The UK’s Engineering and Physical Sciences Research Council (EPSRC) has awarded funding totaling £1m to two universities – University College London (UCL) and Bristol – to support the development of compound semiconductor solar cells based on quantum-dot structures.
Professors Huiyun Liu and Alwyn Seeds of the Department of Electronic and Electrical Engineering at UCL have been awarded £600,519 (EPSRC reference EP/K029118/1) to develop low-cost, high-efficiency III-V quantum-dot solar cells based on silicon substrates.
Their work will be in collaboration with the University of Bristol, which was awarded £356,624 (EPSRC reference EP/K029665/1) to do the modeling and characterization.
The project ‘Energy and the Physical Sciences: Semiconductor III-V Quantum-Dot Solar Cells on Silicon Substrates’ covers a research period of 42 months, running from April 2013 to end-September 2016.
To help combat climate change, the UK has a target to reduce carbon emissions by 80% by 2050. This is an enormous task requiring changes to energy generation and supply. To limit the impact on scarce natural resources and the environment, these reductions need to be delivered by providing affordable green energy.
“This research project will address this problem by developing high-efficiency and low-cost solar cells by growing III-V compound semiconductor self-organized QD structures on cheap and plentiful silicon substrates,” comment UCL’s Liu and Seeds.
“We aim to exploit the advantages of both QD technology and germanium-on-silicon substrates to develop both multi-junction solar cell and intermediate-band solar cell designs. UCL will carry out the epitaxial material growth of III-V on germanium-on-silicon substrates by the molecular beam epitaxy (MBE) facility at the department of Electronic and Electrical Engineering, and the device fabrication in the London Centre for Nanotechnology.”
The Bristol University component will be led by principal investigator professor J Rorison of the Department of Electrical and Electronic Engineering. Project partners are the Defense Science & Tech Lab (DSTL), IQE Silicon Compounds Ltd, and Sharp Laboratories of Europe Ltd.
The project leaders agree that the widespread implementation of photovoltaics and solar cells will be a notable means of achieving sustainable energy production for the planet, together stating: “These green energy developments will require vast areas of semiconductor materials to be structured into PV cells in order to capture the power of sunlight.”
There are two general approaches to be taken: either to use very large-area, low-cost and low-efficiency semiconductor materials (such as organic materials) or to use small-area highly efficient but expensive semiconductor materials and concentrate the light into the small-area concentrator photovoltaic (CPV) cells.
The cost of the housing is a significant cost of the PV cell and therefore making the material cheaper for large-area PV does not improve cost below a certain value. The efficiency of the CPV cells is being improved continuously by improved design, growth and fabrication.
Experimentally, III-V compound semiconductor CPV cells have recently achieved efficiencies of more than 40%, making them the highest-efficiency PVs available in any technology. Further increase of efficiency for CPVs is the key for utilizing solar energy worldwide, say the researchers.
There are two main design approaches towards developing inorganic III-V CPV solar cells: multi-junction SCs (MJSCs) and intermediate-band solar cells (IBSCs). In MJSCs a number of semiconductor material junctions are connected in series, each designed to efficiently absorb a section of the solar spectrum appropriate to its bandgap with the largest-bandgap material placed at the front and the smallest-bandgap material placed at the back.
A single-junction SC has a maximum predicted efficiency of 30%, while a double-junction SC consisting of two optimized bandgaps increases the predicted efficiency to 41%. Much effort has gone into designing a number of MJSCs with an increased number of junctions. Intense effort is going into investigating materials to absorb near the peak of the spectrum around 1.0eV.
The research partners say they plan to employ 1.0eV-bandgap quantum dots as a solution for this. In short, a QD is one semiconductor embedded into another, arising from self-organized growth. QDs enable material combinations to be grown together that would not normally occur in a planar environment as strain is incorporated into the interface. This approach allows novel materials to be combined in a QD system – opening up new material combinations and allowing these materials to be grown on silicon using only a thin germanium sandwich layer.
In IBSCs, an intermediate energy band is introduced into the energy gap of the single semiconductor material junction, introducing three possible optical transitions. The photo-generated carriers in the intermediate level must only link to the host material through optical transitions for the IBSC to function correctly.
The IBSC with one intermediate band (IB) level is predicted to have ultra-high conversion efficiency (up to 63%) while increasing the number of IB levels up to 4 is predicted to increase efficiencies up to 80%. However, these high efficiencies are not observed experimentally. The researchers say that they will hence investigate using QD systems to make IBSCs.
