Solar energy is one of the most important alternative energy sources available to us. In Europe, alongside an increasing number of wind turbines, houses with solar panels are a familiar sight. These solar cells are, however, not particularly efficient compared to the state of the art. One reason for this is their simple construction: they are made from silicon, which has a particular electronic structure. Because of this, electricity production is possible for the entire range of visible light, but the solar cell is only really efficient for a narrow range of wavelengths.

It's possible to work around this by carefully engineering materials so that each absorbs only part of the solar spectrum, which it converts to electricity efficiently. These cells operate with efficiencies of up to 40-50 percent—and cost, per square meter, more than a gold plated Mercedes.

In contrast, silicon solar panels are now so cheap that the limiting cost factors are the supporting structure and power conversion electronics rather than the cells themselves. Reducing those costs is proving difficult, so increasing the efficiency of the solar cell could become rather important. In a recent publication, researchers have used the power and magic of plasmonics to show that maybe, just maybe, one can enhance the efficiency solar cells rather cheaply.

The limits of efficiency

Solar cells are based on semiconductor materials. Non-conductors—into which we will carelessly toss semiconductors—have what is called a bandgap. This is the energy required to excite an electron from a bound state to a conducting state. In the bound state, electrons stay in the vicinity of the atoms to which they are attached, while in the conducting state, they are free to move. Solar cells use sunlight to excite electrons from the bound state to the conducting state, and the electrons give up that energy when they perform work for us.

Now that we know that, let's imagine that we have a material with a bandgap where the energy corresponds to green light. If we shine green light on the material, it is efficiently absorbed and electrons flow and work is extracted. If we shine red light on the material, the energy of the individual photons is too low to excite electrons into a conducting state, so the light just passes through and no work is done. At the other end, if we shine blue light on the semiconductor, we give the electron too much energy. It quickly releases the excess energy as heat and falls to the bandgap energy. So, we only get the same useable energy as we would with green light.

You can see how a stacked solar cell would work. The front surface absorbs the blue light, the back surface absorbs the red light, and, sandwiched between the two is a material that absorbs green light.

What if we could convince a semiconductor to absorb photons with lower energy than the bandgap? Then we could tune the bandwidth to a higher energy and use the same material to grab the redder colors of light. At the same time, we'd lose less energy from blue light.

This is exactly what researchers West Virginia University have claimed to do. Though, at the moment, they demonstrate this with photocatalysis not solar electrical energy production. However, to explain how this works will require pulling in numerous concepts—prepare for a winding explanatory road.

The magic of quantum mechanics

The trick that the researchers used was inspired by a common phenomena exhibited by dye molecules. Imagine that I have a dye molecule that absorbs blue light and emits green light plus a second dye molecule that absorbs green light and emits red light. If I mix them together and shine blue light into the mixture, I will mostly observe green light. Yet, if I look carefully, I will occasionally see some red.

I can enhance that red by using some chemistry to link the dye molecules, keeping them in very close proximity to each other (almost touching, whatever that means in this context). If all of the blue-green dye molecules are paired with green-red dye molecules, then shining a blue light into the mixture will result in a lot of red light.

The trick is that the proximity and overlap between the excited states of the two dye molecules is very large, so the energy is transferred to the second dye molecule in a non-radiative process—that is, no light is emitted, but energy is transferred. If it helps, you can imagine that one electron drops immediately to the ground state and emits a photon that is immediately absorbed by the neighboring molecule. However, this photon is what is referred to as a virtual photon—it is never observed.

The other key feature of this process is that it is a one-way street: we flow from high energy blue photons to low energy red photons and never the other way.

Making water run uphill

There is a myth in quantum mechanics, one that I repeat often. An atom has an electronic structure that is discrete in nature. To excite an atom from one state to another, you need to provide just the right amount of energy: that is, the photon energy (the color of the light) should match the energy required by the transition.

This is a big fat lie. This energy corresponds to the point where a single photon has the highest probability of being absorbed and inducing a transition. But lower and higher energies all have a non-zero probability. The function that describes the probability never formally goes to zero, even though it can fall off very sharply outside of the maximum value.

Normally, this doesn't matter. But if the color of the light is close to the correct color and the light is intense enough, then the interaction between the electrons and the light field act to distort the energy level structure. Think of it like this: the exact value of the energy required for transitions between states is determined by what is referred to as the Hamiltonian. Among other things, the Hamiltonian is set by the attractive force between the positively charge nucleus and the negatively charged electrons.

