Irfan

Strong and Stable Doping of Carbon Nanotubes and Graphene by MoOx for Transparent Electrodes

MoO(x) has been used for organic semiconductor doping, but it had been considered an inefficient and/or unstable dopant. We report that MoO(x) can strongly and stably dope carbon nanotubes and graphene. Thermally annealed MoO(x)-CNT composites can form durable thin film electrodes with sheet resistances of 100 Ω/sq at 85% transmittance plain and 85 Ω/sq at 83% transmittance with a PEDOT:PSS adlayer. Sheet resistances change less than 10% over 20 days in ambient and less than 2% with overnight heating to 300 °C in air. The MoO(x) can be easily deposited either by thermal evaporation or from solution-based precursors. Excellent stability coupled with high conductivity makes MoO(x)-CNT composites extremely attractive candidates for practical transparent electrodes.

Work function recovery of air exposed molybdenum oxide thin films

We report substantial work function (WF) recovery of air exposed molybdenum oxide thin films with vacuum annealing. We observed a sharp reduction in the MoOx WF (from 6.8 eV to 5.6 eV) as well as a very thin layer of oxygen rich adsorbate on the MoOx film after an hour of air exposure. The WF of the exposed MoOx film started to gradually recover with increasing annealing temperature in vacuum, and the saturation in the WF recovery was observed at 450 °C with WF ∼6.4 eV. We further studied the interface formation between the annealed MoOx and copper phthalocyanine (CuPc). The highest occupied molecular orbital (HOMO) level of CuPc was observed to be almost pinned to the Fermi level, strongly suggesting the possibility of efficient hole injection with the vacuum annealed MoOx film.

Role of molybdenum oxide for organic electronics: Surface analytical studies

Extensive studies have been conducted on molybdenum oxide since it has outstanding properties as an insertion layer for efficient charge injection and extraction in organic semiconductor devices. Efficient charge transfer at semiconductor and electrode interface is one of the most crucial issues for the performance of organic electronic device. A lot of efforts have been spent to address this issue, but there are still many unclarified issues to understand the physical mechanisms. In this review, the authors summarize surface analytical investigations on the mechanisms that govern the effectiveness of the insertion layer. Measurement results on the electronic structure, composition, and morphology are presented. It is found that the high work function of MoOx is the dominant factor for the device performance improvement. Compromising environmental effects and methods to recover or prevent such effects are described. Finally, the criteria for MoOx insertion layer to be effective are provided by comparing t...

Effects of exposure and air annealing on MoOx thin films

Thermally deposited molybdenum oxide films are investigated with X-ray photo-emission spectroscopy, ultra-violet photoemission spectroscopy and inverse photoemission spectroscopy. The Interface between MoO x and copper pthalocynine (CuPc) is studied and the previously reported device performance improvement is explained, with the help of interface energy level alignment. The effect of oxygen and air exposure on MoOx films and growth of gap states with exposures are studied. The surface chemical compositions of MoOx films, of varying thicknesses from 1 nm to 50 nm, have also been investigated. For all the investigated film thicknesses, the thermally evaporated films are found to be oxygen deficient. It is believed, that the oxygen vacancies can be subdued to a great extent by annealing at elevated temperatures. We annealed the MoO x thin films in air, at 300 °C for 20 h, and investigated the changes induced by the air annealing.

Protection of MoO3 high work function by organic thin film

The effects of air exposure are investigated for molybdenum trioxide (MoO3) covered with organic thin films using ultraviolet photoemission spectroscopy. It is found that the severe drop of the work function of MoO3 by air exposure is substantially reduced by the organic thin films. Both CuPc and C60 are used for the investigations. The results indicate that the MoO3 surface can be passivated by approximately two monolayers of organic thin films against exposure to air.

Methods to protect and recover work function of air exposed transition metal oxide thin films

Insertion of high work function (WF) transition metal oxide (TMO) layers between the anode and the hole transport layer is established to substantially enhance the performance of organic light emitting diodes (OLED). The high WF of transition metal oxide layer has been demonstrated to be the most crucial for the enhancement. The WF of a TMO layer decreases substantially with air exposure, and noticeably by the ambient even inside a low vacuum system. In the present work we discuss various methods to protect and recover the high WF after a TMO thin film has been exposed to air. We report covering a thin organic layer on top of MoOx to protect the high work function. We found that a thin layer of 1-2 nm organic layer was sufficient to protect the work function of MoOx thin film underneath. We further report methods to recover already decreased TMO WF due to air exposure. We performed oxygen plasma cleaning of air exposed MoOx film and found out that oxygen plasma could substantially recover the WF of as deposited MoOx film. We also performed annealing of air exposed MoOx film inside an ultra high vacuum system and observed a thin layer of oxygenrich adsorbate layer, which desorbed upon annealing that in turn substantially recovered the MoOx WF. We discuss the vacuum annealing and the effect of resulting surface on the interface energy level alignment.

The effect of molybdenum trioxide inter-layer between indium tin oxide (ITO) and organic semiconductor on the energy level alignment

We investigated 0 to 300 A thick stepped molybdenum trioxide (MoO 3 ) inter-layer between in-situ oxygen plasma treated conducting indium tin oxide (ITO) and chloro-aluminum pthalocyanine (AlPc-Cl) layer-by-layer evaporated up to 228 A, with ultra-violet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES). The MoO3 inter-layers were observed to increase the surface workfunction. The workfunction increase was observed to saturate at 20 A of MoO 3 coverage. The increased surface workfunction causes hole accumulation and band bending in the subsequently deposited AlPc-Cl. A possible explanation of reduction in series resistance by the insertion of the MoO 3 insulating layer is discussed based on these observations and energy level alignment.

Improvement of Charge Transfer Between Electrode and Semiconductor by Thin Metal Oxide Insertion

Efficient charge transfer at semiconductor and electrode interface is one of the most crucial issues for the performance of any electronic device. A counter intuitive phenomenon of transfer improvement by insertion of a thin metal oxide film at the semiconductor and electrode interface has gained much attention recently. In this chapter, we will describe our understanding of the mechanism of performance improvement with such insertions based on our surface analytical investigations. We will start by introducing the measurement techniques utilized in our investigations. We will discuss results on the insertion of a thin layer of MoOx between indium tin oxide (ITO) and two well studied organic semiconductors, and demonstrate that the optimum insertion layer thickness is just a few nanometers. We will also illustrate the importance of high vacuum during the deposition of such insertion layers and the impact of exposure on device performance.