The water-dipping effect of branched poly(ethylene imine) interfacial layers on the performance and stability of polymer:nonfullerene solar cells

Abstract

Here we report the water-dipping effect of polymeric interfacial layers on the performance of inverted-type polymer solar cells (PSCs) with polymer:nonfullerene bulk heterojunction (BHJ) layers. Aqueous solutions of branched poly(ethylene imine) (bPEI) were spun on indium-tin oxide (ITO)-coated glass substrates to make the interfacial layers with various thicknesses (0.1–5 nm) for the investigation of the bPEI thickness effect. The water-dipping process was carried out by immersing the 2 nm-thick bPEI layer-coated ITO-glasses in deionized water for various dipping times (0–90 min). The inverted-type PSCs were fabricated by spin-coating the BHJ layers on the water-dipped bPEI layers, followed by successive deposition of hole-collecting layers and silver top electrodes. The results showed that the open circuit voltage (V_OC) of PSCs could be greatly increased by the insertion of bPEI layers (all thicknesses) thanks to the formation of dipole layers. However, the limited thickness range (0.1–1.5 nm) of bPEI layers could only deliver an enhanced short circuit current density (J_SC) owing to the influence of high electrical resistance in the case of thicker bPEI layers. The water-dipped bPEI layers resulted in enhanced V_OC and J_SC due to the well-maintained work functions and reduced thickness, leading to 2.7-fold improved power conversion efficiency by water-dipping for 30 min.