Sub-0.6 eV Inverted Metamorphic GaInAs Cells Grown on InP and GaAs Substrates for Thermophotovoltaics and Laser Power Conversion

We present inverted metamorphic Ga0.3In0.7As photovoltaic converters with sub-0.60 eV bandgaps grown on InP and GaAs substrates. The compositionally graded buffers in these devices have threading dislocation densities of 1.3x10^6 cm^-2 and 8.9x10^6 cm^-2 on InP and GaAs, respectively. The devices generate open-circuit voltages of 0.386 V and 0.383 V, respectively, at a current density of ~10 A/cm^2, yielding bandgap-voltage offsets of 0.20 and 0.21 V. We measured their broadband reflectance and used it to estimate thermophotovoltaic efficiency. The InP-based cell is estimated to yield 1.09 W/cm^2 at 1100 degrees C vs. 0.92 W/cm^2 for the GaAs-based cell, with efficiencies of 16.8 vs. 9.2%. The efficiencies of both devices are limited by sub-bandgap absorption, with power weighted sub-bandgap reflectances of 81% and 58%, respectively, which we assess largely occurs in the graded buffers. We estimate that the thermophotovoltaic efficiencies would peak at ~1100 degrees C at 24.0% and 20.7% in structures with the graded buffer removed, if previously demonstrated reflectance is achieved. These devices also have application to laser power conversion in the 2.0-2.3 micron atmospheric window. We estimate peak LPC efficiencies of 36.8% and 32.5% under 2.0 micron irradiances of 1.86 W/cm^2 and 2.81 W/cm^2, respectively.Comment: 14 pages, 6 figure

Pulsed laser annealed Ga hyperdoped poly‐Si/SiOx passivating contacts for high‐efficiency monocrystalline Si solar cells

Polycrystalline Si (poly-Si)-based passivating contacts are promising candidates for high-efficiency crystalline Si solar cells. We show that nanosecond-scale pulsed laser melting (PLM) is an industrially viable technique to fabricate such contacts with precisely controlled dopant concentration profiles that exceed the solid solubility limit. We demonstrate that conventionally doped, hole-selective poly-Si/SiOx contacts that provide poor surface passivation of c-Si can be replaced with Ga- or B-doped contacts based on non-equilibrium doping. We overcome the solid solubility limit for both dopants in poly-Si by rapid cooling and recrystallization over a timescale of ~25 ns. We show an active Ga dopant concentration of ~3×1020 cm-3 in poly-Si which is six times higher than its solubility limit in c-Si, and a B dopant concentration as high as ~1021 cm-3. We measure an implied open-circuit voltage of 735 mV for Ga-doped poly-Si/SiOx contacts on Czochralski Si with a low contact resistivity of 35.5 ± 2.4 mΩ∙cm2. Scanning spreading resistance microscopy and Kelvin probe force microscopy show large diffusion and drift current in the p-n junction that contributes to the low contact resistivity. Our results suggest that PLM can be extended for hyperdoping of other semiconductors with low solubility atoms to enable high-efficiency devices

Synthesis and Calculations of Wurtzite Al_1–<i>x</i>Gd_<i>x</i>N Heterostructural Alloys

Al1–xGdxN is one of a series of novel heterostructural alloys involving rare earth cations with potentially interesting properties for (opto)­electronic, magnetic, and neutron detector applications. Using alloy models in conjunction with density functional theory, we explored the full composition range for Al1–xGdxN and found that wurtzite is the ground-state structure up to a critical composition of xc = 0.82. The calculated temperature-composition phase diagram reveals a large miscibility gap inducing spinodal decomposition at equilibrium conditions, with higher Gd substitution (meta)­stabilized at higher temperatures. By depositing combinatorial thin films at high effective temperatures using radio-frequency cosputtering, we have achieved the highest Gd3+ incorporation into the wurtzite phase reported to date, with single-phase compositions at least up to x ≈ 0.25 confirmed by high-resolution synchrotron grazing incidence wide-angle X-ray scattering. High-resolution transmission electron microscopy on material with x ≈ 0.13 and x ≈ 0.24 confirmed a uniform composition polycrystalline film with uniform columnar grains having the wurtzite structure. Spectroscopic ellipsometry and cathodoluminescence spectroscopy measurements are employed to probe the optoelectronic properties, showing that the band gap decreases with increasing Gd content x and that this effect causes the ideal Gd substitution level for cathodoluminescence applications to be low. Expanding our calculations to other rare earth cations (Pr3+ and Tb3+) reveals similar thermodynamic stability and solubility behavior to Gd. From this and previous studies on Al1–xScxN, we elucidate that both smaller ionic radius and higher bond ionicity promote increased incorporation of group IIIB cations into wurtzite AlN. This work furthers the development of design rules for new alloys in this material family

3D and Multimodal X-Ray Microscopy Reveals the Impact of Voids in CIGS Solar Cells

Small voids in the absorber layer of thin-film solar cells are generally suspected to impair photovoltaic performance. They have been studied on Cu(In,Ga)Se2 cells with conventional laboratory techniques, albeit limited to surface characterization and often affected by sample-preparation artifacts. Here, synchrotron imaging is performed on a fully operational as-deposited solar cell containing a few tens of voids. By measuring operando current and X-ray excited optical luminescence, the local electrical and optical performance in the proximity of the voids are estimated, and via ptychographic tomography, the depth in the absorber of the voids is quantified. Besides, the complex network of material-deficit structures between the absorber and the top electrode is highlighted. Despite certain local impairments, the massive presence of voids in the absorber suggests they only have a limited detrimental impact on performance

Increased Optoelectronic Quality and Uniformity of Hydrogenated pInP Thin Films

The thin-film vapor-liquid-solid (TF-VLS) growth technique presents a promising route for high quality, scalable, and cost-effective InP thin films for optoelectronic devices. Toward this goal, careful optimization of material properties and device performance is of utmost interest. Here, we show that exposure of polycrystalline Zn-doped TF-VLS InP to a hydrogen plasma (in the following referred to as hydrogenation) results in improved optoelectronic quality as well as lateral optoelectronic uniformity. A combination of low temperature photoluminescence and transient photocurrent spectroscopy was used to analyze the energy position and relative density of defect states before and after hydrogenation. Notably, hydrogenation reduces the relative intragap defect density by 1 order of magnitude. As a metric to monitor lateral optoelectronic uniformity of polycrystalline TF-VLS InP, photoluminescence and electron beam induced current mapping reveal homogenization of the grain versus grain boundary upon hydrogenation. At the device level, we measured more than 260 TF-VLS InP solar cells before and after hydrogenation to verify the improved optoelectronic properties. Hydrogenation increased the average open-circuit voltage (VOC) of individual TF-VLS InP solar cells by up to 130 mV and reduced the variance in VOC for the analyzed devices