Principle of a silicon singlet fission solar cell with incorporated organic crystals (Credit: M. Künsting/HZB)Scientists at Helmholtz-Zentrum Berlin have improved the efficiency of silicon solar cells by integrating layers of organic molecules into the cells using a quantum mechanical process called “singlet exciton fission”. This process divides some of the green and blue photons so that the electrical current of the cell can double in that energy range and with a theoretical efficiency limit of around 40%.
In solar cells, for each incident photon a pair of excitons, comprised of a negative and a positive charge carrier, is produced. The two opposite charges move freely in the semiconductor until they reach the charge-selective electrical contacts, where one just allows positive charges to pass through while the other just negative charges. This generates a direct electrical current that can then be utilised externally.
In this study, reported in Materials Horizons [MacQueen et al. Mater Horizons (2018) DOI: 10.1039/c8mh00853a], a way to develop silicon solar cells so that certain high-energy photons can be used to generate two pairs of charge carrier simultaneously was demonstrated. For the multiplier effect to work, charge carrier pairs have to fulfill some quantum physical conditions – ie, all their spins have to be parallel, meaning charge carrier pairs termed triplet exciton.
“The challenge was to separate the triplet pairs at the silicon interface without significantly disrupting the current flow of the silicon solar cell”Klaus Lips
These triplet excitons are comparatively durable and very well bound together, making it problematic to divide them at an interface to silicon. The researchers therefore integrated a 100-nanometer thick layer of singlet fission-capable tetracene crystals into the surface of a silicon solar cell. With spectroscopy, they detected triplet charge carrier pairs in the thin tetracene layer, characteristic of singlet fission. As team leader Klaus Lips said “The challenge was to separate the triplet pairs at the silicon interface without significantly disrupting the current flow of the silicon solar cell”.
The electrical performance of the first silicon singlet fission solar cell demonstrated that tetracene can absorb the blue–green portion of the light, while low-energy photons are absorbed by the silicon. Based on simulation, they estimated that currently about 5–10% of the triplet pairs produced in the tetracene layer could be added to the output power.
The splitting is successful when a specific organic conductor was incorporated, so that a further organic layer is required. It was crucial that adding the further organic layer didn’t constrain the electrical performance of the silicon cell, which is important for fabricating an efficient device. The team are now looking to carry out further experiments, around the principle of a silicon singlet fission solar cell with incorporated organic crystals, and are already working to increase the yield of separated triplet excitons by up to 200%.