Songwang Yang

Forest-like TiO2 hierarchical structures for efficient dye-sensitized solar cells

Forest-like TiO2 hierarchical structures are prepared by the acid-assisted hydrothermal method without the use of any template or additive. The performances of dye sensitized solar cells (DSSCs) fabricated with the nanoforest structures are much higher than those assembled with nanorod array films. Crystalline nanorods and nanobranches act as preferential electron pathways for efficient charge collection. Nanobranches filling in the spaces among the nanorods increase the amount of dye-loading and enhance the light-harvesting property. Thus, our photoanode can take advantage of both fast electron transport and high surface area. DSSCs made of nanoforest films with a thin layer of nanocrystalline TiO2 as the adhesive show the maximum energy conversion efficiency. Without any mediator, we can also fix films on FTO glasses through physical effects, which suggests potential applications in flexible solar cells.

Template-free synthesis of hierarchical TiO2 structures and their application in dye-sensitized solar cells.

We demonstrate here the synthesis of a hierarchical TiO(2) architecture without any surfactants or templates. Two kinds of structure existed simultaneously, the ordered nanoarrays at bottom provided direct conduction pathway for photo generated electrons, while the upper micro-flowers consisted of nanobelt as building units increased the light harvesting ability as the scattering part. The formation mechanism of the hierarchical architecture has been proposed by studying the morphology evolution processes upon reaction time. The performance of dye-sensitized solar cells based on the obtained hierarchical anatase TiO(2) has been also studied, giving a J(SC) = 12.44 mA cm(-2), V(OC) = 0.64 V, FF = 69.05%, and η = 5.53%, which is superior than commercial TiO(2) (P25). The UV-vis results prove that the obtained morphology is beneficial to light-scattering and thus increases the light harvesting ability. This hierarchical TiO(2) structure offers great potential for the development of high-efficiency DSSCs.

Characterization of Perovskite Obtained from Two-Step Deposition on Mesoporous Titania.

The properties of perovskite films are sensitive to the fabrication method, which plays a crucial role in the performance of perovskite solar cell. In this work, we fabricate organo-lead iodide perovskite on mesoporous TiO2 films through two different two-step deposition methods, respectively, for the purpose of studying the crystal growth of perovskite film and its effect on light harvesting efficiency, defect density, charge extraction rate, and energy levels. The crystal growth exerts a significant influence on the morphology and hence the film properties, which are found to correlate with the performance of solar cells. It is found that vapor deposition of methylammonium iodide in the PbI2 lattice gives a more complete coverage on mesoporous TiO2 with a flatter surface and Fermi level closer to the middle of the band-gap, resulting in higher light absorption in the visible spectral region, lower defect density, and faster charge extraction, as compared to the sequential solution deposition. For this reason, the vapor-processed perovskite film achieves higher short-circuit photocurrent and power conversion efficiency than the solution-processed film.

A facile way to prepare nanoporous PbI2 films and their application in fast conversion to CH3NH3PbI3

In this report, we demonstrate a facile way to prepare PbI2 films with interpenetrating nanopores. The nanoporous PbI2 (n-PbI2) films were prepared by the solvent–solvent extraction (SSE) method, in which the DMF solvent was effectively extracted by isopropanol (IPA) within seconds, resulting in well-crystallized n-PbI2 films without annealing. The mechanism involved in the preparation of n-PbI2 films using the SSE method was studied further, and some universal rules for fabricating n-PbI2 films with the SSE method were proposed. The interpenetrating nanoporous morphology enabled fast penetration of the CH3NH3I (MAI) solution, so most of the PbI2 converted into CH3NH3PbI3 within 10 s even with a perovskite overlayer of 300 nm. Moreover, the perovskite layer was pinhole-free and smoother than that based on a conventional PbI2 film. Consequently, perovskite solar cells based on n-PbI2, with the setup as FTO/compact TiO2/bilayer CH3NH3PbI3/P3HT/Ag, delivered an impressive power conversion efficiency of 10.1%, compared with 5.9% for its counterpart based on a conventional compact PbI2 film. This work unveils the PbI2-morphology-related reaction kinetics in the two-step method, and will contribute to understanding the role PbI2 films play in the preparation of perovskites.

Fast and Controllable Crystallization of Perovskite Films by Microwave Irradiation Process

The crystal growth process significantly influences the properties of organic-inorganic halide perovskite films along with the performance of solar cell devices. In this paper, we adopted the microwave irradiation to treat perovskite films through a one-step deposition method for several minutes at a fixed output power. It is found that the specific microwave irradiation process can evaporate the solvent directly and heat perovskite film quickly. In comparison with the conventional thermal annealing process, a microwave irradiation process assisted fast and controllable crystallization of perovskite films with less energy-loss and time-consumption and therefore resulted in the enhancement in the photovoltaic performance of the corresponding solar cells.

