Relating the physical structure and optical properties of conjugated polymers using neutron reflectivity in combination with photoluminescence spectroscopy

Abstract

Understanding the effect of physical structure and the role of interfaces is critical for gaining insight into the optoelectronic properties of conjugated polymers and their behavior in semiconductor devices such as organic light-emitting diodes and photovoltaic cells. We have developed an in situ neutron reflection measurement that allows the direct relationship between film photoluminescence and structure to be studied. In addition, we have found that by judicious deuteration of the conjugated polymers, the polymer/indium tin oxide (ITO) interface can be probed. Critically for both poly[2-(2-d17-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] and poly[9,9′-(2-d17-ethylhexyl)-2,6-fluorene] of thickness of order 140–150 nm on ITO, we found that a thermally stable low-density layer of 20 Å thickness was present between the polymer film and the ITO. The presence of the low-contact layer means that measurements involving these two families of polymers directly deposited onto ITO may need re-evaluating, and suggests why poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) may be so beneficial for polymer light-emitting diodes.

Side-Chain Influence on the Mass Density and Refractive Index of Polyfluorenes and Star-Shaped Oligofluorene Truxenes

“…We have also evaluated the density of poly[9,9'-(2-d17 -ethylhexyl)fluorene] (PF26) based on neutron reflectivity results 39 . The scattering length density (SLD) is defined as:…”

Multi-Scale Modeling of Bulk Heterojunctions for Organic Photovoltaic Applications

“…Scanning probe techniques mostly give information about the top surface, though some charge transport information can be derived from specialized scanning probe techniques (Groves et al, 2010;Pingree et al, 2009;Reid et al, 2008). The vertical segregation of materials can be determined non-destructively using refraction techniques (including ellipsometry (Campoy-Quiles et al, 2008;Madsen et al, 2011), X-ray (Andersen, 2009;Germack et al, 2010), and neutron scattering (Lee et al, 2010;Mitchell et al, 2004)). Alternatively, the composition of exposed surfaces can be determined using X-ray photoelectron spectroscopy (Xu et al, 2009).…”

Photovoltaic Polymer Materials

“…8.9. Of these four classes, the most widely studied and commercially significant have been the PPVs and PFs [45], although the PTs have recently garnered much interest due to their high hole mobility and performance in BHJ organic photovoltaic devices [37].…”