1. Powered Paint: Nanotech Solar Ink
Hot Science - Cool Talk # 69
Dr. Brian Korgel
December 3, 2010
Produced by and for Hot Science - Cool Talks by the Environmental Science Institute. We request that the use of these materials include an acknowledgement of the presenter and Hot Science - Cool Talks by the Environmental Science Institute at UT Austin. We hope you find these materials educational and enjoyable.

2. Powered Paint: Nanotech Solar Ink
Brian A. Korgel
Department of Chemical Engineering, Texas Materials Institute,
Center for Nano- and Molecular Science and Technology
The University of Texas at Austin
December 3, 2010

3. To Lower the Cost of Solar Energy…

4. To Lower the Cost of Solar Energy…
Change the way solar cells are made
Slow, high temperature vacuum processes

5. To Lower the Cost of Solar Energy…
Change the way solar cells are made
Print like newspaper
Slow, high temperature vacuum processes

6. To Lower the Cost of Solar Energy…
Change the way solar cells are made
Print like newspaper
To lower the cost of solar energy, we need to change the way solar cells are made. The idea is to spray paint a solar cell; similar to interior paint, nano-crystals are dispersed in a solvent, the liquid is then coated onto substrate or surface, the solvent evaporates, and you are left with solid inorganic layer of material (photo reactive nano-crystals).
How does a solar cell work? Solar cells are made with silicon; silicone has to be made under high temps and high pressure. The process is very slow and too expensive. Can we make solar cells the same way we print newspaper? Can substrates be printed with an ink that can absorb sunlight and create power? Can they be mass produced as a kind of “solar ink”?
Slow, high temperature vacuum processes
Photovoltaic Paints…?

7. To Lower the Cost of Solar Energy…
Change the way solar cells are made
Brittle and heavy

8. To Lower the Cost of Solar Energy…
Change the way solar cells are made
Another issue: Solar panels are relatively brittle and heavy. If we can manufacture lightweight, flexible plastic solar cells using substances like polyethylene, we can dramatically lower the cost of manufacturing and change the way they are installed. Example: a roll of solar cells that can be rolled out on a roof for installation.
Brittle and heavy
Light and flexible

9. A Photovoltaic Device
How it works:

10. A Photovoltaic Device How it works:
A semiconductor
How a photovoltaic device works: Start with a semiconductor. Example of semiconductor: silicone, gallium nitride, germanium, etc. Substances that absorb light at different wavelengths.
Start with a semiconductor…
(Examples of semiconductors include silicon, GaN, germanium…)

11. The semiconductor absorbs the light from the sun
A Photovoltaic Device
How it works:
Photons
(Light)
A semiconductor
The semiconductor absorbs the light from the sun.
The semiconductor absorbs the light from the sun

12. In the semiconductor, electrons are tied up in bonds between atoms
In the semiconductor, electrons are tied up in bonds between atoms (sharing electrons).

13. But when the semiconductor absorbs a photon
(Light)

14. But when the semiconductor absorbs a photon, a free electron is created
Photon energy “excites” an electron
But when the semiconductor absorbs a photon, a free electron is created. In other words, an electron is taken out of its bonding orbital, creating a “free” electron.

15. But when the semiconductor absorbs a photon, a free electron is created and a hole
Excited electron leaves behind a hole
Once an electron is excited, it leaves behind a hole (the absence of the electron).

16. Both the electron and hole can move to create a photogenerated electrical current across the semiconductor e-
Excited electron and hole can move to create a current
Both the electron and hole can move to create a photogenerated electrical current across the semiconductor. If the hole and electron recombine, you get no power. h+

17.  Light absorption creates an electron and hole
A Photovoltaic Device
How it works:
Photons
(Light)
Semiconductor 2 e- h+
Light absorption creates an electron and hole.
 Light absorption creates an electron and hole

18. A Photovoltaic Device How it works:
Both the electron and hole can move
Photons
(Light)
Semiconductor 2 e- h+
Both the electron and hole can move; but we need a force that will separate the electron and hole to create an electric current.
But…need a force that will separate the electron and hole to create an electric current

19. A Photovoltaic Device How it works:
Photons
(Light)
Semiconductor 2 e- h+
To do this, another semiconductor layer is needed.
Another semiconductor layer is needed

