Freeform optics has great potential for delivering highly effective solar concentrators and lighting systems, but in some cases it can be challenging to implement. A numerical method is described for calculating 3D flowline concentrator shapes in a way that provides complete freedom over the specification of arbitrary source and receiver objects. This lends the approach to a range of practical design problems involving asymmetric systems, non-lambertian and extended light sources. The method reproduces the hyperbolic and hyperparabolic concentrator geometries identified in the literature, operating close to the thermodynamic limit of concentration. A practical example is given in the optimisation of a secondary concentrator for a concentrator photovoltaic array receiving light from a field of heliostats. The secondary improves the overall capture efficiency of the photovoltaic receiver at noon, and is expected to deliver further improvement at other times of day.
We investigate the integration of Al nanoparticle arrays into the anti-reflection coatings (ARCs) of commercial triple-junction GaInP/ In0.01GaAs /Ge space solar cells, and study their effect on the radiation-hardness. It is postulated that the presence of nanoparticle arrays can improve the radiation-hardness of space solar cells by scattering incident photons obliquely into the device, causing charger carriers to be photogenerated closer to the junction, and hence improving the carrier collection efficiency in the irradiation-damaged subcells. The Al nanoparticle arrays were successfully embedded in the ARCs, over large areas, using nanoimprint lithography: a replication technique with the potential for high throughput and low cost. Irradiation testing showed that the presence of the nanoparticles did not improve the radiation-hardness of the solar cells, so the investigated structure has proven not to be ideal in this context. Nonetheless, this paper reports on the details and results of the nanofabrication to inform about future integration of alternative light-scattering structures into multi-junction solar cells or other optoelectronic devices.
Over the last couple of decades, there has been an intense research on strain balanced semiconductor quantum wells (QW) to increase the efficiency of multi-junction solar (MJ) solar cells grown monolithically on germanium. So far, the most successful application of QWs have required just to tailor a few tens of nanometers the absorption edge of a given subcell in order to reach the optimum spectral position. However, the demand for higher efficiency devices requiring 3, 4 or more junctions, represents a major difference in the challenges QWs must face: tailoring the absorption edge of a host material is not enough, but a complete new device, absorbing light in a different spectral region, must be designed. Among the most important issues to solve is the need for an optically thick structure to absorb enough light while keeping excellent carrier extraction using highly strained materials. Improvement of the growth techniques, smarter device designs - involving superlattices and shifted QWs, for example - or the use of quantum wires rather than QWs, have proven to be very effective steps towards high efficient MJ solar cells based on nanostructures in the last couple of years. But more is to be done to reach the target performances. This work discusses all these challenges, the limitations they represent and the different approaches that are being used to overcome them.
Hot carrier solar cells have the promise to increase photovoltaic conversion efficiency beyond the Shockley-Quiesser limit and towards the thermodynamic maximum of 85%. The concept relies on the ability to extract photo-generated electrons from an absorber region faster than they can lose energy to the lattice in a process termed thermalisation. We have previously presented a realization of such a cell under limited operating conditions, in particular at low temperature, for narrowband illumination and with low total absorption of light. In this work we present the idea of a metallic absorber to address some of these limitations and show how such an absorber is a promising candidate to realize the hot carrier solar cell. In addition to a theoretical justification of the metallic hot carrier solar cell, we show device fabrication and experimental current-voltage characteristics of an initial cell, showing absorption of light in a thin-film metal region and a photo-current driven by this absorption.
For high band gap solar cells, organic molecule based upconverter materials are promising to reduce transmission losses of photons with energies below the absorption threshold. We investigate the approach of embedding the organic upconverter DPA:PtOEP directly into each second layer of a Bragg stack to achieve an enhancement of upconversion performance. The two major effects that influence the upconversion process within the Bragg stack are simulated based on experimentally determined input parameters. The locally increased irradiance is simulated using the scattering matrix method. The variation of the density of photon states is obtained from calculations of the eigenmodes of the photonic crystal using the plane wave expansion method. A relative irradiance enhancement of 3.23 has been found for a Bragg stack of 31 layers including λ/8-layers on both sides. For suppressing the loss mechanism of direct sensitizer triplet decay via variations of the density of photon states, a different design of the Bragg stack is necessary than for maximum irradiance enhancement. In order to find the optimum design to increase upconversion quantum yield, both simulation results need to be coupled in a rate-equation model. The irradiance enhancement found in our simulation is significantly higher than the one found in the simulation of a grating-waveguide structure, which achieved an increase of upconversion quantum yield by a factor of 1.8. Thus, the Bragg structure is very promising for upconversion quantum yield enhancement.
