Toward Stabilization of Domains in Polymer Bulk Heterojunction Films

With the purpose of studying the effect of stabilizing film morphology on polymer photovoltaic cell performance, the morphology and characteristics of bulk heterojunction devices fabricated using binary blends of an azide-functionalized graft copolymer of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C 61 -butyric acid methyl ester (PCBM) were examined. A thermal, solid state reaction between the azide groups attached to P3HT and PCBM, confirmed by Fourier transform infrared spectroscopy (FTIR) and UV−vis spectrocopies, rendered the films largely insoluble and stabilized the morphology, as evidenced by limited growth of macroscopic crystals of PCBM over time, although transmission electron microscopy (TEM) analysis revealed no dramatic changes in morphology at the nanoscopic level. Photovoltaic (PV) devices prepared from these stabilized layers exhibited 1.85% power conversion efficiencies (PCE) which fell to 0.93% over 3 h at 150 °C, whereas native P3HT/PCBM devices, initially displaying 2.5% PCE dropped to 0.5% over the same period. The extent to which the morphology of the bulk heterojunction can be stabilized by this route is discussed.