Na/Mo co-doped PbTiO3 for photocatalytic water oxidation and Z-scheme overall water splitting under visible light.

Herein, we demonstrate a sodium/molybdenum (Na/Mo) co-doped ferroelectric PbTiO3 for efficient photocatalysis under visible light. Doped with a high concentration of Mo6+, quasi-continuous new energy levels are successfully introduced below the conduction band minimum of PbTiO3, giving rise to a band-to-band redshift of the absorption edge. The valence state difference of Mo6+ and Ti4+ in the doped PbTiO3 is compensated by the Na dopant, thus effectively suppressing the formation of the recombination centres caused by Mo4+. Combined with the intrinsic built-in electric field in PbTiO3, this Na/Mo co-doping strategy enables PbTiO3 to exhibit superior water oxidation activity under visible light with threshold wavelength up to 550 nm, which also promotes overall water splitting under visible light in a Z-scheme photocatalytic system. This strategy provides a generally applicable solution to extend the visible light absorption spectrum and engineer electronic structures of ferroelectric materials for photocatalysis and other energy conversion applications.

Effective nitrogen doping of TiO2 polymorphs at mild temperatures for visible-light-responsive hydrogen evolution.

Herein, a mild-temperature nitrogen doping route with the urea-derived gaseous species as the active doping agent is proposed to realize visible-light-responsive photocatalytic hydrogen evolution both for the anatase and rutile TiO2. DFT simulations reveal that the cyanic acid (HOCN), derived from the decomposition of urea, plays a curial role in the effective doping of nitrogen in TiO2 at mild temperatures. Photocatalytic performance demonstrates that both the anatase and rutile TiO2 doped at mild temperatures exhibit the highest hydrogen evolution rates, although the ones prepared at high temperatures possess higher absorbance in the visible range. Steady-state and transient surface photovoltage characterizations of these doped TiO2 polymorphs prepared at different temperatures reveal that harsh conditions (high temperature reaction) typically result in the formation of intrinsic defects that are detrimental to the transport of the low-energy visible-light-induced electrons, while the mild-temperature nitrogen-doping could flatten the pristine upward band bending without triggering the formation of Ti3+, thus achieving enhanced visible-light-responsive hydrogen evolution rates. We anticipate that our findings will provide inspiring information for shrinking the gap between the visible-light-absorbance and the visible-light-responsiveness in the band engineering of wide-bandgap metal-oxide photocatalysts.

Gradient tungsten-doped Bi3TiNbO9 ferroelectric photocatalysts with additional built-in electric field for efficient overall water splitting.

Bi3TiNbO9, a layered ferroelectric photocatalyst, exhibits great potential for overall water splitting through efficient intralayer separation of photogenerated carriers motivated by a depolarization field along the in-plane a-axis. However, the poor interlayer transport of carriers along the out-of-plane c-axis, caused by the significant potential barrier between layers, leads to a high probability of carrier recombination and consequently results in low photocatalytic activity. Here, we have developed an efficient photocatalyst consisting of Bi3TiNbO9 nanosheets with a gradient tungsten (W) doping along the c-axis. This results in the generation of an additional electric field along the c-axis and simultaneously enhances the magnitude of depolarization field within the layers along the a-axis due to strengthened structural distortion. The combination of the built-in field along the c-axis and polarization along the a-axis can effectively facilitate the anisotropic migration of photogenerated electrons and holes to the basal {001} surface and lateral {110} surface of the nanosheets, respectively, enabling desirable spatial separation of carriers. Hence, the W-doped Bi3TiNbO9 ferroelectric photocatalyst with Rh/Cr2O3 cocatalyst achieves an efficient and durable overall water splitting feature, thereby providing an effective pathway for designing excellent layered ferroelectric photocatalysts.

Ion-Diffusion Management Enables All-Interface Defect Passivation of Perovskite Solar Cells.

