cc  >> Nadia Dominguez: All right,  welcome to Wednesday Nite at the  Lab at the UW-Madison Biotech  Center.  My name is Nadia Dominguez.  Tonight we are welcoming  Dr. Mike Arnold of the  Department of Material Science  and Engineering.  Dr. Arnold earned his bachelor  of science degree in electrical  engineering from the University  of Illinois at Urbana-Champaign.  He also earned his PhD  from Northwestern University  in Material Science and  Engineering.  His postdoctoral research at the  University of Michigan at Ann  Arbor, he studied carbon-based  electronic materials for  high-efficiency white lighting  and photovoltaics.  Dr. Arnold joined the Department  of Material Science and  Engineering as an assistant  professor in August 2008,  so relatively recently.  He currently directs the  Advanced Materials for Energy  and Electronics group at UW  Madison and leads research into  novel materials for next  generation solar photovoltaic,  optoelectronic and semiconductor  logic devices.  Wow, that's a mouthful there.  So tonight please join me in  welcoming Dr. Mike Arnold to  Wednesday Nite at the Lab. 

 [APPLAUSE]  >> Mike Arnold: Thank you.  Can you all hear me?  Is the microphone working?  Great.  As you heard, I've made my tour  around the Big Ten of four out  of the 11 Big Ten schools so  far.  And Wisconsin has been my  favorite and hopefully my final  stop on my tour.  So tonight I'm going to try to  answer the question for you why  our electricity doesn't come  from the sun or from sunlight.  The sun is in abundance supply  or free energy to us.  And if we can find a way to  better collect it and convert it  to electricity, it could  potentially solve a lot of our  energy needs.  Obviously, right now the  materials that are used in  collecting sunlight and  converting it to electricity  aren't sufficient, aren't  efficient enough and aren't  cheap enough to allow for solar  energy to play a big part in our  society.  But with future research, solar  electricity could become a very  important part of our economy  and our way of life.  So I'm going to divide the talk  into four sections. 

 First, I'm just going to give a  very general introduction as the  sun as the source of our energy  and prove to you there's a lot  of energy coming from the sun.  Then I'm going to tell you about  how a photovoltaic solar cell  works and what it does and what  photovoltaic even means.  And I'm going to then tell you  about the most common material  used in photovoltaic cells,  which is silicon.  And I'm going to compare the  economics of silicon solar cells  as a means for producing  electricity to that of, say,  coal-produced electricity.  And finally, I'll tell you about  new materials beyond silicon  that have the potential to make  solar photovoltaics more  practical.  So to talk about energy, I think  it's important to first discuss  how much energy we as people  use.  So this is a plot from the  Department of Energy.  It's a diagram that shows the  inputs of energy into our  country and the outputs.  So you see the sources of energy  are coal, natural gas, oil, the  outputs, about a quarter of our  energy uses are in residential,  commercial and industrial and  transportation sectors.  So this is the energy we use  every year. 

 The font is a little bit small  but we use 100 quads of energy  every year.  So what is a quad?  A quad means one quadrillion  BTU.  One quad is 10 to the 18th  joules.  So we use 10 to the 20th joules  of energy every year.  I'll try to put that into  context to maybe something you  can understand a little bit  better in the next couple of  slides.  So one thing to ask is how much  energy is that per person?  Well, as I just mentioned we use  10 to the 20th joules per year  as a country.  If you spread that out even over  time, that's three  trillion-watts which means  three million joules per second.  If you assume the population of  the United States is 300 million  then that's 10,000-watts  per person or 10-kilowatts  per person.  So that's about a hundred  100-watt light bulbs per person  of energy that we're using.  So I have a question for you. 

