Design and analysis of solar cells by coupled electrical - optical simulation

Careful electrical design and optical design are both crucial for achieving high-efficiency solar cells. It is common to link these two aspects serially; the optical design is first done to minimize reflection and maximize light trapping, and then the resulting optical generation rate is input to the electrical simulation. For very high efficiency solar cells that approach the Shockley-Queisser limit, however, electrical and optical transports are tightly coupled in both directions. Photons generated by radiative recombination can be reabsorbed to create additional electron-hole pairs (so-called photon recycling), which decreases losses. A variety of novel photon management schemes are currently being explored. To achieve the promise of these new approaches, a self-consistent simulation framework that rigorously treats both photons and electrons is needed. In this work, the thin-film GaAs solar cell, the single nanowire solar cell, and the GaInP/GaAs tandem solar cell are investigated. For solar cell characterization, this work examines the validity of the reciprocity theorem and quantitative lifetime parameter extraction using Time-Resolved Photoluminescence (TRPL) and Photoluminescence Excitation Spectroscopy (PLE). Overall, this thesis work has created a new simulation tool for advanced photovoltaic devices based on the self-consistent coupling of wave optics with electronic transport, which lead to accurate predictions of the characteristics and performance. Optimization of photon recycling facilitates improved design strategies to approach the Shockley-Queisser limit, which will eventually pave the way for extension to advanced designs, capable of approaching or even exceeding the Shockley-Queisser limit in the future

Limitations of zT as a Figure of Merit for Nanostructured Thermoelectric Materials

Thermoelectric properties of nanocomposites are numerically studied as a function of average grain size or nanoparticle density by simulating the measurements as they would be done experimentally. In accordance with previous theoretical and experimental results, we find that the Seebeck coefficient, power factor and figure of merit, zT, can be increased by nanostructuring when energy barriers exist around the grain boundaries or embedded nanoparticles. When we simulate the performance of a thermoelectric cooler with the same material, however, we find that the maximum temperature difference is much less than expected from the given zT. This occurs because the measurements are done in a way that minimizes Joule heating, but the Joule heating that occurs in operating devices has a large effect for these kinds of materials. The same nanocomposite but without energy barriers at the grain boundaries has a lower measured zT but a higher maximum temperature difference. The physical reason for these results is explained. The results illustrate the limitations of zT as a figure of merit for nanocomposites with electrically active grain boundaries

On the Use of Rau’s Reciprocity to Deduce External Radiative Efficiency in Solar Cells

Rau’s reciprocity relation has been used to deduce the external radiative efficiency of a wide variety of solar cells using just standard solar cell measurements, but it is based on a number of assumptions, some of which may not be valid for typical thin-film solar cells. In this paper, we use rigorous optical simulations coupled with carrier transport simulations to examine some common thin film solar cells. The results provide guidance on when the Rau relation can be used, why it can fail, and on the magnitude of the errors that can be expected in practice

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

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