The efficiency of a solar cell is one of its most important parameters. It indicates what percentage of the solar energy radiated into the cell is converted into electrical energy. The theoretical limit for silicon solar cells is 29.3 percent due to physical material properties. In the journal Materials Horizons, researchers from Helmholtz-Zentrum Berlin (HZB) and international colleagues describe how this limit can be abolished.
The trick: they incorporate layers of organic molecules into the solar cell. These layers utilise a quantum mechanical process known as singlet exciton fission to split certain energetic light (green and blue photons) in such a way that the electrical current of the solar cell can double in that energy range.
The principle of a solar cell is simple: per incident light particle (photon) a pair of charge carriers (exciton) consisting of a negative and a positive charge carrier (electron and hole) is generated. These two opposite charges can move freely in the semiconductor.
When they reach the charge-selective electrical contacts, one only allows positive charges to pass through, the other only negative charges. A direct electrical current is therefore generated, which can be used by an external consumer.
The team around HZB researcher, Prof. Klaus Lips, gives now a solution to build the solar cell in such a way that certain high energy photons are used to generate two pairs of charge carrier simultaneously. The effect they used resides in some organic crystals and is known as “singlet exciton fission” (SF). In order for this multiplier effect to be possible, charge carrier pairs have to fulfill certain quantum physical conditions: all their spins have to be parallel meaning charge carrier pairs called triplet exciton.
These triplet excitons are quite durable and very strongly bound together. The challenge was to split apart them at an interface to silicon. This unbinds the positive and negative charge carriers, permitting them to contribute to the solar cell’s current.
In a pioneering experiment the researchers have shown the splitting to be possible. "With implementing this concept successfully, we can make a silicon solar cell with a maximum quantum efficiency of 200 percent (double the normal limit), and a theoretical efficiency limit of around 40 percent," says Dr. Rowan MacQueen, an Australian researcher who joined the HZB team two years ago and is realizing the charge carrier multiplier solar cell at the HZB.
In the recently-reported work, the HZB researchers integrated a 100 nanometer thick layer of singlet fission-capable tetracene crystals into the surface of a silicon solar cell. Using spectroscopic investigations, triplet charge carrier pairs in the thin tetracene layer were detected, a signature of singlet fission.
"The challenge was to separate the triplet pairs at the silicon interface without significantly disrupting the current flow of the silicon solar cell," explains Klaus Lips, since a poorly conducting organic layer borders on the well conducting silicon layer, a situation that could greatly impede the current flow.
The splitting succeeds with an additionally introduced organic conductor called PEDOT:PSS meaning another organic layer is necessary. “The interfaces play a special role in this structure,” says Dr. MacQueen, which is why the researchers used X-ray light from the BESSY II@HZB synchrotron to study the interface properties. They then fabricated a series of working tetracene-silicon solar cells. A key finding was that the addition of the organic layer did not impede the electrical performance of the silicon cell, which is critical for producing an efficient device.
The electrical performance of the first silicon singlet fission solar cell showed that tetracene absorbs the blue-green portion of the light, while low-energy photons are absorbed by the silicon. Using a simulation, the researchers were able to estimate that currently about 5-10 percent of the triplet pairs generated in the tetracene layer could be added to the output power.
For Klaus Lips this is a great success but he is already considering follow-up experiments: "With this solar cell structure, we have shown that the approach works in principle, and delivered a workhorse design. And we already know what we have to do to increase the yield of separated triplet excitons to up to 200 percent".
The race for the best efficiencies in silicon technology
Despite many new developments, silicon is currently the material of choice in solar cell production. The production technologies are mature, the best solar cells today reach efficiencies of around 25 percent. The theoretical maximum efficiency of the semiconductor of 29.3 percent is almost reached. Research groups worldwide do not want to settle for this. They are looking for different ways to increase the limit.
Most of the loss is due to converting some of the incident light energy into heat and thus losing it. Only part of the light spectrum can be used to actually generate electrical power. A frequently used approach for reducing heat losses is therefore the so-called tandem solar cell. It combines silicon with an additional material that can utilize other areas of the light spectrum than silicon. Promising are currently silicon perovskite tandems, which HZB is also researching in.
For example, see this report:
Klaus Lips and his team are taking different paths and want to work without incorporating a new solar cell material. A few years ago, they showed experiments on so-called photochemical upconversion. Here are two low-energy photons, which actually remain ineffective in the solar cell, bundled into a high-energy photon, which then contributes to the output power.
The singlet fission approach presented here is a new, promising method of quantum physics that can give photovoltaic technology a new boost.
Dr. Ina Helms | Helmholtz-Zentrum Berlin für Materialien und Energie GmbH
Lade-PV Project Begins: Vehicle-integrated PV for Electrical Commercial Vehicles
03.04.2020 | Fraunhofer-Institut für Solare Energiesysteme ISE
Harnessing the rain for hydrovoltaics
03.04.2020 | Max-Planck-Institut für Polymerforschung
Drops of water falling on or sliding over surfaces may leave behind traces of electrical charge, causing the drops to charge themselves. Scientists at the Max Planck Institute for Polymer Research (MPI-P) in Mainz have now begun a detailed investigation into this phenomenon that accompanies us in every-day life. They developed a method to quantify the charge generation and additionally created a theoretical model to aid understanding. According to the scientists, the observed effect could be a source of generated power and an important building block for understanding frictional electricity.
Water drops sliding over non-conducting surfaces can be found everywhere in our lives: From the dripping of a coffee machine, to a rinse in the shower, to an...
90 million-year-old forest soil provides unexpected evidence for exceptionally warm climate near the South Pole in the Cretaceous
An international team of researchers led by geoscientists from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) have now...
The bacteria that cause tuberculosis need iron to survive. Researchers at the University of Zurich have now solved the first detailed structure of the transport protein responsible for the iron supply. When the iron transport into the bacteria is inhibited, the pathogen can no longer grow. This opens novel ways to develop targeted tuberculosis drugs.
One of the most devastating pathogens that lives inside human cells is Mycobacterium tuberculosis, the bacillus that causes tuberculosis. According to the...
An international team with the participation of Prof. Dr. Michael Kues from the Cluster of Excellence PhoenixD at Leibniz University Hannover has developed a new method for generating quantum-entangled photons in a spectral range of light that was previously inaccessible. The discovery can make the encryption of satellite-based communications much more secure in the future.
A 15-member research team from the UK, Germany and Japan has developed a new method for generating and detecting quantum-entangled photons at a wavelength of...
Together with their colleagues from the University of Würzburg, physicists from the group of Professor Alexander Szameit at the University of Rostock have devised a “funnel” for photons. Their discovery was recently published in the renowned journal Science and holds great promise for novel ultra-sensitive detectors as well as innovative applications in telecommunications and information processing.
The quantum-optical properties of light and its interaction with matter has fascinated the Rostock professor Alexander Szameit since College.
02.04.2020 | Event News
26.03.2020 | Event News
23.03.2020 | Event News
03.04.2020 | Materials Sciences
03.04.2020 | Life Sciences
03.04.2020 | Life Sciences