Regional data for C 1s, N 1s, O 1s and S 2p are given in Figure and full XPS survey between 0 and 600 eV in Supplementary Figure . Complete elucidation of the decomposition mechanism and resulting structure is omitted due to being beyond the scope of this study. We instead focussed on qualitatively assessing the change in chemical environments due to surface treatment. The regional C 1s data is displayed in Figure where the alternating single and double C-C bonds showed a peak at 284.4 eV in the pristine film, which slightly shifted to higher energy at 284.6 eV in the film treated for 600 s. This could be an indication of broken conjugation which would cause a shortening of the polymer chains, as corroborated by the reduction in the ICT band in UV-vis spectra. An additional band at lower energy (~285 eV) intensified after UVO exposure, corresponding to the formation of C-O and C=O functional groups after oxidation (typically reported between 285-289 eV). The amide nitrogen in the DPP lactam ring exhibited a peak at ~400 eV in the N 1s spectra (Figure ), which broadened with UVO exposure above 30 seconds. This could be explained by oxidation of the adjacent sidechain carbon. The S 2p spectra for pristine pDPP3T displayed a single peak at 164.2 eV with unchanged relative intensity up to 60 seconds of UVO treatment (Figure ). At this exposure time a second band at 168.6 eV associated with sulphur oxidation (S=O) emerged and increased in intensity with UVO exposure. This phenomenon is likely to contribute to the reduction in D-A character, as seen via UV-Vis absorbance, due to the sulphur lone pair in thiophene no longer being available to donate into the ring. The surface elemental composition obtained from XPS clearly demonstrated oxidation of the CP films (Supplementary Figure ), resulting in the formation of more polar functional groups on the polymer surface, which provides further evidence for to the observed changes in polar and total surface energies discussed previously. Atomic Force Microscopy (AFM) was used to gain insight into the topographic changes upon treating the CP films with UVO by capturing height images (Figure ). The pristine film exhibited a relatively flat, homogeneous surface with a roughness (arithmetical mean height, Ra) of 0.855 nm. The height images revealed smoothing of the polymer film surface which causes gradual reduction in surface roughness down to Ra = 0.271 nm after 600 s (values listed in Supplementary Table ). This can be visualized by plotting the displacement of the AFM tip along the xz axis (Figure ) highlighting the increased surface homogeneity with prolonged UVO exposure time. After exposing the films for t ≥ 60 s, the formation of pin holes in the films became apparent in the height images, suggestive of the physical breakage of polymer chains which is concurrent with reduction in film thickness (Supplementary Table ). It has been shown that surface roughness contributes to poor wettability in intrinsically hydrophobic surfaces (θH2O > 90°), whereas in intrinsically hydrophilic (θH2O < 90°) films wettability is enhanced with surface roughness. Consequently, the UVO smoothed film surfaces are likely to contribute to improved wettability in films treated for short times (t = 5 and 30 s), in addition to the increased surface energy introduced by partial oxidation of the surface.