Nanotechnology Application for Solar Cells: Using Quantum Dots to Modify Absorption Properties Prepared by James Fodor Kwok Mak Viet Huynh

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

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. Ryne Raffaelle Dr. Sheila Baily  Problem Statement – Determining the most optically absorbent semiconductor material  Problem Solution Explanation of Thoery Results

How Classical Solar Cells Operate 1,2

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. Ryne Raffaelle Dr. Sheila Baily  Problem Statement – Determining the most optically absorbent semiconductor material  Problem Solution Explanation of Thoery Results

Absorption Coefficient α – Definition and Relevance of α 3  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 α 3  Unit of Absorption Coefficient α The units of α are per length (cm -1 )

Absorption Coefficient α – Definition and Relevance of α 3  Unit of Absorption Coefficient α The units of α are per length (cm -1 )

Absorption Coefficient α – Definition and Relevance of α 4  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 α 4  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 α 4  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 α 5,6  Optical Transmission Measurement t – Measured transmission l – Sample thickness R - Reflectance

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  Why We Are Interested  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 Nano-coating Dr. Ryne Raffaelle Dr. Sheila Baily  Problem Statement – Determining the most optically absorbent semiconductor material  Problem Solution Explanation of Theory Results

Light Absorption of Quantum Dots – Why We Are Interested 7,8,13  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 Dot 9 Quantum Dot

Light Absorption of Quantum Dots – Definition of a Quantum Dot 9 Quantum Dot Layer

Light Absorption of Quantum Dots – Definition of a Quantum Dot 9 Quantum Dot Layer

Light Absorption of Quantum Dots – Formula 7  _  V av = Average Dot Volume  p fi = 2d momentum matrix element  a = polarization of light  N(ћω) = density of states

Light Absorption of Quantum Dots – Formula 12  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: T n =(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 Dot 9

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 7

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. Ryne Raffaelle  Dr. Sheila Baily  Problem Statement – Determining the most optically absorbent semiconductor material  Problem Solution Explanation of Theory Results

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

Researchers Working on Light Absorption of Quantum Dot Layers 11  Dr. Ryne Raffaelle  Rochester Institute of Technology  NanoPower Laboratories  Organic and Plastic Solar Cells Combined with Quantum Dot Layers

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. Ryne Raffaelle Dr. Sheila Baily  Problem Statement – Determining the most optically absorbent semiconductor material  Problem Solution Explanation of Theory Results

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

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.

Problem Statement – Determining the most optically absorbent semiconductor bulk  Absorption coefficient of InP and GaAs  Required constants by material 14 MaterialElectron Mass (mo) Hole Mass (mo) Calculated reduced mass (mo) Eg (eV) Lattice Constant (A) Refractive index (nr) Gallium Arsenide, GaAs 0.067m hh * = 0.45m r * = Indium Phosphide, InP 0.073m hh * = 0.45m r * =

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. Ryne Raffaelle Dr. Sheila Baily  Problem Statement – Determining the most optically absorbent semiconductor material  Problem Solution  Explanation of Theory  Results

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

Conclusion  How Classical Solar Cells Operate  Absorption Coefficient (α) Definition and Relevance of α Physical Techniques for Measuring α  Light Absorption of Quantum Dots 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. Ryne Raffaelle Dr. Sheila Baily  Problem Statement – Determining the most optically absorbent semiconductor material  Problem Solution Explanation of Theory Results

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