Effect of two-step annealing on the performance of ternary polymer solar cells based on P3HT:PC71BM:SQ

• We introduce the two-step annealing treatment to ternary PSCs.
• By optimizing the two-step annealing processes, we successfully obtained a relatively high performance of ternary PSCs.
• We performed a detailed analysis of the performance of two-step annealing treated ternary PSCs.
• We give the main reason for the enhancement or limitation of ternary PSCs performance.

The effect of two-step annealing, including solvent annealing and further-step thermal annealing on the inverted ternary polymer solar cells (PSCs), which are composed of poly(3-hexylthiophene-2,5-diyl) (P3HT):[6,6]-phenyl C71-butyric acid methyl ester (PC71BM):2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ), was systematically investigated. The film morphology of ternary blends was characterized by X-ray diffraction and atomic force microscope. The results showed that a dramatic enhancement of power conversion efficiency from 1.32% to 4.42% was obtained due to the improved light harvesting, optimized self-assembled morphology and thermally induced crystallization of ternary blends. Especially, the optimized temperature of the thermal annealing process was remarkably decreased to 80 °C. It was also found that the two-step annealing process was crucial in achieving high performance, which could not only control the domain size of SQ but also maintain a well-ordered nanoscale structure within the ternary blends of P3HT:PC71BM:SQ.