Zachary S. Bittner

Epitaxial lift-off of quantum dot enhanced GaAs single junction solar cells

InAs/GaAs strain-balanced quantum dot (QD) n-i-p solar cells were fabricated by epitaxial lift-off (ELO), creating thin and flexible devices that exhibit an enhanced sub-GaAs bandgap current collection extending into the near infrared. Materials and optical analysis indicates that QD quality after ELO processing is preserved, which is supported by transmission electron microscopy images of the QD superlattice post-ELO. Spectral responsivity measurements depict a broadband resonant cavity enhancement past the GaAs bandedge, which is due to the thinning of the device. Integrated external quantum efficiency shows a QD contribution to the short circuit current density of 0.23 mA/cm2.

Effect of electric field on carrier escape mechanisms in quantum dot intermediate band solar cells

Carrier escape and recombination from quantum dot (QD) states reduce the probability of two-step photon absorption (TSPA) by decreasing the available carrier population in the intermediate band (IB). In order to optimize the second photon absorption for future designs of quantum dot embedded intermediate band solar cells, the presented study combined the results of simulations and experiments to quantify the effect of electric field on the barrier height and the carrier escape from the QDs in InAs/GaAs quantum dot solar cells with five-layer QD superlattices. The electric field dependent effective barrier heights for ground state electrons were calculated using eight band k·p theory at short circuit conditions. With an increase in electric field surrounding the QDs from 5 kV/cm to 50 kV/cm, the effective barrier height of the ground state electrons was reduced from 147 meV to 136 meV, respectively. Thus, the increasing electric field not only exponentially enhances the ground state electron tunneling rate...

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.

Characterization of InAlAs solar cells grown by MOVPE

Epitaxial layers of InAlAs are prime candidates for the top cell in triple-junction photovoltaics (PV). Growth conditions during metalorganic vapor phase epitaxy (MOVPE) of InAlAs affect the material properties and subsequently the device characteristics of the epilayers. Impurity concentrations in InAlAs epilayers grown under various conditions are analyzed by secondary-ion mass spectrometry (SIMS) in order to assess impurity incorporation. Deep-level transient spectroscopy (DLTS) is used to assess the energy level and concentration of carrier traps. The effect of defects (traps) on the device characteristics are modeled with a Sentaurus simulation. Devices were fabricated and tested in a solar simulator before and after contact etch. Spectral response (SR) and electroluminescence (EL) are also measured. Final experimental results showed an efficiency of 9.74% without an antireflective coating.

Improved radiation resistance of epitaxial lift-off inverted metamorphic solar cells

The inverted metamorphic (IMM) solar cell has a high specific power compared to traditional germanium-based multi-junction solar cells, which may prove beneficial for space applications where costs are weight-driven. In addition, the mechanical flexibility of the IMM cell may be beneficial for new technologies, such as high-power, flexible, deployable arrays currently under development. However, IMM solar cells have not yet demonstrated radiation resistance equal to that of traditional Ge-based multi-junction cells, largely due to degradation in the InGaAs bottom subcell. A structure and process have been developed to incorporate a back surface reflector on the epitaxial lift-off (ELO) IMM solar cell, permitting the InGaAs subcell to be thinned whilst maintaining high optical absorption. The thinner subcell can better tolerate degraded base diffusion lengths following irradiation. In this manner, a significant improvement in the end of life efficiency of ELO IMM solar cells is demonstrated following irradiation with 1 MeV electrons at a fluence of 1×1015 cm-2.

