DLI-CVD of TiO2–Cu antibacterial thin films: Growth and characterization

TiO2–Cu nanocomposite films were grown by pulsed direct liquid injection chemical vapor deposition (DLICVD) on stainless steel, silicon and glass substrates with the goal to produce bactericidal surfaces. Copper bis (2,2,6,6-tetramethyl-3,5-heptadionate), Cu(TMHD)2, and titanium tetra-iso-propoxide, TTIP, were used as metalorganic precursors. Liquid solutions of these compounds in xylene were injected in a flash vaporization chamber connected to a cold wall MOCVD reactor. The deposition temperature was typically 683 K and the total pressure was 800 Pa. The copper content of the layers was controlled by the mole fraction of Cu (TMHD)2 which was adjusted by the injection parameters (injection frequency and concentration of the starting solution). The chemical, structural and physical characteristics of the films were investigated by XRD, XPS, FEG-SEM and TEM. Copper is incorporated as metal particles with a relatively large size distribution ranging from 20 to 400 nm (with a large majority in 20–100 nm) depending on the copper content of the films. The influence of the growth conditions on the structural features and the antibacterial properties of the thin films are reported and discussed

Comparative study of antibacterial efficiency of M-TiO2 (M = Ag, Cu) thin films grown by CVD

M-TiO2 (M = Ag, Cu) nanocomposite layers were grown by pulsed direct liquid injection chemical vapor deposition (DLICVD) on various substrates to produce bactericidal surfaces with long term activity. Monodisperse Ag nanoparticles (NPs) with an average size of 5-10 nm are embedded in an anatase matrix. A bactericidal behavior determined by the JIS Z 2801 standard test was found for Ag-TiO2 films for Ag ≤ 1 at. % and above. Higher Ag content is not necessary since efficiency is already at its maximum (relative activity 100%). By contrast, using Cu as antibacterial agent, a larger size distribution of metal particles was found (20 to 400 nm). Cu-TiO2 films exhibit a bactericidal behavior if their thickness is higher than 100 nm and Cu content ≥ 3.5 at. %. These coatings are still antibacterial after 5 months of aging and their efficiency has decreased by only 35%

Antibacterial properties of TiO2–Cu composite thin films grown by a one step DLICVD process

The correlations between microstructural features, chemical compositions and antibacterial properties of coatings containing metallic Cu particles embedded in a titanium dioxide matrix have been determined. A Direct Liquid Injection Chemical Vapor Deposition (DLICVD) processwas used for the one step growth of TiO2–Cu composite coatings on various substrates. Titanium tetra-iso-propoxide (TTIP) and copper bis(2,2,6,6-tetramethyl-3,5-heptationate) (Cu(tmhd)2) were used as titanium and copper molecular sources, respectively. This growth process allows a good control of the quantity of metalorganic precursors injected into the CVD reactor and thus of the coating composition. The deposition occurs at 683 K under low pressure (800 Pa). The influence of the main features of the coatings on their antibacterial properties was investigated in order to produce bactericidal surfaces that are durable, non-toxic and containing aminimumamount of active agent. The antibacterial activity on Staphylococcus aureus without any photon activation was measured according to the JIS Z 2801:2000 standard method. An antibacterial activity was detected for a low metal content of ca. 1 at.% Cu, and was found to increasewith the Cu content. It wasmaximal for 3.5 at.% Cu, i.e. TiO2–Cu composite coatings exhibit bactericidal behavior against S. aureus for this optimal composition (relative activity = 100%). In order to better characterize the microbiological behavior of the coatings more discriminating methods derived from the literature were tested to assess the performances of these CVD coatings in terms of efficiency, release of antibacterial agent and accelerated aging

Homogeneous and Graded Ag Alloying in (Cu1‐x_{1‐x}Agx_x)2_2ZnSnSe4_4 Solar Cells

International audienceCu$_2$ZnSn(S,Se)$_4$ (CZTSSE)-based solar cell performances are limited by band tailing due to a large amount of CuZ$_n$ antisite defects. Partially replacing the Cu atoms by larger Ag ones can significantly reduce the prevalence of these defects,which are particularly detrimental close to the front interface. Herein, the possibility of synthesizing (Cu$_{1‐x}$Ag$_x$)$_2$ZnSnSe$_4$ absorbers with various Ag contents by vacuum-based processes is demonstrated. Although the synthesis of high-quality materials is demonstrated, their use in thin film photovoltaic devices does not exhibit performance improvement compared with efficient pure CZTSSE-based solar cells. Moreover, the comparison with literature data reopens the debate of the beneficial effect of homogeneous Ag alloying in kesterite. On the contrary, a new method is proposed to fabricate graded (Cu$_{1‐x}$Ag$_x$)$_2$ZnSnSe$_4$) absorbers with increased Ag content at the interfaces. The solar cells with graded absorbers exhibit better performances than the reference Ag-free ones. Particularly, improved current collection at the back contact and slight reduction of the front interface recombination are demonstrate

