The electrical and optoelectrical properties of naphthalene bis-benzimidazole (NBBI) based organic thin film phototransistors (photoTFTs) with divinyl tetramethyl disiloxane-bis (benzo-cyclobutene) (BCB) as a gate insulator layer were investigated in detail under dark and various white light illumination conditions. One of the most apparent effects on devices under illumination is the shifting of the threshold voltage from positive values to negative values with increasing the light intensity. The calculated values by the proposed expression of the threshold voltage under illumination are fitted well with the experimental data. The photoTFTs exhibited a photoconduction behavior with a high photosensitivity value of 5.45 x 10(4) under 80 mW cm(-2) in off-state (V-G = 0 V) and a high responsivity value of 34.4 AW(-1) under 50 mW cm(-2) in the on-state that demonstrate the NBBI is promising material to be used on the optoelectronic applications. The combination of illumination-induced effect and dipole layer has been also studied to see how DOS distribution changes. When the dipole moment increased from P = 2D to P = 6D, the DOS distribution is more broadened and the DOS decrease with increasing the white light intensity. The used model gives a good agreement between the measured current voltage characteristics of the NBBI-photoTFTs and those modeled in all measurement conditions (in dark and under illumination). (C) 2017 Elsevier B.V. All rights reserved.