A thin film planar heterojunction organic photodetector (PHJ-OPD) is demonstrated. Different from a conventional sensitizer-doped photodetector, the limited spatial distribution of sensitizer in a PHJ-OPD enables significantly reduced thickness of the active layer without allowing the formation of unnecessary trap sites and electron percolation pathways. As a result, peak external quantum efficiency (EQE) of 120 700% and detectivity over 1013 Jones are demonstrated with thin active layer thickness of 150 nm, which can be a significant benefit for high-resolution image sensor application. Furthermore, the operating voltage can be decreased to -5 V while maintaining high detectivity over 1012 Jones. Remarkable thermal stability is also observed with minor change in detectivity for 2 h of continuous operation at 60 °C due to morphological robustness of PHJ. This work opens up a possibility of using a thin film PHJ-OPD as a key unit of high-resolution image sensor.