Introduction
All-polymer solar cells (all-PSCs) hold great promise for future
advancements in the field of organic solar cells thanks to their
excellent film-formation and excellent
properties.[1-11] To date, the highest PCE
achieved in all-PSCs has surpassed 18%[12-14]thanks to the rapid development of polymer acceptors, especially the
recent strategy of polymerizing high-performance small molecule
acceptors.[15-25] Nevertheless, the development of
these polymerized small molecule acceptors (PSMAs) is so rapid that
there is a gap between the exhibited performance and the highest
achievable performance of the materials due to the relatively slow pace
of device engineering. For instance, Yang et al. recently reported a
PSMA named PJ1r, whose frontier orbital energy levels are uplifted
compared to the popular PSMA such as PY-IT and PYF-T-o, which should
promote the voltage of the device in principle. However, the best PCE
reported so far of this PSMA was only about
16.1%,[26] achieved using a polymer donor (JD40)
with an uplifted LUMO and HOMO levels than the workhorse donor, PM6.
This could benefit exciton dissociation but it undermined the potential
of the PSMA to achieve high Voc.
Furthermore, the best efficiency of most recent high-performance
all-PSCs was achieved using chloroform as the processing solvent
including PJ1r.[12-13, 27-37] In addition to its
hazardous properties, chloroform has a high vapor pressure at
temperatures relevant to device fabrication, making it unsuitable for
large-scale fabrication techniques such as slot-die coating and
blade-coating.[38-46] To take advantage of the
superior film-formation properties and mechanical properties of all-PSCs
in future large-scale productions, chloroform must be replaced
processing solvent with a high boiling point at standard pressure,
preferentially non-halogen solvents.
In this paper, we combined the sequential processing (SqP) technique and
a solid additive strategy to address the aforementioned issues, namely,
promoting the performance of existing PSMAs and using hydrocarbon
solvent to replace chloroform. Compared to the conventional
blend-casting (BC) method that relies on their inherent crystallization
kinetics to achieve vertical phase separation, the SqP technique
separates the preparation of the donor and acceptor solutions and the
deposition of them, which utilizes the mutual solubility of the upper
solution with the underlying film to form a bicontinuous
interpenetrating network structure known as the bulk heterojunction
(BHJ). On the basis of SqP, we further mix a tiny amount of
PC71BM into the active layer as a solid additive (SA) to
tune the morphology and electrical properties of all-PSCs in this study.
Specifically, we first employ PM6 as the donor polymer and PJ1-γas the acceptor to fabricate binary all-PSC devices. We demonstrate a
17.4% PCE for the SqP PM6:PJ1-r device compared to a 15.8% PCE for the
BC device. Subsequently, when 0.5% PC71BM is introduced
as a solid additive into the active layer, the PCE is further increased
to 18.0%. Film-depth-dependent light absorption spectra (FLAS) reveals
that the addition of the SA improves the vertical phase segregation of
the active layer, leading to enhanced charge carrier lifetime, reduced
carrier extraction time, increased electron mobility, and more balanced
electron/hole transport, which in turn contributes to the high
photocurrent. Moreover, we carry out the same experiments using another
PSMA (PYF-T-o ) and provide side-by-side comparison between the
devices with these two acceptors. We show that the addition of the SA
also increases the performance of the PYF-T-o devices. Finally,
the maximum power point tracking (MPPT) technique is used to study the
device stability. We show that both the incorporation of SA and the SqP
method improve the photo-stability of the device.
Results and Discussion