Linear polyurethane ionenes for stable interlayer of organic photovoltaics

Abstract

Non-conjugated polymer electrolytes (nCPEs) are applied as electron transporting layers (ETLs) in organic photovoltaic (OPV) devices with conventional structures and used as competent interfacial layers. The new nCPEs are prepared from dihydroxyl pyrrolidinium bromide ionic liquid monomers and m-diphenylene diisocyanate to create linear polyurethane ionenes with main-chain ionic salt moieties. The ultraviolet photoelectron spectroscopy (UPS) results reveal that the formation of an interfacial dipole by the nCPEs facilitates electron transfer from the photoactive layer to the electrode. The OPV devices with the new nCPEs show a power conversion efficiency (PCE) comparable to that with the well-known conjugated polymer electrolyte poly(9,9-bis(3'-(N,N-dimethyl)-N-ethylammoinium-propyl-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene))dibromide (PFN), which possesses charged sites on its pendants, as an ETL. The OPV device with the synthesised nCPE exhibits superior stability compared to the PFN-based OPV devices. Our results suggest that the nCPEs containing ionic salt structures on polymer main chains are more suitable ETLs for OPV devices than PFN-based layers with pendant salt moieties.