N-Doping Donor-Dilute Semitransparent Organic Solar Cells to Weaken Donor: Acceptor Miscibility and Consolidate Donor-Phase Continuity.

Lightweight and semi-transparent organic solar cells (ST-OSCs) offer bright promise for applications such as building integrated photovoltaics. Diluting donor content in bulk-heterojunction active layers to allow greater visible light transmittance (AVT) effectively enhances device transparency, yet the ineluctable compromise of the donor-phase continuity is challenging for efficient charge transport. Herein, a trace amount of n-type N-DMBI dopant is incorporated, which facilitates the donor:acceptor (D:A) de-mixing by strengthening both acceptor polarity and D/A crystallization. With the diminution of component inter-mixing, the limited number of donors increasingly self-aggregate to establish the more continuous phases. For the benchmark PM6:Y6-based ST-OSCs, when the donor content is reduced from regular 45 to optimal 30 wt.%, the device AVT is remarkably raised by more than a quarter, accompanied by a marginal drop in power conversion efficiency from 13.89% to 13.03%. This study reveals that by decreasing the donor content to <30 wt%, acceptor excitons induced by Förster resonance energy transfer are prone to severe radiative recombination. This is nonetheless mitigated by dopant inclusion within the acceptor phase by providing extra energy offset and prolonging charge transfer state lifetime to assist exciton dissociation.