science!
Liquid Crystals for Photovoltaics (part 1/?)
Hopefully this short spiel on liquid crystals gets expanded at some point, beyond just a short introduction to liquid crystals. I’m horrible at remembering this sort of thing, but I think that this is a topic that is important and also interesting - not because I am a Ph.D. scientist that works with this “stuff,” but because this is a fast growing field of science where a lot of the growth of commercial applications initially outstripped a great deal of the science that was being conducting to understand this innovative branch of materials. Obviously, science has caught up quite a bit since then, and the cutting edge of liquid crystal technology is branching out into areas well outside the realm of traditional materials science. Nowadays liquid crystals are extremely common in everyday practice and range in products from treated windows for buildings and homes to the microdisplay in your Nintendo DS, or your cellular telephone. Current research in liquid crystals also includes use of liquid crystals in solar applications - a research direction that has come to prominence with the exorbitant pricing of petroleum. Liquid crystals have been used in one way or another by several species of invertebrates, notably Jeweled Scarab beetles that stretch from the southwestern United States through South America. Liquid crystals have a very rich history and because of how pervasive it is in our everyday lives I honestly feel that as a significant part of materials science it is a subject that, if you will forgive the pun, matters.
I guess the best place to begin, if we are to talk about liquid crystals, is what they are. Liquid crystals (often times simply referred to as mesogens) are materials that exhibit a very unique phase of matter that falls between a crystal and a liquid. Liquid-crystalline materials, when in a liquid-crystalline phase, show orientation or some kind of imposed order in the bulk, but continue to flow as if it were a liquid. This might seem very bizarre, but it simply is a matter of having long-range positional order in less than three dimensions. If there is at least one dimension of disorder, the material will be able to flow like a liquid. In an isotropic liquid there is no way to predict the precise position of where one molecule may be based on the position of another molecule in the system, nor can molecular orientation be predicted based on the orientation of a single molecule. In a fully crystalline solid, three dimensions of long-range positional order allow the observer to pick one atom of the crystalline solid and reliably predict exactly where another atom will be based purely on the crystal lattice. Liquid-crystalline phases live in a happy continuum precisely between these two phases of matter.
Figure 1. Basic representations of solid (left), liquid (center), and liquid crystal (right) phases. In the solid, a regular arrangement lets you predict molecular or atomic placement. The chaos of an isotropic fluid does not let you predict molecular placement. In a liquid crystal, some positional order lets you predict where molecules will be in some directions, for example where layers can exist but not where within a single layer.
The easiest way to think about this rather long-winded explanation is to imagine a box of toothpicks. If each toothpick were one molecule of the liquid-crystalline material then the entire ensemble could be assembled into a random smattering of toothpicks, each pointing every which way. If we were to impose a common orientation to each toothpick, so that they all pointed in the same direction, the toothpicks can still rattle about in their box (the bulk) and slide around every which way - this is a common liquid crystal phase known as a nematic phase. If we were to segregate the toothpicks (which are all pointing in the same direction) into layers, that would be yet another type of liquid crystal phase, a smectic. Segregation into layers allows the observer to predict where the average center of a molecule should be from layer to layer based on the position of just one molecule, but there is no way of determining where a molecule’s exact location might be based on the position of one individual molecule. Each layer is still disordered and liquid-like. Naturally, this analogy only extends so far, and isn’t a universal descriptor for liquid crystals (things are much more complex than that, unfortunately), but it is one way of thinking about how you can have a fluid material that has less than three dimensions of positional order to it.
In the real world, there are various types of liquid-crystalline materials that all show liquid-crystalline phases based on the environment they are put in. Some common liquid-crystalline materials that people use every day, and never think about, are lyotropics. Lyotropic mesogens show liquid-crystalline phases based on the ratio of materials in a specific mixture. Soaps are an excellent example of this and a large number of commercial detergents exhibit beautiful lyotropic liquid crystal phases simply by changing the amount of water they are mixed with. Another very common type of liquid crystal (and the one I will be focusing on the most) is the thermotropic type of liquid crystal. These are seen in every LCD in use today, as well as in several beam-steering applications that are coming to market very soon. Thermotropic liquid crystals are materials that show temperature dependence on phase behavior. Changing the temperature the mesogen is exposed to changes the phase exhibited.
Figure 2. Representation of phases of matter as a function of temperature (increasing left to right), including placement of two examples of liquid crystal phases.
So why the big fuss then? Sure, these things are pretty neat when you make a particular kind of “sandwich” out of these things with transparent conductive oxides, electrodes, and then shoot light through them, but where these materials also appear to shine (no pun intended) is in charge transport for solar cells. Unfortunately I can’t go into the same kind of detail about how solar cells work (from a very basic standpoint - I’m not a semiconductor physicist or a chemist specializing in that sort of thing), but I can say that one of the most important factors in implementation of solar cells as a commercial product is ease of production versus the efficiency of the solar cell. The main goal with using liquid crystals for solar applications is to provide a material that can be very efficient that also is very easily processed. One basic approach is to use the ability of thermotropic liquid crystals to “create” order from a disordered isotropic fluid simply by changing temperature. This ability to self-segregate or self-assemble provides an easily manipulated material that can provide a structured chemical environment - an environment that can easily manipulate light, or more importantly, charge.
For the sake of brevity, I’ll stop here for now - I’ll get into some basics of photovoltaics in the near future, but hopefully for you science-y types, you can see where this is going.
