A novel PCBM-based n-type material, [6,6]-phenyl-C(61)-butyric styryl dendron ester (PCBSD), functionalized with a dendron containing two styryl groups as thermal cross-linkers, has been rationally designed and easily synthesized. In situ cross-linking of PCBSD was carried out by heating at a low temperature of 160 degrees C for 30 min to generate a robust, adhesive, and solvent-resistant thin film. This cross-linked network enables a sequential active layer to be successfully deposited on top of this interlayer to overcome the problem of interfacial erosion and realize a multilayer inverted device by all-solution processing. An inverted solar cell device based on an ITO/ZnO/C-PCBSD/P3HT:PCBM/PEDOT:PSS/Ag configuration not only achieves enhanced device characteristics, with an impressive PCE of 4.4%, but also exhibits an exceptional device lifetime without encapsulation; it greatly outperforms a reference device (PCE = 3.5%) based on an ITO/ZnO/P3HT:PCBM/PEDOT:PSS/Ag configuration without the interlayer. This C-PCBSD interlayer exerts multiple positive effects on both P3HT/C-PCBSD and PCBM/C-PCBSD localized heterojunctions at the interface of the active layer, including improved exciton dissociation efficiency, reduced charge recombination, decreased interface contact resistance, and induction of vertical phase separation to reduce the bulk resistance of the active layer as well as passivation of the local shunts at the ZnO interface. Moreover, this promising approach can be applied to another inverted solar cell, ITO/ZnO/C-PCBSD/PCPDTBT:PC(71)BM/PEDOT:PSS/Ag, using PCPDTBT as the p-type low-band-gap conjugated polymer to achieve an improved PCE of 3.4%. Incorporation of this cross-linked C(60) interlayer could become a standard procedure in the fabrication of highly efficient and stable multilayer inverted solar cells.

A poly(3-hexylthiophene) (P3HT)-based inverted solar cell using indene-C 60 bis-adduct (ICBA) as the acceptor achieved a high open-circuit voltage of 0.82 V due to ICBA's higher-lying lowest unoccupied molecular orbital level, leading to an exceptional power-conversion efficiency (PCE) of 4.8%. By incorporating a cross-linked fullerene interlayer, C-PCBSD, to further modulate the interface characteristics, the ICBA:P3HT-based inverted device exhibited an improved short-circuit current and fill factor, yielding a record high PCE of 6.2%.Polymeric solar cells (PSCs) offer great potential for fabrication of large-area, lightweight, and flexible organic solar cells by using low-cost printing and coating technologies.1 A conventional bulk heterojunction (BHJ) PSC with an active layer sandwiched by a lowwork-function aluminum cathode and a hole-conducting poly(3,4-ethylenedioxylenethiophene):poly(styrenesulfonic acid) (PEDOT:PSS) layer on top of an indium tin oxide (ITO) substrate is the most widely used and researched device configuration. Utilizing this device architecture, devices incorporating a blend of a regioregular poly(3-hexylthiophene) (P3HT) and a fullerene derivative, [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM), have achieved power-conversion efficiencies (PCEs) approaching 5%.2 During the past 2 years, several important low-band-gap polymers with enhanced absorption abilities have appeared. Researchers made a breakthrough in fabricating PSC devices with PCEs of up to 5-7% based on these polymers. 3Alongside high performance, long-term stability is a primary area of concern for PSCs. 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 ameliorate these problems, and improve device lifetime, is to fabricate inverted PSCs. By reversing the polarity of charge collection in a regular cell, air-stable Ag combined with an adjacent PEDOT:PSS layer can substitute for airsensitive Al as the anodic electrode for efficient hole collection. Despite a dramatic improvement in operational lifetime, standard inverted PSCs suffer from a trade-off between performance and stability. The relatively lower performance is attributed to the unfavorable energetics and incompatible chemical interfaces. Extensive efforts to improve the efficiency of inverted PSCs by modifying the interface include inserting Cs 2 CO 3 to reduce the ITO work function, 4 using metal oxides such as TiO x and ZnO to function as electron-selecting layers, 5 modifying the TiO x or ZnO surface with a self-assembled C 60 monolayer, 6 using MoO 3 as the hole-extracting buffer, 7 and employing an optical spacer to redistribute the optical field intensity.8 However, PCEs of P3HT/PCBM-based inverted PSCs are mostly in the range of ca. 2-4%, which is inferior to that of conventional PSCs. So far, PCE values greater than 5% are unreported in any form of inverted PSC. Recently, a cross-link...

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

Pentacyclic diindeno[1,2-b:2',1'-d]thiophene (DIDT) unit is a rigid and coplanar conjugated molecule. To the best of our knowledge, this attractive molecule has never been incorporated into a polymer and thus its application in polymer solar cells has never been explored. For the first time, we report the detailed synthesis of the tetra-alkylated DIDT molecule leading to its dibromo- and diboronic ester derivatives, which are the key monomers for preparation of DIDT-based polymers. Two donor-acceptor alternating polymers, poly(diindenothiophene-alt-benzothiadiazole) PDIDTBT and poly(diindenothiophene-alt-dithienylbenzothiadiazole) PDIDTDTBT, were synthesized by using Suzuki polymerization. Copolymer PTDIDTTBT was also prepared by using Stille polymerization. Although PTDIDTTBT is prepared through a manner of random polymerization, we found that the different reactivities of the dibromo-monomers lead to the resulting polymer having a block copolymer arrangement. With the higher structural regularity, PTDIDTTBT, symbolized as (thiophene-alt-DIDT)(0.5)-block-(thiophene-alt-BT)(0.5), shows the higher degree of crystallization, stronger π-π stacking, and broader absorption spectrum in the solid state, as compared to its alternating PDIDTDTBT analogue. Bulk heterojunction photovoltaic cells based on ITO/PEDOT:PSS/polymer:PC(71)BM/Ca/Al configuration were fabricated and characterized. PDIDTDTBT/PC(71)BM and PTDIDTTBT/PC(71)BM systems exhibited promising power-conversion efficiencies (PCEs) of 1.65 % and 2.00 %, respectively. Owing to the complementary absorption spectra, as well as the compatible structures of PDIDTDTBT and PTDIDTTBT, the PCE of the device based on the ternary blend PDIDTDTBT/PTDIDTTBT/PC(71)BM was further improved to 2.40 %.