Enhancing Seebeck Effects by Using Excited States in Organic Semiconducting Polymer MEH-PPV Based on Multilayer Electrode/Polymer/Electrode Thin-Film Structure

This article reports the experimental studies on using photoexcitation-generated excited states as a new approach to increase Seebeck coefficient as well as electrical conductivity in an organic semiconducting polymer MEH-PPV (poly(2-methoxy-5-(2′-ethylhexyloxy)- p -phenylene vinylene) with multilayer electrode/polymer/electrode thin-film architecture. Our experimental results show that a photoexcitation can lead to a large Seebeck coefficient of 305 μV/K in the ITO/MEH-PPV/Au device under the light intensity of 16 mW/cm 2 . Simultaneously, the electrical conductivity is increased to 8.2 × 10 –5 S/cm from the dark conductivity of 3.6 × 10 –6 S/cm. Clearly, using excited states can lead to a simultaneous increase on Seebeck coefficient and electrical conductivity, as compared to opposite phenomenon induced by doping: increasing electrical conductivity but decreasing Seebeck coefficient. Our experimental studies suggest that excited states can increase both the entropy difference between high and low-temperature surfaces through electron–phonon coupling and the charge density through dissociation to simultaneously enhance Seebeck coefficient and electrical conductivity in the ITO/MEH-PPV/Au device. The underlying mechanism can be attributed to the fact that excited states can largely affect the dominating electrical-transporting factor, namely, interchain electrical transport, but negligibly changes the dominating thermal-transporting factor, namely, interchain thermal transport in an organic semiconducting polymer. As a result, using photogenerated excited states presents a promising approach to develop high Seebeck effects in organic semiconducting materials based on multilayer electrode/polymer/electrode thin-film design.