One-pot synthesis of quasi-block fluoropolymers for graded heterojunctions via dual resurfacing

Graded heterojunctions (GHJs) featuring a gradient donor/acceptor distribution represent an ideal architecture for organic solar cells (OSCs), yet remain challenging to realize. Here, we report a series of fluoropolymers that can form GHJs through surface-energy-driven resurfacing in solution processes. The distinct chemical reactivity of fluorinated versus standard monomers enables the formation of quasi-block copolymers in one-pot polymerization, which rivals the two-step synthesized block copolymer in polymer properties and photovoltaic performance. These fluoropolymers can resurface due to the low-surface-energy fluorinated blocks. During the sequential deposition that small-molecular acceptor solution is cast atop the underlying fluoropolymer film, the acceptor penetrates downwards accompanied by donor resurfacing, consequently generating a vertical donor/acceptor gradient. Moreover, a post-treatment namely fluorous solvent vapor annealing (FSVA) is adopted to further regulate the distribution. The unique solubility of fluoropolymers in fluorous solvents enables fluoropolymers to resurface again, yielding an optimal GHJ to deliver an impressive efficiency of 19.60% in OSCs. Notably, block-containing copolymers achieve superior device performance and more optimal GHJ than the random fluoropolymer, highlighting the importance of monomer sequence. Our findings provide an efficient approach to create GHJs by leveraging surface energy, and also attract attentions to explore the precise structure of copolymers, in particular for fluoropolymers.