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.

0 0.2 0.4 0.6 0.8

2

4

6

8

1010

0

t( )

10 t

Absorption Coefficient α –Physical Techniques for Measuring

α5,6

Optical Transmission Measurement t – Measured transmission l – Sample thickness R - Reflectance

t1 R( )

2e

l

1 R2e2 l

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

Quantum Dot

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

z = propagation direction nr = refractive index omega = frequency alpha = absorption coefficient Laws of Conservation

Energy Momentum

)2

exp()](exp[z

tc

niEE r

o

Figures based on Singh textbook

Photon Absorption

Photon Emission

)(|)*(|1 2

2

2

cvif

oro

Npamcn

e

32

2/12/3* )()(2)(

gr

cv

EmN

cvif ppa 22

32|)*(

eVm

p

o

cv24~20

2

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 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 mhh* = 0.45 mr

* =0.058 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

References

1. Seale, Eric. Solar Cells: Shedding a Little Light on Photovoltaics. 28, Feb. 2002. Solarbotics. <http://www.solarbotics.net/starting/200202_solar_cells/200202_solar_cell_physics.html>. Accessed 03/20/2005.

2. Pierret, Robert F. Semiconductor Device Fundaments. Addison Wesley Longman, 1996. pp 198-205.3. Anonymous. Absorption Coefficient. Undated. LaborLawTalk.

<http://dictionary.laborlawtalk.com/absorption_coefficient>. Accessed 04/01/2005.4. Anonymous. Transmission (T). Undated. Photonics Directory.

<http://www.photonics.com/dictionary/lookup/XQ/ASP/url.lookup/entrynum.5189/letter.t/pu./QX/lookup.htm>. Accessed 04/01/2005.

5. Augustine, G.  Jokerst, N.M.  Rohatgi, A. Absorption measurements of doped thin film InP for solar cell modeling. IEEE: Indium Phosphide and Related Materials, 1992., Fourth International Conference on. 21-24 April 1992.

6. Gerber, D.S.   Maracas, G.N.  A simple method for extraction of multiple quantum well absorption coefficient from reflectance and transmittance measurements. Quantum Electronics, IEEE Journal of. Volume: 29 , Issue: 10. Oct. 1993.

7. Kochman, B; Singh, J; et al. Absorption, Carrier Lifetime, and Gain in InAs-GaAs Quantum Dot Infrared Photodetectors. IEEE Journal of Quantum Electronics. Volume 39, Number 3. March 2003.

8. Anonymous. Photovoltaics. Evident Technologies. Undated. <http://www.evidenttech.com/applications/quantum-dot-solar-cells.php>. Accessed 04/14/2005.

9. Singh, J. Modern Physics for Engineers. John Wiley & Sons, Inc. 1999. pp 34, 156.10. Wu, Y. Singh, J. Polar Heterostructure for Multifunction Devices: Theoretical Studies. IEEE

Transaction on Electron Devices. VOL. 52, NO. 2, FEBRUARY 200511. Raffaelle, R. Profile of Ryne P. Raffaelle. RIT Department of Physics. Undated.

<http://www.rit.edu/~physics/facstaff/profiles/raffaeller.shtml>. Accessed 04/10/2005.12. Blood, P. On the Dimensionality of Optical Absorption, Gain, and Recombination in Quantum-Confined

Structures. IEEE Journal of Quantum Electronics. Vol. 36, No. 3, March 2000.13. D. Pan, E. Towne, and S. Kennerly. Strong normal-incident infrared absorption and photo-current

spectra from highly uniform (In,Ga)As/GaAs quantum dot structures. IEEE Electronic Letters. 14th May 1998 Vol. 34 No. 10.