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(3from 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 Xa and a linear drop in the Xc 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 Xa or Xc. For the 7.5 nm-thin P3HTT film (30 mol % of thiophene), the mu(h) was as high as 0.017 cm(2) x V-1 x s(-1). In order to explain the observed mu(h) 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 mu(h) 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-twodimensional 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.