Herein, we address the reduction in the external quantum efficiency (EQE) of solution-processed organic photodetectors caused by the room temperature phase demixing of components in the composite material of the photoactive layer. The reduction takes place under ambient conditions and after the completion of device fabrication. As a model system, we study photoactive blend films that consist of the electron acceptor N,N'-bis(alkyl)-3,4,9,10-perylene tetracarboxylic diimide) (PDI) and the electron donor polymer poly(9,9'-dioctylfluorene-co-benzothiadiazole) (F8BT). The ambient ageing of these photoactive layers is a consequence of the PDI component segregation; however, the final PDI domain size remains smaller than the resolution limit of optical microscopy. We find that the photophysical properties of the aged F8BT: PDI layer and the EQE of the aged device are significantly altered. The fabrication of F8BT: PDI layers from solvents of increasing boiling point allows for the spectroscopic monitoring of the ageing-induced phase segregation (a-PSG) process. For each solvent used, the extent of a-PSG is correlated with the PDI dispersion in the F8BT matrix as received immediately after layer deposition. The tendency for room temperature phase demixing becomes stronger as PDI is more finely dispersed in the freshly spun F8BT: PDI layer. The evolution of the room temperature phase segregation of PDI has a negative impact on the photophysical processes that are essential for charge photogeneration in the F8BT: PDI photoactive layer.