Hojun Yoon

40% efficient metamorphic GaInP∕GaInAs∕Ge multijunction solar cells

An efficiency of 40.7% was measured and independently confirmed for a metamorphic three-junction GaInP∕GaInAs∕Ge cell under the standard spectrum for terrestrial concentrator solar cells at 240 suns (24.0W∕cm2, AM1.5D, low aerosol optical depth, 25°C). This is the initial demonstration of a solar cell with over 40% efficiency, and is the highest solar conversion efficiency yet achieved for any type of photovoltaic device. Lattice-matched concentrator cells have now reached 40.1% efficiency. Electron-hole recombination mechanisms are analyzed in metamorphic GaxIn1−xAs and GaxIn1−xP materials, and fundamental power losses are quantified to identify paths to still higher efficiencies.

III-V multijunction solar cells for concentrating photovoltaics

Concerns about the changing environment and fossil fuel depletion have prompted much controversy and scrutiny. One way to address these issues is to use concentrating photovoltaics (CPV) as an alternate source for energy production. Multijunction solar cells built from III–V semiconductors are being evaluated globally in CPV systems designed to supplement electricity generation for utility companies. The high efficiency of III–V multijunction concentrator cells, with demonstrated efficiency over 40% since 2006, strongly reduces the cost of CPV systems, and makes III–V multijunction cells the technology of choice for most concentrator systems today. In designing multijunction cells, consideration must be given to the epitaxial growth of structures so that the lattice parameter between material systems is compatible for enhancing device performance. Low resistance metal contacts are crucial for attaining high performance. Optimization of the front metal grid pattern is required to maximize light absorption and minimize I2R losses in the gridlines and the semiconductor sheet. Understanding how a multijunction device works is important for the design of next-generation high efficiency solar cells, which need to operate in the 45%–50% range for a CPV system to make better economical sense. However, the survivability of solar cells in the field is of chief concern, and accelerated tests must be conducted to assess the reliability of devices during operation in CPV systems. These topics are the focus of this review.

High-efficiency space and terrestrial multijunction solar cells through bandgap control in cell structures

Using the energy bandgap of semiconductors as a design parameter is critically important for achieving the highest efficiency multijunction solar cells. The bandgaps of lattice-matched semiconductors that are most convenient to use are rarely those which would result in the highest theoretical efficiency. For both the space and terrestrial solar spectra, the efficiency of 3-junction GaInP/GaAs/Ge solar cells can be increased by a lower bandgap middle cell, as for GaInAs middle cells, as well as by using higher bandgap top cell materials. Wide-bandgap and indirect-gap materials used in parasitically absorbing layers such as tunnel junctions help to increase transmission of light to the active cell layers beneath. Control of bandgap in such cell structures has been instrumental in achieving solar cell efficiencies of 29.7% under the AMO space spectrum (0.1353 W/cm/sup 2/, 28/spl deg/C) and 34% under the concentrated terrestrial spectrum (AM1.5G, 150-400 suns, 25/spl deg/C), the highest yet achieved for solar cells built on a single substrate.

Next-generation, high-efficiency III-V multijunction solar cells

Next-generation solar cell approaches such as AlGaInP/GaAs/GaInNAs/Ge 4-junction cells, lattice-mismatched GaInP/GaInAs/Ge, concentrator cells, and improved 3-junction device structures hold the promise of greater efficiency than even todays highly successful multijunction cells. Wide-bandgap tunnel junctions, improved heterointerfaces, and other device structure improvements have resulted in several record-efficiency GaInP/GaAs/Ge cell results. Triple-junction (3J) cells grown in this work have demonstrated 29.3% efficiency for space (AMO, 1 sun). Space concentrator 3J cells have efficiency up to 30.0% at low concentration (AMO, 7.6 suns), and terrestrial concentrator cells grown at Spectrolab and processed at NREL have reached 32.3% (AM1.5D, 440 suns).

Advances in High-Efficiency III-V Multijunction Solar Cells

The high efficiency of multijunction concentrator cells has the potential to revolutionize the cost structure of photovoltaic electricity generation. Advances in the design of metamorphic subcells to reduce carrier recombination and increase voltage, wide-band-gap tunnel junctions capable of operating at high concentration, metamorphic buffers to transition from the substrate lattice constant to that of the epitaxial subcells, concentrator cell AR coating and grid design, and integration into 3-junction cells with current-matched subcells under the terrestrial spectrum have resulted in new heights in solar cell performance. A metamorphic Ga 0 .44 In 0 .56 P / Ga 0.92 In 0.08 As/ Ge 3-junction solar cell from this research has reached a record 40.7% efficiency at 240 suns, under the standard reporting spectrum for terrestrial concentrator cells (AM1.5 direct, low-AOD, 24.0 W/cm 2 , 25 ∘ C ), and experimental lattice-matched 3-junction cells have now also achieved over 40% efficiency, with 40.1% measured at 135 suns. This metamorphic 3-junction device is the first solar cell to reach over 40% in efficiency, and has the highest solar conversion efficiency for any type of photovoltaic cell developed to date. Solar cells with more junctions offer the potential for still higher efficiencies to be reached. Four-junction cells limited by radiative recombination can reach over 58% in principle, and practical 4-junction cell efficiencies over 46% are possible with the right combination of band gaps, taking into account series resistance and gridline shadowing. Many of the optimum band gaps for maximum energy conversion can be accessed with metamorphic semiconductor materials. The lower current in cells with 4 or more junctions, resulting in lower I 2 R resistive power loss, is a particularly significant advantage in concentrator PV systems. Prototype 4-junction terrestrial concentrator cells have been grown by metal-organic vapor-phase epitaxy, with preliminary measured efficiency of 35.7% under the AM1.5 direct terrestrial solar spectrum at 256 suns.

