Donor–acceptor mutually diluted heterojunctions for layer-by-layer fabrication of high-performance organic solar cells

The photoactive layer of organic solar cells consists of p-type electron donors and n-type electron acceptors, which phase separate to form fine and continuous networks for charge transport. The impact of the donor–acceptor interaction on the microstructure and optoelectronics of the photoactive layer remains unclear. In this work, a tiny amount (1 wt%) of donor PM6 is added into the non-fullerene acceptor (NFA) C8- R or L8-BO (or vice versa) to form a donor (or acceptor) diluted heterojunction. The structural order is improved through dipole–dipole interaction between the donor and the acceptor owing to their opposite electronegativity. We fabricate a pseudo-bilayer heterojunction solar cell based on NFA-diluted donor (that is, donor + 1% NFA) and donor-diluted NFA (that is, NFA + 1% donor) layers: the device exhibits superior power conversion efficiencies compared with their bulk heterojunction and conventional pseudo-bilayer counterparts. We demonstrate an efficiency of 19.4% (certified 19.1%) and 17.6% for 100 and 300 nm-thick PM6 + 1% L8-BO/L8-BO + 1% PM6 solar cells, respectively.