James G. Mutitu

Thin film silicon solar cell design based on photonic crystal and diffractive grating structures

In this paper we present novel light trapping designs applied to multiple junction thin film solar cells. The new designs incorporate one dimensional photonic crystals as band pass filters that reflect short light wavelengths (400 - 867 nm) and transmit longer wavelengths(867 -1800 nm) at the interface between two adjacent cells. In addition, nano structured diffractive gratings that cut into the photonic crystal layers are incorporated to redirect incoming waves and hence increase the optical path length of light within the solar cells. Two designs based on the nano structured gratings that have been realized using the scattering matrix and particle swarm optimization methods are presented. We also show preliminary fabrication results of the proposed devices.

Angular Selective Light Filter Based on Photonic Crystals for Photovoltaic Applications

In this paper, we present a new angular selective filter design that is based on photonic bandgap engineering principles. The operation of the filter is based on the occurrence of angular-dependent photonic stopbands and passbands in a photonic crystal structure. Such a filter allows for the propagation of normally incident light while disallowing the propagation of obliquely incident light waves. When the filter is applied to a solar cell structure that consists of a diffraction grating structure on the back surface, a high-efficiency light trap can be formed. Hence, the light-trapping capacity of the new structure is dependent on the photonic band structure of the light filter rather than on refractive optical properties of the active photovoltaic material. This paper presents a model of such a structure and investigates the possibilities afforded by the new structure.

Light trapping designs for thin silicon solar cells based on photonic crystal and metallic diffractive grating structures

We present novel light trapping designs applied to thin (5 micron) silicon solar cells. The design structures incorporate diffractive gratings to increase the optical path length of light within the solar cells. We incorporate a combination of dielectric and metallic materials to create the gratings. We form a one dimensional photonic crystal stack with the dielectric materials to which we then add the metallic layers. The combination of the two materials enhances the reflective properties of the gratings and thus increasing their effectiveness in light trapping. We use the particle swarm optimization and scattering matrix methods to realize the design structures.

Light trapping enhancement in thin silicon solar cells using photonic crystals

Recent developments in light trapping designs based on photonic crystal structures are presented. Design structures that take advantage of the photonic stop-bands in specially engineered photonic crystal materials are described. To this end, the new materials allow for the propagation of normally incident light while disallowing the transmission of obliquely incident light waves. Hence, an angular selective light filter is formed that regulates the passage of light in and out of the solar cell structure. When combined with diffraction grating structures, the angular light filter provides a new type of light trap that differs from traditional geometric optical designs, such as texturing. This paper explores the science and possibilities afforded by the new photonic crystal enhanced solar cell.

Light Trapping in Thin-Film Cu(InGa)Se

A fundamental optical analysis of thin-film Cu(InGa)Se2 solar cell structures is presented, wherein spectroscopic ellipsometry measurements were performed to acquire material optical constants, which were then used as input parameters to perform electromagnetic simulations. The accuracy of the electromagnetic simulation tools, and thus, the validity of the material optical constants, were verified by comparing the values determined from the simulations with experimental measurements obtained using a spectrophotometer. The verified optical modeling tools were then used to analyze thin, <;0.7-μm Cu(InGa)Se2 solar cell structures, which do not absorb all incident light within a single optical path length, and hence, the need to incorporate light trapping. To this end, a superstrate device configuration was employed in which the metallic back contact is deposited last, giving rise to an opportunity to incorporate photonic engineering device concepts to the back surface layer of the solar cell. Simulations of superstrate Cu(InGa)Se2 solar cell designs, complete with light trapping structures were then performed and analyzed.

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In this paper we present the design, optimization and characterization of a light trapping architecture that is applied to a thin, 50 micron, silicon solar cell structure. We combine a number of novel photonic engineering device concepts to realize the structure, including a single period hybrid dielectric-metallic back surface reflector and a host of diffraction gratings. We present a novel deep ultraviolet lithography process used in the realization of these gratings and a simulated analysis of their light trapping performance in the solar cell structure.

Solar Cells

Metal organic chemical vapor deposition (MOCVD) tools are integral to many technologies, including the growth of high-efficiency multijunction III-V solar cells. Veeco MOCVD has recently developed new tool designs that allow increased MOCVD growth rates that could drastically increase solar cell throughput and reduce manufacturing costs. It is important, however, to understand the trade-offs between increased throughput and decreased material quality and device performance. We fabricate multijunction III-V solar cells from materials grown by both standard and fast growth rate techniques. We analyze the open circuit voltage, short circuit current, fill factor, efficiency, and ideality factor of both types of devices. Comparison of these devices reveals that the existing fast growth protocols result in solar cells with similar performance to standard growth cells. The results suggest that increased growth rates can enable higher throughput fabrication of solar cells without significant performance sacrifices.

Light trapping in thin silicon solar cells

The application of photonic engineering design concepts to photovoltaic devices is not entirely trivial; if anything, it introduces formidable complexity. This paper investigates some of the opportunities afforded by the amalgamation of the two sciences.