ZnS nanostructures: From synthesis to applications

Native point defects in ZnS: First-principles studies based on LDA, LDA + U and an extrapolation scheme

ZnS is a candidate to replace CdS as the buffer layer in Cu(In,Ga)Se2 (CIGS) solar cells for Cd-free commercial product. However, the resistance of ZnS is too large, and the photoconductivity is too small. Therefore, the thickness of the ZnS should be as thin as possible. However, a CIGS solar cell with a very thin ZnS buffer layer is vulnerable to the sputtering power of the ZnO : Al window layer deposition because of plasma damage. To improve the efficiency of CIGS solar cells with a chemical-bath-deposited ZnS buffer layer, the effect of the plasma damage by the sputter deposition of the ZnO : Al window layer should be understood. We have found that the efficiency of a CIGS solar cell consistently decreases with an increase in the sputtering power for the ZnO : Al window layer deposition onto the ZnS buffer layer because of plasma damage. To protect the ZnS/CIGS interface, a bilayer ZnO : Al film was developed. It consists of a 50-nm-thick ZnO : Al plasma protection layer deposited at a sputtering power of 50 W and a 100-nm-thick ZnO : Al conducting layer deposited at a sputtering power of 200 W. The introduction of a 50-nm-thick ZnO : Al layer deposited at 50 W prevented plasma damage by sputtering, resulting in a high open-circuit voltage, a large fill factor, and shunt resistance. The ZnS/CIGS solar cell with the bilayer ZnO : Al film yielded a cell efficiency of 14.68%. Therefore, the application of bilayer ZnO : Al film to the window layer is suitable for CIGS solar cells with a ZnS buffer layer. Copyright © 2012 John Wiley & Sons, Ltd.

Improvement of the cell performance in the ZnS/Cu(In,Ga)Se2 solar cells by the sputter deposition of a bilayer ZnO : Al film

Well-aligned arrays of polycrystalline ZnS nanotubes with close-tips have been successfully prepared by using ZnO nanorod arrays grown on zinc substrate as sacrificial precursors. The method is based on chemical conversion and inward etching of the ZnO sacrificial precursors. Field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images show that the highly ordered ZnS nanotube arrays are 300 nm in average diameter, 6–8 μm in length and about 60 nm in wall thickness. A room temperature photoluminescence-type gas sensing device based on the nanostructured arrays has been established to investigate their humidity sensing properties. ZnS nanotube array-based sensor presents higher response and quicker response/recovery than the intermediate ZnO/ZnS nanorod arrays and the precursor ZnO nanorod arrays, respectively. Moreover, the ZnS nanotube array-based sensor exhibits good linearity in response to relative humidity (RH) and reliable reproducibility in a wide range of RH at room temperature. Compared with powder-form nanomaterials, the as-prepared nanotube arrays as the humidity sensing materials can provide much more open surfaces between neighboring nanotubes and inner surfaces inside the nanotubes and allow for easy diffusion and efficient transportation of sensed gas. As a prototype sensor, the nanotube arrays can avoid tedious process of fabricating a sensor from powder and may hold great potential applications in humidity sensing.

An inward replacement/etching route to controllable fabrication of zinc sulfide nanotube arrays for humidity sensing

Doping semiconductor nanocrystals is a powerful tool to impart new and beneficial optical and electrical properties to the host nanocrystals. Doping has been used to improve the performances of nanocrystal-based devices in applications as diverse as optics, magnetism, electronics, catalysis and sensing. In this work we present a low temperature colloidal synthesis of zinc sulfide (ZnS) nanocrystals doped with indium. Through optimization of the reaction parameters and the doping level, quantum confined (∼2 nm in size) crystalline colloids with highly tunable optical properties are achieved. Using a suite of characterization techniques including X-ray diffraction, high-resolution transmission electron microscopy, optical spectroscopies (absorption, emission, and Raman), compositional analyses and first principles simulations, we investigate the structural, morphological and optical properties of the synthesized nanocrystals. Indium dopants are found to heavily influence the band gap of ZnS. This strategy in addition to traditional methods of size control enables the synthesis of nanocrystals with finely tunable band gaps between ∼3.8 eV–4.3 eV. These doped ZnS nanocrystals are fabricated into selective UV thin-film absorbers and discriminatory proof-of-concept UVA–UVB/C photodetectors.

Augmented band gap tunability in indium-doped zinc sulfide nanocrystals

By employing first-principle total-energy calculations, a systematic study of the dopability of ZnS to be both n- and p-types compared with that of ZnO is carried out. We find that all the attempted acceptor dopants, group V substituting on the S lattice site and group I and IB on the Zn sites in ZnS, have lower ionization energies than the corresponding ones in ZnO. This can be accounted for by the fact that ZnS has relative higher valence band maximum than ZnO. Native ZnS is weak p-type under S-rich condition, as the abundant acceptor V-Zn has rather large ionization energy. Self-compensations by the formation of interstitial donors in group I and IB-doped p-type ZnS can be avoided when sample is prepared under S-rich condition. In terms of ionization energies, Li-Zn and N-S are the preferred acceptors in ZnS. Native n- type doping of ZnS is limited by the spontaneous formation of intrinsic V-Zn(2-); high efficient n-type doping with dopants is harder to achieve than in ZnO because of the readiness of forming native compensating centers and higher ionization energy of donors in ZnS. (C) 2009 American Institute of Physics. [DOI 10.1063/1.3103585]

The bipolar doping of ZnS via native defects and external dopants

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The bipolar doping of ZnS via native defects and external dopants