Hassanet Sodabanlu

Loss mitigation in plasmonic solar cells: aluminium nanoparticles for broadband photocurrent enhancements in GaAs photodiodes

We illustrate the important trade-off between far-field scattering effects, which have the potential to provide increased optical path length over broad bands, and parasitic absorption due to the excitation of localized surface plasmon resonances in metal nanoparticle arrays. Via detailed comparison of photocurrent enhancements given by Au, Ag and Al nanostructures on thin-film GaAs devices we reveal that parasitic losses can be mitigated through a careful choice of scattering medium. Absorption at the plasmon resonance in Au and Ag structures occurs in the visible spectrum, impairing device performance. In contrast, exploiting Al nanoparticle arrays results in a blue shift of the resonance, enabling the first demonstration of truly broadband plasmon enhanced photocurrent and a 22% integrated efficiency enhancement.

A quantum-well superlattice solar cell for enhanced current output and minimized drop in open-circuit voltage under sunlight concentration

Insertion of quantum wells (QWs) extends the absorption edge to a longer wavelength than the value of a p-i-n cell without the QWs, which is preferable for the improved current matching of a InGaP/GaAs/Ge multijunction cell. The QWs, however, reduce the open-circuit voltage (Voc) and degrade the fill factor; the latter is significant for a large number of QWs that are mandatory for sufficient light absorption. As a structure to minimize these drawbacks, a QW superlattice, a strain-balanced In0.13Ga0.86As (4.7 nm)/GaAs0.57P0.43 (3.1 nm) stack, was implemented by metalorganic vapour-phase epitaxy. It brought about an enhancement in short-circuit current density (3.0 mA cm−2) with a minimal drop in Voc(0.03 V) compared with a p-i-n cell without the superlattice. The collection efficiency of photocarriers from the wells to an external circuit was evaluated: the efficiency was above 0.95 for the superlattice, while it was below 0.8 at a large forward bias for a conventional QW cell with thicker barriers. With the fast electron–hole separation in the superlattice owing to tunnelling transport, the superlattice cell exhibited a steeper increase in Voc as a function of the sunlight concentration ratio than the conventional QW cell: at the concentration ratio of 50, the value of Voc for the superlattice cell was almost equivalent to the value of the GaAs p-i-n cell without QWs. As a possible mechanism behind such an enhancement in Voc, photocurrent generation by two-step photon absorption was observed, using the electron ground state of the superlattice as an intermediate state.

Blueshift of intersubband transition wavelength in AlN/GaN multiple quantum wells by low temperature metal organic vapor phase epitaxy using pulse injection method

AlN/GaN multiquantum wells (MQWs) were grown at different growth temperatures via a metal organic vapor phase epitaxy (MOVPE) system using a pulse injection method and their intersubband transition (ISBT) properties were investigated. Strong ISBT at 1.58 μm measured at room temperature was realized with MQWs grown at 770 °C and its absorption properties was the best reported in MOVPE system using GaN buffer layer. Clear blueshift of ISB absorption wavelength by lowering growth temperature was observed, which suggests that interdiffusion within MQWs was suppressed at lower growth temperatures.

Strain-compensation measurement and simulation of InGaAs/GaAsP multiple quantum wells by metal organic vapor phase epitaxy using wafer-curvature

Precise strain compensation for lattice-mismatched quantum wells is crucial for obtaining high performance devices such as quantum well solar cells. High-accuracy in situ curvature monitoring is a more efficient tool to adjust growth conditions for perfect strain balancing, and we have achieved curvature measurement during growth of InGaAs/GaAsP multiple quantum wells by metal organic vapor phase epitaxy. We have also developed the curvature calculation model taking into account of thermal expansion and lattice relaxation effects based on Stoney’s equation. The measured periodical curvature behavior corresponds to the growth of compressive InGaAs well layers and tensile GaAsP barrier layers and fits perfectly with a theoretical curve assuming the structural parameters (thicknesses and atomic contents) obtained by x-ray diffraction analysis, confirming correctness of the developed calculation method. Considering the proper thermal expansion coefficients for InGaAs and GaAsP, we have obtained much accurate ...