The high conversion efficiencies demonstrated by multi-junction solar cells over the past three decades have made them indispensable for use in space and are very attractive for terrestrial concentrator applications. The multi-junction technology consistently displays efficiency values in excess of 30%, with record highs of 37.8% under 1 sun conditions and over 44% under concentration. However, as material quality in current III-V multi-junction technology reaches practical limits, more sophisticated structures will be required to further improve on these efficiency values. In a collaborative effort amongst several institutions we have developed a novel multi-junction solar cell design that has the potential to reach the 50% conversion efficiency value. Our design consists of a three junction cell grown on InP substrates which achieves the optimal bandgaps for solar energy conversion using lattice matched materials. In this work, we present the progress in the different subcells comprising this multi-junction structure. For the top cell, InAlAsSb quaternary material is studied. For the middle, InGaAlAs and InGaAsP materials and devices are considered and for the bottom, a multi-quantum well structure lattice matched to InP for fine bandgap tunability for placement in an InGaAs cell is demonstrated.
In this work, we use an analytical drift-diffusion model, coupled with detailed carrier transport and minority carrier lifetime estimates, to make realistic predictions of the conversion efficiency of InP-based triple junction cells. We evaluate the possible strategies for overcoming the problematic top cell for the triple junction, and make comparisons of the more realistic charge transport model with incumbent technologies grown on Ge or GaAs substrates.
Photovoltaics (PV) offer a solution for the development of sustainable energy sources, relying on the sheer
abundance of sunlight: More sunlight falls on the Earth’s surface in one hour than is required by its inhabitants in a
year. However, it is imperative to manage the wide distribution of photon energies available in order to generate
more cost efficient PV devices because single threshold PV devices are fundamentally limited to a maximum
conversion efficiency, the Shockley-Queisser (SQ) limit. Recent progress has enabled the production of c-Si cells
with efficiencies as high as 25%,<sup>1</sup> close to the limiting efficiency of ∼30%. But these cells are rather expensive, and ultimately the cost of energy is determined by the ratio of system cost and efficiency of the PV device. A strategy to radically decrease this ratio is to circumvent the SQ limit in cheaper, second generation PV devices. One promising approach is the use of hydrogenated amorphous silicon (a-Si:H), where film thicknesses on the order of several 100nm are sufficient. Unfortunately, the optical threshold of a-Si:H is rather high (1.7-1.8 eV) and the material
suffers from light-induced degradation. Thinner absorber layers in a-Si:H devices are generally more stable than
thicker films due to the better charge carrier extraction, but at the expense of reduced conversion efficiencies,
especially in the red part of the solar spectrum (absorption losses). Hence for higher bandgap materials, which
includes a-Si as well as organic and dye-sensitized cells, the major loss mechanism is the inability to harvest low
Hot carrier solar cells have a fundamental efficiency limit well in excess of single junction devices. Developing a
hot carrier absorber material, which exhibits sufficiently slow carrier cooling to maintain a hot carrier population
under realistic levels of solar concentration is a key challenge in developing real-world hot carrier devices.
We propose strain-balanced In<sub>0.25</sub>GaAs/GaAsP<sub>0.33</sub> quantum wells as a suitable absorber material and present
continuous-wave photoluminescence spectroscopy of this structure. Samples were optimised with deep wells and
the GaAs surface buffer layer was reduced in thickness to maximise photon absorption in the well region. The
effect of well thickness on carrier distribution temperature was also investigated. An enhanced hot carrier effect
was observed in the optimised structures and a hot carrier distribution temperature was measured in the thick
well (14 nm) sample under photon flux density equivalent to 1000 Suns concentration.
This work uses simulations to predict the performance of InAlAsSb solar cells for use as the top cell of triple
junction cells lattice matched to InP. The InP-based material system has the potential to achieve extremely high
efficiencies due the availability of lattice matched materials close to the ideal bandgaps for solar energy conversion.
The band-parameters, optical properties and minority carrier transport properties are modeled based on literature
data for the InAlAsSb quaternary, and an analytical drift-diffusion model is used to realistically predict the solar cell
The modeling of high efficiency, multijunction (MJ) solar cells away from the radiative limit is presented. In the model,
we quantify the effect of non-radiative recombination by using radiative efficiency as a figure of merit to extract realistic
values of performance under different spectral conditions. This approach represents a deviation from the traditional
detailed balance approximation, where losses in the device are assumed to occur purely through radiative recombination.
