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Title: Solution-processed metal oxide interlayers for hybrid organic-inorganic optoelectronic devices
Author: Morbidoni, Maurizio
ISNI:       0000 0004 7657 6575
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2017
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Research on optoelectronic devices based on organic semiconductors has seen a steady rise in the last 20 years. Recently the incorporation of inorganic materials such as transition metal oxides as charge transport layers in these devices was found to be highly beneficial in terms of energy level alignment, stability and lifetime. Roll‐to‐roll processing of organic electronics and the use of flexible plastic substrates poses some limitations on the deposition of these oxides: they should be processed using solution‐based methods and not require high temperature post‐deposition treatments. In this work, thin films of four different transition metal oxides (MoO3‐x, V2O5‐x, WO3‐x, CoOx) were deposited using facile solution processes. The chemistry of the precursor solutions was carefully chosen to ensure the formation of the oxide of interest without the need for high temperature post‐deposition treatments (i.e. < 150 °C). The oxides were incorporated in organic solar cells and light‐emitting diodes as hole transport layers and the effect of the solvo‐thermal processing conditions of the oxides on the behaviour of the devices was studied. All performance metrics were compared with those of poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), a widely adopted hole transport material. Great improvements in both types of devices were recorded: in solar cells, the power conversion efficiency was up to 22% higher with a WO3‐x interlayer when compared with devices incorporating PEDOT:PSS; in light‐emitting diodes, the luminous efficacy was 24% higher using a MoO3‐x interlayer instead of a PEDOT:PSS one. In addition to this, the possibility of improving the characteristics and performances of PEDOT:PSS by blending it with MoO3‐x was explored. Different degrees of mixing were investigated, and the effect of increasing MoO3‐x percentage in the PEDOT:PSS/MoO3‐x hybrid on the behaviour of optoelectronic devices was studied. When compared to simple PEDOT:PSS, the hybrid produced an increase of 10% in the power conversion efficiency of solar cells and of 23% in the luminous efficacy of light‐emitting diodes. This thesis is divided into six chapters. Chapter 1 provides an insight into the underlying principles of device operation together with a review of the main characteristics of transition metal oxides and their incorporation in organic electronic devices. Moreover, a detailed analysis of different solution‐based methods that can be utilised for their deposition is given. Chapter 2 lists the different materials, recipes, and characterisation techniques used in this work. Chapters 3‐5 contain the results obtained in this research: Chapter 3 focuses on MoO3‐x and V2O5‐x oxides, Chapter 4 deals with the PEDOT:PSS/MoO3‐x hybrids and finally Chapter 5 contains results from WO3‐x and CoOx. Conclusions are laid out in Chapter 6, together with ideas and prompts for future expansions of this work.
Supervisor: McLachlan, Martyn Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral