Elucidating Vertical Phase Separation of Active Layers in Polymer Solar Cells via NEXAFS

Using synchrotron-based near-edge x-ray absorption fine structure (NEXAFS) spectroscopy, we have quantified the surface compositions of bulk-heterojunction active layers comprising poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester. By delaminating the active layers from the underlying substrates, we have also quantified the compositions at the once-buried film-substrate interface. For active layers on hydrophilic substrates, the surface composition is 97wt\% P3HT. In comparison, the P3HT composition is lower at the film- substrate interface (65wt\%). By increasing the hydrophobicity of the substrates through the adsorption of phosphonic acid derived self-assembled monolayers, we observe an enrichment of P3HT (89wt\%) at the film-substrate interface of the active layers. Accordingly, devices of the conventional architecture fabricated on hydrophobic surfaces show improved performance.