Ramesh B. Laghumavarapu

Hybrid conjugated polymer solar cells using patterned GaAs nanopillars

In this work, we study hybrid solar cells based on poly(3-hexylthiophene)-coated GaAs nanopillars grown on a patterned GaAs substrate using selective-area metal organic chemical vapor deposition. The hybrid solar cells show extremely low leakage currents (I≅45 nA @−1V) under dark conditions and an open circuit voltage, short circuit current density, and fill factor of 0.2 V, 8.7 mA/cm2, and 32%, respectively, giving a power conversion efficiency of η=0.6% under AM 1.5 G illumination. Surface passivation of the GaAs results in further improvement, yielding η=1.44% under AM 1.5 G illumination. External quantum efficiency measurements of these polymer/inorganic solar cells are also presented.

GaSb thermophotovoltaic cells grown on GaAs by molecular beam epitaxy using interfacial misfit arrays

There exists a long-term need for foreign substrates on which to grow GaSb-based optoelectronic devices. We address this need by using interfacial misfit arrays to grow GaSb-based thermophotovoltaic cells directly on GaAs (001) substrates and demonstrate promising performance. We compare these cells to control devices grown on GaSb substrates to assess device properties and material quality. The room temperature dark current densities show similar characteristics for both cells on GaAs and on GaSb. Under solar simulation the cells on GaAs exhibit an open-circuit voltage of 0.121 V and a short-circuit current density of 15.5 mA/cm2. In addition, the cells on GaAs substrates maintain 10% difference in spectral response to those of the control cells over a large range of wavelengths. While the cells on GaSb substrates in general offer better performance than the cells on GaAs substrates, the cost-savings and scalability offered by GaAs substrates could potentially outweigh the reduction in performance. By fu...

Structural and optical properties of InAs/AlAsSb quantum dots with GaAs(Sb) cladding layers

We investigate the effect of GaAs1−xSbx cladding layer composition on the growth and properties of InAs self-assembled quantum dots surrounded by AlAs0.56Sb0.44 barriers. Lowering Sb-content in the GaAs1−xSbx improves the morphology of the InAs quantum dots and reduces cladding layer alloy fluctuations. The result is a dramatic increase in photoluminescence intensity from the InAs quantum dots, with a peak at 0.87 eV. The emission energy exhibits a cube root dependence on excitation power, consistent with the type-II band alignment of the quantum dots. The characteristics of this quantum dot system show promise for applications such as intermediate band solar cells.

Improved quantum dot stacking for intermediate band solar cells using strain compensation.

We use thin tensile-strained AlAs layers to manage compressive strain in stacked layers of InAs/AlAsSb quantum dots (QDs). The AlAs layers allow us to reduce residual strain in the QD stacks, suppressing strain-related defects. AlAs layers 2.4 monolayers thick are sufficient to balance the strain in the structures studied, in agreement with theory. Strain balancing improves material quality and helps increase QD uniformity by preventing strain accumulation and ensuring that each layer of InAs experiences the same strain. Stacks of 30 layers of strain-balanced QDs exhibit carrier lifetimes as long as 9.7 ns. QD uniformity is further enhanced by vertical ABAB… ordering of the dots in successive layers. Strain compensated InAs/AlAsSb QD stacks show great promise for intermediate band solar cell applications.

Effects of GaAs(Sb) cladding layers on InAs/AlAsSb quantum dots

The structural and optical properties of InAs self-assembled quantum dots buried in AlAs0.56Sb0.44 barriers can be controlled using GaAs1−xSbx cladding layers. These cladding layers allow us to manage the amount of Sb immediately underneath and above the InAs quantum dots. The optimal cladding scheme has a GaAs layer beneath the InAs, and a GaAs0.95Sb0.05 layer above. This scheme results in improved dot morphology and significantly increased photoluminescence (PL) intensity. Both power-dependent and time-resolved photoluminescence confirm that the quantum dots have type-II band alignment. Enhanced carrier lifetimes in this quantum dot system show great potential for application in intermediate band solar cells.

Investigation of optical transitions in InAs/GaAs(Sb)/AlAsSb quantum dots using modulation spectroscopy

InAs quantum dots (QDs) were grown in an AlAs0.56Sb0.44/GaAs matrix in the unintentionally doped (uid) region of an In0.52Al0.48As solar cell, establishing a variety of optical transitions both into and out of the QDs. The ultimate goal is to demonstrate sequential absorption, where one photon is absorbed, promoting an electron from the valence band into the QD, and a second photon is absorbed in order to promote the trapped electron from a QD state into the host conduction band. In this study, we directly investigate the optical properties of the solar cell using photoreflectance and evaluate the possibility of sequential absorption by measuring spectral responsivity with broadband infrared illumination.

The effect of passivation on different GaAs surfaces

The surface passivation of semiconductors on different surface orientations results in vastly disparate effects. Experiments of GaAs/poly(3,4-ethylenedioxythiophene/indium tin oxide solar cells show that sulfur passivation results in threefold conversion efficiency improvements for the GaAs (100) surface. In contrast, no improvements are observed after passivation of the GaAs (111B) surface, which achieves 4% conversion efficiency. This is explained by density-functional theory calculations, which find a surprisingly stable (100) surface reconstruction with As defects that contains midgap surface states. Band structure calculations with hybrid functionals of the defect surface show a surface state on the undimerized As atoms and its disappearance after passivation.

High-density InAs/GaAs1−xSbx quantum-dot structures grown by molecular beam epitaxy for use in intermediate band solar cells

InAs quantum-dot structures were grown using a GaAs1−xSbx matrix on a GaAs(001) substrate. The use of GaAs1−xSbx for the buffer and cap layers effectively suppressed coalescence between dots and significantly increased the dot density. The highest density (∼3.5 × 1011/cm2) was obtained for a nominal 3.0 monolayer deposition of InAs with an Sb composition of x = 13–14% in the GaAs1−xSbx matrix. When the Sb composition was increased to 18%, the resulting large photoluminescent red shift (∼90 meV) indicated the release of compressive strain inside the quantum dots. For x > 13%, we observed a significant decrease in photoluminescence intensity and an increase in the carrier lifetime (≥4.0 ns). This is attributed to the type-II band alignment between the quantum dots and matrix material.

Electromagnetic Characterization of High Absorption Sub-Wavelength Optical Nanostructure Photovoltaics for Solar Energy Harvesting

This paper presents a detailed numerical electromagnetic characterization of GaAs photovoltaic (PV) nanopillar array solar cells recently developed for solar energy harvesting. Through electromagnetic theory, full-wave simulations, and an optical measurement, a deeper understanding of the electromagnetic operation of these nanostructure arrays is achieved by revealing the mechanisms that allow for its inherent improvement of optical absorption over conventional PV solar cells. Initial investigations include incorporating and verifying material optical properties through measurements of bulk GaAs samples, simulating the effects of nanopillar geometry and configuration, and an analysis of the optical absorption mechanism of the nanopillar arrays through the graphical visualization of the electric fields in the vicinity of the nanopillars. These investigations will offer critical insights into the effects of pillar dimensions and configuration that can significantly increase optical solar energy absorption approximately 1.5 times that of conventional solar cells spanning the entire visible spectrum and for angles of incidence up to 60

GaSb solar cells grown on GaAs via interfacial misfit arrays for use in the III-Sb multi-junction cell

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