The mechanisms for introduction of n-dodecylthiol to modify the P3HT/PCBM morphology

The mechanisms for introduction of the additive to improve the poly(3-hexylthiophene) (P3HT):methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) organic solar cells performance are investigated. We add n-dodecylthiol to P3HT/PCBM solution and find that it both improves P3HT crystallinity and enhances P3HT/PCBM phase separation extent. UV–vis absorption spectroscopy (UV–vis) and dynamic light scattering (DLS) results reveal that n-dodecylthiol reduces the P3HT chains entanglement in the solution, which facilitates coil-to-rod transformation, a premise condition for P3HT crystallization. Also, it discloses that the influence of PCBM on the P3HT molecular interaction is diminished by adding n-dodecylthiol, this consequently benefits the P3HT crystallization during the film-forming process. Furthermore, the enhanced P3HT crystallinity reduces the PCBM amount which dissolved in the amorphous regions of P3HT, then promotes the aggregation of PCBM, which contributes to the controlled scale of phase segregation. Thus an increased P3HT crystallinity and a commensurate scale of phase separation balancing for exciton dissociation and charge transport and/or collection are finally obtained. This results in an almost threefold increase in device efficiencies when the volume fraction of additive is 2.0%, compared with reference devices without any additive in the photoactive layer as a consequence.