Thierry Pauporté

Electrodeposition of semiconductors for optoelectronic devices: results on zinc oxide

Electrodeposition of polycrystalline semiconductor thin films is mainly used for photovoltaic applications. Most of the work concerns chalcogenide compounds like CdTe, CuInSe2, CdS… This route is also emerging for semiconductor oxide preparation. Results concerning the electrodeposition of zinc oxide layers by reduction of dissolved oxygen in presence of Zn(II) ions are presented. Epitaxial growth can be achieved in zinc chloride solutions on single crystal GaN layers. Bandgap (Eg) variations and effects on photoluminescence are observed by changing the deposition conditions and post-deposition treatments. For instance, changing the potential from −1.1 to −1.5 V versus MSE in chloride solutions increases Eg from 3.4 to 3.55 eV. Changing chloride to perchlorate anions shifts Eg from 3.52 to 3.38 eV at −1.4 V. A thermal annealing in air further decreases the Eg down to the classical value (3.27 eV). This is correlated to a decrease in ZnO lattice parameters. A concomitant variation of the photoluminescence emission in the blue is observed. The highest yield is achieved on the epitaxial layers after annealing (500°C, 1 h). Specific aspects about the deposition mechanism are also discussed.

Low-voltage UV-electroluminescence from ZnO-nanowire Array/p-GaN light-emitting diodes.

Nanowire (NW)-based light-emitting diodes (LEDs) have drawn great interest because of their many advantages compared to thin-fi lm-based devices. Marked improved performances are expected from nanostructured active layers for light emission. Nanowires can act as direct waveguides and favor light extraction without use of lenses and refl ectors. Moreover, the use of wires avoids the presence of grain boundaries and then the emission effi ciency is boosted by the absence of nonradiative recombinations at the joint defects. The junctions between the nand p-type semiconductors can also be of very high quality with low interfacial strain and defect density as a result of a reduced contact area between the two materials in the case of epitaxial heterojunctions. Integration of nanowires is especially promising for the preparation of short-wavelength emitters such as superluminescent UV-light-emitting diodes and laser diodes. GaN is the main wide-bandgap semiconductor ( E g = 3.39 eV) used for the preparation of blue and UV LEDs. However GaN nanowire arrays are diffi cult to grow. [ 1 , 2 ] Alternative materials for the preparation of nanostructured UV LEDs mainly include ZnO. [ 3–5 ] ZnO has a similar wide bandgap of 3.37 eV and a larger exciton binding energy of 60 meV (compared to 29 meV in the case of GaN) that should favor light emission at room temperature. [ 3 , 6 ] ZnO and GaN share the same wurtzite hexagonal structure. A lot of research efforts have focused on the preparation of p-type ZnO to realize LEDs based on homojunctions made of this semiconductor. [ 7–9 ] For the moment, however, reproducible and stable p-type ZnO material with suffi cient high conductivity and carrier concentration is still at the development phase. An alternative promising approach is to grow n-type ZnO NWs on p-type GaN. [ 10–17 ] ZnO NWs can be grown by various techniques. Most of these efforts for LED applications have been focused on vapor phase growth. Recently, devices have

Toward laser emission of epitaxial nanorod arrays of ZnO grown by electrodeposition

Arrays of epitaxial, vertically oriented nanorods of ZnO are grown at low temperature (80°C) on the (0002) plane of GaN single crystals by electrodeposition in an aqueous solution. The freestanding nanocolumns are prepared by a template-free method based on simple solution chemistry. At room temperature, the nanorods present an amplified ultraviolet emission centered at 381nm with an excitation threshold at 4.4MWcm−2. The amplified emission of the ZnO columns is dominated by the radiative recombination of excitons.

Electrochemical nitric oxide microsensors: sensitivity and selectivity characterisation

In this study, we have prepared two nitric oxide (NO) microsensors using two combinations of nickel tetrasulfonated phthalocyanine (NiTSPc), o-phenylenediamine (o-PD) and Nafion® based layers to modify the surface of 8 μm diameter carbon fiber electrodes. We have compared the performances of the obtained composite microsensors (carbon/NiTSPc/Nafion®, and carbon/Nafion®/o-PD, respectively) in our home made operating conditions. We have developed the sessile drop contact angle measurement technique as well as the use of the electrochemical quartz crystal microbalance (EQCM), differential normal pulse voltammetry (DNPV) and differential normal pulse amperometry (DNPA) to correlate the hydrophobicity, mass deposit of the polymer coatings and the sensitivity of the examined microsensors. By comparing the permeability of the microsensors to various interfering analytes, such as L-arginine, ascorbate, nitrite, serotonine, dopamine, acetamidophenol, 4-met-catechol, epinephrine, norepinephrine, dopac and 5-hydroxyindol, we have discussed the molecular sieving exclusion of the deposited membranes in term of molecular weight cutoff (MWCO).

Cathodic Electrodeposition of ZnO/Eosin Y Hybrid Thin Films from Oxygen-Saturated Aqueous Solution of ZnCl2 and Eosin Y

Electrodeposition of ZnO/eosin Y hybrid thin films from aqueous mixed solution of zinc chloride and eosin Y as promoted by reduction of oxygen has been studied. Highly oriented crystalline hybrid thin films with two distinctive structures have been obtained depending on the redox state of eosin Y. Deposition at potentials more positive than that of eosin Y reduction resulted in a formation of compact ZnO crystals into which eosin Y molecules are entrapped, while that accompanied with the reduction of eosin Y yielded a film consisting of sponge-like ZnO crystals with internal nanoporous structure to which eosin Y molecules are adsorbed. The addition of eosin Y accelerated the film growth both in oxidized and reduced forms due to its catalysis toward the reduction of oxygen. Photoelectrochemical measurement in an I - /I 3 redox electrolyte solution resealed that the hybrid thin film in the latter structure performed as an efficient sensitized photoelectrode because of its porous nature.

