Doped SnO2thin films fabricated at low temperature by atomic layer deposition with a precise incorporation of niobium atoms.

Nb-doped SnO2(NTO) thin films were synthesized by atomic layer deposition technique at low temperature (100 °C). For an efficient incorporation of the Nb atoms, i.e. fine control of their amount and distribution, various supercycle ratios and precursor pulse sequences were explored. The thin film growth process studied byin-situQCM revealed that the Nb incorporation is highly impacted by the surface nature as well as the amount of species available at the surface. This was confirmed by the actual concentration of the Nb atom incorporated inside the thin film as determined by XPS. Highly transparent thin films which transmit more than 95% of the AM1.5 global solar irradiance over a wide spectral range (300-1000 nm) were obtained. In addition, the Nb atoms influenced the optical band gap, conduction band, and valence band levels. While SnO2thin film were too resistive, films tuned to conductive nature upon Nb incorporation with controlled concentration. Optimal incorporation level was found to be ⩽1 at.% of Nb, and carrier concentration reached up 2.5 × 1018cm-3for the as-deposited thin films. As a result, the high optical transparency accompanied with tuned electrical property of NTO thin films fabricated by ALD at low temperature paves the way for their integration into temperature-sensitive, nanostructured optoelectrical devices.

Quantitative in situ synchrotron X-ray analysis of the ALD/MLD growth of transition metal dichalcogenide TiS2 ultrathin films.

We present the results of a full quantitative analysis of X-ray absorption spectroscopy (XAS) performed in situ during the growth of ultrathin titanium disulfide (TiS2) films via an innovative two-step process, i.e. atomic layer deposition/molecular layer deposition (ALD/MLD) followed by annealing. This growth strategy aims at separating the growth process from the crystallization process by first creating an amorphous Ti-thiolate that is converted later to crystalline TiS2via thermal annealing. The simultaneous analysis of Ti and S K-edge XAS spectra, exploiting the insights from density functional theory calculations, allows us to shed light on the chemical and structural mechanisms underlying the main steps of growth. The nature of the bonding at the base of the interface creation with the SiO2 substrate is disclosed in this study. Evidence of a progressive incorporation of S in the amorphous Ti-thiolate is given. Finally, it is shown that the annealing step plays a critical role since the transformation of the Ti-thiolate into nanocrystalline TiS2 and the loss of S are simultaneously induced, validating the two-step synthesis approach, which entails distinct growth and crystallization steps. These observations contribute to a deeper understanding of the bonding mechanism at the interface and provide insights for future research in this field and the generation of ultra-thin layered materials.

Improvement of carrier collection in Si/a-Si:H nanowire solar cells by using hybrid ITO/silver nanowires contacts.

Optoelectronic devices based on high aspect ratio nanowires bring new challenges for transparent electrodes, which can be well addressed by using hybrid structures. Here we demonstrate that a composite contact to radial junction nanowire solar cells made of a thin indium-tin oxide (ITO) layer and silver nanowires greatly improves the collection of charge carriers as compared to a single thick ITO layer by reducing the series resistance losses while improving the transparency. The optimization is performed on p-i-n solar cells comprising of dense non-vertical nanowires with a p-doped c-Si core and an ultra-thin a-Si:H absorption layer grown by plasma-enhanced chemical vapor deposition on glass substrates. The optimal hybrid contact developed in this work is demonstrated to increase the solar cell conversion efficiency from 4.3% to 6.6%.

ALD of ZnO:Ti: Growth Mechanism and Application as an Efficient Transparent Conductive Oxide in Silicon Nanowire Solar Cells.

