[BMP]+[BF4]--Modified CsPbI1.2Br1.8 Solar Cells with Improved Efficiency and Suppressed Photoinduced Phase Segregation.

With the rapid progress in a power conversion efficiency reaching up to 26.1%, which is among the highest efficiency for single-junction solar cells, organic-inorganic hybrid perovskite solar cells have become a research focus in photovoltaic technology all over the world, while the instability of these perovskite solar cells, due to the decomposition of its unstable organic components, has restricted the development of all-inorganic perovskite solar cells. In recent years, Br-mixed halogen all-inorganic perovskites (CsPbI3-xBrx) have aroused great interests due to their ability to balance the band gap and phase stability of pure CsPbX3. However, the photoinduced phase segregation in lead mixed halide perovskites is still a big burden on their practical industrial production and commercialization. Here, we demonstrate inhibited photoinduced phase segregation all-inorganic CsPbI1.2Br1.8 films and their corresponding perovskite solar cells by incorporating a 1-butyl-1-methylpiperidinium tetrafluoroborate ([BMP]+[BF4]-) compound into the CsPbI1.2Br1.8 films. Then, its effect on the perovskite films and the corresponding hole transport layer-free CsPbI1.2Br1.8 solar cells with carbon electrodes under light is investigated. With a prolonged time added to the reduced phase segregation terminal, this additive shows an inhibitory effect on the photoinduced phase segregation phenomenon for perovskite films and devices with enhanced cell efficiency. Our study reveals an efficient and simple route that suppresses photoinduced phase segregation in cesium lead mixed halide perovskite solar cells with enhanced efficiency.

Hole transport free carbon-based high thermal stability CsPbI1.2Br1.8 solar cells with an amorphous InGaZnO4 electron transport layer.

Due to their low cost, tunable band gap and excellent thermostability, all-inorganic halide perovskites CsPbX3 (X = Br, I) have become a kind of promising photovoltaic material. However, compared to the organic-inorganic hybrid perovskite solar cells, the performance of CsPbX3 solar cells still needs to be improved. In this work, for the first time, we applied the sol-gel derived amorphous InGaZnO4 film as electron transport layers (ETLs) in CsPbX3-based devices. In these devices, the carbon electrode deposited by screen printing replaced the unstable hole transport layer and the expensive metal electrode to obtain hole transport free carbon-based devices, which significantly simplifies the preparation process and reduces the production cost. With the application of amorphous InGaZnO4 films, devices show a relatively high power conversion efficiency (9.07%) and excellent thermal stability. Compared with the reported CsPbX3 devices using SnO2 or TiO2 ETLs, the performance of amorphous InGaZnO4 based devices has been significantly improved. This work provides a promising route to prepare highly thermally stable all-inorganic perovskite solar cells using a-IGZO films.

Hole transport free flexible perovskite solar cells with cost-effective carbon electrodes.

Low temperature derived carbon electrodes are employed to fabricate low cost hole transport layer free perovskite solar cells, in which perovskite films annealed in glovebox and ambient air are used as the absorbers, respectively. Results suggest that the air annealed sample has bigger crystal grains and higher crystallinity, and the existence of a small amount of lead iodide which passivates grain boundaries contributes to a lower trap density. As a result, a maximum power conversion efficiency (PCE) of 13.07% was obtained on the air annealed device, which is higher than those of devices annealed in glovebox (11.25%). Furthermore, the stability of unencapsulated devices stored in wet (with humidity around 90% ± 5%) air atmosphere are investigated and the results prove that our devices exhibit good stability. In addition to rigid devices, flexible perovskite solar cells are also fabricated using the same procedure. The highest PCE of 11.53% is demonstrated on the champion flexible device, and 69% of its initial PCE can be maintained even after 2000 bending cycles with a bending radius of 2 mm. Our work provides a promising and simple rout for low-cost, air-stable, high-efficiency carbon perovskite solar cells for both large area production and flexible electronic devices industry.

