J. J. Carapella

Lattice-mismatched approaches for high-performance, III-V photovoltaic energy converters

We discuss lattice-mismatched (LMM) approaches utilizing compositionally step-graded layers and buffer layers that yield III-V photovoltaic devices with performance parameters equaling those of similar lattice-matched (LM) devices. Our progress in developing high-performance, LMM, InP-based GaInAs/InAsP materials and devices for thermophotovoltaic (TPV) energy conversion is highlighted. A novel, monolithic, multi-bandgap, tandem device for solar PV (SPV) conversion involving LMM materials is also presented along with promising preliminary performance results.

Effective electron mass and plasma filter characterization of n-type InGaAs and InAsP

We measured the infrared reflectance of thin films of degenerate n-type InxGa1−xAs and n-InAsyP1−y as a function of doping for compositions that correspond to x=0.53, 0.66, and 0.78 (band gaps of 0.74, 0.60, and 0.50 eV, respectively) and y=0.00, 0.31, 0.52, and 0.71 (band gaps of 1.34, 1.00, 0.75, and 0.58 eV, respectively). We then used the Drude theory and Hall measurements to determine the effective electron mass for these samples, and checked the results using Raman spectroscopy. The effective electron mass for these compositions increases abruptly as a function of free-electron density and converges at 5×1019 electrons/cm3. Consequently, it is difficult to attain plasma edges at wavelengths shorter than 5 μm using these materials, and the plasma edge is nearly independent of composition at large electron density levels. Results from similar studies on InP, InAs, and GaAs have been compiled and compared with our data. It is clear that the Kane band model offers an accurate description of the conducti...

Recombination lifetimes in undoped, low-band gap InAsyP1−y/InxGa1−xAs double heterostructures grown on InP substrates

High-quality, thin-film, lattice-matched (LM) InAsyP1−y/InxGa1−xAs double heterostructures (DHs) have been grown lattice mismatched on InP substrates using atmospheric-pressure metalorganic vapor-phase epitaxy. The low-band gap InxGa1−xAs layers in the DHs have room-temperature band gaps that range from 0.47 to 0.6 eV. Both the optical and electronic properties of these films have been extensively measured. The band-to-band photoluminescence is quite strong and comparable to that found for LM InP/In0.53Ga0.47As DHs grown on InP. Recombination lifetime measurements of undoped DH structures show minority-carrier lifetimes in excess of 1 μs in most cases. The earlier properties make the band gap-flexible InAsyP1−y/InxGa1−xAs DH system attractive for applications in high-performance, infrared-sensitive devices.

Metal Pillar Interconnection Topology for Bonded Two-Terminal Multijunction III–V Solar Cells

Metal-interconnected multijunction solar cells offer one pathway toward efficiencies in excess of 50%. However, if a three- or four-terminal configuration is used, optical losses from the interfacial grid can be considerable. Here, we examine an alternative that provides an optimal interconnection for two-terminal bonded devices. This “pillar-array” topology is optimized by minimizing the sum of all power losses, including shadow losses and numerically computed electrical losses. Numerical modeling is used to illustrate the benefit of a pillar-array interfacial metallization for some two-terminal configurations.

Novel processing and device structures in thin‐film CuInSe2‐based solar cells

We have developed a sequential process for the fabrication of enhanced-grain thin-film CuInSe[sub 2]-based photovoltaic devices. In this process, exact control of the composition during fabrication is not necessary. Analysis of the film structures suggests enhanced grain size, transport, and photoconductivity. Device structures vary from one-sided heterojunctions to buried homojunctions where the window layer is not required for photo-response. We have also developed a fabrication process that incorporates variable elemental fluxes and flux ratios, and a substrate temperature of 550 [degree]C. Preliminary results include a fill-factor of 75%, a diode factor of 1.2, and enhanced spectral response in the near-infrared region. We present an update on this work and attempt to describe the relationship between the growth dynamics and the resultant material and device performance. The results suggest new approaches to the fabrication of high efficiency CuInSe[sub 2]-based solar cells.

Recent Advances in Low‐Bandgap, InP‐Based GaInAs/InAsP Materials and Devices for Thermophotovoltaic (TPV) Energy Conversion

Salient advances in the development of thermophotovoltaic (TPV) energy converters based on low‐bandgap, InP‐based, GaInAs/InAsP heterostructures are presented and discussed. InP‐based materials are well‐suited and advantageous for TPV converter applications. Substantial improvements in the quality of lattice‐mismatched (LMM) heterostructures have been realized through an enhanced understanding of the relaxation behavior, and associated microstructure, of InAsP compositionally graded layers and GaInAs/InAsP interfaces. Double‐heterostructure, GaInAs/InAsP test structures with bandgaps as low as 0.5 eV (1.6% lattice mismatch) have been demonstrated with exceptional low‐injection, minority‐carrier lifetimes (several μs) and large estimated diffusion lengths — comparable to those for lattice‐matched materials. The advances in material quality have contributed to a number of notable TPV device achievements. A record in‐cavity efficiency of 23.6% was reported for a 0.6‐eV, GaInAs/InAsP monolithic interconnected module. Additionally, 0.52‐eV GaInAs/InAsP TPV converters were demonstrated with near‐unity internal quantum efficiencies and reverse‐saturation current densities nearly equaling the best reported for lattice‐matched, 0.52‐eV GaInAsSb/GaSb devices. Furthermore, InP‐based, 0.74/0.63‐eV, monolithic, series‐connected, tandem TPV converters are also under development and show promising performance; an in‐cavity efficiency of 11% has been reported for preliminary devices.

