Optical probes of semiconductor device function

Lead Research Organisation: University of Oxford
Department Name: Oxford Physics

Abstract

Understanding performance optimization in solution-processed semiconductor diodes for display, lighting, electronic and solar energy conversion applications requires knowledge of the properties of the individual layers in a device stack and how they interact with each other.

Electrical probes, such as current vs voltage (as a function of temperature and layer thickness), provide insight into charge carrier injection/extraction and transport but the influence of different layers can be difficult to disentangle. In particular, the field distribution among those layers and how it changes during device operation and degradation is important to establish. Theoretical approaches can be used but they require a detailed knowledge of many physical parameters that are not always available for new materials being screened for utility before synthesis scale-up begins. In addition, unique solutions may be hard to identify in such multi-parameter spaces.

Direct probes of the electrical fields are relatively limited but one optical approach has been used to good effect in the past by Professor Bradley's research group - specifically electroabsorption (EA) or DC Kerr spectroscopy. In a typical EA measurement, a combined ac and dc bias V equals Vdc plus Vacsin (wt) is applied to the device and resulting changes in the transmission of a probe beam are monitored using phase sensitive lock-in detection. The fractional change in transmission is proportional to the imaginary part of the third order DC Kerr susceptibility and the square of the electric field. Thus if one has previously determined the Dc Kerr for a particular material it is possible to deduce the value of Edc as a function of applied bias for that layer. In a multilayer stack one can then spectrally deconvolute the response for each layer and hence extract their individual internal electric fields. In addition, one can readily identify field screening and the onset of charge injection.

What makes this project especially timely is the recent strong progress with a number of device types using interlayer materials to enhance performance. These include the use by Professor Bradley and colleagues of CuSCN as a hole injection/collection layer for light emitting diodes and solar cells, of PEOz and related dipolar imines as electron collection layers for inverted architecture solar cells, of dipolar SAMs to create ohmic contacts in ultra high frequency diode rectifiers and of conjugated polyelectrolytes for electron injection/collection in LEDs and solar cells. No studies have been undertaken to date on these systems using EA. Furthermore, the technique can be applied to studies of the perovskite family of solar cell materials, for which Oxford has an unrivalled activity (led by Professor Henry Snaith). Moreover, with the advent of the EPSRC National Thin Film Cluster Facility (EP/M022900/1) of which Professor Bradley is a Co-I, there will be an additional opportunity to probe hybrid devices combining vacuum and solution processed organics, inorganics including 2-D materials, metal oxides and so on.

The availability of new and versatile solid-state light sources such as supercontinuum fibre lasers offers the opportunity to undertake a redesign and capability update of the electroabsorption measurement apparatus to provide additional versatility. Time-resolved measurements may also prove of interest to study situations where ion migration and/or charge trapping phenomena play a role.

The project will involve construction of a new EA apparatus and its use to probe the properties of multilayer devices incorporating novel interlayer materials. Materials for study will both be sourced from commercial suppliers and be supplied by a range of collaborators including the Sumitomo Chemical Company/Cambridge Display Technology Ltd.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509711/1 01/10/2016 30/09/2021
1734270 Studentship EP/N509711/1 01/10/2016 31/03/2020 Nikol Lambeva
 
Description The discovery of electroluminescence from conjugated polymers was followed by tremendous academic and commercial interest in the field of polymer optoelectronics. Conjugated polymer materials have been shown to possess beneficial qualities for device fabrication such as low-cost, large area manufacturing and the potential for realizing flexible applications. Fluorenes, in particular, are a class of conjugated polymers which have been realized in a number of applications such as organic light emitting diodes, solar cells and field-effect transistors. The widespread commercialization of such products critically depends on improving device efficiencies. This can be accomplished by both advancing device engineering and improving material design. For the realization of the latter, fundamental understanding of the interplay between chemical structure and photophysical properties is required. This motivated the work supported by this award which explored the effect of polyfluorene copolymerisation for improved material design. One approach to tailor the optoelectronic properties of conjugated polymers consists in the copolymerisation of different amounts of functional moieties into the polymer backbone. A copolymer structure can combine functionality without the unwanted phase separation often found in blends. The materials investigated were based on the polyfluorene poly(9,9-dioctylfluorene) (PFO). PFO constitutes a fascinating system in itself due to its ability to adopt several different chain conformations, therefore, enabling the study of film microstructure on fundamental photophysical properties. We have investigated the copolymerisation of PFO with three different functional moieties. Their effect on the optical and electrical characteristics of PFO was examined in order to establish the structure-property relationships that govern the observed modifications. Firstly, the hole charge transport of four fluorene-arylamine copolymers was investigated. It was shown that transport in these copolymers is controlled over a large range by the copolymerisation of different amounts of a lower ionisation potential transporting moiety. Next, an alternating fluorene-phenoxazine copolymer was studied. The copolymerisation of phenoxazine was demonstrated to lift the highest occupied molecular orbital (HOMO) level of polyfluorene leading to enhanced hole accepting and transporting qualities compared to the polyfluorene homopolymer PFO. Finally, the copolymerisation of PFO with a cyclometalated platinum complex was investigated. The heavy platinum atom increases spin-orbit coupling leading to the radiative decay of the triplet state. It was shown that at low temperatures kinetic frustration of the triplet state is lifted in the planarized beta phase of the copolymers demonstrating that triplets diffuse more effectively along highly conjugated polymer chains than along materials with short conjugation segments. We hope that this will provide valuable insight for future targeted material design.
Exploitation Route The outcomes of this funding can be used to guide future material synthesis for device applications.
Sectors Electronics

 
Description Platinum Containing Polyfluorene Copolymers 
Organisation Nanjing Tech University
Country China 
Sector Academic/University 
PI Contribution We performed a thorough spectroscopic characterisation of a series of platinum containing polyfluorene copolymers. We studied the phosphorescence spectral diffusion with temperature in the two phase conformations of the copolymers and analysed the results with the aim to learn more about the triplet exciton transfer in disordered polymer films with different conjugation lengths.
Collaborator Contribution The platinum containing polyfluorene copolymers studied were kindly provided by the research group of Professor Youtian Tao at Nanjing Tech University.
Impact It was shown that at low temperatures kinetic frustration of the triplet state is lifted in the planarized beta phase of the copolymers demonstrating that triplets diffuse more effectively along highly conjugated polymer chains than along materials with short conjugation segments.
Start Year 2019
 
Description Oxford Sparks Animation 
Form Of Engagement Activity Engagement focused website, blog or social media channel
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Schools
Results and Impact We have made an animation titled 'Soluble Semiconductors - A Revolution in Printing for the 21st Century?' with Oxford Sparks describing the research in our lab to the wider public. This is a resource available to teachers, students and interested individuals to engage with the exciting science taking place across Oxford University. It has had over 7000 views so far.
Year(s) Of Engagement Activity 2018,2019,2020
URL https://www.oxfordsparks.ox.ac.uk/content/soluble-semiconductors-revolution-printing-21st-century