Jose Alvarez

The reported record efficiencies of silicon heterojunction solar cells using hydrogenated nanocrystalline silicon (nc-Si:H) as carrier-collector layers have increased interest in this material due to low activation energy. Band-bending in silicon creates a difference in work function (Φ) between the bulk and the surface of the film. However, Φ values of nc-Si:H have only been reported as estimates, with few direct measurements performed. Given the lack of reported data on the Φ of nc-Si:H films, we compared various measurement techniques to establish reference values and evaluate the reliability of each method. Using Kelvin probe (KP), ultraviolet photoelectron spectroscopy, and electron beam repulsion (EBR), we measured the Φ of intrinsic, n-type and p-type nc-Si:H layers. We discuss the effect of surface treatments based on ozone and hydrofluoric acid on the Φ. EBR revealed that the Φ of p-type films between 10 and 45 nm thick decreases with increasing thickness, dropping from 5.4 to 4.5 eV. This trend was not evident in KP measurements of thinner films. A similar plateau was observed for films thicker than 45 nm by both techniques. We also compared Φ values measured using these techniques with those estimated from the activation energy obtained via temperature-dependent conductivity measurements.

Atomic layer deposition enables the fabrication of uniform, conformal, and pinhole-free thin films with precise sub-nanometer thickness control. This work presents the first report of thermal ALD of NiO_x using two nickel precursors –Alanis™ and [Ni(ⁱpki)₂]– in combination with two different oxidising agents (H₂O and O₃), over a temperature range of 75-250°C. The Alanis/O₃ composition exhibited one of the highest growth per cycle (1.2Å) and a broad ALD window between 100-175°C. In contrast, the Alanis/H₂O and [Ni(ⁱpki)₂]/O₃ combinations yielded lower growth rates of 0.74Å at 150°C and 0.40Å at 250°C, respectively. Comprehensive structural, morphological, optical, and chemical characterisations revealed that the choice of precursor combination and the reaction temperature significantly impact the film composition, strongly influencing its suitability for various applications. Notably, those parameters closely determined the photoelectrochemical performance and the stability of the Si/NiO_x-based photoanode towards oxygen evolution reaction. Photoelectrodes fabricated with Alanis/O₃ at 200°C demonstrate stability exceeding 24H and exhibit a remarkable OER onset potential of 1.15V vs RHE for a photocurrent density of 1mA·cm^-2.

Nanowires, such as the ones used in energy applications, pose unique challenges in characterization due to their complexity and fragility. AFM provides a multimetrological platform with its various feedback methods to tackle this issue with finesse.

Lead halide perovskites are semiconductor materials which are employed as nonintentionally doped absorbers inserted between two selective carrier transport layers (SCTL), realizing a p‐i‐n or n‐i‐p heterojunction. In our study, we have developed and investigated a lateral device, based on methylammonium lead iodide (MAPbI₃) in which the p‐i‐n heterojunction develops in the horizontal direction. Our research suggests that the effective doping level in the MAPbI₃ film should be very low, below 10¹² cm⁻³. Along the vertical direction, this doping level is not enough to screen the electric field of the buried heterojunction with the SCTL. The perovskite work function is therefore affected by the work function of the SCTL underneath. From drift‐diffusion simulations, we show that intrinsic perovskite‐SCTL structures develop mV range surface photovoltages (SPVs) under continuous illumination. However, perovskite‐SCTL structures can develop SPVs of hundreds of mV, as confirmed by our measurements. We therefore analyzed the compatibility between low doping and low defect densities in the perovskite layer and such high SPV values using numerical modeling. It is shown that these high SPV values could originate from electronic processes due to large band offsets in the buried perovskite‐SCTL heterojunctions, or at the SCTL‐transparent conductive oxide (TCO) buried heterojunction. However, such electronic processes can hardly explain the long SPV persistence after switching off the illumination.

Modulated photoluminescence (MPL) is an optoelectronic characterization technique of semiconductor materials. Going to high frequencies enables one to characterize fast phenomena, and so materials with a short lifetime such as chalcogenides or III–V absorbers. Some typical signatures have already been experimentally observed. However, physical mechanisms and quantitative analyses are not well understood yet. Here, using both an analytical approach and a full numerical modeling, we study how the energy position of a defect level, its electron and hole capture cross sections, its density, influence the frequency dependence of the MPL phase. We show that quantitative information can be extracted. We also study the effect of additional surface recombination, and of non homogeneities created by carrier generation profiles or asymmetric top surface and bottom surface recombination velocities, where diffusion of the carriers plays a role and can be limiting at high frequency. Finally we apply our model to an experimental result to extract defect parameters of the sample. Our analysis highlights the usefulness of MPL and the importance of having a proper modeling of the experiment.

The origin of the high leakage current measured in several vertical-type diamond Schottky devices is conjointly investigated by conducting probe atomic force microscopy and confocal micro-Raman/photoluminescence imaging analysis. Local areas characterized by a strong decrease of the local resistance (5–6 orders of magnitude drop) with respect to their close surrounding have been identified in several different regions of the sample surface. The same local areas, also referenced as electrical hot-spots, reveal a slightly constrained diamond lattice and three dominant Raman bands in the low-wavenumber region (590, 914 and 1040 cm⁻¹). These latter bands are usually assigned to the vibrational modes involving boron impurities and its possible complexes that can electrically act as traps for charge carriers. Local current–voltage measurements performed at the hot-spots point out a trap-filled-limited current as the main conduction mechanism favouring the leakage current in the Schottky devices.