Molecular-Scale and Nanoscale Morphology of P3HT:PCBM Bulk Heterojunctions: Energy-Filtered TEM and Low-Dose HREM

The performance of bulk heterojunction organic photovoltaic devices is critically dependent on the morphology of the active layer. Here we describe the combination of two electron microscopy techniques to quantitatively examine the molecular level structure and mesoscopic domain morphology of the active layer of poly(3-hexylthiophene):[6,6]-phenyl-C(61)-butyric acid methyl ester P3HT: PCBM bulk heterojunction solar cells. Energy-filtered transmission electron microscopy (EFTEM) revealed the nanoscopic, interpenetrating fibrillar structure of the phase separated blend, providing unique assignments of the P3HT-rich and PCBM-rich regions. Low-dose high-resolution electron microscopy (LD-HREM) provided direct images of the P3HT crystals and their orientation within the P3HT-rich domains. The high mobility [010] crystallographic direction of these crystals coincides with the P3HT fibril axis. Additionally, the width of the P3HT crystallite coincides with the width of the P3HT-rich fibril, and is less than that of P3HT crystals in comparably processed pure P3HT films. The local crystallite structure within the blend is commensurate with the constraints of the nanoscale interpenetrating morphology and confirms the intimate relationship between processing protocols, which define the mesoscale phase-separated domains, and the molecular level ordering within the domains, which determines local transport characteristics.