Kun Gao

Thermal effects drive asymmetric electron–hole excitations and lattice perturbations, which cooperatively promote the spontaneous charge separation of excited states, and can be amplified by strengthening the electronic push–pull potential.

Chiral structures have substantial potential for spin information utilization because of their distinctive spatial symmetry-breaking features and the consequent strong spin–charge correlation. One of the limitations in the development of chiral spintronics for both theoretical and practical applications is the limited range of chiral-related signal transmission, which is only tens of nanometers and is usually determined by the conductivity of the chiral materials and device applicability. In addition, the role of circularly polarized light, an important chiral parameter, in chiral signal transmission has been typically neglected. In this study, targeted chiral induction was conducted using the semiconductor polymer PCDTPT ([4-(4,4-dihexadecyl-4 H -cyclopenta[1,2-b:5,4-b′]dithiophen-2-yl)-alt-[1,2,5]thiad-iazolo [3,4-c] pyridine]), which was further used to fabricate a field-effect transistor device. In a dark environment, the chiral-related signal could be transmitted up to 10 μm with an external magnetic field. Through the synergistic effects of spin-selective transition and chiral spin filtering under illumination, the chiral signal expression was fundamentally switched to another form, and the transmission distance was notably increased to the millimeter scale. In addition, owing to the flexible external field tunability (gate voltage, temperature, and polarized light) in multiple scenarios, the chiral signals reflected detailed dynamic changes. The achievement of long-distance chiral signal transmission and circularly polarized light-induced signal conversion and extension is expected to promote the development of chiral spintronics for both theoretical and practical applications.

The commercialization of organic solar cells (OSCs) requires eliminating halogenated solvents. However, the disordered molecular aggregation of nonfullerene acceptors (NFAs) in nonhalogenated solvents hinders achieving high power conversion efficiencies (PCEs). Herein, this problem is addressed using an external physical field modulation strategy involving the sonication of NFA solutions to induce a transformation from disordered molecular aggregation to ordered J-aggregation via microstreaming and shear stress. The aggregation is effectively preserved within films, enhancing π-π stacking interactions. Moreover, sonication promotes the conversion of localized excitons to intra-moiety delocalized excitons and suppresses molecular vibrations. The PCEs of PM6:L8-BO-based binary and ternary devices fabricated using o-xylene are 19.43% and 20.41% (certified 19.84%), ranking among the highest values reported for nonhalogenated solvent-processed OSCs. The binary devices also exhibit high thermal stabilities (T₈₀ lifetime > 10000 h). This universally applicable strategy effectively controls disordered aggregation in NFA solutions, thus paving the way for practical industrial applications.

In the pursuit of advancing the commercialization of organic solar cells (OSCs), stability emerges as a paramount challenge. Herein, we show that the electron transport connectivity is a key factor determining the electron transport and device stability of OSCs. When compared to small molecular acceptors (SMAs), the larger-size polymeric acceptors (PAs) are likely to establish an electron transport network with superior connectivity. This enhanced connectivity enables more robust electron transport during potential device degradation. Our findings indicate that PA-integrated devices sustain elevated electron mobilities, even under reduced acceptor ratios (or higher impurity doping) over prolonged device operation. Furthermore, we employ the refined Su-Schrieffer-Heeger tight-binding model, in tandem with a random electron passing test and algebraic connectivity evaluations of molecular configurations, to conclusively validate the pivotal role played by the electron transport connectivity. These revelations are poised to offer new perspectives for material choices and methodologies for improving stability of OSCs.

The commercialization of organic solar cells (OSCs) requires eliminating halogenated solvents from their production. However, the disordered molecular aggregation of nonfullerene acceptors in nonhalogenated solvents hinders the realization of OSCs with high power conversion efficiencies (PCEs). Herein, this problem is addressed using an external physical field modulation strategy involving the sonication of nonfullerene acceptor solutions. The application of ultrasound field induces a transformation from disordered molecular aggregation to ordered J-aggregation via microstreaming and shear stress in nonhalogenated solvents. The aggregation is effectively preserved within films, causing the formation of a fibril network with enhanced π-π stacking interactions. Moreover, sonication promotes the conversion of localized excitons to intra-moiety delocalized excitons and suppresses molecular vibrations, thus favoring charge separation and reducing nonradiative recombination. Consequently, the PCEs of PM6:L8-BO-based binary and ternary devices fabricated using o -xylene as a nonhalogenated solvent are 19.43% and 20.41% (certified 19.84%), which is among the highest values reported for OSCs produced without halogenated solvents. The binary devices also exhibit high thermal stabilities, with the T ₈₀ lifetime exceeding 10000 h. The developed strategy is applicable to various small-molecule acceptor and polymer acceptor systems, facilitating control disordered aggregation in nonfullerene acceptor solutions and thus paving the way for practical industrial applications.

