b S Supporting Information ' INTRODUCTIONPolymeric solar cells (PSCs) have emerged as a promising alternative technique for producing clean and renewable energy due to their potential for fabrication onto large areas of lightweight flexible substrates by low-cost solution processing. To maximize the donor-acceptor heterojunction interfacial area for efficient exciton dissociation, mainstream PSC devices adopt the concept of a bulk heterojunction (BHJ), where an active layer contains a p-type donor and an n-type acceptor to form an interpenetrating nanoscale network. 1 A conventional BHJ PSC with an active layer sandwiched by a low-work-function aluminum cathode and a holeconducting poly(3,4-ethylenedioxylenethiophene):poly(styrene sulfonic acid) (PEDOT:PSS) layer on top of an indium tin oxide (ITO) substrate is the most widely used and investigated device configuration. On the basis of this device architecture, high powerconversion efficiencies (PCEs) of ca. 4-5% have been achieved for a blend containing a regioregular poly(3-hexylthiophene) (P3HT) and a fullerene derivative, [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM). 2 Along with high performance, long-term stability is a primary area of concern for PSCs. However, it is highly challenging to develop a PSC that can achieve a high PCE while maintaining good ambient stability of the device. Prolonged exposure to air rapidly reduces the performance of unencapsulated conventional devices. Rapid oxidation of the low-work-function metal cathode and etching of ITO by the acidic PEDOT:PSS layer are the most common reasons for instability in conventional unencapsulated devices. An effective approach to solve these problems, and improve device lifetime, is to fabricate inverted PSCs. 3 By reversing the polarity of charge collection in a regular cell, airstable Ag combining with an adjacent PEDOT:PSS layer can substitute for air-sensitive Al as the anodic electrode for efficient hole collection. In such an inverted configuration, it is necessary to insert an inorganic metal oxide (TiO x or ZnO) between ITO and the active layer to function as an electron-selective contact. 4 Despite dramatic improvement in the operational lifetime, inverted solar cells still suffer from a trade-off between stability and performance. Recently reported inverted devices based on P3HT/PCBM composite exhibited PCEs in the range of ca. 2-4%, which is inferior to that of regular solar cells. The relatively lower performance is

A new PC61BM‐based fullerene, [6,6]‐phenyl‐C61 butyric acid pentafluorophenyl ester (PC61BPF) is designed and synthesized. This new n‐type material can replace PC61BM to form a P3HT:PC61BPF binary blend or serve as an additive to form a P3HT:PC61BM:PC61BPF ternary blend. Supramolecular attraction between the pentafluorophenyl group of PC61BPF and the C60 cores of PC61BPF/PC61BM can effectively suppress the PC61BPF/PC61BM materials from severe aggregation. By doping only 8.3 wt% PC61BPF, device PC61BPF651 exhibits a PCE of 3.88% and decreases slightly to 3.68% after heating for 25 h, preserving 95% of its original value. When PC61BP with non‐fluorinated phenyl group is used to substitute PC61BPF, the stabilizing ability disappears completely. The efficiencies of PC61BP651 and PC61BP321 devices significantly decay to 0.44% and 0.11%, respectively, after 25 h isothermal heating. Most significantly, this strategy is demonstrated to be effective for a blend system incorporating a low band‐gap polymer. By adding only 10 wt% PC61BPF, the PDTBCDTB:PC71BM‐based device exhibits thermally stable morphology and device characteristics. These findings demonstrate that smart utilization of supramolecular interactions is an effective and practical strategy to control morphological evolution.

