A nonfullerene acceptor as a solid additive realizing a record efficiency of 17.74% in quasi-layered all-polymer solar cells

Quasi-layered all-polymer solar cells (QLA-PSCs) were fabricated based on wide bandgap polymer PM6 as a donor and narrow bandgap polymer PY-IT as an acceptor. A nonfullerene acceptor, L8-BO, is deliberately selected as a solid additive due to its similar chemical structure to the segment of polymer acceptor PY-IT. The power conversion efficiency (PCE) of the QLA-PSCs is increased from 16.14% to 17.74% by incorporating 2 wt% L8-BO into the PY-IT layer, benefiting from the synergistically increased short circuit current density (24.45 mA cm-2vs. 23.41 mA cm-2) and fill factor (76.38% vs. 72.60%). The contribution of L8-BO as a solid additive on the performance improvement of the QLA-PSCs can be summarized as follows: (i) it induces more ordered molecular orientation of PY-IT confirmed from GIWAXS; (ii) it enlarges the exciton diffusion length in the PY-IT layer and facilitates efficient hole transfer from PY-IT to PM6 as demonstrated by transient absorption; (iii) it increases the exciton dissociation interface as evidenced by the contact angle and photoluminescence of PM6/PY-IT without or with L8-BO. It should be highlighted that the 2000 h storage stability of the QLA-PSCs can be significantly improved, with PCE retention increasing from 85.1% to 90.2% of the initial value upon incorporating L8-BO as a solid additive. The effect of the universality of the nonfullerene acceptor as a solid additive on the performance improvement of QLA-PSCs can also be confirmed by the boosted PCE to 16.71% or 16.79% with Y6 or BO-4F as a solid additive. This work shows that a small molecular nonfullerene acceptor may induce a polymer acceptor molecular arrangement to further improve the performance of QLA-PSCs. The nonfullerene acceptor L8-BO as solid additive was incorporated into QLA-PSCs based on PM6 and PY-IT. The molecular orientation and exciton diffusion length in PY-IT layer can be optimized, resulting in the PCE enhanced from 16.14% to 17.74%.