-Qiang Wu

Hydrothermal fabrication of hierarchically anatase TiO2 nanowire arrays on FTO glass for dye-sensitized solar cells.

Hierarchical anatase TiO2 nano-architecture arrays consisting of long TiO2 nanowire trunk and numerous short TiO2 nanorod branches on transparent conductive fluorine-doped tin oxide glass are successfully synthesized for the first time through a facile one-step hydrothermal route without any surfactant and template. Dye-sensitized solar cells based on the hierarchical anatase TiO2 nano-architecture array photoelectrode of 18 μm in length shows a power conversion efficiency of 7.34% because of its higher specific surface area for adsorbing more dye molecules and superior light scattering capacity for boosting the light-harvesting efficiency. The present photovoltaic performance is the highest value for the reported TiO2 nanowires array photoelectrode.

Multistack Integration of Three-Dimensional Hyperbranched Anatase Titania Architectures for High-Efficiency Dye-Sensitized Solar Cells

An unprecedented attempt was conducted on suitably functionalized integration of three-dimensional hyperbranched titania architectures for efficient multistack photoanode, constructed via layer-by-layer assembly of hyperbranched hierarchical tree-like titania nanowires (underlayer), branched hierarchical rambutan-like titania hollow submicrometer-sized spheres (intermediate layer), and hyperbranched hierarchical urchin-like titania micrometer-sized spheres (top layer). Owing to favorable charge-collection, superior light harvesting efficiency and extended electron lifetime, the multilayered TiO2-based devices showed greater J(sc) and V(oc) than those of a conventional TiO2 nanoparticle (TNP), and an overall power conversion efficiency of 11.01% (J(sc) = 18.53 mA cm(-2); V(oc) = 827 mV and FF = 0.72) was attained, which remarkably outperformed that of a TNP-based reference cell (η = 7.62%) with a similar film thickness. Meanwhile, the facile and operable film-fabricating technique (hydrothermal and drop-casting) provides a promising scheme and great simplicity for high performance/cost ratio photovoltaic device processability in a sustainable way.

Hierarchical Oriented Anatase TiO2 Nanostructure arrays on Flexible Substrate for Efficient Dye-sensitized Solar Cells

The vertically oriented anatase single crystalline TiO2 nanostructure arrays (TNAs) consisting of TiO2 truncated octahedrons with exposed {001} facets or hierarchical TiO2 nanotubes (HNTs) consisting of numerous nanocrystals on Ti-foil substrate were synthesized via a two-step hydrothermal growth process. The first step hydrothermal reaction of Ti foil and NaOH leads to the formation of H-titanate nanowire arrays, which is further performed the second step hydrothermal reaction to obtain the oriented anatase single crystalline TiO2 nanostructures such as TiO2 nanoarrays assembly with truncated octahedral TiO2 nanocrystals in the presence of NH4F aqueous or hierarchical TiO2 nanotubes with walls made of nanocrystals in the presence of pure water. Subsequently, these TiO2 nanostructures were utilized to produce dye-sensitized solar cells in a backside illumination pattern, yielding a significant high power conversion efficiency (PCE) of 4.66% (TNAs, JSC = 7.46 mA cm−2, VOC = 839 mV, FF = 0.75) and 5.84% (HNTs, JSC = 10.02 mA cm−2, VOC = 817 mV, FF = 0.72), respectively.

Ultra-long anatase TiO2 nanowire arrays with multi-layered configuration on FTO glass for high-efficiency dye-sensitized solar cells

A convenient and operable fabrication technique has been demonstrated for self-assembled vertically aligned anatase TiO2 nanowires on FTO glass with tunable length in the range of 15–55 μm for multi-layered configuration photoanode based DSSCs application. DSSCs based on 47 μm-long multi-layered anatase TiO2 nanowire arrays attain a power conversion efficiency of 9.40%, which is the highest value for nanowire array based DSSCs to date.

Maximizing omnidirectional light harvesting in metal oxide hyperbranched array architectures

The scrupulous design of nanoarchitectures and smart hybridization of specific active materials are closely related to the overall photovoltaic performance of an anode electrode. Here we present a solution-based strategy for the fabrication of well-aligned metal oxide-based nanowire-nanosheet-nanorod hyperbranched arrays on transparent conducting oxide substrates. For these hyperbranched arrays, we observe a twofold increment in dye adsorption and enhanced light trapping and scattering capability compared with the pristine titanium dioxide nanowires, and thus a power conversion efficiency of 9.09% is achieved. Our growth approach presents a strategy to broaden the photoresponse and maximize the light-harvesting efficiency of arrays architectures, and may lead to applications for energy conversion and storage, catalysis, water splitting and gas sensing.

