OSCs have mainly employed bulk heterojunction (BHJ) structures in the photoactive layers, in which the blend casting (BC) of donor (D) and acceptor (A) materials can form interpenetrating networks with a large D/A interface area for exciton dissociation. However, it is challenging to delicately balance the self-aggregation and miscibility of the two components during the one-step deposition, involving complicated dynamic and kinetic processes. [12] Accordingly, the photovoltaic performances of BC devices depend strongly on the conditions of host solvents, [13] blending ratio of D:A, [14][15][16][17][18] processing additives, [19][20][21][22] and post-treatment. [23] Thus, it is difficult to control the film morphologies, especially the D/A distribution in the vertical direction of BC films, [12] which is closely related to the charge transport and collection.To tailor vertical phase distribution efficiently, the two-step deposition of D and A materials in a sequence, namely, the sequential deposition (SD) method, is considered as an alternative to the BC process. [24][25][26][27][28][29][30][31][32][33] Since the deposition of D and A can be performed independently, the SD OSCs offer unique advantages, including a favored vertical phase distribution and improved film morphology, which provides sufficient D/A interface area, and direct transport pathways for charge carriers. [34][35][36] Obviously, it is beneficial to exciton dissociation and chargeThe variation of the vertical component distribution can significantly influence the photovoltaic performance of organic solar cells (OSCs), mainly due to its impact on exciton dissociation and charge-carrier transport and recombination. Herein, binary devices are fabricated via sequential deposition (SD) of D18 and L8-BO materials in a two-step process. Upon independently regulating the spin-coating speeds of each layer deposition, the optimal SD device shows a record power conversion efficiency (PCE) of 19.05% for binary singlejunction OSCs, much higher than that of the corresponding blend casting (BC) device (18.14%). Impressively, this strategy presents excellent universality in boosting the photovoltaic performance of SD devices, exemplified by several nonfullerene acceptor systems. The mechanism studies reveal that the SD device with preferred vertical components distribution possesses high crystallinity, efficient exciton splitting, low energy loss, and balanced charge transport, resulting in all-around enhancement of photovoltaic performances. This work provides a valuable approach for high-efficiency OSCs, shedding light on understanding the relationship between photovoltaic performance and vertical component distribution.

Zhan et al. reported a fused ring electron acceptor (FREA) ITIC with an acceptordonor-acceptor (A-D-A) type structure. [1] Subsequently, numerous FREAs are developed by researchers, which largely promoted the photovoltaic performance of OSCs. Recently, Zou et al. developed a series of A-DA′D-A type FREAs (Y6 series). Up to now, the highest power conversion efficiency (PCE) of OSCs based on FREAs has reached 18%, demonstrating the bright future for practical application. [2][3][4][5][6][7][8][9] Although FREAs can achieve high efficiency, they are expensive and their syntheses are complicated, which usually includes low-yield ring-closure reactions. For the commercialization of OSCs, it is highly desired to develop highperformance and low-cost nonfullerene acceptors. Therefore, we want to develop simple nonfused ring electron acceptors (NFREAs) to replace the high-cost FREAs. In 2017, we first postulated and proved that intramolecular noncovalent interactions could be used to partially replace chemical bonds to synthesize NFREAs. [10] Since then, small fused ring building blocks such as indacenodithiophene (IDT), [11][12][13][14] cyclopentadithiophene (CPDT), [15][16][17][18][19][20][21][22][23][24] etc., were often used for the construction of NFREAs. Later, researchers also developed fully nonfused ring electron acceptors. [25][26][27] However, the photovoltaic performance of the solar cells based on NFREAs is still far behind the FREAs.Typical FREAs consist of a fused ring core, side chains, and two electron-withdrawing terminal groups. [28][29][30][31] The planar fused ring core is beneficial for the electron delocalization. The side chain can increase the solubility and affect the molecular stacking of acceptors. The electron-withdrawing terminal group such as 3-(1,1-dicyanomethylene)-5,6-difluoro-1-indanone can induce a strong intramolecular charge transfer (ICT) effect in the acceptor molecule, which can remarkably broaden the absorption, lower the energy levels (especially the lowest unoccupied molecular orbital (LUMO)), and enhance the molecular stacking. [32][33][34] Besides these basic characteristics, recent reports begin to focus on molecular stacking model. There is growing evidence that high-performance FREAs possess a 3D network packing structure, being in favor of facilitating multiple charge transport, achieving small exciton binding energy, and reducing energy loss in OSCs. [5,[35][36][37][38][39] Unlike rigid FREAs, the molecular Three nonfused ring electron acceptors (NFREAs; 2Th-2F, BTh-Th-2F, and 2BTh-2F) with thieno[3,2-b]thiophene bearing two bis(4-butylphenyl)amino substituents as the core, 3-octylthiophene or 3-octylthieno[3,2-b]thiophene as the spacer, and 3-(1,1-dicyanomethylene)-5,6-difluoro-1-indanone as the terminal group are designed and synthesized. The molar extinction coefficient of acceptors and the electron mobility of blend films gradually increase with increasing π-conjugation length. Moreover, 2BTh-2F displays a planar molecular conformation assisted by S•••N and S•••O intr...