In the quest for the replacement of indium tin oxide (ITO), Ti-doped zinc oxide (TZO) films have been synthesized by atomic layer deposition (ALD) and applied as an n-type transparent conductive oxide (TCO). TZO thin films were obtained from titanium (IV) i-propoxide (TTIP), diethyl zinc, and water by introducing TiO2 growth cycle in a ZnO matrix. Process parameters such as the order of precursor introduction, the cycle ratio, and the film thickness were optimized. The as-deposited films were analyzed for their surface morphology, elemental stoichiometry, optoelectronic properties, and crystallinity using a variety of characterization techniques. The growth mechanism was investigated for the first time by in situ quartz crystal microbalance measurements. It evidenced different insertion modes of titanium depending on the precursor introduction, as well as the etching of Zn-Et surface groups by TTIP. Resistivity as low as 1.2 × 10-3 Ω cm and transmittance >80% in the visible range were obtained for 72-nm-thick films. Finally, the first application of ALD-TZO as TCO was reported. TZO films were successfully implemented as top electrodes in silicon nanowire solar cells. The unique properties of TZO combined with conformal coverage realized by the ALD technique make it possible for the cell to show almost flat external quantum efficiency (EQE) response, surpassing the bell-like EQE curve seen in devices with a sputtered ITO top electrode.

In Situ Microgravimetric Study of Ion Exchanges in the Ternary Cu-In-S System Prepared by Atomic Layer Deposition.

Reaction mechanisms during the growth of multinary compounds by atomic layer deposition can be complex, especially for sulfide materials. For instance, the deposition of copper indium disulfide (CuInS2) shows a non-direct correlation between the cycle ratio, the growth per cycle of each binary growth cycles, i.e., CuxS and In2S3, and the film composition. This evidences side reactions that compete with the direct Atomic Layer Deposition (ALD) growth reactions and makes the deposition of large films very challenging. To develop a robust upscalable recipe, it is essential to understand the chemical surface reactions. In this study, reaction mechanisms in the Cu-In-S ternary system were investigated in-situ by using a quartz crystal microbalance system to monitor mass variations. Pure binary indium sulfide (In2S3) and copper sulfide (CuxS) thin film depositions on Al2O3 substrate were first studied. Then, precursors were transported to react on CuxS and In2S3 substrates. In this paper, gas-phase ion exchanges are discussed based on the recorded mass variations. A cation exchange between the copper precursor and the In2S3 is highlighted, and a solution to reduce it by controlling the thickness deposited for each stack of binary materials during the CuInS2 deposition is finally proposed.

Highly efficient MoOx-free semitransparent perovskite cell for 4 T tandem application improving the efficiency of commercially-available Al-BSF silicon.

In this work, the fabrication of MoOx-free semitransparent perovskite solar cells (PSC) with Power Conversion Efficiencies (PCE) up to 15.7% is reported. Firstly, opaque PSCs up to 19.7% were fabricated. Then, the rear metal contact was replaced by a highly transparent and conductive indium tin oxide (ITO) film, directly sputtered onto the hole selective layer, without any protective layer between Spiro-OMeTAD and rear ITO. To the best of our knowledge, this corresponds to the most efficient buffer layer-free semitransparent PSC ever reported. Using time-resolved photoluminescence (TRPL) technique on both sides of the semitransparent PSC, Spiro-OMeTAD/perovskite and perovskite/TiO2 interfaces were compared, confirming the great quality of Spiro-OMeTAD/perovskite interface, even after damage-less ITO sputtering, where degradation phenomena result less important than for perovskite/TiO2 one. Finally, a 4-terminal tandem was built combining semitransparent PSC with a commercially-available Aluminium Back Surface Field (Al-BSF) silicon wafer. That silicon wafer presents PCE = 19.52% (18.53% after being reduced to cell size), and 5.75% once filtered, to generate an overall 4 T tandem efficiency of 21.18% in combination with our champion large semitransparent PSC of 15.43%. It means an absolute increase of 1.66% over the original silicon wafer efficiency and a 2.65% over the cut Si cell.

Deposition of ultra thin CuInS2 absorber layers by ALD for thin film solar cells at low temperature (down to 150 °C).

