The photoinduced energy transfer dynamics in poly[9,9-di-(2-ethylhexyl)- fluorenyl-2,7-diyl]/6,6-phenyl-C61-butyric acid methyl ester (PDFA/PCBM) and poly(9,9-dioctylfluorenyl-2,7-diyl) (PF)/PCBM systems have been investigated. Ultrafast intra-chain energy transfer on the time-scale of a few hundred femtoseconds was found. A decay process that occurs in the picosecond time-scale may indicate an energy transfer process from the polymer to PCBM. This may be associated with a torsional relaxation (in the case of PF) or vibrational relaxation (in the case of PDFA). Bulk heterojunction solar cells have been fabricated using the polymer/PCBM systems. The cell made from PDFA/PCBM blend demonstrated a 30% higher conversion efficiency than the cell made from PF/PCBM blend, consistent with their absorption profiles.