Allen Barnett

Development of a 16.8% Efficient 18-μm Silicon Solar Cell on Steel

Thin crystalline silicon solar cells have the potential to achieve high efficiency due to the potential for increased voltage. Thin silicon wafers are fragile; therefore, means of support must be provided. This paper reports the design, development, and analysis of an 18-μm crystalline silicon solar cell electrically integrated with a steel alloy substrate. This ultrathin silicon is epitaxially grown on porous silicon and then transferred onto the steel substrate. This method allows the independent processing of each surface. The steel substrate enables robust handling and provides a conductive back plane. Three groups of cells with planar and textured structures are compared; significant improvements in Jsc, Voc, and fill factor (FF) are achieved. The best cell shows an efficiency of 16.8% with an open-circuit voltage of 632 mV and a short-circuit current density of 34.5 mA/cm2.

Analysis of tandem III–V/SiGe devices grown on Si

This paper introduces the modeling developed to assess the potential of a III-V/SiGe tandem device. Demonstration of value will be executed via materials and solar cell device models. III-V top cell candidates are evaluated and a high-value composition is identified. Initial windowless GaAsP solar cells demonstrate a bandgap-voltage offset of 0.58.

The Effect of Spectrum Variation on the Energy Production of Triple-Junction Solar Cells

We analyze the energy production of a current-matched, two-terminal triple-junction solar cell under different spectrum conditions. We compare this energy production with that of a separate-terminal three subcell system. Under real-spectrum conditions, the energy loss of the two-terminal system can be greater than 9% because of the current mismatch that frequently occurs in real meteorological conditions. The quantitative analysis of energy production between two-terminal and separate-terminal triple-junction solar cells opens up important opportunities in the design and optimization of concentration photovoltaic solar cells and modules.

Single-junction GaAsP solar cells grown on SiGe graded buffers on Si

We have investigated the microstructure and device characteristics of GaAs0.82P0.18 solar cells grown on Si0.20Ge0.80/Si graded buffers. Anti-phase domains (APDs) were largely self-annihilated within the In0.39Ga0.61P initiation layer although a low density of APDs was found to propagate to the surface. A combination of techniques was used to show that the GaAs0.82P0.18 cells have a threading dislocation density of 1.2u2009±u20090.2u2009×u2009107u2009cm−2. Despite these extended defects, the devices exhibited high open-circuit voltages of 1.10–1.12u2009V. These results indicate that cascading a GaAs0.82P0.18 top cell with a lower-bandgap Si0.20Ge0.80 cell is a promising approach for high-efficiency dual-junction devices on low-cost Si substrates.

Quantification of trap state densities in GaAs heterostructures grown at varying rates using intensity-dependent time resolved photoluminescence

Time-resolved photoluminescence is an established technique for characterizing carrier lifetimes in semiconductors, but the dependence of lifetime on excitation fluence has been only qualitatively investigated. We develop a quantitative approach for fitting fluence-dependent PL decay data to a Shockely-Read-Hall model of carrier recombination in order to extract the trap state density. We demonstrate this approach by investigating growth rate-dependent trap densities in gallium arsenide-indium gallium phosphide double heterostructures. The techniques developed here can be applied for rapid, non-destructive quantification of trap state densities in a variety of materials.

Material and Device Improvement of GaAsP Top Solar Cells for GaAsP/SiGe Tandem Solar Cells Grown on Si Substrates

With its wide bandgap and good diode performance, GaAsP is an excellent candidate for the top cell in a silicon-based multijunction tandem device. Even though the material is not lattice matched to silicon, inclusion of a graded SiGe buffer between the GaAsP layer and the Si substrate has previously been demonstrated to enable lattice matching. The SiGe layer may then serve as a high-quality current-matched bottom cell to form a tandem dual-junction structure. This paper describes the design, fabrication, analysis, and improvement of the GaAsP top solar cell in a three-terminal GaAsP/SiGe tandem solar cell on a silicon substrate. Uncertified GaAsP top cell efficiencies have been improved from 8.4% to 18.4% with bandgap voltage offsets (Woc) of 0.48 and 0.31 V under concentration factors of 1 and 20 ×, respectively. This progress is made by improved III-V material quality, reduced series resistance, and an addition of antireflection coating. Improving the optics, material quality, and fill factor (FF) should further improve the efficiency of the GaAsP top cell in this tandem structure grown on an Si substrate.

Design of Anodic Aluminum Oxide Rear Surface Plasmonic Heterostructures for Light Trapping in Thin Silicon Solar Cells

A metal-dielectric heterostructure that provides the combined capability of light trapping and surface passivation is reported. The light-trapping layer employs a porous aluminum anodic oxide (AAO) with metal nanoparticles formed in the pores on the rear surface of a thin crystalline silicon solar cell. Numerical finite-difference time domain (FDTD) simulations were performed to determine the pore diameter and spacing that would result in optimal light trapping for this metal-dielectric heterostructure. For a 2.5-μm-thick crystalline silicon device, the optimal pore diameter and spacing were determined to be ~250 and ~450 nm, respectively. These conditions resulted in an enhancement of the simulated photocurrent by ~12.6% compared with a device in which the heterostructure was replaced with a homogenous aluminum oxide layer. Simulations also confirmed that the thickness of an underlying dielectric layer should be minimized to 10-20 nm, with the AAO barrier layer being maintained as thin as possible. Finally, it was shown that replacement of silver by aluminum in the pores resulted in a reduction in the photocurrent of 6.3% and would necessitate much larger pore spacing that is difficult to achieve experimentally and would result in thicker AAO barrier layers, which are undesirable.

GaInP window layers for GaAsP on SiGe/Si single and dual-junction solar cells

GaAsP solar cells have been grown on Si substrates facilitated by a SiGe graded buffer layer. Here, single-junction p+/n GaAsP and tandem n+/p GaAsP/SiGe solar cells are reported with an interest in improving efficiency by evaluation of the III-V device passivation layers and pathways to their optimization. Solar cells with varying window thicknesses are reported for both structures and assist in directing focus of future research. The GaAsP/SiGe on Si tandem solar cell demonstrates a result towards AM1.5G 20.8% AR-corrected efficiency.

Dual-junction GaAsP/SiGe on silicon tandem solar cells

GaAsP/SiGe dual-junction solar cells have been grown on silicon substrates which have the potential of achieving tandem efficiencies of 40%. This lattice-matched structure facilitates high performance from the III-V top cell while maintaining the cost advantages of silicon solar cells. The SiGe graded buffer allows for lattice matching of the top and bottom cell while providing a low dislocation interface between the silicon substrate and the device layers. Initial structures have reached an efficiency of 18.9%. Near term improvements to 25.0% under AM1.5G will be described.

15%, 20 Micron thin, silicon solar cells on steel

A method to laminate a thin monocrystalline Si layer to a conductive and fracture-resistant carrier such as steel has been developed, resulting in a practical design for high volume production of robust ultra-thin (10-20 μm) “kerfless” Si wafers. With this technology front and rear cell features based on the world-record PERL cell design have been integrated. A confirmed efficiency of 15.1% has been achieved on a 20-micron thick one-cm2 solar cell. This 15.1% is believed to be the highest confirmed efficiency achieved with ultra-thin silicon integrated with a conducting substrate.