Connecting physical properties of spin-casting solvents with morphology, nanoscale charge transport, and device performance of poly(3-hexylthiophene):phenyl-C₆₁-butyric acid methyl ester bulk heterojunction solar cells

The correlation between the physical properties of spin-casting solvents, film morphology, nanoscale charge transport, and device performance was studied in poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) blends, spin cast with two halogenated aromatic solvents: chlorobenzene (CB) and ortho-dichlorobenzene (1,2-DCB). 1,2-DCB-based blends exhibited fine phase separation of ∼10 to 15 nm length scale with ordered self-assembly of P3HT whereas blends spin cast from CB showed coarse phase separation with large isolated clusters of ∼25 to 100 nm of donor- and acceptor-rich regions. Higher solubility of both P3HT and PCBM in 1,2-DCB and a slower drying rate of 1,2-DCB (because of higher boiling point) facilitated self-organization and ordering of P3HT and promoted finer phase separation. Higher local hole mobility in 1,2-DCB-based blend was attributed to efficient hole transport through the ordered network of P3HT chains. Moreover, higher local illuminated current (dark + photocurrent) in 1,2-DCB-based blend suggested efficient diffusion and dissociation of excitons due to finer phase separation. As a consequence, 1,2-DCB-based devices exhibited higher short circuit current density (Jsc), external quantum efficiency and power conversion efficiency in contrast to the CB-based device. It was also observed that the device performance was not limited by light absorption and exciton generation; rather morphology dependent processes subsequent to exciton generation, primarily charge transport to the electrodes, limited device performance.