Perovskite solar cells (PSCs) have demonstrated over 25% power conversion efficiency (PCE) via efficient surface passivation. Unfortunately, state-of-the-art perovskite post-treatment strategies can solely heal the top interface defects. Herein, an ion-diffusion management strategy is proposed to concurrently modulate the top interfaces, buried interfaces, and bulk interfaces (i.e., grain boundaries) of perovskite film, enabling all-interface defect passivation. Specifically, this method is enabled by applying double interactive salts of octylammonium iodide (OAI) and guanidinium chloride (GACl) onto the 3D perovskite surface. It is revealed that the hydrogen-bonding interaction between OA+ and GA+ decelerates the OA+ diffusion and therefore forms a dimensionally broadened 2D capping layer. Additionally, the diffusion of GA+ and Cl- determines the composition of the bulk and buried interface of PSCs. As a result, n-inter-i-inter-p, i.e., five-layer structured PSCs can be obtained with a champion PCE of 25.43% (certified 24.4%). This approach also enables the substantially improved operational stability of perovskite solar cells.

Reexamining the Post-Treatment Effects on Perovskite Solar Cells: Passivation and Chloride Redistribution.

Post-treatment is an essential passivation step for the state-of-the-art perovskite solar cells (PSCs) but the additional role is not yet exploited. In this work, perovskite film is fabricated under ambient air with wide humidity window and identify that chloride redistribution induced by post-treatment plays an important role in high performance. The chlorine/iodine ratio on the perovskite surface increases from 0.037 to 0.439 after cyclohexylmethylammonium iodide (CHMAI) treatment and the PSCs deliver a champion power conversion efficiency (PCE) of 24.42% (certificated 23.60%). The maximum external quantum efficiency of electroluminescence (EQEEL ) reaches to 10.84% with a radiance of 170 W sr-1  m-2 , forming the reciprocity relation between EQEEL and nonradiative open-circuit voltage loss (86.0 mV). After thermal annealing, 2D component of perovskite will increase while chloride decline, leading to improved photovoltage but reduced fill factor. Hence, it distinguishes that chloride enrichment can improve charge transport/recombination simultaneously and 2D passivation can suppress the nonradiative recombination. Moreover, CHMAI can leverage their roles in charge transport/recombination for better performance than phenylethylammonium iodide (Cl/I = 0.114, PCE = 23.32%), due to the stronger binding energy of Cl- . This work provides the insight that the chloride fixation can improve the photovoltaic performance.

In Situ Passivation on Rear Perovskite Interface for Efficient and Stable Perovskite Solar Cells.

Despite the rocketing rise in power conversion efficiencies (PCEs), the performance of perovskite solar cells (PSCs) is still limited by the carrier transfer loss at the interface between perovskite (PVSK) absorbers and charge transporting layers. Here, we propose a novel in situ passivation strategy by using [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) to improve the charge dynamics at the rear PVSK/CTL interface in the n-i-p structure device. A pre-deposited PCBM-doped PbI2 layer is redissolved during PVSK deposition in our routine, establishing a bottom-up PCBM gradient that is facile for charge extraction. Meanwhile, the surface defects are in situ-passivated via PCBM-PVSK interaction, which substantially suppresses the trap-assisted recombination at the rear interface. Due to the synergistic effect of charge-extraction promotion and trap passivation, the fabricated PSCs deliver a champion PCE of 20.10% with attenuated hysteresis and improved long-term stability, much higher than the 18.39% of the reference devices. Our work demonstrates a promising interfacial engineering strategy for further improving the performance of PSCs.

Material and Interface Engineering for High-Performance Perovskite Solar Cells: A Personal Journey and Perspective.

Hybrid organic-inorganic perovskite solar cells (PSCs) have become a shining star in the photovoltaic field due to their spectacular increase in power conversion efficiency (PCE) from 3.8 % to over 23 % in just few years, opening up the potential in addressing the important future energy and environment issues. The excellent photovoltaic performance can be attributed to the unique properties of the organometal halide perovskite materials, including high absorption coefficient, tunable bandgap, high defect tolerance, and excellent charge transport characteristics. The authors entered this field when pursuing research on dye-sensitized solar cells (DSCs) by leveraging nanorods arrays for vectorial transport of the extracted electrons. Soon after, we and others realized that while the organometal halide perovskite materials have excellent intrinsic properties for solar cells, interface engineering is at least equally important in the development of high-performance PSCs, which includes surface defect passivation, band alignment, and heterojunction formation. Herein, we will address this topic by presenting the historical development and recent progress on the interface engineering of PSCs primarily of our own group. This review is mainly focused on the material and interface design of the conventional n-i-p, inverted p-i-n and carbon electrode-based structure devices from our own experience and perspective. Finally, the challenges and prospects of this area for future development will also be discussed.

