Daniel J. Chmielewski

GaAs

Monolithic, epitaxial, series-connected GaAs0.75P0.25/Si dual-junction solar cells, grown via both molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD), are reported for the first time. Fabricated test devices for both cases show working tandem behavior, with clear voltage addition and spectral partitioning. However, due to thermal budget limitations in the MBE growth needed to prevent tunnel junction failure, the MBE-grown GaAs0.75P0.25 top cell was found to be lower quality than the equivalent MOCVD-grown device. Additionally, despite the reduced thermal budget, the MBE-grown tunnel junction exhibited degraded behavior, further reducing the overall performance of the MBE/MOCVD combination cell. The all-MOCVD-grown structure displayed no such issues and yielded significantly higher overall performance. These initial prototype cells show promising performance and indicate several important pathways for further device refinement.

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The development and demonstration of methodologies for the heteroepitaxy of GaP on Si substrates, free of heterovalent interface related defects, and the subsequent metamorphic grading in the GaAsyP1-y alloy system necessary to achieve target III-V materials at sufficiently high quality, directly enable the achievement of monolithically-integrated multijunction solar cells utilizing both III-V and Si active sub-cells. Such devices hold promise for high photovoltaic performance at significantly reduced costs afforded by the Si platform. In this vein, early-stage prototype all-epitaxial GaAs0.75P0.25/Si dual-junction devices have been grown by a combination of MOCVD and MBE, demonstrating great promise for such an approach, and clear pathways for further improvement.

P

High-quality, heterovalent nucleation of defect-free epitaxial GaP on (100)-oriented Si substrates is an enabling accomplishment toward a pathway for the creation of III-V/Si multijunction photovoltaic devices in which the Si growth substrate can simultaneously act as a near-ideal sub-cell through a monolithic metamorphic GaInP/GaAsP/Si structure. While recent efforts have achieved this goal via molecular beam epitaxy (MBE), the science developed in those efforts is fundamental to the GaP/Si interface. Here this knowledge is utilized to achieve the successful transition from MBE to an all-MOCVD (metal-organic chemical vapor deposition) process, in which all nucleation-related defects are simultaneously and totally avoided for ideal GaP/Si interfaces and subsequent metamorphic III-V materials. Four main topics are presented: (1) GaP/Si(100) grown by MOCVD free of antiphase domains and stacking defects; (2) growth, fabrication, and testing of GaP/active-Si sub-cells; (3) MOCVD/MBE-grown GaAsP/active-Si multijunction structures and component cells having target lattice constants and bandgaps for high efficiency dual and triple junction cells, and (4) comparative interface studies of MBE- and MOCVD-grown III-V/GaP/Si cell architectures.

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Through band gap engineering, metamorphic GaInP/GaAsP/Si multi-junction solar cells are theoretically capable of achieving a substantial increase in efficiency, with a significant decrease in cost, compared to current state-of-the-art Ge-based multi-junction solar cells. While the refinement of the necessary metamorphic materials and development of the associated sub-cells are obvious areas of focus, such devices also require high-performance tunnel junctions to minimize both optical and electrical sub-cell interconnection losses. Here we discuss efforts toward the development of these important component devices, including metamorphic materials growth and analysis, leading to the successful demonstration of metamorphic GaAs09P01 tunnel junctions with peak current densities exceeding 100 A/cm2 and zero-bias resistance-area products of 4.5×104 Ω·αη2, indicating high-quality devices capable for use in future high-concentration GaInP/GaAsP/Si multi-junction devices.

/Si Dual-Junction Solar Cells Grown by MBE and MOCVD

Monolithic, direct epitaxial GaAs0.75P0.25/Si dual-junction (2J) solar cell structures have been grown via both molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD). Fabricated test devices show working tandem behavior, with clear voltage addition and spectral partitioning, in both cases. Due to the thermal sensitivity of the MBE-grown tunnel junction structure, growth conditions necessary to maintain 2J activity yielded reduced quality GaAs0.75P0.25 top cell, while the more robust MOCVD-based tunnel junction enabled higher-quality top cell growth, resulting in overall higher performance 2J behavior. These initial prototype cells show promising performance and suggest several definite pathways for further device refinement.

Epitaxially-grown metamorphic GaAsP/Si dual-junction solar cells

Various metamorphic tunnel junction designs, targeted for application toward Ga_0.56In_0.44P/GaAs_0.9P_0.1/Si triple-junction and GaAs_0.75P_0.25/Si dual-junction solar cells, grown via both molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD), have been demonstrated. Ga_0.56In_0.44P/GaAs_0.9P_0.1 double heterostructure tunnel junctions, grown by MBE on Si substrates at both target lattice constants, yielded at least 50% improved electrical performance (peak tunnel current and resistance-area product), with nominally reduced optical losses, compared to previously demonstrated GaAs_0.9P_0.1 homostructure devices. Initial MOCVD-grown Al_0.4Ga_0.6As_0.9P_0.1/GaAs_0.9P_0.1 heteroface tunnel junctions were also fabricated and exhibited a > 3x electronic performance improvement over comparable GaAs_0.9P_0.1 homostructure devices. Such tunnel junctions reveal great promise for use within Si-based multijunction solar cells for both one-sun and concentrator applications.

