International audienceno abstrac

No abstract

Extremely thin absorber (ETA) solar cells integrating ZnO nanowires have been receiving increasing interest owing to efficient light-trapping phenomena and charge-carrier management, but the chemical instability of ZnO in acidic conditions limits its combination with a variety of absorbing semiconducting shells grown by chemical deposition techniques. By covering the ZnO nanowires grown by chemical bath deposition with a protective, passivating, conformal, thin, anatase-TiO2 layer by atomic layer deposition, we show that a uniform Sb2S3 absorbing shell is formed by chemical spray pyrolysis without structural degradation of the ZnO. The Sb2S3 absorbing shell consists of a very thin, conformal layer together with homogeneously distributed small clusters from the bottom to the top of the ZnO/TiO2 core–shell nanowire arrays. The resulting ETA solar cells integrating these ZnO/TiO2/Sb2S3 core–shell nanowire heterostructures with an Sb2S3 absorbing shell less than 10 nm-thick and P3HT as the hole-transporting material have a photoconversion efficiency of 2.3% with a promising short-circuit current density of 7.5 mA/cm2 and a high open-circuit voltage of 656 mV as one of the largest reported values in ZnO nanowire-based ETA solar cells. The present findings thus reveal the great potential of Sb2S3 as an absorbing, semiconducting shell when coupled with ZnO/TiO2 core–shell nanowire heterostructures, opening the way for new strategies to improve the performance of ZnO nanowire-based ETA solar cells fabricated by low-cost, surface-scalable, easily implemented chemical deposition techniques.

The low-cost fabrication of ZnO nanowire/CuSCN heterojunctions is demonstrated by combining chemical bath deposition with impregnation techniques. The ZnO nanowire arrays are completely filled by the CuSCN layer from their bottoms to their tops. The CuSCN layer is formed of columnar grains that are strongly oriented along the [003] direction owing to the polymeric form of the β-rhombohedral crystalline phase. Importantly, an annealing step is found essential in a fairly narrow range of low temperatures, not only for outgassing the solvent from the CuSCN layer, but also for reducing the density of interfacial defects. The resulting electrical properties of annealed ZnO nanowire/CuSCN heterojunctions are strongly improved: a maximum rectification ratio of 2644 at ±2 V is achieved following annealing at 150 °C under air atmosphere, which is related to a strong decrease in the reverse current density. Interestingly, the corresponding self-powered UV photodetectors exhibit a responsivity of 0.02 A/W at zero bias and at 370 nm with a UV-to-visible (370-500 nm) rejection ratio of 100 under an irradiance of 100 mW/cm(2). The UV selectivity at 370 nm can also be readily modulated by tuning the length of ZnO nanowires. Eventually, a significant photovoltaic effect is revealed for this type of heterojunctions, leading to an open circuit voltage of 37 mV and a short circuit current density of 51 μA/cm(2), which may be useful for the self-powering of the complete device. These findings show the underlying physical mechanisms at work in ZnO nanowire/CuSCN heterojunctions and reveal their high potential as self-powered UV photodetectors.

International audienceMastering the structural ordering of ZnO seed layers by sol–gel process in terms of ultrathin thickness (i.e, <10 nm), strong c-axis texture, low mosaicity, low porosity, and low roughness is a critical challenge for the formation of well-ordered ZnO nanowires in solution. The effects of the solution concentration, of the withdrawal speed, and of the annealing process on the formation mechanisms of ZnO seed layers deposited by single dip process are revealed. The size and density of primary clusters in the sol are found to govern the evolution of the film thickness and nanoparticle average diameter through the solution concentration. The Landau–Levich theory modeling the dragging process accounts for the evolution of the film thickness only before annealing and over a reduced range of withdrawal speeds. The texture mechanisms along the c-axis are driven by particle/particle interactions during annealing and explained in the light of thermodynamic considerations. They are further determined locally by electron backscattered diffraction. Importantly, an alternative annealing process under argon flux is specifically developed for sol–gel process and is shown to form remarkably well-textured, compact ZnO seed layers with a very low mosaicity and porosity as well as a thin thickness as small as 10 nm. These ZnO seed layers lead to the growth of well-ordered ZnO nanowires by chemical bath deposition with a remarkable mean tilt angle smaller than 6° as deduced by X-ray pole figures. These findings represent a significant step toward the more efficient integration of ZnO seed layers grown by sol–gel process into ZnO nanowire-based devices

Synthesis of ultra-thin, well-aligned, monocrystalline, highly luminescent ZnO nanowires on polystyrene beads as templates.

The addition of polyethylenimine (PEI) in the standard chemical bath deposition (CBD) of ZnO nanowires has received an increasing interest for monitoring their aspect ratio, but the physicochemical processes at work are still under debate. To address this issue, the effects of PEI are disentangled from the effects of ammonia and investigated over a broad range of molecular weight (i.e., chain length) and concentration, varying from 1300 to 750 000 and from 1.5 to 10 mM, respectively. It is shown that the addition of PEI strongly favors the elongation of ZnO nanowires by suppressing the homogeneous growth at the benefit of the heterogeneous growth as well as by changing the supersaturation level through a pH modification. PEI is further found to inhibit the development of the sidewalls of ZnO nanowires by adsorbing on their nonpolar m -planes, as supported by Raman scattering analysis. The inhibition proceeds even in the low pH range, which somehow rules out the present involvement of electrostatic interactions as the dominant mechanism for the adsorption. Furthermore, it is revealed that PEI drastically affects the nucleation process of ZnO nanowires on the polycrystalline ZnO seed layer by presumably adsorbing on the nanoparticles oriented with the m -planes parallel to the surface, reducing in turn their nucleation rate as well as inducing a significant vertical misalignment. These findings, specifically showing the effects of the PEI molecular weight and concentration, cast light onto its multiple roles in the CBD of ZnO nanowires.

The successive ionic layer adsorption and reaction (SILAR) technique is found to be of high potential for the formation of ZnO core–shell nanowire heterostructures with high uniformity at moderate temperature.