Comparing strategies for improving efficiencies in vacuum processed Cu2_2ZnSnSe4_4 solar cells

International audienceIn this study, we detail a Cu$_2$ZnSnSe$_4$ based solar cell fabrication process based onthe selenization of metallic precursor stacks with elemental Se. 9.4% efficientdevices without antireflection coating have been obtained. First, reproducibilityissues of the process are carefully shown and discussed. It is demonstrated thatdevice performances are strongly impacted by the precise control of the precursorcomposition. Then, starting from this robust process, a review of existing strategiesto improve kesterite efficiencies is conducted. A significant increase in efficiency(+1.4% absolute efficiency and +50 mV V$_{OC}$) is obtained with absorber surfacetreatment and post-annealing, while no effect of Ge incorporation in the precursorstack is observed. This contradictory result to most of the recent publications raisesthe question of the universality of this strategy to improve kesterite solar cell performance. Finding a universal activation step to boost kesterite efficiencies and bring it to the market remains a crucial need for the communit

Advanced light management based on periodic textures for Cu(In,Ga)Se2_2 thin-film solar cells

International audienceWe have used 3-D optical modelling to investigate light management concepts based on periodic textures and material optimization for photovoltaic devices based on Cu(In,Ga)Se$_2$ (CIGS) absorber material. At first, calibration of the software based on the characterization of a reference (1500-nm thick) CIGS device was carried out. The effects of 1-D and 2-D symmetric gratings on the cell were then investigated, showing significant improvement in anti-reflection effect and in absorptance in the active layer, achieved by excitation of guided modes in the absorber. In addition, device configurations endowed with alternative back reflector and front transparent conductive oxide (TCO) were tested with the goal to quench parasitic absorption losses at front and back side. The use of In$_2$O$_3$:H (IOH) as front and back TCO, combined with an optimized 2-D grating structure, led to a 25% increase of the optical performance with respect to an equally-thick flat device. Most of the performance increase was kept when the absorber thickness was reduced from 1500 nm to 600 nm

Analysis of Failure Modes in Kesterite Solar Cells

International audienceIntense research has been carried out in the past few years to improve efficiencies and understand limitations in kesterite-based solar cells. Despite notable efforts to determine and list the different failure modes affecting the photovoltaic properties of these devices, very few works have tried to quantify and classify the effects of these failure modes. In this study, an exhaustive literature review has first been conducted to determine the different causes leading to limited efficiencies in kesterite devices, with an additional focus on cadmium-free and critical raw material-free devices. Second, an original approach has been employed to quantify the impact of these failure modes on solar cells, based on the evaluation of feedback from 18 scientific experts working on kesterite technology. The result of this survey is analyzed, which allows us to determine what should be the research priority for the community to improve efficiencies and drive kesterite technology to the market

Effect of Sb and Na Incorporation in Cu2ZnSnS4 Solar Cells

International audienceCu$_2$ZnSnS$_4$-based solar cells suffer from limited power conversion efficiency (PCE) and relative small grain size compared to selenium containing absorbers. Introduction of Na in Cu$_2$ZnSnS$_4$ absorbers either during the synthesis or after this step is used to improve device performances and to determine whether its effect is based on structural properties improvement (grain size enhancement, better crystallization) or on opto-electronic properties improvement (defect passivation). In both cases, presence of Na in the absorber notably improves current and voltage of the solar cells, but the effect is more pronounced when Na is present during synthesis. Quantum efficiency analysis shows that these improvements can be related to longer minority carrier diffusion length and reduced absorber/buffer interface recombination. Introducing Na in the process mostly leads to preferential (112) orientation of the crystal which is clearly correlated with better device performances. Otherwise, the performance limitation due to small grain size is discarded by the joint use of Sb and Na, which has a significant impact on grain size but does not affect solar cells efficiency

CIGS solar cells on flexible ultra-thin glass substrates: Characterization and bending test

International audienc