Dye-sensitized solar cells based on a double layered TiO2 photoanode consisting of hierarchical nanowire arrays and nanoparticles with greatly improved photovoltaic performance

A novel double layered photoanode consisting of oriented hierarchical anatase TiO2 nanowire arrays (HNW) and nanoparticles (NP) has been successfully fabricated on a Ti-foil substrate via a two-step hydrothermal and screen-printing process, respectively. Compared to a NP based cell, the photovoltaic performance of the TiO2 HNW cell highlights several outstanding properties, including superior light scattering, rapid electron transport and lower electron recombination rate. Furthermore, the effect of the TiO2 NP film thickness (top layer) on the power conversion of the DSSCs has been investigated in detail. The DSSC based on a Ti/TiO2 HNW + NP photoelectrode with a total thickness of 31 μm (HNW: 13 μm; NP: 18 μm) shows the highest power conversion efficiency of 7.92%, which is remarkably higher than those of 13 μm HNW (4.52%), 18 μm NP (6.50%) and 31 μm NP (5.99%) photoelectrodes. The enhancement of the efficiency for the double layered photoanode based cell compared to the single NP is mainly attributed to better light scattering capability, faster electron transport and lower electron recombination for the former.

Constructing 3D Branched Nanowire Coated Macroporous Metal Oxide Electrodes with Homogeneous or Heterogeneous Compositions for Efficient Solar Cells

Light-harvesting and charge collection have attracted increasing attention in the domain of photovoltaic cells, and can be facilitated dramatically by appropriate design of a photonic nanostructure. However, the applicability of current light-harvesting photoanode materials with single component and/or morphology (such as, particles, spheres, wires, sheets) is still limited by drawbacks such as insufficient electron-hole separation and/or light-trapping. Herein, we introduce a universal method to prepare hierarchical assembly of macroporous material-nanowire coated homogenous or heterogeneous metal oxide composite electrodes (TiO2 -TiO2 , SnO2 -TiO2 , and Zn2 SnO4 -TiO2 ; homogenous refers to a material in which the nanowire and the macroporous material have the same composition, i.e. both are TiO2 . Heterogeneous refers to a material in which the nanowires and the macroporous material have different compositions). The dye-sensitized solar cell based on a TiO2 -macroporous material-TiO2 -nanowire homogenous composition electrode shows an impressive conversion efficiency of 9.51 %, which is much higher than that of pure macroporous material-based photoelectrodes to date.

Three-Dimensional TiO2/ZnO Hybrid Array as a Heterostructured Anode for Efficient Quantum-Dot-Sensitized Solar Cells

The development of a novel nanoarray photoanode with a heterostructure on a transparent conducting oxide substrate provides a promising scheme to fabricate efficient energy conversion devices. Herein, we successfully synthesize the vertically aligned hierarchical TiO2 nanowire/ZnO nanorod or TiO2 nanowire/ZnO nanosheet hybrid arrays, which are proven to be excellent anode candidates for superior light utilization. Consequently, the quantum-dot-sensitized solar cells based on such hybrid arrays exhibit an impressive power conversion efficiency (PCE) under AM 1.5G one sun illumination with improved short-circuit current density (JSC) and fill factor compared to pristine TiO2 nanowire arrays. Combined with the chemical-bath-deposited Cu2S counter electrode, the eventual PCE can be further optimized to as high as 4.57% for CdS/CdSe co-sensitized quantum dot solar cells.

Recent progress in hybrid perovskite solar cells based on n-type materials

Over the past several years, thin film organic–inorganic halide perovskite solar cells (PSCs) have rapidly improved with their power conversion efficiency (PCE) reaching over 22%, nearing that of polycrystalline silicon solar cells. This emerging photovoltaic technology has shown great potential for large-scale application owing to its high efficiency, low material cost and facile fabrication process. Electron-selective contacts, n-type materials that favor the charge transport and collection from the perovskite light absorbers to the front electrode, are important and seen as critical to fabricate high-performance PSCs. In this review, recent progress in such n-type materials is overviewed, with the emphasis on the most explored inorganic semiconducting metal oxides (including TiO2, SnO2, ZnO and Zn2SnO4). The impact of the structure of the n-type materials on the morphology of the perovskite films, the charge extraction and recombination, as well as device performance is highlighted. Optimization of n-type materials and their application in different device architectures are also summarized. An understanding of the intricate function of the n-type materials in PSC devices will assist in the design of materials to give precise control over the light harvesting and charge collection within PSC devices leading to high-performance.

Hierarchical Macroporous Zn2SnO4–ZnO Nanorod Composite Photoelectrodes for Efficient CdS/CdSe Quantum Dot Co-Sensitized Solar Cells

A hierarchical macroporous Zn2SnO4-ZnO nanorod composite film is prepared through a drop-casting process of PS@Zn2SnO4 and subsequent hydrothermal growth of ZnO nanorod. CdS/CdSe co-sensitized solar cells based on the macroporous Zn2SnO4-ZnO nanorod composite photoelectrode exhibits an enhancement of 34.4% in power conversion efficiency (1.68%) compared to the pristine macroporous Zn2SnO4 photoelectrode (1.25%). Especially worth noting is that the growth of ZnO nanorods contributes greatly to the enlargement of surface area and improvement of light scattering ability of the composite film, which dominates the increase of Jsc values and eventual power conversion efficiency. QDSSCs based on the optimized 9 μm thick composite photoanode film exhibits a power conversion efficiency of 2.08%, which is the highest value for the reported QDs sensitized solar cells based on the Zn2SnO4 photoelectrode.