Volatile solid additives (SADs) are considered as a simple yet effective approach to tune the film morphology for high-performance organic solar cells (OSCs). However, the structural effects of the SADs on the photovoltaic performance are still elusive. Herein, two volatilizable SADs were designed and synthesized. One is SAD1 with twisted conformation, while the other one is planar SAD2 with the S···O noncovalent intramolecular interactions (NIIs). The theoretical and experimental results revealed that the planar SAD2 with smaller space occupation can more easily insert between the Y6 molecules, which is beneficial to form a tighter intermolecular packing mode of Y6 after thermal treatment. As a result, the SAD2-treated OSCs exhibited less recombination loss, more balanced charge mobility, higher hole transfer rate, and more favorable morphology, resulting in a record power conversion efficiency (PCE) of 18.85% (certified PCE: 18.7%) for single-junction binary OSCs. The universality of this study shed light on understanding the conformation effects of SADs on photovoltaic performances of OSCs.

The rapid advance of fused‐ring electron acceptors (FREAs) has greatly promoted the leap‐forward development of organic solar cells (OSCs). However, the synthetic complexity of FREAs may be detrimental for future commercial applications. Recently, nonfused‐ring electron acceptors (NREAs) have been developed to be a promising candidate to maintain a rational balance between cost and performance, of which the cores are composed of simple fused rings (NREAs‐I) or nonfused rings (NREAs‐II). Moreover, “noncovalently conformational locks”, are used as an effective strategy to enhance the rigidity and planarity of NREAs and improve device performance. Herein, a novel series of NREAs‐II (PhO4T‐1, PhO4T‐2, and PhO4T‐3) is constructed as a valuable platform for exploring the impact of the end group engineering on optoelectronic properties, intermolecular packing behaviors, and device performance. As a result, a high power conversion efficiency of 13.76% is achieved for PhO4T‐3 based OSCs, which is much higher than those of the PhO4T‐1 and PhO4T‐2‐based devices. Compared with several representative FREAs, PhO4T‐3 possesses the highest figure‐of‐merit value of 133.45 based on a cost‐efficiency evaluation. This work demonstrates that the simple‐structured NREAs‐II are promising candidates for low‐cost and high‐performance OSCs.

The large energy loss (E loss ) 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 (E CT ) and thus the E loss for boosting the efficiency of OSCs. The elevated E CT in the ternary OSCs reduce the energy loss for charge generation (𝚫E CT ), and promote the hybridization between localized excitation state and CT state to reduce the nonradiative energy loss (𝚫E nonrad ). Together with the optimal morphology, the ternary OSCs afford an impressive power conversion efficiency of 19.22% with a significantly improved open-circuit voltage (V oc ) of 0.910 V without sacrificing short-cicuit density (J sc ) and fill factor (FF) in comparison to the binary ones. This contribution reveals that precisely tuning the E CT via donor alloy strategy is an efficient way to minimize E loss and improve the photovoltaic performance of OSCs.