Molecular dispersion enhances photovoltaic efficiency and thermal stability in quasi-bilayer organic solar cells

In comparison to widely adopted bulk heterojunction (BHJ) structures for organic solar cells (OSC), exploiting the sequential deposition to form planar heterojunction (PHJ) structures enables to realize the favorable vertical phase separation to facilitate charge extraction and reduce charge recombination in OSCs. However, effective tunings on the power conversion efficiency (PCE) in PHJ-OSCs are still restrained by the currently available methods. Based on a polymeric donor PBDBT-2F (PBDBT=Poly [[4,8-bis [5-(2-ethylhexyl)-4-fluoro-2-thienyl]benzo [1,2-b:4,5-b ']dithiophene-2,6-diyl]-2,5-thiophenediyl [5,7-bis (2-ethylhexyl)-4,8-dioxo-4H,8H-benzo [1,2-c:4,5-c ']dithiophene-1,3-diyl]-2,5-thiophenediyl]) and a non-fullerene (NF) acceptor Y6, we proposed a strategy to improve the properties of photovoltaic performances in PHJ-based OSCs through dilute dispersions of the PBDBT-2F donor into the acceptor-dominant phase with the sequential film deposition. With the control of donor dispersions, the charge transport balance in the PHJ-OSCs is improved, leading to the expedited photocarrier sweep-out with reduced bimolecular charge recombination. As a result, a PCE of 15.4% is achieved in the PHJ-OSCs. Importantly, the PHJ solar cells with donor dispersions exhibit better thermal stability than corresponding BHJ devices, which is related to the better film morphology robustness and less affected charge sweep-out during the thermal aging.