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.

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 korgel@che.utexas.edu December 3, 2010

To Lower the Cost of Solar Energy…

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

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

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…?

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

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

A Photovoltaic Device How it works:

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…)

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

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

But when the semiconductor absorbs a photon (Light)

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.

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).

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+

 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

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

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

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

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

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

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

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

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

What’s wrong with the existing technology?

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

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

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)

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.

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.

Silicon dominates the solar cell market

Silicon dominates the solar cell market It’s relatively expensive

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.

The Cost of Silicon http://pubs.usgs.gov/fs/2002/fs087-02/ Silicon solar cell costs have reduced since 1980. The technology has gotten better and slightly cheaper. http://pubs.usgs.gov/fs/2002/fs087-02/

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. http://pubs.usgs.gov/fs/2002/fs087-02/

The Cost of Silicon Processing silicon is energy intensive http://pubs.usgs.gov/fs/2002/fs087-02/

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 http://pubs.usgs.gov/fs/2002/fs087-02/

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

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

There are new Technologies on the Horizon:

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

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

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.

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

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).

 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.

 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

15 – 20 nm diameter CuInSe2 nanocrystals 49

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

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.

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.

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

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

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%.

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.

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.

Efficiency of 2% on plastic

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

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.

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.

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%

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)

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.

~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

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.

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%)

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

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.