Joseph Faucher

Comparison of GaAsP solar cells on GaP and GaP/Si

We demonstrate metamorphic ∼1.7 eV GaAsxP1−x (x = 0.71 − 0.73) solar cells on high-quality GaP/Si templates and compare them to cells co-grown on bulk GaP. Both n+-emitter/p-base and p+-emitter/n-base polarities are explored. Cells with n-type bases demonstrate current-voltage characteristics that are similar to p-type base cells, but with blue-shifted peak quantum efficiencies. Threading dislocation densities for cells on GaP/Si were 0.92 − 1.3 × 107 cm−2, significantly lower than previous reports but higher than cells grown on bulk GaP. An open-circuit voltage of 1.12 V was obtained for a 1.71 eV cell on Si, leading to a promising bandgap-voltage offset of 0.59 V.

Adsorption Studies on Clay Minerals. IV. The System Montmorillonite‐Cesium‐Potassium

The ion‐exchange equilibria between a montmorillonite clay and potassium‐cesium from chloride solutions have been studied by chromatographic and other methods. The results are summarized in terms of equilibrium constants and activity coefficients for the mixed clay phase. A tentative discussion is given of the rate of the exchange process, particularly as it affects the behavior of the chromatographic columns.

High Performance Ultrathin GaAs Solar Cells Enabled with Heterogeneously Integrated Dielectric Periodic Nanostructures

Due to their favorable materials properties including direct bandgap and high electron mobilities, epitaxially grown III-V compound semiconductors such as gallium arsenide (GaAs) provide unmatched performance over silicon in solar energy harvesting. Nonetheless, their large-scale deployment in terrestrial photovoltaics remains challenging mainly due to the high cost of growing device quality epitaxial materials. In this regard, reducing the thickness of constituent active materials under appropriate light management schemes is a conceptually viable option to lower the cost of GaAs solar cells. Here, we present a type of high efficiency, ultrathin GaAs solar cell that incorporates bifacial photon management enabled by techniques of transfer printing to maximize the absorption and photovoltaic performance without compromising the optimized electronic configuration of planar devices. Nanoimprint lithography and dry etching of titanium dioxide (TiO2) deposited directly on the window layer of GaAs solar cells formed hexagonal arrays of nanoscale posts that serve as lossless photonic nanostructures for antireflection, diffraction, and light trapping in conjunction with a co-integrated rear-surface reflector. Systematic studies on optical and electrical properties and photovoltaic performance in experiments, as well as numerical modeling, quantitatively describe the optimal design rules for ultrathin, nanostructured GaAs solar cells and their integrated modules.

Single-junction GaAsP solar cells grown on SiGe graded buffers on Si

We have investigated the microstructure and device characteristics of GaAs0.82P0.18 solar cells grown on Si0.20Ge0.80/Si graded buffers. Anti-phase domains (APDs) were largely self-annihilated within the In0.39Ga0.61P initiation layer although a low density of APDs was found to propagate to the surface. A combination of techniques was used to show that the GaAs0.82P0.18 cells have a threading dislocation density of 1.2 ± 0.2 × 107 cm−2. Despite these extended defects, the devices exhibited high open-circuit voltages of 1.10–1.12 V. These results indicate that cascading a GaAs0.82P0.18 top cell with a lower-bandgap Si0.20Ge0.80 cell is a promising approach for high-efficiency dual-junction devices on low-cost Si substrates.

Metamorphic 2.1-2.2 eV InGaP solar cells on GaP substrates

We demonstrate ∼2.1–2.2 eV InyGa1−yP (y = 0.18–0.30) solar cells on GaP substrates for potential use in future high-efficiency multi-junction solar cells. Due to increased direct absorption compared to GaP, the InyGa1−yP solar cells exhibited much higher short-circuit current density than indirect gap GaP solar cells with only a slight decrease in open-circuit voltage. As such, the InyGa1−yP solar cells presented here possessed higher efficiency than comparable GaP solar cells. By taking advantage of strong direct-gap absorption, we believe that metamorphic InyGa1−yP will be an ideal top cell material for future multi-junction devices.

Solar hydrogen production using epitaxial SrTiO3 on a GaAs photovoltaic

We demonstrate an oxide-stabilized III–V photoelectrode architecture for solar fuel production from water in neutral pH. For this tunable architecture we demonstrate 100% Faradaic efficiency for hydrogen evolution, and incident photon-to-current efficiencies (IPCE) exceeding 50%. High IPCE for hydrogen evolution is a consequence of the low-loss interface achieved via epitaxial growth of a thin oxide on a GaAs solar cell. Developing optimal energetic alignment across the interfaces of the photoelectrode using well-established III–V technology is key to obtaining high performance. This advance constitutes a critical milestone towards efficient, unassisted fuel production from solar energy.

