Kenneth M. Edmondson

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.

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.

Development of advanced space solar cells at Spectrolab

High efficiency multi-junction solar cells utilizing inverted metamorphic1,2 and semiconductor bonding technology3 are being developed at Spectrolab for use in one-sun space and near-space applications. Recently that effort has been extended to include low-concentration space applications. This paper will review the present state-of-the-art cell technologies at Spectrolab, with an emphasis on performance characterization data at both 1-sun and low-concentration operating conditions that these cells will experience in flight‥ A cell coupon utilizing IMM solar cells has been assembled and subjected to thermal cycling. Pre-and post thermal cycling data have been collected and there is no performance degradation or mechanical issues after the test.

Toward 40% and higher solar cells in a new Cassegrainian PV module

We demonstrate a 40% efficient multijunction (MJ) cell capable of being used to make a 33% efficient solar concentrator PV module using the new Cassegrainian PV module design. This very high efficiency module can then produce cost competitive solar electric power.

High-voltage, low-current GaInP/GaInP/GaAs/GaInNAs/Ge solar cells

Four-junction GaInP/GaAs/GaInNAs/Ge solar cells are a widely-pursued route toward AM0 efficiencies of 35% and above, and terrestrial efficiencies of up to 40%. Extensive research into the new material system of GaInNAs has so far yielded subcells with AM0 current densities far below the /spl sim/17 mA/cm/sup 2/ needed to current match the other subcells in the stack. A new multijunction structure, a 5-junction GaInP/GaInP/GaAs/GaInNAs/Ge cell, divides the solar spectrum more finely in order to relax this current matching requirement, by using an optically thin, high-bandgap GaInP top subcell, with an additional thick, low-bandgap GaInP subcell beneath it, in combination with a GaInNAs subcell. In this way, the 5-junction cell design allows the practical use of GaInNAs subcells to increase the efficiency of multijunction cells. Light I-V and external quantum efficiency measurements of the component subcells of such 5-junction cells are discussed. Experimental results are presented for the first time on GaInP/GaInP/GaAs/GaInNAs/Ge cells with the top four junctions active, with measured V/sub oc/ of 3.90 V.

Metamorphic and Lattice-Matched Solar Cells Under Concentration

Metamorphic III-V semiconductor materials offer access to bandgaps that span key portions of the solar spectrum, enabling new bandgap combinations in multijunction solar cells, and increasing both theoretical and practical efficiency limits for terrestrial concentrator cells. Experimental results are given for the quantum efficiency of metamorphic GaInAs solar cells with bandgap from 1.1 to 1.4 eV, and for metamorphic GaInP with both ordered and disordered group-III sublattices. Variable intensity Jsc vs. Voc measurements are used to compare recombination components due to n=1 and n=2 mechanisms in metamorphic and lattice-matched GaInAs, GaInP, and 3-junction solar cells. A record efficiency metamorphic GaInP/GaInAs/Ge 3-junction solar cell has been produced with 38.8% efficiency independently confirmed (241 suns, AM1.5D, low-AOD, 25degC), essentially equaling the performance of a lattice-matched 3-junction cell with 39.0% efficiency, the highest efficiency yet demonstrated and verified for a solar photovoltaic conversion device. With the combination of high-quality metamorphic materials that are increasingly less controlled by recombination at dislocations, and the higher efficiency limits afforded by freedom of lattice constant selection, practical terrestrial concentrator cell efficiencies well over 40% are expected in the near future

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

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/.