20. The two semiconductors form a p-n junction
A Photovoltaic Device
How it works:
n-type
Photons
(Light)
Semiconductor 2 e- h+
p-type
The two semiconductors form a p-n junction

21. A Photovoltaic Device How it works:
n-type
Photons
(Light) e- h+
p-type
The two semiconductors form a p-n junction that separates the electron and hole; this is generates the photovoltaic effect.
The two semiconductors form a p-n junction that separates the electron and hole; this is the photovoltaic effect

22. Electrical power can be generated
A Photovoltaic Device
How it works:
Photons
(Light) e- h+
Electrical power can be generated.
Electrical power can be generated

23. But we need metal electrodes on each side to extract the charge
A Photovoltaic Device
How it works:
Metal (anode)
Photons
(Light) e- h+
Metal electrodes are needed on each side to extract the charge.
Metal (cathode)
But we need metal electrodes on each side to extract the charge

24. And a mechanical support
A Photovoltaic Device
How it works:
Metal (anode)
Photons
(Light) e- h+
Metal electrodes are needed on each side to extract the charge, as well as mechanical support.
And a mechanical support

25. A Photovoltaic Device How it works:ç
A Photovoltaic Device
How it works:
Photons
(Light) e- h+
This is the basic design of every solar cell made.
This is the basic design of every solar cell

26. What’s wrong with the existing technology?

27. What’s wrong with the existing technology?
A Solar Farms of PVs (of silicon)

28. What’s wrong with the existing technology?
It’s too expensive

29. What’s wrong with the existing technology?
It’s too expensive
What’s wrong with the existing technology? Comparing costs, solar production is ten times the cost of other kinds of energy production.
Production cost of energy (DOE, 2002)

30. What’s wrong with the existing technology?
To compete with fossil fuels:
Need < $1/Wp module cost
Current cost is $4.27/Wp
Cost of power from fossil fuels is <¢4-10/kWh
-Solar power stands at ¢20/kWh
Costs need to be lowered with energy production using solar technologies. One challenge is that power is not generated at night; no sun, no power. Power generation is discussed in terms of peak watt (Wp); to compete with other forms of energy production, solar power needs to be < $1 per Wp. The current average cost of energy produced by solar technologies is much more expensive at $4.27 per Wp.

31. What’s wrong with the existing technology? Need < $1/Wp module cost
Current cost is $4.27/Wp
-Corresponds to ~¢20/kWh
55% of the cost is in manufacturing the module
An additional challenge: we need to learn how to lower costs in manufacturing of solar cells and how we employ them. We need to get away from heavy, brittle panels and manufacture more flexible, lightweight modules to lower both manufacturing and installation costs.
SolarBuzz.com
Kazmerski LL, J Electron Spectroscopy, 2006; 150:103–135.

32. Silicon dominates the solar cell market

33. Silicon dominates the solar cell market
It’s relatively expensive

34. Silicon dominates the solar cell market
It’s relatively expensive
and mature
Silicon dominates the solar cell market. It is a mature technology; it is a substance that is used extensively in computer components with extensive research in how it functions.

35. The Cost of Silicon http://pubs.usgs.gov/fs/2002/fs087-02/
Silicon solar cell costs have reduced since The technology has gotten better and slightly cheaper.

36. The Cost of Silicon The cost of silicon is high
The primary cost of the silicon solar cell is silicon extraction itself. Silicon comprises 28% of the earth’s crust. It is found in sand as SiO2; never found as Si. We need it in Si form/pure form to utilize it to its full potential. It takes an incredible amount of money to extract silicone from sand and re-crystalize/purify it to get solar cells to work. It also takes an incredible amount of energy to process silicon to make a solar cell. On average, solar cells need about 10 years of use before they make environmental sense for use. Produce an incredible amount of waste, like hydrochloric acid, and other hazardous materials in processing.

