Boosting Organic Solar Cell Electrical Performance by Introducing Large Aromatics onto Small‐Molecule Peripheral Side‐Chains

It is reported herein that electrical performance of diketopyrrolopyrrole based small‐molecule solar cells are boosted via increasing the conjugation size fraction (δ) on lateral side‐chains and on the mainchain of small‐molecule donor. The conjugation size fraction is defined as δ = N _p/N _m, where N _p is the number of five‐ or six‐member aromatic rings on the peripheral side‐chains and N _m is that on the mainchain. As the DPP‐dimer's core structure varies from 5,10‐bis((5‐(2‐ethylhexyl)thiophen‐2‐yl)dithieno[2,3‐d:2′,3′‐d′]benzo[1,2‐b:4,5‐b′]dithiophene (DT‐BDT) to 4,8‐bis((5‐(2‐ethylhexyl)thiophen‐2‐yl)‐BDT (2T‐BDT), and then 4,8‐bis(5′‐(2‐ethylhexyl)‐(2,2′‐bithiophen)‐5‐yl)‐BDT (4T‐BDT), the term of δ increases from 2/13 to 2/11 and then 4/11. In the blended film with the fullerene acceptor, the donor phase size decreases from 35 to 24 and 15 nm, becoming more and more approaching to the effective exciton diffusion length. Consequently, the obtainable maximum generation rate of electron–hole bound pairs increases from 7.0 × 10²⁷ to 7.3 × 10²⁷ and then 7.7 × 10²⁷ m⁻³ s⁻¹. Meanwhile, reduction of donor crystallinity with the increase of δ leads to a higher effective carrier mobility, which suppresses recombination losses and leads to a larger carrier collection probability. Promotions of both charge separation and transport give a larger short‐circuit current density, a higher fill‐factor, and ultimately a larger efficiency.