For lattice matched multijunction solar cells, the model predicts efficiency values of 37.1% for AM0 conditions and
52.8% under AM1.5D at 1 sun and 500X, respectively. In addition to the theoretical study, we present an experimental
approach to achieving these high efficiencies by implementing a lattice matched triple junction (TJ) solar cell grown on
InP substrates. The projected efficiencies of this approach are compared to results for the state of the art inverted-metamorphic
(IMM) technology. We account for the effect of metamorphic junctions, essential in IMM technology, by
employing reduced radiative efficiencies as derived from recent data. We show that high efficiencies, comparable to
current GaAs-based MJ technology, can be accomplished without any relaxed layers for growth on InP, and derive the
optimum energy gaps, material alloys, and quantum-well structures necessary to realize them.
Intrinsic loss mechanisms are quantified for a single junction device under one sun illumination. Thermalisation,
below Eg and Boltzmann losses are shown to be the dominant loss mechanisms in a device with Eg = 1.31eV ,
accounting for 30%, 25% and 9% of the incident solar radiation respectively. Alternative device designs which
target these dominant intrinsic losses are considered. Concentrator (and restricted emission) devices target
Boltzmann loss. This loss mechanism is shown to have a logarithmic relationship with concentration and as
such, a small increase in absorption solid angle equates to a large increase in fundamental limiting efficiency. A
multi-junction device resolves the mismatch between the broad solar spectrum and single threshold absorption,
and thus targets below Eg and thermalisation losses. A greater number of junctions allows the device absorption
profile to better match the solar spectrum, increasing device efficiency. Boltzmann loss slightly increases with
junction number and as such, concentration will be proportionally more effective at increasing efficiency in a
multi-junction device. A hot carrier device targets thermalisation loss. This loss mechanism is eliminated in the
impact ionisation model used in this paper, allowing for enhanced device efficiency.
The on-screen and audio presentation of this paper can be played by clicking the multimedia PDF link at the bottom right hand of this page.
We have investigated a photochemical up-conversion system comprising a molecular mixture of a palladium
porphyrin to harvest light, and a polycyclic aromatic hydrocarbon to emit light. The energy of harvested
photons is stored as molecular triplet states which then annihilate to bring about up-converted fluorescence.
The limiting efficiency of such triplet-triplet annihilation up-conversion has been believed to be 11% for some
time. However, by rigorously investigating the kinetics of delayed fluorescence following pulsed excitation, we
demonstrate instantaneous annihilation efficiencies exceeding 40%, and limiting efficiencies for the current system
of ≈60%. We attribute the high efficiencies obtained to the electronic structure of the emitting molecule, which
exhibits an exceptionally high T<sub>2</sub> molecular state. We utilize the kinetic data obtained to model an up-converting
layer irradiated with broadband sunlight, finding that ≈3% efficiencies can be obtained with the current system,
with this improving dramatically upon optimization of various parameters.
It is possible to tailor the band gap of the strain-balanced quantum well solar cell to match the local solar spectral
conditions by altering the quantum well depth. This has led to a recent single-junction world-record efficiency of 28.3%,
as well as giving advantages for current matching in multi-junction solar cells. Radiative recombination is the dominant
loss mechanism for the strain-balanced quantum well solar cell, so practical improvements focus on techniques for light
management in the cell, such as enhancing the optical path length with epitaxial mirrors. Furthermore, the compressive
strain in the quantum wells suppresses emission into TM-propagating modes, reducing the overall optical loss and
increasing the cell efficiency. As biaxial strain can only be engineered into a cell on the nanoscale, quantum well solar
cells are seen to have a fundamental efficiency advantage over bulk semiconductor cells.
Incorporating quantum wells into multi-junction III-V solar cells provides a means of adjusting the absorption
edge of the component junctions. Further, by using alternating compressive and tensile materials, a strain-balanced
stack of quantum well and barrier layers can be grown, defect free, providing absorption-edge / lattice
parameter combinations that are inaccessible using bulk materials. Incomplete absorption in the quantum wells
has been addressed using a distributed Bragg reflector, extending the optical path length through the cell and
enabling photon recycling to take place. State of the art single-junction quantum well solar cells have now
reached an efficiency of 27.3% under 500X solar concentration and are projected to reach 34% in a double