Subgap optoelectronic characterization of trap states in semiconducting polymers and their bulk heterojunction blends

Semiconducting polymers are expecting to find applications in polymer-based light- emitting diodes (PLEDs) and organic photovoltaic (OPV) cells.[1-2] Development of polymeric electronic devices, however, is hampered by the presence of defects that can act as charge traps. The presence of these traps lead to undesirable losses. Electrical energy is wasted in form of heat rather than light for PLEDs; in an OPV cell, solar energy is wasted as in form of heat, too, as traps act as recombination centers for charge carriers. Further improvement of polymeric devices, therefore, requires a deeper understanding of these defects. This proposal outlines an extremely sensitive technique, known as photothermal deflection spectroscopy, for measuring the subgap optical absorptions of semiconducting polymers and their bulk heterojunction blends. Measurement of subgap absorption can lead to the identification of traps, and provide direct measurements of their energetic positions, distributions, and densities. The outcome of this proposal should lead to better understanding of defects in polymeric semiconductors. Minimization of these defects (by, e.g., intentional doping) should reduce loss in PLEDs and OPV cells and lead to devices with improved performances.