Competition between phase separation and structure confinement in P3HT/PCDTBT heterojunctions: Influence on nanoscale charge transport

Abstract

A strong impact of crystal morphology on hole mobility is evidenced due to competition between phase separation and structure confinement in P3HT/PCDTBT heterojunctions. We find that domain sizes of both components decrease as film thickness decreases, suggesting an initial advantageous scenario for the efficiency of bulk heterojunction solar cells. However the P3HT/PCDTBT (1:1) films with a thickness of 165 nm and thicker present a dense crystal needle-like morphology while no evidence of needle-like motifs appears in the films 58 nm thick, indicating that confinement inhibiting crystallization takes place due to the very thin P3HT domains of only 40 nm in the thinner sample. The structural studies were correlated with nanoscale charge transport by conductive-AFM. A significant zero-field hole mobility increase is observed for the P3HT/PCDTBT films with increasing thickness. Even more interesting is the fact that a zero-field hole mobility increase of about two orders of magnitude is observed for P3HT domains in a 165 nm blend film compared to a neat P3HT film with similar thickness. This observation is related to the highly conductive network induced in P3HT by the presence of the PCDTBT phase, consisting of needle-like crystals growing from the P3HT domains and acting as bridges through the PCDTBT domains.