Ngai Lam Alvin Chan

Design of an achievable, all lattice-matched multijunction solar cell using InGaAlAsSb

A design for a realistically achievable, multijunction solar cell based on all lattice-matched materials with >50% projected efficiencies under concentration is presented. Using quaternary materials such as InAlAsSb and InGaAlAs at stochiometries lattice-matched to InP substrates, direct bandgaps ranging from 0.74eV up to ∼1.8eV, ideal for solar energy conversion, can be achieved. In addition, multi-quantum well structures are used to reduce the band-gap further to <0.7 eV. A triple-junction (3J) solar cell using these materials is described, and in-depth modeling results are presented showing realistically achievable efficiencies of AM1.5D 500X of η ∼ 53% and AM0 1 Sun of η∼ 37%.

Modeling and analysis of multijunction solar cells

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.

Optimal Bandgap Combinations—Does Material Quality Matter?

The balance of photogeneration and recombination gives rise to an optimum bandgap for any solar cell. The radiative limit represents the lowest permissible level of recombination in a solar cell and, therefore, places an upper limit on the voltage that can be attained. Introducing additional nonradiative recombination results in a loss in voltage that can only be compensated for by moving to higher bandgaps. Consequently, the optimal bandgap for solar energy conversion will rise with increasing nonradiative recombination rate. This balance was recognized by Shockley and Queisser for single-junction solar cells and is here extended to multijunction solar cells. A rise in optimal bandgaps has been observed in simulated single-, double-, and triple-junction devices as nonradiative recombination increases. Optimal bandgaps between excellent and poor diode quality devices are shown to differ by 100s of meV under 1-sun illumination with both terrestrial and extraterrestrial spectra but exhibit no significant change at high concentration due to the dominance of the radiative component in the recombination dynamics.

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.

Quantifying the impact of individual atmospheric parameters on CPV system power and energy yield

The performance of concentrator photovoltaic systems can be characterized by the power output under reference conditions and the output energy yield under realistic solar illumination. The impact of air mass, aerosols and precipitable water on the rated power of a particular concentrator system design has been quantified, with aerosol becoming important at certain locations. Time-resolved simulations under cloudless-skies are used to estimate the energy yields harvested by a real concentrator system at a range of locations when levels of knowledge regarding atmospheric parameters are limited. We demonstrate an uncertainty of up to 75% in energy yield between the simplest and most complex cases can occur over an annual period for the same concentrator system design.

Variation in spectral irradiance and the consequences for multi-junction concentrator photovoltaic systems

The most fundamental figure of merit for a solar collector is its power rating under a standard solar spectrum. For concentrator systems that employ highly efficient multi-junction solar cells, the rating of the panel becomes sensitive to the spectrum of the direct beam sunlight. By considering typical spectral conditions at a measurement site, we show that the power rating can vary by up to 16%. Further, we consider to what extent a single reference spectrum can be used to characterize the irradiance at a particular location. Electrical energy yields produced using both synthesized and standard reference spectra are compared, with standardized references shown to be unsuited for accurate energy yield predictions under realistic spectral conditions.

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.

Performance enhancement from radiative coupling considering realistic spectral conditions for optimal and sub-optimal bandgap multijunction solar cells

We calculate the practical limit of performance enhancement due to the radiative coupling effect for multi-junction solar cells (MJSCs) under a wide range of atmospheric conditions encountered in potential sites for concentrator photovoltaic (CPV) systems. MJSCs with both optimal and sub-optimal band gap combinations with respect to the AM1.5D spectrum were considered. Under realistic atmospheric conditions the relative enhancement to power due to radiative coupling is found to be less than 1% for current matched triple-junction solar cells, whereas enhancement of up to 21% can be expected for non-current matched quad-junction devices. The energy yield improvement over an annual period is calculated.