Moran Haddad

GaInP∕GaAs dual junction solar cells on Ge∕Si epitaxial templates

In this study, we report synthesis of large area (≫ 2 cm2) crack-free GaInP/GaAs double junction solar cells on 50 mm diameter Ge/Si templates fabricated using wafer bonding and ion implantation induced layer transfer techniques. Defect removal from the template film and film surface prior to epitaxial growth was found to be critical to achievement of high open circuit voltage and efficiency. Cells grown on templates prepared with chemical mechanical polishing in addition a wet chemical etch show comparable performance to control devices grown on bulk Ge substrates. Current-voltage (I–V) data under AM 1.5 illumination indicate that the short circuit current is comparable in templated and control cells, but the open circuit voltage is slightly lower (2.08V vs. 2.16V). Spectral response measurements indicate a drop in open circuit voltage due to a slight lowering of the top GaInP cell band gap. The drop in band gap is due to a difference in the indium composition in the two samples caused by the different miscut (9° vs. 6°) of the two kinds of substrates.

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.

Multijunction Solar Cells for Dense-Array Concentrators

A major step forward has been made towards cost reduction of terrestrial PV. World-record multijunction III-V solar cells have been integrated into a commercial concentrator photovoltaic (CPV) system. A dense array of high-efficiency solar cells in the receiver of a high-intensity (~500X) concentrator system has been identified as a viable, cost-effective system. Concentrator ultra triple junction (CUTJ) cells have been developed for use in the Solar Systems CS500 solar electric power generator. The cell is designed for efficient conversion of the specific solar spectrum delivered to the system receiver while minimizing cell cost. Cells are optimized for maximum active area in a Solar Systems dense-array cell module. Solar Systems modules using CUTJ dense-array cells have demonstrated module efficiencies of over 35%. Field testing of CUTJ dense-array cells in a CS500 CPV dish unit at the Hermannsburg solar power plant in Australia was initiated in December 2005. A full multi-junction receiver in a CS500 dish has delivered over 30kW with an efficiency of almost 30%. Following qualification, these systems are slated for entry into the terrestrial market in 2006

Triple-junction GaInP/GaAs/Ge solar cells-production status, qualification results and operational benefits

In 1999 Spectrolab completed design and qualification, and began production on the next generation of multijunction solar cells-a triple-junction GaInP/GaAs/Ge. With over 20% AM0 conversion efficiency at an operating temperature of 60/spl deg/C, this cell provides 8-11% more power than competing dual-junction designs in GEO orbit after 15 years (6/spl times/10/sup 14/ 1-MeV electron equivalence). Spectrolab is currently qualifying an improved triple-junction cell capable of delivering over 22% AM0 conversion efficiency under these same conditions, with a beginning-of-life operating efficiency of 27%.

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.

Advancements in GaInP/sub 2//GaAs/Ge solar cells - production status, qualification results and operational benefits

In 2001 Spectrolab completed design and qualification, and began production on the third-generation multijunction solar cell - the improved triple-junction GaInP/sub 2//GaAs/Ge. With over 21% AMO conversion efficiency at an operating temperature of 60/spl deg/C at end-of-life, this cell has 16% more power than competing dual-junction designs in GEO orbit after 15 years (7/spl times/10/sup 14/ 1-MeV electron equivalence). Spectrolab is currently qualifying the fourth-generation triple-junction solar cell capable of delivering over 22% AMO conversion efficiency under these same conditions, with a beginning-of-life operating efficiency of 28%.

Multijunction solar cells with subcell materials highly lattice-mismatched to germanium

The performance of a series of metamorphic GaInP and GaInAs solar cells grown on Ge with lattice-mismatch ranging from 0% to 2.4% is reported, with emphasis on device structures with 0.5% and 1.6% mismatch. Dual-junction cells with moderately lattice-mismatched (0.2% and 0.5%) structures have already reached electrical performance comparable to lattice-matched devices, at about 26% efficiency under AM0, 1-sun condition. Development efforts to date on highly lattice-mismatched (1.6% mismatch) structures have resulted in 22.6% efficiency dual-junction cells, with many improvements still possible. Spectral response measurements reveal excellent quantum efficiency (QE) for metamorphic GaInP and GaInAs materials, with a measured internal QE of over 90%. The offsets between the bandgap voltage (E/sub g//q) and the open-circuit voltage (V/sub OC/) of GaInP and GaInAs metamorphic cells were kept below 550 mV and 450 mV, respectively. Experimental results indicate that lattice-mismatched GalnP/GalnAs dual-junction cells can achieve higher energy conversion efficiency than lattice-matched GaInP/GaInAs dual-junction solar cells.

GaInP/GaAs dual junction solar cells on Ge/Si epitaxial templates

In this study, we report synthesis of large area (≫ 2 cm2) crack-free GaInP/GaAs double junction solar cells on 50 mm diameter Ge/Si templates fabricated using wafer bonding and ion implantation induced layer transfer techniques. Defect removal from the template film and film surface prior to epitaxial growth was found to be critical to achievement of high open circuit voltage and efficiency. Cells grown on templates prepared with chemical mechanical polishing in addition a wet chemical etch show comparable performance to control devices grown on bulk Ge substrates. Current-voltage (I–V) data under AM 1.5 illumination indicate that the short circuit current is comparable in templated and control cells, but the open circuit voltage is slightly lower (2.08V vs. 2.16V). Spectral response measurements indicate a drop in open circuit voltage due to a slight lowering of the top GaInP cell band gap. The drop in band gap is due to a difference in the indium composition in the two samples caused by the different miscut (9° vs. 6°) of the two kinds of substrates.

The effects of electron irradiation on triple-junction Ga/sub 0.5/In/sub 0.5/P/GaAs/Ge solar cells

Ga/sub 0.5/In/sub 0.5/P/GaAs/Ge solar cells have been fabricated at Spectrolab under the Multijunction Solar Cell Manufacturing Technology (Mantech) program, sponsored by the US Air Force and NASA. The cells were irradiated with increasing 1 MeV electron fluences, and the degradation of their PV parameters was characterized using light I-V and external QE. Analysis of the PV parameters of the GaInP top subcell showed little degradation, and was not a limitation for triple junction (3J) cell performance. Furthermore, the radiation degradation of the Ge subcell PV parameters was almost negligible. The GaAs subcell I/sub sc/, however, did limit the device performance as is traditionally documented. The final-to-initial maximum power ratio (P/P/sub 0/) of 3J cells was near 0.833 at a fluence of 1/spl times/10/sup 15/ e/sup -//cm/sup 2/, and matches Spectrolabs presently established value of 0.83 for standard production of space-qualified dual-junction cells.