Ping-Ping Sun

Core–Shell Bimetallic Carbide Nanoparticles Confined in a Three-Dimensional N-Doped Carbon Conductive Network for Efficient Lithium Storage

Carbides represent a class of functional materials with unique properties and increasing importance. However, the harsh conditions in conventional synthetic strategies impede subtle control over size and morphology of carbides, which is highly imperative for their practical applications. Herein, we report a facile, simple approach to prepare porous Co3ZnC/N-doped carbon hybrid nanospheres. In this structure, the Co3ZnC nanoparticles exhibit a core-shell structure and they are uniformly confined in N-doped carbon conductive networks forming rather uniform nanospheres. The hybrid nanospheres have a specific surface area as high as 170.5 m(2) g(-1). When evaluated as an anode material for lithium ion batteries, they show an excellent lithium storage performance, which can be attributed to the combined effect of the core-shell Co3ZnC nanoparticles, the pore structure and the highly conductive and elastic N-doped carbon networks. This work provides an efficient route for the facile production of nanoscale carbides with desirable manipulation over size and morphology for many of important applications.

Dynamic Characteristics of Aggregation Effects of Organic Dyes in Dye-Sensitized Solar Cells

Two organic dyes (LS-1 and IQ4) containing identical electron donor and acceptor units but distinct π units result in significantly different power conversion efficiency of the corresponding dye-sensitized solar cells (DSSCs): LS-1, 4.4%, and IQ4, 9.2%. Herein, we combine first-principle calculations and molecular dynamics to explore the aggregation effects of LS-1 and IQ4 by comparing their optical properties and intermolecular electronic couplings. The calculated absorption spectra are in good agreement with the experimental observations and reveal them to be evidently affected by the dimerization. Furthermore, molecular dynamics simulations show that steric hindrance induced by the diphenylquinoxaline unit in IQ4 can elongate the distances between intermolecular π units or electron donors, which are responsible for the fact that the intermolecular electronic coupling of LS-1 is about 10 times larger than that of IQ4. More importantly, the aggregated IQ4 remains almost perpendicular to the TiO2 surface, whereas LS-1 gradually tilts during the dynamic simulation, impacting electron injection and recombination in several ways, which clarifies why IQ4 leads to larger photocurrent and higher conversion efficiency. The deep understanding of the dye aggregation effects sheds new light on the complex factors determining DSSC function and paves the way for rational design of high-efficiency self-anti-aggregation sensitizers.

A strategy to improve the efficiency of hole transporting materials: introduction of a highly symmetrical core

The electronic, optical and hole transport properties of three new hole transporting materials (HTMs) with a planar core have been investigated by using density functional theory and Marcus theory. A reliable semi-rational formula was adopted to calculate the highest occupied molecular orbital (HOMO) levels of new HTMs. The results showed that the HOMO levels of new HTMs were 0.07-0.30 eV lower than those of Spiro-OMeTAD, and their absorption peaks appeared outside or close to the visible region and overlapped slightly with the absorption band of perovskites. Moreover, the Stokes shifts of the designing molecules were calculated to lie in a range from 72 to 124 nm, meaning that they could undergo large geometrical changes on excitation. More importantly, the hole mobility of new HTMs (0.099-0.27 cm2 v-1 s-1) was approximately two orders of magnitude higher than that of Spiro-OMeTAD (0.0056 cm2 v-1 s-1) due to strong hole coupling from a face-to-face packing pattern. Our results indicated that planar core-based HTMs could become potential candidates to replace the widely established Spiro-OMeTAD.

How to regulate energy levels and hole mobility of spiro-type hole transport materials in perovskite solar cells

Methoxyaniline-based organic small molecules with three-dimensional structure have been proven as the most promising hole conductor for state-of-the-art perovskite devices. A fundamental understanding of the electronic properties and hole transport behavior of spiro-CPDT analogues, which is dependent on the number and position of the -OCH3 groups, is significant for their potential applications as hole transport materials of perovskite solar cells. Our results from density functional theory calculations indicate that meta-substitution is more beneficial to reduce the highest occupied molecular orbital (HOMO) levels of molecules compared with ortho- and para-substitution. Furthermore, the hole mobility can be improved by ortho-substitution or mixed ortho- and para-substitution. Most interestingly, it is found that the improvement in hole mobility is at the expense of raising the HOMO level of spiro-CPDT analogues. These results can be useful in the process of designing and synthesizing excellent hole transport materials with suitable HOMO levels and high hole mobility.

A promising candidate with D-A-A-A architecture as an efficient sensitizer for dye-sensitized solar cells.

A series of metal-free organic dyes with electron-rich (D) and electron-deficient units (A) as π linkers have been studied theoretically by means of density functional theory (DFT) and time-dependent DFT calculations to explore the effects of π spacers on the optical and electronic properties of triphenylamine dyes. The results show that Dye 1 with a structure of D-A-A-A is superior to the typical C218 dye in various key aspects, including the maximum absorption (λmax =511 nm), the charge-transfer characteristics (D/Δq/t is 5.49 Å/0.818 e(-) /4.41 Å), the driving force for charge-carrier injection (ΔGinject =1.35 eV)/dye regeneration (ΔGregen =0.27 eV), and the lifetime of the first excited state (τ=3.1 ns). It is thus proposed to be a promising candidate in dye-sensitized solar cell applications.