Harvey Guthrey

Metal-Insulator-Metal Diodes: Role of the Insulator Layer on the Rectification Performance

Metal–Insulator–Metal (MIM) diodes are actively investigated as high-frequency rectifi ers [ 2 , 4 , 15–18 ] for these applications, complemented by other related devices such as metal/double-insulator/ metal (MIIM) diodes, [ 13 , 19 ] geometric diodes, [ 20 , 21 ] and molecular diodes. [ 22 , 23 ] Quantum-based electron-transport (tunneling based) are targeted for the rectifi cation mechanism in suitably designed MIM structures [ 24 ] because the tunneling process yields time constants in the range of femto seconds (10 − 15 s). A well-designed MIM structures can therefore potentially rectify frequencies as high as 10 15 Hz. Signifi cant progress has been made to fabricate nano-sized MIM diodes, a geometric requirement for THz frequency applications. [ 15 , 16 ] For MIM systems, numerous works have been reported which provide an understanding of the transport mechanism, [ 25 , 26 ] the theory of device operation [ 27 , 28 ] and suitability of MIM for energy harvesting applications. [ 29 ] However, systematic experimental studies to correlate material properties to MIM rectifi cation performance have been largely absent. Consequently, materials-design rules to help choose suitable materials for the MIM stack that favor the desired rectifi cation behavior are lacking. In a recent review article, Miskovsky et al., highlighted the importance of investigating material properties and designing new materials to optimize MIM high-frequency rectifi cation performance. [ 30 ]

Cathodoluminescence spectrum imaging analysis of CdTe thin-film bevels

We conducted T= 6u2009K cathodoluminescence (CL) spectrum imaging with a nanoscale electron beam on beveled surfaces of CdTe thin films at the critical stages of standard CdTe solar cell fabrication. We find that the through-thickness CL total intensity profiles are consistent with a reduction in grain-boundary recombination due to the CdCl2 treatment. The color-coded CL maps of the near-band-edge transitions indicate significant variations in the defect recombination activity at the micron and sub-micron scales within grains, from grain to grain, throughout the film depth, and between films with different processing histories. We estimated the grain-interior sulfur-alloying fraction in the interdiffused CdTe/CdS region of the CdCl2-treated films from a sample of 35 grains and found that it is not strongly correlated with CL intensity. A kinetic rate-equation model was used to simulate grain-boundary (GB) and grain-interior CL spectra. Simulations indicate that the large reduction in the exciton band intensit...

Diode-coupled Ag nanoantennas for nanorectenna energy conversion

Arrays of nanorectennas consist of diode-coupled nanoantennas with plasmonic resonances in the visible/near-infrared (vis/nir) regime, and are expected to convert vis/nir radiative power into useful direct current. We study plasmonic resonances in large format (~ 1 mm2 area) arrays, consisting of electron beam-patterned horizontal (e.g., parallel to the substrate) Ag lines patterned on ultrathin (< 20 nm) tunneling barriers (NiO, NbOx, and other oxides). Our e-beam fabrication technique is scalable to large dimensions, and allows us to easily probe different antenna dimensions. These tunneling barriers, located on a metallic ground plane, rectify the alternating current generated in the nanoantenna at resonance. We measure the plasmonic resonances in these nanoantennas, and find good agreement with modeling, which also predicts that the electric field driving the electrons into the ground plane (and therefore the rectification efficiency) is considerably enhanced at resonance. Various metal-insulator-metal tunneling diodes, incorporating the afore-mentioned barrier layers and different metals for the ground plane, are experimentally characterized and compared to our conduction model. We observe ~ 1 mV signals from NiO-based nanorectenna arrays illuminated by 532 nm and 1064 nm laser pulses, and discuss the origin of these signals.

Quantification of atomic scale defects in poly Si PV devices using atom probe tomography

Characterization of defect locations and their effects on transport in polycrystalline Si photovoltaics is readily accomplished using optical and electrical characterization. Information on the elemental nature of these defects is more difficult due to both the low concentrations and highly localized positions. This work demonstrates the ability to locate and elementally analyze electronic defects in these devices using correlative electron microscopy and spectroscopy within a focused ion beam specimen preparation tool followed by 3-D atom probe tomography.

