Hong Liu

Controllable current oscillation and pore morphology evolution in the anodic growth of TiO2 nanotubes

The spatial heterogeneities and temporal instabilities in the anodic growth of TiO₂ tubes are very important for nanostructure fabrication, but few ordered cases have been reported. In this work, we represent a new current oscillation with pore morphology evolution in the formation of anodic TiO₂ tubes. Small (less than 8.0% of the minimum value) and fast (period ~10⁰ s) current oscillation was formed under static conditions in a wide range, while significant morphological change such as periodical narrowing, abruption and small pits appeared in the pore with characteristic length scale of 10¹-10² nm. Surprisingly, the roughness in the pore would be totally eliminated instead of being enhanced by high speed stirring or periodically modulated voltage with the current oscillation still being enhanced, which indicates an important involvement of the ion transport process. It has also been found that the growth rate could be significantly accelerated by tuning the stirring rate or the periodical modulation of the voltage. The mechanism has been described with consideration of the local reactions and the ion transport with a key involvement of the convection process, which can be strongly influenced by the mechanical stirring and the modulated voltage.

Chemical assisted formation of secondary structures towards high efficiency solar cells based on ordered TiO2 nanotube arrays

Amongst all types of photoanodes in sensitized solar cells, nanotube arrays (NTs) have become a suitable choice that can balance the dye absorption, electron transport, and effective thickness. However, the maximum performance they have shown now is still not comparable with porous films. In this work, we have established a simple yet effective method towards high performance solar cell based on NTs. Significantly increased carrier generation by better dye absorption can be realized by the particle-like secondary structures grown on TiO2 tubes. Nevertheless, this effect was often followed by a sacrifice of the electron transport properties of the NTs, which may greatly weaken the total net increase of efficiency by the secondary structures. With a subsequent TiCl4 treatment and slightly modulated outer conditions (anodic voltage and stirring rate), we are able to adequately resolve this paradox. The short circuit current can be raised to over 110% higher, while the fill factor and open circuit voltage remain unharmed. As a result, an efficiency of 4.35% can be achieved in the back-side illuminated dye-sensitized solar cells, which is ∼114% higher than the basic efficiency of the plain cell. The formation of the secondary structures can be explained by the reversible dissolution-reprecipitation of TiO2 with the reaction-diffusion process of F− and other fluoride species during the treatment.

Passivation of nanocrystalline silicon photovoltaic materials employing a negative substrate bias

Hydrogenated nanocrystalline silicon (nc-Si:H) shows great promise in the application of third-generation thin film photovoltaic cells. However, the mixed-phase structure of nc-Si:H leads to many defects existing in this important solar energy material. Here we present a new way to passivate nc-Si:H films by tuning the negative substrate bias in plasma-enhanced chemical vapor deposition. Microstructures of the nc-Si:H films prepared under a negative bias from 0 to -300xa0V have been characterized using Raman, x-ray diffraction, transmission electron microscope, and optical transmission techniques. A novel passivation effect on nc-Si:H films has been identified by the volume fraction of voids in nc-Si:H, together with the electrical properties obtained by electron spin resonance and effective minority lifetime measurements. The mechanism of the passivation effect has been demonstrated by infrared spectroscopy, which illustrates that the high-energy H atoms and ions accelerated by an appropriate bias of -180xa0V can form more hydrides along the grain boundaries and effectively prevent oxygen incursions forming further Si-O/Si interface dangling bonds in the nc-Si:H films. The detrimental influence of a bias over -180xa0V on the film quality due to the strong ion bombardment of species with excessively high energy has also been observed directly from the surface morphology by atomic force microscopy.

Controllable light-induced conic structures in silicon nanowire arrays by metal-assisted chemical etching.

Silicon nanowires (SiNWs) have long been considered a promising material due to their extraordinary electrical and optical properties. As a simple, highly efficient fabrication method for SiNWs, metal-assisted chemical etching (MACE) has been intensively studied over recent years. However, effective control by modulation of simple parameters is still a challenging topic and some key questions still remain in the mechanistic processes. In this work, a novel method to manipulate SiNWs with a light-modulated MACE process has been systematically investigated. Conic structures consisting of inclined and clustered SiNWs can be generated and effectively modified by the incident light while new patterns such as bamboo shoot arrays can also be formed under certain conditions. More importantly, detailed study has revealed a new top-down diverting etching model of the conic structures in this process, different from the previously proposed bending model. As a consequence of this mechanism, preferential lateral mass transport of silver particles occurs. Evidence suggests a relationship of this phenomenon to the inhomogeneous distribution of the light-induced electron-hole pairs beneath the etching front. Study on the morphological change and related mechanism will hopefully open new routes to understand and modulate the formation of SiNWs and other nanostructures.

A composite CdS thin film/TiO2 nanotube structure by ultrafast successive electrochemical deposition toward photovoltaic application

Fabricating functional compounds on substrates with complicated morphology has been an important topic in material science and technology, which remains a challenging issue to simultaneously achieve a high growth rate for a complex nanostructure with simple controlling factors. Here, we present a novel simple and successive method based on chemical reactions in an open reaction system manipulated by an electric field. A uniform CdS/TiO2 composite tubular structure has been fabricated in highly ordered TiO2 nanotube arrays in a very short time period (~90xa0s) under room temperature (RT). The content of CdS in the resultant and its crystalline structure was tuned by the form and magnitude of external voltage. The as-formed structure has shown a quite broad and bulk-like light absorption spectrum with the absorption of photon energy even below that of the bulk CdS. The as-fabricated-sensitized solar cell based on this composite structure has achieved an efficiency of 1.43% without any chemical doping or co-sensitizing, 210% higher than quantum dot-sensitized solar cell (QDSSC) under a similar condition. Hopefully, this method can also easily grow nanostructures based on a wide range of compound materials for energy science and electronic technologies, especially for fast-deploying devices.

