Kan-Hua Lee

Bridging electromagnetic and carrier transport calculations for three-dimensional modelling of plasmonic solar cells

We report three-dimensional modelling of plasmonic solar cells in which electromagnetic simulation is directly linked to carrier transport calculations. To date, descriptions of plasmonic solar cells have only involved electromagnetic modelling without realistic assumptions about carrier transport, and we found that this leads to considerable discrepancies in behaviour particularly for devices based on materials with low carrier mobility. Enhanced light absorption and improved electronic response arising from plasmonic nanoparticle arrays on the solar cell surface are observed, in good agreement with previous experiments. The complete three-dimensional modelling provides a means to design plasmonic solar cells accurately with a thorough understanding of the plasmonic interaction with a photovoltaic device.

Loss mitigation in plasmonic solar cells: aluminium nanoparticles for broadband photocurrent enhancements in GaAs photodiodes

We illustrate the important trade-off between far-field scattering effects, which have the potential to provide increased optical path length over broad bands, and parasitic absorption due to the excitation of localized surface plasmon resonances in metal nanoparticle arrays. Via detailed comparison of photocurrent enhancements given by Au, Ag and Al nanostructures on thin-film GaAs devices we reveal that parasitic losses can be mitigated through a careful choice of scattering medium. Absorption at the plasmon resonance in Au and Ag structures occurs in the visible spectrum, impairing device performance. In contrast, exploiting Al nanoparticle arrays results in a blue shift of the resonance, enabling the first demonstration of truly broadband plasmon enhanced photocurrent and a 22% integrated efficiency enhancement.

InAs/GaAs quantum dot solar cell with an AlAs cap layer

We report the effects of the deposition of an AlAs cap layer (CL) over InAs quantum dots (QDs) on the performance of QD solar cells (QDSCs). The growth of AlAs CL over InAs QDs led to the elimination of the wetting layer absorption and hence the enhancement of the open-current voltage, Voc, of a 20-layer InAs/GaAs QDSC from 0.69 V to 0.79 V. Despite a slight reduction in short-circuit current, Jsc, for the QDSC with AlAs CL, the enhancement of the Voc is enough to ensure that its efficiency is higher than the QDSC without AlAs CL.

Demonstration of Photon Coupling in Dual Multiple-Quantum-Well Solar Cells

Multiple-quantum-well (MQW) top cells can enhance the performance of multi-junction solar cells since the absorption edge of top and middle subcells can be tuned with the MQWs to maximize the efficiency. The radiative dominance of MQW top cells can enhance photon coupling, which can potentially reduce the spectral sensitivity of the device and, thus, raise the energy harvest. We present experimental results on photon coupling in dual-junction cells with GaInP top cells containing GaInAsP quantum wells along with theoretical calculation based on a detailed balance model. It is observed that at high concentration, approximately 50% of the dark current of an MQW top cell is transferred to the photocurrent of the cell in the bottom, which is much higher than any previously reported values.

Extensible modelling framework for nanostructured III-V solar cells

The use of nanostructures has been shown to provide practical performance enhancements to high-efficiency III-V based solar cells by permitting sub-bandgap tuneable absorption. Nanostructures present a fertile ground for new solar cell technologies, and an improved understanding of fundamental processes may even lead to functional intermediate band and hot-carrier devices. As the fundamental processes occurring in nanostructured solar cells are complex and not easily observable, the study of such devices often requires the analysis of data derived from experimental characterisation techniques using computer models. Models exist for many individual aspects of these nanostructured solar cells, but as yet no comprehensive modelling solution exists. We report on our progress to produce an extendable abstract modelling framework written in the high-level programming language Python. The framework is intended for deployment both as back-end to a variety of interfaces for specialised modelling purposes, and as a library of methods and classes for use at source-code level, allowing adaptation to a wide variety of research problems. Significant code abstraction, such as sequestering complex materials parameterisation behind a simple material object allows simple scripts to do complex work. Modules underway cover several device simulation tiers, including fundamental processes such as quantum well and dot absorption and recombination, as well as device level simulations such as spatial bias mapping using equivalent circuits and multijunction IV characteristics. These simulations correlate with and derive experimental data from characterisation techniques including spatially and temporally resolved electro- and photoluminescence spectroscopy, fourier-transform infrared spectroscopy, and others.

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.

Multiple quantum well top cells for multijunction concentrator solar cells

High efficiency quantum well GaAs solar cells have been successfully applied in commercial multijunction concentrator cells to increase the absorption in the infrared and provide variability of the absorption edge to optimise energy harvesting. Multiple quantum well (MQW) top cells can further improve the performance of multijunction solar cells since the absorption edge of top and middle subcells can be tuned with the MQWs to maximize the efficiency. Also, our simulations show that photon coupling resulting from the radiative dominance of the MQW top cell can make the multijunction cell less sensitive to variations in the incoming spectrum, thus further improving energy harvesting. New results on the characterisation of a novel MQW top cell will be presented along with electro- and photo-luminescence studies relevant to the photonic coupling.

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.

Dual-junction solar cells with multiple-quantum-well top cells

Multiple-quantum-wells (MQW) in multijunction solar cells allow the absorption edges of the subcells to be tuned individually to maximize the energy yields. In this paper, we present the properties a quantum well material for InGaP quantum well solar cells and the performances of the dual-junction dual-MQW devices. We demonstrate that this dual-junction device can achieve 28% efficiency at 100 suns, which can be further improved by reducing the series resistance in the device.

Investigation of Carrier Recombination Dynamics of InGaP/InGaAsP Multiple Quantum Wells for Solar Cells via Photoluminescence

The carrier recombination dynamics of InGaP/InGaAsP quantum wells is reported for the first time. By studying the photoluminescence (PL) and time-resolved PL decay of InGaP/InGaAsP multiple-quantum-well (MQW) heterostructure samples, it is demonstrated that InGaP/InGaAsP MQWs have very low nonradiative recombination rate and high radiative efficiency compared with the control InGaP sample. Along with the analyses of PL emission spectrum and external quantum efficiencies, it suggests that this is due to small confinement potentials in the conduction band but high confinement potentials in the valence band. These results explain several features found in InGaP/InGaAsP MQW solar cells previously.