The structural order and aggregation of non‐fullerene acceptors (NFA) are critical toward light absorption, phase separation, and charge transport properties of their photovoltaic blends with electron donors, and determine the power conversion efficiency (PCE) of the corresponding organic solar cells (OSCs). In this work, the fibrillization of small molecular NFA L8‐BO with the assistance of fused‐ring solvent additive 1‐fluoronaphthalene (FN) to substantially improve device PCE is demonstrated. Molecular dynamics simulations show that FN attaches to the backbone of L8‐BO as the molecular bridge to enhance the intermolecular packing , inducing 1D self‐assembly of L8‐BO into fine fibrils with a compact polycrystal structure. The L8‐BO fibrils are incorporated into a pseudo‐bulk heterojunction (P‐BHJ) active layer with D18 as a donor, and show enhanced light absorption, charge transport, and collection properties, leading to enhanced PCE from 16.0% to an unprecedented 19.0% in the D18/L8‐BO binary P‐BHJ OSC, featuring a high fill factor of 80%. This work demonstrates a strategy for fibrillating NFAs toward the enhanced performance of OSCs.
This paper demonstrates a facile and low-cost carbothermal reduction preparation of monodisperse FeO/C core-shell nanosheets (NSs) for greatly improved microwave absorption. In this protocol, the redox reaction between sheet-like hematite (α-FeO) precursors and acetone under inert atmosphere and elevated temperature generates FeO/C core-shell NSs with the morphology inheriting from α-FeO. Thus, FeO/C core-shell NSs of different sizes ( a) and FeO/C core-shell nanopolyhedrons are obtained by using different precursors. Benefited from the high crystallinity of the FeO core and the thin carbon layer, the resultant NSs exhibit high specific saturation magnetization larger than 82.51 emu·g. Simultaneously, the coercivity enhances with the increase of a, suggesting a strong shape anisotropy effect. Furthermore, because of the anisotropy structure and the complementary behavior between FeO and C, the as-obtained FeO/C core-shell NSs exhibit strong natural magnetic resonance at a high frequency, enhanced interfacial polarization, and improved impedance matching, ensuring the enhancement of the microwave absorption. The 250 nm NSs-paraffin composites exhibit reflection loss (RL) lower than -20 dB (corresponding to 99% absorption) in a large frequency ( f) range of 2.08-16.40 GHz with a minimum RL of -43.95 dB at f = 3.92 GHz when the thickness is tuned from 7.0 to 1.4 mm, indicating that the FeO/C core-shell NSs are a good candidate to manufacture high-performance microwave absorbers. Moreover, the as-developed carbothermal reduction method could be applied for the fabrication of other composites based on ferrites and carbon.
Tailoring of the chemical structure is an effective method to tune the aggregation and optoelectronic properties of organic photovoltaic materials to boost the performance of organic solar cells (OSCs). Here, four non-fullerene electron acceptor materials, namely, BTP-4F-C8-16, BTP-4F-C7-16, BTP-4F-C6-16, and BTP-4F-C5-16, with different lengths of alkyl chain on the bithiophene units were synthesized, and the impact of chain length on the intermolecular stacking, nanoscale phase separation with polymer donors, optoelectronic properties, and device performance were investigated. Molecular dynamics simulations and experimental exploration show that reducing the chain length from n-octyl (C8) to n-pentyl (C5) can enhance the molecular planarity, shorten the π−π stacking distance, and improve the electron mobility, consequently leading to enhanced structural order, charge mobility, and appropriate phase separation in the blend with PM6, contributing to the achievement of the best power conversion efficiency of 18.20% with a V OC of 0.844 V, a fill factor of 77.68%, and a J SC of 27.78 mA cm −2 , which is one of the highest efficiencies of single-junction binary OSCs reported in the literature so far.
Side chain engineering is a widely explored strategy in the molecular design for non-fullerene acceptors (NFAs). Although the relationship between side chain structures and optoelectronic properties of NFAs is well clarified, the effect of side chain structures on the stability of NFAs and their corresponding organic solar cells (OSCs) is rarely reported. Herein, a series of Y-family NFAs with varying side-chains are studied to investigate their degradation upon multiple stresses including water, oxygen from ambient, chemical environment from ZnO electron transport layer, temperature, and ultraviolet light. The results show that all of these Y-family NFAs are highly stable against water and oxygen in ambient dark condition, while their photochemical and thermal stabilities decrease with the increasing side chain length. NFAs with shorter side chains are not only more resistant to photooxidation and photocatalytic reactions, but also can hamper the formation of large phase-separated NFA domains upon storage in both glovebox and ambient conditions. As such, the PM6:NFA OSC with short side-chain NFA also exhibits superior operational stability, associating with a higher T 80 lifetime. This study demonstrates that the side chains must be considered to obtain stable OSCs.
Chemical design and physical control of the molecular aggregation of organic semiconductors have been demonstrated to be efficient strategies to prepare high performance organic solar cells (OSCs). Starting from the non-fullerene acceptor (NFA) BTP-4Cl-C9-12, two NFAs named BTP-4Cl-C9-16 and BTP-4Cl-C9-20 with the alkyl chains of 2-ethylhexyl and 2octyldodecyl attached on the pyrrole rings are synthesized in this work. Through molecular dynamics simulations and experimental characterizations, we show that favorable three-dimensional (3D) honeycomb networks, which are beneficial for charge transport, can be formed in NFAs with the moderate alkyl chain length (BTP-4Cl-C9-12 and BTP-4Cl-C9-16), while two-dimensional honeycomb networks form in BTP-4Cl-C9-20 with long alkyl chains. 1,8-Diiodooctane solvent molecules adsorb on all alkyl chains of NFAs, reducing the adsorption energy between NFAs to promote their intermolecular interactions, especially in NFAs with longer alkyl chains. As a result, the synergistic effect of the 3D network and the appropriate domain size leads to a promising power conversion efficiency of 18.0% and 15.9% in thin-(100 nm) and thick-(300 nm) PM6:BTP-4Cl-C9-16 binary OSCs. This work presents a comprehensive understanding of the interaction between the NFA and solvent additive and provides rational guidance for the molecular design and morphology regulation of NFA-based OSCs toward higher performance.
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