Fabrication of Ag2O-Bi2Sn2O7 Heterostructured Nanoparticles for Enhanced Photocatalytic Activity.

Ag2O-Bi2Sn2O7 composites were prepared by a chemical co-precipitation method. The microstructural, morphological and optical properties of the as-prepared composites were characterized and studied. Effects of Ag2O contents on photocatalytic activity of the Ag2O-Bi2Sn2O7 composites were also investigated in detail. Compared with pure Bi2Sn2O7, the 0.03Ag2O-Bi2Sn2O7 composite exhibits the highest photocatalytic activity for the degradation of Rhodamine B aqueous solution under visible light irradiation. The enhanced photocatalytic activity of the Ag2O-Bi2Sn2O7 composite can be attributed to the existence of the Ag2O-Bi2Sn2O7 heterojunction, which is propitious to an effective separation of the photogenerated electron-hole pairs.

MoS2 P-type transistors and diodes enabled by high work function MoOx contacts.

The development of low-resistance source/drain contacts to transition-metal dichalcogenides (TMDCs) is crucial for the realization of high-performance logic components. In particular, efficient hole contacts are required for the fabrication of p-type transistors with MoS2, a model TMDC. Previous studies have shown that the Fermi level of elemental metals is pinned close to the conduction band of MoS2, thus resulting in large Schottky barrier heights for holes with limited hole injection from the contacts. Here, we show that substoichiometric molybdenum trioxide (MoOx, x < 3), a high work function material, acts as an efficient hole injection layer to MoS2 and WSe2. In particular, we demonstrate MoS2 p-type field-effect transistors and diodes by using MoOx contacts. We also show drastic on-current improvement for p-type WSe2 FETs with MoOx contacts over devices made with Pd contacts, which is the prototypical metal used for hole injection. The work presents an important advance in contact engineering of TMDCs and will enable future exploration of their performance limits and intrinsic transport properties.

Hole selective MoOx contact for silicon solar cells.

Using an ultrathin (∼ 15 nm in thickness) molybdenum oxide (MoOx, x < 3) layer as a transparent hole selective contact to n-type silicon, we demonstrate a room-temperature processed oxide/silicon solar cell with a power conversion efficiency of 14.3%. While MoOx is commonly considered to be a semiconductor with a band gap of 3.3 eV, from X-ray photoelectron spectroscopy we show that MoOx may be considered to behave as a high workfunction metal with a low density of states at the Fermi level originating from the tail of an oxygen vacancy derived defect band located inside the band gap. Specifically, in the absence of carbon contamination, we measure a work function potential of ∼ 6.6 eV, which is significantly higher than that of all elemental metals. Our results on the archetypical semiconductor silicon demonstrate the use of nm-thick transition metal oxides as a simple and versatile pathway for dopant-free contacts to inorganic semiconductors. This work has important implications toward enabling a novel class of junctionless devices with applications for solar cells, light-emitting diodes, photodetectors, and transistors.

Modulation of Photoinduced Phase Segregation and Stress-Driven Nanoscale Cracking in Hybrid Halide Perovskite Solar Cells.

The negative effect of photoinduced halide segregation (PIHS) on the properties of hybrid halide perovskites poses a major obstacle for its future commercial application. Therefore, the in-depth understanding of halide-ion segregation and its causes is an urgent and intractable problem. When PIHS reaches a certain threshold, it will aggravate the deterioration of the film surface morphology and form nanoscale cracks. Herein, the formation mechanism and types of cracks are revealed by exploring the stress distribution in the film. Using the femtosecond time-resolved transient absorption spectroscopy, the ultrafast formation of the iodine rich phase is observed, which appears earlier than the bromine rich phase. In addition, the introduction of organic ligand didodecyldimethylammonium bromide can significantly inhibit PIHS and improve the surface morphology of the film, which can promote the device efficiency from 9.63 to 11.20%. This work provides a novel perspective for the exploration of the PIHS.