Current promoted micro-annealing in anodic TiO2 tube arrays and its application in sensitized solar cells

Crystallization in nanostructured materials is an important basis of solar cells and other nanoscience fields. For instance, crystalline TiO2 for photovoltaic and photocatalysis applications is normally formed by high-temperature annealing above 450 °C for a long period of time (normally more than one hour). In this work, we have established a method to artificially induce crystallization in ordered anodic TiO2 tube arrays at around room temperature for the first time. Assisted by the existence of an electric field in the reaction system, crystallization could take place at a much lower temperature than isothermal annealing. Aided by the low thermal conductivity of TiO2, the sample surface temperature could be limited to an even lower value (below 130 °C). The as-fabricated samples could show significant efficiencies of up to 2.05% after being installed into back-side illuminated dye sensitized solar cells. Furthermore, a nearly monocrystalline-like structure could be formed through some simple physical and chemical manipulation. As a result, an efficiency of 3.51% could be achieved, together with VOC = 0.63 V, JSC = 13.03 mA cm−2, and FF = 0.47. This method can easily realize high quality crystallization in nanostructures like TiO2 tube arrays of a particular size. It can be helpful for the development of new solar cells and other opto-electronic devices, and hopefully for mechanistic studies of other materials as well.

Controllable in situ photo-assisted chemical deposition of CdSe quantum dots on ZnO/CdS nanorod arrays and its photovoltaic application

Compound semiconductors have been widely applied in the energy field as light-harvesting materials, conducting substrates and other functional parts. Nevertheless, to effectively grow them in various forms toward objective applications, limitations have often been met to achieving high growth rate, simplicity of method and controllability of growing processes simultaneously. In this work, we have grown a uniform CdSe layer on ZnO/CdS nanorod arrays by a novel in situ photo-assisted chemical deposition method. The morphology and quality of the as-formed material could be significantly influenced by tuning the optical parameters of the injected light. Due to the effect of injected light on the key reactions during the growth, a modified natural light with removal of the UV and IR components seems to be more suitable than monochromic light. An efficiency of 3.59% was achieved without any additional treatment, significantly higher than the efficiency of 2.88% of the sample by conventional CBD method under similar conditions with growth rate one order of magnitude higher. In general, the result has suggested its potential importance for other compound materials and opto-electronic devices.

Temperature gradient induced instability of perovskite via ion transport

Perovskite has been known as a promising novel material for photovoltaics and other fields because of its excellent opto-electric properties and convenient fabrication. However, its stability has been a widely known haunting factor that has severely deteriorated its application in reality. In this work, it has been discovered for the first time that perovskite can become significantly chemically unstable with the existence of a temperature gradient in the system, even at temperature far below its thermal decomposition condition. A study of the detailed mechanism has revealed that the existence of a temperature gradient could induce a mass transport process of extrinsic ionic species into the perovskite layer, which enhances its decomposition process. Moreover, this instability could be effectively suppressed with a reduced temperature gradient by simple structural modification of the device. Further experiments have proved the existence of this phenomenon in different perovskites with various mainstream substrates, indicating the universality of this phenomenon in many previous studies and future research. Hopefully, this work may bring deeper understanding of its formation mechanisms and facilitate the general development of perovskite toward its real application.

A Convenient and Effective Method to Deposit Low-Defect-Density nc-Si:H Thin Film by PECVD

Hydrogenated nanocrystalline silicon (nc-Si:H) thin film has received a great deal of attention as a promising material for flat panel display transistors, solar cells, etc. However, the multiphase structure of nc-Si:H leads to many defects. One of the major challenges is how to reduce the defects conveniently. In this work, we developed a simple and effective method to deposit low-defect-density nc-Si:H thin film. This method is simply by tuning the deposition pressure in a high-pressure range in plasma-enhanced chemical vapor deposition (PECVD) process. Microstructures of the nc-Si:H were characterized by Raman, AFM, and SEM. Furthermore, we focused on the defect density which was the key characteristic for photovoltaic materials and achieved the defect density of 3.766u2009×u20091016xa0cm−3. This defect density is lower than that of previous studies on the fabrication of low-defect-density nc-Si:H by other complex methods in PECVD process. The minority carrier lifetime of nc-Si:H is thus greatly improved. Moreover, we demonstrated the mechanism about the effect of deposition pressure on the ion bombardment and proved that the defect density is the key characteristic for nc-Si:H photovoltaic material.

Ultrafast Electrochemical Fabrication of Multicomponent Photovoltaic Materials

Recently, organic-inorganic hybrid perovskite materials such as CH3NH3PbI3, CH3NH3PbBrI2 and CH3NH3SnICl2I etc. have attracted great attention for its outstanding opto-electric properties toward application in the field of photovoltaic as well as illumination and photocatalysis.1–7 Normally, like many other opto-electronic materials, these multi-component perovskites were fabricated either by sequential liquid phase chemical deposition (one-step, two step, and successive ion layer absorption known as SILAR) or by gaseous phase chemical vapor deposition (with many variations).1–7 The as-fabricated materials have demonstrated excellent performances especially on efficiencies up to over 20%, yet with efforts continuously paid on stability and mechanisms. However, those fabrication methods have either suffered from complicated and slow processes or high instrumental requirements. Therefore a simpler, controllable and low requirement method would be hopefully helpful for future development