Inverted Type Polymer Solar Cells with Self-Assembled Mono layer Treated ZnO

The work function and surface property of ZnO can be simply tuned by the self-assembled monolayer (SAM) molecules derived from benzoic acid such as 4-methoxybenzoic acid (MBA), 4-tert-butylbenzoic acid (BBA), and 4-fluorobenzoic acid (FBA), which have different dipole orientation and magnitude. MBA, BBA, and FBA treated ZnO layers were used as an electron injection/transporting layer for inverted type polymer solar cells (PSCs) with a structure of ITO/SAM treated ZnO/active layer (P3HT:PC61BM)/MoO3/Ag. The power conversion efficiency (PCE) of PSCs based on MBA and BBA treated ZnO reaches 3.34 and 2.94%, respectively, while the PCE of the device based on untreated ZnO is 2.47%. In contrary, the PCE of the device with FBA treated ZnO is 1.81%. The open circuit voltage (V-oc) of the device with MBA, BBA, and FBA treated ZnO is 0.63 and 0.62 V, respectively, while the V-oc of PSC with untreated ZnO is 0.60 V. Contrarily, the V-oc of the device with FBA treated ZnO is 0.53 V. The PCE and V-oc of PSCs based on MBA and BBA treated ZnO are better than those of the other devices. This seems to be related with the direction of dipole moment of benzoic acid derivatives. Also, the morphology of the active layer seems to be affected by the substituent on the 4-position of benzoic acid. The active layer on MBA treated ZnO shows optimized morphology, and its device shows the best performances. We demonstrate that the work function and morphology of the active layer can be controlled by SAM treatment of the ZnO surface with different dipole orientation and a substituent on the 4-position of benzoic acid. These are very simple and effective methods for improving the performances of PSCs. The results provide an alternative strategy to improve the interface property between inorganic and organic materials in organic electronic devices.