Seth M. Hubbard

Effect of strain compensation on quantum dot enhanced GaAs solar cells

GaP tensile strain compensation (SC) layers were introduced into GaAs solar cells enhanced with a five layer stack of InAs quantum dots (QDs). One sun air mass zero illuminated current-voltage curves show that SC results in improved conversion efficiency and reduced dark current. The strain compensated QD solar cell shows a slight increase in short circuit current compared to a baseline GaAs cell due to sub-GaAs bandgap absorption by the InAs QD. Quantum efficiency and electroluminescence were also measured and provide further insight to the improvements due to SC.

Near 1 V open circuit voltage InAs/GaAs quantum dot solar cells

Ten-layer InAs/GaAs quantum dot (QD) solar cells exhibiting enhanced short circuit current (Jsc) and open circuit voltage (Voc) comparable to a control GaAs p-i-n solar cell are reported. 1 sun Jsc is enhanced by 3.5% compared to that of the GaAs control, while the Voc is maintained at 994 mV. Results were achieved using optimized InAs QD coverage and a modified strain balancing technique, resulting in a high QD density (3.6×1010 cm−2), uniform QD size (4×16 nm2), and low residual strain (103 ppm). This enhanced Voc is a promising result for the future of InAs QD-enhanced GaAs solar cells.

Nanostructured photovoltaics for space power

Quantum dot enhanced solar cells have been evaluated both theoretically and experimentally. A detailed balance simulation of InAs quantum dot (QD) enhanced solar cells has been performed. A 14% (absolute) efficiency improvement has been predicted if the middle junction of a state-of-the-art space multi-junction III-V solar cell can be bandgap engineered using QDs. Experimental results for a GaAs middle junction enhanced with InAs QDs have shown an 8% increase in short circuit current compared to a baseline device. The current enhancement per layer of QD was extracted from device spectral response (0.017 mA per QD layer). This value was used to estimate the efficiency of multi-junction solar cells with up to 200 layers of QDs added to the middle current-limiting junction. In addition, the radiation tolerance of QD cells, key to operation of these cells in space environments, shows improved characteristics. Open circuit voltage (VOC) in QD devices was more resilient to both alpha and proton displacement damage, resulting in a 10X reduction in the rate of VOC degradation.

Luminescence of GaN nanocolumns obtained by photon-assisted anodic etching

GaN nanocolumns with transverse dimensions of about 50 nm were obtained by illumination-assisted anodic etching of epilayers grown by metalorganic chemical vapor deposition on sapphire substrates. The photoluminescence spectroscopy characterization shows that the as-grown bulk GaN layers suffer from compressive biaxial strain of 0.5 GPa. The majority of nanocolumns are fully relaxed from strain, and the room-temperature luminescence is free excitonic. The high quality of the columnar nanostructures evidenced by the enhanced intensity of the exciton luminescence and by the decrease of the yellow luminescence is explained by the peculiarities of the anodic etching processing.

Electrodeposited CdS on CIS pn junctions

We have been investigating the electrochemical deposition of thin films and junctions of cadmium sulfide (CdS) and copper indium diselenide (CIS). We show that it is possible to fabricate pn junctions based on n-type CdS and p-type CIS entirely by electrodeposition. CIS is considered to be one of the best absorber materials for use in polycrystalline thin-film photovoltaic solar cells. CdS provides a closely lattice-matched window layer for CIS. Electrodeposition is a simple and inexpensive method for producing thin-film CdS and CIS. We have produced both p- and n-type CIS thin films, as well as a CdS on CIS pn junction via electrodeposition. Elemental analysis of the CdS and CIS thin films was performed using X-ray photoelectron spectroscopy and energy dispersive spectroscopy. Optical band gaps were determined for these films using optical transmission spectroscopy. Carrier densities of the CIS films as a function of their deposition voltage were determined from capacitance vs. voltage measurements using Al Schottky barriers. Current vs. voltage characteristics were measured for the Al on CIS Schottky barriers and for the CdS on CIS pn junction.