Photovoltaic activity of ZnO nanorods arrays co-sensitized by CdS and CuInS2 quantum dots.

One-dimensional ZnO nanorods arrays were self-assembly grown on a ZnO thin film, and then CdS quantum dots were deposited on the ZnO nanorods arrays by a successive ionic layer adsorption and reaction process. Scanning electron microscopy and transmission electron microscopy results indicate that the CdS quantum dots can be uniformly deposited on the ZnO nanorods arrays and the thickness of the CdS shell can be controlled through varying the number of the adsorption and reaction cycle. For a typical sample prepared by the adsorption and reaction cycles of 10 times, the thickness of the CdS is about 4.0 nm. Monodispersed CulnS2 quantum dots with a size of 3.5 nm were synthesized by a solvothermal route and then deposited on the ZnO nanorods arrays coated with the CdS quantum dots by using an electrophoretic deposition technique. Optical and electrical properties indicate that the as-fabricated ZnO/CdS/CulnS2 heterojunction structure not only exhibits a high absorption of the incident light in visible region but also can reduce a leakage current as compared to the ZnO/CdS heterojunction structure. Electrical impedance spectroscopy is used to analyze the electrochemical reaction of the interfaces.

Photoinduced Phase Segregation Leading to Evident Open-Circuit Voltage Loss in Efficient Inorganic CsPbIBr2 Solar Cells.

The photoinduced phase segregation (PIPS) of mixed-halide perovskites (MHPs), due to halogen migration, has reaped considerable attention for its retroaction on film photostability and photovoltaic output. Nevertheless, the original mechanism is still unclear. Herein, taking the representative CsPbIBr2 material as an example, a confocal laser scanning microscope (CLSM) technique was adopted to track the PIPS and dark recovery procedures. Besides the aggregation of iodide-rich (I-rich) domains at grain boundaries (GBs), some sporadic iodide "islands" with a swifter light response also appear throughout the polycrystalline films. It illustrates again that GBs are not essential for iodide aggregation. Furthermore, the iodide "islands" have substantial influence on a device's open-circuit voltage (Voc), resulting in an obvious plunge in the first tens of seconds. Results reveal the internal reason for the failure to reach the larger Voc outputs expected from wide-bandgap perovskites. Importantly, this finding can help promote the exploration of an efficient means to stabilize MHPs.

Additive-assisted one-step formed perovskite/hole conducting materials graded heterojunction for efficient perovskite solar cells.

Solar cells based on organometallic perovskite materials have been intensively investigated as the most promising next-generation photovoltaic technology. The quality of perovskite film and the heterojunction between perovskite and charge transporting materials dominate the performance of resulting devices. Herein, we report a facile additive-assisted method to form perovskite/2, 2', 7, 7'-tetrakis (N, N-di-p-methoxyphenylamine)-9, 90-spirobifluorene (spiro-OMeTAD) graded heterojunction by one step instead of spin-coating two layers separately. The additives concentration in anti-solution is optimized to form a mixed layer where spiro-OMeTAD is dispersive in upper perovskite films with a vertical gradient, and a capping layer with appropriate thickness. The incorporation of spiro-OMeTAD in anti-solution tremendously improve the crystallinity of perovskite films while the graded heterojunction and the derived capping layer contribute to reduced interfacial losses. Moreover, poly(methyl methacrylate) as the second additive in anti-solution further passivates defects in perovskite films. As a result, we realize perovskite solar cells with a power conversion efficiency of 15.72% based on perovskite-graded heterojunction, which is far beyond the control devices. This study demonstrates an effective extension of heterojunction engineering to fabricate efficient perovskite solar cells using simplified procedures.

Low Temperature-Derived 3D Hexagonal Crystalline Fe3O4 Nanoplates for Water Purification.

