The efficiency of bulk heterojunction solar cells based on either
poly(9,9- dioctylfluorene-alt-bithiophene) (F8T2) or
poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl]
[3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl] (PTB7)
and PC61BM is found to significantly increase upon insertion of an
interlayer made of 4,7-diphenyl-1,10-phenanthroline (Bphen) between the active layer and the electron collecting electrode. That increase is
particularly high when LiF/Al electrodes are used in comparison with
Ca/Al. The obtained values surpass or compare to those reported for
cells based on the same polymers and PC71BM. In particular, our cells
of PTB7:PC61BM rival with most efficient BHJ polymer solar cells based
on PTBT:PC71BM and both optimized anode and cathode interlayers (8.42 %) and are among the highest efficient organic cells reported to date with conventional structures. The morphology of the BPhen interlayer was
investigated leading to rather unexpected nanostructures. We will
discuss the possible influences of this morphology on the devices
performance.

We acknowledge the financial support from FCT-Portugal under the
contracts M-ERA.NET/0001/2012 and UID/EEA/50008/2013 and for a PhD grant to JF