We report synthesis of a new poly(4-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2-yl)-7-(4,4-bis(2-ethylhexyl)-6-(selenophene-2-yl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2-yl)-5,6-difluorobenzo[c][1,2,5]thiadiazole (PDFDSe) polymer based on planar 4,7-bis(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophen-2-yl)-5,6-difluorobenzo[c][1,2,5]thiadiazole (DFD) moieties and selenophene linkages. The planar backboned PDFDSe polymer exhibits highest occupied molecular orbital and lowest unoccupied molecular orbital levels of −5.13 and −3.56 eV, respectively, and generates well-packed highly crystalline states in films with exclusive edge-on orientations. PDFDSe thin film was incorporated as a channel material in top-gate bottom-contact organic thin-film transistor with a solid-state electrolyte gate insulator (SEGI) composed of poly(vinylidene difluoride-trifluoroethylene)/poly(vinylidene fluoride-co-hexafluroropropylene)/1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, which exhibited a remarkably high hole mobility up to μ = 20.3 cm2 V–1 s–1 corresponding to effective hole mobility exceeding 5 cm2 V–1 s–1 and a very low threshold voltage of −1 V. These device characteristics are associated with the high carrier density in the semiconducting channel region, induced by the high capacitance of the SEGI layer. The excellent carrier mobility from the PDFDSe/SEGI device demonstrates a great potential of semiconducting polymer thin-film transistors as electronic components in future electronic applications.