Hopefully this short spiel on liquid crystals gets expanded at some point, beyond just a short introduction to liquid crystals. I’m horrible at remembering this sort of thing, but I think that this is a topic that is important and also interesting - not because I am a Ph.D. scientist that works with this “stuff,” but because this is a fast growing field of science where a lot of the growth of commercial applications initially outstripped a great deal of the science that was being conducting to understand this innovative branch of materials. Obviously, science has caught up quite a bit since then, and the cutting edge of liquid crystal technology is branching out into areas well outside the realm of traditional materials science. Nowadays liquid crystals are extremely common in everyday practice and range in products from treated windows for buildings and homes to the microdisplay in your Nintendo DS, or your cellular telephone. Current research in liquid crystals also includes use of liquid crystals in solar applications - a research direction that has come to prominence with the exorbitant pricing of petroleum. Liquid crystals have been used in one way or another by several species of invertebrates, notably Jeweled Scarab beetles that stretch from the southwestern United States through South America. Liquid crystals have a very rich history and because of how pervasive it is in our everyday lives I honestly feel that as a significant part of materials science it is a subject that, if you will forgive the pun, matters.
I guess the best place to begin, if we are to talk about liquid crystals, is what they are. Liquid crystals (often times simply referred to as mesogens) are materials that exhibit a very unique phase of matter that falls between a crystal and a liquid. Liquid-crystalline materials, when in a liquid-crystalline phase, show orientation or some kind of imposed order in the bulk, but continue to flow as if it were a liquid. This might seem very bizarre, but it simply is a matter of having long-range positional order in less than three dimensions. If there is at least one dimension of disorder, the material will be able to flow like a liquid. In an isotropic liquid there is no way to predict the precise position of where one molecule may be based on the position of another molecule in the system, nor can molecular orientation be predicted based on the orientation of a single molecule. In a fully crystalline solid, three dimensions of long-range positional order allow the observer to pick one atom of the crystalline solid and reliably predict exactly where another atom will be based purely on the crystal lattice. Liquid-crystalline phases live in a happy continuum precisely between these two phases of matter.
Figure 1. Basic representations of solid (left), liquid (center), and liquid crystal (right) phases. In the solid, a regular arrangement lets you predict molecular or atomic placement. The chaos of an isotropic fluid does not let you predict molecular placement. In a liquid crystal, some positional order lets you predict where molecules will be in some directions, for example where layers can exist but not where within a single layer.
The easiest way to think about this rather long-winded explanation is to imagine a box of toothpicks. If each toothpick were one molecule of the liquid-crystalline material then the entire ensemble could be assembled into a random smattering of toothpicks, each pointing every which way. If we were to impose a common orientation to each toothpick, so that they all pointed in the same direction, the toothpicks can still rattle about in their box (the bulk) and slide around every which way - this is a common liquid crystal phase known as a nematic phase. If we were to segregate the toothpicks (which are all pointing in the same direction) into layers, that would be yet another type of liquid crystal phase, a smectic. Segregation into layers allows the observer to predict where the average center of a molecule should be from layer to layer based on the position of just one molecule, but there is no way of determining where a molecule’s exact location might be based on the position of one individual molecule. Each layer is still disordered and liquid-like. Naturally, this analogy only extends so far, and isn’t a universal descriptor for liquid crystals (things are much more complex than that, unfortunately), but it is one way of thinking about how you can have a fluid material that has less than three dimensions of positional order to it.
In the real world, there are various types of liquid-crystalline materials that all show liquid-crystalline phases based on the environment they are put in. Some common liquid-crystalline materials that people use every day, and never think about, are lyotropics. Lyotropic mesogens show liquid-crystalline phases based on the ratio of materials in a specific mixture. Soaps are an excellent example of this and a large number of commercial detergents exhibit beautiful lyotropic liquid crystal phases simply by changing the amount of water they are mixed with. Another very common type of liquid crystal (and the one I will be focusing on the most) is the thermotropic type of liquid crystal. These are seen in every LCD in use today, as well as in several beam-steering applications that are coming to market very soon. Thermotropic liquid crystals are materials that show temperature dependence on phase behavior. Changing the temperature the mesogen is exposed to changes the phase exhibited.
Figure 2. Representation of phases of matter as a function of temperature (increasing left to right), including placement of two examples of liquid crystal phases.
So why the big fuss then? Sure, these things are pretty neat when you make a particular kind of “sandwich” out of these things with transparent conductive oxides, electrodes, and then shoot light through them, but where these materials also appear to shine (no pun intended) is in charge transport for solar cells. Unfortunately I can’t go into the same kind of detail about how solar cells work (from a very basic standpoint - I’m not a semiconductor physicist or a chemist specializing in that sort of thing), but I can say that one of the most important factors in implementation of solar cells as a commercial product is ease of production versus the efficiency of the solar cell. The main goal with using liquid crystals for solar applications is to provide a material that can be very efficient that also is very easily processed. One basic approach is to use the ability of thermotropic liquid crystals to “create” order from a disordered isotropic fluid simply by changing temperature. This ability to self-segregate or self-assemble provides an easily manipulated material that can provide a structured chemical environment - an environment that can easily manipulate light, or more importantly, charge.
For the sake of brevity, I’ll stop here for now - I’ll get into some basics of photovoltaics in the near future, but hopefully for you science-y types, you can see where this is going.