Efficiency improved for inverted polymer solar cells with electrostatically self-assembled BenMeIm-Cl ionic liquid layer as cathode interface layer

The interlayer inserted between active layer and ITO has been a key issue for improving electron extraction in inverted polymer solar cells (IPSCs), while the ideal interlayer for IPSCs has not been well developed. In this work, we presented a spontaneous vertical phase separation (SVPS) self-assembled bilayers structure with BenMeIm-Cl ionic liquid (IL) interfacial bottom layer and a photoactive top layer via a single spin-coated step of BenMeIm-Cl IL and organic donor-acceptor composite and achieved a PCE as high as 8% based on IPSCs with PTB7 as the donor. The presence of BenMeIm-Cl IL reduces the work function of ITO and leads to a better energy-level matching for efficient charge-transfer. The driving force of SVPS self-assembled structure is from the relative surface energy difference between organic materials and BenMeIm-Cl ILs, together with their interactions with the substrates. This self-assembled process procedure pave the way to simplify the manufacturing of low-cost and large-area organic electronic devices. •The inserted of BenMeIm-Cl ionic liquids as cathode interface layer for inverted polymer solar cells.•The ionic liquids own the advantages of low temperature fabrication, variety category, simple synthesis and environment-friendly nature.•The nanoscale thickness of ionic liquids can be controlled by a spontaneous vertical phase separation self-assembled structure.•This one-step self-assembled procedure greatly simplifies the fabrication of IPSCs.