We exploit two commercially available nucleating agents to promote hole mobility within semicrystalline poly(9,9-dioctylfluorene) (PFO) films as probed by time-of-flight (ToF) photocurrent spectroscopy. Guided by the general principles of crystal growth kinetics, we investigate the impact of varying nucleant loadings (e.g. 0.15 versus 1 wt%) and post-deposition thermal treatments (e.g. isothermal versus dynamic annealing) on the bulk microstructure and the recorded ToF mobility. Our results showed that in the best case, of low weight fraction of nucleating agent and isothermal crystallisation, the ToF hole mobility could be increased to ~2 × 10⁻² cm² V⁻¹ s⁻¹ [1], which is approximately an order of magnitude higher than values obtained from conventional, pristine semicrystalline PFO [1,2]. This value also edges towards the highest ToF hole mobility measured for fully glassy-state PFO (~3 × 10⁻² cm² V⁻¹ s⁻¹) [1]. Our results suggest that optimising the trace addition of nucleating agents and thermal protocol can further improve hole transport properties of the processed layer, by enhancing percolation through the crystalline phase.

By considering crystalline domains that are either i) interconnected to facilitate fast carrier motion or ii) poorly coupled thus acting as structural traps for carriers, we put forward a mechanistic interpretation of the photocurrent transients observed in the present study.

Finally, we discuss the extent to which crystallinity necessarily benefits the electronic transport properties of (macro)molecular semiconducting layers.

[1] X. Shi et al., Phys. Rev. X. 9, 021038 (2019);
[2] T. Kreouzis et al., Phys. Rev. B. 73, 235201 (2006).