Yong Ding

Inside-out Ostwald ripening: A facile process towards synthesizing anatase TiO2 microspheres for high-efficiency dye-sensitized solar cells

A facile inside-out Ostwald ripening route to the morphology-controlled preparation of TiO2 microspheres is developed. Here, TiO2 hollow microspheres (HM) and solid microspheres (SM) are prepared by adjusting the volume ratio of isopropanol (IPA) to acetylacetone (Acac) in the solvothermal process. During the formation process of HM, precipitation of solid cores, subsequent deposition of outer shells on the surface of cores, and simultaneous core dissolution and shell recrystallization are observed, which validate the inside-out Ostwald ripening mechanism. Design and optimization of the properties (pore size, surface area, and trap state) of TiO2 microspheres are vital to the high performance of dyesensitized solar cells (DSSCs). The optimized TiO2 microspheres (rHM and rSM) obtained by post-processing on recrystallization, possess large pore sizes, high surface areas and reduced trap states (Ti3+ and oxygen vacancy), and are thus ideal materials for photovoltaic devices. The power conversion efficiency of DSSCs fabricated using rHM photoanode is 11.22%, which is significantly improved compared with the 10.54% efficiency of the rSM-based DSSC. Our work provides a strategy for synthesizing TiO2 microspheres that simultaneously accommodate different physical properties, in terms of surface area, crystallinity, morphology, and mesoporosity.

High-Efficiency and UV-Stable Planar Perovskite Solar Cells Using a Low-Temperature, Solution-Processed Electron-Transport Layer

Efficient metal-halide perovskite solar cells (PSCs) with a regular structure typically use high-temperature-processed TiO2 electron-transport layers (ETLs), which suffer from high electron recombination and inherent UV instability. Herein, we present low-temperature solution-processed lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI)-doped C60 (Li-C60 ) ETLs for high-efficiency and UV-stable planar PSCs with an n-i-p architecture. We found that simple Li-TFSI doping ensured a better energy match between the active layer and the cathode, considerably enhanced the electron mobility of the ETL, and even improved CH3 NH3 PbI3 crystallization finally to increase the power conversion efficiency (PCE) from 15.3 to 17.8u2009% with a minor hysteresis effect. Moreover, it was demonstrated that replacing TiO2 with Li-C60 resulted in PSCs that were much more stable under UV light under an air atmosphere with almost no degradation after 3000u2005h under a nitrogen atmosphere.

Surface states in TiO2 submicrosphere films and their effect on electron transport

Owing to their special three-dimensional network structure and high specific surface area, TiO2 submicrospheres have been widely used as electron conductors in photoanodes for solar cells. In recent years, utilization of TiO2 submicrospheres in solar cells has greatly boosted the photovoltaic performance. Inevitably, however, numerous surface states in the TiO2 network affect electron transport. In this work, the surface states in TiO2 submicrospheres were thoroughly investigated by charge extraction methods, and the results were confirmed by the cyclic voltammetry method. The results showed that ammonia can effectively reduce the number of surface states in TiO2 submicrospheres. Furthermore, in-depth characterizations indicate that ammonia shifts the conduction band toward a more positive potential and improves the interfacial charge transfer. Moreover, charge recombination is effectively prevented. Overall, the cell performance is essentially dependent on the effect of the surface states, which affects the electron transfer and recombination process.

Shape-controlled synthesis of single-crystalline anatase TiO2 micro/nanoarchitectures for efficient dye-sensitized solar cells

