Paul Sharps

Development of a large area inverted metamorphic multi-junction (IMM) highly efficient AM0 solar cell

We have identified the inverted metamorphic multi-junction (IMM) structure as the best vehicle to achieve increased solar cell efficiency. In this paper, advantages of this approach are listed, challenges are discussed, and results for component and integrated sub-cells are presented. The inverted dual junction lattice matched (IDJLM) cell demonstrated an AM0 Voc and efficiency of 2.451 V and 26.4%, respectively. A 1.0 eV filtered inverted metamorphic (IM) cell exhibited an AM0 Voc and efficiency of 0.505 V and 6.0%, respectively. The component cells were combined into a 3J-IMM 2cm × 2cm cell exhibiting an AM0 (135.3 mW/cm2) efficiency of 32.0±1%. A 26.3 cm2 cell achieved ≫30% AM0 efficiency

Experimental results from performance improvement and radiation hardening of inverted metamorphic multi-junction solar cells

This paper discusses results from continued development of inverted metamorphic multi-junction (IMM) solar cells with air mass zero (AM0) conversion efficiencies greater than 34%. An experimental best four-junction IMM (IMM4J) design is presented. In an effort to improve IMM performance in space radiation environments, 1-MeV electron irradiation studies are conducted on the individual IMM4J subcells. This data is used to engineer an IMM4J structure with beginning of life (BOL) AM0 conversion efficiency of approximately 34% and an end of life (EOL) remaining factor greater than 82%, where EOL is defined as performance after exposure to 1-MeV electron irradiation at 1E15 e/cm2 fluence. Next generation IMM designs are explored and an avenue toward AM0 conversion efficiencies of greater than 35% is presented.

Temperature dependent spectral response measurements for III-V multi-junction solar cells

Temperature coefficients for the integrated current of all three subcells in a production InGaP/InGaAs/Ge solar cell structure have been measured at temperatures ranging from 5/spl deg/C to 100/spl deg/C. The InGaP, InGaAs, and germanium temperature coefficients are 0.011, 0.009, and 0.044 mA/cm/sup 2///spl deg/C, respectively. This data can be used to design multi-junction solar cells for optimum performance at any specified operating temperature in this range. The predicted current mismatch for a similar triple junction operating at 100/spl deg/C but designed to be current matched at 28/spl deg/C is approximately 3%.

Consideration of High Bandgap Subcells for Advanced Multijunction Solar Cells

Multijunction solar cell theoretical modeling has been performed as a function of the subcell bandgap energies. This modeling guides the development of advanced multijunction cells. In this report we focus on analyzing the sensitivity of 3, 4, 5, and 6 junction solar cells to the bandgap energy of the high bandgap subcell(s) in the device. This work is motivated by the importance of high bandgap subcells for achieving high efficiency multijunctions, and the lack of proven high bandgap photovoltaic materials above 1.97eV. Modeling of 3-6 subcell multijunctions shows the importance of achieving high performance high bandgap subcells for high efficiency, particularly as the number of junctions increases. In this paper several practical situations are analyzed, including the use of the AM0 spectrum for space applications and low AOD (aerosol optical depth) direct spectrum for terrestrial concentrator applications

Development of a four sub-cell inverted metamorphic multi-junction (IMM) highly efficient AM0 solar cell

We have identified the IMM structure as the best vehicle to achieve increased AM0 solar cell efficiency beyond the conventional 3-junction lattice matched GaInP/GaAs/Ge architecture. Building on our efforts to develop a 3J-IMM III–V based cell presented at the 33rd PVSC [1], we have developed a 4J-IMM AM0 solar cell. The top three sub-cells are identical to that incorporated in our 3J-IMM cell. The fourth sub-cell is composed of 0.70eV GaInAs. This cell structure required an extension of the transparent metamorphic (MM) buffer layer to accommodate an additional 2% mismatch, a metamorphic tunnel diode, and the fourth (MM) sub-cell‥ The processing steps to fabricate this cell are very similar to those previously discussed with regard to the 3J-IMM cell. At 28°C, our best CICed (cover-glass interconnected cell) 4J-IMM 2×2 cm2 cell achieved a 33.9% AM0 (135.3 mW/cm2) efficiency.

