Despite considerable advances devoted to improving the operational stability of organic solar cells (OSCs), the metastable morphology degradation remains a challenging obstacle for their practical application. Herein, the stabilizing function of the alloy states in the photoactive layer from the perspective of controlling the aggregation characteristics of non‐fullerene acceptors (NFAs), is revealed. The alloy‐like model is adopted separately into host donor and acceptor materials of the state‐of‐the‐art binary PM6:BTP‐4Cl blend with the self‐stable polymer acceptor PDI‐2T and small molecule donor DRCN5T as the third components, delivering the simultaneously enhanced photovoltaic efficiency and storage stability. In such ternary systems, two separate arguments can rationalize their operating principles: (1) the acceptor alloys strengthen the conformational rigidity of BTP‐4Cl molecules to restrain the intramolecular vibrations for rapid relaxation of high‐energy excited states to stabilize BTP‐4Cl acceptor. (2) The donor alloys optimize the fibril network microstructure of PM6 polymer to restrict the kinetic diffusion and aggregation of BTP‐4Cl molecules. According to the superior morphological stability, non‐radiative defect trapping coefficients can be drastically reduced without forming the long‐lived, trapped charge species in ternary blends. The results highlight the novel protective mechanisms of engineering the alloy‐like composites for reinforcing the long‐term stability of NFA‐based ternary OSCs.
The underlying hole-transfer mechanism in high-efficiency OSC bulk heterojunctions based on acceptor–donor–acceptor (A–D–A) nonfullerene acceptors (NFAs) remains unclear. Herein, we study the hole-transfer process between copolymer donor J91 and five A–D–A NFAs with different highest occupied molecular orbital energy offsets (ΔE H) (0.05–0.42 eV) via ultrafast optical spectroscopies. Transient absorption spectra reveal a rapid hole-transfer rate with small ΔE H, suggesting that a large energy offset is not required to overcome the exciton binding energy. Capacitance–frequency spectra and time-resolved photoluminescence spectra confirm the delocalization of an A–D–A-structured acceptor exciton with weak binding energy. Relative to the hole-transfer rate, hole-transfer efficiency is the key factor affecting device performance. We propose that holes primarily stem from weakly bound acceptor exciton dissociation, revealing a new insight into the hole-transfer process in A–D–A NFA-based OSCs.
The kinetic aggregation of nonfullerene acceptors under nonequilibrium conditions can induce electron–phonon interaction roll‐off and electronic band structure transition, which represents an important limitation for long‐term operational stability of organic solar cells (OSCs). However, the fundamental underlying mechanisms have received limited attention. Herein, a photophysical correlation picture between intermolecular electron–phonon coupling and trapping of electronic excitation is proposed based on the different aggregation behaviors of BTP‐eC9 in bulk‐heterojunction and layer‐by‐layer processed multicomponent OSCs. Two separate factors rationalize their correlation mechanisms: 1) the local lattice and/or molecular deformation can be regarded as the results of BTP‐eC9 aggregates in binary system under continuous heating, which brings about attenuated intermolecular electron–phonon coupling with intensified photocarrier trapping. 2) The higher density of trap states with more extended tails into the bandgap give rise to the formation of highly localized trapped polarons with a longer lifetime. The stabilized intermolecular electron–phonon coupling through synergistic regulation of donor and acceptor materials effectively suppresses unfavorable photocarrier trapping, delivering the improved device efficiency of 18.10% and enhanced thermal stability in quaternary OSCs. These results provide valuable property–function insights for further boosting photovoltaic stability in view of modulating intermolecular electron–phonon coupling.
Fluorination and chlorination have yielded a novel class of materials and achieved tremendous progress in enhancing photovoltaic efficiency in organic solar cells (OSCs). However, their effects on photocarrier dynamics remain elusive in these organic photovoltaic systems. Herein, a comprehensive study on the underlying mechanisms is conducted based on a 2 × 2 photovoltaic matrix, consisting of PBDB‐T, PBDB‐T‐2Cl, ITIC, and IT4F. Chlorination of donors enhances exciton migration and relaxation rates and promotes the extraction of polarons. The more efficient charge transfer and a larger proportion of long‐lived polarons are observed in fluorine‐containing acceptor‐based systems, which are in favor of charge generation in the actual devices. According to the enlarged dielectric constant in the PBDB‐T‐2Cl:IT4F blend, the improved exciton delocalization, the decreased exciton binding energy, and Coulomb capture radius are obtained relative to other three binary systems, which can increase charge separation efficiency and reduce the probability of bimolecular recombination. The simultaneous fluorination and chlorination can optimize molecular packing and nanoscale phase separation, facilitating effective exciton diffusion, exciton dissociation, and charge transport. These results highlight the important role of fluorination and chlorination on these fundamental mechanisms, possibly resulting in some new molecular design principles toward high‐performance OSCs.
efficiencies (PCEs) of OSCs have been approaching 20%, [5][6][7][8][9] lifting its limitations for industrialization. These encouraging efficiencies are usually obtained from toxic solvent by spin coating process, which is not compatible to production upscaling. On switching to scalable printing techniques and environmentally friendly solvents, there is usually a notable drop in PCEs, which deserve further research attentions to bridge the gap. Strategies including modifying the molecular structures of photoactive materials, [10,11] incorporating additives, [12,13] adopting new device structures, [14] adjusting film formation kinetics with third component [15,16] or substrate temperature [17] or gas flow [18] have been attempted to achieve high PCEs with scalable and halogen-free solvent processing. Revealing the molecular design rules and film formation dynamics is vital to control the thin-film microstructures and finally the photovoltaic performance.In this work, we systematically investigate the fundamental processingmicrostructure-function relationship during transforming from spin coating with halogenated solvent to doctor blade coating with halogen-free solvent for high-performance photoactive materials based on polymer donor D18 [19] and two representative nonfullerene acceptors (NFAs), i.e., BTP-eC9 [11] and Y6. [20] The current power conversion efficiencies of laboratory-sized organic solar cells (OSCs), based on the spin-coating process with halogenated solvents, have exceeded 19%. Environmentally friendly printing is needed to bridge the gap between laboratory and industrialization by being compatible with rollto-roll large-area production. Here, the molecular design rules are revealed for enhancing the green printing potential of the state-of-the-art photovoltaic martial systems by investigating the detailed structure formation dynamic and the key determining factors. By comparing two model systems based on D18:Y6 and D18:BTP-eC9, it is found that disordered preaggregation in liquid state can result in over-sized domains with reduced crystallinity and disordered molecular orientation, which significantly limits device performance. By systematically tuning the length of the inner alkyl side chains with multiple Y-series materials, the authors demonstrate that molecular side-chain engineering can effectively supress the detrimental disordered preaggregation in liquid state during environmentally friendly printing process, leading to enhanced crystallization with preferential faceon molecular orientation, more efficient exciton dissociation and charge carrier transport, and finally high upscaling potential. The work provides deeper insights into molecular engineering and structure formation dynamics toward environmentally friendly production of OSCs.
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