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.

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

We have developed two nitrogen-bridged pentacyclic donor–acceptor dithienopyrrolobenzothiadiazole (DTPBT) and dithienopyrroloquinoxaline (DTPQX) arenes where the two outer electron-rich thiophene moieties are covalently fastened with the central electron-deficient benzothiadiazole and quinoxaline cores by two nitrogen bridges. These rigid and coplanar DTPBT and DTPQX building blocks were copolymerized with fluorene (F), carbazole (C) and cyclopentadithiophene (CPDT) units via Suzuki or Stille coupling polymerization to afford six new alternating copolymers PFDTPBT, PCDTPBT, PCPDTDTPBT, PFDTPQX, PCDTPQX and PCPDTDTPQX, respectively. The nitrogen bridges not only planarize the structure to induce stronger intermolecular π–π interaction but also play an important role in determining the electronic and photophysical properties of the polymers. The device based on PFDTPQX/PC71BM (1:4, w/w) exhibited a open-circuit voltage (V oc) of 0.72 V, a short-circuit current (J sc) of 8.62 mA/cm2 and a FF of 0.55 leading to a decent power conversion efficiency (PCE) of 3.40% due to the lower-lying HOMO energy level, and the highest hole-mobility of PFDTPQX.

Nitrogen-bridged donor-acceptor multifused dithienopyrrolobenzothiadiazole (DTPBT) and dibenzothiadiazolopyrrolothiophene (DBTPT) were successfully synthesized by intramolecular Cadogan annulation. The electron-deficient benzothiadiazole unit in DTPBT can be converted to benzoselenadiazole and quinoxaline moieties through reduction/cyclization to generate dithienopyrrolobenzoselenadiazole (DTPBSe) and dithienopyrroloquinoxaline (DTPQX), respectively. The nitrogen atoms function as the bridges for covalent planarization to induce intermolecular interaction and intramolecular charge transfer.

We have developed a ladder-type multifused thienylÀphenyleneÀ thienyleneÀphenyleneÀthienyl (TPTPT) unit where each thiophene ring is covalently fastened with the adjacent benzene rings by a carbon bridge, forming four cyclopentadiene rings embedded in a nonacyclic structure. This rigid and coplanar TPTPT building block was copolymerized with electron-deficient acceptors, dibromobenzothiadiazole (BT) or dibromodithienyldiketopyrrolopyrrole (DPP), via Stille polymerization. By varying the feed ratio of the monomers, a new series of random copolymers PTPTPTBT11, PTPTPTBT12, PTPTPTDPP11, PTPTPTDPP12, and PTPTPTDPP13 with tunable optical and electronic properties were prepared. The PTPTPTDPP12/PC 71 BM (1:4, w/w) based device exhibited the highest short circuit current (J sc ) of 10.78 mA/cm 2 with a good power conversion efficiency (PCE) of 4.3% due to the much boarder absorption ability and the highest hole mobility of PTPTPTDPP12. The devices based on PTPTPTDPP13, PTPTPTDPP11, PTPTPTBT12, and PTPTPTBT11 polymers also displayed promising efficiencies of 4.1%, 3.6%, 3.1%, and 2.8%, respectively. Most importantly, PTPTPTDPP12 has been demonstrated as a superior low-band-gap material for polymer solar cell with inverted architecture, achieving a high PCE of 5.1%.