Effect of self-assembled monolayer treated ZnO as an electron transporting layer on the photovoltaic properties of inverted type polymer solar cells

The work function and the surface property of ZnO can be simply tuned by the self-assembled monolayer (SAM) treatment derived from benzoyl chloride such as 4-fluorobenzoyl chloride (FBC) and 4-tert-butyl benzoyl chloride (BBC). The work function of FBC treated ZnO is 4.75 eV, which is larger than that of untreated ZnO (4.55 eV). This is due to the formation of unfavorable interface dipole, which is shifted away from the ZnO surface. SAM treated ZnO is used as an electron collection layer for inverted polymer solar cells (iPSCs) with a structure of ITO/ZnO/SAM/active layer (P3HT:PC61BM)/WO3/Ag. Therefore, the open circuit voltage (Voc) of the device based on FBC treated ZnO is 0.53 V, which is smaller than that of the device based on bare ZnO (0.58 V) and BBC treated ZnO (0.58 V). The power conversion efficiency (PCE) of iPSC with FBC treated ZnO is 1.78%, which is smaller than that that of the device with untreated ZnO. The short circuit current (Jsc), the PCE, and the fill factor (FF) of the device with BBC treated ZnO are −9.00 mA/cm2, 2.90%, and 55.6%, respectively, which are higher than those of the device with untreated ZnO (−8.63 mA/cm2, 2.51%, and 50.2%). Enhancement of the Jsc and FF in the device with BBC treated ZnO are achieved by SAM treatment leading to the reduction of a Schottky barrier at the interface and improvement of morphology of the active layer.