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

Growth of GaSb with low threading dislocation density directly on GaAs may be possible with the strategic strain relaxation of interfacial misfit arrays. This creates an opportunity for a multi-junction solar cell with access to a wide range of well-developed direct bandgap materials. Multi-junction cells with a single layer of GaSb/GaAs interfacial misfit arrays could achieve higher efficiency than state-of-the-art inverted metamorphic multi-junction cells while forgoing the need for costly compositionally graded buffer layers. To develop this technology, GaSb single junction cells were grown via molecular beam epitaxy on both GaSb and GaAs substrates to compare homoepitaxial and heteroepitaxial GaSb device results. The GaSb-on-GaSb cell had an AM1.5g efficiency of 5.5% and a 44-sun AM1.5d efficiency of 8.9%. The GaSb-on-GaAs cell was 1.0% efficient under AM1.5g and 4.5% at 44 suns. The lower performance of the heteroepitaxial cell was due to low minority carrier Shockley-Read-Hall lifetimes and bulk shu...

Optimization in wide-band-gap quantum dot solar cells

Quantum dots (QDs) have been under extensive study as a promising material to realize the concept of the intermediate band solar cell (IBSC). Because thermal escape is the dominant mechanism of carrier escape at room temperature, wide-band-gap (WBG) semiconductor can be used to suppress thermal escape by increasing barrier height for InAs quantum dots. Embedded InAs QD in the wide-bandgap matrix (InGaP and AlGaAs) is demonstrated with increased sub-band-gap carrier collection. The deeper confinement and larger activation energy is a move towards realizing an IBSC Additionally, activation energy extracted from temperature dependent external quantum efficiency (TDEQE) of InAs/AlGaAs is 324 meV, which is closer to the transition between IB to CB in an ideal IBSC.

Modeling the effects of using polycrystalline substrates for low cost III–V photovoltaics

Recent interest in growing high efficiency solar cells on polycrystalline “virtual” recrystallized substrates requires an understanding of the impact of substrate quality and growth nucleation characteristics. This work provides a study on the effects of substrate material, roughness, and crystallinity on efficiency of a GaAs solar cell. A variety of devices were grown on both monocrystalline and polycrystalline GaAs and Ge substrates, and the results were used to predict minority carrier diffusion length as a function of crystal grain size, material quality, and nucleation defect densities.

Correlation between quantum dot morphology and photovoltaic performance

The use of nanostructures, such as quantum dots (QD) or quantum wells within photovoltaic (PV) devices has demonstrated enhanced current generation, but often at the expense of open-circuit voltage. QD morphology and optical quality have a direct impact on PV performance and optimizing the epitaxial growth of QDs is critical to achieve the desired benefits of QD-enhanced PV. The spatial uniformity of QD epitaxy can determine the PV performance across a large area wafer. In this paper, we demonstrate a correlation between the spatial uniformity of QD size distribution and photovoltaic conversion efficiency. A spatially varying Voc measured on QD-enhanced GaAs solar cells correlates with the presence of coalesced QDs. The results suggest the presence of large, coalesced QDs is a significant cause for a reduced Voc in QD-enhanced GaAs p-i-n solar cells.

Gallium phosphide solar cells with indium gallium phosphide quantum wells for high temperature applications

In order to increase thermal stability of solar cells for high temperature applications, wide bandgap semiconductors such as GaP are being investigated. The addition of nanostructures, such as quantum wells to the solar cell is expected to extend sub-host bandgap absorption and photocurrent generation. Increasing current generation in wide bandgap single-junction solar cells is needed to take advantage of the benefits of wide bandgap materials. In this study, GaP solar cells were grown via OMVPE with and without InGaP/GaP multiple quantum wells (MQWs). A GaP solar cell, including 5 period InGaP/GaP MQW showed a 8% increase in integrated short circuit current density (Jsc) beyond the direct band edge at 446 nm compared to a GaP solar cell without quantum wells fabricated for this study. Low temperature electroluminescence showed a peak shift from 2.16 eV to 2.12 eV due to the addition of MQWs. An additional GaP solar cell was grown with 5× InGaP/GaP MQW and a GaAs contact layer. This cell had a measured AM0 Jsc, of 2.56 mA/cm2, an open circuit voltage of 1.29 V, and an AM0 efficiency of 1.83%. The normalized temperature dependence of efficiency for a GaP solar cell was shown to have a value of of 2.78 × 10−3 °C−1, demonstrating an increase in efficiency with temperature.