Sequentially N-Doped Acceptor Primer Layer Facilitates Electron Collection of Inverted Non-Fullerene Organic Solar Cells

In most non-fullerene organic solar cells comprising bulk-heterojunction active layers, the inter-domain connectivity of small-molecule acceptors is generally inferior to those of polymeric donors due to their intrinsic short-range ordering. This issue is even exacerbated by the physiochemical mismatch between acceptor-phases and metal-oxide electron transport layers in most inverted n-i-p devices, leading to inefficient electron collection. By pre-depositing an ultra-thin acceptor primer layer, it develops a novel acceptor-enriched-bottom active layer to reinforce the acceptor-phase continuity. It is however challenging to preserve the primer layer during non-orthogonal solvent processing. Thus, sequential n-type doping is implemented on the surface of the primer layer, which allows to slightly reduce the acceptor solubility by polarity regulation, as well as stabilize the film structure via strong π–π interaction between dopant/host acceptor. Upon acceptor enrichment, higher interfacial electron density enhances the built-in potential while the enlarged domains suppress both charge-transfer state and bimolecular recombination. Consequently, the champion device efficiency is greatly improved from ca. 16.1% to 18.0%, mainly resulting from the simultaneously elevated fill factor and short-circuit current density.