Tailoring Nanoporous Structures in Bi2Te3 Thin Films for Improved Thermoelectric Performance.

Thin-film thermoelectrics (TEs) with unique advantages have triggered great interest in thermal management and energy harvesting technology for ambient temperature microscale systems. Although they have exhibited a good prospect, their unsatisfactory performances still seriously hamper their widespread application. Tailoring the porous structure has been demonstrated to be a facile strategy to significantly reduce thermal conductivity and enhance the figure of merit (ZT) of bulk TE materials; however, it is challenging for thin-film TEs. Here, the nanoporous Bi2Te3 thin films with faceted pore shapes and various porosities, pore sizes, and pore intervals are carefully designed and fabricated by evacuating the over-stoichiometry Te atoms. The dependence of the carrier mobility and lattice thermal conductivity on the pore characteristics is investigated. In the case of the pore interval longer than the electron mean free path, the porous structure greatly reduces the lattice thermal conductivity without affecting the electrical conductivity obviously. Phonon specular backscattering that is highly related to the pore characteristics is suggested to be mainly responsible for thermal conductivity reduction, resulting in ∼60% enhancement in ZT at room temperature, that is, from ∼0.42 for the dense film to ∼0.67 for the nanoporous film. The enhanced ZT value is comparable to that of commercial bulk TEs and can be further improved by optimizing the carrier concentrations. This work provides a general approach to fabricate high-performance chalcogenide TE thin-film materials.

Flexible layer-structured Bi2Te3 thermoelectric on a carbon nanotube scaffold.

Inorganic chalcogenides are traditional high-performance thermoelectric materials. However, they suffer from intrinsic brittleness and it is very difficult to obtain materials with both high thermoelectric ability and good flexibility. Here, we report a flexible thermoelectric material comprising highly ordered Bi2Te3 nanocrystals anchored on a single-walled carbon nanotube (SWCNT) network, where a crystallographic relationship exists between the Bi2Te3 <[Formula: see text]> orientation and SWCNT bundle axis. This material has a power factor of ~1,600 µW m-1 K-2 at room temperature, decreasing to 1,100 µW m-1 K-2 at 473 K. With a low in-plane lattice thermal conductivity of 0.26 ± 0.03 W m-1 K-1, a maximum thermoelectric figure of merit (ZT) of 0.89 at room temperature is achieved, originating from a strong phonon scattering effect. The origin of the excellent flexibility and thermoelectric performance of the Bi2Te3-SWCNT material is attributed, by experimental and computational evidence, to its crystal orientation, interface and nanopore structure. Our results provide insight into the design and fabrication of high-performance flexible thermoelectric materials.

Cellulose Fiber-Based Hierarchical Porous Bismuth Telluride for High-Performance Flexible and Tailorable Thermoelectrics.

Porous modification is a general approach to endowing the rigid inorganic thermoelectric (TE) materials with considerable flexibility, however, by which the TE performances are severely sacrificed. Thus, there remains an ongoing struggle against the trade-off between TE properties and flexibility. Herein, we develop a novel strategy to combine Bi2Te3 thick film with ubiquitous cellulose fibers (CFs) via an unbalanced magnetron sputtering technique. Owing to the nano-micro hierarchical porous structures and the excellent resistance to crack propagation of the Bi2Te3/CF architectures, the obtained sample with a nominal Bi2Te3 deposition thickness of tens of micrometers exhibits excellent mechanically reliable flexibility, of which the bending deformation radius could be as small as a few millimeters. Furthermore, the Bi2Te3/CF with rational internal resistance and tailorable shapes and dimensions are successfully fabricated for practical use in TE devices. Enhanced Seebeck coefficients are observed in the Bi2Te3/CF as compared to the dense Bi2Te3 films, and the lattice thermal conductivity is remarkably reduced due to the strong phonon scattering effect. As a result, the TE figure of merit, ZT, is achieved as high as ∼0.38 at 473 K, which competes with the best flexible TEs and can be further improved by optimizing the carrier concentrations. We believe this developed technique not only opens up a new window to engineer flexible TE materials for practical applications but also promotes the robust development of the fields, such as paper-based flexible electronics and thin-film electronics.

Dislocation-induced nanoparticle decoration on a GaN nanowire.