 So when you take a 20-pound  dumbbell, this is how I'm going  to relate to you how much energy  is used in a joule, when you  take a 20-pound dumbbell and you  curl it like this, let's say I'm  lifting it about three yards or  a meter, how much energy is in  that?  How many joules are in that?  Well, 20 pounds is about 10  kilograms and 10 kilograms times  one meter times gravity which is  9.8 meters per second squared  gives you about 100 joules.  So if you wanted to somehow  convert your efforts in lifting  this dumbbell into energy  producing and you could harness  the energy of a hundred joules  each time, by the way I went to  the gym all last week so I could  do this.  [LAUGHTER]  So if you were able to harness  this energy with 100%  efficiency, then you would have  to lift this dumbbell 380,000  times per hour to supply the  amount of energy you're using.  So every person in the country  would have to do this 24 hours  a day, every day of the year.  So that hopefully explains to  you a little bit more clearly in  how much energy we really use as  mankind.  So the US uses about 20% of the  worldwide energy consumption.  So we use three times 10 to the  12th watts of electricity  averaged out over time or three  terawatts, the world uses around  15 terawatts.  So is the sun a viable source of  supplying 15 terawatts of energy  or power to us?  Well, the sun is emitting quite  a lot of energy constantly.  The surface of the sun is around  6,000 degrees Kelvin and it's  constantly emitting in the form  of light 4 times 10 to the 26  watts of energy.  So if you remember terawatt is  10 to the 12th. 

 So 12 orders of magnitude more  energy is being emitted from the  sun, which is a factor of a  trillion, than we use here on  earth.  But, of course, earth is 93  million miles from the sun so  only a small fraction of that  energy is incident upon our  planet.  The diameter is around 8,000  miles, I do the rough  calculation, about 3 times  10 to the 17th watts  are incident on our planet.  We use 10 to the 12th, roughly,  so that's a factor of 10 to  100,000 more energy incident  upon the earth than we use.  So how much reaches the surface?  That's another question.  The atmosphere, of course,  absorbs some of the light but  it's insignificant.  So taking that wattage and  dividing it by area, the sun is  illuminating in outer space at  about an intensity around 1,400  watts per meter squared and a  good fraction of that which is  around 1,000 watts per meter  squared reaches the surface.  That's at noon on a sunny day  near the equator.  Of course, the amount of energy  we reach here in Wisconsin  is not quite as large as that.  So geography and weather will  determine how much actual solar  energy is available to us. 

 So here's a map of the global  radiance in kilowatt hours per  meter squared per year.  The yellow colors represent the  most sunlight and the red and  bluish colors are the least.  So you can see on every  continent there are regions that  receive over 2500 kilowatt hours  per meter squared per year.  So for example in the United  States the southwest receives  the most sunlight.  In China and the desert region  in the west, there's significant  areas of intense solar  radiation.  All of Africa receives a great  deal of sunlight.  So what about Wisconsin?  In Wisconsin we receive around  3.5 kilowatt hours per meter  squared per day of energy.  Which is about half of that in  Arizona.  If you average over the entire  state that means the entire  state is continuously,  on average, this is averaging  between day and night, between  cloudy days and between sunny  days, and winter and spring, and  fall and summer, on average  we receive 20 terawatts of power  from the sun.  Whereas the entire country uses  three terawatts. 

 In Arizona, which is a little  bit bigger area, they receive 86  terawatts on average of energy.  So if you wanted to supply the  entire US's energy from solar,  and you can make a solar power  plant as big as you wanted,  you would only need to make it  a 140 by 140 miles somewhere  in the American southwest and  to use a reasonable solar cell  efficiency of 20%.  So you could buy 20% solar cells  right now.  So in this area, if you made an  enormous solar farm or solar  power plant, you could supply  the entire country's energy  needs.  So what I've hoped to convince  you of in this first section is  while mankind's energy needs are  enormous but the sun is an even  more enormous source of energy  and that sunlight could power  our society if the proper  technology were available.  Worst case, if we run out of  coal and oil, the sun could  prevent us from doing a whole  bunch of weight lifting.  So now I'm going to tell you  about how a solar cell works and  what even it does.  So the very simplest definition,  a photovoltaic solar cell  transduces or converts light,  which are electromagnetic waves,    into electricity.  So that's kind of obvious.  So I think if right now, at this  stage I'm going to call a solar  cell just a black box and we're  going to discuss what this black  box really is. 

 But you illuminate your black  box and whatever material is  inside the black box, its  function could be to generate  positive and negative charge and  to separate them and to cumulate  the positive and negative charge  on different terminals, just  like a battery there's a  positive and negative terminal.  Then you could connect your  solar cell to a circuit.  Here I've drawn a toaster oven.  And the light induces the  formation of charged particles  and those charged particles flow  through your circuit back to  form a loop.  So you can kind of think of a  solar cell as kind of like a  battery except for the energy is  coming from the conversion and  transduction of light to  electricity.  So you've probably all used  solar cells before, or at least  a lot of you have probably used  pocket calculators with solar  cells.  The advantage of having a solar  cell on a calculator is after  taking your calculus exam and  your battery runs out, you have  a solar cell to power you, and  you can still get an A in your  class.  But the idea is it's just like a  battery.  It's taking light, making  electricity.  So what is in this black box?  What is inside it?  So a solar cell is made from  semiconductors. 

