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The side‐chain has a significant influence on the optical properties and aggregation behaviors of the organic small molecule acceptors, which becomes an important strategy to optimize the photovoltaic performance of organic solar cells. In this work, we designed and synthesized three brand‐new nonfused ring electron acceptors (NFREAs) OC4‐4Cl‐Ph, OC4‐4Cl‐Th, and OC4‐4Cl‐C8 with hexylbenzene, hexylthiophene, and octyl side chains on the π‐bridge units. Compared with OC4‐4Cl‐Ph and OC4‐4Cl‐Th, OC4‐4Cl‐C8 with linear alkyl side chain has more red‐shift absorption, which is conducive to obtaining higher short‐circuit current density. Additionally, the OC4‐4Cl‐C8 film exhibits a longer exciton diffusion distance, and the D18:OC4‐4Cl‐C8 blend film displays faster hole transfer, weaker bimolecular recombination, and more efficient exciton transport. Furthermore, The D18:OC4‐4Cl‐C8 blend films may effectively form interpenetrating networks that resemble nanofibrils, which can facilitate exciton dissociation and charge transport. Finally, OC4‐4Cl‐C8‐based devices can be created a marvellously power conversion efficiency (PCE) of 16.56%, which is much higher than OC4‐4Cl‐Ph (12.29%)‐ and OC4‐4Cl‐Th‐based (11.00%) ones, being the highest PCE among the NFREA based binary devices. All in all, we have validated that side‐chain engineering is an efficient way to achieve high‐performance NFREAs. [ABSTRACT FROM AUTHOR]

The large energy loss (Eloss) is one of the main obstacles to further improve the photovoltaic performance of organic solar cells (OSCs), which is closely related to the charge transfer (CT) state. Herein, ternary donor alloy strategy is used to precisely tune the energy of CT state (ECT) and thus the Eloss for boosting the efficiency of OSCs. The elevated ECT in the ternary OSCs reduce the energy loss for charge generation (ΔECT), and promote the hybridization between localized excitation state and CT state to reduce the nonradiative energy loss (ΔEnonrad). Together with the optimal morphology, the ternary OSCs afford an impressive power conversion efficiency of 19.22% with a significantly improved open‐circuit voltage (Voc) of 0.910 V without sacrificing short‐cicuit density (Jsc) and fill factor (FF) in comparison to the binary ones. This contribution reveals that precisely tuning the ECT via donor alloy strategy is an efficient way to minimize Eloss and improve the photovoltaic performance of OSCs. [ABSTRACT FROM AUTHOR]

Two new fused‐ring electron acceptor (FREA) isomers with nonlinear and linear molecular conformation, m‐BAIDIC and p‐BAIDIC, are designed and synthesized. Despite the similar light absorption range and energy levels, the two isomers exhibit distinct electron reorganization energies and molecular packing motifs, which are directly related to the molecular conformation. Compared with the nonlinear acceptor, the linear p‐BAIDIC shows more ordered molecular packing and higher crystallinity. Furthermore, p‐BAIDIC‐based devices exhibit reduced nonradiative energy loss and improved charge transport mobilities. It is beneficial to enhance the open‐circuit voltage (VOC) and short‐current current density (JSC) of the devices. Therefore, the linear FREA, p‐BAIDIC yields a relatively higher efficiency of 7.71% in the binary device with PM6, in comparison with the nonlinear m‐BAIDIC. When p‐BAIDIC is incorporated into the binary PM6/BO‐4Cl system to form a ternary system, synergistic enhancements in VOC, JSC, fill factor (FF), and ultimately a high efficiency of 17.6% are achieved. [ABSTRACT FROM AUTHOR]

The donor/acceptor interaction in non‐fullerene organic photovoltaics leads to the mixing domain that dictates the morphology and electronic structure of the blended thin film. Initiative effort is paid to understand how these domain properties affect the device performances on high‐efficiency PM6:Y6 blends. Different fullerenes acceptors are used to manipulate the feature of mixing domain. It is seen that a tight packing in the mixing region is critical, which could effectively enhance the hole transfer and lead to the enlarged and narrow electron density of state (DOS). As a result, short‐circuit current (JSC) and fill factor (FF) are improved. The distribution of DOS and energy levels strongly influences open‐circuit voltage (VOC). The raised filling state of electron Fermi level is seen to be key in determining device VOC. Energy disorder is found to be a key factor to energy loss, which is highly correlated with the intermolecular distance in the mixing region. A 17.53% efficiency is obtained for optimized ternary devices, which is the highest value for similar systems. The current results indicate that a delicate optimization of the mixing domain property is an effective route to improve the VOC, JSC, and FF simultaneously, which provides new guidelines for morphology control toward high‐performance organic solar cells. [ABSTRACT FROM AUTHOR]

