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

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

3. How Classical Solar Cells Operate1,2

4. How Classical Solar Cells Operate1,2

5. How Classical Solar Cells Operate1,2

6. How Classical Solar Cells Operate1,2

7. How Classical Solar Cells Operate1,2

8. How Classical Solar Cells Operate1,2

9. How Classical Solar Cells Operate1,2

10. How Classical Solar Cells Operate1,2

11. 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.

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

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

14. 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.

15. 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.

16. 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.

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

18. 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%

19. Light Absorption of Quantum Dots – Definition of a Quantum Dot9

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

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

22. Light Absorption of Quantum Dots – Formula7_
Vav = Average Dot Volume
pfi = 2d momentum matrix element
a = polarization of light
N(ћω) = density of states

23. 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

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

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

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

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

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

29. 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

30. 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

31. 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

32. 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.

33. 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

34. Problem Solution – Results: GaAs Bulk

35. Problem Solution – Results: InP Bulk

36. Problem Solution – Results

37. Problem Solution – Results: GaAs Quantum Dot Layer

38. Problem Solution – Results: InP Quantum Dot Layer

39. References
Seale, Eric. Solar Cells: Shedding a Little Light on Photovoltaics. 28, Feb Solarbotics. < Accessed 03/20/2005.
Pierret, Robert F. Semiconductor Device Fundaments. Addison Wesley Longman, pp
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Anonymous. Transmission (T). Undated. Photonics Directory. < Accessed 04/01/2005.
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 April 1992.
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
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.
Anonymous. Photovoltaics. Evident Technologies. Undated. < Accessed 04/14/2005.
Singh, J. Modern Physics for Engineers. John Wiley & Sons, Inc pp 34, 156.
Wu, Y. Singh, J. Polar Heterostructure for Multifunction Devices: Theoretical Studies. IEEE Transaction on Electron Devices. VOL. 52, NO. 2, FEBRUARY 2005
Raffaelle, R. Profile of Ryne P. Raffaelle. RIT Department of Physics. Undated. < Accessed 04/10/2005.
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.
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 Vol. 34 No. 10.