Benjamin Cho

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.

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 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.

The future of high efficiency, multi-junction space solar cells

Space power generation has involved the continual improvement of device designs to achieve higher efficiencies. The improvements have involved both evolutionary as well as revolutionary changes in device design. Currently, the lattice matched, GaInP2/GaAs/Ge multi-junction solar cell is the device of choice for space power generation. The current design of this architecture has reached the 30% efficiency level, the expected maximum performance level for this architecture in a manufacturing environment. However, the band gaps for the GaInP2/GaAs/Ge device are not the ideal set of three junctions for maximum power conversion. We present theoretical modeling for 3-, 4-, 5-, and 6-junction devices. Other criteria for space power generation, including reliability, weight, and cost are discussed. Different approaches to higher efficiencies are discussed and are evaluated not only in the light of power conversion efficiency, but also in light of the other criteria.

The One-per-wafer ZTJ solar cell from Emcore- confidence testing and volume manufacturing performance

The Emcore One-per-wafer ZTJ solar cell, with a cell area of approximately 60cm2, is based on the 29.5% efficiency ZTJ triple-junction structure. The performance of this cell has been enhanced via grid design improvements, resulting in a 0.3% absolute efficiency increase. To confirm electrical performance and investigate reliability with respect to the grid metal and form factor changes implemented in the One-per-wafer ZTJ cell, Emcore has performed a series of confidence tests, which the cells successfully passed. Electrical measurements for approximately 14,000 One-per-wafer ZTJ cells manufactured at Emcore show a median AM0 efficiency of 29.3% (1353 W/m2).

Air Force ManTech qualification of the 30% class GaInP2/Ga(In)As/Ge solar cell to the AIAA S-111 standard: Results and recommendations

We report the results of qualification testing of Emcores sixth generation III–V multi-junction solar cell — the 30% class GaInP2/Ga(In)As/Ge ZTJ cell. The ZTJ cell has undergone space qualification within the Air Force ManTech program utilizing the American Institute of Aeronautics and Astronautics (AIAA) S-111-2005 standard. The S-111 tests comprise characterization tests and qualification tests. The objective of the characterization tests is to provide electrical, mechanical, and environmental data at the cell level to aid in the accuracy of on-orbit power predictions. The qualification test objective is to validate the present materials and manufacturing processes used in ZTJ cell and CIC level assemblies for Geosynchronous Earth Orbit (GEO) applications. The ZTJ cell used in this qualification program is characterized by a beginning of life (BOL) maximum power point efficiency of 29.5% (135.5 mW/cm2) under simulated AM0 illumination at an operating temperature of 28 °C.

Path to a drop-in replacement for current technologies with the 33%, large area, IMM cell

The triple-junction inverted-metamorphic (IMM-3) air-mass-zero (AM0) solar cell is a high-efficiency photovoltaic device with nearly optimized band-gaps for the solar spectrum. We discuss the path forward for a drop-in replacement of the conventional, 3J lattice-matched germanium based solar cell with an IMM-3 space solar cell. The IMM space solar cell can support several form factors, from flexible to rigid carriers, but a configuration similar to standard space solar cells could be quickly adopted into current panel configurations. The goal of the IMM development team has been to provide a stable processing platform that is also compatible with current solar panel configurations. The superstrate configuration has been the platform used for much of the IMM development to date. We discuss this form factor, as well as other alternatives to achieve a robust cell replacement form factor capable of successfully completing the AIAA-S111 space qualification.

Designing for ESD survivability of a monolithically protected GaInP2/GaInAs/Ge solar cell

We report on the design and performance of a monolithically protected GaInP2/Ga(In)As/Ge cell. This solar cell known as the ZTJM is a companion cell to the 30% class GaInP2/Ga(In)As/Ge ZTJ solar cell. The ZTJ cell is characterized by a beginning of life (BOL) maximum power point efficiency of 29.5% (135.3 mW/cm2) under simulated AM0 illumination with an operating temperature of 28 °C. [1] The primary design objectives of the ZTJM cell were to maintain performance parity with the ZTJ cell and to demonstrate improved electrostatic discharge (ESD) hardness of the bypass diode relative to those in previous monolithically protected triple junction products. Through the course of the diode development we identified two key factors required for ESD robustness. With these findings we have achieved a monolithically protected GaInP2/Ga(In)As/Ge cell with an average 1-sun AM0 efficiency of 29% capable of withstanding at least 10 successive pulses of 30A intensity and 100us duration.

Manufacturing improvement of IMM solar cells and flex string arrays on the AFRL Mantech program

The manufacturing improvement of IMM solar cells and flex string arrays (FSAs) is a focus of the AFRL-funded IMM Mantech program. Significant manufacturability improvements have been achieved to date, with focus on reduction in materials cost and touch labor, process automation, and demonstration of large area IMM cells (≥ 60cm2) on 4” and 6” wafers. Confidence testing of FSAs encompassing multiple roll/unroll cycles, hot and cold rolled soak, vibration testing, thermal vacuum (TVAC) and hot GEO thermal cycling has shown negligible degradation of power output.