Organic Electrolytes Doped ZnO Layer as the Electron Transport Layer for Bulk Heterojunction Polymer Solar Cells

ZnO layers doped with small molecule viologen derivatives, i.e., 1,1-bis(4-hydroxypropyl)-[4,4-bipyridine]-1,1-diium bromide (VOH) or 1,1-bis(2,3-dihydroxypropyl)-[4,4-bipyridine]-1,1-diium bromide (V2OH), are prepared and used as the electron transport layer in inverted polymer solar cells (iPSCs). The presence of VOH (or V2OH) in ZnO layer and the formation of homogeneous VOH (or V2OH) doped ZnO layer are confirmed by X-ray photoelectron spectroscopy. The electron mobilities of doped ZnO layers are comparable to those of pristine ZnO because the crystallinity of the ZnO layer is not significantly affected by the doping process. Kelvin probe microscopy measurements show that the work function of doped ZnO layers is in the range of -4.2--4.3eV, which is higher than that of pristine ZnO (-4.5eV). This is due to the formation of interface dipoles at the interface between the ZnO layer and the active layer. The water contact angle data reflect the existence of quaternary ammonium bromide on the surface, and unreacted hydroxyl groups are pointed away from the surface of the ZnO layer. iPSCs based on VOH doped ZnO and V2OH doped ZnO exhibit power conversion efficiencies (PCEs) up to 9.0% and 8.6%, which are dramatically enhanced compared to the device based on pristine ZnO (PCE=7.4%).