Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV16)
Publication date: 28th March 2016
Molybdenum-oxide (MoOx) thin-films have attracted a lot of attention in the past years due to their unique ability to act as interfacial layers in novel electronics and energy applications. In the work presented here, large tuning possibilities in the electronic and optoelectronic properties of MoOx thin-films deposited by means of reactive sputtering using different oxygen partial pressures, sputtering powers and annealing conditions are demonstrated.
MoOx thin-films deposited under low oxygen partial pressure (1.00x10-3 mbar) at high sputtering powers (250W) present a high conductivity of around 3.22 S.cm-1, indicating that although the [O]/[Mo] ratio is low (~2.57 as extracted from Rutherford Backscattering Spectrometry), a semiconductor characteristic is still present in the films. As the oxygen partial pressure increases by only ~4.00 x10-4 mbar, the conductivity of the resulting films drops by around 5 orders of magnitude (to 1.60x10-5 S.cm-1) having an [O]/[Mo] ratio of around 3.00. As the oxygen partial pressure is further increased up to 2.70 x10-3 mbar, the conductivity drops further and reaches values of insulating materials of around 4.00x10-10 S.cm-1 with an [O]/[Mo] ratio greater than 3.00. Optical absorption measurements show drastic changes mostly within the 0.60 eV - 2.50 eV spectral region as the oxygen concentration in the films is steeply increased. The observed effects are attributed to changes in the electron density at the defect band: as the oxygen partial pressure increases, electrons are released and empty out the defect band. High Resolution Transmission Electron Microscopy (HRTEM) confirms an amorphous structure of the as-deposited films [1].
Ultraviolet Photoelectron Spectroscopy (UPS) and X-Ray Photoelectron Spectroscopy (XPS) investigations are conducted for accessing information about the work function and surface composition of the thin-films. The X-ray spectra registered on the Mo 3d core level reveal how the oxidation state of Mo is affected by the partial presure of Oxygen during the sample preparation. The work function of the films increase with annealing temperature and Oxygen content, and span a tuning range of about 2 eV. The application of the MoOx thin-films in organic optoelectronic devices is further investigated by employing the films as hole transport layers in small molecule based solar cells. The work thus demonstrates an interesting and viable method for tuning the electronic and optoelectronic properties of MoOx thin-films, which can be applied in combination with a wide range of materials e.g. in novel flexible electronics and photovoltaics.
[1] A.L. Fernandes Cauduro, et al., Appl. Phys. Lett. 106, 202101 (2015)
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