Achieving high-performance planar perovskite solar cells with co-sputtered Co-doping NiOx hole transport layers by efficient extraction and enhanced mobility

Perovskite solar cells are some of the most promising photovoltaic devices and they have experienced extraordinary progress in efficiency and fabricating technologies. Herein, we explore the effect of Co-doped NiOx hole transport layers on the electronic structure and photovoltaic properties of PSCs, which were deposited onto FTO substrates via DC magnetron sputtering at room temperature. Appropriate Co-doping can slightly regulate the optical band gap and the Fermi level position, leading to an increased potential cell performance. By virtue of continuously adjusting the power loaded onto the Co target, we can obtain the optimal atomic ratio of the Co:NiOx hole transport layer, and the PSC based on Co:NiOx exhibited a 25% higher efficiency than its undoped counterparts (from 9.46% to 12.61%). Therefore, these results demonstrate that Co is an appropriate dopant and the PSCs based on Co:NiOx layers have a good performance.

Ultrasmooth Perovskite Film via Mixed Anti-Solvent Strategy with Improved Efficiency

Most antisolvents employed in previous research were miscible with perovskite precursor solution. They always led to fast formation of perovskite even if the intermediate stage existed, which was not beneficial to obtain high quality perovskite films and made the formation process less controllable. In this work, a novel ethyl ether/n-hexane mixed antisolvent (MAS) was used to achieve high nucleation density and slow down the formation process of perovskite, producing films with improved orientation of grains and ultrasmooth surfaces. These high quality films exhibited efficient charge transport at the interface of perovskite/hole transport material and perovskite solar cells based on these films showed greatly improved performance with the best power conversion efficiency of 17.08%. This work also proposed a selection principle of MAS and showed that solvent engineering by designing the mixed antisolvent system can lead to the fabrication of high-performance perovskite solar cells.

Room-temperature ferromagnetism in Cu-implanted 6H-SiC single crystal

200 keV Cu+ ions were implanted into 6H-SiC single crystal at room temperature with fluence of 8 × 1015 cm−2. No ferromagnetism (FM)-related secondary phase was found by the results of high-resolution x-ray diffraction and x-ray photoelectron spectroscopy. Positron annihilation lifetime spectroscopy results indicated that the main defect type was silicon vacancy and the concentration of it increased after Cu implantation. The room-temperature ferromagnetism was detected by superconducting quantum interference device. First-principles calculations revealed that the magnetic moments mainly come from the 2p orbitals of C atoms and 3d orbitals of Cu dopant. The origin of the FM has been discussed in detail.

Growth–regime–controlled synthesis of CdS–Bi2S3 and Bi2S3 nanocrystals during the dissolution–recrystallization processes

Inorganic composite nanocrystals have been widely investigated due to their novel properties and potential applications. However, to date, more effort is needed to suppress the homogeneous nucleation and self–growth of the second–component nanocrystals. In our work, we have introduced Bi3+ ions which can react with the CdS mother nanocrystals (NCs) during the dissolution processes of the mother NCs. Subsequently, the nanocrystals which recrystallize on the surfaces of the CdS mother nanocrystals are Bi2S3 nanocrystals rather than the mother ones due to the lower solubility of Bi2S3. Therefore, it is effective to avoid separation between the CdS mother nanocrystals and the second–component Bi2S3 nanocrystals. During this dissolution–recrystallization process, the growth regime can be facilely controlled from the thermodynamic control to the kinetic domination through adding more Bi3+ ions or changing the heating method from an oil bath to a microwave field. Consequently, the obtained CdS–Bi2S3 composite nanostructures can evolve from core–shell to tangentially–bonded structures in the oil bath, while tangentially–bonded CdS–Bi2S3 composite NCs and nest–shaped Bi2S3 NCs can be achieved in the microwave field.

Coaction and competition between the ferroelectric field effect and the strain effect in Pr0.5Ca0.5MnO3 film/0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 crystal heterostructures

The coaction and competition between the ferroelectric field effect and the strain effect in Pr0.5Ca0.5MnO3 (PCMO) film/0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 crystal heterostructures were studied. Based on different types of resistance-electric field hysteresis loops at various temperatures, it is clearly identified that the strain effect dominates over the ferroelectric field effect for temperature T above the charge-ordering temperature TCO of PCMO. With the strong localization of charge carriers for T < TCO, the ferroelectric field effect strongly competes with the strain effect and finally dominates over the latter for T < 0.8TCO. Moreover, the poling-induced strain effect is considerably enhanced by a magnetic field, demonstrating the important role of the phase separation in understanding the strain effect in such heterostructures.The coaction and competition between the ferroelectric field effect and the strain effect in Pr0.5Ca0.5MnO3 (PCMO) film/0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 crystal heterostructures were studied. Based on different types of resistance-electric field hysteresis loops at various temperatures, it is clearly identified that the strain effect dominates over the ferroelectric field effect for temperature T above the charge-ordering temperature TCO of PCMO. With the strong localization of charge carriers for T < TCO, the ferroelectric field effect strongly competes with the strain effect and finally dominates over the latter for T < 0.8TCO. Moreover, the poling-induced strain effect is considerably enhanced by a magnetic field, demonstrating the important role of the phase separation in understanding the strain effect in such heterostructures.