Tran Quang Trung

Enhancement of current-voltage characteristics of multilayer organic light emitting diodes by using nanostructured composite films

With the aim of improving the photonic efficiency of an organic light emitting diode (OLED) and its display duration, both the hole transport layer (HTL) and the emitting layer (EL) were prepared as nanostructured thin films. For the HTL, nanocomposite films were prepared by spin-coating a homogeneous solution of low molecular weight poly(4-styrenesulfonate) (PEDOT-PSS) and surfactant-capped TiO2 nanocrystals onto low resistivity indium tin oxide (ITO) substrates; for the EL, nancrystalline titatium oxide (nc-TiO2)-embedded Poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV+nc-TiO2) conjugate polymers were spin-coated onto the HTL. Also, for a shallow contact of Al/LiF/MEH-PPV instead of Al/MEH-PPV a super LiF thin film was deposited onto the EL by vacuum evaporation. The resulting multilayer OLED had the following structure of Al/LiF/MEH-PPV+nc-TiO2/PEDOT-PSS+nc-TiO2/ITO. Characterization of the nanocomposite films showed that both the current-voltage (I-V) characteristics and the phot...

The effects of hydrogen dilution on structure of Si:H thin films deposited by PECVD

Si:H thin films were deposited using PECVD method at substrate temperature of 250°C with different hydrogen dilution. Structure investigations and hydrogen concentration estimation were performed using FTIR, Raman and UV-VIS spectroscopy. The IR and Raman spectra show various peak shifting when H dilution is changed. Band gaps of films were evaluated from the strong absorption regions in UV-VIS transmission spectra. The results demonstrate a transition from amorphous to nc/μc structure when hydrogen dilution increased.

Enhancement of power efficiency and stability of P3HT-Based organic solar cells under elevated operating-temperatures by using a nanocomposite photoactive layer

With the aim to find out an enhanced operating-temperature range for photovoltaic device parameters, two types of the photoactive layer were prepared: poly(3-hexylthiophene) (P3HT) and P3HT+nc-TiO2 (PTC) thin films. The enhancement obtained for the photoelectrical conversion efficiency of the composite based OSCs is attributed to the presence of nanoheterojunctions of TiO2/P3HT. For the temperature range of 30-70°C, the decrease of the open-circuit potential was compensated by an increase of the fill factor; and the increase in the short-circuit current resulted in an overall increase of the energy conversion efficiency. At elevated temperatures of 60-80°C the efficiency of the P3HT- and PTC-based cells reached a maximum value of 1.6% and 2.1%, respectively. Over this temperature range the efficiency of P3HT-based OSC decreased strongly to zero, whereas for the PTC cells it maintained a value as large as 1.2% at the temperature range of 110-140°C. The improved thermal stability of the composite-based device was attributed to the lowered thermal expansion coefficient of the nanocomposite photoactive layer.

Study of nanostructured polymeric composites used for organic light emitting diodes and organic solar cells

Polymeric nanocomposite films from PEDOT and MEH-PPV embedded with surface modified TiO2 nanoparticles were prepared, respectively for the hole transport layer (HTL) and emission layer (EL) in Organic Light Emitting Diodes (OLED). The composite of MEH-PPV + nc-TiO2 was used for Organic Solar Cells (OCS). The results from the characterization of the properties of the nanocomposites and devices showed that electrical (I-V characteristics) and spectroscopic (photoluminescent) properties of the conjugate polymers were enhanced due to the incorporation of nc-TiO2 in the polymers. The OLEDs made from the nanocomposite films would exhibit a larger photonic efficiency and a longer lasting life. For the OSC made from MEH-PPV + nc-TiO2 composite, the fill factor (FF) reached a value as high as 0.34. Under illumination of light with a power density of 50 mW/cm2, the photoelectrical conversion efficiency (PEC) was found to be of 0.15% corresponding to an open circuit voltage VOC = 1.15 V and a short-cut circuit curre...

Enhancing the performance of organic light emitting diodes by using nanostructured composite films

Recent achievements on the use of nanocomposites such as nanostructured composites with a structure of nanoparticles embedded in polymers (NIP) and nanocomposites with a structure of polymers deposited on nanoporous thin films (PON) for OLEDs are presented in this report. The influence of nanooxides on the photoelectric properties of the NIPs is explained with regard to the fact that TiO2 particles usually form a type-II heterojunction with a polymer matrix, which essentially results in the separation of non-equilibrium electrons and holes. MEH-PPV luminescence quenching is strongly dependent on the nature of nanostructural particles embedded in polymer matrix. Actually, the higher quenching of the polymer fluorescence observed in the presence of titania nanoparticles proves that the transfer of the photogenerated electrons to the TiO2 is more efficient for rods. Characterisation of the nanocomposite films showed that both the current-voltage characteristics and the photoluminescent properties of the NIP nanocomposite materials were significantly enhanced in comparison with the standard polymers. OLEDs made from these layers demonstrate high photonic efficiency. For a PON-like hybrid layer of MEH-PPV/nc-TiO2, the photoluminescence enhancement was observed when the MEHPPV-PON films were excited by a 325 nm wavelength laser, and the excitation of a 470 nm wavelength light resulted in the strong polymer fluorescence quenching. Current-voltage characteristics of laminar layer devices with a structure of Ti/PON/Al-Ag in comparison with that of Ti/MEH-PPV/Al-Ag showed that the turn-on voltage of the devices was lowered considerably. Therefore, PONs are suitable for use in a reverse OLED, where the light goes out through a transparent or semi-transparent cathode; moreover, it is much easier to make ohmic contact to the metallic Ti electrode.