Geoffrey K. Bradshaw

Carrier Transport and Improved Collection in Thin-Barrier InGaAs/GaAsP Strained Quantum Well Solar Cells

Multiple quantum wells (MQW) lattice matched to GaAs consisting of In0.14Ga0.76As wells balanced with GaAs0.24P0.76 barriers have been used to extend the absorption of GaAs subcells to longer wavelengths for use in an InGaP/GaAs/Ge triple-junction photovoltaic cell. Thin barriers with high-phosphorus composition are capable of balancing the strain from the InGaAs wells; thus, creating conditions to allow for thicker wells and for carrier tunneling to dominate transport across the structure. As a result, a larger percentage of the depletion region is occupied by InGaAs quantum wells that absorb wavelengths beyond 875 nm and the indium composition is not limited by thermionic emission requirements. Measurements at elevated temperatures and reverse bias suggest that a thermally assisted tunneling mechanism is responsible for transport through the barriers.

Improved light-emitting diode performance by conformal overgrowth of multiple quantum wells and fully coalesced p-type GaN on GaN nanowires

We demonstrate a light-emitting diode (LED) structure with multiple quantum wells (MQWs) conformally grown on semipolar and nonpolar plane facets of n-GaN nanowires (NWs), followed by deposition of fully coalesced p-GaN on these nanowires. Overgrowth on the nanowires’ tips results in inclusion of high density voids, about one micron in height, in the GaN film. The light output intensity of NWs LEDs is more than three times higher than corresponding c-plane LEDs grown simultaneously. We believe this results from a reduced defect density, increased effective area of conformally grown MQWs, absence of polar plane orientation, and improved light extraction.

Interface properties of Ga(As,P)/(In,Ga)As strained multiple quantum well structures

(In,Ga)As/Ga(As,P) multiple quantum wells (MQWs) with GaAs interface layers have been characterized with photoluminescence (PL) and high resolution scanning transmission electron microscopy (STEM). By growing (In,Ga)As/Ga(As,P) MQWs with asymmetric GaAs interfacial layers, we found that phosphorus carry-over had a profound effect on the absorption edge of the (In,Ga)As wells. Evidence for this phosphorus was initially determined via PL and then definitively proven through STEM and energy dispersive x-ray spectroscopy. We show that the phosphorus carry-over can be prevented with sufficiently thick GaAs transition layers. Preliminary results for GaAs p-i-n solar cells utilizing the improved MQWs are presented.

Minority Carrier Transport and Their Lifetime in InGaAs/GaAsP Multiple Quantum Well Structures

Minority carrier transport across InGaAs/GaAsP multiple quantum wells is studied by measuring the response of p-i-n and n-i-p GaAs solar cell structures. It is observed that the spectral response depends critically upon the width of the GaAsP barriers and the device polarity. Electron tunneling is not as efficient as hole tunneling due to a higher conduction band barrier. The spectral response depends on the relative magnitude of the carrier lifetime as compared with the tunneling lifetime. This paper deduces an estimated electron lifetime of 110 ns in In0.14Ga0.86As wells and 25 ns in In0.17Ga0.83As wells, which agree with published results.

Effects of Barrier Width on Spectral Response of Strained Layer Superlattices for High Efficiency Solar Cells

Characteristics of strained layer superlattices (SLS) consisting of alternating layers In x Ga 1-x As and GaAs 1-y P y are examined for use in high efficiency solar cells. The effects of SLS quantum barrier widths on tunneling probability and short circuit current are discussed through analysis of J-V and spectral response measurements. Results indicate a threshold barrier thickness for which tunneling effects are deleterious. Effect of the number of SLS periods incorporated into a p-i-n structure and maximum number of periods are presented through spectral response and CV analysis. It is demonstrated that SLS show increasing responsivity with increasing number of periods due to higher absorption. CV analysis is performed to determine zero bias depletion widths for verifying appropriate number of SLS periods and fully depleted SLS region.

GaInP/GaAs Tandem Solar Cells With InGaAs/GaAsP Multiple Quantum Wells

Lattice-matched multiple quantum wells (MQWs) consisting of InxGa1-xAs wells with very thin GaAs0.2P0.8 barriers have been incorporated into a GaInP/GaAs tandem solar cell. InGaAs/GaAsP MQWs increase the short-circuit current of the GaAs cell by extending the absorption range, with minimal impact on an open-circuit voltage, thus alleviating current matching restrictions placed by the GaAs cell on multijunction solar cells. MQWs with very thin, tensile strained, high phosphorus content GaAsP barriers allow tunneling to dominate carrier transport across the MQWs and balance the compressive strain of the InGaAs wells such that material quality remains high for subsequent top cell growth. We show that the addition of the QW layers enhances the GaAs cell, does not degrade the performance of the GaInP top cell, and leads to potential efficiency enhancements.