CuGaO2: a promising alternative for NiO in p-type dye solar cells

CuGaO2 is used here as photocathode in place of NiO for a p-type dye-sensitized solar cell with iodide/triiodide (I−/I3−) or tris(4,4′-bis-tert-butyl-2,2′-bipyridine)cobalt (Co2+/Co3+) as redox mediator, and PMI or PMI-NDI as sensitizer. Both photovoltaic characteristics and flat band potentials suggest that CuGaO2 can be viewed as a promising substitute for NiO.

Theoretical procedure for optimizing dye-sensitized solar cells: from electronic structure to photovoltaic efficiency.

A step-by-step theoretical protocol based on density functional theory (DFT) and time-dependent DFT at both the molecular and periodic levels is proposed for the design of dye-sensitized solar cell (DSSC) devices including dyes and electrolyte additives. This computational tool is tested with a fused polycyclic pyridinium derivative as a novel dye prototype. First, the UV-vis spectrum of this dye alone is computed, and then the electronic structure of the system with the dye adsorbed on an oxide semiconductor surface is evaluated. The influence of the electrolyte part of the DSSC is investigated by explicitly taking into account the electrolyte molecules co-adsorbed with the dye on the surface. We find that tert-butylpyridine (TBP) reduces the electron injection by a factor of 2, while lithium ion increases this injection by a factor of 2.4. Our stepwise protocol is successfully validated by experimental measurements, which establish that TBP divides the electronic injection by 1.6 whereas Li(+) multiplies this injection by 1.8. This procedure should be useful for molecular engineering in the field of DSSCs, not only as a complement to experimental approaches but also for improving them in terms of time and resource consumption.

Modeling Dye-Sensitized Solar Cells: From Theory to Experiment

Density functional theory (DFT) and time-dependent DFT are useful computational approaches frequently used in the dye-sensitized solar cell (DSSC) community in order to analyze experimental results and to clarify the elementary processes involved in the working principles of these devices. Indeed, despite these significant contributions, these methods can provide insights that go well beyond a purely descriptive aim, especially when suitable computational approaches and methodologies for interpreting and validating the computational outcomes are developed. In the present contribution, the possibility of using recently developed computational approaches to design and interpret the macroscopic behavior of DSSCs is exemplified by the study of the performances of three new TiO2-based DSSCs making use of organic dyes, all belonging to the expanded pyridinium family.

Effects of ZnO film growth route and nanostructure on electron transport and recombination in dye-sensitized solar cells

The photovoltaic performances of ZnO-based dye-sensitized solar cells (DSSCs) have been studied for ZnO porous films prepared by different techniques. A comparison is made between nanoparticle (NP) films prepared by a sol–gel method and two different electrodeposited (ED) nanoporous films. The D149 indoline dye/ZnO with ZnO prepared by electrochemistry at rather high overvoltage has been found to be the best system. The cell functioning has been studied in-depth by electrochemical impedance spectroscopy (EIS) measurements made over a large potential range, in the dark and under illumination. It is shown that a much deeper sub-conduction band edge density of states (DOS) exists in the case of NP cells compared to ED cells. The electron dynamics have been analysed by determining the charge carrier lifetimes and transport/collection times. In the case of NP films, the charge carrier behaviour is characterized by rather long lifetimes and transport times with a marked voltage response typical of conduction that is governed by trapping–detrapping events. On the contrary, in the case of electrodeposited films, these two parameters are much shorter and the transport is characterized by a straight charge transport. Using photoluminescence data, the charge carrier lifetimes have been related to the surface defects of the ZnO nanostructures. The deduced electron collections were found to be very efficient with values greater than 90%. We have also determined that the ZnO film conductivities were higher than for anatase TiO2, and that the electron diffusion coefficient was higher in the ED film compared to the NP film.

From nanowires to hierarchical structures of template-free electrodeposited ZnO for efficient dye-sensitized solar cells

A new and original method for the electrochemical growth of ZnO nanocrystalline porous layers and multiscale hierarchical structures is described. The structures are designed by simply playing with the growth conditions and without any use of template or additive in the aqueous deposition bath. Two types of hierarchical structures are described combining electrodeposited ZnO nanowire arrays and a nanoporous layer: nanowire arrays covered by a conformal nanoporous layer and nanowire arrays embedded in a nanoporous layer. The global performances of dye sensitized solar cells (DSSCs) fabricated using the hierarchical structures are higher than those found for nanoparticulate sol–gel ZnO films and for the two basic electrodeposited structures. Films made of nanowires embedded in a nanocrystalline matrix show a maximum energy conversion efficiency of ∼4.1%. The wires play several important beneficial roles in the presented structures since they permit the electrodeposition of thick nanoporous ZnO films which immobilize a large amount of dye, they act as preferential electron pathways for efficient charge collection and, due to their size, they enhance the light trapping in the photoanode and hence increase the light diffusion length before its harvesting by the dye. Another interest of the proposed ZnO hierarchical structures is a synthesis as well as an applied post-growth thermal treatment performed below 150 °C in soft environments which are then perfectly compatible with lightweight plastic flexible and other fragile substrates.