Fe3O4 nanoplates were fabricated by an anodic oxidation process and a subsequent water assisted crystallization process at low temperature, which was found to be very efficient and environmentally friendly. The as-prepared Fe3O4 nanoplates have hexagonal outlines with a thickness of about 20 nm. Tremendous grooves were distributed on the entire surfaces of the nanoplates, making the two-dimension nanoplates have a unique 3D morphology. Transmission electron microscopy results confirmed that the single-crystalline nature of the nanoplates was well maintained. Owing to the unique structures and porous morphologies, the as-prepared 3D nanoplates show excellent ability for absorbing solar energy and absorbing organic pollutants, which can be utilized for cleaning up water. Moreover, the Fe3O4 nanoplates show good magnetic properties that enable them to be easily collected and recycled. We believe this study will inspire the application of Fe3O4 nanoplates with 3D structures in energy and environmental areas.

Facile synthesis of ZnO/CuInS2 nanorod arrays for photocatalytic pollutants degradation.

Vertically-aligned ZnO nanorod arrays on a fluorine-doped tin oxide glass substrate were homogeneously coated with visible light active CuInS2 quantum dots by using a controllable electrophoretic deposition strategy. Compared with the pure ZnO nanorod arrays, the formation of high-quality ZnO/CuInS2 heterojunction with well-matched band energy alignment expanded the light absorption from ultraviolet to visible region and facilitated efficient charge separation and transportation, thus yielding remarkable enhanced photoelectrochemical performance and photocatalytic activities for methyl orange and 4-chlorophenol degradation. The ZnO/CuInS2 film with the deposition duration of 80min showed the highest degradation rate and photocurrent density (0.95mA/cm(2)), which was almost 6.33 times higher than that of the pure ZnO nanorod arrays film. The CuInS2 QDs sensitized ZnO nanorod arrays film was proved to be a superior structure for photoelectrochemical and photocatalytic applications due to the optimized CuInS2 loading and well-maintained one-dimensional nanostructure.

Enhanced conversion efficiency in perovskite solar cells by effectively utilizing near infrared light.

Up-conversion ß-NaYF4:Yb(3+),Tm(3+)/NaYF4 core-shell nanoparticles (NYF NPs) with a high luminous intensity in the visible light region were synthesized by a hydrothermal reaction process. Photocurrent densities of the mesoscopic perovskite solar cells fabricated by incorporating up-conversion NYF NPs into the electron transporting layer are effectively enhanced. The effects of the thicknesses of the electron transporting layer and the weight ratio of up-conversion NYF NPs/TiO2 on the power conversion efficiency (PCE) of the as-fabricated devices were also investigated. The results indicate that the PCE of the optimized device achieves 16.9%, which is 20% higher than that of the device without introducing NYF NPs, and the steady-state PCE of the as-fabricated devices is close to its transient-state PCE. The up-conversion effect of NYF NPs is conducive to higher device performance rather than the nanoparticles as scattering centers to increase possible light absorption of the perovskite film or the electronic effect of the NaYF4 shell surface. These results can be further confirmed by finite-difference time-domain simulation. Photoluminescence results suggest that the multiphonon-assistance can accelerate the nonradiative recombination process at a lower temperature. Incorporating NYF NPs into the electron transporting layer opens a new approach to a promising family of electron transporting materials for mesoscopic perovskite solar cells.

Highly Efficient Flexible Perovskite Solar Cells Using Solution-Derived NiOx Hole Contacts.

A solution-derived NiOx film was employed as the hole contact of a flexible organic-inorganic hybrid perovskite solar cell. The NiOx film, which was spin coated from presynthesized NiOx nanoparticles solution, can extract holes and block electrons efficiently, without any other post-treatments. An optimal power conversion efficiency (PCE) of 16.47% was demonstrated in the NiOx-based perovskite solar cell on an ITO-glass substrate, which is much higher than that of the perovskite solar cells using high temperature-derived NiOx film contacts. The low-temperature deposition process made the NiOx films suitable for flexible devices. NiOx-based flexible perovskite solar cells were fabricated on ITO-PEN substrates, and a preliminary PCE of 13.43% was achieved.