A study on the optical and microstructural characteristics of quaternary Cu(In,Ga)Se/sub 2/ polycrystalline thin films

The optical and microstructural properties of polycrystalline CuIn/sub 1-y/Ga/sub y/Se/sub 2/ (CIGS) thin film deposited by coevaporation are reported within the boundaries of an orthogonal experimental design investigating the effects of Cu flux, Ga/(Ga+In) composition. Se rate, substrate temperature, T/sub s/ and substrate type. The optical bandgaps for near-stoichiometric CuIn/sub 1-y/Ga/sub y/Se/sub 2/ are smaller and exhibit bowing behavior which follows the relationship E/sub g/=1.011+0.664y+0.249y(y-1). In comparison, Cu-poor films exhibit a linear variation with zero bowing given by E/sub g/=1.0032+0.71369y. The increase in E/sub g /with decreasing Cu may result in part from lattice shrinkage as measured by X-ray diffraction (XRD). Optical absorption below the band edge appears to be dependent upon both Cu and Ga content. Absorption coefficients of alpha >or=10/sup 3/ cm/sup -1/ within this region are indicative of Cu-rich films. Absorption <or=10/sup 3/ cm/sup -1/ may be dictated more by surface morphology and possible phase separation in films containing >or=50% Ga. The magnitude of alpha varies from equivalent to 2*10/sup 4/ near the band edge tip to 10/sup 5/ cm/sup -1/ at 1 ev above the edge for near-stoichiometric films, with the absorption in Cu-poor films being slightly less.<<ETX>>

Fabrication of two-terminal metal-interconnected multijunction III–V solar cells

A novel approach has been developed to enable the creation of a fully lattice-matched two-terminal four-junction III-V solar cell is which an upper 1.85 eV GaInP/ 1.42 eV GaAs two-junction (2J) tandem cell is bonded to a lower Eg3 eV GaInAsP/ 0.74 GaInAs eV 2J tandem cell. In this configuration, the upper tandem is grown inverted and lattice-matched to a GaAs substrate, and the lower tandem is grown upright and lattice-matched to an InP substrate. Prove of concept devices have been fabricated using Au-Au bonding with either SiO2 or GaInP2 as a filler material. The bonding process is discussed in this paper as well as the result of an inverted GaAs cell bonded on a conducting GaAs wafer. The most complex device fabricated to date is a GaInP/GaAs 2J tandem cell bonded to a GaInAs cell using a GaInP2 optical coupling layer, with a post-bonding Voc of 2.7 eV.

2.0–2.1 eV Ga x In 1−x P solar cells grown on relaxed GaAsP step grades

A high quality solar cell with a bandgap in the range of 2.0–2.1 eV may enable the development of four- and five-junction solar cells for terrestrial and space applications. In this paper we describe a set of 2.0–2.1 eV n+/p solar cells fabricated from GaxIn1−xP and grown on compositional step-grades of GaAs1−yPy, on GaAs substrates. Cells were grown by atmospheric pressure organometallic vapor phase epitaxy. The tensile grades were designed to achieve nearly complete relaxation of the active layers, and the in-situ stress as monitored during growth showed a residual tensile stress of <10 MPa in the best samples. We have fabricated 1.98 eV cells with 1-sun and 70-sun efficiencies of 14.4% and 15.9%, respectively, under the direct spectrum, and 2.07 eV cells with 1-sun efficiencies of 10.7%. Improvements in the grade design that reduce the threading dislocation density below 106 cm−2 are expected to lead to efficiency increases. Matching the lattice constants of the confinement and contact layers to the junction layers is critical to achieving low interface recombination velocities, and can be a challenge in lattice-mismatched structures if the graded layers are not sufficiently relaxed.

A Standards-Based Method for Compositional Analysis by Energy Dispersive X-Ray Spectrometry Using Multivariate Statistical Analysis: Application to Multicomponent Alloys

Given an unknown multicomponent alloy, and a set of standard compounds or alloys of known composition, can one improve upon popular standards-based methods for energy dispersive X-ray (EDX) spectrometry to quantify the elemental composition of the unknown specimen? A method is presented here for determining elemental composition of alloys using transmission electron microscopy-based EDX with appropriate standards. The method begins with a discrete set of related reference standards of known composition, applies multivariate statistical analysis to those spectra, and evaluates the compositions with a linear matrix algebra method to relate the spectra to elemental composition. By using associated standards, only limited assumptions about the physical origins of the EDX spectra are needed. Spectral absorption corrections can be performed by providing an estimate of the foil thickness of one or more reference standards. The technique was applied to III-V multicomponent alloy thin films: composition and foil thickness were determined for various III-V alloys. The results were then validated by comparing with X-ray diffraction and photoluminescence analysis, demonstrating accuracy of approximately 1% in atomic fraction.