Christiana Honsberg

Design and Characterization of GaN/InGaN Solar Cells

We experimentally demonstrate the III-V nitrides as a high-performance photovoltaic material with open-circuit voltages up to 2.4V and internal quantum efficiencies as high as 60%. GaN and high-band gap InGaN solar cells are designed by modifying PC1D software, grown by standard commercial metal-organic chemical vapor deposition, fabricated into devices of variable sizes and contact configurations, and characterized for material quality and performance. The material is primarily characterized by x-ray diffraction and photoluminescence to understand the implications of crystalline imperfections on photovoltaic performance. Two major challenges facing the III-V nitride photovoltaic technology are phase separation within the material and high-contact resistances.

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.

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

Limiting efficiency of an intermediate band solar cell under a terrestrial spectrum

The limiting efficiency of an intermediate band (IB) solar cell under the terrestrial AM1.5 spectrum was calculated by detailed balance for various concentration levels. The results show four energy gap combinations giving similar limiting efficiencies. This is in contrast to the more studied case of an IB solar cell under a blackbody spectrum where a single optimum combination is found. A design with a subenergy gap of ∼0.57eV is found to be viable, leading to the conclusion that the design space for an IB solar cell is larger when under the AM1.5 spectrum than when under a Blackbody spectrum.

Design and Realization of Wide-Band-Gap (

The design of coherently strained InGaN epilayers for use in InGaN p-n junction solar cells is presented in this letter. The X-ray diffraction of the epitaxially grown device structure indicates two InGaN epilayers with indium compositions of 14.8% and 16.8%, which are confirmed by photoluminescence peaks observed at 2.72 and 2.67 eV, respectively. An open-circuit voltage of 1.73 V and a short-circuit current density of 0.91 mA/cm2 are observed under concentrated AM 0 illumination from the fabricated solar cell. The photovoltaic response from the InGaN p-n junction is confirmed by using an ultraviolet filter. The solar cell performance is shown to be related to the crystalline defects in the device structure.

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Iodine-methanol (I2/ME), a chemical passivation method, is extensively used in silicon (Si) solar cell fabrication for measuring minority carrier lifetime in bulk regions. We demonstrate that quinhydrone-methanol (QHY/ME) provides higher lifetimes than I2/ME. For 0.01 mol/dm−3 QHY/ME on float-zone (FZ) wafers at 1×1015 cm−3 injection level, a high lifetime of 3.3 ms and surface recombination velocity of 7 cm/sec on n-type (100 Ω cm) and 1.1 ms on p-type (2 Ω cm) is reported. The surface recombination velocity is also measured out of solution for several days. Chemical characterization results indicate increasing surface oxidation with decreasing passivation, consistent with proposed bonding mechanism.

2.67 eV) InGaN p-n Junction Solar Cell

The InGaN material system is investigated to achieve high efficiency solar cells, using tandem and quantum-well structures to implement high efficiency concepts. Here InGaN p-i-n and quantum-well solar cells are designed, grown by MOCVD and fabricated into mesa devices. They are electrically characterized by I-V response under dark, white light and UV illumination and internal quantum efficiency (IQE). Material characterization is done by X-ray diffraction, photoluminescence and photoemission. InGaN solar cells with high In compositions are grown in two configurations, one incorporating it into the i-region of a p-i-n solar cells, and the other incorporating as the well-region of a quantum-well device. A QE of 8% was measured from these quantum-wells. Solar cells with In lean In/sub 0.07/Ga/sub 0.93/N p-i-n device structures show an IQE of 19% as well as photoemission at 500 nm, confirming the suitability of the material for photovoltaic applications.

High effective minority carrier lifetime on silicon substrates using quinhydrone-methanol passivation

InAs quantum dots grown on GaAsSb buffer layers with varying Sb content have been studied. Atomic force microscopy results show that the dot size is reduced as the Sb content increases with a concomitant increase in number density. Analysis of the size distribution indicates that the spread of dot sizes narrows with increasing Sb content. This is confirmed by photoluminescence measurements showing a significant narrowing of the dot emission peak for a GaAs0.77Sb0.23 buffer compared to a GaAs buffer. The results are attributed to the strained buffer reducing interactions between dots and the Sb acting as a surfactant.

Characterization and analysis of InGaN photovoltaic devices

This paper identifies absorbers for multiple transition solar cells that are implemented with nanostructured heterojunctions [e.g., quantum well solar cells with quasi-Fermi-level variations and quantum dot (QD) intermediate-band solar cells]. In the radiative limit, the solar cells implemented with these absorbers are capable of achieving a conversion efficiency ges50% with a geometric solar concentration of at least 1000times. The technical approach enumerates a set of quantitative design rules and applies the rules to the technologically important III-V semiconductors and their ternary alloys. A novel design rule mandates a negligible valence band discontinuity between the barrier material and confined materials. Another key design rule stipulates that the substrate have a lattice constant in between that of the barrier material and that of the quantum-confined material, which permits strain compensation. Strain compensation, in turn, allows a large number of QD layers to be incorporated into the solar cell because each layer is free of defects. Four candidate materials systems (confined/barrier/substrate) are identified: InP0.85Sb0.15/GaAs/InP, InAs0.40P0.60/GaAs/InP, InAs/GaAs0.88Sb0.12/InP, and InP/GaAs0.70P0.30/GaAs. Resulting from the design features, the candidate systems may also find use in other optoelectronic applications.

Use of a GaAsSb buffer layer for the formation of small, uniform, and dense InAs quantum dots

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