Every solar cell under the Sun relies on exciting electrons with sunlight and extracting them to keep our lights on. We’re actually copying a natural process when we build these devices: plants and other photosynthetic systems operate in the same way. In their case, chlorophyll (or other) antennae absorb light that excites electrons, which then quickly jump along to the heart of the system to catalyze chemical processes that give plants their energy. Plants are actually incredibly efficient in this regard: in most cases, each bit of light absorbed (a quantum) excites a single electron that is used in the chemical production of energy, which physics tells us is the best you can do (except for a couple exceptions, of course)!
Researchers have been trying hard to match this 1:1 ratio of absorbed light quanta to number of electrons extracted for useful purpose. But obstacles abound! One of the more intractable challenges is to prevent electrons from losing their energy before they are extracted. When light excites an electron, this negative charge carrier leaves behind a positively charged hole – these oppositely charged carriers have a strong attraction to one another. When they ‘see’ each other, they recombine and lose their energy that they received from the light.
Preventing this recombination is especially challenging in organic solar cells, which are made of materials that allow electrons and holes to feel one another’s presence very easily, increasing the chance that they’ll find each other. These same materials have the potential to be very cheap and mass-produced, so researchers are trying hard to find ways to slow down the electron-hole recombination time and give the electrons and holes an opportunity to be extracted to do useful work.
Keeping electrons and holes excited by sunlight away from one another is akin to keeping lovers apart: given enough time, it’s impossible. Or so conventional wisdom goes – new research in Science suggests that a novel structure for organic solar cells may prevent the two charge carriers from combining for days or even weeks, blowing away traditional recombination times on the order of milliseconds and avoiding one of the major obstacles to highly efficient photovoltaics.
So what were the researchers able to change about traditional organic solar cells to change the recombination dynamics so dramatically? The key lies in eliminating a phase segregation that normally occurs in such materials. Many organic solar cells use a polymer (often P3HT, see figure below) that absorbs light and excites electrons, acting as the electron donor. A second material, usually a conjugated fullerene (PCBM below), acts as electron acceptor that takes electrons and moves them to the rest of the circuit. An interface between the polymer and fullerene is crucial, as this is where the fullerene takes the excited electron while the hole stays on the polymer, thus achieving charge separation and preventing recombination. But usually these two types of materials segregate into different regions, as shown by the purple and gray regions below:
This means that the electron and hole have to travel farther – usually 10-20 nanometers, which is a long distance at the atomic level – to find an interface where they can separate. This increases the probability that the electron and hole will find each other and recombine during their journey.
To improve this situation, Huber et al have discovered a molecule that self-assembles in water to form a more orderly network of both the electron donor and acceptor. The hard work involved getting the chemistry right to find a pair of materials that assemble in such a way naturally. The result is an ordered structure with the long polymer, poly(fluorene-alt-thiophene) (PFT), shown in green, systematically stretched along one axis with charged fullerene acceptors arranged at regular intervals on the outer part of the structure:
Electrons initially excited on the PFT will hop to fullerenes on the outer part of the structure, consistently separating from holes. The researchers measured recombination times by measuring changes in luminescence of the devices – luminescence decreases as long as an electron-hole pair is excited and stable. Such decreased luminescence lasted for up to weeks, essentially removing electron-hole recombination completely as a barrier to charge extraction and use.
These results are exciting for several reasons. First, obviously, is the possibility of creating organic photovoltaic devices with minimal recombination – this is a huge step towards making these materials commercially viable. But the assembly method that Huber et al used is also encouraging. Self-assembly of such an ordered structure means that manufacturers should, in theory, be able to combine the polymer and fullerene with just the right concentrations in just the right solution and, like magic, the well-ordered structure will assemble itself. This severely reduces costs and makes mass production much more viable. Lastly, they found this assembly occurs in water, which means that possibly toxic solvents aren’t necessary. All good reasons that organic solar cells should be making more of an impact in the future!
Huber, R., Ferreira, A., Thompson, R., Kilbride, D., Knutson, N., Devi, L., Toso, D., Challa, J., Zhou, Z., Rubin, Y., Schwartz, B., & Tolbert, S. (2015). Long-lived photoinduced polaron formation in conjugated polyelectrolyte-fullerene assemblies Science, 348 (6241), 1340-1343 DOI: 10.1126/science.aaa6850