PTB7:PC71BM Bulk Heterojunction Solar Cells with Multiple Additives

Bulk-hetero junction organic solar cells with active layers based on [4,8-biz[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] and phenyl-C71-butyric-acid-methyl-ester (PC₇₁BM) with an additive of diiodooctane (DIO) has attracted a lot of attentions due to their high power conversion efficiencies over 9% [1]. The addition of DIO significantly improved the solar cell performance, which was considered due to the change of the nano-structure configulations by the introduction of DIO. On the other hand,  the introduction of dimethyl sulfoxide　(DMSO) into the dicholobenzen solution of PTB7:PC₇₁BM used to deposit the active layers [2]. However, the influence of the DMSO introduction into the PTB7:PC71BM:DIO systems have not yet been reported to the best of our knowledge.

In this work, active layers were fabricated by spin coating 1,2-dichlorobenzene (500 μl) solutions of PTB7 (5 mg): PC₇₁BM (7.5 mg) containing 0.369 mg (3wt%) of DIO and additional DMSO with different weight ratios to the total weight of PTB7:PC₇₁BM (12.5 mg) from 0 to 30 wt% (0 to 3.69 mg).  It was found that the solar cells with 13wt% (1.60 mg) of DMSO additives exhibited the best solar cell performance of an open circuit voltage of 0.72 V, short circuit current density of 15.63 mA/cm², fill factor of 0.49, and power conversion efficiency of 5.47%. The significant structural changes were found from the surface morphology observations of the active layers depending on the DMSO concentration as observed by atomic force microscope.

[1] Z. He et al., Nat Photonics 6, (2012) 591.

[2] Chu et al., Appl. Phys. Lett. 98, (2011) 253301.