Keywords: energy (303) solar energy (240) BESSY II (266) HZB own research (95)

Science Highlight    02.10.2018

HZB researchers boost the efficiency of silicon solar cells

Principle of a silicon singlet fission solar cell with incorporated organic crystalls

Copyright: M. Künsting/HZB

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". 

 

Further Information:

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:
Perovskite – silicon solar cell research collaboration hits 25.2 percent efficienca

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.

Published in Materials Horizon (2018): Crystalline silicon solar cells with tetracene interlayers: the path to silicon-singlet fission heterojunction devices

DOI: 10.1039/c8mh00853a

IH


           



You might also be interested in
  • <p>The enzyme MHETase is a huge and complex molecule. MHET-molecules from PET plastic dock at the active site inside the MHETase and are broken down into their basic building blocks.</p>SCIENCE HIGHLIGHT      12.04.2019

    "Molecular scissors" for plastic waste

    A research team from the University of Greifswald and Helmholtz-Zentrum-Berlin (HZB) has solved the molecular structure of the important enzyme MHETase at BESSY II. MHETase was discovered in bacteria and together with a second enzyme - PETase - is able to break down the widely used plastic PET into its basic building blocks. This 3D structure already allowed the researchers to produce a MHETase variant with optimized activity in order to use it, together with PETase, for a sustainable recycling of PET. The results have been published in the research journal Nature Communications. [...]


  • <p>Frederike Lehmann earned an award for her presentation at the annual meeting of the German Society for Crystallography.</p>NEWS      11.04.2019

    PhD student of HZB earns Best Presentation Award of the Young Crystallographers

    Frederike Lehmann received a prize for her presentation at the annual conference of the German Society of Crystallography in Leipzig on 28 March 2019. She is doing her doctorate in the Department of Structure and Dynamics of Energy Materials at the HZB under Prof. Dr. Susan Schorr at the Graduate School HyPerCell. [...]


  • <p>The SEM shows Molybdenum sulfide deposited at room temperature.</p>SCIENCE HIGHLIGHT      04.04.2019

    Catalyst research for solar fuels: Amorphous molybdenum sulphide works best

    Efficient and inexpensive catalysts will be required for production of hydrogen from sunlight. Molybdenum sulphides are considered good candidates. A team at HZB has now explained what processes take place in molybdenum sulphides during catalysis and why amorphous molybdenum sulphide works best. The results have been published in the journal ACS Catalysis. [...]




Newsletter