 And silicon is the most common  semiconductor used.  There are also inactive  components such as metal, glass,  structural materials, etc, but  the semiconductor is what does  all the work.  So a computer chip like your  Pentium 4 that Intel makes and a  solar cell are both made from  silicon, and therefore a lot of  the same phenomenon physics and  material science understanding  that goes into understanding how  to make computer chips is also  applicable to making solar  cells.  >> Why do they call the  semiconductor rather than a  conductor?  >> Arnold: That's a great  question, which I will hopefully  answer in these next couple  slides.  So a semiconductor can act both  like a conductor and an  insulator.  So, for example, a conductor  would be copper and an insulator  would be glass.  And a semiconductor you can tune  its properties to be like either  one.  And it has unique functionality  for that reason in the  electronics and solar cells.  So you can also induce a  semiconductor to conduct either  positive or negative charge and  has an energy gap which allows  it to absorb light which you  want for a solar cell as well.  So semiconductors are the  materials that are required for  solar cells, but the reason it's  called a semiconductor is it's  not necessarily a conductor, it  can be an insulator too. 

 So I hope that answers your  question.  So let's look at silicon.  So silicon is element number 14  in the periodic table.  It's a group four element.  It is four electrons short of a  complete shell of eight  electrons if you think of terms  of a high school chemistry level  of thinking.  And silicon forms a crystal with  the same atomic structure as in  diamonds except for a diamond is  made from carbon atoms.  And the difference between  silicon and diamond is that the  atomic spacing between the atoms  is larger in silicon.  So every silicon atom is bonded  tetrahedrally to four other  silicon atoms.  So it's hard.  So I'm going to talk a lot about  silicon in the next three or  four slides. 

 It's hard to draw this  three-dimensional crystal  structure so I've drawn a 2-D  structure here.  So here are silicon atoms and  each silicon atom is kind of  bonded to its nearest neighbor.  So each one is bonded to four  other neighbors.  So all four electrons in the  outer shell are covalently  bonded to the neighbors so there  are no free charges.  All the electrons are  participating in these covalent  bonds.  There are no free charges.  Intrinsic silicon, which is just  pure silicon as I've drawn,  therefore is kind of like an  insulator.  There's very low conductivity.  So an important property of a  semiconductor that you could use  for solar cells is that  semiconductors will absorb light  and create charge.  Oops, went too fast. 

 So here's light coming in and a  positive charge and a negative  charge are created.  And the positive charge, which  I've drawn red here and denoted  by h+, are called holes.  They have a positive charge.  The negative charges are  electrons, I've denoted them  blue, e- here.  So this is exactly what I was  telling you we wanted in our  black box.  We wanted light to come in,  create positive and negative  charge and then use that charge  to run our electrical circuit.  Except you need to be able to  separate the positive and  negative charge, and there's no  mechanism in just an intrinsic  semiconductor to separate  charge.  So one route to separating  charge is to use doping.  So what is doping?  Well, this is not the type of  doping I mean.  [LAUGHTER]  Although this could be  potentially useful, maybe he  could lift the dumbbell 380,000  times an hour. 

 [LAUGHTER]  But in semiconductors, doping  results from the intentional or  unintentional addition of  impurities to the semiconductor.  And I'll explain what that  means.  So silicon is group four.  Good dopants for silicon lie in  column three and column five.  For example, boron and  phosphorous are good dopants for  silicon.  So if you replaced a silicon  atom with a phosphorous atom,  that phosphorous atom would then  become positively charged giving  a free electron to the crystal.  And this free electron would be  free to move.  And therefore, doping, because  it adds free electrons,  increases the conductivity of  the semiconductor, making it  less like an insulator.  Alternatively, you could add  boron to the crystal.  The boron would become  negatively charged, giving up a  positive charge, and the  positive charge also will be  mobile, also increasing  conductivity. 