A Superlattice Solar Cell With Enhanced Short-Circuit Current and Minimized Drop in Open-Circuit Voltage

A quantum-well (QW) solar cell including InGaAs wells is a promising candidate for the purpose of current matching in InGaP/GaAs/Ge tandem solar cells by extending the edge of quantum efficiency to longer wavelengths. Even though QWs increase short-circuit current by the extended effective band edge, they tend to obstruct carrier transport and degrade the efficiency of a cell. Therefore, a superlattice (SL) structure has been proposed to prevent the recombination of carriers inside of the wells and, more importantly, to enable carriers to tunnel to a neighboring well, leading to an efficient carrier transportation in such a photovoltaic device. In this paper, a SL solar cell was implemented with a strain-balancing technique. It exhibited excellent performance: Enhanced photocurrent (3.0 mA/cm

Light-emitting devices based on top-down fabricated GaAs quantum nanodisks.

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Absorption enhancement through Fabry-Pérot resonant modes in a 430 nm thick InGaAs/GaAsP multiple quantum wells solar cell

) with minimized drop (0.03 V) in open-circuit voltage. Behind these achievements, substantial contribution of tunneling transport has been confirmed for the SL cell by external quantum efficiency measurement at 77 K.

InGaAs/GaAsP strain balanced multi-quantum wires grown on misoriented GaAs substrates for high efficiency solar cells

Quantum dots photonic devices based on the III–V compound semiconductor technology offer low power consumption, temperature stability, and high-speed modulation. We fabricated GaAs nanodisks (NDs) of sub-20-nm diameters by a top-down process using a biotemplate and neutral beam etching (NBE). The GaAs NDs were embedded in an AlGaAs barrier regrown by metalorganic vapor phase epitaxy (MOVPE). The temperature dependence of photoluminescence emission energies and the transient behavior were strongly affected by the quantum confinement effects of the embedded NDs. Therefore, the quantum levels of the NDs may be tuned by controlling their dimensions. We combined NBE and MOVPE in a high-throughput process compatible with industrial production systems to produce GaAs NDs with tunable optical characteristics. ND light emitting diode exhibited a narrow spectral width of 38 nm of high-intensity emission as a result of small deviation of ND sizes and superior crystallographic quality of the etched GaAs/AlGaAs layer.

Strain effects on the intersubband transitions in GaN/AlN multiple quantum wells grown by low-temperature metal organic vapor phase epitaxy with AlGaN interlayer

We study light management in a 430 nm-thick GaAs p-i-n single junction solar cell with 10 pairs of InGaAs/GaAsP multiple quantum wells (MQWs). The epitaxial layer transfer on a gold mirror improves light absorption and increases the external quantum efficiency below GaAs bandgap by a factor of four through the excitation of Fabry-Perot resonances. We show a good agreement with optical simulation and achieve around 10% conversion efficiency. We demonstrate numerically that this promising result can be further improved by anti-reflection layers. This study paves the way to very thin MQWs solar cells.

A hot-electron thermophotonic solar cell demonstrated by thermal up-conversion of sub-bandgap photons

Quantum wires (QWRs) form naturally when growing strain balanced InGaAs/GaAsP multi-quantum wells (MQW) on GaAs [100] 6° misoriented substrates under the usual growth conditions. The presence of wires instead of wells could have several unexpected consequences for the performance of the MQW solar cells, both positive and negative, that need to be assessed to achieve high conversion efficiencies. In this letter, we study QWR properties from the point of view of their performance as solar cells by means of transmission electron microscopy, time resolved photoluminescence and external quantum efficiency (EQE) using polarised light. We find that these QWRs have longer lifetimes than nominally identical QWs grown on exact [100] GaAs substrates, of up to 1 μs, at any level of illumination. We attribute this effect to an asymmetric carrier escape from the nanostructures leading to a strong 1D-photo-charging, keeping electrons confined along the wire and holes in the barriers. In principle, these extended lifetim...