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 reactive sputtering using different oxygen partial pressures and annealing conditions are demonstrated along with the implementation of the films in organic photovoltaics. MoOx thin-films deposited under low oxygen partial pressure (1.00x10-3 mbar) present a high conductivity of around 3.22 S.cm-1, however, 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 and the [O]/[Mo] ratio changes from 2.57 to 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 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 thin films. Ultraviolet (UPS) and X-Ray (XPS) Photoelectron Spectroscopy 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 pressure 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 close to 2 eV. Low Energy Electron Microcopy (LEEM) is in addition applied to extract the spatially resolved work function values from the sputtered MoOx thin-films. Finally, the application of the MoOx thin-films in organic optoelectronic devices is demonstrated by employing them as hole transport layers in DBP:C70 based small molecule solar cells.

Crystalline MoOx thin-films as hole transport layer in organic solar cells

253rd National Meeting (2017)

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