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.
This work reports the study of solvent effects on the morphology for P3HT/PCBM films using chlorobenzene, 1,
2-dichlorobenzene and 1, 3-dichlorobenzene. Although extensive research has been focused on investigating devices
using chlorobenzene and 1, 2-dichlorobenzene and it was found that 1, 2-dichlorobenzene led to an improved device
performance, little work has been conducted in the morphology by comparing films fabricated via chlorobenzene, 1,
2-dichlorobenzene and 1, 3-dichlorobenzene. Atomic force microscopy (AFM) was performed to study the film
morphology using chlorobenzene, 1, 2-dichlorobenzene and 1, 3-dichlorobenzene as solvents. Initial studies showed
that the size of nanocrystallites in 1, 2-dichlorobenzene based films is smaller than nanoclusters in the other two films.
Kelvin probe force microscopy (KFM) images, which were used to figure out the electron transport pathway, together
with AFM images, showed the solvent effect on the morphology of these films. In addition, obvious red shifts were
observed in the UV-Vis absorption spectra for the P3HT/PCBM blend from 1, 3-dichlorobenzene and 1,
2-dichlorobenzene compared to the one from chlorobenzene.