Multi-terminal dual junction InGaP 2 /GaAs solar cells for hybrid system

The performance of two and three-terminal solar cells under 13-sun AM1.5G illumination is reported. The solar cells are comprised of monolithically grown InGaP2 and GaAs single junction cells. The two-terminal device consists of single junctions in series. The three-terminal device structure comprises the single junctions independently interconnected by an InGaP2 layer serving as the middle contact. Device optimization was based on modeling of the InGaP2 top junction band gap for various spectral irradiance profiles. It was found that the optimal band gap combination for a 2.6eV filtered AM1.5G spectrum is achieved with 1.84eV and 1.43 eV for the top and bottom junctions, respectively. External quantum efficiency and illuminated current-voltage (I–V) measurements of the component two and three-terminal tandem cells are discussed.

Design and production of extremely radiation-hard 26% InGaP/GaAs/Ge triple-junction solar cells

The authors report the design and testing of extremely radiation-hard high-efficiency large-area InGaP/GaAs/Ge triple-junction solar cells. The solar cell junctions are designed for longer minority carrier diffusion lengths after particle irradiation. The power remaining factors after 5E14 and 1E15 electrons/cm/sup 2/ 1-MeV electron radiation are 92% and 87.5%, respectively. These results are highest reported to date and are extremely desirable for electrical power design of the spacecraft. Furthermore, the InGaP/GaAs/Ge triple-junction solar cells are currently in production at EMCORE Photovoltaics. Minimum average AM0 efficiency for the large-area fight cells is 26%, with efficiencies as high as 27% demonstrated for a large number of cells.

Development of 1.25 eV InGaAsN for triple junction solar cells

Current GaInP/sub 2//GaAs/Ge triple junction solar cells currently starting production at EMCORE have achieved average lot efficiencies of greater than 26%, with an EOL/BOL of 92% for exposure to 1 MeV electrons at a fluence of 5/spl times/10/sup 14/ e/cm/sup 2/. Development of the next generation high efficiency multijunction solar cell will involve the development of new materials lattice matched to GaAs. One material of interest is 1.05 eV InGaAsN, to be used in a four junction GaInP/sub 2//GaAs/InGaAsN/Ge device. Despite several years of effort, the development of the 1.05 eV InGaAsN material has been difficult. As an alternative, they have been looking at 1.25 eV InGaAsN for use in a GaInP/sub 2//InGaAsN/Ge triple junction cell. The authors present results for their work with the 1.25 eV InGaAsN material.

Strain Effects on Radiation Tolerance of Triple-Junction Solar Cells With InAs Quantum Dots in the GaAs Junction

A comparison of quantum dot (QD) triple-junction solar cells (TJSCs) with the QD superlattice under tensile strain are compared with those under compressive strain and baseline devices to examine the effects of strain induced by the InAs QD layers in the middle junction. Theoretical results show samples with tensile-strained InAs QDs have lower defect formation energy while compressive-strained QDs have the greatest. Experimentally, it is found that tensile strain leads to degradation of i-region material at values of -706 ppm. Irradiating with 1-MeV electrons, TJSCs with tensile strain exhibit a faster degradation in Isc of the QD samples and slower degradation in Voc but overall faster degradation in efficiency compared with baseline TJSCs, regardless of the magnitude of tensile strain. Compressively strained QD TJSCs have similar degradation in Isc and slower degradation in Voc compared with baseline TJSCs. From this study, it is determined that a slightly compressive strain in the QD superlattice allows for the best performance pre- and postirradiation for QD TJSCs based upon AM0 IV and quantum efficiency measurements and analysis. Fabricating devices with improvements determined from samples with varying strain leads to QD TJSCs with better radiation tolerance in terms of power output for 5, 10, 15, and 20 layers of QDs.

Investigation of quantum dot enhanced triple junction solar cells

InAs quantum dots have been incorporated into the middle junction of an InGaP/(In)GaAs/Ge triple junction solar cell (TJSC) on four inch wafers, in aims of band gap engineering a high efficiency solar cell to even higher limits. Results of QD growth on 4” diameter Ge templates gave densities near 1×1011 cm−3 and QD height between 2–5 nm. Arrays of 10 layers of InAs QDs have been grown between the base and emitter in the middle cell of a full triple junction solar cell. Control triple junction cells that received growth interrupts without QD growth showed similar results (within 5 mV open circuit voltage) to standard triple junction cells without an interrupt. Integrated current of the (In)GaAs junction with 10 layers of strain balanced InAs QD layers shows a gain of 0.37 mA/cm2 beyond the band edge. One sun AM0 current-voltage measurements of QD TJSC show an efficiency of 26.9% with a Voc of 2.57 V.