Synergistic Effects of Processing Additives and Thermal Annealing on Nanomorphology and Hole Mobility of Poly(3-hexylthiophene) Thin Films

Control of the nanoscale molecular ordering and charge-carrier mobility of poly(3-hexylthiophene-2,5-diyl) (P3HT) was achieved by the combined use of processing additives and thermal annealing. Evaluation of four processing additives (1,8-octanedithiol (ODT), diphenyl ether (DPE), 1-chloronaphthalene (CN), and 1,8-diiodooctane (DIO), which are commonly used for the fabrication of organic solar cells, revealed that the nanoscale molecular ordering and, therefore, the charge-carrier mobility, are largely affected by the additives, as demonstrated by spectral absorption, X-ray diffraction, and atomic force microscopy. Thermal annealing selectively influenced the morphological changes, depending on the solubility of P3HT in the additive at high temperature. In the case of CN, in which P3HT can be dissolved at moderate temperature, significant molecular ordering was observed even without thermal annealing. For DIO, in which P3HT is only soluble at elevated temperature, the mobility reached 1.14 × 10 cm² V s only after annealing. ODT and DPE were not effective as processing additives in a single-component P3HT. This study provides insight for designing the processing conditions to control the morphology and charge-transport properties of polymers.