37. The Cost of Silicon Processing silicon is energy intensive

38. The Cost of Silicon 2009, $4.27/W 2010, $3.59/W
In 2003 costs decreasing; the price of oil increased. Supply of available silicon became depleted. Solar cell costs went back up (supply and demand).
2010, $3.59/W

39. Silicon PV’s work well and dominate the market
Estimated 14,000 MW capacity in 2010

40. Silicon PV’s work well and dominate the market, but are too expensive for the long-term
In short, silicon photovoltaics work well and dominate the market, but they are too expensive for the long-term projections of manufacturing and use.
Estimated 14,000 MW capacity in 2010

41. There are new Technologies on the Horizon:

42. There are new Technologies on the Horizon:
Source IEA PVPS
CdTe-based thin film solar cells:
First Solar claims to have built modules at $0.98/W
There are new technologies on the horizon: using Cadmium telluride (CdTe) crystals; major problem is that they contain cadmium, a heavy metal that is highly toxic and polluting.
Rooftop First Solar CdTe panels

43. There are new Technologies on the Horizon:
Organic materials-based solar cells
Other innovative technologies are using organic materials-based solar cells;example of a hand-bag with a solar panel on the side. This bag is made to power your cell phone; only problem is it recharges very slowly and is very inefficient. Roll-to-roll processing of polymer-based solar cells; these are good examples of flexible materials currently being used.
Roll-to-roll processing of polymer-based solar cells (Mekoprint A/S)
Konarka

44. There are new Technologies on the Horizon:
But the cost of solar energy still needs to be reduced by about a factor of 10.
Note: the cost of solar energy still needs to be reduced by about a factor of 10 to meet efficiency and market demands.

45. Can we make a “solar” paint that can convert sunlight energy into electricity?
Can we make a “solar” paint that can convert sunlight energy into electricity? Current research is to make nano-crystals of a semiconductor that will absorb the light, then disperse nano-crystals in a solvent. They are then coated onto a substrate, solvent evaporates, and you are left with solid inorganic layer of material.
100 nm

46. Copper indium gallium selenide: CIGS
First, we need an ink:
Copper indium gallium selenide: CIGS
What material will one use? First we need an ink; current success has been found with copper indium gallium selenide (CIGS).

47.  Develop a chemical synthesis of CIGS nanocrystals
First, we need an ink:
 Develop a chemical synthesis of CIGS nanocrystals
We are using CIGS in the lab. Take them and make solar cells out of them; not a perfect crystal but still functions very well.

48.  Develop a chemical synthesis of CIGS nanocrystals
First, we need an ink: N2 TC
 Develop a chemical synthesis of CIGS nanocrystals
To get CIGS, we combined 4 elements that worked to form a stable material that is absorbing all incoming light.
oleylamine, 240oC
CuCl + InCl3 + 2Se
CuInSe2 nanocrystals

49. 15 – 20 nm diameter CuInSe2 nanocrystals49

50. Korgel and lab published paper in 2008; noted for breakthrough in combination of elements.

51. Glass or plastic support
Metal
n-type semiconductor
Nanocrystal ink
Metal
Glass or plastic support
This is an example of the device structure using CIGS.

52. Nanocrystal PV Device Fabrication
2. Solution-deposit nanocrystals
1. Deposit metal foil onto a flexible substrate
3. Deposit heterojunction partner layers (CdS/ZnO)
4. Pattern metal collection grid
Example of process; note that the solution-deposit nanocrystals are applied with airbrush device.

53. Nanocrystal Film Formation
For the solar cell, need uniform films of nanocrystals.
One application technique was applying solution through pipette onto glass panels.

54. Efficiency 0.341% Voc 329 mV Jsc 3.26 mA/cm2 Fill Factor 0.318 Glass
Standard Cell
Efficiency
0.341%
Voc
329 mV
Jsc
3.26 mA/cm2
Fill Factor
0.318
Glass Mo
CuInSe2 nanocrystals
ZnO
CdS

55. Efficiency 0.341% Voc 329 mV Jsc 3.26 mA/cm2 Fill Factor 0.318 Glass
Standard Cell
Efficiency
0.341%
Voc
329 mV
Jsc
3.26 mA/cm2
Fill Factor
0.318
Glass Mo
CuInSe2 nanocrystals
ZnO
CdS
After one year of working with materials, we had consistency in manufacturing of nano-crystals to generate a photovoltaic effect. Only challenge is that we can not produce a maximum efficiency: theoretical maximum efficiency for a single junction solar cell is 31%. The current technology can never get to 100%. Korgel’s lab was able to consistently produce .3%.

