Organic layer adds light particles
The new organic material is able to “add up” low-energy photons (red) to yield higher-energy light (blue), which can then be harvested by the solar cell. © Y. Y. Cheng/UNSW
Solar cells can only use photons with energies above a specific threshold to generate electricity. A German-Australian research collaboration has now combined solar cells with an organic material which can “add up” low-energy photons to yield higher-energy light, which can then be harvested by the solar cell. In an invited review paper published in the prestigious journal Energy & Environmental Science the scientists give an overview on the fascinating phenomenon of photonic upconversion and report new results: The organic layers show less photo-degradation than suspected and may also be used for other optoelectronics applications.
Every solar cell only uses a small part of the solar spectrum, mainly because it can absorb only light with energies above a material-specific threshold energy. One approach towards circumventing those losses and using more “colors” of the sunlight is photonic upconversion: The solar cell’s backside is coated with a material which can add up low-energy (“red”) photons to yield higher-energy (“blue”) photons which are radiated back towards the solar cell, thus expanding its current harvest. Since 2011, HZB scientists around Prof. Dr. Klaus Lips are collaborating with The University of New South Wales (Sydney, Australia) in investigating novel upconversion materials which have higher efficiencies than the classically employed rare-earth phosphors.
New class of materials
The novel class of organic upconversion materials is based on so-called triplet-triplet annihilation (TTA), and combines strong absorptivity with potentially high quantum yield. The first lab experiments have been published in 2012 in Energy & Environmental Science (see HZB news report )
“This first report has already been cited over 90 times, which is why we received an invitation to write a review article on this topic” says Dr. Tim Schulze from the Lips team. With an impact factor of >15, Energy & Environmental Science is one of the most distinguished journals in the field.
New data on longevity and efficiency
In the recently published review article, Schulze not only presents a comprehensive overview on the topic but also previously unpublished new data: Together with the Australian partners around Prof. Timothy Schmidt, Schulze has now also investigated the longevity of the TTA molecules under illumination – an extremely critical property for organic materials. The reported results are promising: Even the present not optimized system would last for some years behind a solar cell! “We also have ideas about how to further improve the material”, Schulze says.
At the same time the group reports new record values for the photocurrent enhancement of upconversion-assisted solar cells. “The reached numbers are still far away from a real-world application, but significantly higher than the ones realized with lanthanoid upconversion.”, Schulze explains. And Lips adds: “TTA upconversion is a generic optical technology with potential applications beyond thin-film solar cells. They could also assist photo-electrochemical water splitting or organic LEDs.” The collaboration with Sydney will continue and 2015 will see more joint experiments.
Note: The collaboration, initiated by Lips in 2011 with the help of a DAAD scholarship, has since been greatly intensified: Dr. Tim Schulze was hosted by the Sydney group for one year on an Alexander-von-Humboldt fellowship, advancing materials design and device implementation. At the same time an Australian PhD student stayed at HZB for several weeks on a DAAD scholarship, making use of the BeJEL infrastructure in order to shed light on the interesting spin physics of the TTA systems.
Review-Article 2014: Photochemical upconversion: Present status and prospects for its application to solar energy conversion, Tim Ferdinand Schulze and Timothy W Schmidt, Energy Environ. Sci., 2014
DOI: 10.1039/C4EE02481H
Article 2012: Yuen Yap Cheng, Burkhard Fückel, Rowan W MacQueen, Tony Khoury, Raphaël GCR Clady, Tim F Schulze, NJ Ekins-Daukes, Maxwell J Crossley, Bernd Stannowski, Klaus Lips and Timothy W Schmidt, Energy & Environmental Science 5, 6953-6959 (2012)
- New instrument at BESSY II: The OÆSE endstation in EMILA new instrument is now available at BESSY II for investigating catalyst materials, battery electrodes and other energy devices under operating conditions: the Operando Absorption and Emission Spectroscopy on EMIL (OÆSE) endstation in the Energy Materials In-situ Laboratory Berlin (EMIL). A team led by Raul Garcia-Diez and Marcus Bär showcases the instrument’s capabilities via a proof-of-concept study on electrodeposited copper.
- Green hydrogen: A cage structured material transforms into a performant catalystClathrates are characterised by a complex cage structure that provides space for guest ions too. Now, for the first time, a team has investigated the suitability of clathrates as catalysts for electrolytic hydrogen production with impressive results: the clathrate sample was even more efficient and robust than currently used nickel-based catalysts. They also found a reason for this enhanced performance. Measurements at BESSY II showed that the clathrates undergo structural changes during the catalytic reaction: the three-dimensional cage structure decays into ultra-thin nanosheets that allow maximum contact with active catalytic centres. The study has been published in the journal ‘Angewandte Chemie’.
- Solar cells on moon glass for a future base on the moonFuture settlements on the moon will need energy, which could be supplied by photovoltaics. However, launching material into space is expensive – transporting one kilogram to the moon costs one million euros. But there are also resources on the moon that can be used. A research team led by Dr. Felix Lang of the University of Potsdam and Dr. Stefan Linke of the Technical University of Berlin have now produced the required glass from ‘moon dust’ (regolith) and coated it with perovskite. This could save up to 99 percent of the weight needed to produce PV modules on the moon. The team tested the radiation tolerance of the solar cells at the proton accelerator of the HZB.