Yan Shao

Efficient photovoltaic energy conversion in tetracene-C60 based heterojunctions

We report organic solar cells fabricated with small-molecule organic semiconductor tetracene/C 60 heterojunction as the photoactive layer. The external power conversion efficiency of the devices under AM 1.5 solar illumination at 100 mW/cm 2 s1 sund is 2.3±0.5% with relatively high open-circuit voltage sVoc=0.58±0.06 Vd compared to most of the other small-molecular donor-acceptor sD-Ad heterojunction solar cells reported so far. Using atomic force microscopy and x-ray diffraction we found that tetracene thin films consist of submicron-sized grains with rough surface and well defined molecular order. Therefore, using high mobility polycrystalline tetracene thin films for D-A heterojunction devices dramatically increases area of tetracene and C 60 interface for exciton diffusion to reduce the recombination.

White organic light-emitting diodes prepared by a fused organic solid solution method

This letter demonstrates a white organic light-emitting diode (OLED) with high color stability fabricated by using a single organic white-emitting layer. The dopants were introduced prior to the device fabrication process through organic solid solution process formed by high-temperature and high-pressure fusion process. A high band gap organic material, α-naphthlyphenylbiphenyl diamine, was adopted and precisely doped with several kinds of fluorescent dyes as the emitting material. The most important benefits of using this fused organic solid-solution technique are the precise control of dopants, ultrauniform mixture of dopants in the host, easy fabrication; and, as a result, the fabricated white OLEDs show extremely little color shift with increasing injection current. The simplified device fabrication process is believed to be beneficial to the white OLED display and lighting industrialization.

Enhancement of tetracene photovoltaic devices with heat treatment

An efficient photovoltaic heterojunction of tetracene and fullerene has been investigated, and high performance organic solar cells have been demonstrated by thermal deposition and successive heat treatment. After the heat treatment, the open circuit voltage of the devices was enhanced greatly and at the same time the photocurrent remained almost unchanged. The series resistance of the devices was reduced and the fill factor was slightly enhanced. Consequently, the power conversion efficiency was improved from 1.7% to 2.2%. The preliminary conclusion for this enhancement is due to the part crystallization of the tetracene layer and consequent morphological change, which were supported by atomic force microscopy images, absorption spectra, and x-ray diffraction analysis. The part crystallization results in increase in hole mobility as evidenced by hole mobility measurements.

Naturally formed graded junction for organic light-emitting diodes

In this letter, we report naturally-formed graded junctions (NFGJ) for organic light-emitting diodes (OLEDs). These junctions are fabricated using single thermal evaporation boat loaded with uniformly mixed charge transport and light-emitting materials. Upon heating, materials sublimate sequentially according to their vaporizing temperatures forming the graded junction. Two kinds of graded structures, sharp and shallow graded junctions, can be formed based on the thermal properties of the selected materials. The NFGJ OLEDs have shown excellent performance in both brightness and lifetime compared with heterojunction devices.

Dependence of the performance of the organic electroluminescent devices upon the deposition rate of organic thin films

Abstract Typical organic electroluminescent (EL) devices with triphenyldiamine derivative (TPD) and tris-(8-hydroxyquinoline) aluminum (Alq 3 ) as the hole transport layer and electron transport layer, respectively, have been fabricated. The EL properties as a function of the deposition rate of the organic materials were investigated. It was found that a TPD deposition rate of around 0.2–0.3 nm/s is the optimum for the maximum luminous efficiency. The optimum deposition rate of Alq 3 is 0.3–0.4 nm/s.

A novel asymmetric complex for organic electroluminescence

As a promising technology for ~at panel display organic electroluminescence EL# has attracted more and more attention since 0876[ Many emit! ting materials including low!weight molecules and polymers have been designed and utilized in fab! ricating organic light!emitting diodes OLEDs#[ In order to achieve a device with good performance the organic thin _lm should be amorphous and uniform[ Among the organic EL emitting materials tris7!hydroxyquinoline# aluminium Alq2# has been one of the most popular and e}ective mol! ecules with its excellent thermal stability and good performance forming amorphous _lms[ Because of the commercial prospects and the easy preparation of Alq2 enormous research about Alq2 and its devices has been carried out[ Two geo! metric isomers of Alq2 have been found one pos! sessing C2 symmetry called {facial| and the other possessing C0 symmetry called {meridianal|[ In fact only the meridianal isomer can be characterized in the solid phase[ It is reasonable to think that the asymmetric isomer is more competent in forming uniform _lms[ Our quantum chemistry calculation of these two isomers also indicated that the C2 sym! metry isomer is more unstable and tends to lose its ligands more easily[ However even the C0 sym!