The last two years have witnessed the rapid progress of organic solar cells (OSCs), driven by the newly developed nonfullerene acceptor (NFA) Y6, which contains an electron-deficient-core-based central fused ring....

Symmetric conjugated molecules can be broken through suitable synthetic strategies to construct novel asymmetric molecules, which can largely broaden the material library. In the field of organic solar cells, fused‐ring electron acceptors (FREAs) with the A‐DA'D‐A type backbone structure have attracted much attention and enabled power conversion efficiencies (PCE) exceeding 18%. Among them, Y6 is one of the most classic FREAs that can derive many symmetric and asymmetric molecules and exhibit unique optoelectronic properties. Thus, in this review, the focus is on the recent progress of Y6‐derived asymmetric FREAs containing a dipyrrolobenzothiadiazole segment, which can be classified as the following three categories: asymmetric end group, asymmetric central core and asymmetric side chain. The relationship of the molecular structure, optoelectronic properties, and device performance is discussed in detail. Finally, the future design directions and challenges faced by this kind of photovoltaic materials are given.

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Molecular conformation has a striking function in dominating the molecular orientation, crystallinity, charge mobility, film morphology, and photovoltaic performance for classic donor−π–acceptor (D−π–A) type copolymers. Herein, we systematically investigate this correlation by modulating the chemical structures of conjugated polymers composed of a new emerging electron-donating building block of 4,8-bis­(4-chlorothiophen-2-yl)­benzo­[1,2-b:4,5-b′]­dithiophene (BDT-T-Cl). A D−π–A type copolymer PE31 is synthesized as the reference donor polymer, where BDT-T-Cl, benzotriazole (BTA), and thiophene (T) units are used as the D, A, and π-bridge, respectively. PE31 realizes a moderate power conversion efficiency (PCE) of 7.62% when paired with a nonfullerene acceptor Y6. The methoxy substitutes in the BTA unit leads to a twisted backbone conformation of the final polymer PE32, which has negative effects on the charge transport and results in a slightly reduced PCE of 7.31%. The intra/interchain noncovalent interactions can be modulated by importing fluorine atoms on the BTA unit (J52-Cl) and further changing the π-bridge from thiophene to 3-hexylthieno­[3,2-b]­thiophene (PE4). The backbone of PE4 can be changed to a linear conformation, different from the zigzagged conformation of the other three polymers with thiophene as the π-bridge. PE4:Y6 shows the highest PCE of 14.02% with the highest fill factor (FF) of 0.75, superior to J52-Cl:Y6 combination (PCE = 12.31% and FF = 0.61). Our results clearly indicate that changing the backbone conformation by π-bridge engineering has an important significance of achieving the optimal active layer morphology and promising photovoltaic performance.

A new hole transporting material (HTM) named DMZ is synthesized and employed as a dopant‐free HTM in inverted planar perovskite solar cells (PSCs). Systematic studies demonstrate that the thickness of the hole transporting layer can effectively enhance the morphology and crystallinity of the perovskite layer, leading to low series resistance and less defects in the crystal. As a result, the champion power conversion efficiency (PCE) of 18.61% with JSC = 22.62 mA cm−2, VOC = 1.02 V, and FF = 81.05% (an average one is 17.62%) is achieved with a thickness of ≈13 nm of DMZ (2 mg mL−1) under standard global AM 1.5 illumination, which is ≈1.5 times higher than that of devices based on poly(3,4‐ethylenedioxythiophene)/poly(styrene sulfonic acid) (PEDOT:PSS). More importantly, the devices based on DMZ exhibit a much better stability (90% of maximum PCE retained after more than 556 h in air (relative humidity ≈ 45%–50%) without any encapsulation) than that of devices based on PEDOT:PSS (only 36% of initial PCE retained after 77 h in same conditions). Therefore, the cost‐effective and facile material named DMZ offers an appealing alternative to PEDOT:PSS or polytriarylamine for highly efficient and stable inverted planar PSCs.

Herein, a new bifunctional saddle-shaped organic small molecule named 2,2 0 ,7,7 0 -tetrakis(N, N-di-p-methoxyphenyl-aniline)-α, β-cycloocta[1,2-b:4,3-b 0 :5,6b 0 :8,7-b 000 ]tetrathiophenyl (α, β-COTh-Ph-OMeTAD) is synthesized. When compared with spiro-OMeTAD, a star hole transporting material (HTM) for highly efficient perovskite solar cells, the new material has a deeper highest occupied molecular orbital (HOMO) energy level of À5.30 eV, and a higher hole mobility of 2.88 Â 10 À4 cm 2 V À1 s À1 . With dopant-free α, β-COTh-Ph-OMeTAD as a HTM and an interfacial layer combinined with chlorobenzene (CB) as the anti-solvent, mesoporous perovskite solar cells (PSCs) are fabricated, which exhibit a power conversion efficiency (PCE) of 17.22% under AM 1.5 conditions, which is a little higher than that of devices based-on doped spiro-OMeTAD under the same conditions, which is 16.83%. Notably, the PSCs devices with dopant-free α, β-COTh-Ph-OMeTAD as both the HTM and interfacial layer show better stability, and after being stored in dark and dry air without encapsulation for nearly 800 h, the PCE can still be maintained at 86% of the maximum. This opens a new avenue for efficient and stable PSCs by exploring new dopant-free materials as alternatives to spiro-OMeTAD.

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The third component featuring a planar backbone structure similar to the binary host molecule has been the empirical formula for improving the photovoltaic performance of ternary organic solar cells (OSCs). In this work, we explored a new avenue that introduces 3‐dimensional structured molecules as guest acceptors. Spirobifluorene (SF) is chosen as the core to combine with three different terminal‐modified (rhodanine, thiazolidinedione, and dicyano‐substituted rhodanine) benzotriazole (BTA) units, affording three 4‐arm molecules, SF‐BTA1, SF‐BTA2, and SF‐BTA3, respectively. After adding these three materials into the classical system PM6: Y6, the resulting ternary devices obtained ultra‐high power conversion efficiencies (PCEs) of 19.1%, 18.7%, and 18.8%, respectively, compared with the binary OSCs (PCE = 17.4%). SF‐BTA1‐3 can work as energy donors to increase charge generation via energy transfer. In addition, the charge transfer between PM6 and SF‐BTA1‐3 also acts to enhance charge generation. Introducing SF‐BTA1‐3 could form acceptor alloys to modify the molecular energy level and inhibit the self‐aggregation of Y6, thereby reducing energy loss and balancing charge transport. Our success in 3D multi‐arm materials as the third component shows good universality and brings a new perspective. The further functional development of multi‐arm materials could make OSCs more stable and efficient.