Ideal GaP/Si heterostructures grown by MOCVD: III-V/active-Si subcells, multijuntions, and MBE-to-MOCVD III-V/Si interface science

This work investigates 2.05 eV bandgap (Al)GaInP alloys for use as the top junction of IMM solar cells. We explore balancing alloy composition and lattice constant as two complementary variables to achieve the target bandgap in one material system. Here both MBE and MOCVD growth methods are compared to achieve this goal. The specific compositions are Ga<inf>0.63</inf>In<inf>0.37</inf>P (tensile relaxed with respect to GaAs) and (Al<inf>0.13</inf>Ga<inf>0.38</inf>)<inf>0.51</inf>In<inf>0.49</inf>P (lattice matched to GaAs) in order to determine the relative impact of misfit and increased aluminum content, respectively. Prototype solar cell performance and defect spectroscopy (DLTS/DLOS) are used to evaluate the various alloys, and results suggest that MOCVD metamorphic Ga<inf>0.63</inf>In<inf>0.37</inf>P is promising for a high performance top cell.

Metamorphic tunnel junctions for high efficiency III-V/IV multi-junction solar cell technology

Recent work in the development and demonstration of dual- and triple-junction III-V/Si solar cells, with active Si subcells, is presented. This work includes development efforts spanning the full range of associated materials and device components: Si and metamorphic III-V subcells, III-V/Si heteroepitaxial integration, metamorphic III-V buffers, metamorphic III-V tunnel junctions, and full device integration and processing. To date, GaAs0.75P0.25/Si two-terminal dual- junction and Ga0.56ln0.44P/GaAs0.90P0.10/Si triple-junction devices have been demonstrated for the first time; refinement and optimization efforts are in progress.

GaAsP/Si dual-junction solar cells grown by MBE and MOCVD

Theoretical models for III-V compound multijunction solar cells show that solar cells with bandgaps of 1.95-2.3 eV are needed to create ideal optical partitioning of the solar spectrum for device architectures containing three, four and more junctions. For III-V solar cells integrated with an active Si sub-cell, GaInP alloys in the Ga-rich regime are ideal since direct bandgaps of up to ~ 2.25 eV are achieved at lattice constants that can be integrated with appropriate GaAsP, SiGe and Si materials, with efficiencies of almost 50% being predicted using practical solar cell models under concentrated sunlight. Here we report on Ga-rich, lattice-mismatched Ga0.57In0.43P sub-cell prototypes with a bandgap of 1.95 eV grown on tensile step-graded metamorphic GaAsyP1-y buffers on GaAs substrates. The goal is to create a high bandgap top cell for integration with Si-based III-V/Si triple-junction devices. Excellent carrier collection efficiency was measured via internal quantum efficiency measurements and with their design being targeted for multijunction implementation (i.e. they are too thin for single junction cells), initial cell results are encouraging. The first generation of identical 1.95 eV cells on Si were fabricated as well, with efficiencies for these large bandgap, thin single junction cells ranging from 7% on Si to 11% on GaAs without antireflection coatings, systematically tracking the change in defect density as a function of growth substrate.

High-performance metamorphic tunnel junctions for III–V/Si multijunction solar cells

Bandgap tunability achievable using metamorphic epitaxy enables maximization of photodetector performance at target wavelengths. However, an increase in threading dislocation density (TDD), which is inherent for the growth of relaxed, lattice-mismatched layers, could offset this advantage and severely limit detector performance. In this regard, we are investigating the performance of InxGa1-xAs and InzGa1-zP p-i-n photodetectors as a function of TDD, by utilizing a number of different InxGa1-xAs buffer designs. In particular, internally lattice-matched metamorphic In0.20Ga0.80As and In0.68Ga0.32P individual p-i-n detectors are studied to optimize the buffer design and performance of optically-aligned In0.68Ga0.32P/In0.20Ga0.80As visible/near-infrared dual-photodetectors. Reverse-bias dark current density of In0.68Ga0.32P detectors were found to be extremely sensitive to TDD compared to that observed for In0.20Ga0.80As detectors. Nearidentical spectral response curves were obtained for both detectors as a function of TDD due to the relative insensitivity of the p-i-n detector structure to the minority carrier lifetime. A comprehensive comparison between the different graded buffer designs, TDD achieved and photodetector characteristics are presented.