Single-crystal anatase TiO2 nanocrystals were assembled into different morphological TiO2 micro/nanoarchitectures, including TiO2 hollow microspheres (HM), solid microspheres (SM), and diamond-shaped nanocrystals (NR), by well-controlled solvothermal reactions. The morphology of the resultant TiO2 micro/nanoarchitectures, along with the micro/nanostructural shape, size and crystallinity, can be controlled by varying the concentration of the reactants. It was found that the formation process of TiO2 hollow microspheres might include the hydrolysis of titanium alkoxide with water from the Mannich reaction, the aggregation of TiO2 nanocrystals, the anisotropic crystal growth, and the Ostwald ripening. In addition, we have performed systematic density functional theory (DFT) calculations on the adsorption energy of ethylenediamine molecules on the exposed surfaces and investigated the formation mechanism of single-crystalline nanocrystals. Furthermore, the three TiO2 micro/nanoarchitectures (SM, HM, and NR) were used as photoanode materials in dye-sensitized solar cells (DSSCs) and demonstrated improved light harvesting efficiency and electron collection efficiency, which were much greater than conventional TiO2 nanoparticles under the same conditions. Combined with the large specific surface area and high light scattering ability, the HM-based photoanode achieves the highest power conversion efficiency of 10.66%. Meanwhile, our method provides a versatility for structural engineering of various targeted morphological products to prepare high performance photovoltaic device.

Incorporating 4-tert-Butylpyridine in an Antisolvent: A Facile Approach to Obtain Highly Efficient and Stable Perovskite Solar Cells

The synthesis and growth of CH3NH3PbI3 films with controlled nucleation is a key issue for the high efficiency and stability of solar cells. Here, 4-tert-butylpyridine (tBP) was introduced into a CH3NH3PbI3 antisolvent to obtain high quality perovskite layers. In situ optical microscopy and X-ray diffraction patterns were used to prove that tBP significantly suppressed perovskite nucleation by forming an intermediate phase. In addition, a gradient perovskite structure was obtained by this method, which greatly improved the efficiency and stability of perovskites. An effective power conversion efficiency (PCE) of 17.41% was achieved via the tBP treatment, and the high-efficiency device could maintain over 89% of the initial PCE after 30 days at room temperature.

Highly efficient and stable perovskite solar cell prepared from an in situ pre-wetted PbI2 nano-sheet array film

Due to its good pore-filling and morphology control, a two-step deposition technique has been widely used in perovskite film preparation. However, the initial PbI2 film used for the two-step deposition method is usually dense, crystalline and layered, leading to a poor quality perovskite film with a certain amount of PbI2 residue, thus resulting in an inferior stability and low power conversion efficiency (PCE) of the device. In this work, we developed an in situ pre-wetted PbI2 nano-sheet array film to achieve high-quality CH3NH3PbI3 (MAPbI3) perovskite films via a two-step deposition method. By introducing a small amount of hydrogen chloride isopropanol solution (IPA HCl) into the PbI2 precursor solution, an in situ pre-wetted PbI2 nano-sheet array film was obtained, and then a smooth, continuous, uniform, dense, and PbI2-free perovskite film was formed through a reaction with CH3NH3I. After optimization, the device prepared using the in situ pre-wetted PbI2 nano-sheet array film achieved a promising PCE of 19.19%. Moreover, it also had good reproducibility and long term stability, as the devices based on the in situ pre-wetted PbI2 nano-sheet array film retained over 80% of the initial PCE after exposing to ambient air for two months.

Two-dimensional organic-inorganic hybrid perovskite: from material properties to device applications

The two-dimensional (2D) perovskite (including pure-2D and quasi-2D) is formed by introducing large-group ammonium halides into conventional bulk perovskite. In the past twenty years, 2D perovskite materials were widely developed with the enriched species and advanced physical-knowledge in material characteristics as well as optoelectronic device applications. To review achievments in 2D perovskite, the fundamental mechanism and properties of 2D perovskite are introduced to offer insight into device performance. Moreover, the preparation methods of 2D perovskite films are summarized and compared. The latest successful applications of the 2D perovskite in the solar cells and light-emitting diodes fields, especially the advanced stability of 2D perovskite solar cells (PeSCs) and the efficient 2D perovskite light-emitting diodes (PeLEDs), are also achieved. Furthermore, the challenges and outlook of 2D perovskite materials are proposed.摘要二维(2D)钙钛矿材料(包括纯2D和准2D)是在传统意义上的三维钙钛矿晶格中插入大基团卤化铵形成的. 在过去的20年里, 二维钙钛矿材料种类不断丰富, 相关理论研究不断深入, 在光电器件领域的应用不断拓展. 本综述介绍了2D钙钛矿材料的基本形成机制和性能, 汇总和比较了2D钙钛矿薄膜的制备方法, 并给出了其在太阳电池以及发光二极管领域的应用实例. 最后, 提出了该领域亟待解决的问题, 以及未来的研究趋势.

