Finite-Difference Time-Domain Simulation of Photovoltaic Structures using a Graphical User Interface for MEEP

There is a large and growing need for accurate full-wave optical simulations of complex systems such as photovoltaic (PV) cells, particularly at the nanoscale. A finite-difference time-domain tool known as MEEP offers this capability in principle, through C++ libraries and the Scheme programming language. For expert users, this approach has been quite successful, but there is also great interest from new and less frequent users in starting to use MEEP. In order to facilitate this process, we have developed a graphical user interface (GUI) for MEEP, geared toward simulation of 2D and 3D PV cell geometries, freely available through a Java-based web browser on nanoHUB.org. A software toolkit called Rappture was used to develop our GUI. The tool collects input from the Rappture interface, and uses it to create a Scheme control file to run MEEP on the back end as before. It outputs images of the PV cell structure being simulated; graphs of the transmission, reflection, and absorption; as well as an animation of the fields propagating through the PV cell. This tool was subsequently used to examine and optimize the properties of surface texturing for different classes of PV cells. In conclusion, our new, fully interactive tool saves time and effort for researchers investigating nanophotonic structures, and is freely available to the general public

Performance of TF-VLS Grown InP Photovoltaic Cells

A grand challenge of photovoltaics (PV) is to find materials offering a promising combination of low costs and high efficiencies. While III-V material-based PV cells have set many world records, often their cost is much greater than other commercial cells. To help address this gap, thin-film vapor-liquid-solid (TF-VLS) grown Indium Phosphide (InP) PV cells have recently been developed, which both eliminate a key source of high costs and offer a direct bandgap of 1.34eV with potential to approach maximum theoretical efficiencies. However, the unanticipated phenomenon of open circuit voltage (Voc) degradation has prevented TF-VLS grown InP PV cells from achieving their theoretical efficiencies, which appears to be caused by effective bandgap narrowing in certain portions of the cells. To address this issue, we have developed a 3D model for these PV cells in Xyce, a SPICE-like free circuit modeling software. Our model quantifies lateral variation of TF-VLS grown cells observed in photoluminescence (PL) images with two sets of unit cell parameters. It turns out that the PL intensity correlates to PV cells of different bandgaps (Eg). Based on user-defined cutoffs, we are able to categorize the expected bandgap and reduced bandgap cells. With the addition of an appropriate shunt resistance, it is possible to explain most of current-voltage relationship with this model. Finally, we are building a web-enabled tool to allow users to upload their own heterogeneous PV cell data into our model, using a graphical user interface on nanoHUB.org, an open-access science gateway for cloud-based simulation tools and resources for research and education in nanoscale science and technology

Simulating Nanoscale Optics in Photovoltaics with the S-Matrix Method

In the push to build high-efficiency solar cells with less materials usage, thin-film solar cells have attracted an increasing amount of interest. Thin films are particularly attractive if they could exhibit light trapping and photon recycling capabilities exceeding those of traditional wafer-based cells. Recent work by Alta Devices demonstrating a record single-junction efficiency of 28.8% with a gallium arsenide thin film cell shows the potential. However, most existing simulation tools do not handle these properties well -- particularly photon recycling. In this work, we develop an improved solar cell simulation tool to accurately predict thin-film performance. It is based on a fast layered wave-optical module coupled to a drift-diffusion electronic model. The S-matrix method was used to solve for light absorption at any point in a solar cell given the depths and refractive indices of each layer; these results are then used to calculate initial and recycled photon generation profiles, and coupled self-consistently to an existing solar cell simulator, ADEPT 2.0, available on nanoHUB.org -- an open-access science gateway for cloud-based simulation tools and resources in nanoscale science and technology. In general, this improved simulation technique produced more accurate carrier distributions and a higher open-circuit voltage than predicted by standard models, which was also observed in experiment. Preliminary results are presented that indicate this approach is also capable of accurately modeling the effects of anti-reflection coatings (ARCs) and various back reflectors. This new capability will be made available through a revised version of ADEPT 2.0

Thermophotovoltaic System Efficiency Simulation

Thermophotovoltaic (TPV) power systems, which convert heat into electricity using a photovoltaic diode to collect thermal radiation, have attracted increasing attention in recent work. It has recently been proposed that new optical structures such as photonic crystals can significantly improve the efficiency of these devices in two ways. First, the electronic bandgap of the TPV diode should match the photonic bandgap of the emitter, in order to ensure that the majority of emitted photons can be converted. Second, a photonic crystal short-pass optical filter can be added to the front of the TPV diode to send long wavelength photons back to the hot emitter, which is known as photon recycling. This filter can consist of a quarter wave stack of two materials, or many materials blended together into a so-called rugate filter. Here we present a tool, freely available through nanoHUB.org that allows one to simulate and optimize TPV performance when using these components at a system level. A graphical user interface (GUI) was developed using the Rappture toolkit that allows one to specify the materials and the geometric structure of the selective emitter, filter, and TPV diode. This information is subsequently supplied to two simulations: a finite difference time-domain simulation, known as MEEP, which yields the thermal emission spectrum of the photonic structure; and a Fourier modal method simulation, known as S4, which outputs the filter spectrum. Finally, we explored a constrained range of design parameters to find optimal values that warrant further theoretical and experimental investigation

