The electrical properties of solution-processed 6,13-bis(triisopropylsilylethynyl)pentacene thin-film transistors were investigated under various light irradiation wavelengths. Irradiated light conditions were determined from wavelengths with specific peaks in the absorbance spectrum of the film, and changes in electrical characteristics were analyzed under light irradiation at these wavelengths. At long wavelengths of 695, 640, and 585 nm, a small number of photo-generated excitons lowered the hole injection barrier. At short wavelengths of 450 and 340 nm, a large number of photo-generated excitons threatened the electrically accumulated holes, lowering the decrease in threshold voltage variation. Furthermore, results including the content of the films based on light intensity, according to irradiated wavelengths are presented.