Solar
In order to maintain our quality of life a global average in excess of 15 terawatts of power is consumed. Combustion of fossil fuels currently accounts for 80% of this total, which is unsustainable due to their limited supply and negative environmental impact. Photovoltaic energy conversion provides a sustainable solution to this problem and capturing less than 0.02% of the total sunlight reaching the earth’s surface would be enough to satisfy all current demand. Sharp’s current motto in photovoltaics “The sun is the answer” reflects this vision, more information on which can be found at the Sharp Solar website.
Sharp has been developing solar cells since 1959 and is currently one of the leading manufacturers of solar panels worldwide; producing both “first generation” crystalline Silicon based panels and also “second generation” amorphous Silicon panels. The vision for Sharp is to become a “total solution” company: both creating panels and developing plants to provide the public with electricity. In addition to these large scale panels sharp is also integrating solar cells into mobile phones and outdoor lighting.
SLE supports the work of Sharp Corporation at many levels from idea to device to manufacturing:
- We create challenging new concepts for high efficiency photovoltaics
- We develop demos of new products such as novel nano-structured thin-film devices
- We interact directly with Sharp’s manufacturing bases.
The Solar Team at SLE has researchers from a diverse range of backgrounds, both in terms of the countries we are from and the levels of research experience. People in the team come from as far away as Brazil and China and the levels of experience range from researchers with post-doctoral and industrial experience to those straight out of university, with specializations ranging from Materials Science to Chemistry to Physics and Engineering.
We are currently carrying out research in to high efficiency photovoltaics; investigating ultra-high efficiency concepts such as Multiple-Exciton Generation and Multi-Junction cells. More information about these and other current projects can be found in the following pages: Multi-Junction, Quantum Dots and Theoretical and Simulation. We have our own cleanroom facilities with all the semiconductor deposition and etching equipment required for on-site device fabrication; we also have a solar simulator among other characterisation tools to test the cells we make.
Multi-Junction
Sharp currently hold the world record for efficiency of a multi-junction cell under single sun illumination(35.8%). This is for a triple-junction cell with InGaP, GaAs and InGaAs layers. It has been shown that if a 4th junction with a band-gap of 1eV were included in this structure, the resulting multi-junction cell would have an efficiency in excess of 50% under concentrated illumination.
SLE is developing such a 4th junction cell using the dilute Nitride material system InGaAsN. Challenges faced in incorporating this material in a multi-junction cell include its short minority-carrier diffusion length and the possibility of phase separation. By optimising the growth InGaAsN we aim to overcome these challenges and achieve a cell with the material quality sufficient to realise a 50% efficient multi-junction cell.
Other approaches to achieving high efficiency multi-junction cells are also being pursued. Alternative materials and novel device architectures are being explored to reveal new ways to boost efficiency in these devices.
Figure 1
Quantum Dots
Semiconductor Quantum Dots are nanometre sized semiconductor crystals which have tuneable optical properties, dependant not only on their chemical composition but also on their size and the matrix surrounding them. SLE has the capability to grow both Stranski-Krastanov and colloidal quantum dots for use in various applications, not least novel photovoltaic cells.
SLE is exploring how best to exploit the properties of Quantum Dots in novel devices. Their size tuneable absorption spectrum makes them an ideal choice for use as a sensitizer, replacing the dyes used in a traditional dye-sensitized solar cell. They can also be used in a layered structure, similar to a multi-junction cell, but with the ability to continuously change the absorption of the cell through the system by changing the dot size.
Finally, tentative experimental evidence suggests that Multi-Exciton Generation might be present in certain semiconductor quantum dots. Multi-Exciton Generation is the creation of more than one electron hole pair per incoming photon (if the incoming photon is of sufficiently high energy). In principle this would allow photovoltaic cells with significantly higher efficiencies.
Figure 2
Theoretical and Simulation
In addition to experimental research SLE also carries out fundamental theoretical work, both to provide back-up to current projects and to find new ways to exploit the solar spectrum.
Cutting edge semiconductor device modelling software allows us to predict the benefits of a novel cell design before lab work begins. Using FDTD simulation to model illumination and a self-consistent electrical simulation to obtain I-V characteristics gives us important information about what we can expect from new cells. We can also use modelling to find key parameters for device optimisation; feeding experimental results back in to improve accuracy and understanding.
We also carry out ab initio theoretical work, creating theoretical models to analyse novel approaches to achieve ultra-high efficiency solar conversion.
Figure 3
