Heterostructure is the basic building block for functional optoelectronic devices. Heterostructures consisting of two-dimensional (2D) transition metal dichalcogenides (TMDs) and organic semiconductors are currently attracting great interest for high-performance optoelectronics. However, how to design heterostructure for highly efficient optoelectronic devices remains a big challenge. Here we design high-performance organic semiconductor/WSe₂ heterostructure photodetectors by tailoring the charge transfer effect between 2,2ʹ-((2Z,2ʹZ)-(((4,4,9,9-tetrakis(4-hexylphenyl)-4,9- dihydros-indaceno[1,2-b:5,6-bʹ]dithiophene-2,7-diyl)bis(4-((2-ethylhexyl)oxy)thiophene-5,2-diyl))bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene)) dimalononitrile (IEICO-4F) organic semiconductors with various thicknesses and monolayer WSe₂. With the increase of IEICO-4F layer thickness, the photoluminescence (PL) characteristics of WSe₂ could be completely quenched due to the charge transfer from the lowest unoccupied molecular orbital (LUMO) level of IEICO-4F to the conduction band minimum (CBM) of WSe₂. Benefiting from the exquisite charge transfer behavior, the IEICO-4F/WSe₂ heterojunction photodetector with optimized 6.0-nm thick IEICO-4F shows high performance including the responsivity of 8.32 A/W and specific detectivity of 4.65 × 10¹¹ Jones at incident light of 808 nm. This work demonstrates a simple approach based on PL characteristics to design high-performance IEICO-4F/WSe₂ heterojunction, thus paving the way for the development of excellent optoelectronic devices based on organic/TMD heterostructures.