Yiqiang Zhan

Efficient Spin Injection Through Exchange Coupling at Organic Semiconductor/Ferromagnet Heterojunctions

The schematic visualization of the Alq(3) molecule on the Fe substrate with the optimized geometry at lowest total energy. When the Alq(3) molecule is relaxed on the surface, only two of the wings are lying down on the Fe surface, and the third wing remains perpendicular to the surface, showing a strong hybridization occurance.

Materials capability and device performance in flexible electronics for the Internet of Things

The Internet of Things (IoT) has a broad vision of connecting every single object in the world to form one network. Flexible electronic devices, including RFIDs, sensors, memory devices, displays and power sources, are considered to be the technological basis of the IoT. The development of flexible electronic devices has been extremely rapid in the last decade. Many novel applications have been demonstrated, showing a strong potential impact on human life. In this review, we will summarize the recent progress in the research of flexible electronic devices and related flexible material within the framework of the IoT.

Aggregation and permeation of 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran molecules in Alq

The morphologic and luminescent behaviors of various 4-(dicyanomethylene)-2methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) containing films have been investigated. This involves pure DCM layers deposited on top of a tris-(8-hydroxyquinoline) aluminum (Alq) layer or positioned between two Alq layer, DCM-doped Alq films, and periodically arranged Alq/DCM multilayer structures. The occurrence over a period of time of aggregation and permeation of DCM molecules at room temperature and at temperatures as low as ∼260 K is found in all the cases studied. Such a phenomenon will result in degradation of related organic light-emitting devices and is closely related to the electric polarity of the DCM molecule.

Functionality-Directed Screening of Pb-Free Hybrid Organic–Inorganic Perovskites with Desired Intrinsic Photovoltaic Functionalities

The material class of hybrid organic–inorganic perovskites has risen rapidly from a virtually unknown material in photovoltaic applications a short 7 years ago into an ∼20% efficient thin-film solar cell material. As promising as this class of materials is, however, there are limitations associated with its poor long-term stability, nonoptimal band gap, presence of environmentally toxic Pb element, etc. We herein apply a functionality-directed theoretical materials selection approach as a filter for initial screening of the compounds that satisfy the desired intrinsic photovoltaic functionalities and might overcome the above limitations. First-principles calculations are employed to systemically study thermodynamic stability and photovoltaic-related properties of hundreds of candidate hybrid perovskites. We have identified in this materials selection process 14 Ge- and Sn-based materials with potential superior bulk-material-intrinsic photovoltaic performance. A distinct class of compounds containing NH3C...

Reversible air-induced optical and electrical modulation of methylammonium lead bromide (MAPbBr3) single crystals

The photoluminescence (PL) variations of organic-inorganic hybrid lead halide perovskites in different atmospheres are well documented, while the fundamental mechanism still lacks comprehensive understandings. This study reports the reversible optical and electrical properties of methylammonium lead bromide (MAPbBr3 or CH3NH3PbBr3) single crystals caused by air infiltration. With the change in the surrounding atmosphere from air to vacuum, the PL intensity of perovskite single crystals decreases, while the conductivity increases. By means of first-principles computational studies, the shallow trap states are considered as key elements in PL and conductivity changes. These results have important implications for the characterization and application of organic-inorganic hybrid lead halide perovskites in vacuum.

Temperature-dependent electronic properties of inorganic-organic hybrid halide perovskite (CH3NH3PbBr3) single crystal

In this paper, the temperature-dependent electronic properties of inorganic-organic hybrid halide perovskite (CH3NH3PbBr3) single crystals are investigated. The dynamic current-time measurement results at different temperatures directly demonstrate that the electrical properties of the perovskite single crystal are dependent on the work temperature. We find that the Poole-Frankel conduction mechanism fits the current-voltage curves at small bias voltage (0–1 V) under darkness, which is mainly attributed to the surface defect states. The capability of carriers de-trapping from defects varies with different work temperatures, resulting in an increased current as the temperature increases under both darkness and illumination. In addition, the different transient photocurrent responses of incident light at two wavelengths (470 nm, 550 nm) further confirm the existence of defect states on the single crystal surface.

Hydrazinium Salt as Additive To Improve Film Morphology and Carrier Lifetime for High-Efficiency Planar-Heterojunction Perovskite Solar Cells via One-Step Method

One-step solution process is the simplest method to fabricate organic-inorganic metal halide perovskite thin films, which however does not work well when employed in the planar-heterojunction (PHJ) solar cells due to the generally poor film morphology. Here we show that hydrazinium chloride can be used as an additive in the precursor solution to produce perovskite films featuring higher coverage and better crystallinity. The light absorption ability and charge carrier lifetime are both significantly improved accordingly. Under the optimal additive ratio, the average power conversion efficiency (PCE) of the inverted PHJ perovskite solar cells greatly increases by as much as 70%, and the champion device shows a satisfying PCE of 12.66%. These results suggest that N2H5Cl is a promising additive for fabricating high-efficiency perovskite solar cells via one-step method, which could be of interest in the future commercial solar cell industry.

Effects of side groups on the kinetics of charge carrier recombination in dye molecule-doped multilayer organic light-emitting diodes

The carrier recombination coefficient (gamma) in dye molecule-doped multilayer organic light-emitting diodes was quantified by transient electroluminescence. It was found that gamma and device efficiency were both strongly dependent on the molecular structures of the dopants.

Obvious efficiency enhancement of organic light-emitting diodes by parylene-N buffer layer

A parylene-N (PPXN) buffer layer inserted between anode and organic layers in typical organic light-emitting diodes (OLEDs) based on N, N′-bis(naphthalene-1-yl)-N, N′-bis(phenyl) benzidine (NPB) and tris (8-hydroxyquinolato) aluminum (Alq3) results in significant enhancement of the current efficiency. The enhancement can be 1.7 times higher compared to that of the buffer-free structure. Analyzing indicates that the buffer plays a role of blocking both the hole and electron current. Whether hole injection can be enhanced depends on electron accumulation at the buffer-organic interface. Inserting the PPXN buffer modifies carrier balance in the device, leading to the observed efficiency enhancement.

Insertion of parylene-N films in electron transport layer: An effective approach for efficiency improvement of organic light emitting diodes

Efficiency of tris (8-hydroxyquinolato) aluminum (Alq3) based organic light emitting diodes has been obviously improved by inserting a parylene-N (PPXN) buffer layer with proper thickness and location in the Alq3 layer. For an optimized PPXN inserted structure, current efficiency of 6.89 cd/A and luminous power efficiency of 1.85 lm/W were achieved; both are 42% higher than the 4.84 cd/A and 1.30 lm/W of the buffer free structure, respectively. The light emitting spectra of 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) doped Alq3 (Alq3:DCM) strips showed the electron current enhancement resulted in the observed efficiency improvement of PPXN inserted devices. The device current evolutions were explained by nonuniform potential distribution in the organic layer and the tunneling barrier reduction.