Investigation of Interfaces Formed of High Workfunction Metal Oxides with Conjugated Polymers

Author: Yanting Yin

Yin, Yanting, 2019 Investigation of Interfaces Formed of High Workfunction Metal Oxides with Conjugated Polymers, Flinders University, College of Science and Engineering

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A range of proficient polymer-donor and fullerene acceptor materials for active layers used in solar devices, such as polymer-based solar cells have been developed and the hole transport layer (HTL)/anode buffer layer (ABL) such as MoO3, works properly in such device and appropriate for solar materials engineering. However the charge transfer mechanism at the adjacent metal oxides/organic interfaces has not been fully understood due to the lack of technique to quantifying the dipole at the interfaces. Without a direct characterisation, the schematics of energy band shift and bending are ambiguous. Based on that, three main achievements will be shown in this research and can be applied in the solar cell fabricating and similar material engineering: (a) the technique of direct quantifying the dipole strength and its energetic distribution at the metal oxides/organics interfaces; (b) the working mechanism of charge transfer over the HTL/organics interfaces; (c) the characterisation of the changes upon dipole strength with different treatments, which occur in the device fabrication. To achieve the outcomes and a fundamental understanding of the materials properties and engineering of the interfaces formed with metal oxides and organics, a powerful combination of experimental techniques has been adopted to characterise the energetic and chemical properties of the materials and interfaces.

First, the nature of interface forming at MoO3/P3HT:PC61BM BHJ has been investigated. The mixing phase of components and formation of dipole at the interface are observed. Electron spectroscopy is employed to determine the strength of the dipole at the interface and the minimum deposition thickness to form a closed layer. The energy distribution at the interface is further exhibited. The work shows that upon evaporation of MoO3, the strength of the dipole increases with a higher thickness of the MoO3 layer and saturates at a thickness around 3nm at 2.2 eV. The methodology for a directly characterisation of dipole has been shown and the mechanism for charge transfer is discussed. As such the interfaces forming at V2O5/BHJ and WO3/BHJ are further studied and the comparison in-between has been made.

A range of the value of electrical properties such as workfunction of MoO3 has been reported once air exposure occurs. Investigation into the interface of MoO3/P3HT:PC61BM BHJ upon exposure has been made and both the WF and the dipole forming at such interface are significantly influenced. Electron spectroscopy shows the adsorption of mainly H2O on the metal oxide surface upon exposure, which also diffuses to the interface. The invasion of H2O dielectric affects both the electrical properties of MoO3 and the dipole formation at the interface by free- orientating and encountering the dipole direction. A comprehensive energy structure of the MoO3/BHJ interface upon air exposure has been constructed. From which, the charge injection mechanism can be eliminated after exposure for 30min. The samples of V2O5 and WO3 with conjugated P3HT:PC61BM exposed to atmosphere have been further studied. The nature is similar to MoO3/BHJ and the difference of energy structure construction upon exposure is illustrated. The finding is rather important for solar cell engineering due to the application of general fabrication.

A simulation of vacuum-annealing process of solar cell material engineering has been made on P3HT:PC61BM BHJ and the MoO3/BHJ interface, respectively. Our research shows that upon annealing of BHJ before deposition of MoO3, a sharp interface with less mixing phase can be achieved. The mean dipole strength keeps constant so that the charge transfer would not be affected. On the other side, annealing upon MoO3/BHJ interface causes the reduction of MoOx and a heavier penetration into the BHJ. The interface region has been broadened, however the mean dipole strength is noticeably reduced. The charge injection transport is thus not energetically supported, leading to a conventional “HOMO-VB” transport and the device performance can be altered.

The methodology for characterising the dipole is also applied on the interface formed with MoO3/TQ1:PC71BM BHJ. The penetration of MoO3 into BHJ is observed and the distribution of energy states at the interface is determined. It can be seen that a dipole with higher energy was formed at the MoO3/TQ1:PC71BM interface in comparison with that of MoO3/P3HT:PC61BM interface. The root of the difference has been addressed.

Keywords: spectroscopy, interface, dipole, organic, high workfunction metal oxide, hole transport layer

Subject: Engineering thesis

Thesis type: Doctor of Philosophy
Completed: 2019
School: College of Science and Engineering
Supervisor: Gunther G. Andersson