Quantum dot solar cell tolerance to alpha-particle irradiation

The effects of alpha-particle irradiation on an InAs quantum dot (QD) array and GaAs-based InAs QD solar cells were investigated. Using photoluminescence (PL) mapping, the PL intensity at 872 and 1120nm, corresponding to bulk GaAs and InAs QD emissions, respectively, were measured for a five-layer InAs QD array which had a spatially varying total alpha-particle dose. The spectral response and normalized current-voltage parameters of the solar cells, measured as a function of alpha-particle fluence, were used to investigate the change in device performance between GaAs solar cells with and without InAs QDs.

Evaluation of strain balancing layer thickness for InAs/GaAs quantum dot arrays using high resolution x-ray diffraction and photoluminescence

The impact of strain-balancing quantum dot superlattice arrays is critical to device performance. InAs/GaAs/GaP strain-balanced quantum dot arrays embedded in p-i-n diodes were investigated via high resolution x-ray diffraction (HRXRD) and photoluminescence (PL) as a function of the GaP thickness. A three-dimensional modification of the continuum elasticity theory was proposed and an optimal thickness was determined to be 3.8 ML. HRXRD-determined in-plane strain in superlattices with this range of GaP thickness gave an empirical value for the GaP thickness to be 4.5 ML. Optical characterization indicated the highest integrated PL intensity for the sample at the optimal strain balanced condition.

Persistent photoconductivity and optical quenching of photocurrent in GaN layers under dual excitation

Persistent photoconductivity (PPC) and optical quenching (OQ) of photoconductivity (PC) were investigated in a variety of n-GaN layers characterized by different carrier concentrations, luminescence characteristics, and strains. The relation between PPC and OQ of PC was studied by exciting the samples with two beams of monochromatic radiation of various wavelengths and intensities. The PPC was found to be excited by the first beam with a threshold at 2.0 eV, while the second beam induces OQ of PC in a wide range of photon energies with a threshold at 1.0 eV. The obtained results are explained on the basis of a model combining two previously put forward schemes with electron traps playing the main role in PPC and hole traps inducing OQ of PC. The possible nature of the defects responsible for optical metastability of GaN is discussed.

Analyzing carrier escape mechanisms in InAs/GaAs quantum dot p-i-n junction photovoltaic cells

Intermediate band solar cells (IBSCs) are third-generation photovoltaic (PV) devices that can harvest sub-bandgap photons normally not absorbed in a single-junction solar cell. Despite the large increase in total solar energy conversion efficiency predicted for IBSC devices, substantial challenges remain to realizing these efficiency gains in practical devices. We evaluate carrier escape mechanisms in an InAs/GaAs quantum dot intermediate band p-i-n junction PV device using photocurrent measurements under sub-bandgap illumination. We show that sub-bandgap photons generate photocurrent through a two-photon absorption process, but that carrier trapping and retrapping limit the overall photocurrent. The results identify a key obstacle that must be overcome in order to realize intermediate band devices that outperform single junction photovoltaic cells.

Short circuit current enhancement of GaAs solar cells using strain compensated InAs quantum dots

Tensile strain compensation (SC) layers were introduced into GaAs p-i-n solar cells grown with a five-stack of InAs quantum dots (QDs) within the i-region. The effects of strain within stacked layers of InAs quantum dots (QDs) were investigated using high resolution x-ray diffraction (HRXRD). Analysis of the HRXRD data shows that the average lattice strain is minimized for the optimal SC thickness. One sun air mass zero illuminated current-voltage curves show that SC results in improved conversion efficiency and reduced dark current when compared to uncompensated devices. The strain compensated 5-layer QD solar cell shows a 0.9 mA/cm2 increase in short circuit current compared to a baseline GaAs cell. Quantum efficiency measurements show this additional current results from photo-generated carriers within the quantum confined material.