#4A: Solar Cells Inspired by Nature?

Hey everyone!

Today, I’ve got a pretty special, different episode planned out. Instead of exploring a relatively common topic related to sustainable energy–for example, wind turbines, solar panels, hydroelectric power–I’m actually going to be diving deep into a technology which very few usually look into, or even have heard of.

Many have heard of more popular types of solar energy, ranging from solar heating systems to photovoltaics. But what if I was to tell you there was a third type? That’s right, today, I’ll be documenting my thorough exploration of dye-sensitized solar cells. What’s that? Read on, and you’ll find out.

So, when we consider dye-sensitized solar cells, more commonly referred to as DSSCs, we need to understand where the idea came from. And, as the title of this article suggests, to do that, we need to understand how plants are able to create energy. So, let’s take a quick moment, and do just that.

From our earliest years of education, we learn about the process of photosynthesis. Sure, at first it’s just the notion that sunlight can somehow be turned into energy in plants that fascinate us, but over time, we learn about this process in much more detail and understand how beautifully simple and pragmatic these concepts are.

So, if we have a method of producing energy from sunlight that is so simple, yet effective, why don’t we try to apply that same method into our own methods for harnessing solar energy. Instead of trying to reinvent the wheel, and exploring photovoltaics, we can easily find an alternative just by getting an understanding of how plants actually do carry out that process. So, if we want to design such a model, like a DSSC, we first need to understand how plants actually are able to harness sunlight.

So, if you’re reading this, you’ve probably seen the image above–or some version of it–in the past. It’s included in almost every basic biology textbook, and it shows the chloroplast. The chloroplast is what’s responsible for making plants look green, and for harnessing sunlight and transforming it into energy.

At first, that’s all you learn. But, as time progresses, you are gradually introduced to the chemistry behind this process. More specifically, we understand that plants don’t merely produce an abstract ‘energy’ but rather energy in the form of sugar: glucose. Glucose is a molecule which serves to provide energy to the plant, allowing it to gradually grow, change, as well as resist the elements. This process is given to us by the following chemical equation.

Now, from this, what do we understand? That water and carbon dioxide are reacted with one another to produce glucose and oxygen. This chemical process is the reason why plants are so crucial for life on Earth–they carry out a reverse process to humans and transform carbon dioxide into oxygen. At this point, we understand the chemical and biological functions of photosynthesis quite well. However, we’re still not quite there yet, since ‘sunlight’ still appears to be this arbitrary concept that is necessary for the reaction, but we don’t know why.

Okay, so let’s take a deeper look into this. While most basic (and by basic, I mean high school) biology classes don’t typically go this far, we need to understand how the chloroplast is able to channel sunlight into energy. For that, we again turn to our good buddy, chemistry, and try to understand how the chemical makeup of chloroplasts can answer this.

Just a quick look will reveal the solution: chlorophyll. Chloroplasts contain a molecule in them, called chlorophyll, which allows them to retain the green color. But why is this molecule so important? To answer that, let’s take a look at the image below.

So, the issue with chlorophyll is that it’s a very large molecule, and it has a series of alternating double and single bonds. You can quite clearly see this alternation in the diagram above, and it’s known by chemists as a conjugated system. But why does this even matter?

To understand that, let’s consider a simple single bond, which can be quite easily found within a conjugated system like the one in chlorophyll. It looks something like this.

If you haven’t really explored chemistry before, this can be a little confusing. Why? Because to really understand this, you need to first comprehend how orbitals work. If you need that crash course, take a look at the drawer below. If you don’t skip right ahead.

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Crash Course: Electron Orbitals

So, I assume that we all know that atoms have three types of subatomic particles in them: protons, neutrons, and electrons. Protons are positive, neutrons are neutral, and electrons are negative.

As shown in the picture above, protons and neutrons will be located in the central nucleus of the atom, while the electrons zip around it. Alright, but why does this matter? Well, to understand photosynthesis, we need to understand the motion of electrons. [Huh? I know, I know, the link isn’t really there yet, but trust me for a second.] Electrons are arranged in shells, like in the image below.

The first shell contains two electrons, the next contains eight electrons, then eighteen. When I say ‘contains’ I mean that’s the maximum number of electrons that that shell contains. This is where we gain an understanding of electron configuration as looking something like 2, 8, 18, 18, and so on. Okay, so this is at least a bit of a deeper understanding of this process. But what if we wanted to go deeper? Welcome to orbital theory.