But the light field is causing the electrons to oscillate strongly (a process referred to as a coherence) so the Hamiltonian starts to oscillate as well. That means where there was once a single transition between one energy level and another, there are now two transitions: one at a slightly lower energy than the original and one at a slightly higher energy. One of these new energy levels will correspond exactly to the color of the light field. Once this occurs, the atom is in resonance with the light field and will absorb energy.



While the energy of the photons in the light field are too low to be absorbed by the atom, the presence of many photons shifts the atomic structure so that the energy is correct, allowing photons to be absorbed.

Now, if this could just happen, the world around us would be very different. However, we don't observe this kind of behavior except under fairly carefully arranged conditions. That's because the Hamiltonian has to oscillate at a well-defined frequency. And that means the electrons have to oscillate at a well defined frequency.

This doesn't happen normally because every time an atom collides with another atom, the electrons' oscillation is disturbed, which kills the oscillation. Since it takes some time to build up the oscillation, these disturbances have to happen on a time scale that is slower than the build up of the oscillation to observe the energy level splitting. In the messy world, these disturbances happen much faster than the build up of the oscillations, so we don't normally observe the energy level splitting.

We need more power

This is where the magic of plasmons come in. The researchers made little gold spheres and then enclosed them in a semiconductor material. Gold is a conductor and has some free electrons around. The oscillating field of a light beam will cause these electrons in the gold sphere to slosh back and forth. And if the color of the light is right, that sloshing will become very vigorous, with the electrons piling up and one side of the sphere and then shifting to the other. At the edges of the sphere, the electric field caused by the electrons becomes very very large.



Surface plasmon polaritons One of the current big things in optics is plasmonics. At their heart, surface plasmon polaritons are a combination of light waves and electron motion. What makes this combination attractive is that the electron motion is rather slow, so the speed of a plasmon is quite a bit slower than the speed of light. As a result, the wavelength of the plasmon is much shorter than that of the freely moving light wave. This compression makes the electric fields associated with the plasmon very intense and very local. Read more…

Sweet, golden solar panels

The semiconductor, sitting around the gold, feels this field. It doesn't know or care how the field is generated—all it knows is that its electrons start oscillating in sympathy. And the field is so big that the oscillations build up incredibly fast—on the order of a few femtoseconds (10s), compared to normal lab conditions where this might take a few picoseconds (10s). As a result, the entire bandgap (which is the equivalent of the excited state of an atom) of the semiconductor drops into resonance with the light field, allowing it to absorb photons that it would otherwise ignore.

The paper comes with a lot of supplementary material—it's actually more like an advertisement for the supplementary material where all the genuine science is. Combined, they convince me that they really did manage to shift the bandgap.

Remember, however, that the point of this is to shift it in such a way that photovoltaic devices can use lower-energy photons. And they didn't make a solar electrical device. Instead, they went for photocatalysis. Their semiconductor, copper oxide, will catalyze the breakup of a dye molecule, provided that it has an electron in the conduction band to donate. This provided them with an easy way to measure activity: the sample glows less as the reaction proceeds and there's less dye around, which implies more electrons available in the conduction band.

Since the bandgap of copper oxide has a well known value, it is a trivial exercise to compare photocatalytic activity for illumination that has photon energies greater and smaller than the bandgap.

Now, if this had only been done using lasers, I wouldn't have been that surprised. That's because laser light is the best choice to drive coherent processes, which is exactly what this energy level splitting is. However, they also showed that it works with a normal lamp. (The lamp was filtered, rather than a full solar spectrum, and the details of the filtering were not provided, so it is impossible to tell if this will work under more general circumstances.)

The gold particles actually have two roles. Apart from providing the sloshing electrons that shift the absorption of the semiconductor, they also act as a filter. The gold nanoparticle only responds to a limited range of colors, which is essential to drive the semiconductor coherently. But it's possible to pair a single semiconductor material with a range of nanoparticle materials and sizes to enable efficient absorption over a significant portion of the solar spectrum.

This brings me to another remarkable result: the amount that the bandgap shifted. In atomic physics, normally, shifts of less than a nanometer are observed and used (bigger shifts are possible, but not necessary). In this experiment, the copper oxide bandgap appears to shift from 550nm to 650nm, a shift of 100nm. This is enormous. My feeling, before seeing that, was that although this was interesting, it was unlikely to be practical. But shifts of 100nm are huge and will make for some very interesting developments.

Even if this never comes out of the lab, we will learn a huge amount from the follow up work.

Nature Photonics, 2015, DOI: 10.1038/NPHOTON.2015.142