 So a common device in both  transistors and in all of the  electronics and is very  important to solar cells is  something called P-N junction.  Now a P-type of material refers  to silicon that's doped to have  excess holes, and an N-type  material refers to silicon  that's doped to have excess  electrons.  Now if you bring those two  together, what happens is that,  wait for my animation to start,  by virtue of diffusion, well,  first of all, think you have a  bunch of holes here, they're  going to want to migrate  into the N-type material.  By diffusion, the holes  partially move into the N region  and the electrons partially move  into the P region.  But they don't evenly distribute  throughout the entire crystal  because the dopant atoms which  have taken away the boron and  phosphorous denotations here are  ionized and they're holding the  electrons and holes kind of  close to the original side of  the crystal they're on.  But the migration of the charges  has an important effect.  It creates a built-in electric  field in the P-N junction.  So you can imagine if you  separate positive and negative  charge, they're attracted to  each other by electric field.  If you put a new charge in  between them, let's say you add  an electron between and electron  hole, the new electron would go  away from the electron and  towards the other hole.  So it would want to move. 

 So there's a built-in electric  field in the crystal.  So just thinking about the  built-in electric field is very  important for understanding how  a solar cell works.  So now when light comes in to a  P-N junction with an electric  field, electrons and holes are  created but the built-in field  provides the mechanism for  separating the electrons and  holes.  Which I'm depicting here.  And as more and more photons of  light come into the crystal,  more and more positive and  negative charges are being  created and separated.  And this is the positive  terminal of the solar cell and  this is the negative terminal of  the solar cell.  And you can connect this to an  external circuit and derive  electricity from it.  So our black box is really just  a P-N junction.  So the active material of a  photovoltaic solar cell is a  semiconductor.  In a semiconductor, light  creates positive and negative  charge. 

 A junction between a P-doped and  an N-doped semiconductor, which  is called the P-N junction, is  used to induce the spontaneous  separation of charges that are  generated by light, thus  converting light to electricity.  So there are a few things in  this analysis I glossed over.  If you were to take college  level classes in physics or  electronics, you'd get a hundred  percent correct analysis of what  I just told you.  But this is the spirit of what  happens.  All right, in the next part of  the talk I'm going to talk about  crystalline silicon  photovoltaic, which for the last  30 years have been just about,  has been the dominant materials  used in the field.  And I'll tell you how they  compare in terms of their cost  to coal-based electricity.  So silicon is commonly used in  semiconductor photovoltaics  because it is the second most  common element in the earth's  crust.  We have lots of expertise in  processing silicon to make  microelectronic circuits.  It has the right mix of  properties for photovoltaics.  And it's possible to make solar  cells with 20% efficiency  already, which is a very good  efficiency. 

 So earlier I told you if we  could fill a 140 by 140-mile  square in the southwest  with 20% solar cells, we could  power our entire country.  Well, why don't we just do that?  Well, it's obviously because of  cost.  Single crystal silicon is just  too expensive.  And the reason it's so expensive  is it's very expensive,  to process, to make it.  And I have a YouTube video to  show you how it's made.  This is not my personal video,  it's just one I found when I was  preparing for a class last year.  Let's see if I can get it to  show up.  So it's about two minutes long.  And it shows the process for how  silicon is made.  Let me get the sound going here. 

 I apologize for that.  >> Needs to be absolutely  flawless.  It took decades to perfect the  process of producing silicon  with a perfect mono-crystalline  structure.  They begin with raw polysilicon  or poly and heat it to over 2500  degrees Fahrenheit inside a  special sealed furnace.  >> That's the raw material.  >> Which have been purged with  argon gas to eliminate any air.  >> Melted at thousands of  degrees.  >> The resulting lake of molten  silicon is then spun in a  crucible, and a silicon seed  crystal, roughly the size and  shape of a pencil, is lowered  into it while spinning in the  opposite direction.  As the molten polysilicon is  allowed to cool, the seed  crystal is slowly withdrawn at  around one and a half  millimeters a minute.  The result is a single silicon  crystal, weighing around 440  pounds and with a diameter of  around 200 millimeters. 

