IMPEDANCE SPECTROSCOPY AS A TOOL FOR MONITORING THE DRYING BEHAVIOR OF BULK HETEROJUNCTION SOLUTIONS FOR POLYMERIC PHOTOVOLTAIC DEVICES
Kevin A. Masser, Nayool Shin, Dean Delongchamp, Chad Snyder
National Institute of Standards and Technology
The drying behavior of bulk heterojunction [BHJ] solutions for polymeric photovoltaic devices is studied via impedance spectroscopy. Solutions of poly(3-hexylthiophene) [P3HT] and phenyl-C61-butyric acid methyl ester [PCBM] in chlorobenzene were prepared. Small amounts of diiodooctane (DIO), octanedithiol (ODT), or chloronaphthalene (CN) were added to alter the resultant BHJ film morphology and drying time. The solutions were blade-coated onto interdigitated electrodes, and the impedance was monitored during solvent evaporation. As P3HT and PCBM solidify into a film, the alternating electric field causes an interfacial charge layer to develop on any formed precipitates, which is observed in the measured impedance. Impedance spectroscopy is able to accurately measure the drying time, as well as the onset of precipitate formation, and therefore holds the potential for on-line monitoring and process optimization during commercial manufacturing of large-scale printable solar cells.