as being the available semiconductors to create a solar cell. This solar cell will be a hybrid, consisting of a traditional solar cell created with either InP or GaAs, and coating layers of quantum dots of either InP or GaAs. If maximizing absorption is the only criteria for designing the solar cell, which material should be used for the bulk? Which should be used for the quantum dot layers? Assume the density of states for quantum dot layers of both materials is equal and occurs at the same point, E = .1eV, and that the polarization-momentum product sum is the same in both cases.

Absorption coefficient of InP and GaAs Problem Statement – Determining the most optically absorbent semiconductor bulk Absorption coefficient of InP and GaAs Required constants by material14 Material Electron Mass (mo) Hole Mass (mo) Calculated reduced mass (mo) Eg (eV) Lattice Constant (A) Refractive index (nr) Gallium Arsenide, GaAs 0.067 mhh* = 0.45 mr* =0.058 1.5 5.65 3.65 Indium Phosphide, InP 0.073 1.34 5.87 3

Problem Solution – Results: GaAs Bulk

Problem Solution – Results: InP Bulk

Problem Solution – Results

Problem Solution – Results: GaAs Quantum Dot Layer

Problem Solution – Results: InP Quantum Dot Layer

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