 The crystal is so strong its  entire weight can be supported  by a single thread just three  millimeters across.  But it is brittle and it must  now be cut down to size without  shattering.  After testing with chemicals and  x-rays to check its purity and  molecular orientation, it's fed  to a silicon salami slicer.  This 10-ton wire saw uses a fast  moving web of ultra thin wire to  produce wafers of silicon that  are just two-thirds  of a millimeter thick  and 99.999% pure.  Once cut, there are microscopic  marks left on the wafer's  surface, so it's time for a  buffing using a process called  lapping.  After a twirl in this high  powered polisher, they're still  not smooth enough.  So they're given yet another  buff using a chemical process.  The result is wafers of silicon  with a surface roughness of less  than one millionth of a  millimeter.  Buffed to a sheen, they're  finally ready for etching with  the circuit design.  >> Arnold: Okay, so that entire  process is the process that's  used to make single crystal  silicon a substrate  for the 200-millimeter disks,  after they slice up that  silicon into disks they're about  a millimeter thick, that's the  process they use to make  transistors and it's the process  they use to make silicon for  solar cells. 

 And it's very expensive.  The reason it's so expensive is  because they have to use very  high temperature processing to  melt the silicon, they said  thousands of degrees Fahrenheit,  and that's very costly.  Let me switch back for my  presentation.  So the reason that we don't have  solar cells on our roofs in  great abundance and the reason  we don't have 140 by 140 mile  silicon solar cell farm in the  desert is they're just too  expensive.  And that shows in our energy  diagram map.  So here's the slice.  About 6% of our energy comes  from renewable energy, and I've  added on a little slice that  comes from photovoltaics.  It's only 0.6% of our energy,  or 0.006% of our energy supply  comes from photovoltaics.  And it's just about economics.  >> How much is it?  >> How much energy does that  account for?  >> How much does it cost?  >> Arnold: That's my next slide,  so that's a good question. 

 I could read your mind.  So just how expensive is solar  electricity.  Well, there's a good Web site  called solarbuzz.com  that keeps track  of month by month prices.  Let me load it up.  It's over here.  So in this column here is the  cost of module, this is cost in  cents per kilowatt hour.  Just for your reference it's  about 10 cents per kilowatt hour  for coal-based electricity, and  I haven't looked at my bill  recently.  But it's on the order of  magnitude of 8 to 10 cents per  kilowatt hour for electricity  right now.  So in the year 2000 the cost per  kilowatt hour for a silicon  module ranged between 20 and 40  cents per kilowatt hour.  And that has improved a little  bit over the last decade. 

 If you scroll down to this  January, you see the price has  been between 20 and 35 cents  per kilowatt hour.  This is just for the module  price.  You have to multiply by about a  factor of two once you include  installation of the modules.  So what this means is that--  I have these mixed up here.  So this is the price for solar  energy.  It's around 40 to 80 cents  per kilowatt hour.  Whereas our coal electricity  is seven to eight cents per  kilowatt hour.  So this difference in price, and  people love money and they don't  have enough of it, is why no one  uses these solar panels and  everyone uses coal.  [INAUDIBLE]  >> You don't have to burn up  the chips--  >> Arnold: So why is it measured  per kilowatt hour?  So this taking into account the  lifetime of these cells is  around 30 years.  When you buy a solar cell panel  for your roof or panel for out  in the desert, you pay a very  large up front cost, and then  there's minimal upkeep and it  supplies electricity for you  for 30 years. 

 So you divide by that 30 years  and then you reach this factor  which is still too expensive.  So if you can make your solar  cells last 150 years and make  them the same way you're making  them now, then instantly they're  cost-effective compared with  coal if you have up front money  on hand to pay for 150 years of  electricity in advance,  which is another problem.  So the lifetime of these cells  is very important in reducing  their cost.  You want them to be very  long lasting.  And I'll get to that when I talk  about the material science side  of this.  To me, as an engineer and a  scientist, I see this and I'm  very excited.  A factor of five or 10, that's  nothing.  You can easily engineer  around a factor of five or 10.  And I'm very optimistic that  this could be done sometime in  the next couple decades or in my  lifetime.  Some people are more optimistic  and think this could be done  even sooner. 

 So crystalline silicon is too  expensive, so what are the  alternatives?  Obviously, new materials are  needed and so in comes material  science research.  Very important area of research.  Material science is usually in  the engineering department, it's  science and engineering.  And material scientists study  new materials, they study how  the structure of materials  relate to their properties,  they study how the structure of  materials relate to their  applications, new ways to  process materials.  So material scientists are right  at the center of inventing new  materials for solar  applications.  So there's kind of two groups of  alternatives.  There's a slew of ideas that  have already come out of the  labs and are the development  phase that you could potentially  buy on the market in the next  couple of years, if not right  now.  Well, first of all, there's more  clever ways to utilize single  crystal silicon to make larger  panels per cost.  Additionally, people are  exploring polycrystalline, and  I'll explain the difference  between polycrystalline and  single crystal silicon in a  second.  There's amorphous silicon  solar cells, the solar cell is  then made from other elements  such as elements between column  three and five compounds. 

