Opto-electronic properties of P3HT-nanotube composites

Polymer materials are expected to play a major role in the development of low cost opto-electronic devices. A major advantage of polymers is that they can be mixed with other polymers or nanomaterials in solution to form composites with large area internal junctions. By tuning charge exchange and storage in these junctions one can optimise the opto-electronic properties of these composites. One class of polymer-based composites that holds much promise is polymer-nanotube composites. [1–11] However, probing these properties in detail is not trivial. On the one hand, electronic characterisation that relies on the semiconducting response of the composites cannot be used for composites with nanotube concentration above the percolation limit because the composite’s response becomes metallic. On the other hand, at low nanotube concentrations the optical response of the composites is dominated by that of the polymer making optical characterisation ideal for high nanotube concentrations. [12]In this presentation we show how a combination of electrical and optical characterisations can be used to probe the response of charge at the polymer-nanotube bulk junctions. The samples examined were prepared by mixing P3HT and single wall nanotubes (SWNTs) from dichlorobenzene solutions. Processing and measurements were performed in ambient conditions while the samples were kept at dark between measurements. Current-voltage measurements on composites reveal good dispersion of nanotubes with a percolation threshold of about 0.75%wt. Using capacitance-voltage (C-V) measurements we show that charge trapped or released from the SWNTs can be probed. By varying the measurement frequency we can also assess the time response of the polymer-nanotube junctions.The optical response of the composites was studied using spectroscopic Ellipsometry and transient photoinduced absorption measurements. With the addition of SWNTs excitonic energy levels within the polymer density of states appear to quench progressively 1 faster and always in the sub 5ps timescale. The absorption spectra also show that the addition of nanotubes influences the packing of polymer chains. By probing the response of the composites at high SWNT concentrations using optical methods and at low concentrations using C-V, our method provides a unified approach for studies of composites.