Improve exciton generation and dissociation by increasing fullerene content in the mixed phase of P3HT/fullerene

Designing polymeric bulk heterojunction (BHJ) morphology with good exciton separation efficiency is one of the key factors in obtaining high-efficiency polymer solar cells (PSCs). Cascaded energy formed by the energy level difference (ΔE) between aggregated phase and the mixed phase is thought to encourage spatial separation of the geminate charge pair. However, how the composition of the mixed phase affects the exciton generation and dissociation process is still unknown. In this paper, different amount of fullerene in the mixed phase of poly(3-hexylthiophene) (P3HT)/fullerene was achieved by changing the ratio of two fullerenes ([6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM) and its bisadduct bis-PC₇₁BM), which have different miscibility with P3HT. When the content of PC₇₁BM increasing from 0% to about 60%, the mixed phase containing more fullerenes. More donor/acceptor (D/A) interface was formed, and the ΔE value also increased from 0.19 eV to 0.23 eV. As a result, the maximum exciton generation rate (G_max) increased from 5.36 × 10²⁷ m⁻³/s to 6.48 × 10²⁷ m⁻³/s, and the exciton dissociation probabilities increased from 47% to 81%. Thus, perfect three-phase morphology for high performance solar cells was proposed, in which not only the donor and acceptor have good crystallinity to form interpenetrating network but also have good miscibility with each other to increase the fullerene content in mixed phase.