 There's solar cells made by 2-6  compounds such as cadmium and  tellurium or CIGS which is  cadmium, indium, gallium,  selenium.  These, in particular, cadmium  telluride and CIGS solar cells,  are up and coming materials that  will drive down the cost of  photovoltaics within the next  few years.  And then there's ideas that are  being explored more in the  university such as the ideas  being explored in my research  group that are looking even  further down the line for  materials that have ultra-low  costs and potential for very  high efficiency.  And those include organic  semiconductors and polymers.  So kind of plastic-like  materials to make electricity.  Nanomaterials and novel new  inorganic materials.  So all of these have the  potential to reduce the price of  photovoltaic modules equal to or  less than the price of coal.  So silicon sliver cells, these  were invented by a team in  Australia, I believe.  The idea is to take a silicon  wafer that's about a millimeter  thick and slicing it up into  slivers and then turning these  slivers sideways, and you can  take something that has a modest  area, turn all these slivers  sideways and you have a very  large area of silicon that you  could then turn into larger  panels than you would normally  be able to without these  slivers.  So this is the maximizing the  area of solar cell you get  per kilogram of silicon  that goes in. 

 It's a great idea but this is  kind of like a manual process.  It's too expensive to address  how do you take all these  slivers and assemble them into a  large panel.  Another material being explored  is polycrystalline silicon.  Now polycrystalline silicon is  composed of different grains.  Each grain is a silicon crystal,  but the different grains all  have different orientations.  They can be rotated with respect  to each other.  So it's a whole bunch of small  silicon crystals.  This is a photograph of a  polycrystalline silicon solar  cell.  The different reflectivities and  colors, the different shades of  blue, are different grains.  And the advantage of  polycrystalline silicon is that  you can process it at much lower  temperatures meaning lower cost  but the efficiency is lower. 

 So at the boundaries between the  grain, a lot of the electrons  and holes that light creates are  lost, and, additionally, this  inhibits the transportation of  holes across the area.  Question?  >> The photo of the  polycrystalline cell, what sort  of scale are we looking at  there?  Each one of those rectangles?  >> Arnold: That's a good  question, I'm just going  to be guessing here, but  I believe it's probably on  the couple of inch scale here.  >> Okay.  >> Arnold: So the grains  can be quite large.  Even lower in cost is amorphous  silicon which is completely  distorted.  There's no crystalline ordering  or periodic ordering whatsoever.  In amorphous silicon, most of  the carbon atoms are still  bonded to four other carbon  atoms, but there's just  no overall long-range order.  You can process this at very low  temperatures.  And amorphous silicon is what's  used in your pocket calculators  because of its low cost.  But the efficiency is much lower  and the lifetime is lower. 

 So they degrade over time.  >> What affects lifetime?  [INAUDIBLE]  >> Arnold: So in an amorphous  solid, like I've drawn here,  there are dangling bonds.  If you look at where the laser  pointer is, because of this  ordered nature of the crystal  there's these atoms that aren't  bonded to their nearest  neighbors.  And those lower efficiency  unless you passivate them with  hydrogen.  So you can react all these  dangling silicon atoms with  hydrogen and then make  a half-decent solar cell.  But the light over time slowly  dissociates those bonds  and hydrogen is released,  and that's called  the Wronski-Stabler effect.  It was researched in the '80s  and finally understood then  but no real technological way  around it has been invented.  So this is, of course every  material will have a different  degradation mechanism, but this  is the most important one for  amorphous silicon.  So moving beyond silicon,  there's 3-5 compounds, cadmium  telluride, CIGS.  So 3-5 are a combination of an  element from column three and  column five. 

