Andrew G. Norman

40.8% efficient inverted triple-junction solar cell with two independently metamorphic junctions

A photovoltaic conversion efficiency of 40.8% at 326 suns concentration is demonstrated in a monolithically grown, triple-junction III–V solar cell structure in which each active junction is composed of an alloy with a different lattice constant chosen to maximize the theoretical efficiency. The semiconductor structure was grown by organometallic vapor phase epitaxy in an inverted configuration with a 1.83 eV Ga.51In.49P top junction lattice-matched to the GaAs substrate, a metamorphic 1.34 eV In.04Ga.96As middle junction, and a metamorphic 0.89 eV In.37Ga.63As bottom junction. The two metamorphic junctions contained approximately 1×105 cm−2 and 2–3×106 cm−2 threading dislocations, respectively.

BGaInAs alloys lattice matched to GaAs

We report the epitaxial growth of zinc-blende BxGa1−x−yInyAs and BxGa1−xAs on GaAs substrates with boron concentrations (x) up to 2%–4% by atmospheric-pressure metalorganic chemical vapor deposition. The band gap of BxGa1−xAs increases by only 4–8 meV/%B with increasing boron concentration in this concentration range. We demonstrate an epitaxial BxGa1−x−yInyAs layer deposited on GaAs with a band gap of 1.34 eV that is significantly less strained than a corresponding Ga1−yInyAs layer with the same band gap.

Lattice-mismatched GaAsP Solar Cells Grown on Silicon by OMVPE

We report on lattice-mismatched GaAs_0.7P_0.3 solar cells grown on silicon substrates. This composition of GaAs_0.7P_0.3 has a band gap of about 1.7 eV and is well suited as the top junction of a III-V/Si two-junction tandem solar cell. Using a thin, high-quality GaP nucleation layer, a lattice-matched GaN_0.02P_0.98 buffer layer, and a compositionally graded GaAs_xP_1-x buffer layer, the threading dislocation densities was reduced to less than 10⁸ cm^-2 in the active region. The efficiencies of these single-junction cells without any antireflection coatings were as high has 9.8% under the AM1.5G spectrum. The quality of these solar cells based on V_oc is comparable to the best III-V solar cells ever grown on Si substrates with a III-V buffer

Inverted GaInP / (In)GaAs / InGaAs triple-junction solar cells with low-stress metamorphic bottom junctions

We demonstrate high efficiency performance in two ultra-thin, Ge-free III–V semiconductor triple-junction solar cell device designs grown in an inverted configuration. Low-stress metamorphic junctions were engineered to achieve excellent photovoltaic performance with less than 3 × 106 cm−2 threading dislocations. The first design with band gaps of 1.83/1.40/1.00 eV, containing a single metamorphic junction, achieved 33.8% and 39.2% efficiencies under the standard one-sun global spectrum and concentrated direct spectrum at 131 suns, respectively. The second design with band gaps of 1.83/1.34/0.89 eV, containing two metamorphic junctions achieved 33.2% and 40.1% efficiencies under the standard one-sun global spectrum and concentrated direct spectrum at 143 suns, respectively.

Epitaxial growth of BGaAs and BGaInAs by MOCVD

Abstract The growth of epitaxial zinc-blende B x Ga 1− x As and B x Ga 1− x − y In y As alloys using diborane is complicated by a thermodynamic miscibility gap and complex gas-phase chemistry. We have characterized the growth behavior of these alloys when they are grown using trimethylgallium (TMG), triethylgallium (TEG), trimethylindium (TMI), and arsine by low-pressure and atmospheric-pressure metal–organic chemical-vapor-deposition. Boron incorporation into B x Ga 1− x As exhibits qualitatively different behavior using TEG and TMG, but the incorporation efficiency and maximum achievable boron concentration decrease dramatically at growth temperatures greater than 600°C using either Ga source.

