Optical probes of semiconductor device function

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.