GaN nanowires with homoepitaxial decorated GaN nanoparticles on their surface along the radial direction have been synthesized by means of a chemical vapor deposition method. The growth of GaN nanowires is catalyzed by Au particles via the vapor-liquid-solid (VLS) mechanism. Screw dislocations are generated along the radial direction of the nanowires under slight Zn doping. In contrast to the metal-catalyst-assisted VLS growth, GaN nanoparticles are found to prefer to nucleate and grow at these dislocation sites. High-resolution transmission electron microscopy (HRTEM) analysis demonstrates that the GaN nanoparticles possess two types of epitaxial orientation with respect to the corresponding GaN nanowire: (I) [1̅21̅0]np//[1̅21̅0]nw, (0001)np//(0001)nw; (II) [1̅21̅3]np//[12̅10]nw, (101̅0)np//(101̅0)nw. An increased Ga signal in the energy-dispersive spectroscopy (EDS) profile lines of the nanowires suggests GaN nanoparticle growth at the edge surface of the wires. All the crystallographic results confirm the importance of the dislocations with respect to the homoepitaxial growth of the GaN nanoparticles. Here, screw dislocations situated on the (0001) plane provide the self-step source to enable nucleation of the GaN nanoparticles.

Mesoporous TiO2 single crystals: facile shape-, size-, and phase-controlled growth and efficient photocatalytic performance.

In this work, we have succeeded in preparing rutile and anatase TiO2 mesoporous single crystals with diverse morphologies in a controllable fashion by a simple silica-templated hydrothermal method. A simple in-template crystal growth process was put forward, which involved heterogeneous crystal nucleation and oriented growth within the template, a sheer spectator, and an excluded volume, i.e., crystal growth by faithful negative replication of the silica template. A series of mesoporous single-crystal structures, including rutile mesoporous TiO2 nanorods with tunable sizes and anatase mesoporous TiO2 nanosheets with dominant {001} facets, have been synthesized to demonstrate the versatility of the strategy. The morphology, size, and phase of the TiO2 mesoporous single crystals can be tuned easily by varying the external conditions such as the hydrohalic acid condition, seed density, and temperature rather than by the silica template, which merely serves for faithful negative replication but without interfering in the crystallization process. To demonstrate the application value of such TiO2 mesoporous single crystals, photocatalytic activity was tested. The resultant TiO2 mesoporous single crystals exhibited remarkable photocatalytic performance on hydrogen evolution and degradation of methyl orange due to their increased surface area, single-crystal nature, and the exposure of reactive crystal facets coupled with the three-dimensionally connected mesoporous architecture. It was found that {110} facets of rutile mesoporous single crystals can be considered essentially as reductive sites with a key role in the photoreduction, while {001} facets of anatase mesoporous single crystals provided oxidation sites in the oxidative process. Such shape- and size-controlled rutile and anatase mesoporous TiO2 single crystals hold great promise for building energy conversion devices, and the simple solution-based hydrothermal method is extendable to the synthesis of other mesoporous single crystals beyond TiO2.

A quasi-quantum well sensitized solar cell with accelerated charge separation and collection.

Semiconductor-sensitized solar cell (SSSC) represents a new generation of device aiming to achieve easy fabrication and cost-effective performance. However, the power of the semiconductor sensitizers has not been fully demonstrated in SSSC, making it actually overshadowed by dye-sensitized solar cell (DSSC). At least part of the problem is related to the inefficient charge separation and severe recombination with the current technologies, which calls on rethinking about how to better engineer the semiconductor sensitizer structure in order to enhance the power conversion efficiency (PCE). Herein we report on using for the first time a quasi-quantum well (QW) structure (ZnSe/CdSe/ZnSe) as the sensitizer, which is quasi-epitaxially deposited on ZnO tetrapods. Such a novel photoanode architecture has attained 6.20% PCE, among the highest reported to date for this type of SSSCs. Impedance spectra have revealed that the ZnSe/CdSe/ZnSe QW structure has a transport resistance only a quarter that of, but a recombination resistance twice that of the ZnSe/CdSe heterojunction (HJ) structure, yielding much longer electron diffusion length, consistent with the resulting higher photovoltage, photocurrent, and fill factor. Time-resolved photoluminescence spectroscopy indicates dramatically reduced electron transfer from ZnO to the QW sensitizer, a feature which is conducive to charge separation and collection. This study together with the impedance spectra and intensity modulated photocurrent spectroscopies supports a core/shell two-channel transport mechanism in this type of solar cells and further suggests that the electron transport along sensitizer can be considerably accelerated by the QW structure employed.