Effects of growth temperature and device structure on GaP solar cells grown by molecular beam epitaxy

Gallium phosphide (GaP) is an attractive candidate for wide-bandgap solar cell applications, possessing the largest bandgap of the III-arsenide/phosphides without aluminum. However, GaP cells to date have exhibited poor internal quantum efficiency (IQE), even for photons absorbed by direct transitions, motivating improvements in material quality and device structure. In this work, we investigated GaP solar cells grown by molecular beam epitaxy over a range of substrate temperatures, employing a much thinner emitter than in prior work. Higher growth temperatures yielded the best solar cell characteristics, indicative of increased diffusion lengths. Furthermore, the inclusion of an AlGaP window layer improved both open-circuit voltage and short wavelength IQE.

Initiation strategies for simultaneous control of antiphase domains and stacking faults in GaAs solar cells on Ge

Incorporating a Ge junction into a lattice-matched GaInP/GaAs/GaInNAsSb triple-junction cell grown by molecular beam epitaxy (MBE) could enable concentrated efficiencies of ∼50%. Epitaxial integration allows lift-off and wafer bonding steps to be avoided, but growth of III–Vs on Ge by MBE can lead to antiphase domains (APD) and stacking fault pyramids (SFP), both of which diminish solar cell performance. Initiating growth by migration-enhanced epitaxy (MEE) is typically cited as necessary to obtain high-quality III–Vs on Ge. In this work, the authors show that typical MEE growth conditions force a compromise between APD height (hAPD) and SFP density (ρSFP). As APDs can readily self-terminate while SFPs cannot, a two-step initiation strategy was employed, where MEE is performed under conditions that minimize ρSFP followed by low-temperature MBE conditions that encourage APD termination. By doing so, the authors obtained ρSFP < 104 cm−2 with hAPD ≤ 57 nm. The authors also demonstrated that high-quality GaAs...

High-efficiency AlGaInP solar cells grown by molecular beam epitaxy

AlGaInP is an ideal material for ultra-high efficiency, lattice-matched multi-junction solar cells grown by molecular beam epitaxy (MBE) because it can be grown lattice-matched to GaAs with a wide 1.9–2.2 eV bandgap. Despite this potential, AlGaInP grown by molecular beam epitaxy (MBE) has yet to be fully explored, with the initial 2.0 eV devices suffering from poor performance due to low minority carrier diffusion lengths in both the emitter and base regions of the solar cell. In this work, we show that implementing an AlGaInP graded layer to introduce a drift field near the front surface of the device enabled greatly improved internal quantum efficiency (IQE) across all wavelengths. In addition, optimizing growth conditions and post-growth annealing improved the long-wavelength IQE and the open-circuit voltage of the cells, corresponding to a 3× increase in diffusion length in the base. Taken together, this work demonstrates greatly improved IQE, attaining peak values of 95%, combined with an uncoated AM1.5G efficiency of 10.9%, double that of previously reported MBE-grown devices.

GaAsP solar cells on GaP/Si grown by molecular beam epitaxy

We demonstrate metamorphic 1.73 eV GaAs0.72P0.28 solar cells grown by molecular beam epitaxy on high-quality GaP/Si templates and compare them to cells co-grown on bulk GaP. Cascading such a cell with a 1.1 eV Si junction in the substrate could enable a theoretical efficiency of 37% under the AM1.5G spectrum. Electron beam-induced current studies of our cells reveal a threading dislocation density (TDD) of ~1×107 cm-2 for cells on GaP/Si, which is significantly lower than previous reports. We believe that the combination of a highly optimized GaP/Si starting substrate with a well-designed metamorphic buffer enables these relatively low TDDs. Open-circuit voltages as high as 1.10 V were obtained, leading to a bandgap-voltage offset of 0.63 V. This bandgap-voltage offset is also lower than in previous reports, in qualitative agreement with the observation of lower TDD. Direct comparison with cells on bulk GaP confirm the relation between TDD and bandgap-voltage offset, indicating that more investigations to further reduce TDD in GaAsP single-junction cells are required to fulfill the ultimate goal of dual-junction integration on Si.