Thin-film field-effect transistors (FETs) are widely used to evaluate charge transport properties of semiconducting polymers. Discovery of high performance materials require design and synthesis of new polymers. However, most polymers require multi-step synthesis and are difficult to be obtained in a large scale for comprehensive device evaluations. Here, we report a simple method to cast semiconducting polymer films from solutions with polymer concentration as low as 0.5 mg/mL, which is substantially less than typical values (∼10 mg/mL) used in conventional spin coating method. Here, we demonstrate that using this method, our cast films of a previously-reported polymer (PDPP-TT2T) exhibited field-effect mobility (μhole = 0.89 ± 0.13 cm2 V−1 s−1, μe = 0.025 ± 0.005 cm2 V−1 s−1), which is comparable to the reported values using the same device geometry. Furthermore, we extend this method to examine cast films of a pair of polymers (PDPP-3T-Ref, PDPP-3T-Si) to study the effect of siloxane substitution in the side chains on the molecular packing and their subsequent FET performance. We observed that shorter π-stacking distance (3.61 Å) for the siloxane-terminated polymer, when compared to that for the reference polymer (3.73 Å), resulted in improved FET performance (e.g., μhole = 0.63 ± 0.046 cm2 V−1 s−1 for PDPP-3T-Si vs μhole = 0.17 ± 0.062 cm2 V−1 s−1 for PDPP-3T-Ref). Taken together, this work presents an efficient alternative film-casting approach to produce polymer FETs that consumes much less material for their fabrication, lending viability for evaluation of various polymeric materials.Graphical abstractHighlights► Uniform polymer films were cast using a simple drop-pinned method. ► The casting method consumed much less material than conventional spin casing. ► The polymer films exhibited comparable high charge mobility as spin cast films. ► A pair of polymers with branched alkyl side chains or siloxane group were examined. ► Siloxane substitution led to structural modification in favor of charge transport.