Yanqiong Zheng

Enhanced figure of merit of a porous thin film of bismuth antimony telluride

A porous thin film of Bi0.4Te3Sb1.6 with an enhanced figure of merit of 1.8 at room temperature was fabricated by flash evaporation on an alumina substrate containing hexagonally arranged nanopores with an average diameter of 20 nm, separated by an average distance of 50 nm. The thermal conductivity was significantly reduced compared with standard Bi0.4Te3Sb1.6 films to 0.25 W/(m⋅K) with no major decrease in either the electrical conductivity (398 S/cm) or the Seebeck coefficient (198 μV/K). The reduction in thermal conductivity was rationalized using a model for the full distribution of the phonon mean free path in the film.

Very high open-circuit voltage of 5.89 V in organic solar cells with 10-fold-tandem structure

Organic solar cells (OSCs) based on chloroaluminum phthalocyanine (ClAlPc) as donor and fullerene C60 as acceptor with a multi-tandem structure were fabricated. We demonstrated very high open-circuit voltage (VOC) and enhanced power conversion efficiency (PCE) for the multi-tandem OSCs. Using a fivefold structure, we obtained PCE of 2.49% with a VOC of 3.50 V, in comparison with PCE of ∼2% and VOC of 0.72–0.81 V for the single device. We also fabricated a tenfold-stacked OSC showing an extremely high VOC of 5.89 V. The multi-tandem OSCs with very high VOC have great potential for applications in limited-area low-power electronics.

Concentration-dependent, simultaneous multi-wavelength amplified spontaneous emission in organic thin films using Förster resonance energy transfer

We have demonstrated amplified spontaneous emission (ASE) characteristics of binary blends with green and red laser dyes and ternary blends with three primary color laser dyes. The results show that the stimulated emission rate of donor can be balanced with the Forster resonance energy transfer from the donor to the acceptor by adjusting the doping concentration of donor or acceptor dye, leading to the realization of the multi-wavelength ASE. The binary blend emission is at 493 and 570 nm with threshold at 15.7 and 19.1 μJ/cm2, respectively, while the ternary blend emission is at 452, 510, and 596 nm with threshold at 22.0, 18.6, and 23.8 μJ/cm2, respectively.

Fabrication and characterization of double-sided organic light-emitting diodes using silver and nickel as the metal linking layer

Abstract In this work, a novel double-sided emitting organic light-emitting diode (DE-OLED) was developed, and the effect of several metal linking layers on such DE-OLEDs was investigated. To form a DE-OLED, a metal linking layer was thermally deposited between a bottom-emitting organic light-emitting diode (BE-OLED) and a top-emitting organic light-emitting diode (TE-OLED). A series of metal films of 10-nm Ag, 20-nm Ag, 10-nm Ag/10-nm Ni, and 20-nm Ni were used as the linking layers, and 20-nm Ag based DE-OLEDs show the highest current efficiency and luminance. Two types of DE-OLEDs, including monochromatic and dichromatic, have been fabricated successfully. The EL spectrum of the TE-OLED is significantly narrower than that of BE-OLED because of the microcavity effect in both monochromatic and dichromatic DE-OLEDs. The results indicate that the electroluminescent characteristics of the OLED devices at both (bottom and top) sides can be modulated independently by using the metal linking layer without sacrificing the brightness and color stability of the device.

Super color purity green organic light-emitting diodes with ZrO2/zircone nanolaminates as a distributed Bragg reflector deposited by atomic layer deposition

ZrO2/zircone nanolaminate thin films fabricated by atomic layer deposition were used for a distributed Bragg reflector (DBR) in green organic light-emitting diodes (OLEDs). It is found that the novel ZrO2/zircone DBR structure significantly improves the light purity of green OLEDs without interfering with intrinsic electroluminescence properties. The full width at half maximum (FWHM) of the EL spectral band for the green OLEDs decreases with respect to increasing the ZrO2/zircone pairs. The FWHMs of OLEDs with 0, 2, 4, and 6 pairs of ZrO2/zircone layers are 72 nm, 48 nm, 24 nm, and 12 nm, respectively. A super-narrow FWHM of 12 nm is achieved by using six pairs of the DBR structure. The EQE is increased from 10.7% to 16.1% with four pairs of ZrO2/zircone layers.

Comparative Study of the Influence of Photoluminescence Quantum Yield and Molecular Orientation of Wide-Energy-Gap Small Molecules on the Performance of Organic Schottky Junctions

In this paper, we comparatively investigate the influence of absolute photoluminescence quantum yield (PLQY) and molecular orientation of a series of wide-energy-gap donors, which are usually used in organic light-emitting diodes, on the performance of organic Schottky junctions (OSJs) with low donor concentration. This work shows that PLQY of donors plays a crucial role on the photocurrent and fill factor of these types of OSJs. The PLQY of donors demonstrates a negative effect on the short-circuit current (JSC) of OSJs. The molecular orientation of donors also influences the OSJ performance for both linearor planar-shaped molecules, and horizontal orientation helps to achieve a higher JSC. The influencing priority of molecule characteristics of donors is PLQY>orientation.

Improved performance of graphene by effectively removing surface poly-methyl methacrylate residual during the process of wet-etching transfer

ABSTRACT Ultraviolet (UV) treatment has been demonstrated to be an effective way to removing the poly-methyl methacrylate (PMMA) residual during the process of the wet-etching transfer of graphene. The UV/Ozone irradiation with appropriate time can effectively remove the PMMA residual, resulting in a performance improvement of graphene, including the surface morphology, carrier concentration, and sheet resistance. The work function of graphene could be tuned according to treatment time without loss to the transmittance of graphene. Furthermore, we have fabricated graphene-based OPVs with a power conversion efficiency of 3.33%, which increased by 18.7% compared to the OPV without graphene modification.

Ultrahigh-luminance organic light-emitting diodes using LiF/MgAg as cathode for the application of both surface emission

ABSTRACT The effect of different species of cathode on the performance of organic light-emitting diodes under direct current drive has been investigated in this paper. The organic light-emitting diode using LiF/Al and LiF/MgAg layer as cathode has been fabricated, and the luminance of device with cathode structure of LiF/MgAg was found to realize higher efficiency. The performance of the device was found to be improved greatly due to efficient electron injection from MgAg cathode. Moreover, a configuration of LiF/MgAg/IZO (indium zinc oxide) was developed, finding that both the top-emitting intensity and the bottom-emitting intensity achieve very-high luminance, and in this basis, transparent OLEDs with the extremely high-luminance of about 100,000 cd/m2 has been realized, which is considered as the highest luminance among the previous works.

Enhanced performance by organic electron transporting layers compared with inorganic layers for peroskite solar cells

ABSTRACT Firstly, we fabricated several planar peroskite solar cells (PSCs) with inorganic electron transporting layers (ETLs). However, the photovoltaic performance is not high. To improve the interface contact between active layer/ETL, several organic ETLs were applied. The power conversion efficiency (PCE) is enhanced for average 20%. The PSCs with inorganic 8-hydroxyquinolatolithium (Liq), lithium fluoride (LiF) and organic bathocuproine (BCP) as well as 1,3,5-tris(2-N-phenylbenzimidazolyl) benzene (TPBi) as ETL show PCEs of 5.28%, 2.57%, 6.78%, and 5.43%, respectively. The solution-processed (s-) and evaporation-processed (e-) ETLs were comparatively studied, however, the s-ETLs show relatively lower PCEs than the e-ETLs, mostly ascribed to the humidity-sensitivity of active layer of perovskite films. The surface morphology and water contact angles of ETLs were also investigated.