Crosslinkable Amino-Functionalized Conjugated Polymer as Cathode Interlayer for Efficient Inverted Polymer Solar Cells

A novel crosslinkable aminoalkyl-functionalized polymer, poly[9,9-bis(6-(N,N diethylamino) propyl)fluorene-alt-9,9-bis(hex-5-en-1-yl)-fluorene](PFN-V), is designed and synthesized. The resulting polymer can be rapidly crosslinked by UV-curing within 5 s in a nearly quantitative yield based on the "click" chemistry of alkyene end-groups of the PFN-V side chains and the addition of 1,8-octanedithiol. The crosslinked PFN-V film exhibits excellent solvent resistance property and can act as effective cathode interlayer to modify the indium tin oxide (ITO) electrode, which can thus facilitate the formation of Ohmic contact between cathode and active layer. The surface energy of PFN-V is quite comparable to that of PC71BM, which is favorable for the formation of vertical phase separation in the bulk heterojunction film that can facilitate extraction of charges as verified by transient photocurrent measurements. Based on the resulting PFN-V as the cathode interlayer, the fabricated polymer solar cells with inverted device structure show a remarkable enhancement of power conversion efficiency from 3.11% for the control device to 9.18% for PFN-V based device. These observations indicate that the synthesized PFN-V can be a promising crosslinked copolymer as the cathode interlayer for high performance polymer solar cells.