Adam F. Halverson's research while affiliated with National Renewable Energy Laboratory and other places
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Publications (18)
Understanding the influence of different film structures on electron diffusion in nanoporous metal oxide films has been challenging. Because of the rate-limiting role that traps play in controlling the transport properties, the structural effects of different film architectures are largely obscured or reduced. We describe a general approach to prob...
We report on the synthesis and photoelectrochemical characterization of photocathodes based on CdS-sensitized Ni-NiO core-shell nanowire mesh inverse opals (IOs). Compared to the NiO-CdS nanowire mesh IO electrode, the hole diffusion coefficient of the CdS-sensitized Ni-NiO core-shell nanowire mesh IO photocathode was one order of magnitude larger,...
The performance of polycrystalline thin film CdTe solar cells is limited by as yet poorly understood deep level defects which serve as recombination centers. Electron paramagnetic resonance (EPR) has unrivaled analytical power and sensitivity in the identification of deep level defects in semiconductors; however, very little EPR literature exists w...
Thin-film CdS/CdTe solar cells are a promising materials system for large scale deployment of solar photovoltaic modules. Here we report on an extensive side-by-side characterization effort comparing the device properties of a highly optimized cell made on commercially available transparent conductive oxide/glass with a certified efficiency of 15.2...
Record cells typically use high sheet-resistance transparent conducting oxide (TCO), ultra-thin CdS, and thick CdTe layers to achieve high efficiencies. In this paper, the path to high efficiencies using manufacturable film thicknesses is discussed, demonstrating > 15% cells on commercially available TCO and > 16% cells on non-commercially availabl...
Wire array solar cells benefit from enhanced coupling of light into the active area of the device, significantly decreased collection lengths due to radial charge separation and collection, and easier access to grain boundaries for passivation which may enable future deposition on non-wafer substrates. We report on an analysis of the junction opera...
The apparent carrier density profile measured by the capacitance-voltage technique in CdTe thin-film solar cells frequently displays a distinctive U-shape. We show that, even assuming a uniform carrier density, such a U-shape may arise from deep levels, a non-ohmic back-contact, and a thin absorber, which are commonly present in practical CdTe thin...
This study addresses a long-standing controversy about the electron-transport mechanism in porous metal oxide semiconductor films that are commonly used in dye-sensitized solar cells and related systems. We investigated, by temperature-dependent time-of-flight measurements, the influence of proton intercalation on the electron-transport properties...
We report on the preparation of transparent oriented titania nanotube (NT) photoelectrodes and the effect of illumination direction on light harvesting, electron transport, and recombination in dye-sensitized solar cells (DSSCs) incorporating these electrodes. High solar conversion efficiency requires that the incident light enters the cell from th...
The electronic properties of microcrystalline silicon have been characterized for the first time using transient photocapacitance spectroscopy (TPC) and drive-level capacitance profiling (DLCP). These methods were applied to microcrystalline films deposited by the RF glow discharge method at United Solar. The DLCP method allowed the shallow doping...
We report on the influence of annealing temperature (Ta) on the microstructure and dynamics of electron transport and recombination in dye-sensitized solar cells (DSSCs) incorporating oriented titanium oxide nanotube (NT) arrays. The morphology of the NT arrays was characterized by scanning and transmission electron microscopies and Raman and X-ray...
Hydrogenated nanocrystalline silicon was characterized using the transient photocapacitance (TPC) method. The TPC spectra show these materials to have a mixed-phase nature. At low temperatures, the spectra appeared very microcrystalline, whereas at moderate temperatures they appeared very similar to those for hydrogenated amorphous silicon. These d...
The electronic properties of sulfur containing CIS chalcopyrite alloys have been characterized using junction capacitance methods. Two devices were examined; one containing CuIn(S,Se)2 alloy with a 1:2 S:Se ratio and a bandgap near 1.15eV, and the other an endpoint CuInS2 alloy with a bandgap slightly above 1.5eV. Drive-level capacitance profiling...
The electronic properties of hydrogenated nanocrystalline silicon (nc-Si:H) were studied using junction capacitance methods. Drive-level capacitance profiling (DLCP) measurements revealed significant differences for nc-Si:H layers deposited under constant hydrogen dilution compared to those deposited using hydrogen profiling, with lower DLCP densit...
Citations
... The theoretical modeling of the previous works [7]- [9] may provide better information of how the system works. One of the theoretical modelings is the single-diode circuit model. ...
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... Reduces shunting: As the CdS thickness decreases from 100 to 40 nm, weak diode characteristics are created between CdTe and ZnO, increasing shunting and decreasing the solar cell performance. This is called the pinhole effect [90,91]. The electric field at the CdS/CdTe junction is reduced because of the pinhole effect, which can be prevented by increasing the CdS thickness, creating firm grain boundaries at the CdS/CdTe junction. ...
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... Examples of the effect each experimental variable has on V OC are shown in Fig. 2. The data are from what will be called the Control sample, listed first in Table 1. The V OC is typically greatest at low temperatures and high intensities where it is limited by the difference in the built-in voltages of the front and back junctions (Halverson et al., 2013;Ott et al., 2015). In Fig. 2(a), the simulated effect of increasing non-radiative recombination in the absorber material (c a ) is shown, illustrating the uncertainty in the c a parameter as fit to the measured voltages. ...
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... On the right side, it corresponds to the depletion width under negative bias voltage, where the built-in electric field expands, leading to a wider depletion region, indicating the distribution of carriers near the back buffer layer [50]. The variation of the N cv curve is closely related to the carrier concentration, deep-level defects, back buffer barrier height, tunneling mechanism, and the thickness of the absorber layer [28]. The trend of changing apparent carrier concentration reveals that the carrier concentration is lower at the front interface and higher near the back buffer region. ...
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... As annealing was performed on the flakes only, the effect of the underlying titanium metal was omitted [1,3]. Annealing under a non-oxidizing environment can improve the photocatalytic efficiency of TiO 2 [32,33]. The partial pressure of oxygen in a reducing atmosphere, such as N 2 , is low, and therefore, O 2 vacancies can be created. ...
... The presence of an intercalated dopant is known to generate additional energy levels in the vicinity of the conduction band, thereby improving the conduction of the species. 56,57 We believe a similar phenomenon has taken place in the present case too, as a result of which the conductivity of the Li-doped NCs improves to a large extent. At the same time, there are sufficient reports involving Ni/NiO interfaces, wherein the metallic Ni was responsible for the acceleration in the electron transfer process. ...
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... 6,7 It is worth noting that, in the case of semitransparent TNT-based materials, the adhesion of the deposited Ti film on TCO can be further improved to ensure better stability of TNT on TCO. 9−11 Among others, the most simple strategy consists of the introduction of either a titania 10 or a nanoparticle-doped titania 9 interlayer between the deposited Ti film and the TCO substrate. This strategy has been explored by Kim et al., 9 Buẗtner et al., 11 and Krumpmann et al. 10 to produce Ti/TCO substrates and is considered to be crucial in the fabrication of crack-free and more regular TNT with tunable geometry on TCO. ...