Progress of inverted metamorphic III–V solar cell development at Spectrolab

Inverted metamorphic (IMM) solar cells based on III–V materials have the potential to achieve solar conversion efficiencies that are significantly higher than todays state of the art solar cells which are based on the 3-junction GaInP/GaInAs/Ge design. The 3J IMM device architecture based on (Al)GaInP/GaInAs/GaInAs, for example, allows for a higher voltage solar cell by replacing the low bandgap Ge (0.67 eV) from the conventional 3J structure with the higher bandgap (∼1 eV) metamorphic GaInAs. The inverted growth simply allows the lattice-matched junctions (i.e., (Al)GaInP/GaInAs) to be grown first on the growth substrate, thereby minimizing or shielding them from the defects that arise from the metamorphic layers. Spectrolab has demonstrated 30.5% AM0 efficiency based on the 3J IMM cell architecture grown on a Ge substrate, with Voc = 2.963V, Jsc = 16.9 mA/cm2, and FF = 82.5%. In addition, 4J IMM cells have been demonstrated with Voc of 4.072 V and AM0 efficiency approaching 25%. With additional development, demonstrating 33% AM0 efficiency is expected in the near future. However, the IMM devices demand more complex processing requirements than conventional solar cells, and we demonstrate the capability to fabricate large area solar cells from standard Ge solar cell substrates.

Metamorphic GaInP/GaInAs/Ge solar cells

High-efficiency, metamorphic multijunction cells have been fabricated by growing GaInP/GaInAs subcells that are lattice-mismatched to an active Ge substrate, resulting in GaInP/GaInAs/Ge 3-junction (3J) cells. The efficiency dependence of this 3J cell on lattice-constant of the top two cells and on sublattice ordering in the GaInP top cell is presented. A variety of composition-graded buffers have been explored through X-ray diffraction reciprocal space mapping to measure strain in the cell layers, and transmission electron microscopy to minimize misfit and threading dislocations. Quantum efficiency is measured for metamorphic 1.3-eV Ga/sub 0.92/In/sub 0.08/As (8%-ln GaInAs) cells and 1.75-eV Ga/sub 0.43/In/sub 0.57/P cells grown on a Ge substrate, as well as for the 3J cell based on 4%-in GaInAs. Three-junction Ga/sub 0.43/In/sub 0.57/P/Ga/sub 0.92/In/sub 0.08/As/Ge cells with 0.50% lattice-mismatch to the Ge substrate are measured to have AMO efficiency of 27.3% (0.1353 W/cm/sup 2/, 28/spl deg/C), similar to high-efficiency, conventional GaInP/GaAs/Ge 3-junction cells based on the GaAs lattice constant.

Advanced III-V Multijunction Cells for Space

III-V solar cells have become the dominant power generation technology in space, due to their unparalleled high efficiency, reliability in the space environment, and ability to be integrated into very lightweight panels. As remarkable as these attributes are, new types of space III-V solar cells are continually reaching new heights in performance. Commercially-available multijunction solar cells with 30% conversion efficiency under the AM0 space spectrum are just around the corner. Understanding of radiation resistance and thermal cycling reliability has reached levels never before attained, and is resulting in new standards of reliability. A flurry of research activity has resulted in very-thin, flexible, and extremely lightweight space solar cells and panels in several groups around the world, capable of being folded or rolled into a smaller stowage volume for launch than has been possible to date. This approach combines the very high efficiency and reliability of III-V multijunction cells with the thin, flexible PV blanket functionality normally associated only with thin-film polycrystalline or amorphous PV technology. This paper discusses the latest developments in III-V space solar cell technology, and explores opportunities for still higher performance in the future

High-efficiency multijunction photovoltaics for low-cost solar electricity

Multijunction solar cells divide the solar spectrum into smaller slices, delivering experimental efficiencies over 40%, and enabling theoretical efficiency over 60%. These high efficiency cells have a powerful effect on the cost effectiveness of new concentrator photovoltaic systems now being deployed around the world, making this technology one of the most viable options for plentiful solar-generated electricity.

Minority carrier lifetime and radiation damage coefficients of germanium

We report on the measurement of minority carrier lifetime and on the radiation damage resistance of bulk Ge. Lifetime measurements are performed using the resonance-coupled photoconductive decay (RCPCD) method. Specifically, we examine the dependence of the lifetime as a function of the Ge resistivity and various 1 MeV electron radiation fluences. We measure hole lifetimes ranging from /spl sim/0.9-34 /spl mu/s for n-type Ge samples, corresponding to diffusion lengths of /spl sim/30-400 /spl mu/m. Electron lifetimes in p-type Ge range from /spl sim/0.6-19 /spl mu/s, corresponding to diffusion lengths of /spl sim/30-420 /spl mu/m. Lifetime measurements are also made after exposure to 1 MeV electron fluences ranging from 10/sup 13/ to 10/sup 15/ cm/sup -2/ and these results are used to estimate the minority carrier lifetime and diffusion length damage coefficients K/sub /spl tau// and K/sub L/.