Contributions of Polymer Chain Length, Aggregation and Crystallinity Degrees in a Model of Charge Carrier Transport in Ultrathin Polymer Films

Herein we report a systematic study on the quantitative relationships between molecular structure, crystallinity, aggregation, and charge carrier transport in films of poly­(3-hexylthiophene-co-thiophenes) (P3HTTs) with thickness ranging from 5.7 to 8.0 nm. The P3HTTs contained 10, 20, or 30 mol % of thiophene monomer units. Our UV–Vis, X-ray diffraction, and atomic force microscopy (AFM) studies indicated that the variable thiophene content in the P3HTTs as well as the solution aging prior to film casting permitted a control over aggregation density (X a), crystallinity degree (X c), crystal fragmentation, and overall film morphology. Increasing the thiophene content in P3HTT caused a linear increase in the X a and a linear drop in the X c as well as crystal sizes. Aging the solution prior to casting resulted in a change of film morphology from granular to fibrillar and a noticeable increase in both X a and X c. The hole mobility determined from field-effect transistor characteristics grew nonlinearly with increasing thiophene content in P3HTT and was boosted by the aggregation-induced increase in X a or X c. For the 7.5 nm-thin P3HTT film (30 mol % of thiophene), the μh was as high as 0.017 cm2 × V–1 × s–1. In order to explain the observed μh variation, we have employed a model of charge transport through a series circuit, where crystals, aggregates, and the amorphous phase were considered units with different resistance to charge carrier transport. A systematic analysis of the experimental dataset revealed a quantitative correlation of the μh with the polymer end-to-end distance, interaggregate distance, and crystallinity index. Also, the correlation expressed with a single equation derived in this work enabled estimation of the minimum mobility of the amorphous phase and maximum mobility of the crystal phase in the quasi-two-dimensional films. Our original findings suggest that in quantitative considerations of charge transport in the ultrathin polymer films, the morphology should be considered secondary to molar mass and phase composition.