Alexander Mellor

Some advantages of intermediate band solar cells based on type II quantum dots

Unlike Type I, Type II quantum dots do not have hole bound states. This precludes that they invade the host semiconductor bandgap and prevents the reduction of voltage in intermediate band solar cells. It is proven here that the optical transition between the hole extended states and the intermediate bound states within the host bandgap is much stronger than in Type I quantum dots, increasing the current and making this structure attractive for manufacturing these cells.

The influence of quantum dot size on the sub-bandgap intraband photocurrent in intermediate band solar cells

The effect of quantum dot (QD) size on the performance of quantum dot intermediate band solar cells is investigated. A numerical model is used to calculate the bound state energy levels and the absorption coefficient of transitions from the ground state to all other states in the conduction band. Comparing with the current state of the art, strong absorption enhancements are found for smaller quantum dots, as well as a better positioning of the energy levels, which is expected to reduce thermal carrier escape. It is concluded that reducing the quantum dot size can increase sub-bandgap photocurrent and improve voltage preservation.

Requirements for a GaAsBi 1 eV sub-cell in a GaAs-based multi-junction solar cell

Multi-junction solar cells achieve high efficiency by stacking sub-cells of different bandgaps (typically GaInP/GaAs/Ge) resulting in efficiencies in excess of 40%. The efficiency can be improved by introducing a 1 eV absorber into the stack, either replacing Ge in a triple-junction configuration or on top of Ge in a quad-junction configuration. GaAs0.94Bi0.06 yields a direct-gap at 1 eV with only 0.7% strain on GaAs and the feasibility of the material has been demonstrated from GaAsBi photodetector devices. The relatively high absorption coefficient of GaAsBi suggests sufficient current can be generated to match the sub-cell photocurrent from the other sub-cells of a standard multi-junction solar cell. However, minority carrier transport and background doping levels place constraints on both p/n and p-i-n diode configurations. In the possible case of short minority carrier diffusion lengths we recommend the use of a p-i-n diode, and predict the material parameters that are necessary to achieve high efficiencies in a GaInP/GaAs/GaAsBi/Ge quad-junction cell.

Photon management structures for solar cells

Since micro- and nanostructures for photon management are of increasing importance in novel high-efficiency solar cell concepts, structuring techniques with up-scaling potential play a key role in their realization. Interference lithography and nanoimprint processes are presented as technologies for origination and replication of fine-tailored photonic structures on large areas. At first, these structure origination and replication technologies are presented in detail: With the interference pattern of two or more coherent waves, a wide variety of structures with feature sizes ranging from 100 nm to 100 μm can be generated in photoresist by interference lithography. Examples are linear gratings, crossed gratings, hexagonal structures, three dimensional photonic crystals or surface-relief diffusers. The strength of this technology is that homogeneous structures can be originated on areas of up to 1.2 x 1.2 m2. The structures in photoresist, the so-called master structures, can serve as an etching mask for a pattern transfer, as a template for infiltration with different materials or they can be replicated via electroplating and subsequent replication processes. Especially in combination with replication steps, the industrially feasible production of elaborate structures is possible. As a particularly interesting process, nanoimprint lithography (NIL) is described in detail. As a way towards industrial production, a roller NIL tool is presented. After the description of the basic technologies, three application examples for solar cells are presented with details about the design of the structures, the structuring processes, sample characterization and evaluation: (1) honeycomb structures for the front side texturization of multicrystalline silicon wafer solar cells, (2) diffractive rear side gratings for absorption enhancement in the spectral region near the band gap of silicon, and (3) plasmonic metal nanoparticle arrays manufactured by combined imprint and lift off processes.

A numerical study into the influence of quantum dot size on the sub-bandgap interband photocurrent in intermediate band solar cells

A numerical study is presented of the sub-bandgap interband photon absorption in quantum dot intermediate band solar cells. Absorption coefficients and photocurrent densities are calculated for the valence band to intermediate band transitions using a four-band k·p method. It is found that reducing the quantum dot width in the plane perpendicular to the growth direction increases the photocurrent from the valence band to the intermediate-band ground state if the fractional surface coverage of quantum dots is conserved. This provides a path to increase the sub-bandgap photocurrent in intermediate band solar cells.

