Hao-Lin Feng

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.

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.

Dithienopyrrolobenzothiadiazole-based organic dyes for efficient dye-sensitized solar cells

Four novel D–π–A metal-free organic dyes DTP1–4 containing a dithienopyrrolobenzothiadiazole (DTPBT) unit were synthesized and applied in dye-sensitized solar cells, where DTPBT was employed as a π-spacer for the first time. The photophysical, electrochemical and photovoltaic properties of the dyes were systematically investigated. The dyes DTP1–4 showed broad absorption spectra and high molar extinction coefficient, resulting in high light harvesting efficiency. In addition, the impacts of donors and the thiophene unit as an additional π-spacer were also studied. The results showed that the dye DTP4 with triphenylamine as the donor exhibited better photovoltaic performance than DTP1–3 with phenothiazine as the donor. The linking position of the thiophene unit to the DTPBT unit significantly influenced the photovoltaic performance. A power conversion efficiency of 7.55% with 1 mM CDCA as the co-adsorbent under simulated AM 1.5 G illumination was reached by the DSSC sensitized by the dye DTP4. These results indicate that the DTPBT-based organic dye is a promising candidate for efficient DSSCs.

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.

Three-dimensional hyperbranched TiO2/ZnO heterostructured arrays for efficient quantum dot-sensitized solar cells

The ingenious design of nanostructures and smart integration of specific semiconducting metal oxide materials are closely related to the photovoltaic performance of solar cells. Herein, we successfully fabricate vertically aligned TiO2 nanowire–TiO2 nanosheet–ZnO nanorod (TNW–TNS–ZNR) hyperbranched heterostructured arrays on TCO substrates as excellent anode material electrodes for efficient solar-to-electricity conversion. As a result, the TNW–TNS–ZNR heterostructured array based quantum dot-sensitized solar cells (QDSSCs) have exhibited an improved power conversion efficiency (PCE) under AM 1.5G one sun illumination, along with considerable improvement of short-circuit current density (Jsc), open voltage (Voc) and fill factor (FF) due to their superior light harvesting and excellent charge collection capability. Combined with the chemical bath deposited Cu2S counter electrode, the eventual PCE of CdS/CdSe co-sensitized solar cells can be further boosted to as high as 5.38%.

Recent advances in hierarchical three-dimensional titanium dioxide nanotree arrays for high-performance solar cells

Hierarchical metal oxide nanotree array architectures with tunable three-dimensional (3D) morphologies and homo-/heterogeneous junctions, consisting of 1D/2D nanobranches grown epitaxially on the sidewalls of vertical 1D nanostructured trunks (resembling a tree), have been widely explored to demonstrate their huge potential in the development of high-performance photovoltaic devices. In this review, the growth of a wide variety of TiO2 nanotree array architectures will be discussed, with an emphasis on solution-phase and vapor-phase syntheses. The evolution of electrode materials and recent progress in 3D TiO2 nanotree array architectures for solar cells are reviewed. Furthermore, to highlight the obvious benefits of 3D TiO2 nanotree arrays, the limitations and challenges of these hierarchical array architectures when used in solar cells are addressed. Finally, insight into future directions and opportunities for these fascinating electrode materials in creating a new energy conversion epoch is also provided.

Hierarchical ZnO nanorod-on-nanosheet arrays electrodes for efficient CdSe quantum dot-sensitized solar cells

Two-dimensional (2D) ZnO nanosheet arrays were prepared via vanadium (V)-doping assisted hydrothermal method, and then the nanosheet was successfully converted to a nanorod-on-nanosheet ZnO hierarchical structure by treating with Na2S solution and subsequent hydrothermal reaction. Hierarchical films with different nanorod growth time (1–8 h) were prepared and their photovoltaic properties were also investigated after electrodeposition of CdSe quantum dots. For the hierarchical nanorod-on-nanosheet ZnO films, increasing the ZnO nanorod growth time can enormously enlarge the length of branched nanorods and light-scattering ability, resulting in better light-harvesting efficiency and higher photo-generated electron concentration, which leads to higher short-circuit current density (Jsc) and open-circuit voltage (Voc). However, further increasing nanorod growth time to 8 h leads to the over-dense coverage of nanorods, which is harmful for light-harvesting efficiency and leads to severe electron recombination, eventually diminishes the power conversion efficiency (PCE). With the optimized nanorod modification and Cu2S counter electrode, the PCE reaches a maximum value of 4.26%, which to the best of our knowledge, is among the highest PCE record for CdSe sensitized solar cells based on ZnO photoanodes.

Hierarchical TiO2–B/anatase core/shell nanowire arrays for efficient dye-sensitized solar cells

The fabrication of hierarchically structured photoanode materials is believed to be an effective way to obtain efficient dye-sensitized solar cells (DSSCs). In this paper, hierarchical TiO2–B/anatase core/shell heterojunction nanowire arrays on a titanium plate substrate are synthesized and used as novel photoanode materials for DSSCs. By using H2Ti3O7 nanowire arrays as the precursor template, anatase nanoparticle coated TiO2–B nanowire arrays are prepared via a hydrothermal reaction followed by a calcination process. Photoelectric measurements reveal that the anatase nanoparticle shell makes the pristine TiO2–B nanowire rougher for more dye adsorption and effective light scattering, which can enhance the light harvesting ability and thus the photocurrent density of the photoanode largely. Moreover, the dynamic electron transport and recombination study via electrochemical impedance spectroscopy (EIS) and intensity modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS) measurements reveal that the TiO2–B/anatase composite photoanode based cell has a faster electron transport and higher electron collection efficiency than a TiO2–B based cell. As a consequence, the photoelectric conversion efficiency of the hierarchical composite photoanode was greatly enhanced to 4.88%, which is among the highest reported value for TiO2 nanowire array photoanodes grown on titanium plate substrate.