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.

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.

The photovoltaic performance of inorganic perovskite solar cells (PSCs) still lags behind the organic–inorganic hybrid PSCs due to limited light absorption of wide bandgap CsPbI3‑xBr x under solar illumination. Constructing tandem devices with organic solar cells can effectively extend light absorption toward the long-wavelength region and reduce radiative photovoltage loss. Herein, we utilize wide-bandgap CsPbI2Br semiconductor and narrow-bandgap PM6:Y6-BO blend to fabricate perovskite/organic tandem solar cells with an efficiency of 21.1% and a very small tandem open-circuit voltage loss of 0.06 V. We demonstrate that the hole transport material of the interconnecting layers plays a critical role in determining efficiency, with polyTPD being superior to PBDB-T-Si and D18 due to its low parasitic absorption, sufficient hole mobility and quasi-Ohmic contact to suppress charge accumulation and voltage loss within the tandem device. These perovskite/organic tandem devices also display superior storage, thermal and ultraviolet stabilities.

The additive strategy is widely used in optimizing the morphology of organic solar cells (OSCs). The majority of additives are liquid with high boiling points, which will be trapped within device and consequently deteriorate performance during operation. In this work, solid but volatile additives 2‐(4‐fluorobenzylidene)‐1H‐indene‐1,3(2H)‐dione (INB‐F) and 2‐(4‐chlorobenzylidene)‐1H‐indene‐1,3(2H)‐dione (INB‐Cl) are designed to replace the common 1,8‐diiodooctane (DIO) in nonfullerene OSCs. These additives present during solution casting but evaporate after moderate heating. Molecular dynamics simulations show that they can reduce the adsorption energy to improve π‐π stacking among nonfullerene acceptor (NFA) molecules, an effect that enhances light absorption and electron mobility. Both INB‐F and INB‐Cl enhance efficiency, with INB‐F achieving a maximum efficiency of 16.7% from 15.1% of the reference PBDB‐T‐2F (PM6):BTP‐BO‐4F (Y6‐BO) cell, and outperforming DIO. Remarkably, they can simultaneously enhance the operational stability, with the INB‐F‐treated OSC maintaining over 60% of the initial efficiency after 1000 h operation, demonstrating a T80 lifetime of 523 h, which is a significant improvement over T80 values of 66.2 h for the reference and 6.6 h for DIO‐treated OSC. The simultaneously enhanced efficiency and operational lifetime are also effective in PM6:BTP‐BO‐4Cl (Y7‐BO) OSCs, demonstrating a universal strategy to improve the performance of OSCs.

Besides the intrinsic optoelectronic properties of photovoltaic materials and the device architectures, the nanoscale morphology within the photoactive layer, including molecular packing in molecule level and molecular aggregation in nanoscale, [12][13][14][15][16] represents a vital factor in optimizing the device performance and can be manipulated via various approaches. [17][18][19][20][21][22][23][24] It is known that the pre-aggregates of organic semiconductors in solution have a profound influence toward their morphology in solid thin films, and various physical and chemical approaches, including the modulation of solvent, [25] solvent additive, [26] temperature [27] and molecular structure, [28,29] have been demonstrated in the literature to manipulate the pre-aggregation behavior. [13,30] For example, additives can induce polymer aggregates with short range order in solution, which can then act as the nuclei for polymer crystallization during the solution casting process, leading to many small polymer domains with jagged interfaces, resulting in enhanced light harvesting and charge separation. [31] Co-solvents have been used to induce polymer aggregation and prevent the formation of large domains of fullerene as well as restraining the liquidliquid phase separation in PDPP5T:PC 70 BM system in order to receive high performance. [32] Ma et al. controlled the solution and substrate temperatures during the casting of PM6:Y6 from hydrocarbon solvents to enable similar aggregation states in solutions and solid films compared to those cast from halogenated solution, and therefore maintained the device PCE cast from hydrocarbon solvents. [33] Zhang et al. tuned the molecular weight of PM6 to optimized OSCs with suitable size and purity of the domains to achieve well-balanced charge transport, and found that PM6 with higher molecular weight possessed a stronger aggregation degree in solutions. [34] The temperature-dependent aggregation property of conjugated polymers offers the approach to manipulate the pre-aggregation through temperature control, which is also a facile and low-cost approach. [35,36] Different from previous work on tuning the aggregation in a temperature range from room-temperature to over 100 °C, weThe molecular ordering and pre-aggregation of photovoltaic materials in solution can significantly affect the nanoscale morphology in solid photoactive layers, and play a vital role in determining the power conversion efficiency (PCE) of organic solar cells (OSCs). Herein, a cold-aging strategy is reported to mediate the pre-aggregation of PM6 polymer in solution through a disorder-order transition, which leads to dense and fine PM6 aggregates with enhanced π−π stacking in its blend thin films with either fused-ring and non-fused-ring non-fullerene acceptors (NFAs) including Y6-BO, N3, IT-4F, and PTIC. The fine aggregates of PM6 and slightly enlarged NFA domains improve the continuous networks with enhanced and balanced charge mobility. The resulting OSCs all demonstrate enhanced PCEs compared to the...

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.

Fused‐ring non‐fullerene electron acceptors (NFAs) boost the power conversion efficiencies (PCEs) of organic solar cells (OSCs). Asymmetric and halogenated NFAs have drawn increasing attention in recent years due to their unique optoelectronic properties. Starting from the symmetric NFA ITCC‐M, this work systematically designs and synthesizes an asymmetric counterpart ITCC‐M‐2F, halogenated counterpart ITCC‐Cl, and asymmetric and halogenated counterpart IDTT‐Cl‐2F. Among these NFAs, IDTT‐Cl‐2F shows the shallowest lowest unoccupied molecular orbital energy level, broader absorption range, and the tightest molecular packing. As a result, when blended with the donor PBDB‐T‐2Cl, IDTT‐Cl‐2F‐based OSCs yield the highest PCE of 13.3% with an open‐circuit voltage of 0.96 V, short‐circuit current of 19.20 mA cm–2, and fill factor of 71.1%, which is the highest PCE of OSCs employing 2‐(2‐chloro‐6‐oxo‐5,6‐dihydro‐4H‐cyclopenta[b]thiophen‐4‐ylidene) malononitrile (ClIC) unit terminated NFA. The results demonstrate the synergistic effect of asymmetry and halogenation toward tuning of the optoelectronic properties of NFAs for high performance OSCs.

Despite the substantial climb of the power conversion efficiency (PCE) of organic solar cells (OSCs), the majority of processing solvent remains halogenated and stand as a critical issue for commercialization. Herein, a halogen-free solvent system consisting of toluene (Tol) and 1-phenylnaphthalene (PN) is used to replace the traditional halogenated chloroform (CF) and1chloronaphthalene (CN) for the processing of the PM6:M36 OSC, reducing the maximum PCE from 15.0% to 13.3%. Hot-casting is demonstrated to boost the maximum PCE of halogen-free solvents processed OSCs back to 15.2%. The preheated substrate fastens the evaporation of Tol and enables similar film-forming kinetics to CF, resulting in the inhibition of immoderate molecular aggregation and excessive phase separation. Ternary OSCs, with either another donor or acceptor as the third component, can further improve device PCE to 15.8%, confirming the versatile photovoltaic systems that this hot-casting method can be applied to. Encouragingly, the hot-casting processed binary and ternary OSCs also exhibit retained storage stability. Therefore, hot-casting is demonstrated as a superior strategy to fabricate OSCs without efficiency and stability loss using halogen-free solvents.