Dipolar self-assembled monolayers grafted on ZnO for the tuning of electronic properties of the poly (3-hexylthiophene)- [6,6]-phenyl C61-butyric acid methylester blend

A promising solution to improve the performance of Bulk heterojunction organic photovoltaic devices is the insertion of functionalized self-assembled monolayers (SAMs) at the interface between the Electron Transporting Layer (ETL) and the active layer. By grafting such dipolar SAMs based on Y-PP-COOH (P = Phenyl, YY = CH3O, F)) on a ZnO ETL, both the electronic and the chemical properties of the interface are modified. The growth of the active blend the poly(3-hexylthiophene)-[6,6]-phenyl C61-butyric acid methylester blend deposited onto the SAMs is also altered. The dipole with a component oriented perpendicularly to the ZnO surface modifies the ZnO work function whereas the surface energy of the SAM influences the microstructuration of the active material. When we analyze the inverted photovoltaic cell, we find that the two effects may be distinguished: the CH3O moiety favorably affects the electrical transfer of carriers with mainly an improvement of the open-circuit voltage V-oc whereas fluorine disfavors both electron extraction and blend microstructure quality, with a lower V-oc and electrical current density J(sc).