Nanoparticle Scattering for Multijunction Solar Cells: The Tradeoff Between Absorption Enhancement and Transmission Loss

This paper contains a combined experimental and simulation study of the effect of Al and AlInP nanoparticles on the performance of multijunction (MJ) solar cells. In particular, we investigate oblique photon scattering by the nanoparticle arrays as a means of improving thinned subcells or those with low diffusion lengths, either inherently or due to radiation damage. Experimental results show the feasibility of integrating nanoparticle arrays into the antireflection coatings of commercial InGaP/InGaAs/Ge solar cells, and computational results show that nanoparticle arrays can improve the internal quantum efficiency via optical path length enhancement. However, a design that improves the external quantum efficiency of a state-of-the-art cell has not been found, despite the large parameter space studied. We show a clear tradeoff between oblique scattering and transmission loss and present design principles and insights into how improvements can be made.

GaAsBi: An alternative to InGaAs based multiple quantum well photovoltaics

A series of GaAsBi/GaAs multiple quantum well p-i-n diodes are characterized using IV, photocurrent and illuminated IV measurements. The results are compared to an InGaAs/GaAsP multiple quantum well control device of a design that has demonstrated excellent performance in triple junction photovoltaics. The extended absorption of the GaAsBi/GaAs devices, compared to that of the InGaAs/GaAsP device, suggests that GaAsBi/GaAs could present a viable alternative to InGaAs/GaAsP for quad junction photovoltaics.

Nanoparticle scattering for radiation-hard multi-junction space solar cells

We investigate how an array of nanoparticles embedded in the anti-reflection coating can improve the radiation hardness of multi-junction space solar cells. In space, high-energy electron and proton radiation reduces solar cell efficiency. Most notably, the InGaAs-middle-cell diffusion lengths are degraded, reducing photocurrent. Metal nanoparticles can scatter incident photons obliquely into the semiconductor, reducing their penetration depths and hence causing charge carriers to be photogenerated closer to the junctions. We postulate that this can improve radiation hardness by improving carrier collection at end of life. In this work, GaInP/InGaAs/Ge solar cells with optimised double-layer AlOx/TiOx ARCs were fabricated with regular arrays of Al nanoparticles deposited on top. An electro-optical simulation tool was also developed, and validated by comparison to the measured quantum efficiency and reflectance spectra, with good agreement. Using the validated simulation tool, we predict the photocurrent before and after high energy electron irradiation. The fraction of the initial photocurrent remaining after irradiation is predicted to improve for certain nanoparticle arrays. However, the overall photocurrent both before and after irradiation is reduced by the presence of the particles. Hence a net benefit is not predicted for the studied array dimensions.

Assessing the Nature of the Distribution of Localised States in Bulk GaAsBi

A comprehensive assessment of the nature of the distribution of sub band-gap energy states in bulk GaAsBi is presented using power and temperature dependent photoluminescence spectroscopy. The observation of a characteristic red-blue-red shift in the peak luminescence energy indicates the presence of short-range alloy disorder in the material. A decrease in the carrier localisation energy demonstrates the strong excitation power dependence of localised state behaviour and is attributed to the filling of energy states furthest from the valence band edge. Analysis of the photoluminescence lineshape at low temperature presents strong evidence for a Gaussian distribution of localised states that extends from the valence band edge. Furthermore, a rate model is employed to understand the non-uniform thermal quenching of the photoluminescence and indicates the presence of two Gaussian-like distributions making up the density of localised states. These components are attributed to the presence of microscopic fluctuations in Bi content, due to short-range alloy disorder across the GaAsBi layer, and the formation of Bi related point defects, resulting from low temperature growth.

Simple models for complex devices

Summary form only given. The search for higher efficiencies in photovoltaics has led us to develop increasingly complex solar cell architectures that rely on increasingly complex physical processes. The desire to overcome the Shockley-Queisser efficiency limit has caused us to consider stacks of junctions with cascading bandgaps, quantum dots and impurities for the creation of intermediate bands, amongst others. The need to improve in-coupling and absorption in thinner layers requires the development of surface textures and nanoparticles working either in the geometric- or wave-optical regimes, or a combination of both. Quantum wells have been employed to tune bandgaps or to induce angle-selective luminescent emission. More recently, photonic methods have been used to control the black-body emission of solar cells and thus influence their operating temperature. Understanding these processes, requires a detailed knowledge of the paths incident photons take in the device, complex generation and recombination mechanisms (sometimes involving multiple sequential transitions), carrier drift and diffusion, and the paths of emitted photons (either luminescent or incandescent). This sounds like a job for a supercomputer. However, with a few carefully made assumptions, accurate, predictive and insightful models can be developed that allow calculations to be performed on a laptop computer. We will show how these models can help to develop nano-photonic space solar cells that are more radiation hard; to better understand quantum-dot intermediate-band solar cells and improve them using light trapping; and to understand and control black-body emission in silicon solar cells.