The effect of acceptor end groups on the physical and photovoltaic properties of A–π–D–π–A type oligomers with same S, N-heteropentacene central electron donor unit for solution processed organic solar cells

• Synthesized a series of A–D–A small molecules SM1, SM2 and SM3 with S, N-heteropentacene core.
• Effect of end group acceptor on optical, electrochemical properties investigated.
• Power conversion efficiency of 6.64% for SM2:PC71BM was achieved.

Synthesis and characterization of novel acceptor–donor–acceptor small molecules with different end group acceptors such as dicyanovinyl, cyanoacetate and indenedione, denoted as SM1, SM2 and SM3, respectively, linked with same central S, N-heteropentacene donor is described. These small molecules are used as electron donor along with the PC71BM as an electron acceptor for the fabrication of solution processed bulk heterojunction organic solar cells. The optimized devices based on as cast SM1:PC71BM, SM2:PC71BM and SM2:PC71BM showed power conversion efficiency about 2.01%, 1.60% and 2.65%, respectively and improved to 5.64%, 4.24% and 6.64%, respectively, using active layer processed with two steps annealing, i.e. combination of thermal annealing and subsequent solvent vapor annealing. This increase in the power conversion efficiency was due to the enhancement in Jsc and FF attributed to the enhancement of the light harvesting ability of the active layer, which is reflected by favorable nanoscale morphology.

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