Bulk heterojunction (BHJ) photovoltaic devices made of PCDTBT (poly[N‐9′‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)]) and PC70BM ([6,6]‐phenyl‐C70‐butyric acid methyl ester) are among the most efficient and stable devices studied so far. However, during a short regime called “burn‐in”, a significant decrease of power conversion efficiency was observed. A study of the photochemical mechanisms involved in the PCDTBT:PCBM active layer exposed to light in encapsulated systems is presented. It is found that the photochemical reactions resulting from the absorption of light by PCDTBT involve crosslinking between the 2,7 carbazole unit of PCDTBT and the fullerene unit of PCBM. Those reactions stabilize the BHJ by avoiding the formation of microsized PCBM crystals known to cause failure of BHJ solar cells. Using classical electron paramagnetic resonance spectroscopy (EPR) (without illumination), paramagnetic defects along the polymer chains have been detected. The kinetics of defects intensity show a burn‐in trend. The evolution of their relaxation times upon aging is in good agreement with a structural change (crosslinking) of the BHJ observed from the nanomechanical properties. Finally, light‐induced electron paramagnetic resonance (LEPR) measurements performed on aged samples revealed that electron transfer is not significantly affected upon aging, confirming thus the stabilization of the BHJ in solar cell operating conditions.