P.R. Sharps

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.

Photoluminescence of porous silicon buried underneath epitaxial GaP

Recent observations of visible, room‐temperature photoluminescence in porous Si have stimulated research aimed at the realization of efficient, Si‐based electroluminescent devices. To achieve electroluminescence, it may be beneficial to generate carriers with sufficient energy to populate the states of the quantum‐confined Si structures. A viable method to accomplish this is to utilize a wide‐band‐gap heterojunction injector, such as GaP. Toward that end, we report the successful formation of porous Si buried underneath GaP islands, and we demonstrate that the buried porous Si layer exhibits strong photoluminescence (λ≊7000 A).

Wafer bonding for use in mechanically stacked multi-bandgap cells

Two and three junction monolithic two-terminal solar cells have been developed that have 1-Sun, AM0 efficiencies of greater than 25%. In order to reach 1-Sun efficiencies of 30% and greater, solar cells with more junctions are required. Mechanically stacking junctions with different band gaps provides a means of developing such a cell. The authors propose a four-junction, mechanically stacked cascade solar cell structure that projects to a 34.8% AM0 efficiency. Wafer bonding provides a means of mechanically joining semiconductor materials with different lattice constants. They present optical, electrical and mechanical data on wafer bonding GaAs and InP substrates. The data indicate that wafer bonding can be used to develop a four-junction device.

Radiation response of InP/Si and InGaP/GaAs space solar cells

Abstract An analysis of the radiation response of state-of-the-art InP/Si, InGaP, and dual junction (DJ) InGaP/GaAs space solar cells under both electron and proton irradiated is presented. The degradation data are modeled using the theory of displacement damage dose. For each technology, a characteristic curve which describes the cell degradation in any radiation environment is determined, and the characteristic curves are used to compare the radiation resistance of the different technologies on an absolute scale. The radiation data are used as input to a code which predicts the end-of-life (EOL) performance of a solar panel in earth orbit. The results show that in orbits outside the earths radiation belts, the high-efficiency DJ InGaP/GaAs solar panels provide the highest EOL specific power. However, in orbits which pass through the belts, the radiation hard InP/Si panels provide the highest specific power by as much as 30%.

High-efficiency tandem solar cells on single- and poly-crystalline substrates

Abstract This paper will review and assess the current status of the development of tandem solar cells for space and terrestrial applications. We will also introduce and present results on a new In 0.49 Ga 0.51 P / GaAs tandem cell grown and fabricated on a low-cost, polycrystalline Ge substrate.

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.

The Role of Silicon Monohydride and Dihydride in the Photoluminescence of Porous Silicon and Photoluminescence of Porous Silicon Buried Underneath Epitaxial GaP

Thermal annealing studies of the photoluminescence (PL) intensity and Fourier-transform infrared (FTIR) spectroscopy have been performed concurrently on porous Si. A sharp reduction in the PL intensity is observed for annealing temperatures > 300 °C and this coincides with desorption of hydrogen from the SiH 2 surface species. The role of silicon hydride species on the photoluminescence intensity has been studied. The surfaces of luminescent porous Si samples were converted to a predominate SiH termination using a remote H-plasma. The as-passivated samples were then immersed in various concentrations of hydrofluouric solutions to regulate the recovery of SiH 2 termination on the surface. Photoluminescence measurements and transmission Fourier-transform infrared spectroscopy have shown that predominant silicon monohydride (SiH) termination results in weak photoluminescence. In contrast, it has been observed that the appearance of silicon dihydride (SiH 2 ) coincides with an increase in the photoluminescence intensity. To achieve electroluminescence it will be beneficial to generate carriers with sufficient energy to populate the states of the quantum-confined Si structures. A viable method to accomplish this is to utilize a wide-bandgap heterojunction injector such as GaP. Toward that end we report the successful formation of porous Si buried underneath GaP islands and we demonstrate that the buried porous Si layer exhibits strong photoluminescence.

Development of p/n and n/p thick emitter InP solar cells

Both n/p and p/n InP thick emitter (0.3 /spl mu/m) space solar cells with and without Ga/sub 0.5/In/sub 0.5/P windows are studied. Both polarity cells are considered for possible growth on Ge. While not achieving the high efficiencies of thin emitter InP cells, the thicker emitter cells may provide an advantage in radiation hardness. The Ga/sub 0.5/In/sub 0.5/P window has little effect on cell performance, for either polarity.

Development of an IR‐transparent, inverted‐grown, thin‐film, Al0.34Ga0.66As/GaAs cascade solar cell

Inverted growth and the development of associated cell processing, are likely to offer a significant degree of freedom for improving the performance of many III‐V multijunction cascades and open new avenues for advanced multijunction concepts. This is especially true for the development of high‐efficiency Al0.37Ga0.63As/GaAs cascades where the high growth temperatures required for the AlGaAs top cell growth can cause the deterioration of the tunnel junction interconnect. In the approach of inverted‐grown AlGaAs/GaAs cascade cells, the AlGaAs top cell is grown first at 780 °C and the GaAs tunnel junction and bottom cell are grown at 675 °C. After the inverted growth, the AlGaAs/GaAs cascade structure is selectively removed from the parent substrate. The feasibility of inverted growth is demonstrated by a fully‐processed, inverted‐grown, thin film GaAs cell with a 1‐sun AM1.5 efficiency of 20.3%. Also, an inverted‐grown, thin‐film, Al0.34Ga0.66As/GaAs cascade with AM1.5 efficiencies of 19.9% and 21% at 1‐su...

15.8%-efficient (1-sun, AM1.5G) GaAs solar cell on optical-grade polycrystalline Ge substrate

The authors present the first demonstration of a high efficiency GaAs solar cell on a commercially available, cast, optical-grade, polycrystalline Ge substrate. Under AM1.5G simulation, NREL-measured cell parameters include a V/sub oc/ of 0.9323 V, a J/sub sc/ of 24.87 mA/cm/sup 2/, a fill-factor of 0.6822, and an efficiency of 15.8%. This represents the highest 1-sun total-area efficiency for a polycrystalline solar cell in any compound semiconductor even though it is only an initial attempt at the demonstration of a poly GaAs cell with efficiencies comparable to those obtained in state-of-the-art CdTe and CIS material systems. The cell area is about 1 cm/sup 2/ and was grown by organometallic vapor phase epitaxy. The poly GaAs p/sup +/-n junction has been characterized by dark I-V measurements. The choice of junction polarity (p/sup +/-n vs. n/sup +/-p) for the growth of GaAs solar cell on poly-Ge substrates is discussed based on the reduction of dark currents originating from the grain boundaries. Introduction of an undoped spacer in the p/sup +/-n junctions is shown to significantly improve the V/sub oc/ and fill-factor values in small-area (0.174 cm/sup 2/) cells where series resistance is not limiting the fill factor, Spectral-response measurements indicate further improvement possible in the red response of the poly GaAs solar cells, I-V data indicate series resistance is presently limiting the efficiency of 1-cm/sup 2/-area cells. Further development is likely to lead to 1-sun efficiency approaching over 21 percent and concentrator (500 suns) efficiencies approaching 24 percent in the near-term.<<ETX>>