Nanotechnology - The New Science of Small || 18 - Using Nanotechnology to Capture Sunlight
[Music] thank you we've spoken in the course so far about information and Communications Technologies and the role that nanotechnology has to play in advancing those we spoken of biology biomolecules and how nanobiosciences in nanobiotechnology have a real opportunity to play there as well if you think about some of the big challenges facing the world today another one that comes immediately to mind is the field of energy how do we find ways to capture energy store energy how do we find ways to capture energy in a way that's renewable that's sustainable and that's clean and so for the first lecture in this series on energy we're going to talk about solar energy in particular how do we harness it how do we turn it efficiently into electricity and what role does nanotechnology have to play in that first let's spend a minute on the sun and its properties we need to understand the Sun and its Spectrum in order to think about solar energy properly first on the Spectrum the sun is very very hot as as you're well aware and that is that accounts for the fact that the peak of its intensity the peak of its emission lies within the visible spectrum we were speaking earlier about thermal imaging and how you can see a warm person against a colder background well their wavelengths of emission are much much much much longer than the sun's but it's all based on the same physics called black body emission in the sense case because it's so hot about five or six thousand degrees Centigrade the Sun Peaks its luminescence its intensity a right in the visible wavelength spectrum and as we also discussed earlier that's why our eyes have adapted to see in what we call the visible wavelengths an important thing to know about the sun though is that in addition it has a very significant spectral signature in the infrared and not the very long wavelength low energy infrared there's a major portion of sun energy right up against the visible in what we call the near-infrared wavelengths in fact fully half of the sun's energy lies in the infrared and so a lot of the opportunity for making more efficient solar cells is going to be to make full use of the sun's broad spectrum another important thing to think about with the sun is we think about making a good solar cell is the breadth of that Spectrum it's kind of the opposite of a laser in fact if if the sun were a laser which it's not it's not going to be but if the sun were a laser we'd have an easier time making solar cells because we could tailor our converters of light energy into electrical energy we could tailor them to one specific wavelength we could customize them but inevitably this is the source of energy that we have and we have to make good use of all of its spectral components in order to make an efficient solar cell so this will be one of our challenges and I'll call it the Broadband challenge instead of having a narrow Spectrum we have a broad range of colors a broad set of bandwidths that we need to deal with let's start by asking kind of the economic question though uh the Practical question on planet Earth far enough from the Sun is there actually enough energy to meet a significant portion of our energy needs and if you were look to look today at the way in which we meet our energy needs and you look at fossil fuels and look at sources such as coal look at nuclear look at hydroelectric and then you were to look at the proportion of energy that's met using solar energy you'd find that a very very small percent much less than a percent of our total energy needs are met today using Renewables and you might then ask the question was is there some fundamental reason for that is there a lack of sufficient gift of solar energy reaching the planet Earth and the answer to that is is very much not a couple of Statistics there uh one is if you were to look at the total amount of energy rich in the Earth every hour from the Sun there's enough energy there to meet all of the world's energy needs for a year another way to look at this is if we were to cover the Earth with only a small fraction of the area of Nevada with good solar cells not perfect which is good solar cells we could meet all the world's energy needs so scaling by time or scaling by area the same message is given that the solar resource is abundant and the reason why today we're not making Fuller use of it relates to the fact that we haven't yet found ways to make solar cells that are simultaneously high in their efficiency and also low in their cost and that's what we'll talk about much during this pair of lectures is how nanotechnology is enabling us to seek to break compromises between cost and efficiency in solar cells one more thing just to think about about the sun before we move into the technology of solar cells is how it's distributed obviously the equator there's both a little bit more intensity of sunlight and then it's also a better distributed throughout the year and in some locations at the equator we're also doing better in terms of cloud coverage but the point I want to make with this map of insulation which is the amount of energy per day or per year reaching different regions of the earth is that even if you live where I do in Canada uh there's actually an abundance of solar energy and if you were to compare us with the amount of solar energy reaching say uh right at the peak in Saudi Arabia uh or right near the other other locations right near the equator uh it turns out that we have maybe 60 percent as much solar