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.