High work-function metal oxides, such as molybdenum trioxide (MoO3), have been used to improve charge extraction properties of the contacts and reduce surface recombination (i.e. extraction of the wrong carrier type) in organic photovoltaics based on low mobility materials such as polymer fullerene blends. However, research pioneered by Professor Ronald Österbacka and colleagues from the Åbo Akademi University in Finland showed that MoO3 molecules can diffuse from a thin interfacial layer at the anode through the whole active layer causing unintentional doping. The doping action contributed by MoO3 results in the formation of a depletion region and a neutral (field free) region in the active layer. This doping effects present dire consequences in the whole device performance, since only charges generated in the depletion region contribute to the current. Their work is now published in the peer-reviewed journal Advanced Material Energy.
In order to investigate the doping process caused by the metal oxide, the researchers had to conduct capacitance-voltage measurements. Using Mott-Schotty analysis, the research team was able to demonstrate that the doping-induced capacitive regime Charge Extraction by a Linearly Increasing Voltage (doping-CELIV) technique could be applied in determining the doping concentrations and built-in potentials in sandwich-type diode structures. This was possible since the depletion region width was smaller than the thickness of the device.
Devices without the molybdenum trioxide showed a flat response in the transients that were normalized to the displacement current from the charging of the geometrical capacitance. This indicated very low doping concentrations since here the molybdenum trioxide had been replaced by another compound. A parallel experiment was also being conducted that included the use of the molybdenum trioxide interlayer which showed drastic increase in current response due to its doping effect. It was now clear that the doping was caused by diffusion of MoO3 molecules from the contacts to the active layer.
The team also carried out Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) measurements on polymer-fullerene films in order to test whether the origin of the fixed space charge was the molybdenum trioxide that constantly diffused into the active layer. They observed that molybdenum was present on the surface of the film and had migrated through the whole active layer.
This paper shows that the unintentional bulk doping in diodes and solar cells is caused by molybdenum trioxide that diffuses from the thin interfacial layer past the whole active layer. Such extent of doping is extremely detrimental for any device performance.
M. Nyman, S. Dahlström, O. J. Sandberg, R. Österbacka. Unintentional Bulk Doping of Polymer-Fullerene Blends from a Thin Interfacial Layer of MoO3. Adv. Energy Mater. 2016, 6, 1600670.
Physics/Faculty of Science and Engineering and Center for Functional Materials, Åbo Akademi University, Turku, Finland.Go To Advanced Energy Materials