This paper reports on the photochemical behavior upon exposure to UV-visible light of a poly(2,7-carbazole) derivative for use in high-performance solar cells. Poly[N-9′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) is one of a relatively large class of push-pull carbazole-based copolymers that have been synthesized to better harvest the solar spectrum. The 2,7-carbazole building block of PCDTBT is also used with different electron-accepting units in a large variety of low-band-gap polymers. The photochemical and morphological behavior of PCDTBT thin films is investigated from the molecular scale to the nanomechanical properties. The photo-oxidation mechanism is shown to be governed by chain-scission and cross-linking reactions. It results in dramatic evolution of the morphology, roughness and stiffness of thin PCDTBT films. Based on the identification of several photoproducts formed along the macromolecular chains or released into the gas phase, the main pathways of PCDTBT photochemical evolution are discussed. These processes first involve the scission of the C-N bond between the carbazole group and the tertiary carbon atom bearing the alkyl side-chain. Modifications of the chemical structure of PCDTBT, the evolution of its UV-visible absorbance, and its nanomechanical properties initiated by light irradiation are shown to be closely related.