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.

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.

Organic semiconductors are generally featured with low structure order in solid state films, which leads to low charge transport mobility and strong charge recombination in their photovoltaic devices. In this work, we report a “polycrystal‐induced aggregation” strategy to order the polymer donor (PM6) and non‐fullerene acceptor (L8‐BO) molecules during solution casting with the assistance of PM6 polycrystals that were incubated through a vapor diffusion method, toward improved solar cell efficiency with either thin or thick photoactive layers. These PM6 polycrystals were redissolved in chloroform to prepare PM6 pre‐aggregates (PM6‐PA), and further incorporated into the conventional PM6:L8‐BO blend solutions, which was found to prolong the molecular organization process and enhance the aggregation of both PM6 and L8‐BO components. As the results, with the assistance of 10% PM6‐PA, PM6:L8‐BO solar cell devices obtained power conversion efficiencies (PCEs) from 18.0% and 16.2% to 19.3% and 17.2% with a 100 and 300 nm ‐thick photoactive layer respectively.This article is protected by copyright. All rights reserved

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.

Ternary strategy is identified as an effective method to fine-tune the optoelectronic properties of the photoactive layer of organic solar cells (OSCs) toward high power conversion efficiency (PCE). Although numerous highperformance ternary OSCs have been established, the underlying fundamentals of ternary OSCs are still not fully understood, and a general rule for the ternary system design is highly appreciated. In this contribution, we demonstrate the construction of high-performance ternary OSCs via the cocrystallization of conjugated host donor PM6 and guest donor D18-Cl. Due to the similar chemical structure, larger planarity, and lower surface energy of D18-Cl, it locates in the PM6 phase and organizes with PM6 during solution casting, leading to the formation of co-crystallized fibrillar donor phase with enhanced structural order and charge transport channels. As a result, a series of binary host systems, including PM6:BTP-4F-C6-16, PM6:Y6, and PM6:Y7-BO obtained enhanced efficiency with the presence of D18-Cl as the guest donor, achieving a superior PCE of 18.5% in the PM6:D18-Cl:BTP-4F-C6-16 ternary OSCs. This work is the first demonstration of co-crystallized polymer donor fibrils to boost the charge transport and power conversion of ternary OSCs.

Interlayers play a vital role in achieving high efficiency and stability of organic solar cells (OSCs). Zinc oxide (ZnO) has been widely used as an electron transport layer (ETL) in inverted OSCs; however, its high structural defects and intrinsic photocatalytic nature toward nonfullerene acceptors limit its applications in OSCs. Herein, a low-cost, environmentally-friendly biomolecule, potassium aspartic acid (PAA), is introduced as the interlayer on top of the ZnO ETL. Through experimental results and theoretical calculations, we find PAA not only can tune energy alignments and passivate oxygen vacancy defects and zinc interstitial dangling bonds but also can promote the π–π stacking strength of the active layer, leading to enhanced charge collection and photovoltaic performance in both IT series (e.g., PM6:IT-4F) and Y series (e.g., PM6:BTP-4F-C5-16) OSCs. Moreover, benefiting from the reduced surface defects of ZnO, OSCs based upon the ZnO/PAA ETL exhibit superior stabilities under continuous operation as well as UV-light irradiation, leading to an improved T 80 lifetime of around 4 times compared to OSCs fabricated without the PAA interlayer. This work provides a universal solution to fabricate efficient and stable inverted OSCs.

Ternary strategy is demonstrated as an efficient approach to achieve high short‐circuit current and open‐circuit voltage to boost the performance of organic solar cells (OSCs), however, the realization of high fill‐factor (FF) in ternary OSCs has been rare. In this study, three thiophene terminated non‐fullerene acceptors (NFAs) with methyl or chlorine substitutions on their end‐groups are designed and synthesized, and further incorporated into the state‐of‐the‐art PM6:L8‐BO system to construct ternary OSCs. Subtle changes in their chemical structures significantly modify the molecular packings of these thiophene terminated NFAs. While BTP‐ThMe and BTP‐ThCl have limited forms of dimer, versatile molecular dimers, including “Z” shaped D‐D, “S” shaped A‐A, and “F” shaped A‐D packings exist in BTP‐ThMeCl, which lead to the formation of compact 3D honey‐comb network and this is analogous to the host acceptor L8‐BO. This synergetic molecular packing between BTP‐ThMeCl and L8‐BO contributes to maintain the 3D charge transport network in the ternary system via the formation of NFA co‐crystals at the molecular level, and consequently realizing a maximum power conversion efficiency of 19.1% with a superior FF of 82.2%, which is the highest FF reported so far for OSCs.

Organic semiconductors based upon conjugated frameworks are often isomeric and display distinct optoelectronic properties within minor structural variation. In this work, two isomeric nonfullerene acceptors (NFAs) ThMeCl-1 and ThMeCl-2, having the methyl and chlorine atoms attached on different positions of the electron-withdrawing end group, are synthesized and incorporated as the third component in ternary solar cells. Although these NFA isomers exhibit a similar bandgap, energy levels, and energy loss in their PM6 based binary devices, the efficiency enhancements in ternary devices differ significantly. Compared to ThMeCl-1, the incorporation of ThMeCl-2 in PM6:C5-16 solar cells enables less energy loss, leading to an extra 0.03 eV open-circuit voltage gain and a maximum efficiency increase from 17.8 to 18.9%. Grazing-incidence X-ray diffraction and molecular dynamics simulations reveal that this is attributed to the versatile intermolecular π−π stacking forms between ThMeCl-2 and the host NFA, which result in improved charge transport and suppressed recombination. This work provides a rational guidance for controlling the molecular packing in ternary systems to prepare high performance organic photovoltaics.