Thick polyfluorene-based polyelectrolytes realized by regulation of conjugated backbone as cathode interface layers for efficient polymer solar cells

Abstract Polyfluorene-based conjugated polyelectrolytes as efficient cathode buffer layers have powerfully improved the device performance of polymer solar cells, but the p-type nature of backbones make them work efficiently only in very thin film (2∼5 nm). In this work, three new polyfluorene-based conjugated polyelectrolytes with self-doping effect, namely poly[(9,9-bis(6′-N,N,N-trimethylammoniumbromide)hexyl)fluorene-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c] [1,2,5]thiadiazole)] (PFTBT-NBr), poly[(9,9-bis(6′-N,N,N-trimethylammoniumbromide)hexyl)fluorene-alt-2,5-bis(2-octyldodecyl)-3,6-bis(thiophen-2-yl)-pyrrolo [3,4-c]pyrrole-1,4-dione (PFTDPP-NBr) and poly[(9,9-bis(6′-N,N,N-trimethylammoniumbromide)hexyl)fluorene-alt- N , N ′-bis(2-decyltetradecyl)-1,7-bis(thien-2-yl)-perylene-3,4:9,10-tetracarboxylic-diimide (PFTPDI-NBr) are designed and synthesized as buffer layer for PSCs. These conjugated polyelectrolytes have the same polar side chains, but differ in the conjugated backbones which are composed of fluorene alternated with different electron-withdrawing group, benzothiadiazole (BT), diketopyrrolopyrrole (DPP) and perylenediimide (PDI) respectively. The n-type polarity of the conjugated backbones is found to be substantially strengthened by varying the electron affinity and conjugated planarity of these backbones. As a result, employing these conjugated polyelectrolytes as cathode buffer layers for PSCs can effectively boost the device performance. Notably, an excellent thickness tolerance (∼30 nm) of polyfluorene-based polyelectrolytes, especially for PFTPDI-NBr, is achieved in the PSCs, originating from the higher electron mobility and conductivity, better interface contact between active layer and electrode, and favorable morphology of active layer.