We have investigated the effects of thickness variation and thermal treatment of the electrode polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) in photovoltaic and photodetector devices using conjugated polymer blends as the photoactive material, By variation of the PEDOT:PSS layer thickness between 25 and 150 nm, we found optimum device performance, in particular low dark current and high external quantum efficiency (EQE) and open-circuit voltage (V-oc), at around 70 nm. This has been observed for two different active layers. Annealing studies oil the PEDOT:PSS films, with temperatures varied between 120 and 400 degrees C, showed an optimum device performance, in particular EQE and V-oc at 250 degrees C. This optimum performance was found to be associated with loss of water from the PSS shell of the PEDOT:PSS grains. For annealing temperatures above 260 degrees C, device performance was dramatically reduced. This was associated with chemical decomposition leading to loss of sulfonic acid, although this did not significantly affect the in-plane conductivity.