J.S. Ward

40.8% efficient inverted triple-junction solar cell with two independently metamorphic junctions

A photovoltaic conversion efficiency of 40.8% at 326 suns concentration is demonstrated in a monolithically grown, triple-junction III–V solar cell structure in which each active junction is composed of an alloy with a different lattice constant chosen to maximize the theoretical efficiency. The semiconductor structure was grown by organometallic vapor phase epitaxy in an inverted configuration with a 1.83 eV Ga.51In.49P top junction lattice-matched to the GaAs substrate, a metamorphic 1.34 eV In.04Ga.96As middle junction, and a metamorphic 0.89 eV In.37Ga.63As bottom junction. The two metamorphic junctions contained approximately 1×105 cm−2 and 2–3×106 cm−2 threading dislocations, respectively.

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>>

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.

Triple-junction solar cell efficiencies above 32%: the promise and challenges of their application in high-conceniration-ratio PV systems

Results from Spectrolab-grown Ga/sub 0.5/In/sub 0.5/P/GaAs/Ge structures optimized for the AM1.5D spectrum are described along with progress toward developing next generation multijunction solar cells for high concentration ratios (X). The epitaxially-grown layers were processed into triple junction cells both at Spectrolab and NREL, and I-V tested vs. X. Cells were tested with efficiencies as high as 32.4% near 372 suns. The FF limited the performance with increasing X as a result of the increased role of the series resistance. The V/sub oc/ vs. X showed its log-linear dependence on I/sub sc/ over 1000 suns. Based on cell improvements for space applications, multijunction cells appear to be ideal candidates for high efficiency, cost effective, PV concentrator systems. Future development of new 1 eV materials for space cells, and further reduction in Ge wafer costs, promises to achieve cells with efficiencies >40% that cost

GaxIn1−xAs thermophotovoltaic converters

0.3/W or less at concentration levels between 300 to 500 suns.

Thermophotovoltaic and photovoltaic conversion at high-flux densities

Abstract Preliminary research into the development of single-junction Ga x In 1− x As thermophotovoltaic (TPV) power converters is reviewed. The devices structures are grown epitaxially on single-crystal InP substrates. Converter band gaps of 0.50–0.74 eV have been considered in accordance with modeling calculations. A 1-sun, AMO efficiency of 12.8% is reported for a lattice-matched, 0.74-eV converter. Converters with lower band gaps are fabricated using lattice-mismatched, compositionally graded structures. Functional TPV converters with good performance characteristics have been demonstrated for band gaps as low as 0.5 eV.

A review of recent advances in thermophotovoltaics

We first discuss the similarities between generation of electricity using thermophotovoltaic (TPV) and high-optical-concentration solar photovoltaic (PV) devices. Following this, we consider power losses due to above- and below-bandgap photons, and we estimate the ideal bandgap by minimizing the sum of these, for a 6000 K black-body spectrum. The ideal bandgap, based on this approach, is less than that previously predicted, which could have a significant influence on the performance of devices and systems. To reduce the losses, we show that the low-energy photons may be removed from both types of cells and consider the specific case of a back-surface reflector. This approach to the management of waste heat may offer a useful additional tool with which to facilitate the design of high-photon-flux solar cells. In the case of the high-energy photons and the associated problem of thermalization of hot electrons, however, the heat must be removed by other means, and we consider the applicability of microchannel cooling systems. These appear to have the potential to handle thermal loads at least several times those generated by 1000 times concentrators, or by black-body TPV radiators at a temperature of far greater than 1500 K. We go on to consider the management of the very high currents generated in both concentrator TPV and PV systems and discuss the concept of the monolithically integrated minimodule.

High-efficiency heteroepitaxial InP solar cells

Thermophotovoltaic (TPV) generation of electricity is attracting attention because of advances in materials and devices and because of a widening appreciation of the large number of applications that may be addressed using TPV-based generators. The attractions include the wide range of fuel sources and the potentially high power density outputs. The two main approaches to TPV generators are: (1) broad-band radiators, coupled with converters with bandgaps in the range 0.4-0.7 eV; and (2) narrow-band emitters coupled with lower-cost silicon converters. The key issues in realizing a viable TPV system are the durability, efficiency and properties of the radiant emitter; the recuperation of sub-bandgap photons; the optimization of the converter performance; and the recuperation of waste heat.

A new thin-film CuGaSe/sub 2//Cu(In,Ga)Se/sub 2/ bifacial, tandem solar cell with both junctions formed simultaneously

High-efficiency, thin-film one-sun and concentrator InP solar cells grown on GaAs substrates are discussed. A novel, compositionally graded heterostructure is used to grow high-quality InP layers. One-sun cells have AM0 efficiencies as high as 13.7% at 25 degrees C (equivalent to 15.7% under the global spectrum). For the concentrator cells at 25 degrees C a peak conversion efficiency of 18.9% under 71.8 AM0 suns has been achieved. Under the direct spectrum, the equivalent efficiency is 21.0% at 88.1 suns. At 80 degrees C, the peak AM0 efficiency is 15.7% at 75.6 suns. Temperature coefficient data for the concentrator cells are also presented. Approaches for further improving the cell performance are discussed.<<ETX>>

Pushing Inverted Metamorphic Multijunction Solar Cells Toward Higher Efficiency at Realistic Operating Conditions

Thin films of CuGaSe/sub 2/ and Cu(In,Ga)Se/sub 2/ were evaporated by the 3-stage process onto opposite sides of a single piece of soda-lime glass, coated bifacially with an n/sup +/-TCO. Junctions were formed simultaneously with each of the p-type absorbers by depositing thin films of n-CdS via chemical bath deposition (CBD) at 60/spl deg/C. The resulting four-terminal device is a nonmechanically stacked, two-junction tandem. The unique growth sequence protects the temperature-sensitive p/n junctions. The initial device (/spl eta/ = 3.7%, V/sub oc/ = 1.1 V [AM1.5]) suffered from low quantum efficiencies. Initial results are also presented from experiments with variations in growth sequence and back reflectors.