 These are being researched very  heavily in both the US and  Japan.  Recently there was this paper  published in literature about  this very sophisticated compound  made from stacks of P- and  N-type doped 3-5 semiconductors  that each, so there's a whole  bunch of different P-N junctions  in this stack here that achieved  efficiency over 36%.  So very high efficiency.  So this is considerably higher  than what can be achieved with  silicon.  And that's great except these  3-5 compounds are even more  expensive to process than  silicon.  And these are great for  applications where you need the  most power possible and cost is  not important, for example on a  satellite.  You want, you don't really care  how much your satellite costs  but you want to get as much  power from the sun as you can  when it's up in orbit.  So these are less practical for  terrestrial applications.  There have been some proposals  to try to integrate these in  other strategies using  collectors or concentrators.  So a concentrator is where you  use a big inexpensive lens  that's, let's say, a meter large  and capture a meter of sunlight  and focus it because it's a lens  down to a very tiny area which  is where your solar cell is. 

 In this way you could capture a  meter of light and only have to  pay for a very tiny solar cell.  So it's a very intriguing  strategy for reducing the  overall cost of the solar cell  modules, but the problem with  this is that the sun moves  throughout the day and then  where the lens focuses your  light will move as well, so you  actually have to move the entire  assembly and rotate it and track  the sun throughout the course of  the day and that ends up making  it not very cost-effective.  But a lot of research is being  done still on concentrators  nonetheless.  So two of the most promising  technologies that if you maybe  read the business page of the  newspaper, you might read about  startup companies pursuing these  materials are cadmium telluride  and CIGS.  Both of these could be processed  at modest temperatures and can  achieve efficiencies between 10%  and 20% and have excellent  lifetimes.  And the processing costs are  sufficiently low that they have  made a dent recently in the cost  of solar cell modules, and  they're expected to reduce the  price significantly in solar  cell modules over the next  decade.  So the CIGS, which is copper,  indium, gallium, selenium, is a  P-type material.  And it's usually paired with a  cadmium sulfide N-type material.  Similarly, the cadmium telluride  is paired with cadmium sulfide  as well.  So this is a cross-sectional  micrograph of what the solar  cell looks like. 

 CIGS and cadmium telluride are  very strong light absorbers so  you only need about a micron  thick of material to absorb  light.  So you don't need to use very  much material which is another  reason why they're  cost-effective.  Additionally, both of these are  polycrystalline but the grain  boundaries, which are very  detrimental in polycrystalline  silicon cells, are not as  detrimental in these materials.  So you can make polycrystalline  cells and not have to worry  about the effects of the grain  boundaries as much.  So as I mentioned they have  reasonable efficiencies and cost  and should have a large affect  in the PV market.  So there's a company, First  Solar, based in the United  States, who has been making  cadmium telluride cells over the  last five years.  And if you look them up in the  stock market, you can see their  stock just went crazy a couple  of years ago.  Thousands of percent growth.  They do a lot of business in  Japan and Germany where there  are subsidies for photovoltaics.  CIGS, there's lots of startups  to try to go into the CIGS. 

 One of the most prominent is a  company, Nanosolar, which claims  they can almost achieve grid  parody with CIGS cells, meaning  that their cost is comparable to  that of coal-based electricity,  but they haven't seriously begun  sales yet.  One potential long-term  disadvantage of CIGS and  cad telluride is that, well,  they use toxic materials.  Cadmium is toxic; selenium is  toxic.  First Solar, who makes the  cadmium telluride cells, has  an interesting program in  which they hope to recycle all  cells after 20 years.  And they supposedly set money  aside upon the sale of every  panel they sell that will be  later used to reclaim the cell  even if they go out of business  to try to address these  environmental concerns but it's  hard to tell.  The other issue is supply.  How much of these elements do we  have in the earth's crust?  So there's this report by the  DOE that analyzes how much  material, how much indium would  we need, how much tellurium  would we need for example, to  manufacture 20 gigawatts of  solar cells per year by 2050.  And if you analyze these numbers  here, you'll see that, well,  indium and tellurium, they're  not as abundant as silicon.  You might not be able to scale  these beyond 20 gigawatts per  year.  And if you remember, we use  three terawatts per year in the   US so that's 3,000 gigawatts  per year. 

 So you might not be able to take  these things and extend them to  the nationwide scale.  But right now they're some of  the most promising materials.  So future technologies, I'll  finish up talking a little bit  about some of the research that  my lab is doing.  Well, we're really interested in  three material systems.  The first one is using organic  molecules which are mostly  carbon-based molecules that have  semiconducting behavior.  These can be small molecules  like this one shown here, which  is a -- molecule, or they can be  polymers.  The advantage of these is you  can synthesize them in a  laboratory, like a chemistry  bench top for example.  You can evaporate them at very  low temperatures or you can put  them in solvents and spray them  on with like a spray paint can  for example, or paint them on  with a paint brush.  So they potentially could have  very low processing costs.  I'm interested in nanomaterials. 