energy as they do at the peak and given that there's this huge overabundance of available solar power that's that's a lot even those of us who live you know well into the northern hemisphere have an abundance of solar energy available to us and the challenge is to find the ways to harvest it efficiently and cost effectively and so on that point about harvesting it cost effectively this is where there's been some real advances through nanotechnology in recent years take a look at this picture of a physically flexible solar cell you can see that it's bent it's actually on a plastic substrate and one of the great things about this new emerging area of solar cells that aren't rigid they're not made on panes of glass but instead they're printed onto flexible backings uh is is that we're able to make them a bit the way you'd print a newspaper we're able to use roll to roll processing in order to make these flexible solar cells in a manner that's very cost effective instead of being a sequential serial process like the semiconductor industry you know we've talked about these semiconductor Wafers that go through lithographic fabrication stepwise well instead it's a Serial process a continuous process and these flexible solar cells are offering promise of significant cost reductions as a result of their ease of fabrication the other great thing about being physically flexible is that their ease of deployment is enhanced as well a significant part of the cost of getting a solar cell system installed isn't actually in the manufacturing the solar cell or the module that you put onto your roof it's inputting it onto your roof these heavy materials sometimes there's a need to reinforce the roof as a consequence of trying to install these heavy materials on top of them so lightweight flexible solar cells that can be readily integrated into a building in cases even integrated into the building materials themselves offer great promise for reducing this other half of the cost of the solar system which is often referred to as the balance of systems cost look at another another slice at the kind of Moore's Law concept that we spent lots of time in on technological road maps uh silicon-based solar cells actually have their own Morris law as well a rate of improvement in the efficiency to cost ratio in the solar cells the challenge though is that if one projects this law this this empirical observation for the rate of improvement of power versus cost in these solar cells one would predict that it would be many many decades until we will have based on conventional existing Solar Technologies until we actually have these turn into compellingly cost-effective Technologies so this is where many in the field call for a breakthrough they call for a need for something that will take us off the just extrapolative Improvement in solar cell efficiency and that will take us into a new regime and so here people talk about generations of solar cell a first generation such as the Silicon solar cells that we will talk about together during this lecture uh that that have offered improvements in efficiency over time and reductions in cost but not a disruptive change not a step change a second generation of solar cells that have been these flexible solar cells that have led to the emergence of the much lower cost approach but where the efficiencies have not represented improvement over what silicon can give us and then the vision and this is much of the vision for the present work on going today is for a third generation of solar cells which doesn't have a compromise anymore but which instead achieves lower cost and higher efficiency and it's quite easy to see why this point about efficiency is so important you know you might until you think about these so-called balance of systems considerations you might say surely if I can just make my solar cells cheap enough it's not particularly important that I harvest every Photon from the Sun efficiently the photons from the Sun are free but when you then think about the fact that the cost of installing and maintaining solar cells if you like the cost of ownership and the cost of installation these scale with the area of space that you're filling them into then ultimately you need very much to think about efficiency because if you don't achieve reasonably High efficiencies from solar cells you may be able to overcome the cost of the cells themselves but you're not able to overcome these fixed costs associated with the installation and maintenance of the solar cells and this is the reason why this third generation of solar cells is envisioned to be an area of very big opportunity for the energy sector and so to be specific about the challenge that we're posing for the solar energy field it's the following it's that using existing Technologies which we'll dive into now we've achieved certain efficiency improvements but there are fundamental upper balance physical boundaries that will describe on how efficient you can never get a solar cell based on a single semiconductor Junction to be and we're starting to get reasonably close to those bounds and so with the incumbent Technologies there are limits to our capacity for further Improvement the opportunity for the third generation solar cells is to take us Beyond those limits is to make such better use of the sun's broad spectrum that we overcome the traditional limitations on solar cells based on a single semiconductor Junction so let's now dive in and let's talk about the basics of solar cells how they work how they relate to some of the materials that we've