A Novel Sandwich Needlelike Structure in Annealed P3HT:PCBM Blend Films

A unique needlelike sandwich structure is observed in the P3HT:PCBM blend films after annealing at 220°C. The real-time observation of a growing needle indicates that the needle exhibits 7 μm/min longitudinal growth rate and no lateral growth. Both confocal fluorescence and Raman microscopic mapping measurements reveal that these needles have a PCBM core sandwiched between P3HT edges. According to the eutectic nature of P3HT:PCBM nature, when annealing at high temperature (～220°C), the aggregation of PCBM results in recrystallization of P3HT in the PCBM-depleted regions. These results will give clearer understanding of the melting, diffusion, and recrystallization behavior of the organic eutectic system.