Linfeng Lu

Electrochemically hydrogenated TiO2 nanotubes with improved photoelectrochemical water splitting performance

One-dimensional anodic titanium oxide (ATO) nanotube arrays hold great potential as photoanode for photoelectrochemical (PEC) water splitting. In this work, we report a facile and eco-friendly electrochemical hydrogenation method to modify the electronic and PEC properties of ATO nanotube films. The hydrogenated ATO (ATO-H) electrodes present a significantly improved photocurrent of 0.65 mA/cm2 in comparison with that of pristine ATO nanotubes (0.29 mA/cm2) recorded under air mass 1.5 global illumination. The incident photon-to-current efficiency measurement suggests that the enhanced photocurrent of ATO-H nanotubes is mainly ascribed to the improved photoactivity in the UV region. We propose that the electrochemical hydrogenation induced surface oxygen vacancies contribute to the substantially enhanced electrical conductivity and photoactivity.

Enhanced supercapacitance in anodic TiO2 nanotube films by hydrogen plasma treatment

One-dimensional anodic titanium oxide (ATO) nanotube arrays hold great potential as electrode materials for high-performance electrochemical supercapacitors. However, their poor electronic conductivity limits their practical applications. Here, we develop a hydrogen (H2) plasma treatment method to greatly improve the electrochemical performance of ATO electrodes. Compared with pristine ATO, the nanotubes treated by H2 plasma illumination (ATO-H) present a rough and amorphous layer at the surface of the nanotubes with simultaneously incorporated Ti(3+) and -OH groups. At a current density of 0.05 mA cm(-2) in charge-discharge measurements, the specific capacitance of the ATO-H electrode has substantially increased ~7.4 times, with a value as high as 7.22 mF cm(-2). Moreover, the novel ATO-H electrode has also exhibited excellent rate capability (6.37 mF cm(-2) at a current density of 2 mA cm(-2)) and cycling performance with no degradation after 10,000 cycles.

Enhanced Photoelectrochemical Water Splitting Performance of Anodic TiO2 Nanotube Arrays by Surface Passivation

One-dimensional anodic titanium oxide nanotube (TONT) arrays provide a direct pathway for charge transport, and thus hold great potential as working electrodes for electrochemical energy conversion and storage devices. However, the prominent surface recombination due to the large amount surface defects hinders the performance improvement. In this work, the surface states of TONTs were passivated by conformal coating of high-quality Al2O3 onto the tubular structures using atomic layer deposition (ALD). The modified TONT films were subsequently employed as anodes for photoelectrochemical (PEC) water splitting. The photocurrent (0.5 V vs Ag/AgCl) recorded under air mass 1.5 global illumination presented 0.8 times enhancement on the electrode with passivation coating. The reduction of surface recombination rate is responsible for the substantially improved performance, which is proposed to have originated from a decreased interface defect density in combination with a field-effect passivation induced by a negative fixed charge in the Al2O3 shells. These results not only provide a physical insight into the passivation effect, but also can be utilized as a guideline to design other energy conversion devices.

Roll-to-roll fabrication of large scale and regular arrays of three-dimensional nanospikes for high efficiency and flexible photovoltaics

Three-dimensional (3-D) nanostructures have demonstrated enticing potency to boost performance of photovoltaic devices primarily owning to the improved photon capturing capability. Nevertheless, cost-effective and scalable fabrication of regular 3-D nanostructures with decent robustness and flexibility still remains as a challenging task. Meanwhile, establishing rational design guidelines for 3-D nanostructured solar cells with the balanced electrical and optical performance are of paramount importance and in urgent need. Herein, regular arrays of 3-D nanospikes (NSPs) were fabricated on flexible aluminum foil with a roll-to-roll compatible process. The NSPs have precisely controlled geometry and periodicity which allow systematic investigation on geometry dependent optical and electrical performance of the devices with experiments and modeling. Intriguingly, it has been discovered that the efficiency of an amorphous-Si (a-Si) photovoltaic device fabricated on NSPs can be improved by 43%, as compared to its planar counterpart, in an optimal case. Furthermore, large scale flexible NSP solar cell devices have been fabricated and demonstrated. These results not only have shed light on the design rules of high performance nanostructured solar cells, but also demonstrated a highly practical process to fabricate efficient solar panels with 3-D nanostructures, thus may have immediate impact on thin film photovoltaic industry.

Inverted nanocone-based thin film photovoltaics with omnidirectionally enhanced performance.

Thin film photovoltaic (PV) technologies are highly attractive for low-cost solar energy conversion and possess a wide range of potential applications from building-integrated PV generation to portable power sources. Inverted nanocones (i-cones) have been demonstrated as a promising structure for practical thin film PV devices/modules, owning to their antireflection effect, self-cleaning function, superior mechanical robustness, and so forth. In this work, we have demonstrated a low-cost and scalable approach to achieve perfectly ordered i-cone arrays. Thereafter, thin film amorphous silicon (a-Si:H) solar cells have been fabricated based on various i-cone substrates with different aspect ratios and pitches to investigate the impact of geometry of i-cone nanostructures on the performance of the as-obtained PV devices. Intriguingly, the optical property investigations and device performance characterizations demonstrated that the 0.5-aspect-ratio i-cone-based device performed the best on both light absorption capability and energy conversion efficiency, which is 34% higher than that of the flat counterpart. Moreover, the i-cone-based device enhanced the light absorption and device performance over the flat reference device omnidirectionally. These results demonstrate a viable and convenient route toward scalable fabrication of nanostructures for high-performance thin film PV devices based on a broad range of materials.