Two new processes for the atomic layer deposition of copper indium sulfide (CuInS2) based on the use of two different sets of precursors are reported. Metal chloride precursors (CuCl, InCl3) in combination with H2S imply relatively high deposition temperature (Tdep = 380 °C), and due to exchange reactions, CuInS2 stoechiometry was only achieved by depositing In2S3 layers on a CuxS film. However, the use of acac- metal precursors (Cu(acac)2, In(acac)3) allows the direct deposition of CuInS2 at temperature as low as 150 °C, involving in situ copper-reduction, exchange reaction and diffusion processes. The morphology, crystallographic structure, chemical composition and optical band gap of thin films were investigated using scanning electronic microscope, x-ray diffraction under grazing incidence conditions, x-ray fluorescence, energy dispersive spectrometry, secondary ion mass spectrometry, x-ray photoelectron spectroscopy and UV-vis spectroscopy. Films were implemented as ultra-thin absorbers in a typical CIS-solar cell architecture and allowed conversion efficiencies up to 2.8%.

Synthesis of indium oxi-sulfide films by atomic layer deposition: The essential role of plasma enhancement.

This paper describes the atomic layer deposition of In2(S,O)3 films by using In(acac)3 (acac = acetylacetonate), H2S and either H2O or O2 plasma as oxygen sources. First, the growth of pure In2S3 films was studied in order to better understand the influence of the oxygen pulses. X-Ray diffraction measurements, optical analysis and energy dispersive X-ray spectroscopy were performed to characterize the samples. When H2O was used as the oxygen source, the films have structural and optical properties, and the atomic composition of pure In2S3. No pure In2O3 films could be grown by using H2O or O2 plasma. However, In2(S,O)3 films could be successfully grown by using O2 plasma as oxygen source at a deposition temperature of T = 160 °C, because of an exchange reaction between S and O atoms. By adjusting the number of In2O3 growth cycles in relation to the number of In2S3 growth cycles, the optical band gap of the resulting thin films could be tuned.

Multiple reaction pathways in rhodium-catalyzed hydrosilylations of ketones.

A detailed density functional theory (DFT) computational study (using the BP86/SV(P) and B3LYP/TZVP//BP86/SV(P) level of theory) of the rhodium-catalyzed hydrosilylation of ketones has shown three mechanistic pathways to be viable. They all involve the generation of a cationic complex [L(n)Rh(I)]+ stabilized by the coordination of two ketone molecules and the subsequent oxidative addition of the silane, which results in the Rh-silyl intermediates [L(n)Rh(III)(H)SiHMe2]+. However, they differ in the following reaction steps: in two of them, insertion of the ketone into the Rh-Si bond occurs, as previously proposed by Ojima et al., or into the Si-H bond, as proposed by Chan et al. for dihydrosilanes. The latter in particular is characterized by a very high activation barrier associated with the insertion of the ketone into the Si-H bond, thereby making a new, third mechanistic pathway that involves the formation of a silylene intermediate more likely. This "silylene mechanism" was found to have the lowest activation barrier for the rate-determining step, the migration of a rhodium-bonded hydride to the ketone that is coordinated to the silylene ligand. This explains the previously reported rate enhancement for R2SiH2 compared to R3SiH as well as the inverse kinetic isotope effect (KIE) observed experimentally for the overall catalytic cycle because deuterium prefers to be located in the stronger bond, that is, C-D versus M-D.

Metal silylenes generated by double silicon-hydrogen activation: key intermediates in the rhodium-catalyzed hydrosilylation of ketones.

Rhodium silylenes, which are generated by double Si-H activation at the metal, are involved in a low-activation-barrier mechanism of the hydrosilylation of ketones with R(2)SiH(2). A DFT-based study of reaction mechanisms accounts for the experimental observations, notably the rate enhancement for R(2)SiH(2) over R(3)SiH and an inverse kinetic isotope effect.