Daniel J. Aiken

Direct-bonded GaAs∕InGaAs tandem solar cell

A direct-bonded GaAs/InGaAs solar cell is demonstrated. The direct-bonded interconnect between subcells of this two-junction cell enables monolithic interconnection without threading dislocations and planar defects that typically arise during lattice-mismatched epitaxial heterostructure growth. The bonded interface is a metal-free n+GaAs/n+InP tunnel junction. The tandem cell open-circuit voltage is approximately the sum of the subcell open-circuit voltages. The internal quantum efficiency is 0.8 for the GaAs subcell compared to 0.9 for an unbonded GaAs subcell near the band gap energy and is 0.7 for both of the InGaAs subcell and an unbonded InGaAs subcell, with bonded and unbonded subcells similar in spectral response.

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.

Evolution of a 2.05 eV AlGaInP top sub-cell for 5 and 6J-IMM applications

This paper discusses the evolution of our 2.05 eV AlGaInP top sub-cell. Although we were not successful in introducing a more optimum 1.94 eV AlGaInP alloy into the top sub-cell of our 3 or 4J-IMM (inverted metamorphic multi-junction) structure because of insufficient current density capacity, we did successfully develop a 2.05 eV AlGaInP top sub-cell structure for 5 or 6J-IMM application. To achieve the desired current capacity of 11.3mA/cm2, we fabricated 2.05 eV hetero-junctions composed of a 1.91 eV GaInP emitter and a 2.05 eV AlGaInP base. Such a hetero-junction top sub-cell incorporated in a 3J-IMM iso-type structure exhibited sufficient Jsc but reduced Voc=1.466 V, being only 43 mV greater than the 1.91 eV GaInP homo-junction sub-cell. Minor modification to the emitter of our AlGaInP homo-junction permitted the fabrication of a 2.05 eV sub-cell in a 5J-IMM iso-type structure with sufficient current density and improved Voc=1.537 V. 5 and 6J-IMM cells incorporating these top sub-cells demonstrated maximum Vocs= 4.839 and 5.195 V, respectively.

Lattice-Matched Solar Cells With 40% Average Efficiency in Pilot Production and a Roadmap to 50%

A commercial lattice-matched InGaP/InGaAs/Ge solar cell has reached an average efficiency of 40% at 500 kW/m2. The design changes that lead to this result are discussed. These data are complemented with a presentation of the latest new solar cell development results from the laboratory. Inverted metamorphic multijunction (IMM) solar cells have been prototyped with 42.4% efficiency at 325 suns for concentrator applications and 33.6% efficiency at 1 sun AM0 for space applications. Six subcell devices are now under development. These results are used, along with other experimental data and other industrial constraints, as input to a computer model to predict what practical efficiency might be achievable with this device approach. The computer model suggests that 45% and 50% efficiencies are technologically feasible with a three-junction and five-junction device, respectively, at an irradiance of 500 kW/m2 and 25 °C using known materials, device architectures, and manufacturing methods.

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.

Evolution of the high efficiency triple junction solar cell for space power

Emcore is presently qualifying the fourth generation triple junction (3J) solar cell under the Air Force ManTech program. This cell referred to as the ZTJ, is designed to achieve 30% conversion efficiency under 1 sun AM0 illumination. In addition a companion cell with a monolithically integrated protection diode referred to as the ZTJM is under qualification at Emcore with a target AM0 efficiency of 29.5%. We present performance results of each of these 3J cells from pilot line production. In addition, modeling results of various 3J solar cell designs are discussed to motivate the use of a new 3J architecture that relieves the constraint of an active germanium subcell. This new architecture known as inverted metamorphic multi-junction (IMM) allows one to more easily realize the optimum set of band gaps in monolithically connected n-junction solar cell devices. Finally, recent performance results of our fifth generation 3J cell known as the 3J IMM are reported here.