A powerful insight into the structural and interfacial properties of post-fabricated bulk heterojunction (BHJ) organic photovoltaic (OPV) devices is reported. The nanoscale local structure of integrated devices is revealed by combined X-ray diffraction (XRD) and fluorescence (XRF) investigations. A comparative study is performed on devices using graphene oxide (GO) as the hole transporting layer (HTL) and reference PEDOT:PSS (poly(3,4 ethylenedioxythiophene):poly(styrenesulfonate)) devices. Spatially resolved simultaneous XRD/XRF measurements with nanometre resolution on pristine and aged states of the devices evidence the occurrence of morphological modifications in the poly(2,7-carbazole) derivative (PCDTBT):fullerene derivative (PC₇₁BM) active layer, induced by thermal reduction and solar illumination. Additionally, the results indicate that OPV devices with partially reduced graphene oxide (pr-GO) used as the HTL, exhibit photovoltaic characteristics similar to the PEDOT:PSS based devices but with a significant durability enhancement. This is attributed to the protecting role of the pr-GO film against humidity and indium diffusion from the Indium Tin Oxide (ITO) anode into the photoactive layer. As a result, the devices fabricated with pr-GO-HTL retain approximately 65% of their initial power conversion efficiency over 20 hours, while the efficiency of the reference devices degrades to 45% of the initial value.