spoken about in the course together and then ultimately how we're going to make them better using nanotechnology so let's now talk about the semiconductors themselves we've referred to semiconductors throughout the course uh We've Spoken of their different spectral absorption characteristics we've spoken about how there are Photon energies that are too small to be absorbed within a semiconductor and others that are sufficiently energetic that they do get absorbed let's put a little bit more formalism or or vocabulary around these ideas of absorption inside a semiconductor and I'd actually like to start back at where we began I'd like to start with atoms the atoms that make up the semiconductor when the elements let's take a silicon element silicon atom for example exist in isolation they're not part of a semiconductor but when an atom exists in isolation it has these shells that we talked about these shells that correspond to the energetic levels that electrons fill up that account for silicon's particular membership in a particular column of the periodic table these are what we call discrete energy levels they're very well defined they're distinct only certain transitions energetic transitions are available to say to a silicon atom and in fact those discrete transitions are much of the basis for how we're able to detect certain chemical elements it's based on these very specific transitions Optical energetic transitions that are available inside atoms but in a semiconductor material what matters is not just the behavior of the constituent atoms what matters what happens when they come together and when these atoms come together electrons which as we know are delocalized they have some extent they start to merge and they start to participate in existing over many atoms now that inside a semiconductor is very advantageous it accounts for the good electronic transport in semiconductors the fact that as we discussed earlier in the electronics context these electrons can set up waves that have extent that can propagate along a semiconductor this is the basis for very good electronic transport inside semiconductors what it also leads to is the establishment of what are called Energy bands so instead of having these discrete defined steps now there are ranges entire ranges of energy where electrons are allowed to exist they're allowed to propagate freely and instead of having these original big chasms of energy in which no energetic States existed and now we have in between these bands what's called a band Gap and this too is a Chasm but it's not a Chasm between just a pair of discrete States it's a Chasm between a band and another band and so now there is a range of energies less than this energy gap or this band gap for which photons are not able to excite an electron from a lower band to an upper band and in this case there's no Optical absorption the material is transparent in fact even this concept exists in things we don't think of traditionally as semiconductors so diamond for example also has a band Gap it just happens to have a very large band gap which is why it's transparent to our eyes whereas if you were to look at a piece of silicon to our eyes it looks opaque it's because it's absorbing all of the light across the visible wavelength Spectrum its band Gap lies out in the infrared so you can see through silicon when you look in the infrared but you can't if you look inside the visible but how does this idea of band Gap then relate to how we build a solar cell well we need to match the band gap of our semiconductor to the spectral properties of the sun reaching the Earth to the sun Spectrum and so certain semiconductors such as silicon have a band Gap that's actually very well matched to the emission properties of the Sun or in particular the sun spectrum that reaches the Earth in the case of of silicon it's actually just about the perfect choice it was not something that that we made a conscious choice to make silicon's band Gap this way this is a property of silicon atoms themselves but it's a reason why we use silicon as a very appropriate semiconductor for making a device for making a junction that's able to absorb the sun's light and turn it into electrical power in fact on that it's worth pausing for just a moment and thinking about the relationship between the sensors the light sensors that we spoke about earlier and solar cells but they have quite a bit in common each is responsible for absorbing light each typically in the case of silicon covers the visible and a little bit into the near-infrared wavelengths but in the case of the sun there's a preordained spectrum that we have to work with and there's a preordained intensity uh in fact researchers working in the field don't even speak usually when they speak of the sun they don't even speak in terms of the absolute formal units they talk about one Sun one sun is the unit of intensity to describe how bright the sun is it's such a universal phenomenon that everybody having to dealing to deal with solar energy is conversant in in the case of light sensors typically the intensities that are being dealt with are much much lower and so they're they're both doing this Optical to electronic conversion but typically in different ranges of light intensity many factors of 10 different light intensity now the other thing is that in the case of sensors we are converting information about the physical world into an electronic representation and that means that if we need to apply a little bit of a voltage bias we need to put a little bit of potential across our