Optimizing the photoluminescence quantum yield (PLQY) of non-fullerene acceptor (NFA) molecules is critical for reducing the non-radiative recombination energy loss in NFA-based organic solar cells. In this letter, by developing a non-adiabatic dynamical method combined with different electron population rate equations, we separately simulate the radiative and non-radiative recombination process of the NFA molecular excited state, and thus clarify the quantitative correlations of typical characteristics of NFA molecules with their PLQY, including the analyses for the corresponding mechanisms directed against the conventional "energy gap law". The main findings include: weakening the intramolecular electron-phonon coupling and electronic push-pull potential can optimize the competition between radiative and non-radiative recombinations, thus improving PLQY; furthermore, increasing the intermolecular J-aggregation ratio should be an effective strategy to alleviate the aggregation-induced reduction in PLQY. These findings provide clear directions for the rational design of NFA molecules and morphology optimization toward a higher PLQY.

The incorporation of polymeric insulators has led to notable achievements in the field of organic semiconductors. By altering the blending concentration, polymeric insulators exhibit extensive capabilities in regulating molecular configuration, film crystallinity, and mitigation of defect states. However, current research suggests that the improvement in such physical properties is primarily attributed to the enhancement of thin film morphology, an outcome that seems to be an inevitable consequence of incorporating insulators. Herein, we report a general and completely new effect of polymeric insulators in organic semiconductors: the insulator-donor electron wavefunction coupling effect. Such insulators can couple with donor polymers to reduce the energy barrier level and facilitate intramolecular electron transport. Besides the morphological effects, we observed that this coupling effect is another mechanism that can significantly enhance electron mobility (up to 100 times) through the incorporation of polymeric insulators in a series of donor systems. With this effect, we proposed a polymeric insulator blending approach to fabricate state-of-the-art pseudo-bilayer organic solar cells, and the PM6/L8-BO device exhibits a high efficiency of 19.50% (certificated 19.18%) with an improved interfacial electron transport property. This work not only offers a novel perspective on the quantum effect of polymeric insulators in organic semiconductors, but also presents a simple yet effective method for enhancing the performance of organic solar cells.

Improving the radiative recombination rate of nonfullerene acceptor (NFA) molecular excited states can help to promote their photoluminescence quantum yield and thus reduce the nonradiative energy loss in NFA-based organic solar cells. In this Letter, by developing a nonadiabatic dynamical simulation method, we clarify quantitative correlations of some typical characteristics of NFA molecules with their radiative recombination rates. For a single NFA molecule, the weakening of electron−phonon coupling and the strengthening of electron-push−pull potential can each improve the radiative recombination rate. For different NFA molecular aggregates, their radiative recombination rates are all reduced compared with a single molecule, where the A-to-A and A-to-D type J-aggregates have higher rates than D-to-D type H-aggregate. To further improve the radiative recombination rate of NFA molecular J-aggregates, we should increase the intermolecular distance, such as extending the side chain length.

Ultrafast pump-push-probe/photocurrent experiments have confirmed that free charges can be spontaneously generated in single-component organic solar cells. A deeper understanding of the experimental results is expected to further modulate the charge yield. Herein, the effect of an infrared push pulse on the relaxed exciton in conjugated polymers is theoretically studied. We find that the relaxed exciton can be pushed into different hot excitons depending on the energy of this infrared pulse. In particular, the dynamics of the transition from localized to delocalized excitons is explicitly presented. Moreover, we attempt to demonstrate that the delocalization effect of hot exciton is favorable for charge generation by introducing a driving field. The results suggest that the strength of the driving field and timescale required for the dissociation of hot exciton is significantly reduced compared to this relaxed exciton. Finally, the influence of the photoexcitation conditions on the charge generation is discussed to further elucidate the effect of hot exciton delocalization. Overall, this work has the potential to provide further information for the analysis and control of charge generation by hot exciton dissociation.

The energy loss of hole-charge transfer in nonfullerene-acceptor (NFA) organic solar cells has been significantly reduced by minimizing their interfacial energy offset to near-zero and negative values. However, the physical mechanism of highly efficient hole-charge transfer in these conditions has not been fully addressed. In this work, we theoretically clarify that a moving hole charge along the polymer-donor and NFA interface, generated from the spontaneous dissociation of NFA molecular excited states, tends to transfer from NFA molecules to polymer-donor to keep moving, even their interfacial energy offset is chosen as a negative value. This dynamic viewpoint revises the conventional static viewpoint about the dependence of hole-charge transfer upon the interfacial energy offset. In addition, considering the polaron effect in organic molecules with a hole charge generated, we redefine the interfacial energy offset and further consider the effects of the NFA molecular electron-lattice interaction and lattice elastic constant on the hole-charge transfer.