Elucidating the dynamics of solvent engineering for perovskite solar cells

Researchers working in the field of photovoltaic are exploring novel materials for the efficient solar energy conversion. The prime objective of the discovery of every novel photovoltaic material is to achieve more energy yield with easy fabrication process and less production cost features. Perovskite solar cells (PSCs) delivering the highest efficiency in the passing years with different stoichiometry and fabrication modification has made this technology a potent candidate for future energy conversion materials. Till now, many studies have shown that the quality of active layer morphology, to a great extent, determines the performance of PSCs. The current and potential techniques of solvent engineering for good active layer morphology are mainly debated using primary solvent, co-solvent (Lewis acid-base adduct approach) and solvent additives. In this review, the dynamics of numerously reported solvents on the morphological characteristics of PSCs active layer are discussed in detail. The intention is to get a clear understanding of solvent engineering induced modifications on active layer morphology in PSC devices via different crystallization routes. At last, an attempt is made to draw a framework based on different solvent coordination properties to make it easy for screening the potent solvent contender for desired PSCs precursor for a better and feasible device.摘要光伏领域的研究者们在不断探索可以用于高效太阳能转换的新材料. 研究每种新型光伏材料的主要目的是通过简单的制造工艺和 较低的生产成本来实现更高的能量产出. 新兴的钙钛矿材料也在竞争行列之中. 通过不同的化学计量调控和工艺改进, 钙钛矿太阳电池在 过去的几年中实现了最高的光电转换效率, 这一技术已经成为未来能量转换材料的有力候选者. 到目前为止, 许多研究表明活性层的薄膜 质量在很大程度上决定了钙钛矿太阳电池的光电性能. 当前和潜在的用于制备良好活性层形貌的溶剂工程技术大体上是使用主要溶剂、 共溶剂(路易斯酸-碱加合方法)和溶剂添加剂来实现. 在这篇综述中, 我们详细讨论了多种已报道的溶剂工程动力学对钙钛矿太阳电池活 性层形态特征的影响. 目的是通过不同的结晶过程得到一个关于溶剂工程如何诱导钙钛矿太阳电池活性层形貌的清晰的认知. 最后, 我们 基于不同溶剂的配位性质绘制了一个基本框架, 便于筛选可用来制备钙钛矿前驱体的有效溶剂, 以获得性能更好和可行性更高的光伏器件.

Reducing the Universal “Coffee-Ring Effect” by a Vapor-Assisted Spraying Method for High-Efficiency CH3NH3PbI3 Perovskite Solar Cells

Organic-inorganic perovskite solar cells (PSCs) are one of the most attractive and efficient burgeoning thin-film photovoltaics. The perovskite films have been fabricated via lots of deposition methods, but these laboratory-based fabrication methods are not well-matched with large-area manufacture. Herein, spray coating as a deposition technique was explored to prepare perovskite films and break the bottleneck that plagued large-scale production. However, it is hard to reduce the notorious coffee-ring effect during the process of spraying perovskite films especially in a one-step spraying method. Thus, the vapor-assisted spraying method (VASM), namely, fabricating perovskite films through a vapor-solid in situ reaction between CH3NH3I vapor and sprayed PbI2 films, was creatively applied to the preparation of dense and uniform perovskite films. The surfaces of the sprayed PbI2 films were optimized by adjusting the wettability, viscosity, and contact quality via various methods such as the selection of solvent, solution concentration, and substrate temperature to inhibit the capillary flow and release the pinning contact line. The application of a component solvent could effectively crush the dense structure of the PbI2 film, optimizing the morphology of PbI2 films and reducing the influence of the coffee-ring effect. Integrating the above aspects, the optimized PbI2 films could form uniform perovskite films via an in situ reaction, and a best power conversion efficiency of 17.56% was achieved for planar structure PSCs, which is high among the PSCs fabricated by the spraying method. In addition, the VASM could be applied in the actual conditions for mass production, exhibiting excellent optical and electrical properties and paving the way of the commercialization of PSCs.