Researchers are hard at work looking for ways to improve solar cell efficiency. We as society should be pushing them, too, as more efficient solar cells means more competitive solar cells means a widespread injection into the market and a replacement of fossil fuels.
First-generation solar cells on the market today, usually made of silicon, rely on a single electron excited per photon of light that loses much of its energy prior to being extracted to an external circuit for useful work. These types of solar cells are cheap, mainly because we’ve been perfecting them for the past few decades, but their efficiency is limited around just over 30% – this means we only get 30% of the energy that is inputted from the Sun!
So we’re all looking for better ways to do this simple extraction of photoexcited electrons. Researchers at the University of Pennsylvania may have found an interesting alternative by harnessing plasmons!
What are plasmons? The explanation requires a brief background of how electrons behave in metals. In metals such as gold and silver, we typically imagine electrons moving freely throughout the material. Each electron, moving very quickly, sees its environment as a collection of many other quickly moving electrons, and this has the effect of reducing the interaction between the electrons or between the electrons and ions in the nuclei. This effect is known as screening and leads to electrons in metals acting very similarly to free electrons.
What happens when optical radiation excites these nearly free electrons? The answer is plasmons. The electromagnetic waves of incoming light create dipoles that shift electrons away from their initial positions. The electrons want to return to their equilibrium positions, but the incoming light keeps shifting them away. Back and forth they go, due to this competition between the incoming light and the electron’s desire to return to equilibrium, thus creating oscillations. The radiation at a particular energy, known as the plasma frequency, creates the greatest coupling between the light and the electrons. The collective group of electrons is known as the plasma, and its oscillations at the plasma frequency are known as plasmons.
So how do these plasmons help solar cells? Conklin and colleagues created an optoelectronic device consisting of gold particles linked together using organic molecules called porphyrin. The porphyrin molecules are photoactive, meaning that they are good at absorbing light and photoexciting electrons as well as transporting excited carriers through them. In addition, the gold particles are plasmonic, which means that optical absorption easily leads to plasmon resonance in a network of these particles. Here’s a little picture representing how the gold and molecules connect:
The main question the authors attempted to answer was this: Is it possible to use plasmons to directly extract electrons for useful work in an optoelectronic device, such as a solar cell? By careful manipulation of conditions, the authors have answered with quite a confident yes!
Let’s go into a bit more detail about what they did. The authors clearly found a strong photocurrent, which is proportional to the number of electrons, using the network of gold and molecules as a photoabsorber. However, the main thrust of their results centered around accounting for all possibilities other than plasmons that would lead to this current. They tested each alternative by carefully manipulating environmental conditions to determine whether it is the hot electrons from the plasma directly contributing to the current or something else.
For example, plasmons are known to produce strong local electric fields that can lead to increased absorption of light. This increased light absorption means more excited electrons and therefore could mean higher photocurrent (known as the nanoantennae effect). To test whether this could account for increased photocurrent in their device, the authors simply measured the photocurrent at different wavelengths. Wavelength of red color both excited the plasmon and created the nanoatennae effect, however wavelength of green color only excited the plasmon. In both cases, however, the photocurrent increases – therefore, the effect MUST be from the plasmon, not the increased optical absorption. This is a great example of clever physicists controlling environmental variables to eliminate possible explanations.
The authors tested two other possibilities: 1) the plasmons reducing the tunneling activation energy for electrons, and 2) photoassisted tunneling increased photocurrent. By adjusting variables such as temperature and the number of gold nanoparticles in the network, they showed that neither of these possibilities could lead to increased photocurrent. The last possibility, then, is that the hot electrons excited from the plasma oscillations are directly responsible!
This is impressive because plasmons are known to have short, femtosecond lifetimes. This means that the associated excited electrons in the plasma relax very quickly to their original state, whereas extraction to an outside circuit usually requires lifetimes several magnitudes larger. The paper doesn’t give any detailed explanation for this, which opens the door for others to understand this effect in more detail, however their careful experimental control over various conditions seems to suggest that they are truly extracting hot carriers from plasmon excitations for use in an optoelectronic device. This is exciting because it involves a completely different type of photoexcitation than the traditional silicon solar cell, and provides another avenue for researchers to attempt to improve efficiency.
Conklin D, Nanayakkara S, Park TH, Lagadec MF, Stecher JT, Chen X, Therien MJ, & Bonnell DA (2013). Exploiting plasmon-induced hot electrons in molecular electronic devices. ACS nano, 7 (5), 4479-86 PMID: 23550717