In this work, we successfully applied a fullerene derivative PC 71 BM as the second acceptor in PM6:BTP-BO-4Cl bulk heterojunction organic solar cell system. The device performance can be remarkably enhanced from 16.65% of the PM6:BTP-BO-4Cl binary system to approaching 17.4% of the ternary system with just low PC 71 BM loading. Detailed investigation revealed that introduction of small amount of PC 71 BM can improve the molecular aggregation and film morphology, thus display better exciton dissociation and charge transport. [Display omitted] In this work, we introduced a fullerene acceptor (PC 71 BM) into the binary photo-active layer based on a polymer donor (PM6) and a non-fullerene small molecular acceptor (BTP-BO-4Cl), and as a consequence, the ternary organic solar cells realized a high-power conversion efficiency of 17.39% compared to 16.65% in binary solar cells. The performance enhancement was found to be due to the optimized morphology and hence balanced hole and electron mobilities, which is responsible for the suppressed charge recombination and hence high photocurrent in solar cells. In addition, PC 71 BM shows the complementary absorption with PM6 and BTP-BO-4Cl, which can broaden the absorption range of the photo-active layer and hence more photons from the sunlight can be utilized. Besides, PC 71 BM shows the cascade energy level alignment between PM6 and BTP-BO-4Cl, which is helpful for charge transfer from donor to acceptor. All these merits explain the high performance in ternary solar cells, and also demonstrate that ternary photovoltaics adopting non-fullerene acceptor with the fullerene acceptor as small amount of additive is an efficient strategy to gain high performing organic solar cells. [ABSTRACT FROM AUTHOR]

• Acceptor fluorination leads to increased voltage losses in organic solar cells. • The increased voltage loss is associated with increased reorganization energy. • A side chain modification strategy is proposed to address the voltage loss issue. Acceptor fluorination is a commonly employed molecular modification strategy aimed at enhancing the quantum efficiency and charge carrier transport properties of organic solar cells. However, this approach often leads to a significant reduction in the open-circuit voltage (V oc) of the solar cells, thereby limiting their overall performance. In this study, we investigate the limitation of V oc in organic solar cells based on fluorinated acceptors. Our investigation reveals that the reduced V oc can be attributed to increased voltage losses (V loss), primarily arising from more significant non-radiative voltage losses originating from a decline in the external quantum efficiency of electroluminescence (EQE EL). Furthermore, we establish that the reduced EQE EL is a result of deteriorated molecular packing, which leads to an elevated reorganization energy. To address the high V loss and limited performance observed in solar cells utilizing fluorinated acceptors, we propose a strategy involving acceptor side-chain modifications. These results provide new insights into enhancing the performance of organic solar cells. [ABSTRACT FROM AUTHOR]

• Increasing donor side chain size leads to reduced CT dissociation efficiency. • Increasing acceptor side chain size results in reduced device voltage losses. • Highest PCE is realized when both the donor and acceptor are with large side chains. Increasing the size of the side chains of the donor or the acceptor materials is a highly effective strategy to reduce voltage losses, and increase the open-circuit voltage (Voc) of organic solar cells (OSCs). However, higher Voc does not necessarily translate to improved overall performance. To optimize side-chains and improve the performance of OSCs, the origin of the impact of the increased size of the donor and/or acceptor side side-chains size on the performance of OSCs is investigated. The study demonstrates that increasing the size of donor side chains can increase Voc, but it can also lead to deteriorated molecular packing, limiting overall solar cell performance. On the other hand, increasing the size of acceptor side chains can also increase Voc and improve molecular packing properties of the active layer, resulting in improved solar cell performance. Finally, the study shows that the best photovoltaic performance is achieved when both the donor and acceptor have large side chains. This research establishes the relationship between side chains of active materials, molecular packing properties, and the energetics of the donor/acceptor active layer, which is crucial for overcoming performance bottlenecks in OSCs. [ABSTRACT FROM AUTHOR]

