Photoresistance and photo induced current hysteresis in bulk heterojunction systems P3HT–PCBM–polymethine dye

Abstract

Organic dyes can be used as a polar dipole component to create strong local electrical field and promote exciton dissociation in polymer bulk heterojunction (BHJ) solar cells. Some polymethine dyes are characterized with strong IR absorption and therefore can significantly improve light energy harvesting characteristics in such devices due to extension of the absorption spectrum of the photoconducting layer to the IR region. In a view of these opportunities, photovoltaic properties of BHJ systems constituted with poly-3-hexylthiophene (P3HT) as an electron donor, 6,6-phenyl-C61-butyric acid methyl ester (PCBM) fullerene-type electron acceptor, and cation polymethine or merocyanine dyes were studied depending on the dye concentration and under different light excitation conditions. A new physical effect of photoinduced current hysteresis was observed in dye containing BHJ solar cells, when the dye concentration was in the range of 16–60%. The photocurrent hysteresis is detected only when simultaneously polythiophene and dye molecules are excited with light (white light excitation), while no current hysteresis can be seen in the dark or under selective excitation of the polymer component (490nm) or dye component (740nm) of the BHJ system. In addition, significant reduction of the photocurrent collected from the dye-containing devices under illumination with white light in comparison to a selective excitation of the polymer component only is observed.Therefore, the phenomenon can be described as an illumination induced photoresistance of the composite BHJ layer. It can be explained based on energy diagram consideration as a generation of transition dipole moments at polymethine dye molecules located close to the heterojunction and thus opposing donor–acceptor charge transfer events.