Modeling High-Efficiency Rear Junction Photovoltaic Devices

Solar cells are a renewable energy technology that has begun to supply energy in many regions of the world at a utility scale. Nonetheless, further improvements in solar technology are still needed to help reduce costs and increase adoption. Many researchers have been particularly interested in demonstrating higher efficiencies through new materials and designs. Some of the most efficient technologies are made from III-V materials, which feature strong absorption and radiative recombination. Rear junction III-V solar cell devices are of particular interest, as they have been proven to provide higher efficiencies compared to traditionally structured devices. This is believed to be caused by reduced bulk recombination and enhanced photon recycling. However, their performance is not fully understood at this time. In particular, previous studies produced a discrepancy between previous simulated literature and experimental fitted values, at odds with prior independent measurements of the material properties. To help develop a better understanding of these cells, we have developed a tool to examine the parameters that properly fit experimental data sets. Our tool features a web-enabled graphical user interface that allows the user to set the range of parameters to sweep. It then outputs contour plots to determine which ranges of values are most consistent with experimental results. This simulation tool will provide a more accurate understanding of the behavior of GaInP rear-junction solar cells, and more generally, will help advance our understanding of how unconventional solar cell architectures and materials can help achieve higher efficiencies. Future benefits may include improved design techniques for single-junction photovoltaics for terrestrial use, as well as multi-junction high-performance cells for aerospace applications

Predicting and Optimizing Solar Cell Performance with Material/Surface Characteristics

Renewable energy sources have begun replacing fossil fuels at the utility scale. In particular, photovoltaics has grown rapidly in recent years. To further improve solar technology in terms of cost and efficiency and promote adoption, researchers often seek material and device level advancements. Photovoltaic simulation tools can be utilized to predict device performance before fabrication and experimentation, streamline research processes, and interpret experimental results. Therefore, we developed ContourPV, which simulates various combinations of values of different device characteristics to optimize and predict photovoltaic performance. ContourPV sweeps the inputted range of values for each chosen device or layer characteristic and obtains performance data by utilizing the drift-diffusion solver, ADEPT. ContourPV plots these metrics in contour plots as output. The parameters that can be swept include Shockley-Read-Hall recombination lifetime, doping concentration, radiative recombination coefficient, and surface recombination velocity for front and rear contacts. Open circuit voltage, short circuit current, fill factor and efficiency are available as output. This tool can provide researchers with intuitive simulation results to predict the performance of a solar cell design, determine material properties based on experimental current-voltage measurements, and help predict performance crossover regions between different device designs. Silicon and GaInP are investigated as example materials in ContourPV: silicon because it is the most common material for commercial solar panels, and GaInP because it is a strong candidate for high-efficiency multijunction solar cells. Furthermore, a wide range of other material systems can be simulated in this tool by users of ADEPT

Stanford Stratified Structure Solver (S4) Simulation Tool

The Stanford Stratified Structure Solver (S4) developed in 2012 allows for fast, accurate prediction of optical propagation through complex 3D structures. However, there have been two key challenges preventing wider use to date: the use of a specialized control language, and the difficulty of incorporating realistic materials parameters. In this project, both concerns have been addressed. We have constructed a graphical user interface as an alternative, using the open-source Rappture platform on nanoHUB. This has been combined with a comprehensive materials database known as PhotonicsDB, which incorporates materials optical data drawn from carefully vetted sources. An Octave script file was written to accept the user inputs and then generate and run an S4 control file. The results are then interpreted and displayed on the interface by the xml file. This new S4 GUI was then used to investigate and optimize selective solar thermal absorber designs, which can convert sunlight into heat for direct use in hot water, or for powering mechanical engines. Preliminary results are also presented

Active materials in photonic crystals

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2007.Includes bibliographical references (leaves 129-139).I analyze new phenomena arising from embedding active materials inside of photonic crystal structures. These structures strongly modify the photonic local density of states (LDOS), leading to quantitative and qualitative changes in the behavior of active materials. First, I show that the emission spectrum of point-like sources inside an "omniguide" is strongly modified by features resembling one-dimensional van Hove singularities in the LDOS. The resulting overall enhancement of the LDOS causes radiating dipoles to emit more rapidly than in vacuum (known as the Purcell effect). Second, I study optically pumped lasing in three model systems: a Fabry-Perot cavity, a line of defects in a two-dimensional square lattice of rods, and a cylindrical photonic crystal. It is shown that high conversion efficiency can be achieved for large regions of active material in the cavity, as well as for a single fluorescent atom in a hollow-core cylindrical photonic crystal, suggesting designs for ultra-low-threshold lasers and ultra-sensitive biological sensors. Third, I consider a photonic crystal-based light-trapping scheme, capable of compensating for weak optical absorption of crystalline silicon solar cells in the near infrared. For a 2 pm-thick cell, relative efficiency enhancements as high as 35% are expected. Fourth, I explore a way to achieve full ±900 electronically-controlled beam steering using a linear array of one dimensionally periodic elements containing electro-optic materials. Fifth, I consider switching of a single signal photon by a single gating photon of a different frequency, via a cross-phase modulation generated by electromagnetically-induced transparency atoms embedded in photonic crystals. The exact solution shows that the strong coupling regime is required for lossless two-photon quantum entanglement.(cont.) Finally, I demonstrate that the Purcell effect can be used to tailor the effective Kerr nonlinear optical susceptibility. Using this effect for frequencies close to an atomic resonance can substantially influence the resultant Kerr nonlinearity for light of all (even highly detuned) frequencies. For example, in realistic physical systems, enhancement of the Kerr coefficient by one to two orders of magnitude could be achieved.by Peter Bermel.Ph.D