Enhanced Conversion Efficiencies in Dye-Sensitized Solar Cells Achieved through Self-Assembled Platinum(II) Metallacages.

Two-component self-assembly supramolecular coordination complexes with particular photo-physical property, wherein unique donors are combined with a single metal acceptor, can be utilized for many applications including in photo-devices. In this communication, we described the synthesis and characterization of two-component self-assembly supramolecular coordination complexes (SCCs) bearing triazine and porphyrin faces with promising light-harvesting properties. These complexes were obtained from the self-assembly of a 90° Pt(II) acceptor with 2,4,6-tris(4-pyridyl)-1,3,5-triazine (TPyT) or 5,10,15,20-Tetra(4-pyridyl)-21H,23H-porphine (TPyP). The greatly improved conversion efficiencies of the dye-sensitized TiO2 solar cells were 6.79 and 6.08 respectively, while these SCCs were introduced into the TiO2 nanoparticle film photoanodes. In addition, the open circuit voltage (Voc) of dye-sensitized solar cells was also increased to 0.769 and 0.768 V, which could be ascribed to the inhibited interfacial charge recombination due to the addition of SCCs.

19.2% Efficient InP Heterojunction Solar Cell with Electron-Selective TiO2 Contact.

We demonstrate an InP heterojunction solar cell employing an ultrathin layer (∼10 nm) of amorphous TiO2 deposited at 120 °C by atomic layer deposition as the transparent electron-selective contact. The TiO2 film selectively extracts minority electrons from the conduction band of p-type InP while blocking the majority holes due to the large valence band offset, enabling a high maximum open-circuit voltage of 785 mV. A hydrogen plasma treatment of the InP surface drastically improves the long-wavelength response of the device, resulting in a high short-circuit current density of 30.5 mA/cm2 and a high power conversion efficiency of 19.2%.

Photocatalytic degradation of methyl orange over nitrogen-fluorine codoped TiO2 nanobelts prepared by solvothermal synthesis.

Anatase type nitrogen-fluorine (N-F) codoped TiO(2) nanobelts were prepared by a solvothermal method in which amorphous titania microspheres were used as the precursors. The as-prepared TiO(2) nanobelts are composed of thin narrow nanobelts and it is noted that there are large amount of wormhole-like mesopores on these narrow nanobelts. Photocatalytic activity of the N-F codoped TiO(2) nanobelts was measured by the reaction of photocatalytic degradation of methyl orange. Results indicate that the photocatalytic activity of the N-F codoped TiO(2) nanobelts is higher than that of P25, which is mainly ascribed to wormhole-like mesopores like prison, larger surface area, and enhanced absorption of light due to N-F codoping. Interestingly, it is also found that the photocatalytic activity can be further enhanced when tested in a new testing method because more photons can be captured by the nanobelts to stimulate the formation of the hole-electron pair.

Enhanced photocatalytic activity of ZnO microspheres via hybridization with CuInSe2 and CuInS2 nanocrystals.

ZnO microspheres sensitized by CuInSe(2) and CuInS(2) nanoparticles, which were synthesized by a solvothermal method and have a size about 20 and 3.5 nm, respectively, were used to a photodegradation of rhodamine B under an irradiation of mercury lamp. Results show that the photocatalytic activities of the ZnO/CuInSe(2) and the ZnO/CuInS(2) are much higher than that of the ZnO microspheres because of a formation of the heterojunction in two systems. It is also noted that the ZnO/CuInS(2) exhibits a higher photocatalytic activity than the ZnO/CuInSe(2), which is probably related to more suitable band gap to sunlight for CuInS(2) nanocrystals and the larger specific surface due to a small size. Particularly, the ZnO/CuInSe(2)/CuInS(2) shows the highest photocatalytic activities in all measured photocatalysts, which should be attributed to the formation of double heterojunctions among ZnO, CuInSe(2), and CuInS(2).