Tomos Thomas

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.

InGaAs/GaAsP strain balanced multi-quantum wires grown on misoriented GaAs substrates for high efficiency solar cells

Quantum wires (QWRs) form naturally when growing strain balanced InGaAs/GaAsP multi-quantum wells (MQW) on GaAs [100] 6° misoriented substrates under the usual growth conditions. The presence of wires instead of wells could have several unexpected consequences for the performance of the MQW solar cells, both positive and negative, that need to be assessed to achieve high conversion efficiencies. In this letter, we study QWR properties from the point of view of their performance as solar cells by means of transmission electron microscopy, time resolved photoluminescence and external quantum efficiency (EQE) using polarised light. We find that these QWRs have longer lifetimes than nominally identical QWs grown on exact [100] GaAs substrates, of up to 1 μs, at any level of illumination. We attribute this effect to an asymmetric carrier escape from the nanostructures leading to a strong 1D-photo-charging, keeping electrons confined along the wire and holes in the barriers. In principle, these extended lifetim...

Practical Limits of Multijunction Solar Cell Performance Enhancement From Radiative Coupling Considering Realistic Spectral Conditions

III-V multijunction solar cells (MJSCs) operate close to the radiative limit under solar concentration. In this regime, radiative losses from the semiconductor material in one junction of the solar cell can be absorbed by a subsequent junction, thereby transferring charge from one subcell to another. Under blue-rich solar spectra, radiative coupling can improve the electrical performance by lifting constraints imposed by a series connection of subcells. We calculate the practical limit of performance enhancement due to the radiative coupling effect for MJSCs under a wide range of atmospheric conditions encountered in potential sites for concentrator photovoltaic systems. Three-junction and four-junction solar cells with current matched and current mismatched designs under the AM1.5D spectrum were considered. Under realistic atmospheric conditions, the relative enhancement to power due to radiative coupling is found to be 1% or less for current-matched triple-junction solar cells. Enhancement of up to 21% can be expected for noncurrent-matched quad-junction devices. The energy yield improvement over an annual period is shown to be up to 5% for the best combinations of devices and sites.

GaNAsSb 1-eV solar cells for use in lattice-matched multi-junction architectures

Photovoltaic devices made from a dilute nitride material, GaAsNSb, with band-gap close to 1eV have been developed and characterised. Homojunction devices of n-on-p and p-on-n type as well as an n-on-p GaAs/GaNAsSb heterojunction have been grown by molecular beam epitaxy. Optical and electrical characteristics are reported and a one-dimensional drift-diffusion model of internal quantum efficiency is used to estimate minority carrier diffusion lengths. The GaAs/GaNAsSb heterostructure produced AM1.5G short-circuit current of 23.6 mA/cm2, open-circuit voltage of 0.44V and fill factor of 67%. The model suggests that this performance is limited by both diffusion length and surface recombination.

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.

Demonstrating Dilute-Tin Alloy SiGeSn for Use in Multijunction Photovoltaics: Single- and Multijunction Solar Cells With a 1.0-eV SiGeSn Junction

SiGeSn ternary alloys offer a means to fabricate a 1.0-eV subcell junction for inclusion in a multijunction solar cell. The main advantage of the SiGeSn alloy is a tuneable bandgap energy and variable lattice parameter, enabling the material to be integrated into the existing lattice-matched multijunction architectures. Recent growth, structural, optical, and device results from SiGeSn material, with energy gaps in the vicinity of 1.0 eV and lattice matched to Ge substrates, are presented. An all lattice-matched InGaP/InGaAs/SiGeSn triple-junction cell is presented and compared with a conventional InGaP/InGaAs/Ge solar cell. Comparable short-circuit current values of 13.9 mA/cm2 are obtained for both devices under the AM1.5G spectrum, whereas the open-circuit voltage and fill factor are reduced in the device with the SiGeSn subcell. Peak external quantum efficiency in the SiGeSn single junction in excess of 80% is realized, placing a lower limit on the base minority hole diffusion length of 5

Single and multi-junction solar cells utilizing a 1.0 eV SiGeSn junction

\mu

Advances towards 4J lattice-matched including dilute nitride subcell for terrestrial and space applications

m with surface recombination velocities in close agreement to those found in bulk Ge material.

Addressing reflectivity losses in multi-junction solar cells to achieve 50% power conversion efficiency

Multi-junction photovoltaic technologies lead the way to achieving ultra-high power conversion efficiencies for both space based and terrestrial concentrator applications. However, realizing a lattice matched quad-junction solar cell remains challenging due to a lack of suitable material systems able to achieve the elusive middle sub-cell band-gap energy of 1.0 eV. In this work, we present a potential candidate material to achieve this 1.0 eV sub-cell, the group–IV ternary alloy SiGeSn. Initial simulations of triple and quadruple junction solar cell designs show that this novel material system has the potential to reach efficiencies in excess of 45 and 48% respectively under concentrated illumination. We report on the electrical characterization of both single and triple junction devices based on a 1.0 eV SiGeSn junction. It is shown that the external quantum efficiency performance of both SiGeSn devices is promising given the initial state of development of the alloy, with a fitted minority electron diff...

GaAsBi: An alternative to InGaAs based multiple quantum well photovoltaics

Recent advances on the development of a 4J lattice-matched dilute nitride solar cell for terrestrial and space applications are described. Modeling of the solar cell is carried out using a drift-diffusion model and material parameters extracted from ad hoc electro-optical characterization resulting in an efficiency prediction of 47% for concentrations of 1000 suns AM1.5d G173 spectrum and 33% for 1× AM0. First experimental solar cell results of a dual-junction GaNAsSb/Ge solar cell and a triple-junction GaInP/Ga(In)As/GaNAsSb components of the full 4-Junction are shown.