Determination of carrier recombination lifetime in InGaAs quantum wells from external quantum efficiency measurements

GaAs cells containing multiple quantum wells (MQW) of strained InGaAs/GaAsP can enhance efficiency in multijunction solar cells. Determination of carrier recombination lifetime in the InGaAs well is useful to understand material quality and carrier transport across the structure. GaAs p-i-n structures with and without strain balanced In0.17Ga0.83As wells and GaAs0.25P0.75 barriers were grown by MOCVD on p-type GaAs substrates. The GaAsP barrier thickness was varied between devices to intentionally influence carrier transport. A decrease in EQE was observed as barrier width was increased, which was attributed to an increase in tunneling lifetime, τtn. While this EQE decrease is undesirable in practical devices, it is useful for determining the recombination lifetime, τr, of the InGaAs wells. The decrease in EQE was observed only at wavelengths of light greater than 600 nm, indicating that minority carrier electrons generated in the base are responsible for the reduction in EQE. Shorter wavelengths (<;600 nm) of light are almost completely absorbed before reaching the base and primarily generate holes in the emitter. The tunneling lifetime and the currents generated in the p-i-n structures were modeled to calculate the EQE of a GaAs control and both thick and thin barrier MQW devices. The probability of transport through the entire MQW structure, Ptot, was varied until the calculated EQE fit the experimental data. The value of Ptot was then correlated to the only unknown parameter, the recombination lifetime. Using this method the recombination lifetime in In0.17Ga0.83As in the QW was determined to be 110 ns, which agrees with values found in previous time resolved photoluminescence measurements of metamorphic InGaAs films.

The optimization of high indium and high phosphorus content InGaAs/GaAsP strained layer superlattices for use in multijunction solar cells

InGaAs/GaAsP strained layer superlattices (SLS) with high phosphorus and high indium content inserted into the intrinsic region of GaAs solar cells increases short circuit current with minimal impact on open circuit voltage. Very thin, high phosphorus content barriers provide several key advantages over other methods, namely 1) thin barriers occupy less space in a depletion width-limited SLS, 2) carrier transport is primarily due to tunneling, and 3) the height of the barrier, or depth of the well, is not subject to thermionic emission requirements. Our staggered well concept reduces quantum size effects to increase absorption beyond the GaAs band edge.

Characterization of Strained Layer Superlattice Solar Cells by X-ray Diffraction and Current-Voltage Measurements

InGaAs can be used to enhance the response of solar cells past the 1.43 eV cutoff of GaAs. Strained-layer superlattice (SLS) structures with high indium and phosphorus compositions (up to 35% and 68% respectively) have been grown successfully. SLS solar cells with indium and high phosphorus compositions (up to 15% and 85% respectively) have been grown successfully. The spectral response of the solar cells has been extended to as low as 1.27 eV. This enhancement is also shown by an increase in the short circuit current, with a small reduction in the short circuit voltage as compared to standard GaAs p-n junction for AM1.5 and one sun. Dark current curves show the extent of recombination in the superlattice. The reverse saturation current in the recombination region (0.2-0.8 V) was determined using a non-linear least squares fitting routine. An Arrhenius plot was generated by finding the reverse saturation current over a temperature range of 300-370 K. The low recombination devices show non-ideality constants of 1.7 with activation energies of 1.3-1.4 eV. The high recombination devices have non-ideality constants (˜2.3) and lower activation energies of 1.1 eV.

Tandem InGaP/GaAs-quantum well solar cells and their potential improvement through phosphorus carry-over management in multiple quantum well structures

InGaP/GaAs/Ge multijunction solar cell (MJSC) efficiency can be increased through improved current matching among the subcells with multiple quantum wells (MQWs) being promising for this purpose. In this study we show that InGaAs/GaAsP QWs utilizing high phosphorus composition barriers can be successfully incorporated into the GaAs subcell of an InGaP/GaAs tandem solar cell. This InGaP/GaAs-MQW device has an enhanced short circuit current density when compared to that of a standard InGaP/GaAs tandem device with minimal impact on either GaAs or InGaP subcell open circuit voltage. Additionally, phosphorus carry-over in the MQW structure is investigated through the use of photoluminescence (PL). It is demonstrated that the phosphorus carry-over can be overcome through the utilization of thick GaAs transition layers at the GaAsP→InGaAs interfaces, resulting in a MQW with an extended absorption edge.