Despite remarkable breakthrough made by virtue of “polymerized small‐molecule acceptor (PSMA)” strategy recently, the limited selection pool of high‐performance polymer acceptors and long‐standing challenge in morphology control impede their further developments. Herein, three PSMAs of PYDT‐2F, PYDT‐3F, and PYDT‐4F are developed by introducing different fluorine atoms on the end groups and/or bithiophene spacers to fine‐tune their optoelectronic properties for high‐performance PSMAs. The PSMAs exhibit narrow bandgap and energy levels that match well with PM6 donor. The fluorination promotes the crystallization of the polymer chain for enhanced electron mobility, which is further improved by following
n
‐doping with benzyl viologen additive. Moreover, the miscibility is also improved by introducing more fluorine atoms, which promotes the intermixing with PM6 donor. Among them, PYDT‐3F exhibits well‐balanced high crystallinity and miscibility with PM6 donor; thus, the layer‐by‐layer processed PM6/PYDT‐3F film obtains an optimal nanofibril morphology with submicron length and ≈23 nm width of fibrils, facilitating the charge separation and transport. The resulting PM6/PYDT‐3F devices realizes a record high power conversion efficiency (PCE) of 17.41% and fill factor of 77.01%, higher than the PM6/PYDT‐2F (PCE = 16.25%) and PM6/PYDT‐4F (PCE = 16.77%) devices.
Suppressing the photon energy loss (Eloss), especially the non‐radiative loss, is of importance to further improve the device performance of organic solar cells (OSCs). However, typical π‐conjugated semiconductors possess a large singlet–triplet energy gap (ΔEST), leading to a lower triplet state than charge transfer state and contributing to a non‐radiative loss channel of the photocurrent by the triplet state. Herein, a series of triplet polymer donors are developed by introducing a BNIDT block into the PM6 polymer backbone. The high electron affinity of BNIDT and the opposite resonance effect of the BN bond in BNIDT results in a lowered highest occupied molecular orbital (HOMO) and a largely reduced ΔEST. Moreover, the morphology of the active blends is also optimized by fine‐tuning the BNIDT content. Therefore, non‐radiative recombination via the terminal triplet loss channels and morphology traps is effectively suppressed. The PNB‐3 (with 3% BNIDT):L8‐BO device exhibits both small ΔEST and optimized morphology, favoring more efficient charge transfer and transport. Finally, the simultaneously enhanced Voc of 0.907 V, Jsc of 26.59 mA cm−2, and FF of 78.86% contribute to a champion PCE of 19.02%. Therefore, introducing BN bonds into benchmark polymers is a possible avenue toward higher‐performance of OSCs.
Symmetry-breaking
charge separation (SB-CS) provides a very promising
option to engineer a novel light conversion scheme, while it is still
a challenge to realize SB-CS in a nonpolar environment. The strength
of electronic coupling plays a crucial role in determining the exciton
dynamics of organic semiconductors. Herein, we describe how to mediate
interchromophore coupling to achieve SB-CS in a nonpolar solvent by
the use of two perylenediimide (PDI)-based trimers, 1,7-tri-PDI and 1,6-tri-PDI. Although functionalization at the
N-atom decreases electronic coupling between PDI units, our strategy
takes advantage of “bridge resonance”,
in which the frontier orbital energies are nearly degenerate with
those of the covalently linked PDI units, leading to enhanced interchromophore
electronic coupling. Tunable electronic coupling was realized by the
judicious combination of “bridge resonance” with N-functionalization. The enhanced
mixing between the S1 state and CT/CS states results in
direct observation of the CT band in the steady-state UV–vis
absorption and negative free energy of charge separation (ΔG
CS) in both chloroform and toluene for the two
trimers. Using transient absorption spectroscopy, we demonstrated
that photoinduced SB-CS in a nonpolar solvent is feasible. This work
highlights that the use of “bridge resonance” is an effective way to control exciton dynamics of organic
semiconductors.
Terpolymer fabrication is an effective methodology for molecular engineering and generating high‐performance organic photovoltaic materials to construct highly efficient polymer solar cells. Modification of the polymer PM6 by incorporating a third component resulting in the formation of a ternary copolymer is reported to outperform PM6 in achieving enhanced device performances. However, one of the major challenges in constructing high‐performance terpolymers is to counter the molecular disorder caused by the backbone entropy induced by the third moiety. In this work, double B←N bridged bipyridine (BNBP) is used as the third component, which possesses a strong out‐of‐plane electrostatic dipole owing to the saddle‐shaped B←N fused ring structure. The out‐of‐plane dipole moment introduced in the modified PM6 terpolymer can be used as a means for tuning and optimizing the nanostructures of the blended films. The prepared PM6‐BNBP‐4 blend polymer with 4% of the benzodithiophene dione monomers replaced by BNBP results in excellent power conversion efficiency of 19.13%. This work demonstrates that the out‐of‐plane electrostatic dipole moment in saddle‐shaped molecules is valuable for achieving high‐performance organic photovoltaic donor materials.
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