J.S. Hills

18.2% (AM1.5) efficient GaAs solar cell on optical-grade polycrystalline Ge substrate

In this work, the authors present GaAs material and device-structure optimization studies that have led to achieve a open-circuit voltage of /spl sim/1 volt and a best solar cell efficiency of 18.2% under AM1.5G illumination, for a 4 cm/sup 2/ area GaAs cell on commercially-available, cast, optical-grade polycrystalline Ge substrate. This V/sub /spl infin// is almost 70 mV higher than on their previously-reported best GaAs cell on similar substrates. They discuss the growth of high-quality GaAs-AlGaAs layers, across the various crystalline orientations of a polycrystalline Ge substrate, important for obtaining good device performance. Optimization studies of the minority-carrier properties of GaAs layers on poly-Ge substrates have revealed that lifetime-spread across various grains can be reduced through the use of lower doping for the Al/sub 0.8/Ga/sub 0.2/As confinement layers. The cell-structure optimization procedures for improved V/sub /spl infin// and cell efficiency, include the use of thinner emitters, a spacer layer near the p/sup +/-n junction and an improved window layer. An experimental study of dark currents in these junctions, with and without the spacer, as a function of temperature (77 K to 288 K) is presented indicating that the spacer reduces the tunneling contribution to dark current.

An inverted-growth approach to development of an IR-transparent, high-efficiency AlGaAs/GaAs cascade solar cell

An approach for inverted-grown AlGaAs/GaAs cascade cells, where the AlGaAs top cell is grown first at high temperatures, placing the surface to be illuminated nearest to the substrate, is presented. Following the growth of the top cell, the GaAs tunnel interconnect and the bottom cell are grown at lower temperatures. After the inverted growth, the AlGaAs/GaAs cascade structure is selectively removed from the parent substrate, Ge in this case. Advantages of the inverted-growth approach are discussed.<<ETX>>

Development of 20% efficient GaInAsP solar cells

The authors report on the development of two compositions of a GaInAsP lattice-matched to GaAs for photovoltaic applications. Successful development of cascade solar cells necessitates the development of both high bandgap (1.5 to 1.9 eV) as well as low bandgap (0.7 to 1.4 eV) materials. The GaInAsP lattice-matched to GaAs is an excellent candidate for the high band gap material. Ga/sub 0.84/In/sub 0.16/As/sub 0.68/P/sub 0.32/ cells, with a band gap of 1.55 eV, have been developed that have demonstrated a V/sub oc/ of 1.047 volts, a J/sub sc/ of 22.5 mA/cm/sup 2/, a fill factor of 0.849, and an active area efficiency of 21.8 per cent under AM1.5 direct illumination. A Ga/sub 0.84/In/sub 0.16/As/sub 0.68/P/sub 0.32/ tunnel diode has also been developed with a peak current of 4.33/spl times/102/spl sim/mA/cm/sup 2/ at a voltage of 65 mV. Both the Ga/sub 0.84/In/sub 0.16/As/sub 0.68/P/sub 0.32/ cell and tunnel diode are being used in conjunction with a Ge cell to develop a monolithic Ga/sub 0.84/In/sub 0.16/As/sub 0.68/P/sub 0.32Ge cascade cell. Ga/sub 0.68/In/sub 0.32/As/sub 0.34/P/sub 0.66/ cells, with a band gap of 1.7 eV, have been developed that have demonstrated a V/sub oc/ of 1.161 volts, a J/sub sc/ of 28.9 mA/cm2 a fill factor of 0.86, and an active area efficiency of 21.4 per cent under AMO illumination. The Ga/sub 0.68/In/sub 0.32/As/sub 0.34/P/sub 0.66/ cells have also demonstrated resistance to radiation damage as well as a recovery of preirradiation performance after low temperature annealing.<<ETX>>

Thermal photovoltaic cells

Most current emphasis is on GaInAs alloys or GaSb for thermal photovoltaic converters operating in a band gap range between about 0.50 to 0.75 eV. In this paper the growth and fabrication of GaInAs devices with nominal band gaps of 0.6 eV are described. Yield statistics are presented for the growth of a large number of devices, and I‐V data are presented. Alternative cell structures are also described, and manufacturing issues are discussed.

