Plastic solar panels
In order to make more cost-effective solar panels, and benefit from the carbon-free energy they provide, we need a better understanding of how ‘plastic’ solar panels work. The latest generation of photovoltaic diodes are based on blends of polymeric semiconductors and fullerene derivatives. Inside these devices, the absorption of light fuels the formation of an electron and a positive ion – to provide electricity these must be separated and the electron must move away. If the electron cannot move away fast enough, then it recombines with a positive ion and the chance to produce electricity is lost. The overall efficiency of solar devices compares how many atoms recombine and how many separate.
An international group of collaborators from the University of Cyprus, the University of Montreal, Imperial College, London and the CLF studied the chemistry at work in these reactions that underpin solar energy conversion devices.
For their research, which was published in Nature Communications, the team used the femtosecond Raman spectroscopy (FSRS) technique to reveal the photo-stimulated chemistry that takes place within the solar panels. FSRS uses three laser beams. The first, a green, pulse activates the polymer to create an electronic excited state. Then a pair of near infra-red and white light continuum pulses are used to generate the Raman spectrum that records the vibrational modes of the excited molecules as they change. Using ultra-short pulses enables a time resolution of less than 300 femtoseconds, and this technique gives details on how chemical bonds change during these extremely fast chemical reactions.
Their results open avenues for future research into understanding the differences between material systems that produce efficient solar cells, and those that do not perform as well as expected. Armed with this information, manufacturers will be able to build better solar panels in the future.
Artificial photosynthesis
Scientists are working on a fascinating challenge, to develop a system for artificial photosynthesis that would work as efficiently as plants and would enable us to produce carbon-free fuels that help us meet our growing energy needs, and can even be used for transport.
Nature has covered our planet with green plants that use the energy of sunlight to grow using a complicated chemical process called photosynthesis that converts the solar energy into fuels - this stored energy is in the form of chemical bonds. We can make use of plants’ ability to photosynthesise directly, by growing crops that are turned into biofuels. But politically this is complicated because farm land is needed to grow food, and burning plants for fuel is a very inefficient process. Artificial photosynthesis has the potential to be much more efficient. Work by Sheffield University and the ULTRA team is looking at new molecules that efficiently absorb light and form a “charge separated state”, where a positive and negative charge are spatially separated and can take part in independent chemical reactions. Researchers from the University of Nottingham have synthesised a new molecule that could do just that, with a lifetime (15 ns) long enough for artificial photosynthesis applications. ULTRA’s time-resolved infrared laser capability was used to understand how these molecules responded to light.
Laser-driven fusion
Fusion is the process by which nuclei (with mass numbers less than that of Iron) can join to produce heavier nuclei and also release energy. In stellar environments these reactions involve either mainly hydrogen (stars around the Sun's mass or less), or carbon, nitrogen, and oxygen (in stars somewhat heavier than the Sun). Harnessing fusion for energy on Earth would have to involve using the much faster reactions that can occur between heavy hydrogen isotopes (deuterium and tritium). Since deuterium accounts for 3% of the hydrogen in terrestrial water there is an abundant source of this basic fuel. Fusion could therefore, in principle, create a source that is clean (no carbon), safe (no meltdowns), and long lasting (could provide energy for millions of years).
Creating a viable fusion reactor has been one of the great outstanding problems of modern physics. Currently there are two main approaches. One is to create a quasi-static plasma which is confined in a magnetic device such as a tokamak. This “Magnetic Confinement Fusion” approach is pursued at CCFE, Culham in the UK. The other is to create a highly transient, super-compressed mass of plasma using a laser driver. During its brief life-time one can obtain net energy release from such a highly compressed hot plasma. This “Inertial Confinement Fusion” approaches one area of high energy density physics that has been investigated at the CLF.
The fast ignition (FI) approach, the physics of which has been investigated at the CLF, is an advanced variant of the ICF approach. FI involves using one set of lasers to compress the fuel and then a single high intensity laser pulse to generate a beam of energetic particles that heats the compressed fuel. Laser generated electrons or laser generated protons could be used as the ignitor beam. Copious numbers of multi-MeV electrons are generated in most ultra intense laser-solid interactions, and theoretical studies indicate that conversion of energy into electrons can be high. However there are substantial issues in terms of the guiding and focussing/divergence of these 'fast electron' beams. Protons, on the other hand, have the advantage that they deposit energy in a very well-defined volume, but until recently the conversion of laser energy into the proton beam was never high enough to be viable in the design of a laser-fusion power station. Work at the CLF has opened up ways in which the problems with both the electron and proton based approaches might be overcome.
The National Ignition Facility in the US is the world’s only laser-fusion experiment which is at “full scale”, i.e. it is thought to be theoretically capable of achieving ignition and energy gain. NIF produces vastly more fusion neutrons than any other experiment, and has now demonstrated in excess of 10kJ of fusion energy release. This has to be compared to the 1.8MJ of laser energy that the 192 beams deliver – so net energy gain is yet to be achieved. The NIF team have identified a number of challenges that will need to be overcome in order to reach ignition. A number of UK-NIF collaborations have resulted in UK scientists making significant contributions towards this ultimate goal.
Further information:
- CLF contact: Dr Alex Robinson.
- Theory of fast electron transport for fast ignition, A.P.L.Robinson et al. , Nuclear Fusion, 54, 054003 (2014). http://dx.doi.org/10.1088/0029-5515/54/5/054003 (link opens in a new window)
- High energy conversion efficiency in laser-proton acceleration by controlling laser-energy deposition onto thin foil targets, C.Brenner et al., Appl. Phys. Lett. 104, 081123 (2014). http://dx.doi.org/10.1063/1.4865812 (link opens in a new window)
- Effect of Laser Intensity on Fast-Electron-Beam Divergence in Solid-Density Plasmas, J.S.Green et al., Phys. Rev. Lett. 100, 015003 (2008). http://dx.doi.org/10.1103/PhysRevLett.100.015003 (link opens in a new window)
- Measurements of Energy Transport Patterns in Solid Density Laser Plasma Interactions at Intensities of 5x1020 Wcm-2, K.L.Lancaster et al., Phys. Rev. Lett. 98, 125002 (2007). http://dx.doi.org/10.1103/PhysRevLett.98.125002 (link opens in a new window)
