Our Research

Multiple Threshold Solar Cells

For a single junction solar cell there is a savage trade-off between the loss in excess energy of photons absorbed above the band-gap and the loss of photons that are not absorbed; shown above as the thermalisation and below-Eg losses respectively.  This limitation can be overcome by introducing additional absorption thresholds into the device, effectively sorting photons by energy to absorbers that can more optimally convert the photon energy into electrical power.  

Multi-junction solar cells

By far the most common approach for achieving high efficiency photovoltaic power conversion is the multi-junction solar cell, where several individual sub-cells are stacked and tailored to absorb a specific part of the solar spectrum.  By cascading the band-gap energy through the device, photons are absorbed selectively as light propagates through the material.  The multi-junction solar cell approach presents a conceptually simple path to high efficiency power conversion and is used in a triple junction form for all solar cells used on commercial satellites and terrestrial concentrator systems.  However, to improve upon present technology it is necessary to introduce new semiconductor materials into the stack.  The QPV group pioneered an approach using strain-balance quantum wells, which enable us to optimally match our component junctions to the solar spectrum.  In 2009 a spin-out company from the research group 'Quantasol' set a new world record using this quantum well approach, achieving what was then the highest efficiency from a single junction solar cell of 28.3% under solar concentration. Since then the group has tackled the problem of low-gap semiconductor materials required to approach power conversion efficiencies of 50%.  These include dilute nitride materials, GaAsBi and SiGeSn.  We are also tackling the problem of light trapping in multi-junction solar cells, where light is selectively confined within the solar cell device enabling a very radiation tolerant space solar cell to be made. 

Intermediate Band Solar Cell

The intermediate band solar cell supports similar optical transitions as in a triple junction solar cell yet does so within a single material.  If achieved in a highly manufacturable material this approach could provide a means for attaining high efficiency at low cost.  There have been many attempts at implementing the device since its the inception  in 1998 by Luque and Marti which we reviewed a recently.  To help guide experimental work, we established the experimental signature for an intermediate band solar cell material, showing that it would show a highly unusual PL signature and support strong up-conversion of light, work that later led to us proposing a molecular intermediate band solar cell which has now been demonstrated experimentally.  In addition we established the need for a relaxation stage in realistic intermediate band solar cells, analogous to a ratchet. We showed that a ratchet device fundamentally improves the efficiency of an IB cell under 1-sun illumination and yields an efficiency increase even when absorptivity is sub-optimal. We demonstrated this experimentally using a series of coupled quantum wells to establish a spatial-ratchet and have also demonstrated efficient up-conversion systems that correspond to a spin-ratchet.  

Thermal Photovoltaic Systems.

The thermalisation loss shown above indicates that a great deal of absorbed energy will be dissipated as heat.  The QPV group has tackled this problem from two directions, the first very immediate approach is to transfer the heat supplied by a solar cell towards a useful purpose in a hybrid PV-Thermal collector.  The second is to prevent the thermalisation loss in the first place in a hot carrier solar cell.

Hybrid PV-Thermal Collectors

The combination of solar heat and electricity provides a very rapid means for attaining system efficiencies of 70% or higher.  The majority of this energy is supplied as heat as opposed to electricity and in conventional systems, it is delivered at relatively low temperatures.  Working with the British Company Naked Energy and the Clean Process Engineering group at Imperial College, the QPV group is working to both raise the fluid temperature from the PV-T collector and  mitigate the electrical losses that follow from operation at elevated temperature.  We have established that radiative loss forms an important heat loss in PV-T collectors, determined the thermal emissivity of conventional silicon photovoltaic solar cells and demonstrated a low-emissivity TCO coating on a silicon solar cell that has resulted in a 10% gain in thermal efficiency. 

Hot carrier solar cell.

The most ambitious concept in photovoltaic power conversion is the hot carrier solar cell.  Originally formulated by Ross & Nozik in 1981 it considers the situation where carriers are equilibrate thermally among themselves and do not interact with lattice phonons.   The concept was refined by Wurfel in 1997 where impact ionization and Auger recombination ensure that no electrochemical potential exists in the hot absorber layer.   Our first contribution to this area was to propose an optical means to transport the energy from the hot carrier population, exploiting the phenomenon of hot luminescence.  We followed this up with the a demonstration of a hot carrier photocurrent from a quantum well solar cell.  Since a quantum well device can only ever be narrow band, our work in this area has now moved to consider ultra-thin broadband absorbers. 

Computer models

The QPV group has over the years developed a number of computer models which we have now made available in a Python 3 package called Solcore.  A description of the package was published in 2018 describing the full capability of the code but it is generally well suited to III-V solar cells with the particular ability to simulate the properties of quantum well layers and optical nanostructures.  The code can be installed like any Python package; more information on installation and use is available on the Solcore website.