Non-Fullerene Acceptor-Based Bulk Heterojunction Polymer Solar Cells: Engineering the Nanomorphology via Processing Additives

The performance of bulk heterojunction solar cells made from blends of a non-fullerene acceptor, N,N'-bis(2-ethylhexyl)-2,6-bis(5 ''-hexyl-[2,2';5',2 ''] terthiophen-5yl)-1,4,5,8-naphthalene diimide (NDI-3TH), and poly(3-hexylthiophene) (P3HT) donor is enhanced 10-fold by using a processing additive in conjunction with an electron-blocking and a hole-blocking buffer layers. The power conversion efficiency of P3HT:NDI-3TH solar cells improves from 0.14% to 1.5% by using a processing additive (1,8-diiodooctane) at an optimum concentration of 0.2 vol%, which is far below the 2-3 vol% optimum concentrations found in polymer/fullerene systems. TEM and AFM imaging show that the size and connectivity of the NDI-3TH domains in the phase-separated P3HT:NDI-3TH blends vary strongly with the concentration of the processing additive. These results demonstrate, for the first time, that processing additives can be effective in the optimization of the morphology and performance of bulk heterojunction polymer solar cells based on nonfullerene acceptors.