Effect of incorporating an asymmetric electron-richer dithieno [3,2-b:2',3'-d]pyrrole moiety into perylene-diimide-based polymer backbone on the optoelectronic and photovoltaic properties

Asymmetric 5-H-dithieno[3,2-b:2 ',3 '-d]pyran (DTP) possessing the stronger electron-donating ability than its analogue 4H-cyclopenta[2,1-b:3,4-b']dithiophene (CPDT) was widely applied in fullerene-based solar cells and/ or constructed the A-D-A type nonfullerene-based small molecular acceptors. However, limited efforts have devoted to develop perylene-diimide (PDI)-based polymeric acceptors in the field of all-polymer solar cell (AllPSCs). Herein, two donor-acceptor (D-A) type N,N-bis(2-hexyldecyl)-PDI-based polymer acceptors, PPDI-CPDT and PPDI-DTP utilizing 4,4-bis(2-ethylhexyl)-CPDT and 5,5-bis(4-hexylphenyl)-DTP as D moiety, were synthesized. Replacing CDT donor moiety with DTP led to the decreased thermal- and photo-stability. Regardless of an enhanced absorption around 500 nm and 0.02 eV up-shifted ELUMO, however, decreased VOC from 0.79 to 0.56 V and down-shifted JSC from 8.18 to 1.89 mA cm-2, fallen FF from 40.80% to 28.48% and thus 87.79% decreased PCE from 2.62% to 0.30% were observed. The great drop of device efficiency was mainly attributed to the weak aggregation and interaction as a result of inserting the large electronegative oxygen into the inner penta-ring of CPDT unit, which resulted in insufficient separation and restricted the photocurrent and efficient charge transfer process. These researching results suggested that it should be cautious to introduce the strong electron-rich moiety to tune the optoelectronic property and aggregation for obtaining the boosted device efficiency in AllPSCs.