Silicon and GaAs/Ge concentrator power plants: a comparison of cost of energy produced

Until now virtually all of the development of terrestrial concentrator PV power plants has utilized silicon cell technology. If silicon concentrator technology achieves its predicted potential performance and cost, it would be very close to commercial viability as defined by DOE goals of 12 c/kWh. Also until now, GaAs cell technology has not been a viable alternative for these power plants, largely due to the cost of cells grown on GaAs substrates. The ability to grow high-efficiency GaAs solar cells on germanium substrates presents the strong possibility of reducing their cost of energy produced. The cost of large-grain, optical-grade, polycrystalline germanium substrates is potentially 15 times less than single-crystal GaAs substrates, and cell conversion efficiencies approaching 35% are likely for tandem-junction GaAs cells. This analysis shows that for comparable 50 MW Fresnel lens plants, the 30-year, levelized cost of energy (COE) from GaAs/Ge cells can be

High-temperature performance and radiation resistance of high-efficiency Ge and Si/sub 0.07/Ge/sub 0.93/ solar cells on lightweight Ge substrates

0.118/kWh compared to

Advances in the development of an AlGaAs/GaAs cascade solar cell using a patterned germanium tunnel interconnect

0.140/kWh for a 27.4% Si back-contact cell.

Back surface fields for n/p and p/n GaInP solar cells

Ge and Si/sub 0.07/Ge/sub 0.93/ alloys are potential candidates for low-bandgap junctions in multijunction solar cells. The radiation resistance of these materials to 1 MeV electrons has been measured, yielding end-of-life/beginning-of-life ratios of about 0.85 at a fluence of 10/sup 16/ electrons/cm/sup 2/. For Ge, the temperature coefficient of efficiency has been determined to be -0.106%/ degrees C under a full AM0 spectrum and -0.048%/ degrees C under a GaAs filter. Si/sub 0.7/Ge/sub 0.93/ cells have shown no performance degradation after exposure to temperatures as high as 500 degrees C, and the temperature coefficient of efficiency for Si/sub 0.07/Ge/sub 0.93/ junctions is about -0.045%/ degrees C, about the same as Si in spite of a lower bandgap by almost 200 meV.<<ETX>>

Graded-band-gap AlGaAs solar cells for AlGaAs/Ge cascade cells

Abstract In this paper, we discuss various aspects of the development of an inverted-grown AlGaAs/GaAs cascade solar cell incorporating a patterned germanium tunnel junction. Topics include the development of the Al 0.37 Ga 0.63 As top cell, the growth of the GaAs bottom cell over the patterned germanium tunnel junction, and a technique for selective removal of thin AlGaAs/GaAs heterostructures after lattice-matched growth of germanium substrates. The problems to be overcome for the achievement of around 30% efficiencies in the AlGaAs/GaAs cascade cell under concentrator applications are also discussed.

GaInAsP lattice matched to GaAs for solar cell applications

The authors have grown n/p/p/sup +/ and p/n/n/sup +/ GaInP/sub 2/ top cells, lattice matched to GaAs for multijunction tandem solar cells. They have studied the effect of different types of back surface field (BSF) layers on the cell parameters of each cell. These layers are namely, abrupt strained GaInP/sub 2/, graded strained GaInP/sub 2/, disordered GaInP/sub 2/ and AlGaAs layers. The measurements were done under 1-sun AMO spectrum. The results show that the abrupt strained GaInP/sub 2/, BSF outperforms the disordered GaInP/sub 2/ BSF. Likely related to material quality, the AlGaAs layer clearly produce the least efficient BSF. It has been found that the abrupt strained Ga/sub x/In/sub 1-x/P BSF with x=0.56 gives better results than the case with x=0.63 for the p/n/n/sup +/ structure.