Xindong Shi

Highly conductive and uniform graphene oxide modified PEDOT:PSS electrodes for ITO-Free organic light emitting diodes

We have successfully obtained a highly transparent and conductive film by doping poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) with graphene oxide (GO) and sodium dodecyl benzene sulfonate (SDBS). The PEDOT:PSS hybrid film mixed with 0.02% GO:SDBS by weight exhibits a sheet resistance of 85 ohm per square and a transmittance of 87% at 550 nm. The improvement of conductivity is mainly attributed to the weakening of coulombic attraction between PEDOT and PSS by the functional groups on GO nanosheets, and forming an extended conductive network by connecting PEDOT chains with GO nanosheets. Using the optimized PEDOT:PSS hybrid film as an anode, indium tin oxide (ITO)-free organic light emitting diodes (OLEDs) have been demonstrated on glass and polyethylene naphthalate (PEN) substrates, showing better performance than that with ITO as the anode. This proves that such PEDOT:PSS and GO:SDBS hybrid films are promising alternatives to ITO for low cost flexible OLEDs.

Highly Efficient and Stable Electron Injection Layer for Inverted Organic Light-Emitting Diodes

A highly efficient and stable electron injection layer (EIL) for inverted organic light-emitting diodes (OLEDs) is developed. A 1 nm-thick Al is deposited between indium tin oxide cathode and commonly used Cs2CO3 EIL, which can significantly improve the stability. The Al may react with evaporated Cs2CO3 and form a much stabler Al-O-Cs complex, avoiding Cs oxidization by air according to X-ray photoemission spectroscopy measurement. When the Al is evaporated after Cs2CO3 layer, although such a Al-O-Cs complex also forms, the inferior electron injection at Al/4,7-diphenyl-1,10-phenanthroline interface leads to a joule heat-induced resistance that adversely affects the air stability of the device. It is expected that the developed Al/Cs2CO3 EIL promotes high efficiency and stable active-matrix OLEDs based on n-type thin film transistor.

Inverted Tandem Phosphorescence Organic Light-Emitting Diodes Based on

A multilayer charge generation unit (CGU) MoO₃/Al/Cs₂CO₃ has been developed for inverted tandem organic light-emitting diodes (OLEDs). The current efficiency of inverted tandem phosphorescence OLED with MoO₃ (8 nm)/Al (1 nm)/ Cs₂CO₃ (0.5 nm) CGU is almost twice as high as that of individual electroluminescent (EL) device. Charges generate at hole-transporting host material/ MoO₃ interface and MoO₃ bulk layer, and electrons are extracted by Cs₂CO₃/electon-transporting layer (ETL) interface. The significantly enhanced performance of the tandem device with MoO₃/Al/Cs₂CO₃ CGU compared with a MoO₃/Cs₂CO₃-based one may be attributed to preventing chemical reaction of MoO₃ and Cs₂CO₃ by the inserted ultrathin Al layer. Moreover, the current efficiency and EL spectrum of another inverted tandem OLED combining red and green stack further prove the effective charge generation and separation with the multilayer MoO₃/Al/Cs₂CO₃ CGU. It is expected that such a CGU is a promising candidate for inverted tandem white OLED for active matrix organic light-emitting diode display driven by an n-type thin-film transistor backplane.