Three selenophene-incorporated quaterchalcogenophenebased donor−acceptor copolymers PFBT2Th2Se, PFBT2Se2Th, and PFBT4Se are designed and synthesized. To systematically fine-tune the molecular properties and investigate the chalcogen effect, PFBT2Th2Se and PFBT2Se2Th hybridize two thiophenes and two selenophenes as the donor with different isomeric main-chain placement while thiophene-free PFBT4Se uses quaterselenophene as the donor. On account of the selenophene's advantageous features such as higher quinoidal population and higher molecular polarizability, the three polymers show good light-harvesting ability, strong intermolecular interactions, high crystallinity, and high charge mobilities. Bulk-heterojunction solar cells incorporating these selenophenecontaining polymers have exhibited promising photovoltaic performance with impressive current densities over 20 mA/cm 2 . The device with the PFBT2Se2Th:PC 71 BM blend showed a PCE of 9.02% with a J sc of 21.02 mA/cm 2 . In addition, the device using quaterselenophene-based PFBT4Se:PC 71 BM blend exhibited a PCE of 8.92% with a superior J sc of 22.63 mA/cm 2 which represents one of the highest current densities from polymer:fullerene-based solar cells reported in the literature.

A new thieno­[3,2-b]­thiophene-incorporated acceptor TTC has been developed. The TTC acceptor was installed in a haptacyclic ladder-type core (BDCPDT) to furnish an n-type BDCPDT-TTC. The standard PBDB-T:BDCPDT-IC device showed a PCE of 9.33% with a V oc of 0.86 V and a J sc of 16.56 mA/cm2. By molecular engineering of the acceptor unit, the BDCPDT-TTC:PBDB-T-based device delivered an enhanced efficiency of 10.29% with a simultaneously enhanced V oc of 0.94 V and J sc of 17.72 mA/cm2. Incorporation of the electron-donating thieno­[3,2-b]­thiophene unit into the acceptor moiety decreases the electron-accepting strength, thereby upshifting the HOMO/LUMO energy levels to decrease the ΔE HOMO and E loss, achieving a larger V oc. Second, the extended conjugated bicyclic thieno­[3,2-b]­thiophene ring beneficially induces an additional optical transition at short wavelengths, leading to improvement of J sc. Alternatively, BDCPDT-FIC installed with the fluorinated acceptor shows more red-shifted absorption to achieve a high J sc of 19.12 mA/cm2.

Benzene-based 1,1-dicyanomethylene-3-indanone (IC) derivatives have been widely utilized as the end-group to construct acceptor−donor−acceptor type nonfullerene acceptors (A−D−A type NFAs). The extension of the end-group conjugation of nonfullerene acceptors (NFAs) is a rational strategy to facilitate intermolecular stacking of the end-groups which are responsible for efficient electron transportation. A bicyclic benzothiophene-based end-group acceptor, 2-(3-oxo-2,3-dihydro-1H-benzo[b]cyclopenta [d]thiophen-1-ylidene)malononitrile, denoted as α-BC was designed and synthesized. The Knoevenagel condensation of the unsymmetrical 1,3-diketo-precursor with one equivalent of malononitrile selectively reacts with the keto group attached at the α-position of the thiophene unit, leading to the isomerically pure benzothiophene-fused α-BC. The well-defined α-BC with extended conjugation was condensed with three different laddertype diformylated donors to form three new A−D−A NFAs named BDCPDT-BC, DTCC-BC, and ITBC, respectively. The corresponding IC-based BDCPDT-IC, DTCC-IC, and ITIC model compounds were also synthesized for comparison. The incorporation of the electron-rich benzothiophene unit in the end-group upshifts the lowest unoccupied molecular orbital energy levels of the NFAs, which beneficially enlarges the V oc values. On the other hand, the benzothiophene unit in α-BC not also imparts an optical transition in the shorter wavelengths around 340−400 nm for a better light harvesting ability but also promotes the antiparallel π−π stacking of the end-groups for efficient electron transport. The organic photovoltaic cell devices using a PBDB-T polymer and BC-based NFAs all showed the improved V oc and J sc values. The BDCPDT-BC-and DTCC-BCbased devices exhibited a power conversion efficiency (PCE) of 10.82 and 10.74%, respectively, which outperformed the corresponding BDCPDT-IC-, and DTCC-IC-based devices (9.33 and 9.25%). More importantly, the ITBC-based device delivered the highest PCE of 12.07% with a J sc of 19.90 mA/cm 2 , a V oc of 0.94 V, and an fill factor of 64.51%, representing a 14% improvement relative to the traditional ITIC-based device (10.05%).