Synthesis of ultra-thin MoΟ3 layers by Hot Wire ALD doped with H, and F
Each metal wire is surrounded by a layer of the corresponding oxide. When the wire is heated and if the melting point of the metal is higher than the sublimation temperature of the oxide, then a vapor is formed from metal oxides that can be deposited and give us the corresponding film. The exact stoichiometry of the deposited film depends on the atmosphere of the deposition. Thus, in the presence of hydrogen, oxygen sub-stoichiometric films are deposited. The electronic structure of these films differs significantly from that of stoichiometric ones because intermediate bands (IBs) are formed within their energy gap. Width and position of the IBs within the gap depend on the exact oxygen stoichiometry as well as the presence of impurities (e.g., H and / or F) which may be added during deposition.
Solar cells are based on the extraction of electrons and holes photo-produced inside a semiconductor. These carriers are collected from electrodes that may be either metallic or composed of transparent and conductive materials (e.g., SnO2, ZnO, ITO, etc.). The IBs can be used to align the energy levels of the semiconductor with those of the electrodes to facilitate the collection of carriers. The IB can also be used to empede one type of carrier from approaching the electrode and to facilitate the other. In particular, molybdenum oxide films are used as hole-transport layers (HTL).
Similar phenomena apply for light emitting diodes (LEDs) but now the carriers (electrons and holes) are injected into the semiconductor where they recombine producing photons. Again molybdenum oxide films are used as hole-transport layers
Details about solar cells and LEDs based on organic semiconductors and perovskites are given by the team of Dr. Vasilopoulou and Dr. Argitis.