In the image above, there are a lot of orbitals, and they represent the position of electrons in the surrounding shells. I won’t get too much into the theory of how these orbitals are derived (it relates to some more advanced modern physics), but to understand the gist, it essentially breaks down the regular configuration into pairs.

So, instead of having 2, 8, 18, we get something that looks like this:

Okay, so before you start freaking out, this is just a crash course, so I’m only going to explain what’s strictly relevant. All you need to know about to continue is the different classes. There are four different classes in here, s, p, d, and f. As shown above, each orbital class has its own shape, based again, on modern physics.

To continue, we need to know only about the first two classes, and their shape. First is the s class, which is found in every single atom on the periodic table. 1s2 represents a filled first electron shell, for instance, and all atoms have at least one electron shell. The s orbital looks like a bubble, as shown below.

Next, the p orbital, which occurs in the second shell first, but after an s suborbital. So, the first filled p orbital would occur in an atom with a configuration of 1s2 2s2 2p6. It’s a bit confusing, so you may want to reread this again or do some extra background research before continuing.

Anyway, the p orbital looks like a 3D figure-eight, as shown in the image below. This is extremely important for understanding our DSSC functionality later on down the road.

The electrons move around within that volume. Alright, so now that we know everything we need to know about all of these fancy electron orbitals. Now, let’s try to understand why they’re important to our topic at hand.

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So, when we consider single and double bonds, we need to understand what these are at an electron level. In essence, we have two types of bond being formed. The first is called a sigma bond and occurs when there is a head-on overlap of s or p orbital, kind of like in the picture below.

Then, there’s also a thing called a pi bond, which is the side overlap of two p orbitals. So, instead of just being a regular bond, like the one seen above, it’s more complex and looks like the diagram below.

This ends up creating two clouds of electrons, one above the bond axis, and one below it. To know if a bond contains a sigma has a sigma or pi bond in it, you just need to see whether it’s a single, double, or triple bond. All bonds contain a sigma bond. However, the double bond also contains one pi bond, and the triple bond contains two pi bonds. But, again, why is any of this important?

Let’s go back to our conjugated system. If we recall, it has an alternation of single and double bonds. This is key to understanding why chlorophyll can harness sunlight. When single and double bonds alternate, that means that we also have an alternation of sigma and sigma-pi bonds.

What does this mean? It means that every other bond will have a cloud of electrons around the main bond axis. This is why chlorophyll is good at its job.

You see, practice doesn’t really align with theory, in chemistry. So, instead of every other bond having this cloud of electrons around its sigma bond, we have a different effect that takes place: electron delocalization. That means that in practice, the clouds of electrons are actually going to spread over all of the atoms in the conjugated system. This helps create a more balanced molecule, instead of one with alternating bond strengths and lengths.

A good example of this phenomenon is benzene, as shown above, where this concept was first discovered. The Kekule model of benzene was developed in the 19th century, and Kekule’s calculations revealed that there should be an alternation of single and double bonds in this molecule. However, some more data, including the use of X-ray crystallography revealed this process of delocalization, by proving that all bonds had the same length and the same strength (a sort of 1.5 bond if you will).

Anyway, this same phenomenon occurs in chlorophyll. That means that in every chloroplast, in every plant, there is a cloud of free, delocalized electrons floating above the molecule. If you’ve read any of my previous articles on this blog, you’ll definitely know where I’m going with this.

When sunlight hits these delocalized electrons, they are able to be excited, given that photons are essentially just packets of energy. The energy provided by the electrons is dependent on the wavelength of the light, as given by the following equation from basic physics.

When an electron in chlorophyll is excited in this manner, it is able to rise to a higher energy state, which means that it can more easily be transferred to another molecule. In the main reaction of photosynthesis, electrons are transferred from water to carbon dioxide, in a redox reaction. We know that electrons are needed in redox reactions, and chlorophyll accelerates this process. The excited electrons undergo a series of transfer steps that serve to accelerate the reaction.

And, there you go. That’s how plants use solar energy in order to help provide us with oxygen and get rid of our excess carbon dioxide. But, how does this relate to dye-sensitized solar cells? To learn about that, please visit part B of this episode, where I’ll explain the reactions occurring in one of these cells, and how they are inspired by those in photosynthesis. Check that out, now!

Hope you enjoyed reading! I wanted to keep this a bit short because making too long would have gotten a bit tiring, I think. I’ll be back next time with a detailed explanation of how this process intricately relates to the overall method of producing electricity via DSSCs, and why this method could really be the solution to *some* of our energy needs, especially if we invest in it. Anyway, hope you enjoyed, and until next time…see ya!