56. Nanocrystal Film Formation
For the solar cell, need uniform films of nanocrystals.
Using nanocrystals formed with CIGS, we realized that heavy metals do not need to be used, we are not limited to a heavy substrate like glass, and that we can apply by spray painting, etc.

57. CIS Nanocrystal PV device
Efficiency of 3.1%
This realization enabled Korgel’s lab to achieve higher efficiencies. To make an impact, have to achieve 10% efficiency.
V. A. Akhavan, M. G. Panthani, B. W. Goodfellow, D. K. Reid, B. A. Korgel, “Thickness-limited performance of CuInSe2 nanocrystal photovoltaic devices,” Optics Express, 18 (2010) A411-A420.

58. Efficiency of 2% on plastic

59. Solar inks can be chemically synthesized
Accomplished to date:
Solar inks can be chemically synthesized
Accomplishments to date: solar inks can be chemically synthesized.

60. Solar inks can be chemically synthesized
Accomplished to date:
Solar inks can be chemically synthesized
Solar cells can be fabricated with solar inks
Accomplishments to date: solar inks can be chemically synthesize; solar cells can be fabricated with solar inks.

61. Solar inks can be chemically synthesized
Accomplished to date:
Solar inks can be chemically synthesized
Solar cells can be fabricated with solar inks
Solar cells can be fabricated with solar inks on light-weight flexible substrates
Accomplishments to date: solar inks can be chemically synthesize; solar cells can be fabricated with solar inks; solar cells can be fabricated with solar inks on light-weight flexible substrates.

62. Solar inks can be chemically synthesized
Accomplished to date:
Solar inks can be chemically synthesized
Solar cells can be fabricated with solar inks
Solar cells can be fabricated with solar inks on light-weight flexible substrates
Accomplishments to date: solar inks can be chemically synthesize; solar cells can be fabricated with solar inks; solar cells can be fabricated with solar inks on light-weight flexible substrates.
The current challenge is to try to improve the power conversion efficiency up to >10%.
The current challenge is to try to improve the power conversion efficiency up to >10%

63. 3.1% Korgel group milestone chart for CIGS Nanocrystal PVs Project
Graph of efficiency vs. time regarding Korgel’s lab progress.
Project
conception
(Sept., 2006)

64. Extracting the photogenerated electrons and holes efficiently is currently the biggest challenge
Photons
(Light) e- h+
Extracting the photogenerated electrons and holes efficiently is currently the biggest challenge.

65. ~200 nm thick layer of nanocrystals on glass disc
The highest efficiency devices have very thin nanocrystal layers that do not absorb all of the light
The highest efficiency devices have very thin nanocrystal layers that do not absorb all of the light.
~200 nm thick layer of nanocrystals on glass disc

66. Thicker nanocrystal layers absorb more light, but are less efficient
120 nm
250 nm
Measurement is determined by the Internal quantum efficiency; every time the nanocrystal layers absorb a photon and create an electron and hole, what fraction of the electrons and holes make it out of the device as current. This is not same as a measurement known as the power conversion efficiency: total energy in the sun converted to total power of your device.
400 nm
V. A. Akhavan, M. G. Panthani, B. W. Goodfellow, D. K. Reid, B. A. Korgel, “Thickness-limited performance of CuInSe2 nanocrystal photovoltaic devices,” Optics Express, 18 (2010) A411-A420.

68. Korgel’s research was published in the Reader’s Digest annual list of 20 inventions that will save your life, came in at number 12 on the list.
The challenge is to demonstrate commercially viable efficiencies of >10% (currently, the devices function at 3%)

69. National Science Foundation
Special Acknowledgement to:
Vahid Akhavan
Brian Goodfellow
Matt Panthani
Danny Hellebusch
Dariya Reid
Funding from Robert A. Welch Foundation; Air Force Research Laboratory;
National Science Foundation

70. Dr. Brian Korgel
Brian Korgel's research lab studies Nanotechnology, the field of applied science at the atomic and molecular scale. His group focuses on investigating size-tunable material properties, and the self-assembly and fabrication of nanostructures. This multidisciplinary research finds applications in microelectronics, photonics, photovoltaics, spintronics, coatings, sensors and biotechnology.