[Display omitted] • Three small molecular acceptors were designed and synthesized. • Noncovalent Conformational Locks and Side-Chain Engineering can adjust the properties of acceptors. • Significantly different power conversion efficiencies of 12.09%, 12.30% and 14.25% are achieved. Three novel small molecular acceptors (SMAs) BO-2F , BO-C8-2F and BO-C2C6-C8-2F were designed and synthesized. These acceptors all possess planar molecular backbone because of the introduction of intramolecular noncovalent interaction. Among them, BO-C2C6-C8-2F with four side chains, which has the highest solubility, can form more appropriate nanomorphology when blending with PBDB-T. Hence, organic solar cells (OSCs) based on PBDB-T: BO-C2C6-C8-2F displayed an outstanding power convesion efficiency (PCE) of 14.25%, which is higher than BO-2F and BO-C8-2F based ones. To the best of our knowledge, the PCE of 14.25% is one of the highest for the OSCs based on A-π-D-π-A type SMAs. Our results have demonstrated that the combination of side-chain engineering and intramolecular conformation lock has a great prospect in the design of high performance SMAs for OSCs. [ABSTRACT FROM AUTHOR]

[Display omitted] • We can modularly synthesize two fully nonfused ring acceptors with only four high-yield steps. • A simple fully nonfused ring electron acceptor can achieve a high PCE of over 12%. • The high FOM of 4T-OEH indicates its great potential for future practical applications. We designed two simple fully nonfused ring electron acceptors 4T-OEH and 4T-EH with 3,4-bis(alkoxy)thiophene and 3,4-dialkylthiophene π-bridge units, respectively, which can be modularly synthesized with only four high-yield steps. With the help of intramolecular S-O noncovalent interaction, 4T-OEH tends to form a planar molecular backbone, which is beneficial for the electron delocalization and charge transport. Besides, the neat 4T-OEH film displays a more ordered molecular packing and obviously red-shifted absorption after thermal annealing. Compared with 4T-EH , 4T-OEH based devices can form more homogenous phase morphology, higher and more balanced hole and electron mobilities. The optimal OSCs based on 4T-OEH can generate an excellent PCE of 12.12%, which is much higher than 4T-OEH based ones (7.36%). It is worth noting that the figure-of-merit values of 4T-OEH is much higher than the star acceptors (ITIC and Y6), demonstrating its high potential for future practical application. Our work has demonstrated that we can tune the backbone planarity, solubility and packing behavior of acceptor molecules via optimizing their lateral substituents to obtain high efficiency and low-cost fully nonfused ring electron acceptors. [ABSTRACT FROM AUTHOR]

• Minor terminal and central changes endow DO-2F and FO-N complementary absorption. • Asymmetrical FO-N effectively suppress the self-aggregation of DO-2F. • Acceptor alloy is found between the FO-N and DO-2F. • Alloy state acceptors produce suitable morphology with reduced V nr of 0.23 eV. • FO-N is an excellent third component for highly efficient OSCs of 14.10% Nonfused ring acceptors have shown great potential for future commercial applications of organic solar cells (OSCs) due to their simple structure and low synthetic costs. Herein, a novel asymmetrical nonfused ring acceptor FO-N with one branched lateral chain and one fluoro substituent at the central phenylene core was designed and introduced into PBDB-T : DO-2F binary system to fabricate high-efficiency ternary OSCs. The high-lying LUMO level of FO-N is conducive to improve V oc for ternary devices. More importantly, the introduction of FO-N as the third component can efficiently improve the compatibility of the ternary system via tuning the crystal size and optimizing the blend film morphology, which is beneficial to charge separation and reduced the non-radiative energy loss (Δ E nonrad). Finally, the ternary device achieved a low Δ E nonrad of 0.23 eV and a greatly improved PCE of 14.10% and with a V oc of 0.88 V, a J sc of 21.48 mA/cm2 and an FF of 74.41%. These results imply that the asymmetric nonfused ring acceptor design strategy is an effective way for optimizing the photovoltaic performance of highly crystallized blend system. [ABSTRACT FROM AUTHOR]

[Display omitted] • Two isomeric fully nonfused ring electron acceptors are designed and synthesized. • The location of diphenylamine units can adjust the properties of acceptors. • Significantly different power conversion efficiencies of 12.83% and 0.43% are achieved. We have designed and synthesized two fully nonfused ring electron acceptors LW-in-2F and LW-out-2F with two diphenylamine side chains at different positions of the bithiophene core. The two diphenylamine side chains of LW-in-2F and LW-out-2F are at the outside and inside positions of the central bithiophene unit, respectively. The performance of LW-in-2F and LW-out-2F -based devices is very different, the photoelectric conversion efficiency (PCE) of LW-out-2F -based device is as high as 12.83%, while the efficiency of LW-in-2F -based device is only 0.43%. By GIWAXS test, we found that the change of diphenylamine position affects the way of molecular arrangement. When the diphenylamine side chain is located on the outer side of the central core, the molecular arrangement shows face-on orientation, which is beneficial for perpendicular charge transport, resulting in high and balanced charge mobilities and high PCE. [ABSTRACT FROM AUTHOR]