 I've done a lot of work with  carbon nanotubes.  These are very exciting  materials for photovoltaics.  At least I think so.  And new inorganic semiconductors  like copper sulfide, I'll tell  you about that.  So organic photovoltaics are,  well organic materials are  molecular materials.  They're conjugated with these  aromatic groups in them that  make them conducting unlike your  plastic water bottle which is an  insulator which doesn't have  these aromatic groups.  And they're being explored for  more than just photovoltaics.  There's a large effort in making  white lighting sources with  these for very high efficient  light bulbs.  As very high quality displays, a  lot of new cell phones and like  personal organizers and eBooks  that are coming out all have  organic light emitting displays.  And they're interesting for  electronics and for  photovoltaics. 

 So in an organic solar cell, you  also have a material that takes  the negative charge and the  positive charge except for it's  two different molecules.  So this is the most common  combination of molecules.  This is a fullerene or C60  carbon derivative and a --  derivative.  You blend them together.  Both absorb light.  And the positive charge ends up  on the polymer, and the negative  charge ends up on the fullerene  derivative.  And they separate.  And cost is very low, efficiency  is greater than 7% as  demonstrated so far.  The major research challenge?  Is, again, stability.  So the carbon-carbon bonds in  this case are what the degrade  under light. 

 Carbon nanotubes are  nanomaterials that are 100%  carbon.  They have diameters around one  nanometer, the length could be  as long as a millimeter.  And they have exceptional  properties in all sorts of  fields, not just solar cells.  Mechanically, a carbon nanotube  is very strong.  It's 20 times stronger than  steel.  Its conductivity can be better  than both copper, and charges  can move through it faster than  in silicon.  For photovoltaics they're very  interesting because they have  just about the right band gap  and they're very strong  absorption and, unlike the  polymers and the molecules I was  just showing you about, they're  stable.  Different diameters absorb  different colors of light so you  can capture all the different  colors coming from the sun.  And we're at a very, very basic  fundamental stage in research  here so when a nanotube absorbs  light, positive and negative  charge is created and we're at  the very beginning stages of  understanding how to even  separate that charge.  They have very exciting  properties, but we're at the  very fundamental starting point  of this research. 

 Finally, something my group is  interested in is copper sulfide.  Copper sulfide has very similar  properties to CIGS and cadmium  telluride except for these  elements are very abundant and  it's nontoxic.  The problem is that this is  P-type and there's no N-type  material that's been found yet  to match up with it.  So we're investigating for  different N-type materials you  could put with it so you can  make a P-N junction out of it.  So this is the kind of research  we're doing right now in my lab  and in labs across the world to  try to invent new materials for  photovoltaics.  So just to conclude, all these  new material systems have led to  lower cost and increase in the  PV market.  This is 30 years here.  The cost has come down by an  order of magnitude.  So maybe in another 30 years the  cost will be down another order  of magnitude.  This is the market share of  crystalline silicon cells on top  from 1998 through 2007. 

 And thin-film cells on the  bottom.  You can see the thin-film cells,  which are cheaper to make, are  gaining market share and soon  they'll displace crystalline  silicon cells.  And you'll see this impact the  price of solar cell modules over  the next decade.  So over time, all of these  technologies should become  cheaper and cheaper to make.  These are all the different  solar technologies coming down  in price.  Inflation and energy demands  should increase the cost of  coal-based electricity.  At some point there'll be  convergence where it will be  just as expensive to get your  electricity from coal as from  the sun.  At that point, I think you will  get your electricity from the  sun and I think that will happen  sometime in the next, well,  hopefully in my lifetime.  So to conclude, the sun is an  abundant source of free energy.  A solar cell, a photovoltaic  solar cell is a semiconductor  between a doped P and a doped  N-type region that are kind of  stuck together that separate  negative and positive charges. 

 Solar cells are only an order of  magnitude away from grid parody.  We're actively trying to make  better ones, and I believe that  it can be done sometime in the  next, like I said, decade or so.  So thank you, guys.  (applause)