Simulation of nanostructure-based high-efficiency solar cells: challenges, existing approaches and future directions

Many advanced concepts for high-efficiency photovoltaic devices exploit the peculiar optoelectronic properties of semiconductor nanostructures such as quantum wells, wires and dots. While the optics of such devices is only modestly affected due to the small size of the structures, the optical transitions and electronic transport can strongly deviate from the simple bulk picture known from conventional solar cell devices. This review article discusses the challenges for an adequate theoretical description of the photovoltaic device operation arising from the introduction of nanostructure absorber and/or conductor components and gives an overview of existing device simulation approaches.Comment: Invited paper, accepted for publication in IEEE Journal of Selected Topics in Quantum Electronic

Photon Management for Thin-Film Quantum Dot Solar Cells

L'abstract è presente nell'allegato / the abstract is in the attachmen

Modeling of Solar Cell Efficiency Improvement Using Optical Gratings and Intermediate Absorption Band.

This dissertation is a summary of the research effort for the theoretical study, modeling, design, and optimization of solid state photovoltaic devices. The effect of grating structures on the back reflector is studied using electromagnetic modeling and an optimized structure is proposed to enhance the optical absorbance of these devices. Solar cells with optimized arbitrarily shaped gratings exhibit a 29% improvement over planar cells and 9.0% improvement over the optimal cell with periodic gratings. A new model incorporating carrier transport and recombination is proposed and simulation result shows the significance of this model in the modeling of intermediate band solar cell. The material ZnTeO is used as a numerical example for the intermediate band solar cell model. The optimal impurity concentration is determined to be 1018 cm-3 for an optical absorption cross section of 10-14 cm2. The conversion efficiency of a ZnTe solar cell with a total recombination lifetime of 10 ns is calculated to increase from 14.39 % to 26.87 % with the incorporation of oxygen. Fully coupled solution to partial differential equations provides insight into the operation of intermediate band solar cell. A doping compensation scheme is proposed to mitigate the space charge effects, and the device achieves conversion efficiencies of approximately 40%, similar to the maximum expected values from prior 0-D models. A spectrally decoupled scheme for subbandgap photovolatics is proposed in which, device structures with non-uniform occupation of intermediate electronic states are employed to reduce the dependence of conversion efficiency on spectral overlap. Solar cell conversion efficiencies are calculated for structures where absorption bands are spatially decoupled due to defined occupation of intermediate states. The spectrally-decoupled device provides a means to achieve high theoretical efficiency independent of spectral overlap that approaches the detailed balance efficiency limit of 63.2 % for intermediate state devices without spectral overlap. The analysis of experimental work using the model developed for intermediate band solar cell is conducted and ZnTeO alloy is chosen to be the material for intermediate band solar cell, where oxygen states are served as intermediate sites in the fundamental bandgap.Ph.D.Electrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/75981/1/shihchun_1.pd

Light Trapping in Plasmonic Solar Cells

Subwavelength nanostructures enable the manipulation and molding of light in nanoscale dimensions. By controlling and designing the complex dielectric function and nanoscale geometry we can affect the coupling of light into specific active materials and tune macroscale properties such as reflection, transmission, and absorption. Most solar cell systems face a trade-off with decreasing semiconductor thickness: reducing the semiconductor volume increases open circuit voltages, but also decreases the absorp- tion and thus the photocurrent. Light trapping is particularly critical for thin-film amorphous Si (a-Si:H) solar cells, which must be made less than optically thick to enable complete carrier collection. By enhancing absorption in a given semiconductor volume, we can achieve high efficiency devices with less than 100 nm of active region. In this thesis we explore the use of designed plasmonic nanostructures to couple incident sunlight into localized resonant modes and propagating waveguide modes of an ultrathin semiconductor for enhanced solar-to-electricity conversion. We begin by developing computational tools to analyze incoupling from sunlight to guided modes across the solar spectrum and a range of incident angles. We then show the potential of this method to result in absorption enhancements beyond the limits for thick film solar cells. The second part of this thesis describes the integration of plasmonic nanos- tructures with a-Si:H solar cells, showing that designed nanostructures can lead to enhanced photocurrent over randomly textured light trapping surfaces, and develops a computational model to accurately simulate the absorption in these structures. The final chapter discusses the fabrication of a high-efficiency (9.5%) solar cell with a less than 100 nm absorber layer and broadband, angle isotropic photocurrent enhance- ment. Moreover, we discuss general design rules where light trapping nanopatterns are defined by their spatial coherence spectral density.</p

Nanoantennas for visible and infrared radiation

Nanoantennas for visible and infrared radiation can strongly enhance the interaction of light with nanoscale matter by their ability to efficiently link propagating and spatially localized optical fields. This ability unlocks an enormous potential for applications ranging from nanoscale optical microscopy and spectroscopy over solar energy conversion, integrated optical nanocircuitry, opto-electronics and density-ofstates engineering to ultra-sensing as well as enhancement of optical nonlinearities. Here we review the current understanding of optical antennas based on the background of both well-developed radiowave antenna engineering and the emerging field of plasmonics. In particular, we address the plasmonic behavior that emerges due to the very high optical frequencies involved and the limitations in the choice of antenna materials and geometrical parameters imposed by nanofabrication. Finally, we give a brief account of the current status of the field and the major established and emerging lines of investigation in this vivid area of research.Comment: Review article with 76 pages, 21 figure