Atomic scale characterization of compound semiconductors using atom probe tomography

Internal interfaces are critical in determining the performance of III-V multijunction solar cells. Studying these interfaces with atomic resolution using a combination of transmission electron microscopy (TEM), atom probe tomography (APT), and density functional calculations enables a more fundamental understanding of carrier dynamics in photovoltaic (PV) device structures. To achieve full atomic scale spatial and chemical resolution, data acquisition parameters in laser pulsed APT must be carefully studied to eliminate surface diffusion. Atom probe data with minimized group V ion clustering and expected stoichiometry can be achieved by adjusting laser pulse power, pulse repetition rate, and specimen preparation parameters such that heat flow away from the evaporating surface is maximized. Applying these improved analysis conditions to III-V based PV gives an atomic scale understanding of compositional and dopant profiles across interfaces and tunnel junctions and the initial stages of alloy clustering and dopant accumulation. Details on APT experimental methods and future in-situ instrumentation developments are illustrated.

Defect band luminescence intensity reversal as related to application of anti-reflection coating on mc-Si PV Cells

Photoluminescence (PL) imaging is widely used to identify defective regions within mc-Si PV cells. Recent PL imaging investigations of defect band luminescence (DBL) in mc-Si have revealed a perplexing phenomenon. Namely, the reversal of the DBL intensity in various regions of mc-Si PV material upon the application of a SiNx:H anti-reflective coating (ARC). Regions with low DBL intensity before ARC application often exhibit high DBL intensity afterwards, and the converse is also true. PL imaging alone cannot explain this effect. We have used high resolution cathodoluminescence (CL) spectroscopy and electron beam induced current (EBIC) techniques to elucidate the origin of the DBL intensity reversal. Multiple sub-bandgap energy levels were identified that change in peak position and intensity upon the application of the ARC. Using this data, in addition to EBIC contrast information, we provide an explanation for the DBL intensity reversal based on the interaction of the detected energy levels with the SiNx:H ARC application. Multiple investigations have suggested that this is a global problem for mc-Si PV cells. Our results have the potential to provide mc-Si PV producers a pathway to increased efficiencies through defect mitigation strategies.

Spatially Resolved Recombination Analysis of CuInxGa1-xSe2 Absorbers With Alkali Postdeposition Treatments

In this contribution, we probe spatial variations in charge-carrier recombination in CuInxGa1-x Se2 (CIGS) absorbers grown on soda–lime glass (SLG) and alkali-free sapphire substrates with NaF and KF postdeposition treatments (PDTs). Temperature-and illumination-dependent device measurements are used to track interface recombination and recombination in the quasi-neutral region. The analysis of these data reveals that the benefit of alkali PDTs depends on the substrate: interface recombination is reduced in devices grown on sapphire substrates, whereas recombination in the quasi-neutral regions is reduced in devices grown on SLG substrates. Cathodoluminescence (CL) spectrum imaging is used to study the spatial distribution of recombination with respect to the grain structure. The grain-boundary CL contrast is similar in films with no PDT, NaF PDT, or KF PDT. A reduced grain-boundary contrast is observed with a NaF + KF PDT; however, suggesting a reduced recombination strength at the grain boundaries (GBs) for combined NaF + KF treatment. CL spectra indicate band tailing, consistent with the fluctuating potential model. Fluctuating potentials are believed to reduce open-circuit voltage, but their spatial distribution has not been studied. Here, CL spectrum imaging data are used to generate maps of the root-mean-square value of the potential energy fluctuations—γ. These maps reveal a bimodal γ distribution for all samples: γ is generally in the range ∼15–50 meV or ∼100–180 meV. The higher γ range is more significantly affected by the PDTs; after the PDTs, it is strongly correlated with GBs. The lower γ range is correlated with higher emission intensity regions, typically grain interiors, and increases in area fraction after the PDTs. These results demonstrate how spatially resolved luminescence and device characterization measurements can be used to monitor changes in recombination in CIGS films and photovoltaic devices. Such measurements can complement empirical device optimization and help improve device performance.

