Abstract
Organic optoelectronic device behaviour is heavily dependent on interfacial effects due to the device architecture and thickness. Interfaces between the inorganic electrodes and the active organic layers play a defining role in the all of the electronic and stability processes that occur in organic light emitting diodes (OLEDs) and organic solar cells (OPVs). Amongst the many interlayers introduced at these interfaces to improve charge carrier movement and stability, LiF has proven to be the most successful and it is almost ubiquitous in all organic semiconductor devices. Implemented at both top and bottom contact interfaces, doped into the charge transporting layers, and used as encapsulants, LiF has played major roles in device performance and lifetime. This review highlights the use of LiF at both top and bottom contacts in organic optoelectronics, discusses the various mechanisms proposed for the utility of LiF at each interface, and explores its impact on device lifetimes. From examples relating to charge carrier flow, interfacial electronic level modification, and interfacial stability, a comprehensive picture of the role of LiF in organic devices can be formed. This review begins with a brief overview of the role of the interface in OLEDs and OPVs, and the general properties of LiF. Then, it discusses the implementation of LiF at the top contact electrode interface, followed by the bottom substrate contact electrode, examining both performance and degradation effects in both cases.
Highlights
Organic optoelectronic device behaviour is heavily dependent on interfacial effects due to the device architecture and thickness
Semiconducting devices utilizing π-conjugated organic molecules have been of significant research interest for over 30 years, with generation hardware systems based on organic light emitting diodes (OLED), organic photovoltaics (OPV), organic transistors, organic memory devices, and organic sensors all being developed and optimized
It is common to describe the device lifetime using the time it takes to decay as some percentage of the initial performance, typically 50% or 80% of initial performance for OLEDs (t50 ) and OPVs (t80 ), respectively
Summary
Semiconducting devices utilizing π-conjugated organic molecules have been of significant research interest for over 30 years, with generation hardware systems based on organic light emitting diodes (OLED), organic photovoltaics (OPV), organic transistors, organic memory devices, and organic sensors all being developed and optimized. OLEDs are already available commercially in lighting and displays [1], and organic photovoltaics (OPVs) have reached efficiencies surpassing amorphous silicon, and approaching thin film crystalline Si solar cells [2,3] In all such molecular devices, heterojunctions between electrodes, interlayers, and active materials play a strategic role. LiF interlayers have significant impacts on the stability of organic devices, through control of interfacial chemical interactions and modifications of the interface morphology at either electrode. In this contribution, the aim is to highlight the use of LiF in organic optoelectronic devices to understand its impact on performance and stability. From examples of our own research and others relating to electrical behaviour, interfacial chemical interactions, and interfacial morphological changes, a comprehensive picture of the role that LiF plays at interfaces in devices can be made
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