Multi-Influences of Ionic Migration on Illumination-Dependent Electrical Performances of Inverted Perovskite Solar Cells

ZnO films are employed as the electron transport layers for perovskite solar cells. Such a device exhibits an ultralong time increase in <i>V</i>_oc (∼100 s) and <i>J</i>_sc (∼1000 s) and a weakening hysteresis under continuous illumination. Besides, a slow (∼20 s) <i>V</i>_oc decay when illumination is switched off is also observed. The electrical measurements performed under illumination and under voltage bias before being illuminated, suggest the influences of ionic accumulation/redistribution in causing above phenomena. Ionic accumulation happening in dark and ionic redistribution under illumination lead to band bending which affects the excitons separation and carrier extraction. These can account for the ultralong time increase in <i>V</i>_oc and <i>J</i>_sc as well as the slow <i>V</i>_oc decay. Also, the time-dependent photocurrent response under stepwise scan proves the presence of a capacitive effect in the device which can be dramatically reduced by the ionic redistribution under illumination. The ionic redistribution is also an important reason for the weakening hysteresis

Trapping Behaviors of Photogenerated Electrons on the (110), (101), and (221) Facets of SnO₂: Experimental and DFT Investigations

Spatial separation of photogenerated charges between different crystal facets has been observed in some semiconductor photocatalysts; however, the charge separation mechanism is still ambiguous. As a characteristic parameter of crystal facet, surface energy may be a crucial factor to dictate the flow of photogenerated charges. In this work, the relationship between surface energy and the flow mode of photogenerated charges is investigated by using model photocatalysts, including lance-shaped SnO₂ particles and dodecahedral SnO₂ particles. The former are enclosed by two kinds of crystal facets with a big gap in surface energy, while the latter are composed of two types of crystal facets with nearly equal surface energy. However, the experimental results exhibit that the photogenerated electrons flow to all exposed crystal facets <i>randomly</i> in both two kinds of SnO₂ nanocrystals, which is opposite to what has been observed in extensively investigated semiconductor photocatalysts including TiO₂, SrTiO₃, BiVO₄, BiOCl, and Cu₂O. Our results disqualify surface energy as an appropriate descriptor of preferential charge flow. Furthermore, the experimental results are confirmed by trapping energies and work functions calculated with the first-principles methods, which are proved to be more relevant parameters for describing the charge flow direction. Additionally, the trapping sites on each crystal facet are determined by charge analysis

Highly Efficient Flexible Perovskite Solar Cells Using Solution-Derived NiO_<i>x</i> Hole Contacts

A solution-derived NiO_<i>x</i> film was employed as the hole contact of a flexible organic–inorganic hybrid perovskite solar cell. The NiO_<i>x</i> film, which was spin coated from presynthesized NiO_<i>x</i> 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 NiO_<i>x</i>-based perovskite solar cell on an ITO-glass substrate, which is much higher than that of the perovskite solar cells using high temperature-derived NiO_<i>x</i> film contacts. The low-temperature deposition process made the NiO_<i>x</i> films suitable for flexible devices. NiO_<i>x</i>-based flexible perovskite solar cells were fabricated on ITO-PEN substrates, and a preliminary PCE of 13.43% was achieved

Construction of High-Quality SnO₂@MoS₂ Nanohybrids for Promising Photoelectrocatalytic Applications

High-quality three-dimensional (3D) hierarchical SnO₂@MoS₂ nanohybrids were successfully obtained via a facile but effective wet chemistry synthesis method. Meanwhile, the SnO₂@MoS₂ hybrid film was fabricated through an electrophoretic deposition method to promote photoelectrocatalytic (PEC) efficiency and solve the recovery problem. Compared with the pure SnO₂ and MoS₂ films, the SnO₂@MoS₂ heterostructures could decrease the rate of the photoelectron–hole pair’s recombination, which resulted in the superior PEC pollutant degradation and water splitting activities. Meanwhile, the SnO₂@MoS₂ hybrid films with well-defined 3D hierarchical configurations have large surface areas, abundant active edge sites, and defects on the basal surfaces, which were also advantageous for the PEC activities (for pollutant degradation, apparent rate constant <i>k</i> = 5.91 h^–1; for water splitting, onset potential = −0.05 V and current density = 10 mA/cm²). Therefore, the SnO₂@MoS₂ hybrid film proved to be a superior structure for PEC applications

Photocatalytic Degradation of Methyl Orange over Nitrogen–Fluorine Codoped TiO₂ Nanobelts Prepared by Solvothermal Synthesis

Anatase type nitrogen–fluorine (N–F) codoped TiO₂ nanobelts were prepared by a solvothermal method in which amorphous titania microspheres were used as the precursors. The as-prepared TiO₂ 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 TiO₂ nanobelts was measured by the reaction of photocatalytic degradation of methyl orange. Results indicate that the photocatalytic activity of the N–F codoped TiO₂ 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

New Insights into the Electronic Structure and Photoelectrochemical Properties of Nitrogen-Doped HNb₃O₈ via a Combined in Situ Experimental and DFT Investigation

The nitrogen-doping approach has been intensively adopted to improve various properties of metal oxides, especially for adjusting the energy band structure and extending the photoresponse range of oxide photocatalysts. However, the nitrogen doping behavior is still unintelligible and complex due to the diversity of compositions and crystal structures. In this work, new insights into the electronic structure and photoelectrochemical (PEC) properties of nitrogen-doped HNb₃O₈ were presented. On the one hand, we utilized an in situ experimental strategy to ascertain the effect of nitrogen doping on the energy band and photoelectrochemical (PEC) properties of HNb₃O₈ and nitrogen-doped HNb₃O₈ (N-HNb₃O₈). Their energy band level, donor densities, and interfacial charge transfer properties were studied by Mott–Schottky plots and electrochemical impedance spectroscopy. After nitrogen doping, the conduction band position is unusually descended by 0.23 eV, the valance band position is raised by 0.51 eV, the donor density (<i>N</i>_d) is increased from 3.71 × 10²¹ to 6.46 × 10²¹ cm^–3, and interfacial charge transfer efficiency is reduced, though. On the other hand, density functional theoretical calculations were also conducted, so as to understand the electronic structures of HNb₃O₈ and N-HNb₃O₈. After nitrogen doping, the electronic structure is modified due to the upshift of the valance band edge consisting of hybrid N 2p and O 2p orbitals and the downshift of the conduction band edge consisting of the H 1s and Nb 4d orbitals. Furthermore, these insights into the behavior of nitrogen-doped semiconductors have important guiding significance toward their potential applications

Activating the Single-Crystal TiO₂ Nanoparticle Film with Exposed {001} Facets

TiO₂ films consisting of single-crystal anatase nanoparticles with exposed {001} facets were fabricated from anodized TiO₂ nanotube arrays. The films’ photocatalytic activities were further activated and then enhanced (∼2.5 times) by removing F^– from the {001} facets. This study indicates that fluorine-free crystal surfaces are of great importance for the application of such kinds of single-crystal TiO₂ nanoparticle films with exposed {001} facets in related areas

Regulating Functional Groups Enhances the Performance of Flexible Microporous MXene/Bacterial Cellulose Electrodes in Supercapacitors

Ultrathin MXene-based films exhibit superior conductivity and high capacitance, showing promise as electrodes for flexible supercapacitors. This work describes a simple method to enhance the performance of MXene-based supercapacitors by expanding and stabilizing the interlayer space between MXene flakes while controlling the functional groups to improve the conductivity. Ti3C2Tx MXene flakes are treated with bacterial cellulose (BC) and NaOH to form a composite MXene/BC (A-M/BC) electrode with a microporous interlayer and high surface area (62.47 m2 g–1). Annealing the films at low temperature partially carbonizes BC, increasing the overall electrical conductivity of the films. Improvement in conductivity is also attributed to the reduction of −F, −Cl, and −OH functional groups, leaving −Na and −O functional groups on the surface. As a result, the A-M/BC electrode demonstrates a capacitance of 594 F g–1 at a current density of 1 A g–1 in 3 M H2SO4, which represents a ∼2× increase over similarly processed films without BC (309 F g–1) or pure MXene (298 F g–1). The corresponding device has an energy density of 9.63 Wh kg–1 at a power density of 250 W kg–1. BC is inexpensive and enhances the overall performance of MXene-based film electrodes in electronic devices. This method underscores the importance of functional group regulation in enhancing MXene-based materials for energy storage