Building high-efficiency CdS/CdSe-sensitized solar cells with a hierarchically branched double-layer architecture.

We report a double-layer architecture for a photoanode of quantum-dot-sensitized solar cells (QDSSCs), which consists of a ZnO nanorod array (NR) underlayer and a ZnO nanotetrapod (TP) top layer. Such double-layer and branching strategies have significantly increased the power conversion efficiency (PCE) to as high as 5.24%, nearly reaching the record PCE of QDSSCs based on TiO2. Our systematic studies have shown that the double-layer strategy could significantly reduce charge recombination at the interface between the charge collection anode (FTO) and ZnO nanostructure because of the strong and compact adhesion of the NRs and enhance charge transport due to the partially interpenetrating contact between the NR and TP layers, leading to improved open-circuit voltage (Voc) and short-circuit current density (Jsc). Also, when the double layer was subjected to further branching, a large increase in Jsc and, to a lesser extent, the fill factor (FF) has resulted from increases in quantum-dot loading, enhanced light scattering, and reduced series resistance.

All-solid-state hybrid solar cells based on a new organometal halide perovskite sensitizer and one-dimensional TiO2 nanowire arrays.

A novel organometal halide perovskite (CH3NH3PbI2Br) is synthesized and used as a visible light absorber to sensitize one-dimensional (1D) TiO2 nanowire arrays (NWAs) for all-solid-state hybrid solar cells. It achieved a power conversion efficiency (PCE) of 4.87% and an open circuit voltage (Voc) of 0.82 V, both higher than those of its analogue CH3NH3PbI3.

Density-controlled electrodeposition growth of zinc oxide nanorod arrays.

Density-controlled zinc oxide (ZnO) nanorod arrays were synthesized on indium tin oxide (ITO) substrates via a potentiostatic electrodeposition method under different conditions. The effects of preparing conditions on the density of nanorods were systematically investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). It is demonstrated that the density and diameter of ZnO nanorods can be effectively controlled by varying the electrodeposition parameters, such as electrodeposition potential, concentration of Zn2+ precursor and growth temperature. Pre-treatment of the substrate also has great influence on the morphology of the electrodeposited ZnO nanorods. By coating the ITO substrates with an gold (Au) or ZnO nanoparticle seed layer, ZnO nanorod arrays with an average diameter of about 50 nm were synthesized, and the nanorod density can be controlled to above 50 x 10(8) rod cm(-2), which is several dozen times greater than that of the ZnO nanorod arrays synthesized on original ITO substrates.

Electrodeposition of hierarchical ZnO nanorod-nanosheet structures and their applications in dye-sensitized solar cells.

We present a two-step electrochemical deposition process to synthesize hierarchical zinc oxide (ZnO) nanorod-nanosheet structures on indium tin oxide (ITO) substrate, which involves electrodeposition of ZnO nanosheet arrays on the conductive glass substrate, followed by electrochemical growth of secondary ZnO nanorods on the backbone of the primary ZnO nanosheets. The formation mechanism of the hierarchical nanostructure is discussed. It is demonstrated that annealing treatment of the primary nanosheets synthesized by the first-step deposition process plays a key role in synthesizing the hierarchical nanostructure. Photovoltaic properties of dye-sensitized solar cells (DSSCs) based on hierarchical ZnO nanostructures are investigated. The hierarchical ZnO nanorod-nanosheet DSSC exhibits improved device performance compared to the DSSC constructed using photoelectrode of bare ZnO nanosheet arrays. The improvement can be attributed to the enhanced dye loading, which is caused by the enlargement of internal surface area within the nanostructure photoelectrode. Furthermore, we perform a parametric study to determine the optimum geometric dimensions of the hierarchical ZnO nanorod-nanosheet photoelectrode through adjusting the preparation conditions of the first- and second-step deposition process. By utilizing a hierarchical nanostructure photoelectrode with film thickness of about 7 µm, the DSSC with an open-circuit voltage of 0.74 V and an overall power conversion efficiency of 3.12% is successfully obtained.