Efficient inverted polymer solar cells via pyridine-based organic molecules as interfacial modification layer on sol-gel zinc oxide surface

Pyridine-based organic molecules (POMs), 4-phenylpyridine (4-Py) and 4-tert-butylpyridine (4-tbp) were used to modify the sol-gel ZnO surface for inverted polymer solar cells (PSCs). X-ray photoelectron spectroscopic (XPS) results showed the formation of N–Zn bonding and the reduction of oxygen vacancies (V_o), depicting that POMs chemisorbed on V_o sites of ZnO surface to passivate O defects. From ultraviolet photoelectron spectroscopic (UPS) data, the work function (WF) of ZnO slightly decreased due to interfacial dipole, enhancing the electron extraction from ZnO surface. ZnO surface modified with POMs (ZnO/POMs) possessed smoother morphology and more hydrophobicity than the pristine ZnO surface, providing good interfacial contact with polymeric active layer. The improved surface properties benefited power conversion efficiency (PCE) of bulk-heterojunction (BHJ) PSCs and their stability. As a result, PSCs with ZnO/4-tbp and ZnO/4-Py achieved 9.29% and 9.07% respectively, while 8.16% with the pristine ZnO. Especially, PSCs with ZnO/4-tbp possessed the best PCE because of its intramolecular dipole from electron donating group. This study demonstrates the benefits of POMs modifying the sol-gol ZnO surface for inverted PSCs.