Side-Chain Optimization of Phthalimide-Bithiophene Copolymers for Efficient All-Polymer Solar Cells with Large Fill Factors

A series of phthalimide-bithiophene copolymers with various side-chains have been used in bulk-heterojunction all-polymer solar cells (all-PSCs) by blending with the well-known n-type polymer N2200. Variation in the side-chains on the bithiophene and N-imide groups afforded phthalimide polymers with tunable optoelectrical properties and different self-assembly characteristics. Optimization of the fabrication conditions for the all-PSCs resulted in a power-conversion efficiency (PCE) of 3.77 %, with a remarkable fill factor (FF) of 71.21 % for a cell with an active layer thickness of 100 nm. This FF is one of the highest values for all-PSCs reported to date. Moreover, a greatly improved PCE of 4.7 %, with a good FF of 61.64 %, was obtained by increasing the film thickness to 120 nm. These results indicate that the FFs of all-PSCs could approach those of polymer:fullerene cells, if a favorable blend film morphology can be realized and efficient charge-transport/-extraction can be achieved. These results demonstrate that phthalimide-based polymers are promising donor semiconductors for the construction of efficient all-PSCs and that their side-chains are critical for performance enhancement, in addition to their polymer backbone.