Conduction and rectification in NbOx- and NiO-based metal-insulator-metal diodes

Conduction and rectification in nanoantenna-coupled NbOx- and NiO-based metal-insulator-metal (MIM) diodes (“nanorectennas”) are studied by comparing new theoretical predictions with the measured response of nanorectenna arrays. A new quantum mechanical model is reported and agrees with measurements of current–voltage (I–V) curves, over 10 orders of magnitude in current density, from [NbOx(native)-Nb2O5]- and NiO-based samples with oxide thicknesses in the range of 5–36u2009nm. The model, which introduces new physics and features, including temperature, electron effective mass, and image potential effects using the pseudobarrier technique, improves upon widely used earlier models, calculates the MIM diodes I–V curve, and predicts quantitatively the rectification responsivity of high frequency voltages generated in a coupled nanoantenna array by visible/near-infrared light. The model applies both at the higher frequencies, when high-energy photons are incident, and at lower frequencies, when the formula for classical rectification, involving derivatives of the I–V curve, may be used. The rectified low-frequency direct current is well-predicted in this works model, but not by fitting the experimentally measured I–V curve with a polynomial or by using the older Simmons model (as shown herein). By fitting the measured I–V curves with our model, the barrier heights in Nb-(NbOx(native)-Nb2O5)-Pt and Ni-NiO-Ti/Ag diodes are found to be 0.41/0.77 and 0.38/0.39u2009eV, respectively, similar to literature reports, but with effective mass much lower than the free space value. The NbOx (native)-Nb2O5 dielectric properties improve, and the effective Pt-Nb2O5 barrier height increases as the oxide thickness increases. An observation of direct current of ∼4u2009nA for normally incident, focused 514u2009nm continuous wave laser beams are reported, similar in magnitude to recent reports. This measured direct current is compared to the prediction for rectified direct current, given by the rectification responsivity, calculated from the I–V curve times input power.Conduction and rectification in nanoantenna-coupled NbOx- and NiO-based metal-insulator-metal (MIM) diodes (“nanorectennas”) are studied by comparing new theoretical predictions with the measured response of nanorectenna arrays. A new quantum mechanical model is reported and agrees with measurements of current–voltage (I–V) curves, over 10 orders of magnitude in current density, from [NbOx(native)-Nb2O5]- and NiO-based samples with oxide thicknesses in the range of 5–36u2009nm. The model, which introduces new physics and features, including temperature, electron effective mass, and image potential effects using the pseudobarrier technique, improves upon widely used earlier models, calculates the MIM diodes I–V curve, and predicts quantitatively the rectification responsivity of high frequency voltages generated in a coupled nanoantenna array by visible/near-infrared light. The model applies both at the higher frequencies, when high-energy photons are incident, and at lower frequencies, when the formula for c...

Studying Perovskite-based Solar Cells with Correlative In-Situ Microscopy

Hybrid organic-inorganic perovskite based solar technologies are generating a great deal of interest in the materials community. In this work, we plan to discuss our ongoing work to characterize new synthesis routines and processes to generate sustainable and reliable perovskite-based materials whose properties are significantly better than the current state of the art. In particular, we are utilizing the latest advances in high-resolution analytical and in-situ microscopy to characterize these emerging photovoltaic materials and their interfaces, defects, and discrete paths to crystallization.

Characterization of Photovoltaics: From Cells Properties to Atoms

Photovoltaics are an attractive option to supplement our nations energy supply as they provide continuous power under illumination throughout the lifetime of the device. Making these devices as efficient as possible will inevitably increase their presence in the market as the installation cost to power return ratio will increase. The reality is that the majority of PV technologies have been extensively studied over the last several decades and for many the slope of the efficiency vs. time curve has drastically decreased. Research is now focused on making small improvements in device efficiency. Efficiency measurements provide feedback on the quality of the entire device structure but ultimately device properties are determined at the atomic scale through the interaction of charge carriers and defects. In order to determine how defects are related to the discrepancy between theoretical and actual device efficiency it is necessary to employ a suite of characterization techniques that vary in scale from centimeters to angstroms.