Transition metal dichalcogenides (TMDs) potentially provide opportunities for large-area electronics, including flexible displays and wearable sensors. However, most TMDs suffer from a Schottky barrier (SB) and non-uniform defects, which severely limit their electrical performances. A chemical doping scheme using poly-(diketopyrrolopyrrole-terthiophene) (PDPP3T) compensates the defects and SB of multilayer molybdenum diselenide (MoSe2), exhibiting greatly enhanced electrical characteristics, including on-current (~2,000-fold higher) and photoresponsivity (~10-fold larger) over the baseline MoSe2 device. Based on comprehensive analysis using XPS, GIWAXD, AFM, and NEXAFS, we show that two mechanisms (dipole-induced and charge-transfer doping effects) account for such enhancements in the multilayer MoSe2 device.