photo detector that's perfectly fine because we don't mind expending a little bit of power to run our sensor integrated circuit but the name of the game in solar energy isn't harvesting energy uh and so when our goal is to harvest energy we're looking for this device to take the sun's power its photons and to deliver them to us in the form of an electrical current which can be driven across a load so there's some voltage as well and so the mode of operation of photovoltaic devices and solar cells is different as a result of that need to transfer power from the Sun into some either a system where we want to charge our car or to some means of storage such as the case of a battery or a fuel cell topics that I'll talk about a little bit later when we get to the field of storage let's dive in now and look at the heart of a solar cell a little bit this is called a PN Junction solar cell and uh some of the details of the nomenclature aren't important but what's what's key is to understand that when we build really any solar cell we bring together two materials typically having the same semiconductor properties the same semiconductor band Gap but that are dissimilar in their affinity for electrons so one of these materials will be rich in electrons we will achieve that richness of electrons by introducing selectively a certain number of impurities that add electrons to the lattice that donate electrons to participate in conduction and on the other side of this Junction we will utilize a material that is depleted of electrons and so now we have a structure which is electron rich on one side electron poorer of the other and we've created the potential for a Cascade when we excite a charge on one side of this Junction we now have a propensity for the electrons to want to go from one side to the other as a result of this built-in Cascade this built-in potential as it's called and that is symmetry as one of the crucial conceptual building blocks of of any solar cell the reason is that we're not going to be applying an external electrical bias we don't we don't get to have an applied potential on this device because we're trying to harvest energy instead we've built a device that likes to absorb light and then funnel the energy that's extracted from that light in a specific direction towards a circuit now the other point that we need to think about when we think about solar cells in their use of the spectrum and the limits on their efficiency is that as we've discussed because of this concept of a semiconductor band Gap there will be certain Photon energies below which there's no absorption so these colors of light simply pass through our solar cell and there will be others that just barely cross the band Gap that just barely cross this Chasm these will lead to absorption and in fact this is a very efficient region of the spectrum for these solar cells because uh all of the energy in the photon that impinged upon the semiconductor is utilized ideally is extracted from this however a sun spectrum is Broad and there are photons at much shorter wavelengths corresponding to much higher energies where there's an overabundance of energy in each photon now if we could take that entire Photon energy and harvest it we'd be in great shape but the way semiconductors work is that if you excite an electron well into a band then extremely rapidly it trickles down right to the band Edge we say that these states that arise from Silicon atoms coming together and forming these bands we say that these states within a given band are very well coupled to each other and so very rapidly at room temperature we get a loss of energy and it's just converted into thermal energy not something that we Harvest inside a photovoltaic device now this idea of loss within a band loss of energy from a given photon is very important in understanding the limits on solar cell efficiency because you might say from this uh this lost consideration well okay let's go find a semiconductor having a larger band Gap they're available and then we won't throw away as much energy we'll be able to deliver More Voltage to our device ultimately we'll be able to extract more power however if we were to do that we would also absorb fewer photons because we would have made a larger band Gap and there would be more photons in the sun spectrum that we're not able to breach that band Gap so this leads essentially to a trade-off it leads to our choosing a semiconductor band Gap that is small enough that we Harvest many of the Suns a good fraction of the sun's photons but large enough that we don't do too much throwing away of energy from the juiciest from the most energetic of these photons that come in but it is a trade-off and as a result when one works with a semiconductor device a photovoltaic device that is based on a single band Gap a single type of semiconductor one is limited not to 100 or 97 or 95 percent overall power conversion efficiency but instead the upper bound on the efficiency with which one can extract energy from one Sun intensity from the amount of intensity reaching the the earth's surface is closer to 30 percent it turns out that for the right at the right price that's not about efficiency either but it's important to think about the the limitations of solar cells based on a single choice of semiconductor and how they arise from making this choice to work with the material that has a fixed band Gap and that leads to some loss of of energy through this thermalization process within the Bands the concept that I'd like to spend a moment on with respect to solar