Allen M. Barnett

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.

The spectral p-n junction model for tandem solar-cell design

Tandem solar cells can have significantly higher efficiencies than single-junction solar cells because they convert a larger fraction of the incident solar spectrum to electricity. For the design of tandem solar cells the spectral p-n junction model is proposed. It is based on tabulated standard spectra, on the fit of experimentally achieved open-circuit voltages, and assumes a quantum efficiency of unity. By consistent treatment of the energy gap in the diode equation, the model can be quantitatively applied to all tandem solar-cell systems. The special form and use of the reverse saturation current density is discussed in detail. The spectral p-n junction model is rigorously applied based on accepted standard spectra. The tandem solar-cell performance limits based on the model are calculated. A quantitative expression for the increase in efficiency under concentration is derived. Choosing materials with optimum bandgaps, a two-solar-cell two-terminal tandem system can achieve a theoretical maximum efficiency of 38.2-percent (AM1.5 global). A two-solar-cell four-terminal tandem system can have a maximum efficiency of 39.1 percent at the same spectrum. This four-terminal system allows more freedom in choosing the most efficient bandgap combinations. Assuming realistic losses, a configuration consisting of a Si solar cell on the bottom and a solar cell with a bandgap, Eg= 1.85 eV on the top, a maximum efficiency of 32.1 percent (AM1.5 global) can be predicted. Increased efficiency can be obtained from a three-solar-cell six-terminal tandem system. With an optimum bandgap combination the theoretical maximum efficiency is 44.5 percent (AM1.5 global) for the three-solar-cell system. The limits predicted by the model are discussed for tabulated standard spectra. The highest achievable efficiency is 57.3 percent (AM1.5 global) without concentration of the incident light. The increase in efficiency under concentration is evaluated, and it is found that the relative change of the efficiency at any concentration X is linear with In (X).

50% Efficient Solar Cell Architectures and Designs

Very high efficiency solar cells (VHESC) for portable applications that operate at greater than 55 percent efficiency in the laboratory and 50 percent in production are being created. We are integrating the optical design with the solar cell design, and have entered previously unoccupied design space that leads to a new paradigm. This project requires us to invent, develop and transfer to production these new solar cells. Our approach is driven by proven quantitative models for the solar cell design, the optical design and the integration of these designs. We start with a very high performance crystalline silicon solar cell platform. Examples will be presented. Initial solar cell device results are shown for devices fabricated in geometries designed for this VHESC program

The design and fabrication of thin-film CdS/Cu 2 S cells of 9.15-percent conversion efficiency

Thin-film polycrystalline CdS/Cu2S cells with energy conversion efficiencies in sunlight of up to 9.15 percent and areas of ∼1 cm2have been developed. The improvement over previously achieved efficiencies is due to the development of techniques to separately measure and minimize fill factor losses. Specific design and fabrication changes based on a detailed quantitative analysis of the cell operation, were introduced to correct series resistance, shunt conductance and field effect losses. Further increases in efficiency can be expected from the development of a planar junction thin-film CdS/Cu2S cell.

Thin-film solar cells: A unified analysis of their potential

The development and deployment of low-cost thin-film solar cells for the direct conversion of sunlight to electricity can be accelerated by the utilization of loss minimization and cost minimization methodologies. The solar cell is separated into its five constituent layers to provide a common basis for the development of these methodologies. Photovoltaic theory, materials science, and loss analysis are combined to develop the loss minimization methodology which can be used to systematically improve and optimize performance of any solar-cell material system. The techniques of the chemical process industry have been applied to achieve cost minimization. The loss-and cost-minimization methodologies have been combined into a generalized procedure for the accelerated development of all low-cost thin-film photovoltaic material systems.

Nanostructured Solar Cells for High Efficiency Photovoltaics

The use of nanostructures in photovoltaics offers the potential for high efficiency by either using new physical mechanisms or by allowing solar cells which have efficiencies closer to their theoretical maximum, for example by tailoring material properties. At the same time, nanostructures have potentially low fabrication costs, moving to structures or materials which can be fabricated using chemically or biologically formed materials. Despite this potential, there are multiple and significant challenges in achieving viable nanostructured solar cells, ranging from the demonstration of the fundamental mechanisms, device-level issues such as transport mechanisms and device structures and materials to implement nanostructured solar cells, and low cost fabrication techniques to implement high performance designs. This paper presents the challenges and approaches for using nanostructured solar cells in devices which can approach the thermodynamic limits for solar energy conversion

The effect of dislocations on the open-circuit voltage of gallium arsenide solar cells

The effect of dislocations on GaAs solar cell performance is modeled with a focus on open-circuit voltage as a measure of device quality. On the basis of the properties of GaAs grain boundaries and surfaces, dislocations in n-type GaAs are assumed to be arsenic-rich regions that form inverted p-type regions. The inverted regions are assumed to form a low-voltage Schottky barrier or heterojunction with the metal or conductive substrate at the n-type surface. This approach successfully predicts the open-circuit voltage of GaAs solar cells on Si substrates at their reported dislocation density. >

Thin-film silicon crystal growth on low cost substrates

Abstract Thin films of crystalline silicon are being grown by us on dissimilar substrates for photovoltaic solar cell application. The approach offers the potential to combine the high performance and stability of crystalline silicon with the low cost of thin films. Growth from saturated solution is being used to meet the specific requirements of large grain growth, benign grain boundaries, and long minority carrier diffusion length. For a good quality crystalline overlayer on a substrate, the growth process consists of five steps: (1) wetting, (2) nucleation, (3) non-impinging crystal growth, (4) fill-in crystal growth, and (5) homoepitaxial film growth. Silicon films with crystals that have a width to thickness ratio of greater than 2 to 1 and an overall thickness of 20μm have been grown on steel and quartz substrates. These films have demonstrated the required morphological characteristics for a high performance thin-film polycrystalline silicon solar cell.

Wide Band Gap Gallium Phosphide Solar Cells

Gallium phosphide (GaP), with its wide band gap of 2.26 eV, is a good candidate for the top junction solar cell in a multijunction solar cell system. Here, we design, fabricate, characterize, and analyze GaP solar cells. Liquid phase epitaxy is used to grow the semiconductor layers. Four generations of GaP solar cells are developed and fabricated with each solar cell structure being designed and improved based on the first principles analyses of the predecessor solar cells. Quantum efficiency and current-voltage measurements are used to analyze the solar cell performance and to develop predictive models. We create a GaP solar cell with an efficiency of 2.42% under AM 1.5G one sun illumination.

Light trapping in Silicon-Film™ solar cells with rear pigmented dielectric reflectors

This paper presents the novel method of using pigmented dielectric reflectors to provide light trapping in thin-film silicon solar cells. This type of reflecting material offers many potential advantages over specular metallic reflectors, including low cost, compatibility with high temperatures common to solar cell processing, and high, broadband and diffuse reflectance. As this is the first time this concept is described, the basic theory of the optical behavior of pigmented materials is presented by connecting the basic material properties of pigment and medium to their light-trapping benefit in thin silicon solar cells. Several general principles leading to maximum light-trapping benefit are identified, and experimental evidence is presented corroborating these general principles. Light trapping is demonstrated in thin silicon solar cells with pigmented dielectric reflectors by measurement and analysis of external quantum efficiency curves. Copyright