Performance enhancement of thin-film amorphous silicon solar cells with low cost nanodent plasmonic substrates

Performance of thin film photovoltaics largely relies on photon absorption capability. Here, we introduce a novel substrate with patterned aluminum nanodent arrays with unique light management capability. Hydrogenated amorphous silicon thin film solar cells have been fabricated on the nano-texturized substrate for optical property study and photovoltaic performance evaluation. Our measurements have shown significant enhancement on broadband light absorption using these patterned substrates via both geometrical light trapping and plasmonic coupling. Particularly, the enhancement factor reaches as high as 5–30 times at wavelength near the band edge. Numerical simulations confirm the measurements and uncover the mechanisms of the enhancement. More importantly, photovoltaic measurements on nanodent solar cells present improvements of over 31% and 27% in short circuit current and energy conversion efficiency respectively compared with planar solar cells. Therefore, the novel patterned substrates are promising candidates for low cost and high performance thin film solar cells.

Large scale, flexible and three-dimensional quasi-ordered aluminum nanospikes for thin film photovoltaics with omnidirectional light trapping and optimized electrical design

Nanostructured photovoltaics has attracted an enormous amount of attention in recent years owing to its potency for significant device performance enhancement over the conventional technologies. Nonetheless, conventional fabrication approaches for nanostructured scaffolds rely on glass or silicon substrates which are costly, brittle and have limited scalability. Meanwhile, rational design guidelines for optical and electrical performance optimization of solar cells are of urgent need for their practical applications. In this work, flexible and quasi-ordered three-dimensional (3-D) nanospike (NSP) arrays are fabricated on a reasonable large scale with well controlled geometry. Systematic investigations by experiments discovered that photovoltaic devices based on NSPs with optimal geometry can accommodate the trade-off between optical absorption and electrical performance, demonstrating a power conversion efficiency of 7.92%, which is among the highest efficiency reported for single junction a-Si:H solar cells on a flexible substrate. Furthermore, we have demonstrated the superior omnidirectional device performance by utilizing such a 3-D NSP. This unique feature is of paramount importance for practical photovoltaic applications.

Morphology Defects Guided Pore Initiation during the Formation of Porous Anodic Alumina

Aluminum (Al) anodization leads to formation of porous structures with a broad spectrum of applications. Naturally or intentionally created defects on Al surfaces can greatly affect pore initiation. However, there is still a lack of systematic understanding on the defect dependent morphology evolution. In this paper, anodization processes on unpolished, polished, and nanoimprinted Al substrates are investigated under high voltages up to 600 V in various acid solutions. A porous structure is obtained on the unpolished and nanoimprinted Al foils with rough surface texture, whereas a compact film can be rationally obtained on the polished Al foil with a highly smooth surface. The observation of surface roughness dependent oxide film morphology evolution could be originated from the high voltages, which increases the threshold requirement of defect size or density for the pore initiation. Electrostatics simulation results indicate that inhomogeneous electric field and its corresponding localized high current induced by the surface roughness facilitate the initiation of nanopores. In addition, the porous films are utilized as templates to produce polydimethylsiloxane nanocone and submicrowire arrays. The nanoarrays with different aspect ratios present tunable wettability with the contact angles ranging from 144.6° to 56.7°, which hold promising potentials in microfluidic devices and self-cleaning coatings.

3D periodic multiscale TiO2 architecture: a platform decorated with graphene quantum dots for enhanced photoelectrochemical water splitting

Micropatterned TiO2 nanorods (TiO2NRs) via three-dimensional (3D) geometry engineering in both microscale and nanoscale decorated with graphene quantum dots (GQDs) have been demonstrated successfully. First, micropillar (MP) and microcave (MC) arrays of anatase TiO2 films are obtained through the sol-gel based thermal nanoimprinting method. Then they are employed as seed layers in hydrothermal growth to fabricate the 3D micropillar/microcave arrays of rutile TiO2NRs (NR), which show much-improved photoelectrochemical water-splitting performance than the TiO2NRs grown on flat seed layer. The zero-dimensional GQDs are sequentially deposited onto the surfaces of the microscale patterned nanorods. Owing to the fast charge separation that resulted from the favorable band alignment of the GQDs and rutile TiO2, the MP-NR-GQDs electrode achieves a photocurrent density up to 2.92 mA cm(-2) under simulated one-sun illumination. The incident-photon-to-current-conversion efficiency (IPCE) value up to 72% at 370 nm was achieved on the MP-NR-GQDs electrode, which outperforms the flat-NR counterpart by 69%. The IPCE results also imply that the improved photocurrent mainly benefits from the distinctly enhanced ultraviolet response. The work provides a cost-effective and flexible pathway to develop periodic 3D micropatterned photoelectrodes and is promising for the future deployment of high performance optoelectronic devices.

High‐Performance and Omnidirectional Thin‐Film Amorphous Silicon Solar Cell Modules Achieved by 3D Geometry Design

High-performance thin-film hydrogenated amorphous silicon solar cells are achieved by combining macroscale 3D tubular substrates and nanoscaled 3D cone-like antireflective films. The tubular geometry delivers a series of advantages for large-scale deployment of photovoltaics, such as omnidirectional performance, easier encapsulation, decreased wind resistance, and easy integration with a second device inside the glass tube.