cells is how do we think about this efficiency in terms of a current delivered and in terms of a voltage well it turns out that the power the electrical power that can be delivered say to your car for charging or to a fuel cell or to a battery the power of this is given by the product multiplied together the product of the current and the voltage and so it's very important in solar cells to get as much current as possible and to get as much voltage as possible and it's really managing this trade between those two through the choice of band Gap that leads to the optimal bandgap device and leads to the best management of this trade-off so I don't like to start in and talk about some Nano Concepts that allow us to make solar cells more efficient and the first one of these I'd like to talk about relates to the management of light to the management of photons in fact these concepts are already being utilized in solar cells that can be purchased today that are that are available and uh one of these Concepts is is it's actually quite simple to think about in terms of fabrication it involves trying to make a rough surface trying to make a textured solar cell and the reason for doing this is the following within silicon and we talked about this in the context of photodetectors a silicon is a reasonably good absorber it's actually a very good absorber of blue photons of the more energetic visible photons but it gets much weaker in its absorption of light as we go to longer wavelengths even those above its band Gap the name for this phenomenon is that silicon is an indirect band Gap semiconductor now when we try to make an efficient solar cell clearly we're trying to absorb all of the light across the Sun spectral region that we are absorbing at all anything that's above the band Gap and uh it turns out to be necessary in Silicon in particular to make a very thick wafer a thick wafer of Semiconductor in order to absorb all of the light in the infrared because of this weak Optical absorption that occurs in the infrared wavelengths and that is one of the things that's driven up the cost of silicon solar cells historically is the need for a very thick quantity a very very pure very very high temperature fabricated material and what happens when we roughen the surface of a silicon solar cell is that instead of light simply passing into the cell and then if it sees a mirror bouncing back out we instead create the opportunity for multiple passes through this device so light is able to pass in and then it sees this roughened surface if you like it sees a rough mirror and instead of turning right back around and bouncing back out of our solar cell and losing its chance for absorption it's typically reflected or scattered off in a different direction and so instead of just leveraging the thickness of the device for absorption if we can couple in and then couple sideways we're able to increase the interaction length we're able to overcome this weakness in the rate of absorption that occurs within silicon by using a textured substrate it turns out that the exact shape that you want to do this with is actually very important you want to think about the Angles and even the sharpness of the angles that are used for scattering of light in fact this phenomenon where we get light into one of these semiconductor materials and then we get it trapped in there and we get it given multiple opportunities to bounce around it's called exactly that it's called light trapping and many interesting techniques have been devised in order to achieve this of course if you had to make these spiky structures you know everyone exactly perfectly the same as all the others that would be a challenging and probably a costly semiconductor patterning and etching process it would start to look like what we do in microelectronics and nanoelectronics but for solar cells we're trying to make things that are big we're trying to make things that can be spread across a roof and do so in low cost and fortunately a number of techniques have been discovered to make structured silicon surfaces without having to go in and actively pattern every little Spike Every Little Valley in order to make one of these light trapping structures and the technique actually uses a chemical etching where a silicon has these particular directions like all crystals silicon has these particular directions known as as facets these are like the facets of a diamond that you can see and when you introduce a chemical etchant they can chemically Etch in a particular direction they can etch along a particular facet and so they're naturally prone to taking a planar surface and starting to carve it into these triangles into these Hills and Valleys or spiky mountain tops and valleys and as a result there are now very low cost and convenient methods to make nanostructured silicon surfaces that are very prone to want to trap light so we've covered the basics of solar cells we've covered the huge opportunity that solar energy harvesting represents we've talked about the underlying principles of matching the behavior of a semiconductor used to capture light with the broad solar spectrum and we've just started to touch in on how nanotechnology can be our friend in enabling the efficient capture of energy from the Sun in our next lecture we're going to dive in further and really spend time on how we're able to take advantage of the nanoworld control over the coupling of photons to electrons inside semiconductors to make solar cells that break the historic compromise between cost and performance
2023-02-12 06:25
