Samar Sinharoy

Multi-Junction Solar Cell Spectral Tuning with Quantum Dots

We have theoretically analyzed the potential efficiency improvement to multi-junction solar cell efficiencies which are available through the incorporation of quantum dot using detailed balance calculations. We have also experimentally investigated the Stranski-Krastanov growth of self-organized InAs quantum dots and quantum dot arrays on lattice-matched GaAs by metallorganic vapor phase epitaxy (MOVPE). The morphology of the quantum dots were investigated as a function of their growth parameters by atomic force microscopy (AFM). Photoluminescence and optical absorption measurements have demonstrated that the incorporation of InAs quantum dots (QD) into a GaAs structure can provide sub-GaAs bandgap electronic states

InGaAsP/InGaAs Tandem TPV Device

Power conversion in a thermophotovoltaic (TPV) system can be accomplished with single p‐n junctions which are interconnected on‐wafer (monolithic interconnected module (MIM) approach) or off‐wafer (individual chip module approach) to form large area arrays. Using a MIM architecture, 0.6 eV InGaAs diodes grown lattice mismatched to InP have produced a power conversion efficiency of 23% and a power density of 0.65 W/cm2 at cell and radiator temperatures of 25°C and 1000°C, respectively. A shortcoming of a single p‐n junction is inefficient use of the incident spectrum due to over‐excitation losses from high, above bandgap energy photons. In order to overcome these losses, tandem TPV devices have been proposed which comprise two or more p‐n junctions of differing bandgaps. In this work we report on the growth, fabrication and characterization of InGaAsP/InGaAs (0.72/0.60 eV) tandem TPV diodes. Electrical measurements using a grey body source reveal an open circuit voltage of 0.504 V/cell and a fill factor of...

Development of a high efficiency metamorphic triple-junction 2.1 eV/1.6 eV/1.2 eV AlGaInP/InGaAsP/InGaAs space solar cell

We report our progress in the development of a high efficiency metamorphic triple-junction tandem solar cell that is predicted, on the basis of modeling calculations, to have a maximum practical room temperature air mass zero one-sun (AM01S) efficiency of 31.5%, and an efficiency of 36.5% under 100 suns. Cell structures are grown using the metal organic vapor phase epitaxy (MOVPE) process. Although the three cells are lattice-matched to each other, the bottom InGaAs cell is grown on a lattice-mismatched GaAs substrate using a proprietary buffer layer. This unique buffer layer pins most defects in the vicinity of the substrate-buffer interface, allowing the growth of nearly defect-free subsequent layers.

1.62 eV/1.1 eV InGaP/InGaAs dual-junction solar cell development on lattice-mismatched GaAs

The authors report progress towards achieving a high efficiency monolithic dual-junction solar cell consisting of a 1.62 eV InGaP top cell and a 1.1 eV InGaAs bottom cell grown on a lattice-mismatched GaAs substrate using a proprietary buffer layer scheme. They have achieved air mass zero (AMO), one-Sun efficiencies of 17.5% for the 1.1 eV InGaAs and 15.7% for the 1.6 eV InGaP standalone cells on lattice-mismatched GaAs. The predicted AMO, one-Sun efficiency of a 1.62 eV/1.1 eV n/p dual-junction cell is 27%. To date, they have achieved an efficiency of 19% in their dual-junction cells. Further process optimization experiments are currently underway, aimed at achieving the 27% efficiency goal.

Solar cells for NASA RAINBOW concentrator

The RAINBOW concentrator system is based on a concept of splitting the solar spectrum and focussing each portion on a solar cell having a bandgap matching the input spectral portion. Efficiencies over 40% are predicted for systems using a four-bandgap cell assembly under 20/spl times/ concentration. Reported here are the results of materials growth, processing, and testing of four different solar cells designed to populate the RAINBOW testbed, under development at JPL. The cells are 0.74-eV InGaAs on lattice-matched InP, 1.1-eV InGaAs on lattice-mismatched GaAs, 1.43-eV GaAs on GaAs, and 1.85-eV InGaP on lattice-matched GaAs. Quantum efficiencies between 0.8 and 1.0 have been realized for the spectral region from 0.5 to /spl sim/1.5 /spl mu/m.

InAs quantum dot development for enhanced InGaAs space solar cells

The metamorphic or lattice mis-matched triple junction cell under development by ERI and its partners has an InGaAs junction (bottom cell) of the three-cell stack. This junction is the current limiting, and therefore efficiency limiting, junction due to current matching which must be maintained through the device. This situation is further exacerbated when these devices are used in space, due to the bottom junction being the most affected by radiation degradation. This limitation may be addressed through the incorporation of InAs quantum dot array into the depletion region of an InGaAs cell. The InAs quantum dots in the InGaAs cell will provide sub-gap absorption and thus improve its short circuit current. This cell could then be integrated into the three-cell stack to achieve a space solar cell whose efficiency exceeds current state-of-the-art standards. A theoretical estimate predicts that a InGaAlP(1.95 eV)/InGaAsP(1.35 eV)/InGaAs(1.2 eV) triple junction cell with an improved bottom cell current could have an efficiency exceeding 40%. In addition, there is now a growing body of work that theoretically and experimentally indicates that the use of quantum dot structures may also hold ancillary benefits such as improved temperature coefficients and better radiation tolerance. These benefits are extremely important considering the intended space utilization of these devices. In this study, InAs quantum dots have been grown on lattice-mismatched InGaAs (1.2 eV bandgap) grown epitaxially on GaAs by metalorganic chemical vapor deposition (MOCVD) and characterized via photoluminescence (PL) and atomic force microscopy (AFM). Arrays of these InAs Quantum dots have been incorporated into prototype InGaAs devices. The photovoltaic efficiency under simulated 1 sun intensity and air mass zero (AM0) illumination was measured. The spectral response demonstrated that sub-gap photoconversion in InGaAs cells is possible through the incorporation of the InAs quantum dots.