M. W. Wanlass

High-efficiency GaInP∕GaAs∕InGaAs triple-junction solar cells grown inverted with a metamorphic bottom junction

The authors demonstrate a thin, Ge-free III–V semiconductor triple-junction solar cell device structure that achieved 33.8%, 30.6%, and 38.9% efficiencies under the standard 1sun global spectrum, space spectrum, and concentrated direct spectrum at 81suns, respectively. The device consists of 1.8eV Ga0.5In0.5P, 1.4eV GaAs, and 1.0eV In0.3Ga0.7As p-n junctions grown monolithically in an inverted configuration on GaAs substrates by organometallic vapor phase epitaxy. The lattice-mismatched In0.3Ga0.7As junction was grown last on a graded GaxIn1−xP buffer. The substrate was removed after the structure was mounted to a structural “handle.” The current-matched, series-connected junctions produced a total open-circuit voltage over 2.95V at 1sun.

High‐efficiency indium tin oxide/indium phosphide solar cells

Solar cells have been fabricated by rf sputter depositing indium tin oxide onto single crystal p‐type indium phosphide. Four different substrate doping densities have been used but in all cases the dopant was zinc and the wafers were 〈100〉 oriented. The optimum doping density from the range studied was 3×1016 cm−3 and devices based on such substrates have yielded total area efficiencies up to 16.2% using the air mass 1.5 spectrum normalized to 100 mW cm−2, which correspond to active area efficiencies of 19.1%. A doping density less than the optimum yielded devices with excessive series resistance. Higher doping densities led to a marked loss of red response.

Lattice-mismatched approaches for high-performance, III-V photovoltaic energy converters

We discuss lattice-mismatched (LMM) approaches utilizing compositionally step-graded layers and buffer layers that yield III-V photovoltaic devices with performance parameters equaling those of similar lattice-matched (LM) devices. Our progress in developing high-performance, LMM, InP-based GaInAs/InAsP materials and devices for thermophotovoltaic (TPV) energy conversion is highlighted. A novel, monolithic, multi-bandgap, tandem device for solar PV (SPV) conversion involving LMM materials is also presented along with promising preliminary performance results.

Recombination lifetime of In0.53Ga0.47As as a function of doping density

We have fabricated devices with the structure InP/In0.53Ga0.47As/InP, with a InGaAs doping range varying from 2×1014 to 2×1019 cm−3. These isotype double heterostructures were doped both n and p type and were used to measure the minority-carrier lifetime of InGaAs over this doping range. At the low doping end of the series, recombination is dominated by the Shockley–Read–Hall effect. At the intermediate doping levels, radiative recombination is dominant. At the highest doping levels, Auger recombination dominates as the lifetime varies with the inverse square of the doping concentration. From fitting these data, the radiative- and Auger-recombination coefficients are deduced.

Advanced high-efficiency concentrator tandem solar cells

Computer modeling studies of two-junction concentrator tandem solar cells show that infrared (IR)-responsive bottom cells are essential to achieve the highest performance levels in both terrestrial and space applications. These studies also show that medium-bandgap/low-bandgap tandem pairs hold a clear performance advantage under concentration when compared to high-bandgap/medium-bandgap pairs, even at high operating temperatures (up to 100 degrees C). Consequently, two novel concentrator tandem designs that utilize low-bandgap bottom cells have been investigated. These include mechanically stacked, four-terminal GaAs-0.95-eV-GaInAsP tandem, and monolithic, lattice-matched. three-terminal InP-0.75-eV-GaInAs tandem. In preliminary experiments, terrestrial concentrator efficiencies exceeding 30% have been achieved with each of these designs. Methods for improving the efficiency of each tandem are discussed.<<ETX>>

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

Direct-bonded GaAs∕InGaAs tandem solar cell

A direct-bonded GaAs/InGaAs solar cell is demonstrated. The direct-bonded interconnect between subcells of this two-junction cell enables monolithic interconnection without threading dislocations and planar defects that typically arise during lattice-mismatched epitaxial heterostructure growth. The bonded interface is a metal-free n+GaAs/n+InP tunnel junction. The tandem cell open-circuit voltage is approximately the sum of the subcell open-circuit voltages. The internal quantum efficiency is 0.8 for the GaAs subcell compared to 0.9 for an unbonded GaAs subcell near the band gap energy and is 0.7 for both of the InGaAs subcell and an unbonded InGaAs subcell, with bonded and unbonded subcells similar in spectral response.

InGaAs/InP double heterostructures on InP/Si templates fabricated by wafer bonding and hydrogen-induced exfoliation

Applications of InP-based materials are numerous, and thus integration of InP on Si may enable realization of powerful integrated III‐V-on-Si systems. InP and its lattice matched quaternary counterpart In12xGaxAsyP12y are direct gap semiconductors, which have high carrier mobilities, therefore finding applications in lasers, multijunction solar cells 1 and high-speed devices. Additionally, they cover the low dispersion and minimum loss wavelengths for optical fiber communication at 1.3 and 1.5mm, respectively, making them attractive materials for fabricating semiconductor lasers and detectors for telecommunications applications. However, InP is mechanically fragile, is not available in large substrates, and is expensive. Integrating InP thin films on Si substrates improves its mechanical strength and may also allow InP integration on large substrates by a process of tiling transferred thin films. Most importantly, a viable approach to InP/Si may enable cost-effective integration of infrared optoelectronic devices with well-established silicon electronics.

Inverted GaInP / (In)GaAs / InGaAs triple-junction solar cells with low-stress metamorphic bottom junctions

We demonstrate high efficiency performance in two ultra-thin, Ge-free III–V semiconductor triple-junction solar cell device designs grown in an inverted configuration. Low-stress metamorphic junctions were engineered to achieve excellent photovoltaic performance with less than 3 × 106 cm−2 threading dislocations. The first design with band gaps of 1.83/1.40/1.00 eV, containing a single metamorphic junction, achieved 33.8% and 39.2% efficiencies under the standard one-sun global spectrum and concentrated direct spectrum at 131 suns, respectively. The second design with band gaps of 1.83/1.34/0.89 eV, containing two metamorphic junctions achieved 33.2% and 40.1% efficiencies under the standard one-sun global spectrum and concentrated direct spectrum at 143 suns, respectively.

Monolithic, Ultra-Thin GaInP/GaAs/GaInAs Tandem Solar Cells

We present here a new approach to tandem cell design that offers near-optimum subcell bandgaps, as well as other special advantages related to cell fabrication, operation, and cost reduction. Monolithic, ultra-thin GaInP/GaAs/GaInAs triple-bandgap tandem solar cells use this new approach, which involves inverted epitaxial growth, handle mounting, and parent substrate removal. The optimal ~1-eV bottom subcell in the tandem affords an -300 mV increase in the tandem voltage output when compared to conventional Ge-based, triple-junction tandem cells, leading to a potential relative performance improvement of 10-12% over the current state of the art. Recent performance results and advanced design options are discussed