Atom Probe Analysis of III–V and Si-Based Semiconductor Photovoltaic Structures

The applicability of atom probe to the characterization of photovoltaic devices is presented with special emphasis on high efficiency III-V and low cost ITO/a-Si:H heterojunction cells. Laser pulsed atom probe is shown to enable subnanometer chemical and structural depth profiling of interfaces in III-V heterojunction cells. Hydrogen, oxygen, and phosphorus chemical profiling in 5-nm-thick a-Si heterojunction cells is also illustrated, along with compositional analysis of the ITO/a-Si interface. Detection limits of atom probe tomography useful to semiconductor devices are also discussed. Gaining information about interfacial abruptness, roughness, and dopant profiles will allow for the determination of semiconductor conductivity, junction depletion widths, and ultimately photocurrent collection efficiencies and fill factors.

0.7-eV GaInAs Junction for a GaInP/GaAs/GaInAs(1eV)/GaInAs(0.7eV) Four-Junction Solar Cell

We discuss recent developments in III-V multijunction solar cells, focusing on adding a fourth junction to the Ga0.5In0.5 P/GaAs/Ga0.75In0.25As inverted three-junction cell. This cell, grown inverted on GaAs so that the lattice-mismatched Ga0.75In0.25As third junction is the last one grown, has demonstrated 38% efficiency, and 40% is likely in the near future. To achieve still further gains, a lower-bandgap GaxIn1-xAs fourth junction could be added to the three-junction structure for a four-junction cell whose efficiency could exceed 45% under concentration. Here, we present the initial development of the GaxIn1-xAs fourth junction. Junctions of various bandgaps ranging from 0.88 to 0.73 eV were grown, in order to study the effect of the different amounts of lattice mismatch. At a bandgap of 0.88 eV, junctions were obtained with very encouraging ~80% quantum efficiency, 57% fill factor, and 0.36 eV open-circuit voltage. The device performance degrades with decreasing bandgap (i.e., increasing lattice mismatch). We model the four-junction device efficiency vs. fourth junction bandgap to show that an 0.7-eV fourth-junction bandgap, while optimal if it could be achieved in practice, is not necessary; an 0.9-eV bandgap would still permit significant gains in multijunction cell efficiency while being easier to achieve than the lower-bandgap junction

Dielectric function spectra and critical-point energies of Cu2ZnSnSe4 from 0.5 to 9.0 eV

We present dielectric function ɛ = ɛ1 + iɛ2 spectra and critical-point energies of Cu2ZnSnSe4 determined by spectroscopic ellipsometry from 0.5 to 9.0 eV. We reduce artifacts from surface overlayers to the maximum extent possible by performing chemical-mechanical polishing and wet-chemical etching of the surface of a Cu2ZnSnSe4 thin film. Ellipsometric data are analyzed by the multilayer model and the ɛ spectra are extracted. The data exhibit numerous spectral features associated with critical points, whose energies are obtained by fitting standard lineshapes to second energy derivatives of the data. The experimental results are in good agreement with the ɛ spectra calculated within the GW quasi-particle approximation, and possible origins of the pronounced critical-point structures are identified.

Use of a GaAsSb buffer layer for the formation of small, uniform, and dense InAs quantum dots

InAs quantum dots grown on GaAsSb buffer layers with varying Sb content have been studied. Atomic force microscopy results show that the dot size is reduced as the Sb content increases with a concomitant increase in number density. Analysis of the size distribution indicates that the spread of dot sizes narrows with increasing Sb content. This is confirmed by photoluminescence measurements showing a significant narrowing of the dot emission peak for a GaAs0.77Sb0.23 buffer compared to a GaAs buffer. The results are attributed to the strained buffer reducing interactions between dots and the Sb acting as a surfactant.

Strain-dependent morphology of spontaneous lateral composition modulations in (AlAs)m(InAs)n short-period superlattices grown by molecular beam epitaxy

The nature of spontaneous lateral composition modulation and its relationship to surface morphology during the growth of (AlAs)m(InAs)n short-period superlattices by molecular beam epitaxy are investigated as a function of the global strain between the short-period superlattice and (001)InP substrate. For samples grown under tension, transmission electron and atomic force microscopy reveal composition modulations along directions close to 〈310〉 coupled to a surface cusping. For samples grown under compression, we observe composition modulations roughly along the elastically soft 〈100〉 directions coupled to a surface rippling. For high strains (⩾0.7%), with individual InAs layer thicknesses ⩽1.6 monolayers, we observe weak or no composition modulations.