Effects of Cathode Confinement on the Performance of Polymer/Fullerene Photovoltaic Cells in the Thermal Treatment

Polymeric photovoltaic (PV) cells based on poly(3-hexylthiophene-2,5-diyl):[6, 6]-phenyl C₆₁ butyric acid methyl ester (P3HT:PCBM) with the cathode confinement in the thermal treatment show better performance than the PV cells without the cathode confinement in the thermal treatment. The functions of the cathode confinement are investigated in this paper by using X-ray photoelectron spectroscopy, atomic force microscopy, optical absorption analysis, and X-ray diffraction analysis. It is found that the cathode confinement in the thermal treatment strengthens the contact between the active layer and the cathode by forming Al-O-C bonds and P3HT-A1 complexes. The improved contact effectively improves the device charge collection ability. More importantly, it is found that the cathode confinement in the thermal treatment greatly improves the active layer morphology. The capped cathode effectively prevents the overgrowth of the PCBM molecules and, at the same time, increases the crystallization of P3HT during the thermal treatment. Thus, a better bicontinuous interpenetrating network is formed, which greatly reduces the exciton loss and improves the charge transport capability. Meanwhile, the enhanced crystallites of P3HT improve the absorption property of the active layer. All these aforementioned effects together lead to the great performance improvement of polymeric PV cells.